~~Cutting out / Backfiring at high RPM!!~~~

[FecalFajita]

So I finally got the boat (1987 Sunsport, 351 w/ clip down distributor cap) running reliably after almost two years of steady work on it.

One problem remains though, When I get the motor up to 3200rpm and above, it randomly backfires and knocks, losing power. Sometimes not at all, sometimes up to once every 10 seconds. It seems to do it more with weight in the boat, and I could be imagining, but it also seems to happen when I hit big waves.

The boat rips and runs great besides this...

It did it before replacing or changing anything, but things that are new;

plugs (properly gapped, don't remember the spec.)
wires
distributor cap
rotor
electronic ignition
coil
timing set to 6 degrees

fuel lines
fuel filter
carb was recently rebuilt

Anyone have any ideas on what's going on here? 90% of the time it runs great and it's hard to keep the boat in the driveway with the way the weather has been...but I'm sure this isn't too healthy. Thanks for any input

[Longhorn21]

knocks, losing power, and backfire are all signs of the timing being off or worn chain. I'm not saying thats what it is, but could be. What kind of boat do you have cause i thought most pcm engine are set to 10 degrees

[87SunSportMikeyD]

Hey Sean - To be honest bud this is kind of what mine is doing too. Sometimes it starts to sputter once or twice a second which causes power loss and surging unless I throttle back.

They thought it was a transmission damper plate, but not the problem. They checked the chain and say it seems looks but fine. He is going to start tearing into the motor and is looking around to price out new motors.... Gulp....sniffle.....

Longhorn is there any way this can be electrical related like distributer that is affecting my ability to set timing? We have a timing light and have adjusted timing many times.
Both boats are 87 SunSports w/ pcm 351

[FecalFajita]

Wow, how did I forget that?

Boat is a 87 sunsport.
351 with a clip down distributor cap.

[fastab]

I ran a 1990 Comp up till this year and know a number of others with similar boats. We would run it with lots of weight and started having this problem. It would run irratic etc. It was not an electrical problem but rather water in the fuel. On the 90 the fuel vent is located at the back of the boat just below the gunnel molding and back splash or all of the people on one side can cause it to push water into the tank.

Water does not flow the same as gas, it has a hard time flowing thru the jets.
It is heavier than the fuel and will sit in the bottom, at times of high load the fuel is going to flow fast(more demand) the water in the carb float bowl can cause the engine to sputter, die, run poorly depending on how much is in the carb and fuel filter. When you shut the boat of the water will settle to the bottom and the boat may run fine.

If you have a fuel vent on the back or side below the gunnel I would relocate it to where there is no chance of water getting into it.

[FecalFajita]

Water is one thing that I haven't thought of. The vent for the tank is on the port side in the same area as the handle/rope tie off in the rear. It doesn't seem like water could get in there easily, but who knows.

What's the best way to go about checking for water in the gas? Run it low and dump the remainder of the tank, fuel lines, and whatever is in the carb? Or is there an easier way?

Also, google has brought up a few cases where people have had the plug wires in the wrong places, so I've been looking into what the correct firing order is, and I can only find info on the newer style (bolt on cap) distributors. Is the order the same for the older style (clip down cap) ?

I only had a minute to look at it today and I could be wrong, but it doesn't seem like it matches the patterns I found.

[Longhorn21]

Go to plant nautique website and they have the manual their for free. It's got the firing order in it

[fastab]

If the fuel vent is at the back of the boat when you have people moving around all to one side the vent can go under and take in some water if this happens several times you continue to buildup water in the fuel. The water sits on the bottom of the tank. The fuel is picked up from the bottom of the tank, so if there is enough water it will pick it up first.


Water will sit at the bottom, while there is no movement. If you have a water/fuel filter that has a cartridge you can take it off remove the cartridge and see how much water is in the cartridge and other dirt, if it had a spin on you can dump it into a clean container to inspect. Let it settle the water will go to the bottom. You can also get rust from the filter housings. Rust/dirt can do similar things. You would have to pull the carb apart if there is anything in it other than water or fuel. On mine it had a Holley and I would pull one of the bottom bolts for the float bowls draining the fuel into a clean container, I would usually cut the top of a emty pop can. You may need a new gasket with for the bolt so you may want to find this before you do it.

If it is a water problem the solution is to clean the fuel tank, carb and reroute the vent.

[rollenrock33]

My 87 Sunsport is doing the same thing. Only at high rpm's though. The water in the fuel makes sense but I really don't see how fuel can go through the one way vent on the port side gunwale. I thought being that its one way (only air goes out, nothing can go in) this would prevent water getting into the fuel tank? Anyone try putting dry gas in the tank? I know we use it in the winter to get rid of condensation. Would be an easy fix. Any other ideas?

[87SunSportMikeyD]

We double checked our plug wire order a bunch of times and replaced them too. We spent some time checking the spark from each plug and noticed there seemed to be two plugs that were not sparking strongly or barely at all. We were considering replaceing the distributer cap or possible the whole distributer assembly.

I will call and have him double check for water in the gas right now as well as debris in the carb and integrity of inside of fuel line.

Just called mechanic and he seemed to think that the water would only affect the motor for a few minutes and then would be gone.

[Supra-in-steamboat]

I'm with Mikey's mechanic. The water would have an effect, but the symptoms (knock, backfire, power loss, etc) would not occur so consistantly as your describing them- ie. only when pulling, or weighted. I hope you guys get this dialed soon. Continued good luck.

[Salty87]

are you guys running holley's?...checked your power valves?

[87SunSportMikeyD]

Yes and Yes

[fastab]

Your fuel tank vent is 2 ways, it needs to let air in to replace the fuel as it is burned and it needs to let pressure or fuel out if the tank is full or changes temps.

[fastab]

I'm with Mikey's mechanic. The water would have an effect, but the symptoms (knock, backfire, power loss, etc) would not occur so consistantly as your describing them- ie. only when pulling, or weighted. I hope you guys get this dialed soon. Continued good luck.

A small amount of water will not be that consistant, however if you are getting water in the fuel tank vent, it will sit at the bottom of the tank, the fuel is picked up from the bottom of the fuel tank, it will then travel to the fuel/water strainer. When this is full it will pass and go to the carb, it will continue to come to the carb untill it is all gone. On some units the strainer bowl will rust, this will also cause similar problems in the carb. When you shut the motor off (fuel flow) the water will sit at the bottom of the floatbowl until there is enough of a draw to get it to the jets.

It is easy to see, check your water fuel seperator, drain the float bowls, collect any fuel that comes out into a clean container, it should not have anything other than clean fuel.

[rludtke]

We double checked our plug wire order a bunch of times and replaced them too. We spent some time checking the spark from each plug and noticed there seemed to be two plugs that were not sparking strongly or barely at all. We were considering replaceing the distributer cap or possible the whole distributer assembly.

I will call and have him double check for water in the gas right now as well as debris in the carb and integrity of inside of fuel line.

Just called mechanic and he seemed to think that the water would only affect the motor for a few minutes and then would be gone.

A couple of thought Mikey,

Water could be a longer-term problem if there is alot of it in the tank. It could accumulate over time, and it won't necisarily leave, because it is much heavier than gas, and will sink to the bottom of the tank. If it does leave the tank, it will have to travel through the carb...

I also wonder if your coil may be the culprit. When I first read this post, I immediatly thought that it sounded like an ignition problem...

[Satchmo]

I'm a newbie to the forum w/an 87 SunSport in Louisiana...I had a similar problem many moons ago; and, it turned out to be a cracked cylinder head...you might try doing a compression check on the cylinders...I replaced mine w/a '78 rebuilt head from a junk yard for about $150 total...

[87SunSportMikeyD]

Hey all and thank you thank you THANK YOU for your comments. I know this forum can be tedious to read and try and diagnose other people's problems, but sometimes it feels like you are all alone with a broken boat and summer passing you by and it makes me sick inside!!!!!

You guys are a big big help in deciding on a course of action moving forward. Like I said, my mechanic is wanting to drop in a newly rebuilt (used) engine for $1500-2000 (just the engine, not including labor or the $500 in labor he has already done for me in diagnostics). He is claiming that the labor cost of him ripping into my engine will come close to the cost of a newly rebuilt engine. In addition a new rebuilt engine will bring possibly higher resale value for me in the future or faster sale and less possible engine problems in the future.

I did ask my mechanic to do a check of the fuel and lines. He said he checked the fuel filter/water seperator, pulled a few gallons of fuel from the tank, inspected the carb screens and fuel lines and found no evidence of water whatsoever (damnit). So we are still looking.

He said he would reconnect the trans and conduct a compression test per my request, However he does not feel the problem could be connected to the cylinders or will be revealed in a compression test. He said he will continue to price out new engines as well as look for more problems.

Here's what my dad said this morning about getting MY engine rebuilt: "Matt of Matts Engines can do it for $1700 and Wagamon Bros Marine Engines is $4500. Most marine dealers do not have the machinery to rebuild a motor and need to send the engine block out to a third party machine shop. Matt has all the equipment in-house. Not sure what Mike will want to do." (I had been thinking I would get a different engine that had been rebuilt to avoid having to wait while my engine is rebuilt):

Satchmo thanks for adding your experience with a cracked cylinder head. I will ask my mechanic about it this morning in an hour or two. Hopefully he has already done the test and I will report back shortly. If that is the only problem...well.. that would be great!

I REALLYYYYY dont wanna be broken down all year that is a nightmare ahhhhhhhhhhhhhhhwhahahaa

[87SunSportMikeyD]

Okay guys we are back from a long talk with my mechanic, Doug Miller in Forest Lake, MN. His labor AND parts are half the big dealers and he didn't jerk me around with scheduling. Let me know if you want more info on him.

Basically I am farked. The compression test has revealed poor compression all around. All cylinders should be about 150psi. For my motor, cylinders 1&2 were only at 25psi which explains my lack of spark on those cylinders and many other problems. Cyl's 3&4 were at 135psi, and 5&6 were at about 120.

He said there is reason to believe there is a problem with a cylinder head and/or gasket, piston ring, or similar. The clunking noise is likely caused by a thrown bearing, main bearing, or rod.

The engine will need to be rebuilt or replaced. I can send my motor off to be rebuilt but I will probably order a different longblock engine to be delivered to the mechanic and installed. He is also replaceing the dampner plate on the trans and my starter which he said is shot (again) and was way too small.

My dad thinks this is caused by surfing at low RPM's under heavy load. Can anyone confirm their surfing RPM's and speed with a 1:1 trans? He thinks a 10.5 prop or a gear reduction transmission should help boost the RPM's into the engine's sweet spot.

He said another option we may wish to ask about is a stronger cam shaft designed for a large truck or RV. Anyone??? Please all serious comments welcome we are starting to get somewhere now, but it is so painfull.....

[FecalFajita]

Wow that is pretty horseshit man, Sorry to hear that.

I really (REALLY) hope that isn't what's going on with mine.

I'm gonna have to compression test it when I get a chance. I'm still trying to find time to look into the other suggestions here too, I'll let you guys know what I find.

[rludtke]

My dad thinks this is caused by surfing at low RPM's under heavy load. Can anyone confirm their surfing RPM's and speed with a 1:1 trans? He thinks a 10.5 prop or a gear reduction transmission should help boost the RPM's into the engine's sweet spot.

He said another option we may wish to ask about is a stronger cam shaft designed for a large truck or RV. Anyone??? Please all serious comments welcome we are starting to get somewhere now, but it is so painfull.....

I'm sorry to hear about your troubles Mikey.

When it comes to overhauling your engine, I've got a couple of thoughts:

A "standard" overhaul will probably give you years of service, and is a viable option. This should be considered if the budget is tight.

If your worried about high manifold pressures, then consider removing some additional pitch from your prop to bring up the RPM's a little more at your surf speed. Changing the gear ratio is probably expensive.

Tuning your engine with a cam shaft designed for lower end performance might make sense. I actually don't know if this would improve the life of the engine, after all you will continue to run the same or simular power settings, but you might prefer the performance charactoristics of the new cam.

I see three ways to get your engine overhauled:

1) "Contract" the machine work your self with local machine shops
This means you transport the parts to and from the shops, shop for and order the parts, and do your own assembly. Could be the best price, but lots of effort on your part, and some risk of not doing something right

2) Find a reputable local shop capable of doing everything "in house" for you.
This might be the most expensive, but may also offer the highest quality workmanship. This may be the list risk option.

3) Mail order exchange
Likely the lowest price, as these shops have small margins, and make it on volume. The internet is full of these shops. They'll ship right to your door. This might be the quickest route, as these shops often have the common engines ready to ship as seed units. There is some risk though: you could get a good engine, or you could get crap. You might want to do some research and search for refferals/recomendations to reduce this risk.

[87SunSportMikeyD]

Thanks Rick. Sean sorry if I have taken over your thread? :) I can move it to my own thread if you wish. I was kinda assuming we had similar noises/issues.

My mechanic has an engine source of the 'mail order exchange' variety. He can have an engine delivered to him and give them my engine for about $2k.

Just today we found a 351W on Craigslist. Someone ordered it mail exchange for their muscle car and cannot afford to complete the repairs. It would likely not come with a warranty (maybe though) but it's only $975. http://minneapolis.craigslist.org/wsh/pts/1879289148.html

[FecalFajita]

It's all good, the thread can help anyone with similar issues.

As for the 351 you found on craigslist, Will that work? I mean, I know it will bolt in and what not. But, I thought the marine engines had different cams and were in a whole different class as far as durability goes. I could be wrong here though.

[87SunSportMikeyD]

That is what I am determining now. Basically if the motor has been beefed up for a musclecar it may work for us, but my mechanic thought likely not. He is going to call them for me and inquire.

[mysledblows]

I had a jasper 350 chev motor in my old 78 truck. I put 50,000 miles on it without an issue. Ran like a top and made way better power than the tired old piece that ate the rods and mains on a 35 below night at 2 in the morning. That was a cold half mile walk before some guy going to work picked me up and dropped me at home. Yes this was before cell phones were everywhere:)

[mysledblows]

As for the motor going bad due to surfing, I was talking to a friend of mine the other night whose brother in law works at one of the local marine dealers. Said that they are seeing all sorts of motor issues with surfing being so popular. Apparently with the boat listing to one side trying to make a nice wave, its causing the oil to flow away from the pickup in the pan and they are seeing an increase in burned motors due to lack of lubrication. They are telling all the people surfing to run a quart over full so that there is some extra oil in the pan thus allowing it to cover the pickup.

[rludtke]

As for the motor going bad due to surfing, I was talking to a friend of mine the other night whose brother in law works at one of the local marine dealers. Said that they are seeing all sorts of motor issues with surfing being so popular. Apparently with the boat listing to one side trying to make a nice wave, its causing the oil to flow away from the pickup in the pan and they are seeing an increase in burned motors due to lack of lubrication. They are telling all the people surfing to run a quart over full so that there is some extra oil in the pan thus allowing it to cover the pickup.

This is probably realy good advice, or switch to a high capacity oil pan (my preference if it will clear the bilge). There may be other things that could be done with the oil pickup to move it to a better location (such as farther aft in the pan- many engines put the pick up in the front of the low part of the pan).

Could someone make a point of monitoring their oil pressure gauge while surfing to see if the pressure ever surges or drops off?

[87SunSportMikeyD]

Talked to mechanic again yesteday. Mentioned the oil and surfing issue mentioned above and he said it would be a good idea to go a bit overfull on the oil. He said no way a bigger oil pan will fit. I pointed out that we have noticed rear plugs fouling before front plugs, and a small amount of oil near the rear of the valve covers. Right away he said 'do you notice pressure drops or surges when surfing?" I said we would watch for that.

He said he still thinks we did not do anything wrong, he thinks it was either hard use on a worn engine or may have possible had poor compression when we bought it.

So we are very close to ordering the engine from craigslist. My mechanic called the guy and they have several engines I guess, rebuilt by Jasper engines locally. He said it should work just fine and there would be no parts in the marine version that were stronger or better for our uses. He said the only difference will be that this engine will be tuned to go up to 5000 rpms instead of 4500, so the sweetest rpm band to produce maximum power would also be slightly higher. Since we are surfing, we are already running extra low rpm's, and I am concerned this would put more stree on the engine. He said not to worry about it beacuse the new engine will have so much more power

[rludtke]

Because we don't have more than one gear ratio to select from, the propellor becomes our "load controller".

Manifold pressure is the measure of the pressure in the intake manifold. You can think of it as a throttle valve position indicator. At idle (throttle closed), the manifold pressures would be at it's lowest (this would be the highest on a vacuum gauge), and at wide open throttle, manifold pressure would be at its highest, approaching ambient pressure (30 inches of mercury on a "standard day").

High engine loads can occur when the engine is asked to perform at high manifold pressures and low rpms (think bogging your car down in 4th gear- throttle is held far open, but RPMs are low, and the engine is below its optimum power on the curve, and is incapable of developing the power needed to bring RPMs up into power range).

High engine wear occurs at high engine RPMs (simply due to the engine speed- every turn of the crankshaft causes wear).

Your propellor diameter and pitch control the balance of these two elements. In airplane reciprocating engines, the pilots utilize the manifold pressure and the rpms to set power settings. Power settings are expressed in % of total power capable by the engine. The settings are developed by the engine manufacturer, and published in the manual. Take off power = 100%. Cruise power = 65% for fuel economy etc.

For surfing our boats wake, we select the throttle position (RPM) to give us the desired boat speed. Unfourtunalty we don't know the corrosponding manifold pressure we have selected, just RPM. If the manifold pressure is too high for the RPM selcted then we are operating simular to our car in too high of a gear for the lower speed we are tying to drive. For our boats, the only solution would be to decrease the pitch of our prop and allow the engine to speed up (increase RPMs). Then, when traveling at the desired speed, the engine will be in a better spot on the power curve. Unfourtunatly, when traveling at a higher boat speed, the engine will be turning faster than desired (causing faster wear), and peak power will be experienced at a lower boat speed. Because we lack a multi-speed transmission, or an adjustable propellor, we have to decide which power setting to optimize for. The rest of the speed range will not be ideal.

The manufactuer probably selected a middle of the road prop to try and accomidate both low end pull, and yet offer some top speed. Those who weight the boats for surfing will not be able to live with this prop setting. Decreased pitch (increased RPM) is no doubt required to operate your boat this way, but it will come at a sacrifice of top-end boat speed, and accelerated engine wear.

Increasing prop diameter, or adding more blades complicates the prop formula a little bit, but more blades and/or bigger diameter adds more load on the engine (increases manifold pressure), requiring the pitch to be reduced to compensate. The rule of thumb for fixed pitch airplane propellors is that every inch in diameter increase requires a reduction of one inch in pitch to maintain the balance. If this rule of thumb transfers to hydronamic properties of our boats, than I think the weighted surfers running 14" diameter props may need to drop 2-3 inches in pitch to idealize their surfing power settings.

Mickey, I know you re-pitched your prop already, but you might consider reducing the pitch a little more. Even if you don't, I think you will get many years of service out of this engine. We are limited in what we can do, considering that we operate these boats through a wide speed range.

[87SunSportMikeyD]

Hey all, back for another update. Sorry I should have took pics! Drove out with Dad on a blazing 90 degree day. Yay. Picked up the new rebuilt engine from the craigslist guy. He had about 10 engines across various years in case someone else needs one! Took it over to our mechanics.

Boy did he have some neat and rare odd vehicles! He had a two-person-wide seater snowmobile with a fiberglass t-top. It looked kinda like an old VW Beetle. It had two independantly steering tracks for winter, wheels for summer and it was submersible. He thought it was the only one in the US.

He said the new engine looks great. He is also installing a new starter, a whole new distributer, new transmission damper plate, and the engine. I will also have him tune it up once it's running again. He said there is a chance it could be done this weekend. Crossing my fingers!

We talked a bit about surfing and the engine and he doesn't think I "did anything" to cause this failure. He thinks it was just an old engine and may have had poor compression since I bought it. He said he did not think that I was pushing the engine too hard, or that I 'did something wrong' in surfing the boat. I asked him about several of the ideas voiced here and he did agree it may be a good idea to run the oil a bit overfull. Also extremly helpful in his opinion would be a re-pitched prop, power transmission (1:1.23), and the 454 if possible. He said the 454 is practically bulletproof and would really be well suited for surfing.

[Blackntan90]

WOW! A ton of info guy's! I am much more enlightend after reading through this thread! Any way the remark about the big-block is right on, 454's run at lower RPM's, but produce alot of torque! And maybe a dry sump oiling system for people that surf? Hell I want one regardless! Well, best of luck to you, and don't sweat it too much- it's been at or near 100 degrees for weeks here!

[87SunSportMikeyD]

Here are comments I got from my mechanic regarding the new distributor:

[Slightly edited by me] In my opinion, some brands can be hard to find replacement parts for and/or may have a higher failure rate. It also has a vacuum and mechanical advance for automotive applications. The one I would recommend has been ordered ordered from DIM should be here any day.

[wotan2525]

To each his own... sounds like you've found a mechanic you can trust and it's worth listening to him. My mechanic whole-heartedly endorsed the ProComp distributors and I can tell from comparing it to the one that I pulled out that the coil and rotor look the same.

I haven't been following this thread but for $975 you got a realllllly good deal. When I rebuilt my engine I did everything myself (except for the revalving and the associated machine work.) I also had the machine shop inspect and magnaflux my block and they gave it a thumbs-up... but... bottom line is that I had WAAAAY more money than $975 into it without any labor cost. I always overfill my oil by a quart and I'm pretty good about checking it. My rebuild wasn't without issue (it was the first time I'd rebuilt an engine) but the block and bottom has been rock solid.

[wotan2525]

Oh also -- if it was me, I'd at least pull the oil pan off the old engine (would take 10 minutes) to see how the bearings on your bottom end look. It's just a curiosity thing but if those look good then you probably had (at the very least) a cracked block towards the front of the engine.

[87SunSportMikeyD]

Thanks wotan I really appreciate the advice, you are obviously more knowledgeable about the engines than I am! I should have edited my post above from my mechanic, I didn't mean for it to sound quite so critical (I will edit now) of the other brand. I think he was speaking kinda 'off the cuff'.

[87SunSportMikeyD]

Boat was KINDA suppossed to be done today so I could take it to the cabin this weekend. Now it will be done next week. Sigh.

[87SunSportMikeyD]

Hopefully getting her back tonight or tomorrow (my birthday yay). Crossing fingers.

[Supra-in-steamboat]

Happy B-day. I hope you celebrate by burning some gasoline. Ride on.