Poor performing 351?

[chrisk]

There used to be a thread about this a couple months ago so I wanted to bring it up again.

My trying to determine if my boat is underperforming. The reason I'm wondering is because the likes of CSuggs has essentially the exact same motor, hull, and prop and he has mentioned that he's getting around 44 mph at 4400 rpm's. My boat will only get 37-38 mph at about 4000 rpm's.

I understand there are a lot of other variables that go into this and my boat may actually be meeting its potential, but I'm just curious.

I did a compression check and each cylinder came in between 110-120 psi. Is this normal?

I'm going to check the timing with a timing light tonight or tomorrow, what else can I do?

Thanks for the help guys.

[Hematoma]

I can't remember off the top of my head but when I did a compression check on my 351 it was around 160 across the board. IIR it's been awhile. I own a motorcycle shop and my rule of thumb is anything under a 100 and you lose performance and it gets harder to start. It's getting close on yours. Everyone has their own way of things but thats MY minimum. What about plugs, wires and clean ignition system? Maybe it's got a valve thats not seating properly, Can you do a leak down test? That is WAY better than compression test.

[michael hunter]

What prop are you running compared to csuggs. Both engines are turning the same RPMs I think the difference would be the prop.

[chrisk]

We have the same prop, the Acme 13x11.5 CNC. And I don't believe both engines are turning the same RPM's, he's 400 above me.

I'm a noob at most of this engine stuff, what is a leak down test? Also, if it were a compression issue, wouldn't just one of them be struggling, not all 8?

[chrisk]

Also, I plan on replacing the plugs, haven't done that since I bought the boat in 2010 and not sure when the PO replaced them before that.

[Hematoma]

Also, I plan on replacing the plugs, haven't done that since I bought the boat in 2010 and not sure when the PO replaced them before that.A leak down test helps determine if you have any valves that are not seating properly. If any air passes a shut valve you lose compression. Basically you'll put each cylinder under the compression stroke so the valves for that cylinder are closed. Apply 5-7 psi air pressure and watch to see how long it takes for the air to bleed off. Good rings and tight valves should take a good amount of time if at all to bleed away. also you can listen in the intake and exhaust and or dipstick tube to tell if anything is leaking. If you hear air leaking out the bottom end (dip stick tube ) it's a good indication that you may have a stuck ring. If you hear anything out the intake or exhaust than more than likely you have a bad valve.

Change the plugs first, I had some bad plugs and the boat would idle good and start fine but under load it just would'nt get there lol if you know what I mean. I changed the plugs and it was fine.

[jet]

Well his engine is all FRESH to, right??. I think he just went thru his. #1 If your carb hasnt been rebuilt in the last 5 yrs I would start there. #2 Electronic ignition?? Everything else is based off this base-line and if there old, everything is just a lil off.

[chrisk]

Yes, electronic ignition and yeah his is basically a brand new set up.

So what you're saying is I just need to buck up and start buying all new parts, right??? You guys are so good at spending my money.

Spark plugs and a timing test are going to happen tomorrow so I'll report back after that.

[suprasam]

Well since were on the subject of spending your money......ah screw it, swamp the boat give me your SS# and I will bring you a whole new setup with a bigger and better motor an newer. That right there will solve all your problems. BTW I might use some of your money for some new upholstery work I need done on the bow seating.

[jet]

No man, Im frugal too! If there is a $300 part and it says marine on it. I will spend 1-2 wks making my own, buying it somewhere where it doesnt say marine on it, or I will re-design it and built it myself. The hunt and re-engineering is what challenges me and I love the hunt. We had to make sure those 2-items were stable BEFORE you go tinkerin with adjustments. Its just on tune ups, guys dont want to mess with the carb rebuilds and you cant get around it and becuase one effects the other.

tune up tip: take your dist cap off and lightly sand down FLAT all of the terminals on the inside and get the dust off of them. Then sand down FLAT the rotor tip and get them back to new looking and you can reuse them again. And a little barrymans in the gas tank once a month will keep the valves clean from bad gas. Tip of the day by Handy Andy!! Hiddee HO'

[Hagman]

Where is your trim tab set. If it’s down to fare the nose of the boat will plow slightly slowing you down. If you carry your weight to the back this could help raze the nose. Hence less drag more speed. Is your boat heavy -water ,people, beer. A heavy boat is a slower boat. Craggs has one of the finest boat on the forum , it’s been newly rebuilt its going to be as light as it was when it was new.

[csuggs]

Where is your trim tab set. If it’s down to fare the nose of the boat will plow slightly slowing you down. If you carry your weight to the back this could help raze the nose. Hence less drag more speed. Is your boat heavy -water ,people, beer. A heavy boat is a slower boat. Craggs has one of the finest boat on the forum , it’s been newly rebuilt its going to be as light as it was when it was new.

Hey thanks for the compliment Hagman! My motor is not all that "fresh", and by that I mean it has not been rebuilt. I have rebuilt the carb and the electronic ignition is nearly new as are the plugs and wires. My prop is the Acme 13x11.5 that has been re-pitched to the spec. (the prop was new last spring and after I ran over my tow rope and bent a blade, I had it repaired. The shop I use not only straightens and balances the prop, but they "pitch" the blades to spec!). After I got the prop back I noticed an improvement in top end of about 2mph. Also, I think there is something to the weight issue with regard to a newly rebuilt hull and floor. Without knowing Chris' situation, I'm sure my boat is light compared to his if he has not had his floor and stringers done. The other factor is the speedo. Both of mine are relatively new and have new tournament style pitot tubes with new lines run all the way back. Even the bronze tubes have been repaired. I'm pretty certain that my speedos are correct, although they have not been checked.

Chris - you don't have to spend a fortune, but I'd start with a tune-up. Carburetor and ignition.

Clint

[cadunkle]

110-120 psi and never rebuilt? I'd say you've got a genuinely worn out engine. Be happy your only 10 psi from high to low, they've worn evenly. Either be content with the performance you've got or rebuild it. Ignition and carb will not help much with such low cranking psi but may show some gains. Either rebuild next winter or just run it until it dies.

[chrisk]

CSuggs, I'm fairly certain the previous owner replaced the stringers but I'm not sure about which method he used. Also, I'm basing my speed off of GPS on my phone, not the speedos.

I'm content with the performance of the boat, only reason I'm looking for more top end is to be able to barefoot. I suppose if it can't go fast enough for me to barefoot it'll prevent me from getting injured again ; )

I'm going to continue with checking timing, replacing plugs, and cleaning the dist cap contacts. Just the easy stuff to see if anything comes of it. I'm not going to pursue a rebuild since it's running so dang well aside from this.

[cadunkle]

Yep give it a good tune up, timing, carb tune and call it a day. You'll probably get a few more seasons out of it before performance really drops off. When you eventually go through the engine it'll be like night and day and you'll be able to barefoot. Well cared for engines wear out evenyl as rings lose tension and cylinders taper... So no worries if it's otherwise running good.

[chrisk]

Just an update: Replaced the plugs with platinum Autolite's and didn't see any performance change. Currently have sea foam running through the engine but I haven't taken the time to test timing and clean distro cap contacts. Still starts, idles, and runs like a top so I'm not too worried.

[DAFF]

Given a choice of boats .... One with a fresh engine and a worn out non ajusted carb and the other engine worn but tuned correctly the old engine will will typicially out perform the new.

I would keep the factory old school plug style in the boats, those fancy ones will cause too much greif down the road. I have seen way too many bad running older vehicles where the owners put platnums in during a tune up to drop there vehicle off for rough running and hard starts. Spark systems are designed specifically for a certain plug, even changing name brands can cause grief once in a while.

I would start on the carb and fuel mixtures..... Perhaps the boat is running out of fuel at the higher RPM's.

[chrisk]

Resurrecting an older thread here but these thoughts are back in my mind.

During a long boat trip over the weekend I pulled out of a lock and dam and hammered down to WOT. I was able to get almost 4300 rpm's and it felt like we were really flying. I had 6 people and gear in the boat. Normally, the whole time I've owned the boat I can normally only get barely 3900 rpm's. Right now I have two thoughts:

1) The carb needs rebuilt. I don't know much about 4 barrels and whatnot so this may be a dumb question but could this be an issue where the 4 barrel isn't opening all the way at WOT? In your opinion, will a rebuild kit show an improvement?

2) My second theory is simpler. There was a chop on the water when I achieved 4300. I know speed boats and even skis and wakeboards are sometimes designed without continual surfaces to break up the water tension on the bottom of the hull. Is it possible that reduced water tension on the hull allowed the boat to spin higher RPM's?

Also, the ignition system is all good. I've since learned that all parts were replaced right before I purchased 3 years ago and the timing is good.

[chrisk]

Also, this thread really belongs in Service if a Mod wants to move it over, that's my bad.