PCM 351 Vapor Lock Issues

[Supra_Comp]

Well guys, had the boat out for the first time and it ran great when cold start and cruising, but after sitting and pulling skiers it gets the dreaded vapor lock. Even died on me when I idling back into the marina. I can restart it but as soon as I put it in gear it stumbles and dies.

I searched and see tons of different answers so I thought I would start a fresh topic to get all ideas.

This is a fresh rebuilt 351 so I can eliminate that. Carbed motor.

1. What is the correct thermostat, 140 degree or 160 degree? Mine runs 160 and pushes 170-175 when the issue happens.

2. Phenolic spacer and heat shields- are these band aid fixes or do they actually work?

3. Mechanical fuel pumps - obviously this is bolted to the side of the block and soaks a lot of heat. How did they deal with this in the older days? Is ethanol content in fuel becoming a big issue?

4. Fuel line insulation - I assume this will help? Any one gone this route?

[lively]

Do your self a favor and a cheap one . Put a inline pressure gauge and see what the actual fuel pressure is . I live in tx and it's very hot most days . Never had vapor lock . But how do you know it's vapor lock ? Do you have clear lines ?


Sent from my iPhone using Tapatalk

[TitanTn]

I'm thinking your thermostat should be closer to 160. Mine is a 145, but for some reason they had a colder one in the big block. I think the spacer will help (it's helped others), and the fuel lines insulation helped me. I'm a little curious about the temp running up to 175 though. This shouldn't be happening, and isn't related to fuel. Has the impeller and thermostat been replaced lately?

[Supra_Comp]

Titan, Temp runs up to 170 ish max only after it has been sitting in the sun. Once I fire it up again it goes back to 160. New impeller and t stat when the engine was replaced.

Lively, I'm only making an assumption on vapor lock as it only happens when the engine is hot. Cold starts it runs like a champ and doesn't stumble. Could anything else cause similar issues?

[lively]

Stumble like pop back through carb ? You need to WOT when it stumbles for few short burst . To see what is actually happening . Lean will pop back through the carb , rich will load the engine up . Check the color of your plugs start with cyl #1 and just note what color is what . Chiltons manual or the enter web will have charts to tell colors of plugs . I have a stumble from time to time on my edlebrock but fuel pressure is solid and the cap was the culprit . Also if you have a volt meter check your voltage at the coil + when you see the engine stumble . Not hot enough spark will cause the same symptoms I always hold a flat head to the block and back off 1" to see if it will spark to the block and make sure it's fat spark . Touch your coil how hot is it ?


Sent from my iPhone using Tapatalk

[TitanTn]

I agree that it doesn't sounds like a vapor lock issue. I really don't think you'll get vapor lock when it's running, so having issues when idling isn't vapor lock. It could be anything that Lively just mentioned and I'd start with the carb. Get the mixture set accurately with a vacuum gauge and then go from there.

[Supra_Comp]

No pop back that I can hear. If I WOT it will usually just die, or slowly creep up in RPM but as soon as I give it load (forward/reverse) it dies.

This is only when hot tho, cold start it is just fine and doesn't stumble at all.

[Supra_Comp]

In the morning I will pull the plugs out and see what they look like.

One thing I didn't mention is that I have an inline water separator that the marina guys said should be installed...not sure if that makes any difference?

[TitanTn]

In the morning I will pull the plugs out and see what they look like.

One thing I didn't mention is that I have an inline water separator that the marina guys said should be installed...not sure if that makes any difference?

It'll be good to get that installed, but no, that's not your problem. If the carb didn't get rebuilt with the motor, do that now. If it did get rebuilt, then get it tuned.

[Supra_Comp]

It'll be good to get that installed, but no, that's not your problem. If the carb didn't get rebuilt with the motor, do that now. If it did get rebuilt, then get it tuned.

It was rebuilt about 3 years ago on the old motor but I don't think it was ever tuned properly when they put it on the new motor. I assume the carb guy set it for a baseline and that was about it. Any key points that are critical for these boats?

[lively]

Start it up , turn the adjusters on the side of the metering block all the way in one side at a time motor should idle out and almost die , then 2 turns out CCW that would be default and then you need a vaccum gauge to see what jets and power valve you need to be using . Cadunkle on here knows quiet a bit about the holleys I would PM him and see what he recommends . I bought a edelbrock for mine cause I got tired of the Holley and having to adjust float levels and such


Sent from my iPhone using Tapatalk

[Supra_Comp]

The more you guys talk about tuning the more things make sense. Obviously the boat runs great with the new motor/foam/stringers but it does still labour a bit when pulling a skier with a couple people in the boat. I thought the vapor lock was the cause but that's what Internet reading does lol

Since I haven't done carb tuning before I will send him a PM and post some updates. I would pull the plugs today but the black clouds overhead aren't good.


EDIT: I pulled the plugs and they were all gapped to about .028-.030", re gapped them to .035" per the pcm manual. Going by the charts I would say they are on the slightly hot/lean side, insulator is pretty white with little to no carbon buildup on the electrode.

[lively]

Plugs tell it all , I would invest in the blue holly gaskets for the metering block that allow you to remove many times and are used for race applications when changing out power valves and jets that way as you're tuning you won't have to keep cleaning the base and the metering block to keep from buying a bunch of gaskets , it sounds like to me you need to go down a size in jets but I would make damn sure first to check your pressure and get a vacuum gauge to check how much negative vacuum you're pulling


Sent from my iPhone using Tapatalk

[Wulphie]

I had a similar problem on a 2001 I had. It would run great for a while then start stumbling and die. Let it sit for a while and it would start and run great again for a while, repeat.

After thinking all the stuff you are it turned out to be a clogged fuel tank breather.

Can't hurt to check the easiest stuff first.

[chris young]

I'm going to throw my.02 in here as well. Mine was similar and it was a float level issue. It's almost always the forward bowl but you can check them both at the same time. Remove the flame arrestor, and look down the carb while the boat is idling. If you see any fuel dripping from any of your boosters ( the small rings in the middle of the barrels) then you have a float level issue. I chased it all last summer, thought I had it handled and after I rebuilt the carb, without touching the forward float level, it returned yet again. Rather than further adjust the float, I leveled the carb with wedge spacers and now it starts, runs, and restarts on a bump of the key. I actually have it idling too low just because I haven't needed to mess with the curb idle. The other thing that may be at issue is your PCV. If you don't have fuel dripping you may have a vacuum leak. Pull the PCV out of the valve cover and plug it with your thumb and see what happens.

All that said, I'll bet a dozen donuts on float level being the culprit. Good luck.

With all due respect to lively on this one, over sized jets won't cause bad idle and start, but if the boat has no power then the jets could be causing that.

Good luck

[92SupraComp]

EDIT: I pulled the plugs and they were all gapped to about .028-.030", re gapped them to .035" per the pcm manual. Going by the charts I would say they are on the slightly hot/lean side, insulator is pretty white with little to no carbon buildup on the electrode.

How old are the plugs? Also, if you are getting the hot/lean signs, go up a size or two in the jets... Usually these things are jetted rich from PCM and Holley...

[lively]

I'm not a carb guy buy no means , injector pulse and PCM programming I can make it work like no other . But I always ask dad what he would do on carbs and learn from that lol


Sent from my iPhone using Tapatalk

[Supra_Comp]

I'm not a carb guy buy no means , injector pulse and PCM programming I can make it work like no other . But I always ask dad what he would do on carbs and learn from that lol


Sent from my iPhone using Tapatalk

All help is good help! I understand how carb's work but know nothing about tuning! Thanks for all the help. I have some pointers from above and cadunkle so I will be hitting the lake in the next couple of days to test some theory's!

Basic idea now is to set the carb back to stock settings (mixture screws) and go from there. Then check float levels, and if that doesn't help check vacuum and fuel pressure.

[lively]

That's right ! I always had an issue with my float levels and spilling over the boosters causing a rich environment . But I just Bent the tangs Down a bit to compensate . Good luck and take a video to help guide you through it . Always helps my active thinking


Sent from my iPhone using Tapatalk

[Supra_Comp]

Here is a good question, how do you adjust the float levels on these carbs? I don't think they have an adjustment screw at the top of the bowl.

http://www.holley.com/data/products/pictures/large0-80319-1.jpg

[Wulphie]

You have to remove the float bowl

[chris young]

First, if you don't see fuel dripping from a booster, then adjusting the floats is probably not needed. But once you get the bowl off, then you bend the tang on the float.

[lively]

You take the bowls off and hang them upside down as if they were to be seating the needle . The object of the game is to make the float level with the needle seated . That's why I said get the blue Holley gaskets so you don't have to buy a bunch of gaskets . Once you have found if the floats are correct or not you have to bend the metal tang that holds the float to make the adjustment . Look up 4160 adjustments


Sent from my iPhone using Tapatalk

[cadunkle]

Phenolic spacers do help if you in fact have a heat related issue with fuel boiling or vaporizing in the carb. I run one, fairly thin to fit under the doghouse I believe it may be 1/2", but it made a big improvement it starting when hot after sitting briefly.

[biggsie]

My tstat is a 143. Temps run about 150-160.


Sent from my brain using Tapatalk.

[Supra_Comp]

So on the 4160 model, how do you know when the floats are 100% when there are no sight plugs/screws?

http://i678.photobucket.com/albums/vv142/GT40_Sally/th_054ce2defcb7a3c20f475c9179b6470d_zpsa0a3f738.jp g

http://i678.photobucket.com/albums/vv142/GT40_Sally/th_5f601234af27ed303993a290fe08552c_zps8ba44a46.jp g

[lively]

Remove them as stated and turn the bowl upside down and look at the float , if it is not level with the base of the bowl ( where it mounts to the side of the carb) then you will need to adjust it . Remove the bowls .. You will see from there .


Sent from my iPhone using Tapatalk

[Supra_Comp]

Just to narrow out one more item....sorry I know I am digging here LOL

I noticed my fuel line from the tank to the pump is literally touching the oil filter and it is only a rubber hose. Normally 351 engine oil temps are anywhere from 85-95*C which is about 190-200*F (maybe 10* cooler through the casing of the filter). With the ethanol fuel of today boiling points are as low as 150-160*F. Just a thought?!

http://i678.photobucket.com/albums/vv142/GT40_Sally/IMG_20140707_174115_zps822aad82.jpg

[lively]

No , keep moving , if it bothers you move it . But not the issue


Sent from my iPhone using Tapatalk

[Supra_Comp]

Check lol

I am going to order some gaskets for the fuel bowls. Guess I'll be taking the dive and try my hand at carb adjustments....tried to avoid the inevitable

[biggsie]

Don't be intimidated.... Read my post about Holley carb tuning. There isn't much to it, good luck!


Sent from my brain using Tapatalk.

[chris young]

I'll second what biggsie says. BTW if you go to the Holley site and the Summit racing site, read everything, and watch all the videos regarding tuning the 4150/4160 vacuum secondary carburetor, you will have all the info you need. Start with the Holley videos, they are short and break the carb down into all it's individual circuits, and how they work.

I still haven't heard, have you looked into the barrels to see if there is fuel dripping? If it is and it continues to drip for a bit after the motor is shut down, you have float issues, if it doesn't, don't mess with the floats they are probably ok. It is highly unlikely that they are adjusted so low the you are running lean, and running rich due to float adjustment is obvious by the dripping.

Good luck.

[CJD]

A little tip from those like me, who couldn't go a week without tweeking their Holleys...

The bowl and valve block gaskets are made from a paper material with a coating that seems to glue them to the surfaces, making them stick hard so they tear and cannot be re-used...not to mention that they are a real pain to clean off the faces of the parts. Spray them with silicone spray before installing and they come off easily and can be re-used a handfull of times before they get too messed up. Saves a lot of money while you get the jets, PV's, and ports all tuned.

[lively]

They make Holley blue gaskets for race application that you can remove them as many times as you like


Sent from my iPhone using Tapatalk

[Supra_Comp]

Ok folks, an update.

So I had the carb looked at by the company who re-built it and he said the floats are set perfectly, he even had it on a test motor and said it works just fine.

So being completely baffled, I called SKIDIM and talked to Vince. Walked through the situation with him twice and he started talking about ignition issues that can arise. So the boat does have an electronic ignition conversion (Mallory) with the 3 wire hook up. He said the coil could be bad and is heating up which causes symptoms similar to heat soak and carb problems, and it is also the wrong application (Accel Super Stock 1.4 ohm) which tests out ok using a multimeter according to specs but Vince also said it could be the wrong resistance for the dizzy.

Now comes the head scratching part. As I was looking at the wiring, I began to ponder about how all of this is hooked up. I found the link below showing how a normal PCM 351 should be connected with an electronic ignition:

http://www.mastercraft.com/teamtalk/attachment.php?attachmentid=84826&stc=1&d=1348855200

So when I look at my setup, I have the following.

Red wire from dizzy is direct to + of coil.
'Pink' wire from ignition source is direct to + of coil.
I then have a purple wire coming from + of coil to one side of ballast resistor, but other side of resistor is only hooked up to the choke on the carb and what looks to be the old 'igniter' wire for the old dizzy that is abandoned.
Green Wire from dizzy goes to correct - of coil.
Brown wire from dizzy goes to correct ground.

As a summary, the ballast resistor is essentially doing nothing because the coil and distributor are getting a full 12V source and the power to one side of the resistor is coming via the coil. Now I have read that the resistor may not be needed for certain coils that are 1.4 ohms and higher (internally resisted?) but my assumption now is to eliminate this issue by buying the correct lower resistance (0.7 ohm) coil and hooking up the ballast resister in the fashion shown above.

Apologies for the long post! Anyone have any comments on this?

[ssa]

Mine has a Accel Super Stock coil installed also by a previous owner. It isn't wired according to the above diagram but it works and I figured if its working leave it alone. For whatever reason the red wire coming from the ignition module is connected to the positive terminal of the coil and the coil is fed through the ballast resistor. So you may be able to wire either according to the diagram or how mines hooked up without replacing anything. Your ignition module may also be the problem if it was the kind requiring the resistor and it hasnt been wired in.

[lively]

Ok folks, an update.

So I had the carb looked at by the company who re-built it and he said the floats are set perfectly, he even had it on a test motor and said it works just fine.

So being completely baffled, I called SKIDIM and talked to Vince. Walked through the situation with him twice and he started talking about ignition issues that can arise. So the boat does have an electronic ignition conversion (Mallory) with the 3 wire hook up. He said the coil could be bad and is heating up which causes symptoms similar to heat soak and carb problems, and it is also the wrong application (Accel Super Stock 1.4 ohm) which tests out ok using a multimeter according to specs but Vince also said it could be the wrong resistance for the dizzy.

Now comes the head scratching part. As I was looking at the wiring, I began to ponder about how all of this is hooked up. I found the link below showing how a normal PCM 351 should be connected with an electronic ignition:

http://www.mastercraft.com/teamtalk/attachment.php?attachmentid=84826&stc=1&d=1348855200

So when I look at my setup, I have the following.

Red wire from dizzy is direct to + of coil.
'Pink' wire from ignition source is direct to + of coil.
I then have a purple wire coming from + of coil to one side of ballast resistor, but other side of resistor is only hooked up to the choke on the carb and what looks to be the old 'igniter' wire for the old dizzy that is abandoned.
Green Wire from dizzy goes to correct - of coil.
Brown wire from dizzy goes to correct ground.

As a summary, the ballast resistor is essentially doing nothing because the coil and distributor are getting a full 12V source and the power to one side of the resistor is coming via the coil. Now I have read that the resistor may not be needed for certain coils that are 1.4 ohms and higher (internally resisted?) but my assumption now is to eliminate this issue by buying the correct lower resistance (0.7 ohm) coil and hooking up the ballast resister in the fashion shown above.

Apologies for the long post! Anyone have any comments on this?


this is a PCM wiring diagram and you need to use this with a coil that is externally resisted . They used them for points and condenser dizzys but like mine and a lot others I have the YL Mallory dizzy electronic . It does not like internally resisted coils at 1.4 ohm . Buy a cheap 20$ coil from napa should say use with external resistor on top . And wire from solenoid to dizzy the way this diagram shows http://img.tapatalk.com/d/14/07/17/8u8e8y6e.jpg



Sent from my iPhone using Tapatalk

[CJD]

Just my feeling...if you cannot find direct documentation from the coil manufacturer saying the coil can take the full 12v...then I'd go safe and use the resister. Also, as I am sure you know at this point, it sounds like you need to replace the coil you have!

[Supra_Comp]

Great info guys, found the diagram Lively posted.

I am a bit confused only because the coil would say to 'use with an external resistor' but the diagram shows to put a by-pass for electronic distributor. Any clues? My theory is to wire the resistor in without the by-pass and see what happens. Found a 0.7 ohm coil, so I will re-wire and give it a shot.

http://www.mastercraft.com/teamtalk/attachment.php?attachmentid=43269&stc=1&d=1230475845

[lively]

I talked to skidim when I did mine , HEI like full 12v but these external mount coils and Mallory dizzys do not . I ran into the same issue like a month ago and I posted videos of my findings . Mine had a accel coil 1.4ohm internally resisted and the coil was suffering from heat of excess voltage and would not provide enough spark for long periods of time . I then ordered a ballast resistor and coil from napa and wired it just like that diagram shows . Do not follow the msd or dizzy diagram because you want the + input from dizzy to + coil and the. To ballast . My coil without this setup was reading like 189 degrees at operating temp of 180 but now that I changed it . It runs at 90-120 degrees . And my spark is big and fat from 1" away from ground .


Sent from my iPhone using Tapatalk

[Supra_Comp]

I just watched your videos, I wish I had watched your thread….you had the exact same problem as me! When yours started acting up I immediately knew.

Lively, great information. Well done sir. I bought a new coil and matching ballast resistor tonight (actually got an MSD from my local store because it was what they carried) and it is a .7 ohm to be used with the resistor.

I will wire mine up tonight (basically just switching wires around) and provide an update!!

[Supra_Comp]

Wired it tonight, and did a quick dry start and it fired immediately...hasn't done that in many years! With no pumps. Shut it down though, I drained the fuel separator during my investigations so I'd rather have it hooked up to water.

So far so good. Taking it to the lake on Saturday for a trial run!

[lively]

Make sure timing is correct as well , I run 8 degrees BTDC , others here will run 10 degrees BTDC . And once you start it on the water let it warm up and then just briefly touch the resistor to check for warmth . If it's hot you know it's working ! Good luck and let us know how it turns out !


Sent from my iPhone using Tapatalk

[Supra_Comp]

Well guys, I think we have success. *knock on wood*

Dropped the boat in the water and it runs incredible. Fired it up and immediately checked voltage at the + of coil and saw anywhere from 9.3 to 9.7 volts. Ballast resistor was hot and the coil was relatively warm/hot but not scorching. Drove out of the harbor and gave a few pulls from a idle...I can easily say I have never felt the boat move that quickly from a hole shot before!

The other thing I did notice was the reliability. I did about 10 hole shots and not once did it hesitate or stumble. It also pulls harder once up on plane too, honestly never gave it more than 1/2 to get on plane and the throttle response while cruising was very crisp. Even after getting back to the dock and letting it sit, it fires right up with out any stumbling.


Would just like to say thanks to all you guys for your help!

[lively]

Your voltage is spot on and as long as the resistor is hot it's working . Glad to help and enjoy


Sent from my iPhone using Tapatalk

[Supra_Comp]

Had the boat out this week and it ran fantastic! Great throttle response and never gave out on my once. Pulls me (215 lbs) on a slalom from a deep start with ease.

Since I had some flat conditions, I thought I would give it a full out run. When giving it full throttle and going for max speed, at around 35-37 mph I get a bit of a hesitation (when the vacuum secondaries open up) for a second and then it kicks back in and I am able to get it up to about 40-41 mph (roughly, speedo's aren't perfect). My assumption here is a lack of fuel and some minor carb tuning. Has anyone found a good combination of pump cam and vacuum spring?. Still the stock 4160 carb.

Other than that, we are running good!

[lively]

Check timing and set 10* BTDC , full advance at 3600 rpm should be 32*-36* degrees , hesitation could be from bowls running dry . Adjust float levels .

[cadunkle]

You may also try a stiffer secondary diaphragm spring. No accelerator pump on secondary side so if they open too quickly it'll cause a lean stumble on the transition. Failing that you may try going up 2-4 jet sizes on the secondary side. If this is a 4160 your options are more limited and you'll need a different metering plate or to enlarge the holes in yours slightly (not reversible). You may try slightly raising the float level on the secondary side as this will slightly richen the mixture. going richer will not likely cure a lean stumble from secondaries opening too quickly though.

Retarded timing or late/jerky advance will also cause a stumble.

[covertcombatant]

Is there any coil that you can pick up from napa / autozone / oreilly that will work?
My coil is the same as the one on skidim R117001and it has the prestolite electronic conversion without a resistor. Would like to pick one up before I put it on the water tomorrow.
Thanks.

[Supra_Comp]

Here is what I am running, you should be able to get it at most stores.

http://www.msdperformance.com/Products/Coils/Stock_Replacement/8203_-_Blaster_2_Coil_w/Ballast___Hardware.aspx

Notice that it has 0.7 ohms resistance on the primary where as the one you are running has 1.5 ohms. I switched to the lower resistance and ballast resistor as I was having issues at operating temp. Just be sure to find out what works best with the prestolite conversion.

[codyb209]

Whats was does the wiring looking like? Im having the same issue mine might be wired wrong too