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 ?


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[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 ?


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[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?