1979 3.7 470 Mercruiser not start after hours of perfect use?

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slatham

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Hello Dr:

I have a 1979 Marquis Carabbean 18', 4 cylinder 4 stroke. Start and runs fine for hours. Park it for 30 to 60 miniutes. Re-start..... cuts off after 1 - 2 minutes. May try to start a few times , but acts like it gets chocked off. Normal running and WOT are all in specs when operational. Idle ranges from 475-750 after short warm-up. No signs of flooding.

First time it did this it restarted perfectly about 3 to 4 hours later.

Our altitude is about 2000 ft. msl. Is the fuel ethenol content causing vapor lock?

It has been cleaned. If internal prob., I can rebuild. I think a 2 barrel Rochester?

I wish to open carb today and check fuel flow.


Any ideas?

Thanks,

Scott
 
havasuboatman

havasuboatman

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

Your idle roams between 475 and 750 rpms? It sounds like you have a vacuum leak and during the hours of running it has gotten worse.
 
S

slatham

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

It may be that. however, idle is stable after warm-up. This happens after use and not immedeate re-start. If I leave it in water for 30-60 minute and the engine is still warm. It starts, runs, and cuts off a few times without starting. When it did re-start on the first occurance, it did 3 hours later and ran perfectly. Then it repeated itself day before yesterday in a thunderstorm! LOL. Murphy's Law!
 
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DJ

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

Fuel or spark? You're losing one of them. Spark is easy enough to check.
 
S

slatham

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

I used Stay Bil first tank. Just put Sea Foam in 2nd tank. It runs to well to cut off spark, but you never know. Opening carb and see if sticking internally.

Scott

Used starter cleaner and it kicked over up to 750-900 rpm @ idle. Do not know if it will repeat, but will carry starter can from now on!

Thanks,

Scott
 
Last edited:
havasuboatman

havasuboatman

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

That starter fluid is NOT the answer. It washes the oil from the cylinder walls, and puts a tremendous strain on the rings. Very bad for the engine.
 
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chevydan04

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Apr 9, 2010
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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

I would look into the coil itself, it may have some internal problem. Have you check for spark from the coil?

Maybe you have a bad resistance wire on the coil?

You are going to need to duplicate the problem to troubleshoot the problem properly. Sounds like things go wrong after the engine has had time to warm up and reach peak operating temps. after running for a long period of time.

Good luck keep us updated!
 
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chevydan04

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

Also you may want to look into the shift interupt switch.

might as well clean all the electrical connections as well.
 
S

slatham

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

It has taken time,but it seems to be a vapor lock problem. I have changed fuel octance and booster up and down. when I ran it up on a new tank, problem did not occur. Next tank, staight lower octane with Ethanol.... run it, let it sit for awhile and it runs gas out of carb and quits. Used starter fluid to draw it back in line to restart.

2 things i will try:

1) Open engine box and run blower for a few minutes when parking after use to cool engine area.

2) use only 92 octane.

If this fixes prob I will let you know. When it runs it runs perfect?

Scott
 
lowkee

lowkee

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Re: 1979 3.7 470 Mercruiser not start after hours of perfect use?

This may sound too simple, but are the vents on the tank clogged? It's more common than you may think, especially with the "runs fine for 20 mins, then won't restart for a good while" issue.

Easy diagnosis.. take the fuel cap off while trying to restart it.
 
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