350 will not rev when hot

I built a 350 with fairly aggressive cam edelbrock intake and standard heads for my boat. When it gets to operating temp it refuses to rev past idle. Sits and makes a chugging sound. When it does this the tach drops very low and the voltmeter is all over. When the engine is cold no problems runs like a champ.i tried replacing the coil thinking it was getting and shorting out no change. Tried slowly adding fuel into the carb when its doing this thinking if its a fuel issue adding fuel would bring it out of it. If i leave it with throttle but refusing to rev it will backfire hard and shut off. Restarts immediately.

Vapor lock?

I pulled the fuel lines as far away from the block as I could thinking that. Could it be vapor locking inside the carb? It did the same with an edelbrock carb i thought it was something wrong with that carb. Not much chance in two carbs being bad. I did notice today the fuel pump was hot but I revert back to if it was fuel issue it would've livened up when I poured fuel down the carb. Also note the engine runs around 150degrees since its fresh water cooled.

The way it backfiring could there be something with the timing or my adjustments on the rockers are off? No matter where I put the timing it has the issue.

150 degrees is not a good "normal operating temperature". far to cool. Does it not have a thermostat at the intake manifold? You could be creating condensation in the basepan.
As far as the stalling, I think the voltage jumping around is telling you the story. Put a voltmeter onto the + side of the coil, run the engine until it begins reacting, see what the voltage is at the coil.

It does have a thermostat. In a boat they tend to run cooler a lot with 140 degree tstat. Are you thinking bad alternator?

This sounds like a bad/corroded electrical connection. Check your motor grounds.

A note on Thermostats. You're in WI so I'd say you should be running at least a 160F or 170F thermostat. The reason for a 140/150F is for running in salt water, where salt starts crystallizing out at around 180F, so hot spots in the heads can lead to plugging, and 140 to 150F operating temperature is right for ocean running. You've got the biggest radiator there is, so your T-stat's job is to keep the engine warm.

You said you built this engine. Has it EVER run right when warm, or is this an initial startup issue? I like Mike's suggestion of checking voltage at the coil when it's "chugging", and 36 sedan's suggestion to check grounds. Boats are terrible for corrosion, especially galvanic from copper/brass on aluminum or steel.

Also, if it's been backfiring you've likely got a blown power valve in the carb, but find the reason for the backfiring before you change it, or else buy a dozen.

Apples and oranges but my 01 Indian Chief developed a heat soak issue with the ignition module. Are you running points?

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Originally Posted by rspears

A note on Thermostats. You're in WI so I'd say you should be running at least a 160F or 170F thermostat. The reason for a 140/150F is for running in salt water, where salt starts crystallizing out at around 180F, so hot spots in the heads can lead to plugging, and 140 to 150F operating temperature is right for ocean running. You've got the biggest radiator there is, so your T-stat's job is to keep the engine warm.

You said you built this engine. Has it EVER run right when warm, or is this an initial startup issue? I like Mike's suggestion of checking voltage at the coil when it's "chugging", and 36 sedan's suggestion to check grounds. Boats are terrible for corrosion, especially galvanic from copper/brass on aluminum or steel.

Also, if it's been backfiring you've likely got a blown power valve in the carb, but find the reason for the backfiring before you change it, or else buy a dozen.

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This issue began from the beginning. Thats when I had the edelbrock carb on it. I switched to the stock Rochester and it solved it until I put it in the lake. Thats also why I'm questioning if I messed up the adjustments on the rockers or something since I can't really adjust it while running because of the exhaust manifolds. Im going to take a can of stating fluid with me to spray around and see if there are vaccum leaks. I will get a different tstat for it and check the voltage at the coil when its having the issue if i have time to take it out today. And I'll clean all the ground connections. Seems like if it was bad ground it would do it whether cold or hot?

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Originally Posted by firebird77clone

Apples and oranges but my 01 Indian Chief developed a heat soak issue with the ignition module. Are you running points?

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This is mercruiser version of hei. Has a sensor in the dizzy and a box that controls spark curve. Thunderbolt iv is what its called

If the voltmeter is jumping around when the problem occurs, it definitely sounds like a basic electrical problem. One simple test you can try is just run a jumper wire from the charging terminal on the back of the alternator directly to the positive side of the coil, or to the distributor if the coil is in the cap. Run another jumper wire from the alternator housing to the distributer housing. If that makes it run better, that will narrow the problem down to a bad wiring connection. While you are running it that way you can also use a volt meter to confirm the alternator output voltage.

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Originally Posted by Driver50x

If the voltmeter is jumping around when the problem occurs, it definitely sounds like a basic electrical problem. One simple test you can try is just run a jumper wire from the charging terminal on the back of the alternator directly to the positive side of the coil, or to the distributor if the coil is in the cap. Run another jumper wire from the alternator housing to the distributer housing. If that makes it run better, that will narrow the problem down to a bad wiring connection. While you are running it that way you can also use a volt meter to confirm the alternator output voltage.

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7.9 v at the coil when its doing it will let it cool then try what you said with the jumpers. Will it work to go from the battery instead of alternator? Very hard to get to the alternator where its at. I did check 14v at the battery always.

7.9 volts at the coil is way too low. If you are maintaining 14 volts at the battery you have a wiring problem, probably a bad connection somewhere, maybe a bad ignition switch.

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Originally Posted by Driver50x

7.9 volts at the coil is way too low. If you are maintaining 14 volts at the battery you have a wiring problem, probably a bad connection somewhere, maybe a bad ignition switch.

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I tried with the jumper wire from the battery to the coil no change.

Try this, disconnect the harness wire from the pos. + side of the coil.

Connect your jumper from the battery pos. + side to the coil pos. +side and crank it over.

If it runs good, then you know there is a problem in the power supply from the ignition switch and in the harness.

Sorry for long to get back. I will have to try it like that. Could the low voltage be the electronic brain box mercrusier uses failing athigh heat?

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Originally Posted by thestigmach1

Could the low voltage be the electronic brain box mercrusier uses failing athigh heat?

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YES!! Just like the older ignition modules on Ford trucks & cars!

And my Indian.