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4.3LX (Thunderbolt HEI) surging/dieing

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me2633

New member
Some of you might recall, that I had a problem a few weeks ago with this engine running way too rich. Problem was solved with rebuilt carb (Rochester Quadrajet). Most of my run times since then have been short (10-15 minutes). Over the weekend I was out for about an hour with heavy waves from boat traffic and engine started to die at about 1/2-3/4 throttle, then surge, then start to die. I had to cut the throttle back to about 1/8-1/4 throttle to be sure it would get me home. It was pretty hot over the weekend.

I let it cool down and then it ran fine for the 15 minutes that I ran it.

I checked the pickup tube, checked the "ceramic filter" at the fuel pump, checked the filter at fuel inlet to the carburetor - everything was spotless. Choke was fully open. Disconnected fuel line at carb and a few seconds of engine cranking fills up a pint glass container. Let fuel sit for a few hours and then overnight; no evidence of water in the gas. Gas is pretty fresh since I have burned about 20 gallons over the last two weeks.

I did finally remember that the original carb had a 1/8" spacer at the bottom that I assume was a thermal insulator to keep the carb from getting too hot. I forgot to install the spacer.

The coil is new so I doubt the coil is failing when it heats up. The cap, rotor, plugs, wires are also new.

My thoughts are that either the fuel is partially boiling in the carb because of the missing spacer or the fuel tank vent/overflow is clogged.

Any other ideas? Anyone had a problem with an engine with a missing spacer.
 
I will install the spacer this weekend. I am just looking for other things to check this weekend if installing the spacer does not resolve the problem.
 
Had a similar problem, also in rough water, with engine surging, stuttering, and then not going over 3000rpm. My mechanic thought is was most likely the carb floats were adjusted too high, and the rough water led to a temporary flooding condition. Was there smoke when you restarted the engine?
Roger
 
i like rochesters they are almost bullet proof. I know i know ask 100 guys in a room what carb what oil or what filter you will get 150 different opinions, just saying i like rochesters.

Pump giving you want psi ?

When running the motor spray carb cleaner at the base of the carb the motor run differently you will be able to tell instantly, then maybe its sucking air around the base.

Maybe the fuel line is sucking air, the hose you said is clean, is it cracked somewhere maybe under a clamp and you can't see it ?

About the dist the new rotor fit tight no play in the shaft and all ends pushed down and seating good ?

Listen run the boat in pitch black darkness if you see sparks you see the problem.

Let us know after you double check everything.
 
Just to complete loop, I installed the missing thermal spacer under the carburetor and ran (fast, slow, etc) the boat all weekend with no problems. Obviously, the spacer does have an effect when the weather is really hot and the boat is moving slowly.
 
The spacer allows better fuel atomization at low engine speeds. The Mercarb 2 BBL w/a single mixture screw uses a thick gasket to do the same.
 
If that is the spacers only purpose, I wonder why installing the spacer solved the surging/dieing problem at 1/2-3/4 throttle when the engine was hot and why the engine performed well (at 1/2-wot) without the spacer when the engine was still fairly cool.
 
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