Trouble running rebuilt engine

[davidsys]

I have recently rebuilt an old style Atomic 4 engine (dole recirculating thermostat, round tranny cover, etc.). Everything was taken apart, cleaned, painted and replaced as necessary. It starts quickly and runs smoothly at about 1200 or so (by the ear), after the choke is open. But, suddenly after 20 to 50 seconds it drops to a rough 600 or so rpm, and stays there. The engine will rev up, but roughly. A hot restart will be at the rough 600. A cold restart goes through the 1200 smooth to 600 rough switch after 20-50 seconds.

Everything on the fuel and electric side have been changed:
The carb is a new Zenith from Moyer
The fuel pump is a new low-pressure electric with filter
The fuel is straight connect from a remote outboard motor tank
The distributor is a Prestolite with electronic ignition, and it was traded out
for one with points and condenser.
The cap, rotor, wires and plugs are new.
The coil is a Flamethrower for electronic ignition, and it was changed out for
a new one.
The batteries have automatic charger and are at 13.4 volts.
The engine does not eject water when cold, and the Dole thermostat was
cleaned and tested ok.
The prop shaft does not turn in neutral when the engine is running, but
does go to reverse when asked to.

Both the owner and I are at a loss for what is changing the operation of the engine. It is predictable, in that it happens every time, and is consistent in its results or affect on the engine operationally. Any suggestions would be appreciated.

David

[ndutton]

Things I would check:

1. Timing - and carefully. Use a timing light.
2. Use the light to check the timing advance at high RPM.
3. Check for free movement, spring integrity and lubrication of the advance weight mechanism.
4. Measure the internal resistance of the new coil and the output voltage of the alternator at high RPM. Please report back.
5. Measure the temperature of the coil during rough running and report back.

[Al Schober]

Wow! At least it's repeatable....
If you don't have one, I'd add a pressure gauge at the fuel inlet to the carb. See if that changes when the RPMs change.
A second thing would be a voltage check with a good digital meter. Is the voltage into/out of the coil changing?
Also wondering if this is exhaust related? If you have a water lift, perhaps it's filling up?

[davidsys]

Replies and Suggestions

Neil and Al,

Thank you for taking the time to reply to my posting.

I should have explained a little more. The engine was run with 2 different coils, 2 different distributors, 2 different fuel pumps, and the results were the same. It never got to high rpm, and it never got hot. Results were the same regardless of which equipment was hooked with the engine.

Still thinking, It the problem gets solved, I will certainly post it.

David

[Dave Neptune]

David, welcome to the forum. Everything sounds good ign wise. And a new carb. However I do wonder about fuel flow, do you have a pressure gage in front of the carb. I have seen week electric pumps prime the carb but once started the "flow" is to little to maintain float level ~~~run
rough~~~die~~~~refill~~~~start~~~low on fuel again.
Might be worth a look.

Dave Neptune

[davidsys]

Fuel Pressure

Dave,

Thanks for the suggestion. I will look into that. I had been more concerned about over pressure, but under pressure or flow would fit the symptoms fairly well.

David

[ndutton]

I should have explained a little more. The engine was run with 2 different coils, 2 different distributors, 2 different fuel pumps, and the results were the same. It never got to high rpm, and it never got hot. Results were the same regardless of which equipment was hooked with the engine.

OK but that does not address at least 4 of the 5 suggestions I offered.

edit: have you tried running with the outboard fill cap removed?