'89 340 DA Engine issue at low RPM

[amayes]

Hello all, New to the forum, and new to boating.
I have a new to me this year '89 340 DA. Twin 7.4L with velvet drives. Having issue with starboard engine.
Starts fine, runs fine at no wake speeds. Get it up on plane with no problems. Run around for a while with no issues. Head back into marina at slow speed, but as soon as i throttle back to no wake speed(approx 800 rpm) it starts miss firing and will hardly run. Sounds like its only running on 4 cylinders. Put it in neutral and it will rev up after you get past 1000 rpm but wont run below 1000 rpm? Just chuggs at 200-250 rpm with throttles closed. Get back to the dock and shut it down. 10-15 minutes later it will start up and run fine again. This whole process had repeated over multiple outings.
New fuel filters installed. I had a brand new ignition coil that i installed just cause i had it. No difference. I rebuilt the carb (full cleaning and new carb kit components). No difference. Swapped carb from port engine just to rule the carb out. Same problem.
Almost seems heat related, although both engines run at the same temps 145°. Doesn't seem fuel related at least as far as the carb is concerned.
Anyone had anything similar happen. Kind of stumped!
Any help appreciated,
Adam

 

[scoflaw]

What ignition system? What about spark plugs, wires, cap and rotor. Could be the pickup inside the Diz.

 

[amayes]

Thunderbolt IV i believe. Wires look good. plugs were new at end of last year. New Cap and rotor at beginning of this season.
Seems odd that it only happens on my way back in at slow speed.

 

[scoflaw]

What about the ignition sensor in the distributor. Does sound heat related. FYI, just because your wires look good doesn't mean they are.

 

[Pyrojodge]

Yes, try and swap distributor cap 1st and even possible coil from one engine to the other and see if issue follows If it does then what ever you swapped I'd replace. Great boat by the way!

 

[amayes]

What about the ignition sensor in the distributor. Does sound heat related. FYI, just because your wires look good doesn't mean they are.

I agree about the wires. Could be suspect but i would think that would give me issues at all RPM? Maybe the ignition sensor in the distributor could be heat affected. Thanks

 

[amayes]

Yes, try and swap distributor cap 1st and even possible coil from one engine to the other and see if issue follows If it does then what ever you swapped I'd replace. Great boat by the way!

Distributor cap is new as are coils(also did swap coils from port to stb) Love the boat but its frustrating me right now.
Intermittent issues are a pain.

 

[kap2]

Could be vapor lock. Take a close look at your fuel lines.

 

[amayes]

Could be vapor lock. Take a close look at your fuel lines.

 

[amayes]

When you say vapor lock, are you meaning fuel vaporizing in the lines and not able to feed the carb properly?
I'm assuming excessive heat build up in the fuel lines? Is this a common thing on these 7.4L engines.
If it is vapor locking what could be the cause.
Still new to this boating thing and learning every day.
Thanks for ideas!