7.4L Mercruiser won't idle properly or run below 1000 rpm when hot

[amayes]

I have twin 7.4 L Mercruisers with thunderbolt IV ignition and v-drives in my new to me 89 SeaRay.
Issue with starboard engine only. Engine starts fine, idles fine runs out from the marina at no wake speed fine, gets up on plane and runs great. When coming back to the marina and slowing to below 1000 rpm it starts to run rough. Sounds like someone pulled off 2 or 3 plug wires. Won't idle but never stalls. Tach will bounce around at 200-400 rpm.
Get back to dock and let it sit for 20 minutes and it will fire right up and run fine. Does this like clockwork every time I go out.
Both carbs rebuilt and tuned(even swapped carb from other engine, same issue) so I ruled out carb.
New cap, and rotor this season.
Changed fuel filters.
New coils.( one on port side had a crack so I replaced both)
Plug wires seem newish. Have misted them with water in a spray bottle while running and it doesn't cause any issue.
Tested engine with vacuum gage( little low at idle but the engines are a little tired and have 1700 hrs. Port engine has same reading on vac gage).
Compression is on the lower side but again the hours are up there and the other engine is the same.
Seems heat related but not sure what.
Engine temps are 145 all day long.
Exhaust elbows seem ok for water flow. Hot on the sides but cool to the touch on the top.
Any ideas or has anyone seen a similar issue?
Thanks,
Adam

 

[tpenfield]

:welcome: to iBoats . . .

Run a compression test and cylinder leak test on both engines. Record & post the detailed results. I say this because of the number of hours on the engines, and you've tried a bunch of the obvious stuff.

 

[amayes]

Hi Ted,
Thanks for responding. Sounds like the prudent thing to do. Rule out any mechanical issues other than the age of the engines. Only a few weeks left in the season up here in Toronto. So probably do it on the hard as the boat is coming out on Oct 8. Just wondered if anyone had seen anything like this before.
Best Regards,
Adam

 

[Scott Danforth]

I would get compression readings prior to being on the hard.

 

[tpenfield]

Hi Ted,
Thanks for responding. Sounds like the prudent thing to do. Rule out any mechanical issues other than the age of the engines. Only a few weeks left in the season up here in Toronto. So probably do it on the hard as the boat is coming out on Oct 8. Just wondered if anyone had seen anything like this before.
Best Regards,
Adam

My current thought process is that 1700 hours on the engines . . . maybe one of them has been rebuilt or had some major work done . . . that you might be running lower compression on one of the engines. When the engine is cold and/or running above idle speed not a big deal, because of a richer fuel mixture and /or more RPM. But when you get down to idle speed on a warm engine . . . maybe a bunch of misfiring. Also could be some slop in the distributor . . . but that may be more consistent cold/hot.

I do my compression and leak tests on the hard, but you could do them any time, if you got a source of shore power to keep the batteries juiced up during the testing.

 

[amayes]

Hi Ted,
I agree, maybe low compression on that engine, or on one cylinder maybe. The boat has had 2 previous owners so I only know the info from who I bought it from. Engines could be original, not a lot of signs that they have been messed with. I was wondering about the distributor slop as an engine builder friend of mine had also mentioned that. Possibly causing issues with the pickup coil.
But like you said i would think that would be more consistent. I have power and water on the hard so I can do do my compression tests and easier to to do the leak down as I don't have to drag my air compressor to the dock.
Can you tell me what compression range I should be in for one of these engines.
Thanks,
Adam

 

[tpenfield]

IIRC, 7.4/454 compression spec's are 150 psi. you may find an older engine might be more in the 135 range. Both value and consistency across the cylinders are important.

I'm not sure if having the outdrive attached reduced the numbers from spec, or not. I have run a compression test with the outdrives off and got maybe +5 psi better numbers than 'normal' . (I had to rig up a water supply to warm the engines to operating temp, since I could not use the earmuffs)

Anyway, post what you get for both PSI and leak for each cylinder.

 

[amayes]

Hi Ted,
Thanks for the input. I'll post the results of the test when completed