1979 Chrysler 75 HP running rough

[userreeducationtool]

Last year when I had our new-to-me boat out on the water for the first few times, I noticed that our outboard would seem to run fine at idle & full throttle, but had a pretty bad shake/misfire at part throttle. I didn't worry about it too much as we only got the chance to use it a few times before storing it for the winter. This year I am finishing up the 'little things' after a total rebuild of the boat last year, so I am diving in to the engine.

I've done the 'link and sync' procedure, set the timing at 32 degrees BTDC, and cleaned the carburetors. Idle screws are set at 1 1/8 turns out for now. I replaced the fuel line from the pump to the carburetors & between the carburetors with see-through fuel line & verified that there are no bubbles when running so it shouldn't be sucking air. Spark plugs are new Champion L20V's and a compression test seemed good - my gauge won't hold pressure for some reason, but the needle was over 120 PSI on all cylinders.

The idle is still rough on muffs (haven't had it in the water yet, no time!), the best description I can give is that it sounds like its got a 'lumpy' cam in it. I know revving it way up is bad on muffs, but even just off idle it still sounds rough. I'm not getting any fuel spitback through the carbs, so that coupled with the decent compression seems to me like mechanically its fine. I didn't 100% rebuild the carbs because I don't have $$ right now for the rebuild kits, but they all looked completely clean in the bowls & I blew them out the best I could with Gumout.

When I re-set the timing this year I was thinking that was it, it was only set to 20-something degrees BTDC previously. Its better now, but still not 'smooth'. Any ideas as to where I should be looking? I just don't want to start throwing money at guesses here & am hoping to get a pretty good idea of what the problem is.

Thanks in advance...

 

[jerryjerry05]

How did you set the timing?
http://youtu.be/tDoR64jP5l8
video on timing

First few post in this forum cover some TU procedures.
Get a manual and make sure that you did it right?

The carbs if the bowl was clean then they might be ok??
But check the jets anyway.

Some carbs run good on 1 and 1/8th out some need to be out more??
In the water and in gear is the recommended way.

 

[Nordin]

Do you have beltdriven distributor Magnapower I ignition system or Prestolite typ?
Have you set the timing static or dynamic? I would suggest you to retard the timing to 30dgr BTDC.

As jerryjerry says the carbs maybe need some more tuning. The setting of the air/fuel mixture screw should be 1-1 1/2.
I also would suggest to set the mixture and even the idle screw in water not by muffs.
Two strokers need a bit of backpressure for the exhaust from the water and it maybe will idle smoother with some backpressure.

 

[userreeducationtool]

Its a belt-driven Magnapower setup. I can try bumping the timing back to 30 degrees - I followed the Chrysler service manual procedure to set it & if you rock the timing back & forth past the 32 degree mark, you can hear the ignition fire.

On the carbs I did pull the main jets (.066 jets, BTW) and flush them & the main 'tube' running up through the carb with Gumout. I did have it running in a Rubbermaid container of water, but it was barely deep enough to cover the exhaust outlet so that may not have helped that much. I may get a chance over the weekend to splash it & test it out, I'm just hoping to get as much done as I can at home first.

 

[Nordin]

Okey then you have set it by static and it is the right way you made it.
Why retard it to 30dgr? Well back in those days the fuel was better for the engine then today. Now it is ethanol and other stuff that do not make it better.

If you have a timing light you can tune the timing more exactly and set it to 30dgr BTDC at WOT.

Tune the carb in water and setting the air/fuel mixture to 1-1 1/4 would end up good, maybe 1 1/2.
Idle would be about 750-800 RPM in water and in gear.

 

[userreeducationtool]

Well, I am happy to report that things are much better than before. I got a chance to get it out on a small lake locally on Sunday & after getting the timing adjusted, there's a noticeable power difference from when I had it out last year. Starts well, idles well, and the misfire seems to be gone, judging by sound. It still vibrates a lot off-idle, but the engine tone is smoother so it may just be a natural vibration or harmonic that its hitting.