OMC 250 hp rough idle, back fires through carb on load

[Rochester_Rob]

Hi everyone,

I have a 1973 Penn Yan tunnel drive boat with an OMC 250 inboard engine. My dad bought it when I was a kid. The engine is a small block Chevy 350. In 2022, the original manifolds had internal cracks that let water into the cylinders when I was winterizing the boat. I didn’t realize right way and ended up pulling the engine, disassembling, changing the rings, honing the cylinders, and reinstalling it with new manifolds. It ran great for the summer of 2023 but then started to make a noise I never heard before when it was under load. I now believe it was backfiring through the carb. I can’t find anyone locally that will work on this old a motor other than to just rebuild it which it may not need. I have been working on it off and on since. Now it will not even idle well most of the time (although it does idle well sometimes and I can’t figure out why it changes). I have changed the points, condenser, rotor, flat cap distributor cap, set the dwell, and verified timing is to spec at 5 degrees BTDC. I use ethanol free gas and have also tried to run it from a fresh gas can. I have verified fuel pump pressure and flow. I have run compression tests (cold, plugs removed, WOT), leak down tests, and used an indicator to verify the cam lift. The only things I have found to be out of spec is there is a little cam wear and compression variation is above 10%. The intake lobes only have 0-.004’ wear while the exhaust lobes are mostly .003 or .004 of wear but one is at .009. While the larger numbers are out of spec, when I chose to have the cam changed on a 1980 vette it had visually significant lobe wear unlike here. The other thing that is out of spec is compression which varies from 142 to 161 PSI, a little more than 10%. I am attaching a file where I documents 7 pages of history and what I have tried and while I am methodical, with my lack of experience, I must have missed something. I can’t expect anyone to read all that but I am including it. This is a fairly simple engine, what am I missing? I am also not sure how easy this would be to rebuild since it is a reverse rotation with a flat tappet cam. Thank you for any suggestions.

 

[Scott Danforth]

what is the dwell set to on the points?

what does the fuel look like?

two reasons to backfire thru the carb. dwell and timing is off, or the carb is gummed up and its running lean

 

[Lou C]

How about an intake gasket leak either under the carb or one of the actual intake gaskets & maybe an intake valve not seating fully?

 

[Rochester_Rob]

what is the dwell set to on the points?

what does the fuel look like?

two reasons to backfire thru the carb. dwell and timing is off, or the carb is gummed up and its running lean

Hi Scott, thanks for the quick response. The dwell spec is 30 degrees. After several adjustments I have it set to 29 degrees. The fuel was fresh when I started but it is several years old now, though I do use stabilizer. For most of my tests now I have the fuel pump draw fuel directly from a portable gas can with fresh gas. I had good idle performance for the first time I tried the fresh gas but have not been able to get reliable good idle from the portable gas can since. I did rebuild the quadrajet carb. It had previously been rebuilt about 10 years ago and still looked good even before I rebuilt it. I also replaced the quadrajet fuel filter and the fuel filter that is on the bottom of the mechanical fuel pump. I don't know now if I have 2 problems... the original backfiring through the carb and a new problem that is causing the rough idle or if the original problem has become worse. Should I focus on getting a reliably good idle now and then deal with the lean sneeze? How should I do that?

 

[Rochester_Rob]

How about an intake gasket leak either under the carb or one of the actual intake gaskets & maybe an intake valve not seating fully?

Thanks, Lou, for the quick response. I sprayed some starter fluid around the intake gasket while the engine was idling but saw no change in idle speed like you might expect if it sucked in some air but maybe there is a better way to check that. To check the intake valve, I bought a leak down tester. I applied 100 psi to each cylinder and had leakage between 15% and 25%. That seemed high to me but the gauge suggested below 40% is "green". Hopefully I did the test right. For a moment, I thought I had found the issue because the numbers would sometimes suddenly increase but then I realized the applied 100 psi sometimes rotated the engine so the valve then opened.

 

[Lou C]

What do the center electrodes insulators look like on the spark plugs, black?= rich, white?= lean, tan/grey?= about right. Quadrajet carb….are the idle air tubes clean? Float height? Excessive primary throttle shaft play? That can def cause a vacuum leak. You can check it with the throttle cable disconnected with a dial indicator. Up to about .010” is ok, if it’s as much as .020” the primary shaft needs to be re bushed.

 

[Rochester_Rob]

The one local person I found that would work on my boat said he doesn't work on old motors and that he would put in a new motor because he has had some many problems with rebuilt ones. When I told him it was reverse rotation, he "reversed" and said he would put in a rebuilt one. When I asked what if the problem was fuel related or ignition related, he said he could fix that easily after the motor was rebuilt.
I am willing to change the intake gasket if I can prove that is the problem or take the cylinder heads off and have them rebuilt if I can determine that is the problem. I just think something that i tested is really the problem but I didn't recognize it.

 

[Lou C]

I would:
Hook up a vacuum gauge
Get some light motor oil & a squirt can
Run the motor & squirt oil around the throttle shaft for the primaries & observe the gauge. Do the same with the carb gasket & intake gaskets. See which area sucks in the oil and causes the manifold vacuum to rise….

 

[kenny nunez]

You may have either a broken ot some weak valve springs. To replace them you will have to remove the manifolds. They are easy to replace with the KD tool. Are your rocker arm studs positive stop?
Make sure none of the studs have pulled out.

 

[Scott Danforth]

The fuel was fresh when I started but it is several years old now, though I do use stabilizer.

fuel lasts 6-12 months. Stabilizer at the most can extend that up to 6 months.

if you have a floor to varnish, you have a tank full of it.