OMC Sterndrive Idle Stall

"Hello. I have a 1980 vintage

"Hello. I have a 1980 vintage OMC 800 Stern Drive (Stringer), Chevy 350 engine, 260 hp, Rochester Quadrajet carb. My problem is that my engine develops a rough idle after it is warmed up, which eventually causes it to stall at idle rpm which is set to 600. (Not too friendly when trying to dock the boat.) When first started cold, and allowed to idle to warm-up, the engine idle is OK. Then it runs at speed OK (no hesitation or misses). But then when I throttle down after running at speed for a while, the problem occurs. It is not a definite miss, but rather a "stumble" in the engine idle that eventually gets worse until it dies. I suspect the carb, but I'm not sure. My understanding is that the Rochester Quad carb has a separate idle circuit apart from the main carb operation.

The reason I'm not sure about the carb, is that the problem first occurred after I replaced the spark plugs!? Altho the low speed operation of the engine has never been completely smooth (some roughness), at least it didn't stall. I replaced the spark plugs because I developed a hard engine miss - one cylinder out - that was due to a fouled plug, which in turn I suspect was due to oil leaking from the valve covers - another problem. The new plugs cured the engine miss, but now I have the hot idle stall. I replaced the old spark plugs, which were NGK-BR6FS installed by a service shop, with AC-MR43T which were the original plugs called for in my GM engine. The old plugs were gapped to approx .032. I set the gap on the new AC plugs to .035 per the owners manual. But the original OMC factory service manual, which I have, calls for the gap in the 260 engine to be .030. Which is correct? And could the larger spark plug gap in the new plugs possibly cause this problem? In general, what is the effect of the gap difference in spark plugs?

Any thoughts on what could possibly be causing my problem, or how I could further trouble-shoot it, would be greatly appreciated."

"Howard,
Off hand I'd say

"Howard,
Off hand I'd say it's time for a carb cleaning and reseal.
The plug gap could help a little if you close it.

Sounds more like the carb has a problem with the idle circuit. Taking a compression reading is a good idea to rule out the chance you have any weak cylinders.

Leaking valve covers really do not cause plug failure unless the plug wire is oil soaked and shorting out.
Since it is not idleing at a higher rpm than normal I doubt that you have a vacuum leak.

Is there a fair amount of smoke coming from the valve cover breathers? If so you may have blow-by and this is why you are fouling plugs.

The dry / wet compression test will show if there is a problem with the rings passing oil.

Try checking the compression then if it is ok go after the carburetor.
Good luck."

"Charli, thanks very much for

"Charli, thanks very much for your answers. One thing I didn't mention in my original post is that my engine, even though 26 years old, is still "young" in terms of use...it has 240 hours on the clock, and I am the orig owner. That's because the boat sat unused for about 15 years, and during other years usage has been less than 25 hours per year. The engine is perfectly tight, mechanically, doesn't smoke nor burn oil. I appreciate your eliminating possible causes such as the leaking valve covers (they are not bad, but there is a little leakage around the cover nuts) not causing spark plug fouling. But when I did replace the plugs, that cured the one cylinder-not-firing problem. Most significant is your comment about reducing the gap, which makes me think to try another set of plugs with the gap set to .30 per the OMC service manual to see what is the effect. Do I need to reset the timing when changing plugs? I didn't think so or do so, but am not sure? Another aspect of all of this is what ignition settings called for in the 1980 service manual should be modified by the drop in fuel octane since then? The dealers recommend retarding the timing to 7 deg BTDC, versus 10 degrees called for in the manual. And 7 deg is where my timing is set now. And of course that causes some balkiness and hestition in performance which could be mistaken as carb. Are there any other ignition settings (plug or point gaps) that should also me modified due to lower grade fuel?"

"Howarrd,
Sounds like a nice

"Howarrd,
Sounds like a nice boat.
I would still lean towards the carb being a little dirty.
As for the fuel I'd think that 91 octane would be just fine.
I see no reason for retarding the timeing unless the engine has run-on or it turns over hard when it is at operating temp.

This is my opinion but I think people blame fuel for a lot of problems.
It seems that when someone can't find a problem the reason is bad fuel. If this were ture there would be broken down vehicles boats whatever all over the place.
I'm running an 84 350 at 12 BTDC and I haven't had any problems. But I live at 8500ft also.

Anyway, I'd try a vacuum check. This is a good way of finding low rpm problems. Change the idle mixture screws and watch the gauge. I'd also put the timing where the book says it should be and use the recomended octane.

It's also a good idea to install the correct number plugs set at the proper gap.

When all else fails go back to stock calibration."

"OK. Reading directly from my

"OK. Reading directly from my 1980 OMC Owner's Manual under Fuel Grade Requirements, it says "...260 (350 CID V8) (requires) leaded regular fuels of 89 AKI or higher. If low lead or no-lead fuel of 86 AKI to 89 AKI is used, the initial spark advance setting for... 350 CID models must be altered by retarding to 7 deg BTDC." <Normal spark adv setting is 10 deg.> Recalling the days of leaded fuels, the lead acted as an anti-knock agent allowing higher compression and/or spark adv with less octane. Hence, I am thinking that if leaded 89 AKI was acceptable originally, then unleaded 91 AKI might be roughly equivalent. I will give that a try and reset the timing back to 10 deg. My understanding is that as long as engine doesn't knock, it is OK."

"Howard, I have the same engin

"Howard, I have the same engine and drive (1983)
and noticed the same problem with mine. I had my timing at 7 degrees as I use 87 octane and noticed the crappy idle. I replaced my points as it was time and I set the timing to 10 degrees. It runs a lot better now. Soon as I started to advance the timing it smoothed out. I'm not sure if I am getting "engine knock" on 87 octane or not. I can't hear it at all. Seems to run fine. But to be safe I will probably at least use 89 octane or higher."

"Thanks for your info, Bud. I

"Thanks for your info, Bud. I'm pretty sure if you were getting engine knock, you would hear it... an unmistakenable rattling sound under load which is all the time for a marine engine. If you are running the Chevy 350 engine (5.7L), what spark plug gap do you use? There is some conflict across my various service manuals. The OMC owners manual which covers both 4 and 8 cyl, says .035 for all models. The OMC service manual says .035 for 4 cyl and .030 for V-8s. The after market service manuals (Clymer and Seloc) say .035 for all. I replaced my plugs which were gapped to .032 by the dealer who installed them, with new ones gapped to .035, and my idle stall problem got worse, with no other changes. I am going to try .030 per the factory service manual to see if this makes a difference."

"Hey Howard,
I was just wonde

"Hey Howard,
I was just wondering if this engine has a points type ignition system. I'm not sure what year OMC made the change.
If it is points check the point condition / gap. A set of points that are opening to far will make a engine idle very rough but at the same time the engine will have great top end rpm.

A dwell meter is the best way for checking the gap.
As for the fuel I'd think 91 octane should be fine if not a little over kill."

My timing and plug info are on

My timing and plug info are on the valve cover of the engine. Off hand I can't remember what the gap is. I think its .035

"Howard, There is one thing yo

"Howard, There is one thing you might want to check. Most marine engines do not use vacuum advance on the distributer, but all do use mechanical advance. It's possible that the weights are sticking in the advanced position. Pull the cap off the dist. and check the the rotor can be moved in one direction and not the other. You should be able to rotate it maybe 1/10th of a revolution, then it should snap back on it's own. If it doesn't snap back, or if it comes back slowly, it needs servicing. The springs might be getting baggy or the assembly needs to be lubricated so that it pops back freely. I've had this happen in cars before, and after a good clean and lube everything was fine again. Good luck!"

"Thanks for this tip, Dave. I

"Thanks for this tip, Dave. I will definitely check that. I plan to replace points, rotor, condenser as well as regapping (new) plugs to .030, so will have all new ignition components working for me when I go out again."