92' PCM 351HO Idling Problem

[DKJBama92Mariah]

OK guys, i got the drain plug problem fixed, now for the problem that has been eating at me for two years.

Most of the time, the engine idles really rough. It idles perfectly only about 10% of the time. The rough idle is enough to make the engine stall when i put the boat in gear unless i immediately get on the gas a little bit right when i put the boat in gear. Needless to say, this isnt very good for the transmission.

So here is the dilemna. Im not sure whether it is a ignition or carnuretor problem. I had the carb rebuilt about 3 years ago. I'm not completely sure, but the problem seems to have come about around that time. I've ordered another rebuild kit for the carb. AM i barking up the wrong tree with that route.

The ignition on the motor is the Pro Tec distributorless system. Could a have a single coil pack that is intermittently failing? If so, is there any way to test the packs and can they be replaced individually or do i have to buy the complete set of four already in the housing?

Im thinking i'll start with that carb rebuild because that is the cheapest route. Is this something i can do myself? I am reasonably adept with mechanical things, but i just dont want to get stuck carrying a box full of parts from my disassembled carb to the shop because i cant get the darn thing back together.

If that doesnt work, i'm not sure where to go next.

Any thoughts and advice would be greatly appreciated.

Thanks,
DKJ

[Salty87]

what are your rpms at idle? checked the timing? condition of plugs, wires, etc?

[Juice75]

If you only have the problem at idle, then that lesses the possibility of a coil failure. I'm not familiar with the ignition setup you have, but if each coil controls two plugs, then pull your plugs and check them out. If all of your plugs are in the same condition, I'd rule out the coils for now.

Like Salty87 mentioned, check your plugs if you haven't already. You could have a fouled plug. Also like he said, check the timing and that you are at the correct RPMs for idle. When your carb was rebuilt, they could have adjusted the idle screw out to far.

Here are some other things to check before you tear the carb apart. Hook up a vacuum gauge and adjust the air/fuel mixture screws. There should be one on each side at the front of the carb. If you don't have a vacuum gauge, run the screws in as far as you can until the idle gets worse, then back them out slowly until you reach the highest, best idle. With a vacuum gauge, adjust until you reach the highest vacuum reading.

On the vacuum topic, you may have a vacuum leak. Make sure you don't have any open vacuum ports or cracked lines on the carb. To check for leaks around the base of your carb and manifold, have the engine at idle, then spray some WD-40 around the carb base and also where the manifolds connect to the heads. If the idle jumps up, then you have a leak.

As for carb rebuilding difficulty, Holley's are pretty easy, Edlebrocks have a few more parts and are a little harder if you don't know what you're doing. I think anyway.

Hope this helps, and good luck!

[DKJBama92Mariah]

Thanks for the input guys. I have a set of new plug ready to go in, but it will have to wait till summer cause im only on spring break and have to go back to law school. The wires are 13 years old, but they are still pliable and there is no visible cracking in the jeckets. Could the wires still be the problem? When the engine is idling poorly, it idles about 700-800 RPM; when it is running smoothly, it is up around 1100 RPM. I cant lower the idle setting any further because the boat will stall when it is not idling well.

The local boat shop quoted me $195 + parts to rebuild the carb. Is this reasonable? I am thinking i will just take the carb back to school with me in Birmingham, AL and try to find a performance shop to do the rebuild on it. I already bought the kit from skidim and it arrived today.

Thanks for the advice, on locating the problem. It will come in very handy when i come back to deal with it after finals in May.

Thanks again,
DKJ

[Juice75]

Hey DJK - $195 sounds a little steep to me. Especially if you already have the kit and pull the carb off yourself. You could probably find somebody to do it for about $50. Or if you have the time, do some research on the internet and then do it yourself. Just make sure you pay attention to where everything goes when you take it apart and you should be alright.

-Kraig

[DKJBama92Mariah]

Update.

I ended up sending the carb off to the Holley Custom Shop to be remanufactured. It was pretty steep ($425), but apparently the particular carb mine came with isnt made anymore and i didnt want to go with a universal replacement. I know the argument: "you could have gotten a brand new universal one for $75 more." But i didnt want to mess with a good thing, the motor runs great besides the idleing problem. Apparently, the carb will come back looking and working better than new. In addition, i ordered a new set of plugs and wires, and plan on replacing my "canister type" fuel filter with a "screw on" type. Looks like i've got my work cut out for me when finals are over. If, after all this, i've still got the problem, i guess i'll have to take it in to the local boat stealership (not a Supra dealer) and get a leakdown test done to make sure it isnt something internal. I'll keep you posted.

Cheers,
DKJ