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Old     (fox)      Join Date: Jul 2002       06-23-2003, 7:29 AM Reply   
I have a 351 PCM in my 92 Nauti. It's carbed but has electronic ignition, knock sensors and the like. It ran great when I put it away last season. This season it runs like crap. It will idle well, though a bit fast at about 1000rpm. I've slowed it down, but it labors kinda hard starting at 750rpm. The real surprise was at about 3000rpm, it backfires through the carb and sputters and wheazes. If I back down to under 3K it is fine again. Also, when trying to pull a boarder, it coughed and sputtered the whole time. The only things I have done, were to replace the plugs, the impeller, the thermostat, and the plug wires. I've already replaced the gaskets on the carb, and the power valve. Nothing really seems to be working well. Any thoughts?
Old    tommyadrian5            06-23-2003, 8:23 AM Reply   
Most likely you messed up the order of your plug wires, that would cause it to backfire. Did you gap ALL your plugs?
Old     (fogey)      Join Date: Mar 2002       06-23-2003, 9:31 AM Reply   
It sounds like you may have two problems -- carburetion and ignition.

You may have a vacuum leak somewhere. That could cause the fast and/or poor idle and low power. The plugs could help the diagnosis; if they're really white, the mixture is lean which could be caused by a vacuum leak.

The popping could be ignition. Check the distributor cap for signs of arcing. A new cap and rotor might be worthwhile even if you don't see anything.

Also make sure the plug wires are routed in a way that separates as much as possible the wires for two cylinders that fire consecutively in the firing order. The current in one wire can induce a spark in another close-by wire if they run parallel for enough of a distance. I've seen this cause a misfire, especially under load.

(Message edited by fogey on June 23, 2003)
Old     (csquared)      Join Date: Jan 2002       06-23-2003, 9:33 AM Reply   
What Tom said. Check the plug wires, the timing and the timing advance (in that order).
Old     (fox)      Join Date: Jul 2002       06-23-2003, 11:01 AM Reply   
The plugs I took out after running it at the beginning of the season were black, however the boat had been stored all winter with stabil in the gas and fogging oil down the plug holes. So I suppose they are not a good indicator. I will check the new ones I just put it. They were gapped to .005 if I remember what we did correctly. It is an electronic ignition, so there is no "cap and rotor". I did have one plug wire crossed, but we pulled them one at a time to check proper set up and I think we have that licked. I have never heard about the plug wires jumping current. I can try switching them out again to see if it helps. I had been thinking perhaps the mixture screws need to be reset since we rebuilt the carb. I can't get past the idea that it is an electrical problem though.

Thanks for the suggestions so far...

Old    tommyadrian5            06-23-2003, 11:43 AM Reply   
If you rebuilt the carbs you will definitely have to adjust the mixing screws, this must be done before using the boat, a lean or rich mixture can destroy an engine. I don't know what the settings for that engine should be, but an experienced technician familiar with the engine or a haynes or clymer manual will most likely know the approximate settings, and you can fine tune it from there. Having too rich or lean a mixture is most likely causing your problems.
Old     (fox)      Join Date: Jul 2002       06-23-2003, 12:15 PM Reply   
Do you think that I will need to replace the power valve again? I have heard that if the engine backfires through the carb twice, it is shot. I hate to have to tear it all apart again.
Old    tommyadrian5            06-23-2003, 12:48 PM Reply   
I don't know much about the carbs in this engine, my boat is EFI so i haven't dealt with carburated engines in ski/wake boats, just PWC. I'd try to figure out what the problem and fix it before you tear the carbs out again, fix them, just to mess them up again with this problem. Contact someone at they will probably be able to help you figure out what the problem is and how to fix it.

(Message edited by tommyadrian5 on June 23, 2003)
Old     (robandrus)      Join Date: Feb 2002       06-23-2003, 1:44 PM Reply   
I went through the exact same problem earlier this year, I cleaned the carb, replaced powervalves, I even bought a new carb! I replaced the wires with the highest quality ones I could find, and spent about $55 bucks trying different plugs, some of those things helped but none really solved the problem. I bought a Timing light to make sure that it was right where it was supposed to be. All to no avail. Finally I got out my compression tester and did a test, If was a pain cause on mine you have to remove the exhaust manifolds to get the tester screwed in.

Finally I found the problem, I had two cylinders that were bad, one had 75psi and the other only like 22psi. So try that first to make sure you have at least (at the very least) 100psi in each cylinder. You can get a compression tester at any autozone.

My engine had been replaced and shouldn't have been bad yet so I kind of had the attitude of ride it till it dies, cause you could still go wakeboarding speeds.

Luckily a neighbor let me know about a trick. If you pour a bunch of oil into the dead cylinder, then put the tester back in, then turn it over. If the compression jumps up then your rings on the piston are bad and you should be looking for a new block. If however the compression does not change then it means your valves are bad. If you are mechanical you can pull your heads off and get a valve job for like $90. Otherwise you'll be paying for about 4-5 hours of labor. Still better than replacing the whole engine.

In my case the valves were bad on the two cylinders and there was even a crack in the head. You can get refurbished heads from AutoZone for $300. Or in my case I liked the shop so I bought some used heads from a junk yard for $70 then took them to the machine shop and got a valvejob complete with new valves, (It ended up being $180) then put everything back together myself. Wasn't that tough.

Now it runs like brand new. Its a rocket out of the hole and it completely cured the problem.

(Message edited by robandrus on June 23, 2003)
Old     (fox)      Join Date: Jul 2002       06-24-2003, 11:35 AM Reply   
I'm going to start with the mixture screws and then move onto the filters and then the anti-siphon valve. This is the suggestion of Vince at Discount Inboard Marine. I've worked with them before and they have always come through. Thanks for all the good suggestions. They're all going down on the "list" in case Vince's suggestions don't pan out. I really hope I'm not going down your route Rob!

Old    miker            06-24-2003, 4:55 PM Reply   
I had a cracked head that did the same thing
Old     (peter_c)      Join Date: Sep 2001       06-24-2003, 11:14 PM Reply   
The mixture screws only affect idle. The problem being at 3000 RPM is going to be something else. There could very simply be a problem with a crossfire on the coil pack. Why did you replace the wires? The wires should be good for at least a 1000 hours. Is there any kind of corrosion on the coil towers or some kind of carbon tracking? Common problem that shows up as rust on the coil towers.

Power valve will not cause the problem as it is a lean running condition or an ignition missfire. A blown power valve would cause it to run rich.

As for running the plug wires all funky....well you could zip tye them together and it will not affect them. More importantly do not let them rub themselves to a cut on a sharpe corner or such.

What brand of spark plugs are you running and what did you really gap them too? With a good proper plug there should be no need to gap them.

Water in the fuel can also cause the exact problem you describe. Remove the plug on the bottom of your fuel canister and drain the water out. Also check to see how much comes out.

A piece of debris in one of the jets could do the same thing also.

It could also be a mechanical problem that should show up with a compression test.

Good luck!
Old     (fox)      Join Date: Jul 2002       06-25-2003, 7:54 AM Reply   
Thanks for the additional advice Peter. I was told that it is like messing with a moto-x bike. That have the mixture right at idle will ensure the top end as well. Apparently even if the mixture is off, it will do ok through a certain RPM Range.

I replaced the wires because I noticed cracking last season. The old ones were working fine, but call it insurance. I noticed no rust or cracking on any of the coil packs. I am running direct replacement AC Delcos and they are gapped to 35 thousandths. I have not checked for water in the fuel, but I did poor gas dryer into the gas after the rebuild. We replaced the power valve becuase it was bad and was dumping tremendous amounts of fuel at idle. I think the next step may be to take a clean plastic fuel tank and make the motor draw gas off of it, if the mixture doesn't resolve the issue.

I will be sure to post the results. Friday is my day to mess with it...out of town on buisness this week.

Old     (fox)      Join Date: Jul 2002       06-30-2003, 7:24 AM Reply   
Hi all. Turns out I had Three plug wires on the wrong plugs. We originally switched 2 and thought we had it licked. Come to find out one of the 2 we switched and another needed to also be swapped. Now I just need to throw a new power valve in, again (since it was popping through the carb) and re-adjust the carb and all should be well. Thanks for all the suggestions.

Old    nohalfboats            06-30-2003, 10:22 AM Reply   
Had exact same symptoms as you and it was a clogged main jet which is easy to fix as you won't have to remove the carb.


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