pat@osp

Have a '99 Formula 400SS with a pair of 502 Mag MPI's. Having a problem where the port engine seems to be dropping a cylinder randomly. Maybe it's something else but it seems like a dropped cylinder.

Symptoms = Starts fine and seems to idle as well as the other engine, while idling out of the cove with the throttles all the way back it runs fine, if I bring the engine speed up to 1000 RPM the port engine hunts up and down about 100 RPM on the tach and you can hear the exhaust note change, coming up on plane with both engine at the same throttle position the port engine is 300 - 400 RPM behind the stbd engine, then for about the first 10 - 15 seconds after the boat gets up on plane and the RPM is stabilized at 3400 both engine run smoothly and match RPM's, after cruising at 3400 RPM for the first 10 - 15 seconds the port engine drops 100 - 200 RPM lower than the stbd engine with a noticable change in exhaust sound, after that the port engine will pick back up for 5 - 10 seconds every so often and match RPM's the exhaust note cleans up and you can feel the boat accelerate slightly before it then drops back off again.

The engines have 525 hrs, just installed new SSM manifolds and risers with long tails, eliminated the silent choice system, remove the "extra" baffles in the muffler tips on advise from Corsa. The exhaust systems and plug wires are new within the last 5 hrs and I didn't notice this problem until after they where installed. Can't figure why these changes would create the problem so maybe it was there before and I didn't notice as much with the old exhaust? New plugs / caps / rotors / fuel filters / Hardin water pumps with in the last 40 hrs. Checked all the plugs and they look good. Tested the new wires for resistance and continuity OK. Couple cylinders are little weak on compression probably from rusty valve seat due to the leaky old manifolds but all within 30 psi. Haven't checked injectors yet but thinking that should be next. I'm stumped by the fact that it seems to clear up and then drop off again as I'm running which seems more like a wierd wiring related problem?

Sorry for the long post, just wanted to give as much info as possible. Anyone have any ideas. Problem sound familar to anyone?

Thanks, Pat

Tinkerer

Have you checked your fuel pressure - is it consistant.
I am a carb guy.
could be a fuel problem, could be the fuel pickup in the tank.

pat@osp

Haven't checked fuel pressure under load yet, only at the dock.................38 psi at idle, 36 psi at 1200 RPM, and 45 psi at idle with the vacuum line disconnected. I guess that just tells me that the pump functions, the regulator works, and the vac line is OK. Checking fuel pressure under load is a bit of a project on the boat because it doesn't have a sunpad but instead the whole rear seat / trunk storage module swings up to access the engine compartment. That whole deal sways back and forth a bunch with just a little bit of wave action while sitting on the anchor so I can't run with it open. This week I'm planning to rig the gauge up with some extension hoses to run under the lip of the seat and leave it up a bit with a shim on either side or maybe have the guage poke up thru the emergency engine compartment access hatch under the rear seat. I need to get another person to drive or watch the gauge.

Haven't checked the fuel tank pick up but I should. It could explain why the problem comes and goes?

Thanks for the ideas,
Pat

pat@osp

ttt

pat@osp

Update............no water in the fuel, a bit of debris on the filters but only what I'd expect, fuel pressure stays at 40 psi rock steady coming up on plane and underway while I can hear the engine skipping.

I'm thinking it must be ignition or maybe wiring related. I read somewhere on here about a guy replacing a distributor to coil wiring harness?? Anyone hear of this as a common problem?

Thanks, Pat

sonicss42

Part # for the wiring harness is 84-817376T. Listed as NLA but I think the part number is changed to the one I have given.

NorCalracer

try disconnecting the tach sender wire to the motor, sometimes a faulty tach could do that. otherwiser find a good local efi gooroo, he should have a ultrasonic fuel injector cleaner. my local guy on ly charges 10 dollars per injector, he pulls out screen filters replaces them with new ones flows all 8 injectors makes sure none are bleading by basically if ther is a bad injector he will find it.

pat@osp

Sonic,

Did your guy go right for that wiring harness because it's a known common problem or was it something you found while checking everything else? Was there something visibly wrong with it.................melted, corroded, etc? I think your main symptom was a rich idle condition if I remember but did you also have a skip or random dropped cylinder type feel under load? Thanks a bunch for the info and part number.

Pat

pat@osp

NorCal,

I will try the tach wire..............it sure would be nice if that was it! The injectors have been on my list of things to check. Actually being 11 years old with 500 hrs I should probably just do them as maintenance even if it's not causing this particular problem. I think I have a place in CT that can do it but $10 per injector from your guy sure sounds like a smokin' deal.

Thanks, Pat

pat@osp

FIXED! As usual it always seems to end up being the simple things................#7 spark plug was junk for some reason. Both engines have Delco R43T marine plugs brand new at the start of the season.........50 engine hrs. ago. Tracked it down to #7 on the port engine by pulling all the plugs for inspection AGAIN. #7 was dark and a bit wet with fuel but other than that had no visible signs of damage. Checked the terminal on the cap and it was good. Figured it had to be the plug wire or the injector leaking down a bit and fouling that hole. Swapped the stock Merc plug wire with a new MSD test wire I made at the shop and it got much better but not quite fixed all the way................couldn't understand that. Changed the plug and problem solved! To look at the plug there does not appear to be any thing wrong with it but a new plug cleared it all up. Put the stock Merc plug wire back on to confirm and everything is still good. I'm speculating that the MSD wire being a spiral core and having much lower resistance made it easier for the ignition to fire the bad plug than the stock Merc carbon core wires (with much higher resistance) and that's why it improved by at least 50% with the MSD wire? I never realized that the Merc wires are carbon core until doing all my testing trying to track this problem down. The Merc wire has approx 10,000 kilo ohms of resistance vs the MSD with approx 850 kilo ohms of resistance. I can't believe that the Merc electronics are so sensitive that they need the better EMI supression of the carbon wires so I think I'll pitch the Merc wires for some MSD's. Gotta think it can't hurt to let the ignition system see about 12 times less resistance when firing the plugs but I'm sure that will bite me in the ass somehow I did read an old thread on here from 2002 where a guys said that sprial core wires trigger the Merc knock sensors and retard timing. Anyone ever have that experience?

Thanks for all the suggestions,
Pat