OMC 4.3 Cobra performance problems

[Lou C]

Finally got the boat in the water, and I noticed that it tends to idle somewhat roughly and lacks power--seems to miss when a load is put on it. I changed the spark plugs, and checked the dwell, ignition coil, and timing. The coil's secondary resistance is too high by about 2000 ohms, so I am going to put in a new one. Dwell was OK, as was the timing, and the spark appears to be advancing properly. I am going to replace the cap, rotor and wires as well. Any other ideas before looking at the fuel system (Rochester 4bbl). Compression was checked recently and was 180-190 in all 6 cyls. No water in oil, no overheating. Carb accel pump gives a good squirt in each primay barrel.<br />This is a non balance shaft V-6, do they generally run a little rough at idle and low speeds even when everything is in good shape??

 

[Lark40]

Re: OMC 4.3 Cobra performance problems

Mine is so smooth at idle I have to listen to be sure it's running.<br /><br />Keep going down your diagnostic checklist. The problem will turn up.

 

[Lou C]

Re: OMC 4.3 Cobra performance problems

Well I'll keep at it, does anyone have any other ideas of things to check??

 

[Lou C]

Re: OMC 4.3 Cobra performance problems

Come on OMC experts, need some ideas here!!

 

[Boatin Bob]

Re: OMC 4.3 Cobra performance problems

Any chance the ESA is involved? You could quickly disconnect just to be sure

 

[Lou C]

Re: OMC 4.3 Cobra performance problems

You know I thought of that too, it seems to cycle on and off like it is supposed to, so I have to keep looking.

 

[andrewgroup]

Re: OMC 4.3 Cobra performance problems

Have you tried 93+ octance Gas?<br /><br />Rough Idling is a classic symptom of a carbueretor in need of rebuilding...

 

[Lark40]

Re: OMC 4.3 Cobra performance problems

What would 93 octane gas do besides waste his money?<br /><br />OMC specified 89 for that engine.

 

[Lou C]

Re: OMC 4.3 Cobra performance problems

Yep I am running 93 in it. I changed the original plugs from the AC MR43T to a Champion RV15YC4 based on a recommendation from George Van Parys on Boatered. The AC's were all uniformly carbon fouled. I also checked the timing and advance at 3000 rpm, they are both in spec. Last year when I first got it it had the Champ RV8C and it didn't seem to run as badly as it did with the AC's. When I took the old Champs out this spring they were fouled too but I thought it was because of the fogging oil. So we'll see.So far it seems to run better.

 

[metwrench]

Re: OMC 4.3 Cobra performance problems

Lou, did you put the coil on it ? any Difference ? also what did you set your dwell angle at ? and what is the base timing set at ?<br /><br />a Misfire Underload I feel would be Secondary Ignition ( cap, rotor, wires ) Have the wires been done ? Rough Idle also..Did you try cleaning and Blowing out the IDle Mix Jets at the Base of the carb ? There should be 2 Hard to see/reach Idle Mixture jets at the base of the Carb, Counting the number of turns counter clockwise, Back them out and remove them. Take carb cleaner and Blow inside those passages, Then look at the ends of the Needles you took out. are they Black? Clean them, reinstall counting the Number of turns back. restart Motor, Run till it warms up and slowly turn each Screw to obtain the best sounding idle. This will not cure a " Skipping" or " Misfire" but may cure your idle<br /><br />John

 

[Lou C]

Re: OMC 4.3 Cobra performance problems

So far I did the plugs cap rotor and wires. I have the coil, but after testing the primary and secondary resistance of both I find out that the new one checks out exactly the same as the old!! I don't have a coil tester, just a digital engine analyzer (volt ohms amps dwell rpm etc). I took out one of the plugs (new) after idling it for about 1/2 hr and it was not fouled, looked to burning clean. Dwell is set within spec, 37 degrees. I think it may have the initial timing set at 4 degrees instead of 6, so I am going to change that and water test it tomorrow. (I keep it on a mooring). I can get to the mixture screws, but instead of screw heads with a slot, they have a odd-shaped end that requires a special tool. Don't know if this was GM's doing or OMC's, so I have to look around for the tool. Accel pump gives two good squirts.Thanks for all the suggestions.