Need some help on 50hp johnson....again....'72 model

[jbjennings]

Motor is idling good, although it was fouling plugs yesterday and having intermittent spark on one cylinder yesterday when running low speed for any length of time. It would be o.k. for a minute or so after a full throttle run and start fouling a plug again. When applying throttle for full speed, I have to give it full throttle and let speed build up, then back off a little until it hits kind of a "sweet spot", and it will sound smoother and run faster than it will with absolute full throttle. It sounds as if it's "pinging" if I apply full throttle and will seem to bog a little. I have set the timing properly, and tried both retarding the timing and advancing with no positive results. I have recleaned the carbs with no effect either. It takes low speed idle adjustment well on both carbs., and I've rodded out the high speed jets as well. I even tried running it with the cowl off to see if that'd help---it doesn't. It also has a brand new NOS fuel pump. Pumping the primer bulb has no effect.
I'm really at the end of my skill on this motor. I've never had one I couldn't get just right eventually. It has 146psi on one cylinder and 150 on the other (new rings and good pistons), so compression seems good. The old fiberglass boat I am pretty confident has no water inside the hull. It also has a brand new stainless prop of the proper pitch.
Any suggestions woud be greatly appreciated. I put 2 brand new plugs QL77JC4 in it today and didn't have a bad fouling problem. I also have a new thermostat in it as far as I know but don't think it's reaching full operating temp. I suspect that may be the problem with the plug fouling but could it also affect WOT running that badly??? I've also done the choke fix so it's not closing the choke at low speed.
Thanks,
JBJennings

 

[Cofe]

Re: Need some help on 50hp johnson....again....'72 model

When the new rings were installed, did you do a proper break in period running extra oil in the gas? Sounds like the rings may not be broke in.

 

[jbjennings]

Re: Need some help on 50hp johnson....again....'72 model

It's still in the "break-in" phase, although I have not been nearly as careful as I would have been with a brand new motor. How would the rings not being broken in contribute to the high speed problem? Excessive blow-by or what??
Thanks for responding,
JBJ

 

[boobie]

Re: Need some help on 50hp johnson....again....'72 model

Will your spark jump the 7/16" open air gap? Quick way to check mtr temp is to have it warmed up and put your knuckles on top of the block right by the cyl head. If you can hold your knuckles on the block over 10 seconds it's running to cold. Running to cold will foul plugs.

 

[Cofe]

Re: Need some help on 50hp johnson....again....'72 model

Yes extra oil getting by the rings and fowling plugs. Running excessive rpm's, and not enough oil in the fuel before the rings are broke in, will cause them to form a glaze on the rings. When this happens, the rings will never break in.

Only other thing I can think of is some kind of ignition problem. Kinda guessing along with you, as you seem to have all the bases covered.

 

[HighTrim]

Re: Need some help on 50hp johnson....again....'72 model

JBJ, Im having a feeling that you're experiencing detonation. Likely due to you needing to retard the timing, but there are other possibilities.

You could have bad fuel with low octane. Your props pitch could be too high? What is it, and what is your WOT RPM? Your fuel mix could be too lean at WOT. Plugs heat range too hot, but those Champs should not be a problem. Overheating, yet you feel that you are running cool. Carbon deposits on your piston or combustion chamber, but being freshly rebuilt, not likely your issue. Shouldnt need a decarb quite yet.

This abnormal combustion AFTER the normal spark or injection ignition point has occured could definately cause that pinging you hear. Just a guess, but worth looking into I think.

 

[Cofe]

Re: Need some help on 50hp johnson....again....'72 model

HighTrim could be right about the octane being low.
Personally, I would put sea foam in the gas and perform a break in.
Give it some time, it just may need it being a 72 model.