1976 25 hp Fireing on one cylinder

[69 Alumacraft]

Question: Can a clogged up carb cause the top cylinder not to fire? I got a big blue 1/2 inch spark on both cyliders and entirely new ignition as well as a new fuel pump, I switched coils and the problem stays on the top. Changed all ground wires etc. Compression is 125 on both. Carb was serviced but may have reclogged!?Engine starts and runs pretty well on bottom cylinder, i can pull of top plug boot and no change, just keeps running the same, also got rid of play in armature plate, no slop at all!?

 

[AlTn]

Re: 1976 25 hp Fireing on one cylinder

suggest putting a rag over the sparkplug holes of both cylinders and pulling the engine through a few times...fuel mix appear from each cylinder? < good idea to ground both plugwires while doing this>

 

[69 Alumacraft]

Re: 1976 25 hp Fireing on one cylinder

I will try that. When I pull the plugs after I run it, they both appear "gassy".

 

[kbait]

Re: 1976 25 hp Fireing on one cylinder

I take it you changed plugs.. champ UL81C, or ngk B7HS.

You may have a fuel pump leaking into the 'pulse' hole, flooding that cylinder. (not sure which cylinder the fuel pump draws from on that one..if it's the top cylinder, it's a possibility).

Also, reed valves for top cylinder may not be seating correctly so it can't 'suck' fuel in. A quick test would be to run the motor as is for a bit, shut it off, pull top plug and squirt a bit of premix in, replace plug/boot and fire it up. If it runs on both cylinders for a few seconds before reverting to just the bottom one, check the leaf/reed valves mounted on a plate behind the carb.

Good luck!

 

[F_R]

Re: 1976 25 hp Fireing on one cylinder

I agree that the reeds are just about the only thing that hasn't been covered, other than a hole in the crankcase. Reed failure is extremely rare, but possible. A two-stroke must have compression in the crankcase in order to run. Reed open or broken = no crankcase pressure = no run (on that cylinder)

The carburetor cannot feed only one cylinder (reeds excepted).

 

[69 Alumacraft]

Re: 1976 25 hp Fireing on one cylinder

Using champ j4c's , I thought it was that or j6c if you do not troll very often. Second question is how can I check to see of my leaf/ reeds are correctly mounted or opening?

 

[kbait]

Re: 1976 25 hp Fireing on one cylinder

J4C is correct.. my bad. (J6C for mostly trolling). I was thinking they were the same as 9.9/15hp from '76.

Pull the carb, then the intake plate and inspect the reeds. Hopefully you find a broken/misadjusted leaf or something stuck in there so they won't seat properly

 

[69 Alumacraft]

Re: 1976 25 hp Fireing on one cylinder

Pulled carb and recleaned and blew it out w/ compressor. Pulled intake manifold and reeds/ leafs looked brand new although both carb and manifold bolts did not seem to be very tight. Also, (not sure if this has any bearing) discovered no t-stat so I put one in. Put brand new j4c's and it fired and seemed to run on both cylinders. I was able to run it with plug boot pulled off top and plug boot pulled off bottom. Have to get it out on the river saturday to see if this fix worked. I was using lightly used j4c plugs before, maybe that was the issue but I doubt it. Let everyone know how things went, any comments theories always appreciated.

 

[F_R]

Re: 1976 25 hp Fireing on one cylinder

It was the spark plugs. The no-thermostat may have contributed to the fouling. There are other reasons for that motor fouling plugs too. Fuel pump diaphragm is one, but you checked that, right?

 

[69 Alumacraft]

Re: 1976 25 hp Fireing on one cylinder

Yes new pump, just thought the used plugs would work, my 69 sportster runs fine on the same exact plugs!? Must be the different old school ignition. Thanks

 

[69 Alumacraft]

Re: 1976 25 hp Fireing on one cylinder

Running strong, just doesn't like to idle low and smokes. but very happy, thanks guys!!