1957 big twin missfiring, stumped!

[schneider244]

I have a 35hp '57 big twin which seemed to be running well except for the occasional missfire. I rebuilt the carbs and set the points visually inspected the coil and observed good spark on both plugs. Test ran the engine in a tank, started right up seemed to run well.

Mounted it to the boat and went for a cruise. The engine started well but underload wouldnt go above high idle and would soon die out. I ended up having to paddle back to the dock.

Got the boat home and put the engine back in the test tank and it ran fine just smoked more than usual. I then pulled the top cylinder plug and it immediatly died out. Put the plug back on and restarted, pulled the bottom and nothing. it ran exactly the same, not even a hiccup. it seemed to run fine without the bottom all through the rpm band..

Afterwards i noted the top plug was quite hot wheras the bottom relatively cool. both plugs spark, i got a fairly good shock from both plugs when removing the boots. and the bottom plug gave a familiar tick tick tick sound when removing and replacing the boot which leads me to believe it isnt an ignition problem, however they both have good compression................


im stumped


does anyone have any ideas?

THANK YOU

 

[bktheking]

Re: 1957 big twin missfiring, stumped!

Don't these motors have an adjustable high speed jet? You don't mention making any adjustments after rebuilding the carbs.

 

[Faztbullet]

Re: 1957 big twin missfiring, stumped!

Swap plugs to see if problem follows plug, if so its a bad plug. Also check that they are correct plugs and piston has not closed gap on plug. You should have J6C plugs installed

 

[lindy46]

Re: 1957 big twin missfiring, stumped!

Sounds like the coil to the bottom cylinder is bad. Did you test spark with a tester or just look at the plugs? Those motors will idle fine with only one cylinder firing.

 

[schneider244]

Re: 1957 big twin missfiring, stumped!

I didnt test with a tester, I figured I saw a spark and that was good enough. Is that not an accurrate assumption?

I have about 10 j6c plugs laying around and none work on the bottom cylinder.

I dont think the high speed jet would be causing this as it wont fire at any speed, plus there is only one carb for both jugs.

 

[bktheking]

Re: 1957 big twin missfiring, stumped!

We had this problem too on a 64 big twin 40 , had spark but nothing on the plug, one of the wires wasn't in far enough on the coil, once we removed everything and pushed it in farther is started working again. The other thing is grounding the magneto, check for frayed wires under the flywheel.

 

[bktheking]

Re: 1957 big twin missfiring, stumped!

Also, you could try swapping the coils around as a last resort.

 

[schneider244]

Re: 1957 big twin missfiring, stumped!

im on my way out now to pick up a balance puller so I can get the flywheel off. i have a pair of new coils sitting around I will try them out.

 

[bktheking]

Re: 1957 big twin missfiring, stumped!

Got an air chisel?

 

[schneider244]

Re: 1957 big twin missfiring, stumped!

ok, got the balance puller, removed the recoil starter, the peep hole cover, and the large main flywheel bolt. Attached the puller and started pulling the flywheel, the darn thing wont budge! am i missing some other bolt somewhere?

thanks

 

[tx1961whaler]

Re: 1957 big twin missfiring, stumped!

No. The large nut is all that holds it on. You're gonna have to really, really tighten the puller to get the flywheel off. Hopefully you are using Grade 8 bolts in the puller to pull it off. Stripper bolts are the best, but more difficult to find. Lesser bolt are going to snap. An impact wrench helps, if you have one.
Your problem is most likely the coils. The coils of that vintage crack and fall apart. Clean and regap the points while you're under there.

 

[lindy46]

Re: 1957 big twin missfiring, stumped!

ok, got the balance puller, removed the recoil starter, the peep hole cover, and the large main flywheel bolt. Attached the puller and started pulling the flywheel, the darn thing wont budge! am i missing some other bolt somewhere?

thanks

Leave the large nut loosely on the shaft, so the flywheel doesn't go flying when it releases. Tighten the center bolt on the puller, then give it a good rap with a hammer. Tighten the bolt a little more, another rap, etc. until the flywheel releases.

 

[CBGale2]

Re: 1957 big twin missfiring, stumped!

Leave the large nut loosely on the shaft, so the flywheel doesn't go flying when it releases. Tighten the center bolt on the puller, then give it a good rap with a hammer. Tighten the bolt a little more, another rap, etc. until the flywheel releases.

For sure leave the nut on a few turns, I got that tip when I did mine and was sure glad I did. I thought for sure when that thing let go that something broke off, scared the heck out of me.

 

[asdasc]

Re: 1957 big twin missfiring, stumped!

Yeah, they do idle really well with one cylinder if you adjust the carb right. Boy was I surprised when I changed the plugs one day and found I had twice the power. I had to retune the carb again, but what a difference. I figure I ran it over a year on one cylinder and it purred like a kitten.