31.8 giving me fits

[jerry smith]

i hope i am putting this in the right section. i have a early 31.8 with over the hub exhaust i am figuring mid to late 70s. this motor would not run at all unless you sprayed gas into the carb. i wanted to upgrade the carb to a 30hp anyway so i changed the carb to a 30/35 now the motor starts up but now it will back fire and quit the only way to keep it running is to partially choke it. when it back fires it does so through the exhaust it has good spark i swapped the cdi box still the same i even switch back to a good 25 carb with it doing the same thing i checked the key on the flywheel and it looks fine i am confused any help would be greatly appreciated . by the way compression is 120 in both

[17W]

Sounds like both carbs you are playing around with need a to be rebuilt & a fresh kit BRP #396701 installed.

Partial choke creates a rich condition (allows fuel to flow from all orifices due to psi differential)

[jerry smith]

sorry should have put that in there did a complete rebuild on the carb

[Chairman]

I had a similar problem, minus the back firing. Was difficult to start and had to partially cover the carb throat to smooth out the idle. Turned out to be a bad bottom seal. I assume a bad upper seal could cause the same problem.

[MTECHMARINE]

Check the carburetor pickup roller assembly, the portion mounted on the manifold with a screwed in pin. It is probably sticking and not allowing the carburetor butterfly to close completely at slower speeds.:eek:

Seen a lot of this problem, especially around salt water.

[Fastjeff57]

I'm with the "bad crank seal" opinion. That the second carb made no difference was the clue to the real problem. Check this out:

I fixed a 4 hp McCullock for a customer that acting the same way. In this case, it was an air leak--but what a weird deal it was! A plate with six # 10 screws holds the top bearing assembly in place. Each screw had a rubber sealing washer beneath it, but there was enough air leaking in around the threads to cause the problem! I gooped the threads with Permatex and she ran perfectly from then on.

Jeff

[JohnsonM50]

I always look at the easy stuff 1st. When a crank seal leaks it will get pretty far before its more symptomatic than harder than usual starting. If its an upper, oil be on the block from under the time base, the lower will be in the exhaust & water might be found in the lower cyl but not always. A 31.8 will run till the seals are fairly bad, the center seal rarely has any problem & would only be caused by a worse failure to worry about.
What Bill wrote is possible & would still happen even if you got a new carb. The motor will run bad & have a lean idle miss thing going if the carb is up & the time down, can even cough & quit from that. Be sure that cam follower devise is free to return, Its got its own spring but can seize anyway. A bad carb gasket, intake gasket or needle packing can cause secondary air that will cause the motor to rev & drop off repeatedly or any of many possible air leaks before the reeds. The problem is fuel /air related not the ignition.

[jerry smith]

ok all thanks for the great information. after inspecting the reed cage that was out of tolerance and cleaning it then replacing the fuel pump she started running with a miss figured that one out the hard way when #2 coil lit me up :mad: after changing the coils she ran great after that put the 35 carb on and all set to go. i am so happy that i found this site you guys have so much knowledge im learning new stuff everyday.

[Smokin' Joe]

i hope i am putting this in the right section. i have a early 31.8 with over the hub exhaust i am figuring mid to late 70s. this motor would not run at all unless you sprayed gas into the carb. i wanted to upgrade the carb to a 30hp anyway so i changed the carb to a 30/35 now the motor starts up but now it will back fire and quit the only way to keep it running is to partially choke it. when it back fires it does so through the exhaust it has good spark i swapped the cdi box still the same i even switch back to a good 25 carb with it doing the same thing i checked the key on the flywheel and it looks fine i am confused any help would be greatly appreciated . by the way compression is 120 in both

Late response but did you solve the problem? Carbs should run the same, I think, 1976-2000 except that the carbs after about 1986 came with bigger high speed jets and primer instead of choke. 120 psi should be ok.

[jerry smith]

thank you for the reply. yes i got the motor running great it was multiple problems. after fixing reeds fuel pump coils plugs wires we got it to sit and idle so i used a bunch of ideas that i got from you guys thank you very much what an awesome web site.

[Smokin' Joe]

thank you for the reply. yes i got the motor running great it was multiple problems. after fixing reeds fuel pump coils plugs wires we got it to sit and idle so i used a bunch of ideas that i got from you guys thank you very much what an awesome web site.

Good to hear. I specialize in that model because it runs well and because I can lift it.

[JohnsonM50]

Good to hear. I specialize in that model because it runs well and because I can lift it. :cool: Yes, me too, Ive lifted & run worse :rolleyes: you can run it hard all day & put it away ready.:)