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Thread: 31.8 giving me fits

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    Default 31.8 giving me fits

    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

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    Team Member 17W's Avatar
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    Default Carb Issue....

    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)
    Brent 17W

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

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    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.

    Tunnelboat and V-bottom Plans for 15-35 hp, 40-60 hp, 60-80 hp

    Dillon-Racing.com also on YouTube, on Facebook, on Twitter

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    Default Recalcitrant 25/35

    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.

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

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    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

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    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by jerry smith View Post
    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.

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    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.

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    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by jerry smith View Post
    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.
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    Team Member JohnsonM50's Avatar
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    Quote Originally Posted by Smokin' Joe View Post
    Good to hear. I specialize in that model because it runs well and because I can lift it.
    Yes, me too, Ive lifted & run worse you can run it hard all day & put it away ready.

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