1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

[Boater1010]

HI everyone,
I have a 1956 Evinrude 30hp bigtwin in good condition. 120 psi both cylinders. I attempted to convert from 2line fuel to 1 line and added a Mikuni metal fuel pump. I capped off the pressure port on the manifold and drilled and tapped the bypass cover to use as the vaccuum port. I tried the motor and it runs great at idle, however when I put in under load and give it a lot of gas it takes off and then backs down then up again. So basically is surges under load. Would you have any ideas what would be causing this? carb has been rebuilt, newer coils, newer points. Should I have removed the intake manifold and plugged 1 port of the pressure ports? Its seems like its starving for gas at higher rpm's.
any help would be greatly appreciated.
Thanks
Chris

 

[Crosbyman]

Re: 1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

plugging the air pressure nipple should be enough.
I have put a mikuni on a QD but is it good enough a pump for a 30hp ?? can theengine run better if sqeezing the fuel bulb.

if yes the engine may be starved for fuel

 

[kfa4303]

Re: 1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

HI Boater. I think it may be one of two issues. Either you're dropping a cylinder at high speed and/or there is a problem in the fuel delivery of the motor. There are a few simple tests you can do for each. When the motor fails under load, use some insulate needle nose pliers and disconnect each spark plug lead one at a time to see it one of the cylinders is failing to fire. If you can disconnect one of the wires and the motor does not change, then you know it's bad. As for the fuel delivery, I would test the fuel pump. If you can squeeze the primer bulb prior to start up and it gets firm and stays that way for a few minutes, that's a good sign. You can also try manually pumping the ball when the motor fails under load. If the motor picks up, then you know the fuel pump is failing. Here are some handy links that might help too.

Reviving a Vintage Big Twin - Part 1

HOW TO CONVERT 50s motors from two line tanks - YouTube

Converting the Old OMC Dual Fuel

 

[Boater1010]

Re: 1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

plugging the air pressure nipple should be enough.
I have put a mikuni on a QD but is it good enough a pump for a 30hp ?? can theengine run better if sqeezing the fuel bulb.

if yes the engine may be starved for fuel

When Pumping the fuel bulb there is no difference. I did try that. I used the metal mikuni pump that everyone has recommended.

 

[Boater1010]

Re: 1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

Hi
If I do pull the spark plug wire and there is no difference what would it be? Has great compression on both cylinders. Coils? Points, condenser?
thanks

 

[kfa4303]

Re: 1956 evinrude 30hp Bigtwin 2 to 1 fuel line conversion

If you pull the spark plug wire and there is no effect, then that wire was not firing. However, if you pull the wire and the rpms drop dramatically, then you know that cylinder is/was firing properly. Some electrical issues only reveal themselves after the motor has been running a while when the components get hot. Make sure you use pliers with rubber coated handles, or you'll get nice little zap! Here's a great ignition rebuild link. It's for a different model, but all of the steps are the same, except you'll want to use Champion J6C plugs.

3 HP Evinrude Lightwin Ignition System Tune-Up 1952-1967