18 hp vanguard idle down problem

[jacklegjim]

I have a 18 hp vanguard I have built and I am having problem with it idling down after you give it some throtle. it takes it a few seconeds for the rpms to come down

also with the throttle plates closed as far as they can it is idling faster than it should. I can't kill it with throttle and choke closed.
I think this is a carb problem I just don't know where to start.

It has the CPP high peformance cam and high rev kit with one shim under springs
I milled .035 off the heads
briggs metal head gasket
ported intake and exhaust ports.
the carb is a nikki two barrel with around 1.090 bores with no adjustment screws. I dont know what it was off of.
The intake was off a lot bigger motor so I took the one barrel intake I had and made it a two barrel with the bigger intake.
Thanks Jim

[1989Marathon]

One of two things to check.
1. Intake leak between carb and heads.
2. Fatten up the low speed jets.

Do you have a better pic of the carb? If it has low speed adjusters, turn them counterclockwise to richen the mixture and bring the idle speed down.
That will likely solve the return to idle issue too.

[beater cart]

Due to the high idle I would look for an intake leak check the bushings on the throttle blades also. Does the carb have any adjustment needles for idle?

[jacklegjim]

There is no adjustment on this carb.I thought about a leak in the intake. I will check but I dont think that is it. This is my first experience with this type of carb. I have had it apart but I am not sure where the low speed jets are.
I have done a lot of research on these carbs but the more I read the more confused I get.
Jim

[beater cart]

Were there any holes drilled in the throttle blades and did you use the right mounting gasket? I ask about the mounting gasket because on my carburetor the blades would catch the gasket when closing and keep them from returning all the way. I trimmed a little off the gasket to fix issue.

[1989Marathon]

That looks like a modified Briggs Nikki carb. Can you post pics of the sides and the bottom?

[jacklegjim]

I made sure the idle screw was out of the way and looked at the blades and they are completely closed. I will rule out vacuum leak tomorrow.

[1989Marathon]

Originally Posted by jacklegjim

I made sure the idle screw was out of the way and looked at the blades and they are completely closed. I will rule out vacuum leak tomorrow.

When you checked to make sure the throttle plates were closed all the way, did you shine a light through from the back side? One can be closed completely and the other open slightly.

Using the choke to see if you can kill the motor isn't a good test. If you put your hand over the carb inlet to completely close off the air, does the motor die? If not, you have an air leak.

[jacklegjim]

yea they both look to be closed.I can kill it with my hand over the carb but it pulls a lot of gas out of the front. Here is more pictures

[1989Marathon]

What do you mean it pulls alot of gas out of the front?

Since you made the intake manifold, I would suspect first that the mating surface to one or both of the heads is leaking. When the motor is cooler, try closing off the air intake to the carb while spraying carb cleaner around both head/intake mating surfaces.

Make sure the float is set correctly, and that the choke is staying open.