Carb problem - won't take throttle

Sun Nov 17, 2013 3:16 pm

Problem started with the fuel inlet threads - stripped, could not get fuel line to seal

Have a pile of old carb parts, found a Cub carb top in good shape -swapped the top I found with other body/bottom. Put float and needle & seat on replacement top.

Engine starts, runs good at idle, idle mix screw functions - but - I cannot get the engine to take a quick throttle opening - it just dies. If I very slowly increase the throttle it 'sputters' like a lean condition then quits. Haven't tried it at load yet. Acting exactly like a down-draft carb with bad accelerator pump except I can get it to run about 1/2 throttle but it acts 'lean'.

What functions in place of the accelerator pump that you would find in a down-draft carb. I thought the fuel enrichment came somehow from the vacuum drop when the throttle plate opens. Where is that circuit? What am I missing? I did try 'search' and got 2 million hits for carburetor...........

Going out to check the float level again

Re: Carb problem - won't take throttle

Sun Nov 17, 2013 3:44 pm

Dave, since you removed the carb you will need to make sure it is retimed to the governor. That may be your problem. Go to this link and start at page 13 to see how to do this. http://www.cleancomputes.com/Cub/Blue%2 ... index.html

Re: Carb problem - won't take throttle

Sun Nov 17, 2013 4:21 pm

Check the float and fuel level as previously mentioned. Also check the fuel flow into and through the carburetor.

Check for a partially obstructed main jet. If you haven't done so, use carburetor cleaner and blow out all the carburetor passages.

Next question. Ignition timing spot on and ignition advance working properly?

Re: Carb problem - won't take throttle

Sun Nov 17, 2013 5:19 pm

Tractor was running fine - ignition and timing good.

My only problem was a leaking fuel line at the carb. Threads in carb are bad so all I did was change top. Tops are identical.

Governor linkage OK - engine runs great at idle; runs at full throttle for a few seconds then slowly dies.

Full fuel flow to carb, no restrictions through needle and seat, float level correct

Re: Carb problem - won't take throttle

Sun Nov 17, 2013 7:35 pm

In the venturi of the carb you will find 2 tiny holes. These holes provide a smooth transition from idle to hight throttle by way of balancing air vacume pressure . you will find that these 2 tiny holes may be plugged..They are a bugger to get opened. usually a tiny wire and a can of brake cleaner with sprey tube does the trick.

Re: Carb problem - won't take throttle

Mon Nov 18, 2013 6:47 am

Clark Thompson wrote:In the venturi of the carb you will find 2 tiny holes. These holes provide a smooth transition from idle to hight throttle by way of balancing air vacume pressure . you will find that these 2 tiny holes may be plugged..They are a bugger to get opened. usually a tiny wire and a can of brake cleaner with sprey tube does the trick.


Are they the holes right above the throttle plate?

Re: Carb problem - won't take throttle

Mon Nov 18, 2013 9:52 am

Yes, they are about 3/8" down from the top edge of the manifold flange. One above the other, with the top one being a little larger. They are on the gas inlet side of the venturi, or the side that has the small brass plug on the top of the flange.

Bob

Re: Carb problem - won't take throttle

Mon Nov 18, 2013 4:19 pm

Here is some news!!

The carb tops are NOT IDENTICAL! - The original top has a venturi bore dia of 0.48", the top I am using is 0.60"
This makes sense, if my memory is correct, that top came off a Cub Power Unit carb.

This explains the 'lean condition' - the main jet is too small for the increased venturi bore.

I don't know if these venturies are removable - I know the ones in the larger letter series carbs are.

If I can't swap venturies my only other choice is to enlarge the main jet - trial and error....

Comments please

Re: Carb problem - won't take throttle

Mon Nov 18, 2013 4:45 pm

No, the venturis are not interchangeable. Why not fix the problem with the original carb top, instead of looking for a work-around? The fuel inlet threads can be repaired using a 1/2-20 helicoil. Several members here provide this service for a small fee.

Re: Carb problem - won't take throttle

Mon Nov 18, 2013 5:33 pm

FIXED!

Found the old carb body (also found an H-4 magneto I haven't seen in years :shock: ) - swapped main jet and discharge tube (don't ask why I just didn't re-assemble the old carb....)

Engine runs fine, will put it under load tomorrow - and remembering my old hot-rod days more/bigger carbs = more horsepower, right? :D

I will probably get the original top heli-coiled