Carburetor question again

[Gordon]

I have been working on a carburetor again. I am using basically the Duclos design. I am having a problem with the engine running for 10-15 seconds and then dying. It is running our of fuel. My question is what should I be changing? I have been trying bigger and smaller throat diameter. What I currently have is a 1/4 dia throat with a #10 (.190) fuel rail with a #60 jet dia. Should I be trying smaller of larger throat? Should I use a bigger or smaller jet. My modifications do not seem to be obtaining the results I am expecting. I would think that less area between the fuel rail and the throat dia would improve things to get more velocity but it does not seem to be working.

 

[stevehuckss396]

What size engine? My V8 is 25cc and I started out with .094 diameter throat and have opened it to .120.

5/8 x5/8 bore and stroke.

Also is it a problem when hot? If the carb is attached to the head and gets to hot it can cause the carb to vapor lock. Isolating the carb from the engine with a plastic spacer can reduce the temp of the carb and prevent the problem.

 

[Gordon]

The engine has 1 1/8 bore. I am back to working on the Atkinson differential again. The carburetor is similar to Brian's:

www.homemodelenginemachinist.com/threads/a-different-carburetor-thread.31339/#post-324891

I have had this thing running for up to 20 minutes but suddenly it just does not want to run more than a few seconds. It does not run long enough to get vapor lock and the carburetor is not at the combustion chamber.

I end up working on this thing until I get frustrated and then do something else for a while. This engine is certainly one of the most inconsistent beasts I have ever worked on. I think I am gaining and suddenly it just refuses to run at all. Very minor carburetor adjustment or timing seem to be critical at one time and then the thing runs with different settings at a later time.

 

[coulsea]

inconsistency is a curse and probably means that you are looking at the wrong place. when building a carb I like to try it on another engine first, they will work with quite a variety of bore and stroke sizes.
my vertical upshur gave me a lot of problems that turned out to be alignment of the crank big end con rod and piston. if I tightened up the big end the piston would tilt sideways and it would loose compression (o ring) but only on the up stroke and only sometimes. with the right tension on the big end it ran perfectly until the threads in the con rod wore and the bolts would work their way out(after a couple of months of showing people how well it ran). then I took it apart to paint it and put the piston back in the other way around and spent a week trying to get it to run again. in the end I took half a millimetre off each side of the bigend so now it can float around. in the process of getting it to run using an electric drill as a starter the ignition and valve timing slipped at different times. the amount of time I spent trying to tune the carb was ridicules.
you might be unlucky enough to have problem that no one has ever seen before.

 

[Gordon]

inconsistency is a curse and probably means that you are looking at the wrong place. when building a carb I like to try it on another engine first, they will work with quite a variety of bore and stroke sizes.
my vertical upshur gave me a lot of problems that turned out to be alignment of the crank big end con rod and piston. if I tightened up the big end the piston would tilt sideways and it would loose compression (o ring) but only on the up stroke and only sometimes. with the right tension on the big end it ran perfectly until the threads in the con rod wore and the bolts would work their way out(after a couple of months of showing people how well it ran). then I took it apart to paint it and put the piston back in the other way around and spent a week trying to get it to run again. in the end I took half a millimetre off each side of the bigend so now it can float around. in the process of getting it to run using an electric drill as a starter the ignition and valve timing slipped at different times. the amount of time I spent trying to tune the carb was ridicules.
you might be unlucky enough to have problem that no one has ever seen before.

That is interesting. Ray M has also been working on his Atkinson Differential engine and having the same compression, leakage, inconsistency problems and he is convinced that the problem is that the piston are tilting and causing the leakage. I have helped the problem by using o-rings but it still has not eliminated the problem. Compression is better but still marginal. It would seem that with o-rings it would take a lot of tilt to cause leakage. This engine is one of the most frustrating engines I have ever worked on. A slight adjustment in timing or carburetor adjustment seems to get things running and then suddenly things refuse to work at all.