not enough backpressure?

[George Miller]

Okay, what would be a good jet size? The carb started with a 42 and ran very poorly so I tried a 52 I had on hand and it ran better. The next size up I had was 64 and it ran worse. Does anyone have a comparable setup? The carb is a later Holey 94.

What kind of intake are you running, the intake makes a big difference in jetting the carb. But by the sounds of what you are saying I'm thinking ignition problems.

 

[jkeesey]

The intake is a Burns single downdraft. I have been thinking ignition also thats why i went through the ditstributor, everything inside of it checks out good unless the centrifigul advance just isnt going far enough and i need to advance it more to start. I was also pondering the possibilty that it may be the power valve is too weak but its just a thought. I just added the air cleaner so i will test drive it again later to see if that made any difference, but i doubt it.

 

[MikeK]

Okay, what would be a good jet size? The carb started with a 42 and ran very poorly so I tried a 52 I had on hand and it ran better. The next size up I had was 64 and it ran worse. Does anyone have a comparable setup? The carb is a later Holey 94.

Jet size? That is impossible to determine or recommend. No one will have exactly the same intake & exhaust airflow and harmonics, cam profile & timing, valve porting, compression and combustion chamber characteristics as you. Any jet number given is a guess. To make matters worse, your jet changes were ridiculously and wildly huge.

To complicate the issue, in addition to the main jet size in a 94 you have one of many power valves, each with different vacuum actuation profiles. The problem may very well be the PV. There is also a known problem with later manufacture power valves improperly fitting in 94's.

You stated the rear two plugs oily, not the front two. You may have an intake gasket leak. I'd start there. Now, to figure which jets and power valve to use, your best bet is to temporarily install a wide-band AFM.

The HAMB has a lot of 94 tech threads, you might look there.

 

[jkeesey]

MikeK i tried the Hamb but i couldnt get a good thread on the 94 with on a Model A. The jet changes i used were simply the ones I had. I had the PV fitment issue in an older set of 94s but this one it fit in fine. Im thinking of just removing it and plugging it for now. I think the stock PV is rated somewhere around 8 which is what is in there now.

 

[MrTube]

First, Tom and George, the 2 strokes do not use exhaust back pressure they use sound waves to control the cylinder scavenge. This is why the muffler is shaped like 2 cones welded together at the big end. I found out how to tune 2 strokes running them back in the day. We used to make our own stingers to get the best performance from the 2 stroke racers.


Second, the reason a pietenpol runs good is the carb is tuned to the engine without exhaust and the engine is mostly run at a steady speed not up and down like a car. If you run the throttle up on any motor without a muffler or with straight pipes slowly it will respond without problem. It is when you stab the throttle that it hesitates and splutters.


I don't think the problem with this motor is back pressure because even a straight pipe with a bend would present enough to make it run OK.

That is not a muffler, it is an expansion chamber and not all 2 strokes have them. They effectively act as a supercharger and can be very RPM dependent. They are designed for fuel and air to be pulled into the exhaust and then reflected back into the cylinder after the piston has covered the intake holes.

To my knowledge on a 4 stroke less backpressure will require a richer fuel mixture, other than that I know of no other negative effects.

 

[jkeesey]

This is why I prefer the Barn for all Ford related questions. I posted my problem on the Hamb and got 0 responses. I may not have fixed the problem but at least people here are trying to help.

 

[jkeesey]

Problem Solved! Swapped on my known good, shop use Stromberg and shes runnin like a champ! Im gonna take apart the Holley at some point and try to figure out whats goin on. Thanks everyone for the help.