M-25 carb.

[carleton lathrop]

I've got an M-25 on my '40 knuck. Someone told me to make it run good I should use a 1.125" venturi, but don't I have to change some other parts such as jets, etc? any help would be appreciated. AMCA membe, Carl

[Jerry Wieland]

Cotten swears by opening up the venturi on a M-25 but I have had good results just leaving them alone and getting rid of all vacuum leaks. It probably cannot hurt to give them more air. Sometimes tuning a Linkert can be a trial by fire thing and you just keep working at it til you get it right. I would think everything else should be fine with a venturi change.

Jerry

[T. Cotten]

Carl!

My copy of Palmer's lists a 1 5/16" venturi for the M-25,
so 'someone' advised you to go smaller than 'stock'.

For breaking in a motor, or pulling a hack, that would have its benefits.

The M-25 has no fixed jet, and the main nozzle is essentially the same as for models with the 1 1/8" venturi.
So venturies can be swapped at will, as long as they properly fit.

For a limbered-up solo machine however, particularly with today's higher road speeds, I would advocate the correct venturi. And an M-75 nozzle if you can find one (second from left in attachment.)

....Cotten

[RichO]

I don't know if anyone will read this as it's after the fact but here's what I know. 1936 had a 1 1/8" venturi, 1941 had a 1 1/8" venturi, 1940 had a 1 5/16" venturi because it was the first year of the new big port heads and they were getting ready for the 74" model in 1941. They got a surprise though. The bigger ventri slowed the air flow and the motor response bogged down on the EL. The police put in bombsites and that increased the velocity and the response was much better. I ride a 1940 EL and know this to be true and accurate. They wouldn't have gone back to the smaller venturi for the 1941 FL if it wasn't. Bigger is not always better. They didn't come back out with the 1 5/16" venturi till the hydraulic liftered panhead. FLH I believe. My bike was an ex-CHP and had a bombsite in it. I took it back to stock configuration and it wouldn't respond. It has a 1 1/8" venturi in it now and look out!

[T. Cotten]

Dr. Sprocket!

You excite me with 'bombsites' on a police model! Any pics?

At any rate, my best guess at why they went back to 1 1/8" was because Eisenhower's Interstates were nearly two decades in the future.

But H-D was ready.

....Cotten