smaller carb for H ?

I imagine that all the low low 1st gear sets are long gone, so I was wondering if there is a carb with a smaller venturi that will fit on the H manifold. Or is there a smaller venturi that will fit in the H carb ? The idea being to keep the air speed up in the carb at low rpm.

Anyone have any other ideas ?

Thanks

Greg

H carburetors are the most popular carb I rebuild. I believe this is due to the unavailability of any aftermarket or "other application" crossovers. I am sure if you looked long and hard enough you could find something similar but I believe it would be a long row to hoe. As for there being different venturi, you are correct. It does not list it in the parts manual, but there are two sizes. The larger size is what I will call a rarity as I find it only in 10% of the H carbs I rebuild. The other 90% have the small venturi which incidentaly has the #24 stamped in the top of the venturi. I have been unable to find out what application the large venturi was for, my opinion leaning towards high altitude applications. If you need Inside measurements post me back on this site.

Generally a fast or rough low idle is caused by excessive air leakage either in the carb or at a manifold gasket. If the stock carburetor is in good shape and adjusted correctly, and if the governor linkage is set right, you should be able to idle the engine down to it's absolute minimum idle speed (350 RPM or so for an H...) and beyond to stall simply by backing out the idle speed screw. So before you mess around with different venturis, adapting a smaller carb, etc. suggest take a good look at the carb you've got. A few thing things you can do:

Make sure there's NO vacuum leaks at the intake manifold to head, and carb to intake manifold gaskets. If any doubt, replace the gaskets.

Remove the carburetor and check the throttle shaft for wear. If there's any noticeable looseness (radial play) around the shaft, air will be sucked in around the governor tube and into the carb. Solution is put in a new throttle shaft and new bushings if necessary.

While the carb is off check that the throttle plate closes completely. Back out fully or remove the idle speed adjustment screw. Next fish a small light bulb in thru the carb air inlet (or shine a very bright light into the air inlet) and manually close the throttle. Now look for light around the perimeter of the throttle plate. If you see any light at all (except at a port opening in the body casting), loosen the 2 screws that hold the throttle plate to the shaft a turn or so, wiggle the shaft and plate until the plate is shut tight then carefully retighten the screws.

After the carb is reinstalled check that the governor can pull the throttle shaft all the way to the fully closed position. If adjustment is required remove the cover from the governor riser tube, pull the pin from the bellcrank and turn the threaded crank as required to adjust. You may also need to remove the governor cover and adjust (loosen) the low speed stop screw on the input rockshaft.

Finally adjust the carb idle mix and idle speed screws. Run the engine until it is COMPLETELY warmed up (20 – 30 minutes under moderate load). Adjust the idle mix screw for maximum idle speed point, then slow RPM using idle speed screw. You will need to repeat this adjustment 2 or 3 times to get the smoothest and slowest idle. But if you get it right you should be able to slow the motor so you can almost count each fan blade as it goes past...

Hope this helps!

If you had access to a tracer lathe, you could turn a smaller venturi.

Another option might be to dip the venturi in a thick epoxy and trim the excess off the OD after applying a mold release such as a wax.

The governor connection would be the hard part. Can you run without the governor by using a cable throttle? The one I'm using has a 3/4" venturi.

What people don't seem to understand... all carburetors combine fuel vapor with air in very nearly the same proportions. At high speeds, a Quadrajet could be used on a little lawnmower, but it would be difficult to start and idle.

George Willer