Re: smaller carb for H ?

Posted by Bob M on March 08, 2004 at 17:39:29 from (66.67.10.249):

In Reply to: smaller carb for H ? posted by riverbend on March 08, 2004 at 06:23:29:

Greg - A few random thoughts...

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!

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