Gentlemen,
For those of you have seen the videos of my Holt running you know that it runs quite well. This came after a lot of tinkering and adjusting following the fitting of the carb that was originally designed for the engine.
Although it runs well there was still one nagging problem that was carb related. When it idled for extended periods upon throttling it would spray out a lot of unburned fuel that had puddled in the manifold. Try as I might with the needle and air screw adjustments I just couldn't overcome this problem.
While I've had a lull in the summer's activities I thought I would do some experimentation with different carb sizes. The original bore and venturi sizes are .330 dia. bore and .220 venturi. I can't remember how I came up with these sizes other than developing them from the original manifold sizes.
This is a fairly slow running engine (around 500 rpm at idle and 4000 at full song) but for shows it runs at idle or just above. It has a bore of 1.00 and a stroke of 1.25.
The first new carb was going to be a lot smaller both in bore and venturi sizes, .280 and .156 diameters respectively. The old carb was mounted to an adapter that would bolt to the original Holt manifold but rather than go through that I just started with a big enough piece of stock to machine the whole thing in one piece.
The air bleed hole was reduced to what I thought would be a good size for this bore and venturi. (.086 dia.)
I didn't put all the sculpting detail on this carb like the original until I found out if it would make any change to the operation of the engine.
Upon finishing I bolted it up anxious to find out what the results would be. I set the air bleed screw at about 1/2 open and the needle just cracked. I stared turning it over with my cordless drill and slowly opened the needle until the engine started. From there I adjusted the main needle and air bleed until it would idle clean and accelerate without stumbling (too lean). I found that the needle needed to be opened up a little to take the hesitation out when throttling and therefore the air screw had to be wide open. When I close the throttle back down to the idle position the needle needed to be closed up a small amount which told me that the air bleed needed to be a little larger, originally .086 dia. so maybe up to .094.
The change in diameter was made and the carb bolted back up. WOW is all I can say! The idle is dead on as far as adjustment and it will rev so cleanly, and high, that I'm almost afraid to open the throttle wide open.
It's possible that going even a little smaller might yield even better results but I'm so happy with it that I'm going to stop here. And by the way, no more spray of unburned fuel when throttling which was kind of the goal in the first place.
When I get some more time I'll sculpt it like the original carb to give it more character.
I'm attaching the drawing for the new carb should anyone care to use it on their Holt or other engine of similar bore and stroke.
gbritnell
