Markus,
I have zero experience with modding a pull choke carb, but have wondered about the very same thing!
Seems to me that the main restriction in the carb is the pull choke itself! The support piece that goes across the inlet would need to be thinned down. I bet the old timers pulled the whole choke assembly out, plugged the back, and ported the inlet! Living in a warmer climate here in SoCal, I typically just remove the choke on the later carbs and pressurize the tank before I try to start it. A shot of ether in the filter often lights it off, but that is not normally necessary.
What mods are being done to the engine? No governor? What RPM range will it run? The old pull choke workhorse was able to support 5HP at 3600RPM!
I have zero experience with modding a pull choke carb, but have wondered about the very same thing!
Seems to me that the main restriction in the carb is the pull choke itself! The support piece that goes across the inlet would need to be thinned down. I bet the old timers pulled the whole choke assembly out, plugged the back, and ported the inlet! Living in a warmer climate here in SoCal, I typically just remove the choke on the later carbs and pressurize the tank before I try to start it. A shot of ether in the filter often lights it off, but that is not normally necessary.
What mods are being done to the engine? No governor? What RPM range will it run? The old pull choke workhorse was able to support 5HP at 3600RPM!
the pull choke carb will have more restriction in stock form than the thumb lever carb due to the choke itself. I had a pull choke carb I sent to an engine builder to have it board out and made to flow better but I he didn't re tune it for me so I had issues balancing the mix for both idle and wide open but when I got on the gas boy did it pull. he removed the choke, rounded the corner beneath it, thinned the throttle butterfly and made sure it fully opened and board the carb it a little. I also had a primer cap that pressurized the tank to prime the carb but it eventually blew the seals on the tank and it began leaking at the seam where the top and bottom were joined. I later went over to a mikuni vm22 a bigger more tunable and user friendly carb. and put the pull choke carb in with my spare parts. also the raptor racing engine has a different tank mounted carb than the fixed jet thumb lever carb or the adjustable pull choke carb. its made for racing so its probably the best option out of the 3.
thanks for the info, when I pull the briggs stuff back out here soon I will try comparing the carbs to a bit to see what I can do. The carb on the engine I want to use is an already modified choke lever type. De tuning this engine is fine for the bike its intended for I just dont know if a restrictive stock carb is going to mesh well with the engine itself.
I found this Markus...I think its from Jimmy Glenns book on Briggs Flatty's .
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNFvxh484emm0j5imLp_NjJjalEwPw
I'm using the same type old pull choke carb on my Powell build....but I'm going with pump fuel, not alcohol.
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNFvxh484emm0j5imLp_NjJjalEwPw
I'm using the same type old pull choke carb on my Powell build....but I'm going with pump fuel, not alcohol.
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I took a pull choke carb, cut the choke part off and jb welded the end back on the carb and then cut the web out in the throat, used jb weld in the throat to smooth it all out to make a funnel effect, rounded the throat to the bore on the top side for a smoother flow to the bore... of course there is no choke now but the carb will have a lot better air flow to the bore..I also match ported the gasket end to the block.
Hey Markus, I've been working on my old pull choke Briggs carb today and can get it to idle all day long, and revs up ok for a restricted carb with the pull choke still intact. But when I rev it up and let it go back to idle it loads up real bad and usually dies..:censure::censure:. I've taken out the twisted fuel atomizer piece....no difference. I've resurfaced the diaphragm cover and relpaced the diaphragm its self with a new one....still the same thing. Using the stock Briggs jet (not drilled ) made sure both the low and high circuit holes behind the jet was open and clear...still the same thing.
So I tore down a vari jet carb and one of the pull choke carbs...heres the butter flies from both.
Now that I have the one out of the pull choke carb....I see the little stamped out impression on the flap thats between the low and high side circuit holes....I'm going to try and change this tomorrow.
but don't know exactly how yet.
The vari jet flap is in the same position in the body itself...but totally flat.
So I tore down a vari jet carb and one of the pull choke carbs...heres the butter flies from both.
Now that I have the one out of the pull choke carb....I see the little stamped out impression on the flap thats between the low and high side circuit holes....I'm going to try and change this tomorrow.
but don't know exactly how yet.The vari jet flap is in the same position in the body itself...but totally flat.
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