Idle mixture screws have no effect
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videoguy009
- Posts: 23
- Joined: Fri Mar 08, 2013 10:18 am
Idle mixture screws have no effect
Found an issue with the popping carb on my dual 40 idf carbs.the idle mixture screws have no effect when turning them.If I turn them in all the way the carb stays running strong.I have checked for vacuum leaks and there are none.I can get a good idle and a good sync but when I try to adjust the mixture screws to richen it up to take the popping out they do nothing.The linkage is right on as I followed the correct procedure,unhook for syncing and reattach and listen for change in idle.No change.I am running 55 idles and 135 mains.
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The Newf
- Posts: 145
- Joined: Sun Jan 25, 2015 6:03 pm
Re: Idle mixture screws have no effect
You say you have no false air.....so the idle circuit is dirty/plugged,or your idle jets are too small for your application
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videoguy009
- Posts: 23
- Joined: Fri Mar 08, 2013 10:18 am
Re: Idle mixture screws have no effect
If the idle circuit was plugged how come in idles smoothly but I know I am running lean becuase of the slight popping.Strange situation!
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FJCamper
- Moderator
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- Joined: Wed Nov 14, 2007 2:19 pm
Re: Idle mixture screws have no effect
Hi videoguy009,
You know the story about Dr. Porsche in his days with Mercedes just before the Auto Union-Mercedes wars.
The Mercedes came in off the track with a problem and all the German engineers were in a group arguing about the cause, when Porsche walked up and told them the solution.
"How do you know that!" they demanded.
"I looked," Porsche said.
First, clean out your idle jets. If that makes no change, you have dirt in the idle circuits and it has to be blown out. Now, odds are you're getting air into the idle mixture from somewhere if the engine will idle with the idle mixture screws closed. Air typically bleeds in through a loose top gasket, a worn throttle shaft, or a bad manifold gasket (top or bottom).
Your description of a lean pop with unresponsive idle jets alone points to clogged jets and a vac leak.
Good luck -- and with Webers there is rarely a quick fix. We race 14 hour enduros on Kads, and like the song says (almost) we got 99 problems but our Kads ain't one.
FJC
You know the story about Dr. Porsche in his days with Mercedes just before the Auto Union-Mercedes wars.
The Mercedes came in off the track with a problem and all the German engineers were in a group arguing about the cause, when Porsche walked up and told them the solution.
"How do you know that!" they demanded.
"I looked," Porsche said.
First, clean out your idle jets. If that makes no change, you have dirt in the idle circuits and it has to be blown out. Now, odds are you're getting air into the idle mixture from somewhere if the engine will idle with the idle mixture screws closed. Air typically bleeds in through a loose top gasket, a worn throttle shaft, or a bad manifold gasket (top or bottom).
Your description of a lean pop with unresponsive idle jets alone points to clogged jets and a vac leak.
Good luck -- and with Webers there is rarely a quick fix. We race 14 hour enduros on Kads, and like the song says (almost) we got 99 problems but our Kads ain't one.
FJC
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videoguy009
- Posts: 23
- Joined: Fri Mar 08, 2013 10:18 am
Re: Idle mixture screws have no effect
I found my problem.I was trying to use a center mount carb on a dual application.You cannot do this as there are holes drilled in the butterflies on the center mount and the jetting is different.You will have to plug the holes and change the jetting.In all I was getting a big vacuum leak which was bypassing the idle circuit.