Carb adjustment question on webers

pjv911 · pjv911's Garage

I have a lean intake backfire which happens intermitently when crusing at light throttle 1/4 peddle, (just maintaining speed).
I makes the same pop noise that comes from the carbs if your idle circiut is to lean at idle speed. I dont have the pop sound under heavy throttle or idle ONLY at light throttle. How do you set the midrange mixture on the 40mm webers. This is on my 68` 912.

Kurt Williams

john walker's workshop · 02-12-2002, 06:16 PM

at light throttle you're probably still on the idle circuit, so enlarging the idle jets would likely help. what size are they now? a jet gauge doesn't lie, the size number on the jet often is inaccurate because someone may have oversized it. you need at least a .55mm idle jet, maybe a .60mm, but that's a hair large for a 912. proper ignition timing plays a large part in it too. 009 vw centrigugal distributors don't work real well in 912s either.

pjv911 · pjv911's Garage

Distributor is the 050 , and the timing is set at 3 deg BTDC using a test light (not running). Timing feels excellent and there are no vacuum leaks at all. The pop (intake backfire ) happens at random from any one of the 4 venturies. I do have a slight hesitation around 2500 rpm and improved it a little by adjusting the accelerater pump nuts to sqirt more in. Also note that when its warmer (55 deg) outside the poping is much less than when its 30 deg. It has to have a lean mixture under part throttle. The car did sit for a few years and was started up once a month (says the previous owner). The carbs are (WERE) new when the engine was rebuilt which was just before it got laid up in the last owners garage for 10 years. I have been driving it now for 2 months and put about 5 tanks of gas through it.

Kurt Williams

cegerer · cegerer's Garage

I had an identical problem last year. Took out the idle jets and blew them clean with an air hose, then ran a bottle of Techron thru the fuel system. Problem gone. -- Curt

avendlerdp · avendlerdp's Garage

Your carbs are probably out of sync. You know the drill... unhook the linkage set the carbs at idle using sync tool... Here is the rub: Some throttle linkages are so worn that they open the carbs unevenly. So, although you got the idle air flow balanced as soon as you touch the gas pedal, you open one more than the other and pop pop pop. Try using somthing to hold the idle up at the pedal, or the stock fast idle lever, and sync them using the linkage. Alex

john walker's workshop · 02-13-2002, 05:08 PM

i don't much care for the 050 distributors either. when you change distributors, you can no longer use the factory timing spec. do 33-35° total advance, then however many degrees it idles at is what it idles at. the hesitation may be due to not enough advance. do check idle jet size though. also, carbs don't adjust well at low RPM. if you adjust them at 900, you will get better response from the screws than if you did it at 500.