Carb adjustment question

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

josh912 · 02-12-2002, 05:21 PM

At what rpms does it do this? It could be a variety of things. Timing, pluged idle or main circuit, intake leak, or valve adjustment.

Green 912 · 02-12-2002, 06:04 PM

If the carb runs slightly rich on the idle jets and correct to slightly rich on the main jets then you might look into the advancement circ. Given that you only get the lean type f**ting under steady throttle the emulsion tubes are coming into play as well. These are often overlooked. I do not recommend changing the E tubes but they are very sensitive to the fuel level in the bowls. Recommend you check the floats and adjust to spec if needed. 912s with Webbers are also known sometimes have a flat spot with some cams in the 2800 to 3800 range. It can be somewhat reduced with careful jet selection and ignition timing

As Josh so correctly stated it can be a number of things from the cam timing to air leaks to the dist advancement timing to gunk in one or more of little advancement circ holes in the carb throat. I try and start at the simple (read easy to check) end and work to the complex. What dist and static timing are you using?

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

josh912 · 02-13-2002, 03:00 PM

I would richen up the idle screws and see what happens. What it sounds like is a typical weber setup problem. Webers do not come set up properly from the factory, and most likely you have too small of idle jets, among other things. Fortunately they are cheap and easy to change. If richening up the idle screws doesn't work you can try to change the jets and emultion tubes. Email me if you need more information.

pjv911 · pjv911's Garage

Which ones are the idle jets ?

Green 912 · 02-13-2002, 08:36 PM

They are the ones on the side under a flat head brass screw. 55 is a good place to start. I have found that 5 deg for a 050 on a 912 works well.

ron69912t · 02-14-2002, 05:03 AM

Kurt:
Before I would do anything I would check to see if the carbs are syncronized. My 69 912 Solexes pop at low rpms if the carbs are not in sync. I use a Uni-Syn Gage to do the adjustment and follow the shop manual directions TO THE LETTER.

ron69912t · 02-14-2002, 05:05 AM

Kurt:
Before I would do anything I would check to see if the carbs are syncronized. My 69 912 Solexes pop at low rpms if the carbs are not in sync. I use a Uni-Syn Gage to do the adjustment and follow the shop manual directions TO THE LETTER.

Good luck
Ron D

pokemon · 02-14-2002, 10:36 AM

Are the throttle arms the same length? They shouldn't be!
Cylinder #3 and 4 sit more forward then cylinder #1 and 2. Same is true with the carbs. If all arms have the same length it is simply impossible to sync. The left carb will always open more then the right hand side one.
To compensate you need to bring the arm on the left carb more backwards OR the arm from the right hand side carb more forward.
In practice this can only be done on the left hand side carb.
On the right hand side carb the arm will touch the idle adjustment arm and may lock the carb open if we try to bring the arm more forward.
I had to bring the left carb's arm backwards for about an 25mm (0.1inch) with an extension rod.

Good luck,
Patrick

I struggled quite some time to get my carbs synched.
For a good sync the carbs must start to open at the same time AND open up the equal amount all the way to full throttle.
In most articles I read about this, the sync is checked at the start point from idle AND at 3000rpm.
Problem here i believe is that the test at 3000rpm is done without loading the engine. The carbs only have to open very little to get to this 3000rpm.
We better check at a point further away (at least half way open) from the start point.
A tuning shop with a dyno to load the engine and at the same time have access to the carbs may do that.

Green 912 · 02-15-2002, 01:03 PM

Pokemon has hit a very good point that is often missed in a carb conversion.

It is easy to check the carb throttle position at any point. Depress the throttle pedal (motor off) with a C clamp through the wood foot board and you can crank the pedal to any position you want. (I found a long thin light one that fits nice) Better than using your hand on the cross arm as pushing on the arm will slightly change the setting depending on where you put your finger. You can then measure the butterfly gap on both the carbs. I like to check and set the point that the butterflies “crack” then check at about mid throttle and at almost wide open throttle (WOT). It is then good to check that the butterflies are opening completely at WOT as well. Things can go slack over the years and then do not pull the carbs all the way open and we pushrod 4s need all we can get. Then you can check that the throttle pedal is hitting and stopping on the pedal stop so the linkage isn’t taking the strain.

After the carbs are adjusted in the above manner you can synch them up with the motor running using a synchrometer(or two). The moving flap type syncro ($$) is way easer to use and more accurate than the bouncing ball ($) type. I like to synch up one carb front to back then the other, then across.

K ROMO · 02-21-2002, 06:58 AM

I HAVE HAD A SIMILAR PROBLEM WITH MY 66. IT TURNED OUT TO BE THE DISTRIBUTOR WAS NOT ADVANCING. MY PROBLEM WAS WITH THE #3 & #4 CYLINDER SO NATURALLY I WAS THINKING FUEL PROBLEM. ALLTHOUGH I WOULD RECHECK VALVE ADJUSMENT AND TIMING. I THEN FIGURED IT WAS AN AIR LEAK THRU THE THROTTLE SHAFTS OR THE BASE OF THE CARB, WHICH BY THE PREVIOUS MECH/OWNER OVERTIGHTED AND WARPED. PROBLEM STILL EXISTED. FINALLY TOOK TO FRIEND SHOP AND THE FIRST THING HE FOUND WAS THE DIST.(.050) WAS NOT ADVANCING. REPLACED IT AND SYNCHED THE CARBS AND VROOM.GOOD LUCK