TheSamba.com :: View topic - backfiring and IDF44 carb mixture settings

Fri Sep 23, 2011 5:40 pm

New 2020cc with Dual Weber 44IDFs driven for about 200-300km and ran very strong.

Not sure of the cam size, but its fairly large. Distributor is SVDA. Heads are 44x35 dual spring. Exhaust is CSP Python.

Around 300km, it started to backfire when decelerating (downshifting.) We found that the side tin screw (doghouse to cylinder tin) had come loose, fallen into the sparkplug hole, and was apparently grounding it out some how. Removed the screw, and it ran fin... for a while.

50km later it started to fart and pop again. I checked the plugs and found that #1 was quite black so I was obviously running very rich. I replaced all plugs with new ones and re-adjusted carbs on all cyilnders.

About 100km later, it started randomly popping and farting out the exhaust again. Theres nothing that I can feel in the sparkplug hole that could be grounding it out and the new sparkplug does not appear to be foul (in fact, looks pretty good?)

Idle isn't too bad but once warm, there are LOUD random pops from the exhaust. (Once every few seconds.)

Pulling the #1 wire off the distributor while running doesn't make much (any) of a difference to idle speed! (All other wires do make a noticeable difference.) If I hold the wire just above the distributor cap I can see that its definitely is getting spark, or at least the charge is going through the wire but seems like the plug is not lighting up?

I've swapped the #1 wire out. No help.

Checked floats (they were at 11mm) and I reset them to 10mm.

Checked fuel pressure - it oscillates between 3 - 3.5 psi which I understand is ideal for Webers.

Leaning the #1 carb out completely (mixture screw all the way closed) does not lower the idle speed - it actually stops the popping. Opening it up to default setting or beyond causes the popping to come back.

Timing and valves were checked about 100km ago and while I don't think they're far off now, I'll be checking the valves again tomorrow.

Any other ideas? Perhaps exhaust leak? I'm really racking my brain with this. It seems like I'm just not getting spark on #1.

Fri Sep 23, 2011 5:48 pm

Check the #1 idle jet and DO check for exhaust leaks but it sounds like a partially blocked idle jet.

Fri Sep 23, 2011 9:21 pm

Check for exaust leaks. Sometimes the. Nuts loosen from the vibration. Plugged idle jet. Also intake leak sometimes the gaskets get old and need to be replaced especially if u drive it alot.

Fri Sep 23, 2011 9:46 pm

When checking idle jets, make sure lil o-ring is on them and verify all are same size by measuring them. Make sure throttle plates shaft isn't bent, which may cause #1 plate to be in progression circuit. Thats probably why #1 calms down a lil when you close off that a/f mix screw.

Fri Sep 23, 2011 9:55 pm

Very simple. When the idle jets get clogged it does not get enough air and will run rich and then in turn will gas foul your plugs. Make sure you are running a good fuel filter. Pull the idle jets clean and it will run fine. Mine has done this several times. I cut an access panel in the fender well so I don't have to pull the carbs off when I have to do this.

Fri Sep 23, 2011 10:19 pm

Also, check air bypass settings. Should be set the same whether you have them closed or utilized on initial settings. You can fine tune with air bypass later if needed.

Sat Sep 24, 2011 8:15 am

rs58rag wrote: Also, check air bypass settings. Should be set the same whether you have them closed or utilized on initial settings. You can fine tune with air bypass later if needed.

I'm a total rookie as far as carbs are concerned but I fought some popping on mine for awhile and didnt mess with the air bypass screws cause i figured they were set correctly on original set up (and worked well for the first 10k) After closing up my # 1 air bypass my carb (1&2) and thus my engine reacted more to the mixture screw adjustment and was able to tune the engine and get rid of the pops.

Sat Sep 24, 2011 9:39 am

Could be cold idle circuit.

http://www.thesamba.com/vw/forum/viewtopic.php?t=443993