TheSamba.com :: View topic

Sat Oct 29, 2011 10:06 am

I have a new (under 2000 miles) 2110 with dual 40 Dellortos and am wondering about an issue I've noticed with a popping through carb sound when going downhill with partial throttle. Accelerating and cruising is fine, but when the wheels start turning faster than the motor I get some pops through the carb. I'm running a bus with reduction boxes and stock tranny.

Sat Oct 29, 2011 10:20 am

If it has always done it then i would guess timing or air mixture is off. If it ran well at one point (tuned) but is now popping then a plugged idle jet could be a problem.

Sat Oct 29, 2011 10:25 am

The popping may be through the exhaust, which is common. Popping through the carbs will often sound like a huffing or sneezing sound rather than a pop.

What is your jetting? How did you set your mixture screws? Are the carbs synched properly and opening at the same time?

What kind of exhaust? Are you sure there are no exhaust leaks? These suck air in during decel and cause ignition of any unburnt fuel vapors

Sat Oct 29, 2011 11:14 am

I would check the valves too. I had a popping through my carbs the other day, turned out my valves weren't adjusted right and had caused one of my pushrods to come unseated.

Sat Oct 29, 2011 11:27 am

Though I like to assume that people willing to dive into dual carbs have their valves adjusted, points set properly, timing set properly, and fresh ignition parts, you are right. Valves will cause some problems like this as well.

Sat Oct 29, 2011 12:35 pm

Its out the carbs, because I can MAKE it do it while I'm back there and rev it way high and watch the timing change with the pop (affecting timing through the vac advance). I have changed to pointless ignition this week and am dialing in the timing...I think it may be too advanced still.

Valves are a no, its hydraulic and have been adjusted correctly after break in.

The whiole thing was setup on a stand when new. Unfortunately, I don't have the dyno sheet in front of me..its MIA right now so I can't remember the jetting. Its correct and hasn't changed nor have any other carb settings.

I've got an A1 sidewinder...theres no seals on it..just slip fit and such...now that you mention that...it MIGHT be that too.

I THINK its carbs because it seems to affect the timing

Sat Oct 29, 2011 12:43 pm

The carbs don't affect the timing except for if it has vacuum advance connected. Backfires up through the carb will slam the advance diaphragm shut. Definitely check timing. Exhaust has little to nothing to do with carburetors backfiring. Is it on all barrels or is it limited to one side or one barrel?

Sat Oct 29, 2011 1:17 pm

Yes, it is Vac advance, thats why I mention it. Its hard to say, I'n tthe one driving every time. I'll watch for it more carefully next time. Regarding the exhaust, it may be pulling air through there and burning fumes...I never thought of that option. Of course it doesn't affect timing or carbs.

I'll give the timing another round later...my Kennedy plate grenaded and have the engine out of the car right now.