Carburetor Question

[Dave48]

I have a 1980 Special with BS34 carburetors.
It has Uni air filters with shortly mufflers.
Compression on both cylinders is about the same.
There are no vacuum leaks that I can find.
The carburetors were re-build including new throttle shaft seals.
The valves were adjusted, the cam chain was adjusted and the carburetors were balanced.
It has one size over pilot jets.
The idle mixture screw was at 3 1/2 out.
After installing the one size over pilot jet, it idled at about 2000-2500 RPM with the throttle adjustment screw all the way out (too much fuel) (it also bogged down coming off of a stop). To compensate for this I turned the idle fuel mixture screw in 1/2 turn. Still idled to fast so I went to 3/4 turn in. It idles good now and runs strong from a stop and through the gears. It runs great.

Question. I'm getting this backfiring in the left/#1 cylinder exhaust. It backfires at idle and when I let off on the gas to stop or shift. It does not backfire at constant road speed of 25 to 65 MPH.

It's annoying. What am I missing? Any suggestions?

Note: It doesn't seem to backfire as bad as it did before the new pilot jet.

 

[weaselbeak]

First off, pleated filters and CV carbs are a bad combo. Use foam pods. Secondly, shorty mufflers IMO aren't much better than straight pipes for subduing noise. Take the exhaust baffling out of most anything and you'll hear some backfiring type noise. Stick a substantial muffler on there and see how much difference you get. Tuning shorties and pleated pods is a very touchy deal.

 

[5twins]

You should be able to turn the idle speed right down to nothing, basically turn the carbs off at idle and stall the motor. If yours is hanging at 2K even though the idle speed screw on the left carb is backed out so it's not touching the cable arm, your carbs are out of sync. The right carb is turned up way too high. Fiddle with the sync adjusting screw on the linkage between the carbs to bring the idle speed down. You shouldn't use your mix screws for this. They should be set to obtain fastest, smoothest idle and the least amount of popping on decel. On the BS34s, that's usually somewhere in the 3 to 3.5 turns out range. If the idle speed climbs too high, turn it down with the idle speed adjusting screw.

 

[Dave48]

They are foam Uni filters not pleated filters. I also understand the backfire type noise without baffling. This is not normal backfire type noise. This sound's more like a 9mm. And why only the left side exhaust?
There has to be something different between to two sides. That's what i'm trying to figure out.

 

[5twins]

Your left cylinder is running lean because it is essentially shut off. That's why you're getting the large backfires from it.

 

[Dave48]

5twins – Thanks for your reply.

I'm beginning to think your right, it's a sync problem. That seems to be the only thing that makes sense. I did sync the carburetors but that may have been before the bike was running correctly. I may have got them way out of sync. However, the left/#1 cylinder seems to be running overly rich. The mufflers are new and the inside of the left exhaust is black and the right is not.

 

[XSLeo]

When the carbs were rebuilt, who did them, did whoever rebuilt them use carb kits? If so then your carbs may have wrong parts installed.
The BS34 carbs were used on many bikes. Most kits fit the bikes that were most popular. Some of the pats were for them. Our set up needs different parts.
Leo

 

[funky]

im going for float level on this one back fire is to rich cant adjust idle air leak dump the filters get foam ones check balance and float levels also check needle settings and all jets are the same.