I'm baffled. Please Help with backfiring!!

[AaronOregon]

1980 XS650 Special with BS34's and the last era of the brass floats. Right side is backfiring upon revving and not getting very hot. It idles like it's firing on 1 cylinder and than when you rev it up it fires on it, but also backfires. I have pulled the carbs and cleaned them 3 times, installed a complete carb kit along w/ new floats, adjusted valves, timing, swapped and reversed plugs and plug wires to eliminate coil issues. It stays on that side. Where else do I go? Getting very frustrated. Thanks in advance guys!

Aaron

 

[FB71]

have you checked base engine condition? Compression and cylinder leakage on both holes? Compression and valve leakage can cause the issues you are having.

 

[angus67]

carb sync. what did you replace with the carb kit? the pilot styles are usually wrong, as well as the idle circuit needle

 

[plattey]

Try the compression check like FB71 said. Have you over-revved it at any point in the recent history? Could have slightly bent a valve. That happened to a buddy of mine's bike. We ended up finding a ever so slightly bent intake valve. A quick check could be to see if it is spittin' back through the right side intake while it's running. You'll have to take the airboxes off. Good luck.

 

[79xs]

1) check compression

2) synch carbs and check for intake leaks

3) check / replace exhaust gaskets

In my experience, if your motor and carbs are in good shape, a leaking exhaust gasket can cause back-firing. Actually, this just happened on my bike.

 

[AaronOregon]

Thanks for the responses guys. I will look into these things you mentioned and report back. I have synced the carbs and made sure the intakes boots are good by swapping them around. Problem stays on #2. I am really hoping it is the exhaust gasket, but not sure if that would make it want to run on one cylinder until you revved it up? I hope it is something simple. I am just confused. I will check compression first and than gasket. Starts on first kick and has great power for it to be compression though.

Aaron

 

[sseres]

Also, this may sound dumb, and I refuse to admit to why I might know this, but are you SURE you adjusted valves on the compression stroke for the appropriate side?

 

[650Skull]

Note* You didn't answer angus67. A yes or no will make a major difference

 

[angus67]

its ok. im used to it.

 

[XSLeo]

The BS carbs were used on many different bikes. Yamaha wasn't the only one. Every application needed different tuning specs.
Most carb kits are a generic set of parts that may fit some applications but not very often the S650.
Best to not buy anything until you tear down, clean, inspect what you have. That way you buy only what you need. Most often you only need a float bowl gasket. And that not very often.
Leo

 

[AaronOregon]

Sorry guys. Been busy! I changed the gaskets and the needle and seat. I also replaced the brass floats with the Mikesxs floats. I set these at 22mm instead of the 27.3 the brass ones were at. Compression is at 145 per cylinder, brand new exhaust gaskets. I am just baffled and wondering if it is in the electronics somewhere? I am really thinking about just doing the $425 Pamco on it to eliminate all the old stuff. Can't figure out now if it is fuel or bad spark....... Thanks for the help.

Aaron

 

[angus67]

mikes plastic floats seem to have an issue with hanging up, because (ive heard) the pivot isn't sleaved. I pulled a set of tour max plastic floats off of ebay, the pivots are sleaved with brass, and ive had no issue.
You really should do your self a favor, and learn the old stuff, before advancing to the new. although, I could not for the life of me get a 71 with points to hold idle. once I put a standard pamco in it I never looked back. but the mechanical so and so genious in me wonders why I couldn't get it.

 

[willy]

your banging your head against a brick wall, get rid of points ignition and fit pamco or boyer then start from there........Pete

 

[NONclow]

+1 Angus on figuring out what you got, instead of a whole new monster. I'd adjust point gap (after filing and cleaning contacts), check all connections. It could also be a bad condenser.
.......Although, running on one cyl till revved up sounds like pilot circuit/needle jet blockage

 

[nj1639]

+1 Angus on figuring out what you got, instead of a whole new monster.
.......Although, running on one cyl till revved up sounds like pilot circuit/needle jet blockage

That's where I'd bet my money.

 

[Jaydela180]

Stock pipes? Cutting pipes behind relief holes in outer wall of exhaust to weld on straight pipes will allow for air to be sucked in on exhaust stroke and backfire. Happened to me.

 

[hks650]

i just installed my pamco and got it up and running thnaks to guys like pamcopete and xsleo. but i think i might be having the same problem as you, my left is backfiring upon revving as well.

 

[XSLeo]

I might check for exhaust leaks, air leaks in the intake both can lead to lean mix that can cause the backfiring.
Cleaning the carbs never hurts. www.amckayltd.com/carbguide.pdf will help with that. Maybe needs a bit more carb tuning, that's in the carb guide too.
Leo

 

[willy]

like everyone has said before, down on compression on that side, you have to check compression...in my experience after building these bikes, [44] to be exact, the standard set up of carbs stock jets and settings will run in all conditions from a standard motor up to the 750cc conversion without changing anything buy mixture, by changing jets you could be creating problems ..and running points and condenser is a big disadvantage, you gotta go electronic.......good luck, hope you get it sorted........P