Back firing on the over run

Hello all,

I have had this since I bought the bike a while ago (2002 550FC). If I accelerate and then shut off the throttle, the bike backfires. This it does whether hot or cold but never when revving on the sidestand. It will do it even when I have the switch set to the hi map.

The bike is on a Delorto carb which it came with as standard. The idle is fine and the bike starts very easily hot or cold.

Spark seems good and the SEM stator is within tolerance of the resistances.

Any ideas as to what might be causing the back firing? Dont want to fry the valves.

Cheers

George

It might indeed be running slightly rich. Took the spark plug out and it was black as opposed to brown or white.

Will have to check for leaks in the exhaust, might be I need to tighten the manifolds up. After that, I will consider the carburation.

Taffy, I don't quite get the bit about not needing the choke. When the bike is cold, I have to choke it for it to start. When hot it kicks fine. Don't think I have anything wrong there or do I?

As for the mixture screw, I had a fiddle with it because it was hard to start the bike once when warm. I was also hoping that this would cure the backfire but no luck. The hot start got better though.

For the record:

PJ 33
MJ 200
Needle K35
Atomiser DR264
Choke jet 45

The circlip on the needle was on the 3rd notch from the top end. No ovality or wear in the atomiser and needle straight.

Oring on the choke was chewed. Air mixture oring also a bit shagged.

So the question is... Bar the choke jet being too big, where should the circlip of the needle be? I would think it should be raised to reduce the clearance between the atomiser and needle right?

Taffy, I am sorry you feel the need to reply only that you are "out of here". The fact of the matter is, I was not expecting you to reply at all and nobody is obliged to answer. I am sure there are other members of the forum that also learning and others that are willing to give newbies some insight into things. As with all thing, the "don't ask, don't get" rule applies to knowledge too. I will be the first to admit I know fcuk all about carburation as the only other bikes I have had either didn't need fettling or were fuel injected.

From what I have read in the owners docs and the dellorto tuning guide document, your suggestion will probably solve my backfiring problems which as you have rightly pointed out is the bike running too rich.Don't think I do not appreciate you taking the time to write what you did. You are also more than welcome to tell me that I will need to buy things from you, which if I was a ****** that didn't want/need to understand things first, I would surely do in an instant.

If anybody would like to know why I have asked the question about the depth of needle penetration into the atomiser:
it's because I looked at the mechanisms of the carb, rolled the needle on a mirror to check run out, put the atomiser under a magnifying glass to check for wear and made sure all the seals were in good condition. In my line of work I see metal wear and seal failure daily so I know what I am looking for.
If I am not mistaken, the choke jet should not come into play during normal operation right? So as I said, I have a 33 idle jet which is what Taffy has suggested I get. I had already tried the idle mixture screw and made sure the idle was reasonable, so bar getting a smaller main jet or tweeking the slide's cut away or start figuring out what needle and atomiser I need, the only other option is to lean out the mixture be increasing the depth at which the needle goes into the atomiser. Am I missing something? Have I understood the following diagram wrong?


If I have, someone please tell me.

What husabutt is talking about is if you are also out on your timing and super super lean. Had that happen on my BMW RT. it was basically backfiring through the airbox....

Guys, thanks for the responses thus far. I am putting the bike back together so should be able to test the new needle position, hopefully tonight.

Will report back.

So, result! Shifted the needle down two notches, so its one from the top. Tweeked the air mixture screw, and voila! no more back firing. A big thanks to all how helped me solve my bikes little quirk.

Regards

George