MT500 - overrun backfire - PHF34

[happychappy]

This is an issue I've been messing with fro about 2 years and I'm at the end of my rope.

I have a standard MT500 engine with a full stainless exhaust, the end can is free flowing.

The carb is a Dellorto Phf34 with the following settings:
Idle 58
Main 120
Needle k28

The bike stars and runs really well. It can do an indicated 90-95MPH on a flat.

My issue is that if I give it a lot of throttle, then completely cut the throttle, the bang from the exhaust is offensively loud! I've tried all sorts of needle positions and needles (K32, K4). I've also tried a 125 main.

I adore the MT500 but the backfire is really getting me down. I've checked the entire exhaust system and there doesn't appear to be any air leak.

[deadgrass.]

have you tried a different spark plug?

[happychappy]

I have indeed. It made no difference. The cylinder does not appear to misfire at all either, you can almost predict the bang.

[bigfatcat]

Have you tried replacing the exhaust studs? I have the same setup, and I had issues with seized nuts (ah-hem) making me think i was tight on the flange (fnarr) but actually they were causing the stud to turn and bottom out, so weren't holding the exhaust down tubes on tightly even though they felt like they were

[arceye]

I would think you have an air leak somewhere in your exhaust system.

[12royal]

As Mr Arceye suggests, you have an air leak on your exhaust system.

[bigfatcat]

yep, I'm also suggesting air leakage, just trying to explain where mine was and how I fixed it - by fitting new exhaust studs

[bigfatcat]

Did you fix it?