popping exhaust, is it an air leak on the carb?

[baron]

I recently fitted a new exhaust, a mondo muffler on my 1641 with prog carb

it was fine for 5 minutes, but eventually developed a bad backfire. took it to my garage and it seemed to get sorted

however a month on, and the popping backfires have just returned at around 2000rpm, above that it seems to run fine

but as you can imagine it's a mare to drive, and is hating hills

i had a quick listen in the engine bay, and there a loud hiss as the carb opens (prog carb with the usual air filter fitted). is this normal, or could this be an air leak causing my pops? i always had a bit of a flat spot in low rev's, so wondering if this was always there and has just got worse of late

if it is a leak, is there anywhere typical that will be leaking? or it it just a case of hunting it down?

or maybe my exhaust isnt sealed properly? it's just with the hiss i thought it might be air coming in that was the issue not going out

 

[matt-me]

mine was the exhaust to heat exchanger connection, not a great design IMHO. Pulled them apart slightly, pushed in some gun gum type paste, pushed them back together and tightened, no probs yet after 1800miles, touch wood...

 

[noddy]

The air leak is likely to be in the exhaust system, had a back fire issue with my motorbike turned out the exhaust system had an air leak. Air was getting sucked in and mixing with the unburnt fuel it only did it on overrun after hard acceleration.

 

[360andi]

like Noddy says, if it's popping and banging on the over run it's usually an air leak on the exhaust side of things.

If you suspect you have a leak around the carb or inlet area, run it up and give those areas a good squirt of WD40 or something similar. This might help identify the leak area.

Good luck.

 

[Bookwus]

Hiya baron,

Yep, I'd agree with the previous posters concerning a leak in the exhaust plumbing somewhere. This is especially likely if work has just been done (as it has in your case) on the exhaust.

However, there is another possibility that has not been mentioned.........yet. How has the engine been running? If you have had a consistent miss in one cylinder unburnt gas and air can be expelled into your exhaust system where you may well get an afterfire. All your spark plugs firing and your engine in time?

 

[baron]

hey everyone, thanking you kindly for the suggestions, I'm still wondering about an air leak on the carb and will check it out using your suggested approach

BUT.... the reason it was backfiring was that it was about to run out of petrol (broken fuel guage) and was running on dodgy crap at the bottom of the tank (due to some muppet filling my spare fuel tank with 2 stroke a couple of years back....)

i found out the hard way, when it ran out of petrol on the leeds rin road - dual carriageway, no hard shoulder, oops! lucky i have a spare fuel can (a new one) that always contains fuel

DOH!