backfiring/popping mid throttle

[Cash]

Whats up guys, looking for a little direction from the more experienced here. I have a 1981 cb750c, ran like crap when I bought it, had to do some work to it, so I replaced the plugs, re packed the mufflers, replaced seals in forks, replaced intake boots, air filter, and had someone clean and sync the carbs and now the bike runs well.

Plugs and carbs were the main issues. Packing the universal mufflers helped also, there was zero packing left in them and the revs would hang. Now the bike sounds and runs great, as it should, with one exception. There is a bit of popping/backfiring right around 3800 - 4500 rpm (in every gear), anywhere else in the throttle range the bike runs great. It happens on acceleration or steady throttle at that range.

Any ideas? is this typical behaviour of the bike running rich in that throttle range? There are no exhaust leaks, she's sealed up pretty well. The bike came with 4 into 4 with emgo universal shorty mufflers. Any help or suggestions would be awesomely appreciated!

 

[2_DONE_THE_TON]

Is the bike on the bike on the standard exhaust setup?

Has the carb been rejetted?were the jets changed to other sizes?

Does the popping happen when bike is cold or also when its warm?


I guess that there is a malbalance of jetting and exhaust setup.

And most importantly:

Plug colors?

Did you check for air leaks at the intake?(spray a little sonax around the carb boots,will it rev up....)

 

[Cash]

Thanks for the help, I'm not sure if it is the stock setup. I just picked the bike up a couple months ago. It looks like the stock 4 into 4 but I can't confirm that. The mufflers are the universal shorty ones that I think Emgo makes. Ill throw a picture up. I also just re packed the mufflers.

As far as the carbs being re jetted I'm also not sure. I didn't touch the carb set up when I bought it, I'm mechanically inclined and have re built engines before but I don't know much about carbs.

The popping happens even when the bike is at full operating temperature.

I think you are right about the imbalance of jetting and exhaust. If exhaust is flowing faster, how does that affect the jetting? What is happening in the system that would cause the popping?

I don't think they are air leaks as I put brand new intake boots in, but I will double check. I will also check the plugs, the plugs look fine, but I havent checked them by shutting the bike down while in the 3800-4500 ish range. I think that will give me a better reading.

pic of exhaust on the bike

 

[2_DONE_THE_TON]

Maybe the carbs were jetted to the unpacked shorter muffler and now you packed it causing the air to flow slower.

Normally shorter exhaust means higher main jet,removing air box and pod filter means higher main.jet.

Try to run the bike without the packed mufflers and rev in midrange see if popping still happens...

I had new carb boots that didnt exactly fitted my carbs,kawa z 400 and it ran like shit i mean the carb should not easily slip out of the boota they should be tight,the sonax move (spraying around intake boots a lil bit).see what happens.

My ducati used to backfire and pop when the plugs where wet (carbs were synced to rich cuz i changed the exhaust).
I think its gasoline steam residue that gets somewhere.

Please make a video to illustrate your problem...with and without mufflers.

 

[ApriliaBill]

I see a stock air box, but low restriction exhaust. The bike "runs great", but it pops. I've never had an EMGO that didn't "pop" it just depended on where in the power band it popped. When you say pop, do you mean on down shift, or at steady RPM. If it's at down shift, then you need to remember, an engine is an air compressor, you need tom take in a certain amount of air and then release it. with a stock air box and low restriction exhaust, you're probably out of balance. If the popping is at constant RPM, then you might have other issues along with intake and exhaust.

I had a set of EMGO shorties on my Triumph, I finally gave up and put some Dunstals on. The EMGO's had a nice tone, but could never get rid of the popping. If your bike is running well and the plugs look right, it may be the exhaust you have to replace, but then you'll have to maybe rejet or do intake work.....

 

[Cash]

When I first got the bike I ran it with the mufflers off and it didn't seem to like it, backfired quite a bit if I recall.

I haven't had a chance to work on the bike yet but when I do I will check for air leak around the intake boots (although I doubt this is the case I'll double check).

ApriliaBill, what I meant by ran great was that everywhere else in the throttle range the bike seems to run as it should and pull hard/smooth, no backfiring. As soon as I'm b/w 3800 ish to 4500 ish it'll start to pop. It's not too aggressive of a popping/backfire but it is noticeable. It seems to happen while accelerating or if i'm holding the throttle steady in that rpm range. Sounds like its coming from the exhaust.

Hopefully I get a chance to work on it this weekend and maybe get a video up. I really appreciate the help. I have a chance to get a Mac 4 into 1 exhaust for a good price, but it is for a 84-86 Nighthawk 750. Any chance it will fit my bike or will i run into fitment issues around the frame?