lots of backfiring, popping on deceleration | GTAMotorcycle.com

lots of backfiring, popping on deceleration

GateKeeper

Well-known member
1985 YAMAHA FZ750
4-1 exhaust
stock air box, with K&N air filter

bike pulls good and hard, however, upon deceleration I am getting a lot of backfiring, popping etc out the exhaust, might sound cool (not really) but it's not how I want the bike to run and sound

I have checked all the usual suspects, air leaks, exhaust gaskets, carb boots etc, all seems to be OK and tight and no leaks, at least I don't think so, carbs are in sync, and bike idles fine, and starts no problem

so my next thought was a jet kit, many to chose from and not sure which one to get and from who

Factory Pro jet kit

Sigma jet kit

Dynojet jet kit

During my rebuild I did have the carbs out and did a carb clean, not a full tear down ( I should have, I was lazy) so I don't know the current jet sizes, needle position etc, but I am thinking it's all stock, but I would have to tear them apart to make sure

Anyone else with any other thoughts, tips, tricks, opinions, I am not racing this bike, just a commuter, but would like it to run as best as possible, any racers out here have a good contact on some of the jet kits, or know of a shop that would help out,
 
1985 YAMAHA FZ750
4-1 exhaust
stock air box, with K&N air filter

bike pulls good and hard, however, upon deceleration I am getting a lot of backfiring, popping etc out the exhaust, might sound cool (not really) but it's not how I want the bike to run and sound

I have checked all the usual suspects, air leaks, exhaust gaskets, carb boots etc, all seems to be OK and tight and no leaks, at least I don't think so, carbs are in sync, and bike idles fine, and starts no problem

so my next thought was a jet kit, many to chose from and not sure which one to get and from who

Factory Pro jet kit

Sigma jet kit

Dynojet jet kit

During my rebuild I did have the carbs out and did a carb clean, not a full tear down ( I should have, I was lazy) so I don't know the current jet sizes, needle position etc, but I am thinking it's all stock, but I would have to tear them apart to make sure

Anyone else with any other thoughts, tips, tricks, opinions, I am not racing this bike, just a commuter, but would like it to run as best as possible, any racers out here have a good contact on some of the jet kits, or know of a shop that would help out,

cycle bro in brantford smoothed out my carbs, on their dyno, after a rebuild.
 
Check for exhaust leeaks first.
 
I had a DynoJet kit in my FZ750. Bike came very lean from factory, set it according to the instructions in kit and there was a huge difference in engine response. No need for dyno, you'll know when it's spot on.
 
I had a DynoJet kit in my FZ750. Bike came very lean from factory, set it according to the instructions in kit and there was a huge difference in engine response. No need for dyno, you'll know when it's spot on.

Thanks

I don't have a jet kit, but I think it just might be the way to go, especially if I don't find an air leaks

I am leaning towards the Factory Pro kit, have heard good things about them

.
 
I should also add that the 4-1 isn't helping. Yamaha added a lot of backpressure with the OEM exhaust to give it decent mid-range on the street. That engine was made for top-end racing.
 
I should also add that the 4-1 isn't helping. Yamaha added a lot of backpressure with the OEM exhaust to give it decent mid-range on the street. That engine was made for top-end racing.

I think so as well, but the 4-1 looks so much better than the stock setup....and the original pipes are a mess, the pipe I got was in much better condition.....

seems my best and quickest solution will be a jet kit, and a tune afterwards....
 
Good luck. Let us know how it works out. I still miss mine, heavy pig that it was lol.
 
Good luck. Let us know how it works out. I still miss mine, heavy pig that it was lol.

LOL

+1

Put your hand over the outlet, the added pressure inside will allow you to hear the hissing of the leak better.

I will try this as well, and listen for the hissss, but somehow I have a feeling it's not an air leak, exhaust gaskets were new, when the 4-1 pipe went in, but it could happen...

Thanks

if no hissing, then a jet kit will have to be ordered....
 
LOL
I will try this as well, and listen for the hissss, but somehow I have a feeling it's not an air leak, exhaust gaskets were new, when the 4-1 pipe went in, but it could happen...
The leak doesn't have to happen at the block. On my bike (not a Yamaha), I had a poor joint between the exhaust manifold and muffler which caused popping and backfiring on decel. Tightened up the joint using high temp RTV and the problem disappeared. So, leaks in the first 20 inches of the exhaust system could contribute to your problem.
 
Agreed, anywhere in the primary pipe should get some sucking action to draw the air in.
 
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prior to rebuild, everything was fine?? if thats the case, you have intake air leaks, if carb rebuild is what you did. if the exhuast was removed also, engine rebuild? then possible exhaust leaks can be present. theres nothing that rejetting using stock jets cant do that jet kits do, dynojet etc, can do other than use more fuel by richening up the jetting and giving the bike a new feel. the same can be accomplished by jetting yourself with stock jets. you need to read the plugs and jet accordingly, this will give you best performance and fuel economy/engine temps
 
this is the pipe, and it's in 3 sections







so I guess by what you guys are saying any one of the joints could be a cause of the issue I am experiencing

@Skully, when I got the bike it was not running, only long enough to tell me the motor was good, no test ride, just a short running at idle, if throttle was applied it died. the exhaust is new to the bike, the original was taken off and the 4-1 installed, not a full carb rebuild, just enough to clean out the crap in the float bowls, and visual inspection of the rest, no jets were removed, no new O rings, and this could be my issue as well









First order I will check the entire exhaust system for leaks....

Thanks
 
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pops out the exhaust are not neccessarily a bad thing, cam/engine mods can cause this, its the nature of the animal. you have alot going on there, exhaust leaks is not what id be chasing, intake leaks or lean jetting/timing. new plugs and a good run down the hw, read the plugs. if you are certain that the intake is sealed and the carbs are kosher, have a bud with a cigarette round your exhaust ports. the smoke will show you the way

try the link from mik for some good reads too. the bible per say!! link
 
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