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Discussion Starter #1
Well, I've had this bike since March of 2015. It has won battles over and over again. I've put maybe 2 miles on it. Rebuilt/cleaned carbs(like 1000 times), rebuilt starter clutch, kremed the tank, new plugs, new ignition coils, oil change, the works. Still giving me hell.

The issues I'm having is when the bike warms up it backfires under load. I can ride 2 laps around my neighborhood and it doesn't backfire but once it gets warm it starts backfiring. The bike purrs at idle, no issues at all, but when I take off on it and it gets warm it will very very very loudly backfire. Equal to a shotgun blast easy as they are low boy shotgun pipes from Cobra.

When I rebuilt the carbs I put in Max Air PMS screws. I was told by someone they only control idle. But I can turn them in all the way and it doesn't affect idle at all. So I turned them out 3 turns each and went from there.

I'm so close to being able to ride it, it starts every time, runs everytime, but it's almost like it's loading up or something.

What would cause the bike to run fine for a couple minutes at idle and under load and then when it's warm backfire like that? Thoughts or tips? I haven't re-jetted the carb or anything as the bike came as is with custom pipes but it also wasn't running when I got it.

I'm looking for a savior. I dumped $176 on new ignition coils because I thought one might be heating up and breaking down but that proved to be a waste of money because I installed the new ones and still get backfire once its warmed up. On a tight budget so any help is appreciated.
 

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Discussion Starter #4
@nick57 I have custom exhaust on there, they are cobra low boy shotguns. I'm going to turn both in to two turns. Right now I'm at 3 on the rear cylinder and 2-1/2 on the front. When I rode it around at work my boss said it looked like it backfired through the front cylinder exhaust pipe.
@chedobean if they needed to be jetted then why does it run perfectly for a couple minutes then start backfiring?
 

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if stock is about 2 turns out and you put pipes on you will get popping on decelleration. so to get rid of the popping turn the screws out 1/4 turn or until the popping goes away.you say the bike has never run right. you have to determine the carb set up, what size pilot jets, the needle position.what octane fuel.
 

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Discussion Starter #6
@nick57 ok, right now I'm getting backing on acceleration. I'll turn them in to 2 on my lunch break today and work out from there and let you know how it goes. Thanks for the help
 

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@nick57 I have custom exhaust on there, they are cobra low boy shotguns. I'm going to turn both in to two turns. Right now I'm at 3 on the rear cylinder and 2-1/2 on the front. When I rode it around at work my boss said it looked like it backfired through the front cylinder exhaust pipe.

@chedobean if they needed to be jetted then why does it run perfectly for a couple minutes then start backfiring?
I'm not an expert on carburation, but I have managed to rejet and needle a few of my bikes successfully. I'm assuming that when you say it's fine initially, that it's starting to backfire when it gets to operating temperature?
It's been my experience that out of adjustment idle screws are most noticeable by popping and crackling on deceleration [ too lean]and problems just off idle. Sounds like you need a good mechanic to take a look and listen. [though I am one of those stubborn 'Bs' that likes to work things out for himself].
Another possible cause could be a vacuum leek in the manifold?
 

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On an 1100 with stock intake, you really don't need to change any jets....simply adjust the PMS out a bit like Nick suggested.

Please tell me you started this diagnosis by synching the carbs. When things aren't running correctly that is always the starting point. I wouldn't go any further until that is done. There is plenty of ground to cover afterwards if that doesn't solve it......but trying to diagnose an issue on a bike with 2 carbs and you're not sure if they are in synch is going to be a mighty futile excersize.
 

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Also the air injection system is notorious for causing popping and banging on the 1100s. I owned an 1100 vstar and mine popped a fair bit,[ but not loud shotgun bangs like you have described]. On advise from another owner I disconnected the air injection and it fixed that problem.
 

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Alll you really need to do is adjust the PMS out about 1/4 turn on each carb to stop the backfire on decel. You can leave the AIS on. 1100's can be slightly lean on the idle circuit, but it's an easy fix.
 

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Discussion Starter #12
Alll you really need to do is adjust the PMS out about 1/4 turn on each carb to stop the backfire on decel. You can leave the AIS on. 1100's can be slightly lean on the idle circuit, but it's an easy fix.
Ok guys, so I'm still chasing the backfire issue. I took my wheel in to have it mounted at a Yamaha dealer and explained that my backfire is coming from the front cylinder and the rear cylinder is running about 300 degrees hotter than the front. He told me that means the front cylinder isn't firing. Or when it does it has dumped fuel and then finally ignites. I didn't get to pick his brain much as they were busy but does anyone have any idea what would cause the front cylinder not to fire? I put new plugs and wires, new coils, checked the fuse, all that. Brand new, still does the same thing. I rejetted the carb, adjusted them to Dynojet spec, carb is clean, fuel is new and ethanol free. New battery, the works. I feel like something is shorted or unplugged somewhere. Just wondering what are some thighs I should check for if one of my cylinders aren't firing. Thanks guys
 

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Backfire on decel = lean
Backfire during load=rich

No fire on one cylinder is a real possibility. What did the plug look like when you pulled it?
If you have a timing light, connect it to the front and see if you have a steady spark.
PMS will do very little for your problem BTW.
Have the valves been properly adjusted? Check the can timing while you're at it.

Best advice, don't keep changing things and throwing money around chasing this problem. The more you change an swap the harder to diagnose.


Sent from my SM-G930V using Tapatalk
 

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Discussion Starter #14
Ok so I ordered and installed Dynojet jets in the carbs. Set everything to the recommended level. Now it pops through the front cylinder exhaust pipe at idle. It's like the front cylinder isn't firing hardly at all. When it ran before the jet install the rear cylinder was about 300 degrees hotter than the front. I thought I posted earlier asking if anyone had experienced only one cylinder firing and what could it be but I don't see my post on here. I may have posted it in reply to someone else but anyway. Any ideas on what could be the culprit on only one cylinder firing? When I get some time I'm going to do an ignition timing check, Ignitor unit check, and a voltage regulator check. Surely its one of those.
 

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if it's only doing it when hot and under a load your valves may be adjusted to tight when the engine gets hot they get even tighter creating a valve leak, always check compression first if you don't have good and equal compression it will never run right, always start there, if a valve stays tight for too long the valve will burn and never seal right even with the right adjustment
 

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Backfire on decel is caused by a lean condition on the idle circuit....removing the AiS is just a bandaid that reduces the oxygen in the exhaust a slight amount. The real fix is to just to tune the carbs right. We've got hundreds of 1100's with AIS still installed, aftermarket pipes and zero backfire......by simply taking a few minutes to tune the carb approprately.

A real biker knows when something is over his head and allows an expert to take a look at the problem. This does 2 things...the bike get fixed right - right away, and you learn what the issue was, so you may be able to fix it next time. But education is never free....you gotta pay to play, it's only fair.

On this issue, we don't even know if the carbs were synched right, whether anyone checked the exhaust gaskets, or the intake for vacuum leaks......or.......if the carbs were taken apart, that they were even re-assembled correctly. I'm not sure what "Dyno Jet" spec is....but if it's like anything else from them, I'd question it. My normal procedure with carb issues on 1100's with carb issues is to toss all the DJ stuff in the garbage can and use only Mikuni jets and needles.....they are far easier to tune and there is a ton of reliable tuning info available.
 

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everyone is missing the fact that he said it only happens when ( hot and under a load) which is exactly when pickup coils usually start to fail, or valve clearances close up a bit, if it were the jetting wouldn't it be happening all the time ?
 

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Here he says...."Now it pops through the front cylinder exhaust pipe at idle." ..... which to me means I've got to synch the carbs, check for an exhaust leak or a plugged/improper pilot jet in the carb. Nothing at all to do with running hot and under a load....
 

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i bet in the end he'll find the front cylinder has bad compression it's probably pumping up a few times before there is enough pressure to fire. i wonder how the bike runs going down the road all i'm reading here is idle issues, there has to be more to it than just a miss at idle. backfire is an explosion in the intake tract and afterfire is an explosion in the exhaust tract. i'v seen a backfire crack a plastic plenum once
 

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I agree that there is a lot more going on..... without any information or being able to see the bike......who knows?
 
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