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Discussion Starter #1
I put a Chinese knockoff on my 89 and the initial results were better than I expected. After about 400 miles the bike has started to burp as I’m just picking up the throttle or just rolling off. Maybe hiccuping is a better way to describe it. Will adjusting the air/fuel mixture screw help, or is it more likely it needs a different jet, or something else? The knockoff is a clone of the newer style carb. I appreciate any advice. I’d like to resolve this before it takes its winter nap. Cheers.
 

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It’s likely to be the idle that’s the culprit, as 0 to 30% of the throttle will be dependent on this jet – the main comes in at 30 to 100%

Hard to tell, but the roll off is an indicator

Take it out as required, if you get to three turns, it’s maybe time to re-consider sizes – no experience of Chinese, so it’s all a learning curve …..
 

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Discussion Starter #3
Thanks Purple. It was at 3 turns out of the box. Before I installed it, I set it at 2.5 based on my experience with my other two bikes. Maybe I should have left it alone. Tomorrow I’ll see if I can adjust it for the better.

I forgot to mention that it has recently turned colder here which coincides with the start of the burping. Not sure if that makes much of a difference but my other bikes have been more finicky when it turns cold.
 

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Discussion Starter #4
Today I turned it out to 3 turns. Cold start was better, but still plenty of burping. My next move was to go to 2 turns out. That made it much better, although it’s still burping a little bit. Over the winter, I’m gonna take it apart and compare the internals to those on my 01 stock carb. I rejetted my 01 last winter and saved the old main jet. I also added two shims which improved the cold starting. Cheers.
 

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You mention that for the first 400 miles the carb was better than you expected but now all of a sudden it is developing a hiccup. My feeling is that an abrupt swing in the temperature could play a small role but I am leaning more toward a piece of crud stuck in one of the orifices that is causing the issue. Absolutely and always with our carbs the very first place to look and test are the seals around the intake and manifold boots. Any leakage there changes the entire air/fuel/vacuum mixture. The metal tanks are famous for growing a slight rust line at the fuel level inside the tank. As this flakes off it ends up in the carb in tiny specs. An in line fuel filter will help but once these specs get in the carb they start causing havoc.

GaryL
 

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You mention that for the first 400 miles the carb was better than you expected but now all of a sudden it is developing a hiccup. My feeling is that an abrupt swing in the temperature could play a small role but I am leaning more toward a piece of crud stuck in one of the orifices that is causing the issue. Absolutely and always with our carbs the very first place to look and test are the seals around the intake and manifold boots. Any leakage there changes the entire air/fuel/vacuum mixture. The metal tanks are famous for growing a slight rust line at the fuel level inside the tank. As this flakes off it ends up in the carb in tiny specs. An in line fuel filter will help but once these specs get in the carb they start causing havoc.

GaryL

That's exactly what I was thinking. Could just be coincidence about the temperature change.
 

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Carbs "sneezing" (if that's what you mean)- it kind of sounds like a quick "chuff" accompanied by a momentary loss of power is generally from a too-lean condition. That would seem to go along with the issue starting in colder weather. Now the too-lean can either be caused by incorrect carb settings (not supply enough fuel), or an intake leak that's allowing extra air in(cracks or damage to the carb boot). But if you're saying turning the mixture screw in(leaner) improves the symptoms, that's odd. Overly rich carbs tend to spit and sputter, with a "soft", unresponsive throttle and you tend to get a "burbling" exhaust sound that's whiffy and stinks of fuel.

Pull the carb apart and check for any debris or plugged jets first.

From experience though I've chased my tail when it comes to the knockoff carbs and run into some really baffling symptoms I couldn't make heads or tails of. I'll run them for small/equipment engines, but on anything you have to drive, I've never had a lot of luck. Personally I'd rather just get a used OEM.
 

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Discussion Starter #8
I appreciate the advice. Our long term forecast has highs in the low 30s so all the bikes are getting winterized and this winter I’m gonna open the knockoff carb and have a look.
 

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Discussion Starter #9
Update - you guys were right about air leak. I thought I had a good seal on the intake boot, but man was I wrong. The way I rerouted the throttle cables made it such that as I applied throttle, it would actually move the front of the carb a little (thus the burp). Same thing would happen as I rolled off the throttle. I could be wrong about that diagnosis, but the boot definitely needs to be addressed as well as the throttle cable set up. I’m kinda surprised it wasn’t running worse. I could swear that front boot fit very snug when I installed the carb.

I really wish I could get it solved and test it out but right now it looks like this.
 

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Update - you guys were right about air leak. I thought I had a good seal on the intake boot, but man was I wrong. The way I rerouted the throttle cables made it such that as I applied throttle, it would actually move the front of the carb a little (thus the burp). Same thing would happen as I rolled off the throttle. I could be wrong about that diagnosis, but the boot definitely needs to be addressed as well as the throttle cable set up. I’m kinda surprised it wasn’t running worse. I could swear that front boot fit very snug when I installed the carb.

I really wish I could get it solved and test it out but right now it looks like this.
Hi-Jack (ish).

Love your "TW-Lift".


I used this method a few times myself
 
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