Popping on deceleration, running lean. What screws do what?

[Guest]

Mikuni CV carbs on 81 GS850L

Bike ran great, carbs were recently balanced and tuned, then petcock fails.

Replaced petcock, bike now pops on deceleration with engine braking. Plugs also look very white.

I think I'm running lean. I have resealed the airbox and have good boots on both side of the carb. I'm sure there's no air leaks.

My guess is that the petcock was imperceptibly leaking excess fuel into the carb when it was tuned last at a shop. When the damn finally broke and it was apparent the petcock had failed, I replaced it and now the slow fuel leak that the carb was tuned with was fixed, leading to a lean running bike. Does that sound feasible or am I barking up the wrong tree?

Now, my proposed solution is to adjust a screw... but man is there some confusing terminology when it comes to carburetor screws. I have seen the terms "air screw, fuel screw, air/fuel screw, idle mixture screw, pilot screw". Searching the forum just leads to more synonyms for what I think are only two different screws, right?

Here's a picture: http://imgur.com/a/r7e51

I don't know what screw "A" is called. I know it's used to balance the carbs and I don't plan on adjusting that to solve this problem...but what's it called anyways?
Screw "B" I think is called the pilot screw. Is that the same thing as the fuel/air screw, sometimes just called the air screw, sometimes called the fuel screw, sometimes called the idle mixture screw?

So what I *think* I need to do is adjust screw "B" so I'm running a little richer.
This tutorial clearly states "Richen the mixture: rotate the pilot air/fuel screw counter-clockwise" http://members.dslextreme.com/users/storagecliff1/images/Adjusting_Idle_Mixture_on_CV_Carbs.pdf
This tutorial clearly states "If the screw is turned in, it reduces the amount of air and richens the mixture" http://www.iwt.com.au/mikunicarb.htm

So which way do I turn it?

Initially, I think I want to count the turns it takes to lightly seat them then return back to where they just were so I have a reference point(hopefully that's between 1.5 to 2.5 turns right?). Then I'll turn each one half a turn in the richer direction(whatever that is), and ride around and see if that solves my Popping/PseudoBackfire problem. Rinse and repeat until that goes away then plug chop to see if everything looks good. I think I could use the "highest idle rpm" method here but I've tried that before with mixed results so I'm doing this one slow, steady, and make it easy to return back to the starting point.

Thanks for taking the time to read this.

-John

 

[chuck hahn]

Usually one of 2 things..carbs are tad lean or theres an exhaust gasket leak.

 

[Guest]

Exhaust gasket leak is something I hadn't thought about. The problem arose after the petcock failure and replacement though so I'm trying to find a connection between those. How do I richen up the carbs?

 

[chuck hahn]

They may be sort of ashy but to what degree of ashy is another story. See this read.

https://www.ngksparkplugs.com/about-ngk/faqs/spark-plug-faqs/how-do-i-read-a-spark-plug

 

[chuck hahn]

At the top of the carb section is a STICKY on CV CARB TUNING that will explain it for you.

 

[JTGS850GL]

My bet is an exhaust leak.

 

[Guest]

Hi chuck, I'm sorry I didn't mean overlook the sticky. I had used the search function and in those results it didn't come up under the keywords I was using. I read through it now. Also in case you didn't see it in the other thread, you nailed the kinked fuel line diagnoses. Thank you!

When running through the rpm's as outlined in the carb tune sticky, can I do it all in first gear even for the 7500-redline test?

In the middle of typing this out my brain connected the memory of hearing the popping from my right ear. Inspection of the right exhaust pipe:
http://imgur.com/a/E7y9L
This could mess with the backpressure of the exhaust system and cause the popping, couldn't it?

Can I do a quick and dirty repair job with this? :
http://imgur.com/a/a597D

 

[Big T]

John,

In looking thru the 2 threads you have going, I see a glaring fault. You state that the previous owner rebuilt the carbs and you posted the picture of the carbs asking about the screws.
It looks to me as though the cap over the idle mixture screw is still in place, If so, the carbs have not been properly rebuilt. Also, taking the float bowl off and spraying carb cleaner about is useless.

Am I wrong about the photo?

 

[chuck hahn]

It can cause popping. What happens is he leak lets in cool oxygen rich air that mixes with the unburnt gasses from the engine and the heat makes for an ignition source and she pops off. Usually the gaskets at the head or at a crossover pipe are the main culprits though. I have a day one stock Yamaha TX750 and there were some bad spot where the rear hangers are welded to the cans. My fix was mix up some JB Weld and fill the spots in. At the underside like yours are buggered its gonna be a challenge to keep any filler in till it sets up. Maybe mix the JB and have some duct tape precut and right there. Squish the JB in and throw tape over it to dam the area till the stuff sets up????

 

[Guest]

John,

In looking thru the 2 threads you have going, I see a glaring fault. You state that the previous owner rebuilt the carbs and you posted the picture of the carbs asking about the screws.
It looks to me as though the cap over the idle mixture screw is still in place, If so, the carbs have not been properly rebuilt. Also, taking the float bowl off and spraying carb cleaner about is useless.

Am I wrong about the photo?

Hey Big T.

The factory screw cap has definitely been removed, the pictures don't show it well but I have clear access to the screw heads.
I kinda figured my haphazard carb cleaner spray wouldn't do much Maybe I was hoping to scare the ghost in this machine idk.

Thanks for following the threads and offering your wisdom!

 

[Guest]

Patching those exhaust holes I took a picture of did not solve the popping problem. Onto the next one. Going in the exhaust leak direction as per popular opinion.

Using the search function I read the gaskets where the exhaust bolts onto the head are a common source of leaks. Is there any way to diagnose an exhaust leak that you cannot see, or do I just go in and start replacing things that *could* be the culprit until the problem is solved or I've replaced everything thoroughly enough to rule it out.

 

[JTGS850GL]

You can try to tighten the exhaust bolts but in most cases, the gaskets need to be replaced. They're not expensive and are relatively easy to replace. The only issue you need to be aware of is the exhaust bolts can easily break so work slowly and use lots of heat and penetrating oil. While doing the replacement it's advised to replace the bolts with stud and nuts. Makes future work much easier.

 

[Nessism]

Sorry, but I can't review your links due to firewall issues.

The "pilot screws" are on the top front of the carbs. For your bike these screws should be open between 2-3.5 turns open from lightly seated. 360 degrees = 1 turn.

More open = more fuel. I'd set the screws at 3 turns and then check the popping. You can always go in from there if necessary.

 

[Guest]

Thanks JTG,

I will probably just go ahead and replace those gaskets, studs, and nuts as it'll probably run me less than 30 bucks. 30 bucks could be the difference between making rent and not so I'll do that once I make the money (self employed so not a consistent paycheck, also a student so I'm inherently broke, haha). Is there a method to check for any other exhaust leaks if replacing those gaskets doesn't solve the issue?

 

[Guest]

Sorry, but I can't review your links due to firewall issues.

The "pilot screws" are on the top front of the carbs. For your bike these screws should be open between 2-3.5 turns open from lightly seated. 360 degrees = 1 turn.

More open = more fuel. I'd set the screws at 3 turns and then check the popping. You can always go in from there if necessary.

We're talking about the same screws, I figured that out with the help of these lovely people on the forum.

My screws are currently set at 3.5, maybe 3.75. I knew that was on the edge of too much, but I opened them another half turn on the assumption that I'm lean and not only did the popping not go away, but I had a soft spot in the throttle response between about 2 and 3.5k rpms. Turned it back to how it was and the power comes back. Could I possibly have misdiagnosed and be running too rich instead? All white spark plugs aren't typical of too rich, but I'll turn the screws in a half turn and see what happens to the popping. I will report back!

On a side note, I feel so lucky to have you all help. I'm not technically skilled enough yet to offer repair or maintenance advice, but I wanted to point people towards the only small contribution I was able to make: the GS850 owner's manual with searchable text, like basscliff's factory service manuals. Direct download link here https://mega.nz/#!yUdA1SYI!TWj9QztSz...trA7u_OVqVPlIs
Thread here: http://www.thegsresources.com/_foru...Text&highlight=owner's+manual+searchable+text

One of these days I'll do the rest of them too. School and work has been keeping me challenged :/

 

[Guest]

I turned the fuel mixture screw in half a turn. No change in popping on deceleration, but there was noticeable change in popping while choked warming up. It has done that a bit before, but this time around it was popping every few seconds almost immediately on startup.

 

[Tom R]

Did you replace the fuel line when you replaced the petcock? Make sure it's properly routed and doesn't have any kinks.

 

[Guest]

Have you changed the boot O-rings and checked the boots for air leaks? The air box can also cause issues. It's a major problem with these bikes and could definitely make it run lean.

 

[Guest]

Did you replace the fuel line when you replaced the petcock? Make sure it's properly routed and doesn't have any kinks.

New fuel line with the new petcock. No kinks.

 

[Guest]

Have you changed the boot O-rings and checked the boots for air leaks? The air box can also cause issues. It's a major problem with these bikes and could definitely make it run lean.

Air box has been resealed (very thoroughly by me), and the PO had changed the boots and O-rings within the last year. They look to be in excellent condition, and using the carb spray method, there is no indication of air leaks.