Is this correlation or causation?

[Guest]

I used the search function, but couldn't come up with a consensus that way. Perhaps you can help?

Bike was running great. Previous owner rebuilt carbs, new intake boots, and sealed airbox within 3,000 miles prior to my purchase. Upon acquiring the bike I immediately did valve adjustment, inspected the cam chain (ok), both oils changed, then paid a mechanic to adjust and sync carbs (he used a colortune and vacuum carb sync tool). I've put 1,000 miles on it since then and it felt great, fine tuned. Then last week things started going wrong.

I know there are divided opinions on this, but I've had good experiences running seafoam once a year on my 83 KZ750. It smoothed out the idle and sounded cleaner, so I figure I'll do the same on the 81' GS850L. I ran 4-6oz on a full tank of gas and on that tank during a round trip from Oakland to Sacramento, my petcock fails, badly. I limp home getting about 20mpg because it's dumping out the petcock as well as feeding back through the vacuum tube to the #2 carburetor. I assume coincidence because seafoam shouldn't destroy a petcock right? I'm still a novice mechanic so if anyone has information otherwise I'd be happy to be corrected.

I replace the petcock and spark plugs gaped within spec. Now I have a loss of power between 2500 and 4000rpms, and at speeds in 4th and 5th gear it comes and goes when the throttle control is consistent. I also have backfiring when decelerating.

I ran the bike at about 30mph at 3,000 rpms in fourth gear for a few blocks after warming up and then clutch and kill switch. Coast to a stop and pull the plugs. Links to the pictures below, but I'll save you the time cause they're all pretty much chalk white. I'm running lean, which I think is consistent with the midrange loss of power and backfiring.

http://imgur.com/fzN0Up4
http://imgur.com/WinpUvV
http://imgur.com/McAbIhE
http://imgur.com/Hz8uPPE

So my ideas:

1. My petcock has been failing for a while, so when the mechanic adjusted the fuel and air it was compensating for a small leak? Once I replaced the petcock I would be running lean? Can I fix this with a turn of a screw? What would be the procedure for that?

2. I'm having some sort of blockage or other problem in the main jet, which would explain why my power loss is in the rpm band that it is. I really don't want to pull my carbs because I just had them balanced and I don't have the tool, nor possibly the skills, to put them back together and sync them. I'll do it if I have to though. I bought this to be my commuter bike for school between Oakland and Sacramento, and here on week two I'm taking the greyhound again!

3. I have an airleak somewhere in the system that showed itself after the seafoam and petcock events? I don't know if either of those would cause this but maybe?

 

[steve murdoch]

Seafoam is not going to affect the petcock, certainly not in the ratio you described.
With gas puking everywhere any chance you have contaminated the engine oil?
Check the level and smell of the oil.

You could have picked up some bad gas or crud from your tank and clogged up your carbs.

An easy way to check for air leaks is to spray something like WD40 around the carb boots when the bike is running. The revs will increase if a boot has cracked or come dislodged.

 

[chuck hahn]

81 850 has CV carbs ( is supposed to at least that is ) so I fail to see the effectiveness of a Colortune...it more suited for VM where you can actually adjust the air to fuel ratio manually. See the CV carb tuning sticky at the top of the carb section for answers on how to properly tune cv carbs.

Im with Steve on checking for clogged jets first as all that requires is taking off the bowls and unscrewing the jets. if your unsure look at the rebuild tutorials here for guidance. Scroll down and see the right hand margin for tutorials.

http://members.dslextreme.com/users/bikecliff/

 

[posplayr]

It certainly sounds like it is related to carburation.

 

[hillsy]

81 850 has CV carbs ( is supposed to at least that is ) so I fail to see the effectiveness of a Colortune...it more suited for VM where you can actually adjust the air to fuel ratio manually. See the CV carb tuning sticky at the top of the carb section for answers on how to properly tune cv carbs.

Im with Steve on checking for clogged jets first as all that requires is taking off the bowls and unscrewing the jets. if your unsure look at the rebuild tutorials here for guidance. Scroll down and see the right hand margin for tutorials.

http://members.dslextreme.com/users/bikecliff/

You can use a Colortune on CV carbs - it's used to adjust the burn at idle with the idle mixture screws just like you do on manually operated carbs.

 

[chuck hahn]

You can see the flame get weaker as you turn the screw in but other than that you cant adjust air. The fuel and air ratio is mixed down in the bowls and is controlled by the jets. The only thing the mixture screws does is meter the PREmixed fuel and air as its introduced into the venturis. The colortune can only be an "aid" for setting the burn while also listening for the subtle PRM change when doing the "highest idle method"

So if the lean situation is due to too much air there isnt anything the mixture screws are gonna do for him. Only way to lean or richen CV carbs due to improper fuel to air ratio is to change jets. Click on an 50 manual on Bikecliffs site and read the functioning of all the circuits on CV carbs and youll see.

 

[bonanzadave]

I replace the petcock

With what ? Good hoses both feed and vac ? not kinked ? Id hook up a simple gravity gas tank with some fresh gas. If it runs better you can start looking at the old gas / tank or the new petcock. If there is no change then start looking at the carbs.

 

[chuck hahn]

Be sure the fuel line isnt kinked some with the tank setting down on it. Poor fuel flow will let the bowls run at a low fuel level..which in turn makes them run lean.

 

[Guest]

Some excellent suggestions! Thank you all so much! my course of action:

1. Check for kinks in fuel line. It is possible, as there's an inline fuel filter in it so the fuel line is not the normal length and routed unusually. I'll check the inline fuel filter for clogs as well. Don't know why I didn't think to do that already.

2. Then I will check for clogged jets. Very happy to hear I can just remove the float bowls to get to them. It seems like the in line fuel filter would decrease the possibility of clogged jets but of course it's still a possibility.

Regardless, I'll also check for air leaks, and I've got a new airbox lying around that I've been meaning to seal and install anyways so I might as well.

My bike has Mikuni carbs. I don't know why. Thanks again for your suggestions and I'll be back for personal thank you's to whoever diagnosed it correctly. GSResources is great!

 

[habsdoc]

The inline fuel filter may be causing your problems to begin with. Some are very restrictive and will not let our gravity fed fuel flow enough fuel.

 

[hillsy]

You can see the flame get weaker as you turn the screw in but other than that you cant adjust air. The fuel and air ratio is mixed down in the bowls and is controlled by the jets. The only thing the mixture screws does is meter the PREmixed fuel and air as its introduced into the venturis. The colortune can only be an "aid" for setting the burn while also listening for the subtle PRM change when doing the "highest idle method"

So if the lean situation is due to too much air there isnt anything the mixture screws are gonna do for him. Only way to lean or richen CV carbs due to improper fuel to air ratio is to change jets. Click on an 50 manual on Bikecliffs site and read the functioning of all the circuits on CV carbs and youll see.

The only reason you use a Colortune plug is to adjust the idle mixture. It's the same process regardless of whether the carb is CV or VM style.

But you are correct in that if the pilot jets are way off the idle mixture screws will not be able to compensate.

 

[Steve]

True, the Colortune only works on the idle mixture, but most of us here in the US that have tried it on the CV-type carbs have never seen the color change.

The first time I used a Colortune, it was on a set of VM carbs. I borrowed the device, took it home. Used it on another set of VM carbs, it worked perfectly. Then I used it on my wife's bike ('82 850L with CV-type carbs). Saw the same color flame become a bit intermittant, then finally die out as I turned the mixture screw in. As I turned the screw out, the flame started sputtering to life then became steady, but never changed color.

.

 

[chuck hahn]

Actaully Hillsy, you adjust the fuel air mixture at idle and then again around 4,000 RPMs on VMs if the color changes at the higher RPMs f

Secondly, you dont need to fiddle with the bottom PILOT screws ( they are NOT mixture screws as the DO NOT meter a MIXTURE of anything). You set them and walk way and DO NOT have to readjust anything. The common theme from hundreds and hundreds of members is that 3/4 out is the factory setting and 7/8 is about optimal because the factory setting was on the lean side to satisfy the EPA regulations of the day.

Third. you tune the fuel to air mixture EXTERNALLY on VM carbs with the side mixture screws ( commonly callled AIR screws because they meter the air into the fuel externally) where as the CV carbs do it INTERNALLY and AUTOMATICALLY.

These are facts and you are fundamentally wrong.

 

[chuck hahn]

And Steve you are 100% correct as I have done and observed the same things.

 

[Grimly]

Also bear in mind the Colortune was designed to work with SU carbs on relatively low bhp / litre cars, where the factory-installed main jet and needle were a known quantity and pretty much fixed for that car. Once you set the idle mixture, the fuel/air ratio should be right all the rest of the way up (until wear creeps in).
Of course, air leaks, fuel leaks, jet / needle substitution, spring changing, needle height changing, and all bets are off as to which way it will go from idle. Otoh, it's a useful diagnostic aid in its own right.
For a known quantity engine - totally stock, or one you're already familiar with, the Colortune is a great tool, but a modern A/F ratio wideband gauge would be much better for telling you exactly what's happening.

Fwiw, the Colortune was next to useless on the CV carbs of my '80 GS, but was handy on the '79.
It is useful on the SU converted '80, and does a great job on the XJ900.

 

[Guest]

Sorry to bump this thread, but for the sake of anyone having similar problems as well as to give credit to a great suggestion here is an update:

First took off the float bowls and soaked+sprayed the main jets, and anything I could access on the bottom of the carburetor got a good shot of carb cleaner.
No change.

Then I inspected the fuel line and found a very subtle pinching turn. Replaced the fuel line, routed it better and the power came back! Smooth throttle response again. Top winning suggestion was given by bonanzadave and chuck hahn.

The backfiring and lean looking plugs were still there though...

so I inspected the previously sealed airbox and found a few sources of air leaks. I resealed the whole thing, and reseated the intake boots (which are new in the last year, still very "rubbery" and no doubt form a good seal). All air leaks plugged. A very subtle change in the spark plug color, just a very faint, almost unnoticeable tan dusting tells me it's richer than the pure white it was before, but still too lean. There's also still popping upon deceleration/engine braking, though much less than before I sealed the airbox. I still think it's possible that my carbs were unknowingly tuned to compensate for the extra fuel coming from a leaking petcock, so when I finally replaced it these problems logically began. I think I need to adjust some screw on the CV carbs, but I'm not sure what, or how much I need to adjust. If I cannot figure it out with the search function, I will post something in the carb/fuel/exhaust forum, as I think that's where this might have belonged in the first place.

Thank you to all who helped!