81 xs650 difficult tuning

[TDodge7]

I take it back , the guy who owns this bike just came by and we were hanging out , I let him know about the current situation . He decided to run down the street high speed so I could listen . He took it up to 6 or 7k and let off hard , it started popping out the exhaust like crazy on both pipes , the right one being the worst . All the other times I've tried it I've been keeping the speed low and just putting around the neighborhood so I guess I never got it up fast enough to pop like crazy on decel . But how could the optimum mixture not be the optimum mixture ?


I should add that it is , and always has been popping in the pipes , not through the carbs .

 

[5twins]

If you can't cure the popping with the mix screws, going up a size on the pilots is the usual next step.

 

[TDodge7]

So I may need to go even richer ? I was under the impression that since I can adjust it to a point where it's too lean and too rich that the correct mixture should be between them somewhere right ? If the jet is incorrect is it possible for it to skip over the correct mixture and only go too lean or too rich ? It's already one size up from stock , and the mixture screws are currently adjusted to the highest rpm/ blue between rich and lean on the color tune (3 turns out each) .

 

[5twins]

You can achieve the proper mixture according to your tool at idle with the wrong pilots. What you're not getting is the proper mixture during decel. Remember those other 3 holes that draw directly from the pilot. Overall, your mix isn't rich enough yet to supply the engine's needs during decel. You will be able to re-tune for the proper idle mix with the larger pilot (general rule of thumb is about 1/4 less turn out for each size increase) but what it gives you is more flow overall through the 4 holes combined. This is what you need to fix that popping. Decel puts a heavier demand on the pilot circuit than just sitting there idling.

If the 47.5 pilot proves to be too rich, you can fine tune and get to a setting somewhere in between the 45 and 47.5 by changing the air jet. Put the 45 back in and drop the air jet 2 or 3.

 

[TDodge7]

That makes sense , I'll tell my friend to go ahead and order the richer jets ,Thanks .

 

[xjwmx]

With the larger jet you'll turn the mix screw in to compensate so that you get the same mix at idle as you have now. On decel, where the problem you have is, the throttle plate will be shut and everything will be the same as it is at idle. Therefore where's the benefit?

 

[TDodge7]

But aren't the 3 holes on the intake side of the throttle plate where they're still able to deliver fuel even with the throttle closed ? The way I understand it now is that those holes are only metered by the pilot jet , if I richen the pilot then those holes will put out more fuel which should get rid of the pops . I would of course have to readjust the mixture screws for idle but the mixture screws don't control those 3 holes so it would just be a matter of finding the correct mixture again. Correct me if I'm wrong , I'm just trying to wrap my mind around the whole thing , I had completely forgotten about those 3 holes ....by the way , is there a more technical term for them , makes me feel like I'm talking about that part thingie with all the gizmos and do dads .

 

[5twins]

The labeled carb drawing I put in the Carb Guide calls them by-pass holes .....

http://imageshack.us/photo/my-images/119/bs38mainsystemsvs5.png/

The passageway from the pilot jet feeds them and the mix screw. It gets to them 1st so they are unmetered. The only way to change how much they flow is by changing the pilot jet size.

When the butterfly closes, it doesn't close completely. Your idle speed adjustment screw holds it open slightly. That's how idle speed is set and controlled. The butterfly may restrict some of the flow from the 3 holes but it doesn't block it completely. They still flow when you close the throttle.

 

[xjwmx]

TDodge7, the way I understand it is regardless of the location of the holes, the state of everything at idle is the same as the state at decel. If it isn't richer at idle, it won't be richer at decel.

So if richer is the solution, you'll need to idle richer. Maybe do an experiment where you go out a turn using the jet you have, to see if richer helps.

 

[TDodge7]

I tried running the bike at 4 turns out which was pretty much the same as 3.5 turns out , so I bumped it to 4.5 turns out which was as far as I thought I should go , didn't want the screw coming out . At 4.5 turns it doesn't pop MUCH but I still hear it a tiny bit every once in a while . However at 4.5 turns out it seems to idle badly , the idle hunts , it's a lot shakier at idle and it just doesn't sound all that smooth .

 

[xjwmx]

Well, the experiment says there's a good chance richer will help you. You got improvement when you went richer. At the number of turns you went out, the screw might have been out of its normal range, causing some of the badness you saw. So try the larger pilot, which might let you get the screw back in range.

With mine, all stock, I resynched the carbs with a gauge, and it caused it to pop badly. I put the screw back where it was to start with, maybe 1/8 turn from where I had it, and the popping went away, with no symptoms I could see of being out of sync.

 

[emzdogz]

a. this is a great thread. Hope this bike can get sorted soon.

b. any chance this bike has non-stock cams? that would make it idle lumpier?
(hope that's not way too dumb a question)

c. a general question that may help others (plus ME) who may be dealing with mixture screws, and who search this thread.

When reassembling (re-inserting) a new o-ring, washer, spring and mixture screw, is it ok to put them all in together? i.e. put spring, washer and o-ring on tip of screw and stick it all in that way together. I'm doing this with carbs ON the bike, btw. I was able to remove them this way.

OR, must the o-ring go in first, and make sure somehow that it seats level against bottom of bore, then washer, then spring plus screw?

If I put them in together, would it hurt anything to use a tiny bit of grease to hold the o-ring and washer onto end of shaft of screw?

thx

 

[xjwmx]

adjusted float height to factory spec

I would be suspicious of that step, if the factory procedure was used to do it. Best to use a ruler I think instead of the clear tube.

Em, I'll bet it doesn't matter how it gets in there as long as it does I think the spring is strong enough to seat it. But I'm sure I'd push on it with a mini screwdriver to guarantee it was seated.

 

[5twins]

I stick all the parts on the mix screw then put the assembly in the hole. I put some sort of light lube on it like WD40. I don't like assembling any of the carb parts dry, especially o-rings.

 

[emzdogz]

thx, for assembly info.

 

[TDodge7]

My friend is out of town until Thursday so he hasn't ordered the new pilot jets yet . So since today is going to be the only non rainy day for about a week I decided to try messing around with what I already have here . I put the non epa mixture screws back in to see if that made a difference since now the o-rings/washers/springs are in the right order . They must be a good bit richer than the original screws because the popping was gone and the bike was idling like it was pig rich , that was of course just to warm it up so I could do the actual mixture adjustment .


After a ride around the neighborhood to warm it up I started playing with the mixture screws , using the dead cylinder method . I found that anything more than 1.5 turns out and it was so rich it would stumble and konk out . Anything less than 1 and it would start popping and hanging up after a rev so I settled on 1.25 turns out on both which I feel is unacceptable , maybe if I was still running the original pilots (or even something leaner) they would work well . 1.25 turns out may be the spot where it runs best with non epa screws and 45 pilots but it still doesn't run all that great , while the popping has stopped it has new bad habbits , when I come to a stop sometimes the idle drops below 1300 and then comes back up , yet when sitting if you rev it sometimes the idle will hang for a second or two .

Hanging idle usually means lean , though since it's just barely hanging I suppose it could also just mean that these mixture screws are too rich and there's just enough extra fuel returning to idle that it takes a second to burn it all off and return to normal idle , is that possible ? All in all I think it's best to run the epa screws as those give a much wider range of adjustment . So I'm thinking once my friend gets back we'll probably go with the 47.5 pilots and the epa screws and see how it does . If that doesn't do it then I'm finding the best of what I've got and calling it a day , I want this bike out of my shop and he wants to ride .

 

[5twins]

I buy most of my mains and pilots locally. These are just Mikuni jets, nothing special, and many local shops carry them. I've found several shops in my area that have large assortments on hand and sell at the Mikuni retail ($3-$4 per jet). I prefer to use genuine Mikuni jets instead of the knock-offs, especially since the price is pretty much the same. If you do go shopping locally for pilots, just make sure you get the correct BS30/96 style used in the BS34s, not the VM22/210 type. At some of the shops I've gone to, the parts guy doesn't even know the difference so it's pretty much up to you to make sure it's the right jet.