Tuning help needed

devildrums

XS650 Enthusiast
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Hey guys, could definitely use a little assistance with my 77D. I recently had to change out my carbs and went with VM34s, and as many different people i have seen with similar posts, kind of expected it to come nearly ready to roll when I ordered them for a specific year bike. After finding out how wrong i was, I have gone through all sorts of jet changes and adjustments and am looking for a little fine tuning advice.

Currently running:
22.5 - Pilot
6F9 - Jet needle, is in #4 groove
P-6 - Needle jet
2.0 - Throttle slide
180 - Main.
Had a 185 main and it started out smooth, but it was spitting at 3500" rpms, so i switched out to 180. That seemed to fix that issue, but then developed a backfire out of the right carb. I tried tweaking the air screw a little, since it was the most basic, but had no success. During this transition I have done many different jet changes and can't seem to get everything to run right.

Any advice on what direction to go in would be greatly appreciated, as it seems I keep fixing one problem while creating a new one. Thanks.
 
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Where did you get em from?

195 main
6f9 needle
35 pilot
2.5 slide
159 p6 needle jet

1-3/4 turn out on the air screw
2nd step down from the top on my needle.

Big bore kit with open pipes and pods. Runs very nicely.

I’d guess you’re probably a little lean on the low end. Might settle around 130,

I think there is thread where people have posted their set ups.

Search the archives.
 
Where did you get em from?

195 main
6f9 needle
35 pilot
2.5 slide
159 p6 needle jet

1-3/4 turn out on the air screw
2nd step down from the top on my needle.

Big bore kit with open pipes and pods. Runs very nicely.

I’d guess you’re probably a little lean on the low end. Might settle around 130,

I think there is thread where people have posted their set ups.

Search the archives.
Got them from Niche supply in Florida. Kind of used my friends bike as a starter reference, and been trying to dial mine in. Trying to stop causing another problem as i fix one, figured i needed a little more knowledge on what piece is best to address first. I will look for some setup threads, thank you.
 
I tend to start with idle / take off to 1/4/ half throttle

Then wide open.

Is your backfire on deceleration? You give it throttle and it coughs/backfires?

Also, if you didn’t do a good cleaning straight outta the box it’s worth it to do it. Also check the float levels.
 
I tend to start with idle / take off to 1/4/ half throttle

Then wide open.

Is your backfire on deceleration? You give it throttle and it coughs/backfires?

Also, if you didn’t do a good cleaning straight outta the box it’s worth it to do it. Also check the float levels.
No, thats the thing, my adjustment seemed to make the throttle process smooth out, the trouble is mostly when i sit idling, that's when it starts to backfire out of the right carb. And the posts i read say that is usually a lean problem, but today i took out my plugs and they were black, which is supposed to be a rich problem. There has to be a sweet spot somewhere, i just don't have any idea where.
 
Are you using oil?
If griz was here he'd be hammering; ignition, compression, issues cause most "carb issues".
What's been done to the engine? sloppy valves can cause "carb" problems.
Well, when we first changed out the carbs we checked everything else and it came back good. Each issue i have had to deal with comes from changing one of the series of jets, which is why it is so confusing. Wouldn't ignition/compression/valves problems remain consistent regardless of jet change?
 
Whilst it is always satisfying to diagnose and fix a problem this has to be offset by the time, cost and frustration that is involved.
Unless you are a carb guru much of what we do is based on gut feel, guess work or what the tea leaves tell us.
IMO it is far better to base any changes on factual data and to that end I'd always get a dyno base line first.
The initial cost will soon be offset by the time saved and reduction in hair pulled out. :eek: :)
 
Normally for 650 use, guys run these carbs with the air jets removed. If yours are still in place, you might try that. Also, 4th slot from the top is quite rich for the needles. That could be contributing to the black plugs.
I had read the plug problem was most likely rich, but at the same time the backfire through the carbs is supposed to be lean. That is why i was hoping there might be some sort of middle ground solution i was missing.
 
If you started the bike with the choke, that will blacken the plugs right quick. It takes a few miles of running down the road to burn them clean again. For tuning, it's best to have several sets of plugs. Start the bike with the choke and warm it up to the point that it will re-start without the choke. Shut it down and swap in a clean set of plugs, re-start without the choke.

Like I said, the black plugs you're seeing may be misleading if you started the bike with the choke on. I think what I'd do is remove the air jets and move the needles back to their middle (#3) slot. Both of these steps will lean things out. Then I'd increase the pilot a size or two (25 or 27.5). Your 2.0 slide is also richer than the 2.5 WideAWAKE is running. If you can't sort the carbs with just jetting changes, you may need to change the slides as well.

I'll also mention that your mix screw is a true air screw. Opening it up lets more air in and leans the idle mix. I would start the screws out at about one turn out.
 
If you started the bike with the choke, that will blacken the plugs right quick. It takes a few miles of running down the road to burn them clean again. For tuning, it's best to have several sets of plugs. Start the bike with the choke and warm it up to the point that it will re-start without the choke. Shut it down and swap in a clean set of plugs, re-start without the choke.

Like I said, the black plugs you're seeing may be misleading if you started the bike with the choke on. I think what I'd do is remove the air jets and move the needles back to their middle (#3) slot. Both of these steps will lean things out. Then I'd increase the pilot a size or two (25 or 27.5). Your 2.0 slide is also richer than the 2.5 WideAWAKE is running. If you can't sort the carbs with just jetting changes, you may need to change the slides as well.

I'll also mention that your mix screw is a true air screw. Opening it up lets more air in and leans the idle mix. I would start the screws out at about one turn out.
Thanks 5twins, i will try the plug change and air screw removal tomorrow. In the meantime, my biggest concern is trying to sort out the cause of the backfire through the left carb.
After i tried several air screw adjustments to no avail, i changed out to a 20 pilot to cut down on fuel going into it, figured the less going in will stop the not burning before going through (please pardon my wording) and causing the carb problem. Took a ride and performed well through gear changes, but still carb backfire/miss. Could that be a timing problem instead of fuel issue?

And side note, needle was on already on #3, must have forgot to write it down. So Pilot-20, Needle -3, Main - 180. Air screw was at 1 turn and 3/4 turn, both with the same result.

Thanks again everyone
 
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