Yet another carb thread!

[Nine_Iron]

Sorry guys I have searched all over and can't find a problem similar to mine. I have a 75 XS650B......all stock except Pamco High Output Ignition, pod filters and main jets that are one size larger than stock. All the rest is stock. My carbs are synced perfectly, floats are both 24mm from flange, and both carbs have all new parts....jets, gaskets,needles,etc. Both carbs seem to be 100% equal on both sides and totally cleaned. Seem to be!

Anyway, the bike fires up great, with responsive throttle. However, the right side idles somewhat sporadic and back fires under acceleration. The left side runs very very smooth and does not back fire. Funny thing is is the right side with the back fires runs about 30 degrees cooler than the left. The left side seems to be getting way hot....270 degrees or so. The left side is turning my pipe blue. With my carbs set completely equal I can't seem to figure out why such a huge difference in the way the two carbs operate.

I have noticed the left side carb with the choke lever has an extra jet inside the bowl on the front left of the carb hanging down. The right carb has a hole there but no jet hanging down. Not sure what the affect is of this jet or if it is from the choke being on that side or what.

Any ideas guys???

 

[retiredgentleman]

You didn't mention anything about the mixture screw setting.

I would recommend the "dead cylinder" method to adjust the mixture screws.

 

[pa23driver]

could also check for an airleak - mixture screw setting is more likely but it's an easy check.

 

[Nine_Iron]

Not sure that I have a mixture screw....there's the idle pilot screw. It's set at 3/4 turn to 1 full turn. That's where my Haynes book says is factory. I even have the needle clip set on the richest setting.

I let it run a little last night while spraying carb cleaner all around the carb mounts in front for the carbs with no change in idle. I even tried starter fluid with no change.

It feels as if my bike should be running rich yet it back fires on the right cylinder as if it were lean.

 

[littlebill31]

The long brass overflow tube? Ya, you need one in each carb.

Which bowl type do you have? It should be the one on the right in this pic.


I ask 'cuz the later VM type will fit, but as you notice the 78-79 VM22/210 (left in above pic) does not have the tube, the 76-77 type does (below pic).

Yours came with the BS30/96, but may have been switched, which can cause problems with two different type bowls.

 

[gggGary]

The 75 carbs only have one choke that is the choke (enricher) pick up tube you see on the LH carb. You did not mention the magic word synched. the two cable carb sets can be tough to get even, Yamaha went to linked throttle shafts the next year. Backfiring and one hot cylinder seem to point to synching issues. The homemade manomometer seems like a good idea if you don't have a vacuum gauge set.
Cam chain adjusted, valves set, compression test results?

 

[Nine_Iron]

I have the 30/96 type. Both bowls have the overflow tube which isn't the one I was talking about. The tube/jet I'm talking about hangs from the carb itself. It looks similar to the needle jet but it hangs down into that small channel in the front/top of the carb bowls you posted.

 

[littlebill31]

Well at least there is a little bowl type discription.

 

[Nine_Iron]

ok....that's what I thought that tube was, thanks!

I've checked the carbs and both butterflies sit even at idle position. Both idle screws are the same adjusted position and when I twist the throttle both travel at the same time.

I did notice last night when it idles for a few minutes and I twist the throttle it is a little sluggish then revs up real fast. Rich?

 

[gggGary]

No you HAVE to synch with a gauge, do that and see what happens.
My bet is all your problems will disappear, what about the other maintenance and test items I asked about?
How to make a synch gauge, http://www.xs650.com/forum/showthread.php?t=2317
If you don't have vacuum tube ports on your manifolds, the carbs a have screw that covers a vacuum port. You can use locally available metric zerk fittings with the ball drilled out to make adapters. Total investment about $10.00 A smooth running bike you tuned yourself?? Priceless.

 

[Nine_Iron]

I am in the process of adding ports to my carb mounts so I can sync my carbs.

I am just curious though.....my carbs have solid brass unremovable plugs where the mixture screw is on later models. Is that normal?

And does anybody know any links to videos of these bikes running lean and running rich? To see the comparison.

Thanks again fellas!

 

[littlebill31]

Plugs tell the tale. On the road you would get it going full throttle, then without decelerating, shut the engine off, pull the plugs, and check the color. I couldn't ride it once, so I held that sucker wide open for about a 2 minutes and hit the kill switch. Seemed to work just like the road test. The neighbors thought I was crazy, but they're nimrods anyway.

 

[gggGary]

scratch this Early BS38s

 

[pa23driver]

i ordered the motion pro 6 mm adapters for my carb synch gauge a week ago and still haven't received them so i got impatient and tried the zerk fitting in the vacuum ports today, worked great...

i was having a lot of the same issues as the OP, then i synched the carbs with a gauge and what d'ya know, problem disappeard.

 

[Tech7]

Don't forget to check all basics. Compression, spark, valve adjustment. Didn't see that anywhere in this thread.

 

[grizld1]

Amen, Tech7! Ya can't fix the ignition, etc. through the carbies! And defects in other areas will cause misleading signals when you try to make carb adjustments; things have to be done in order.

 

[Nine_Iron]

lol There's so many paths you can take for these bikes running rough. I've been working on this bike for about a year now and have pretty much everything in original spec. Timing is with a timing light to where it should be, valves are all adjusted with feeler gauges, and cam chain tension is set correctly. The Pamco ignition is awesome, I'm getting plenty of spark. I'm not sure on my compression though. I ran a compression test with just the starter and battery turning over the engine and I topped out at about 125 on each cylinder. Seems kind of low?

I've installed some zerk fittings and am getting some clear hose today for my sync test. We'll see what that brings. I'm definitely no pro on working on these bikes but have a good feel of how everything should be set. I really do think I've got it down to fine tuning the carbs and I'll be good. I'm just having a hard time figuring out if I'm lean or rich. Hoping the sync will help me figure that out.


Thanks again fellas!

 

[retiredgentleman]

The sync will not tell you if you are rich or lean. The sync is only to adjust both carbs so that they are each drawing the same amount of air/fuel mixture.

To achieve the right air/fuel mixture, you should start out with the "dead cylinder" method to adjust the mixture screws (pilot screws). The stock or factory position for the mixture screw is only a starting point...............each carb may have a different screw position.

 

[5twins]

Compression testing a cold engine usually doesn't give the best results, warm it up first and you should get better (higher) numbers.

You say you measured your floats from "the flange". Was that the lip that hangs down around the outside of the carb body or the actual bowl gasket surface? They should be measured from the bowl gasket surface. If you measured from that lip, they could be a couple MM too low or lean. You should also measure each float twice - measure both sides or both "bulbs". It's not uncommon for the mount between the floats to be twisted or tweaked, resulting in one side sitting higher or lower than the other. Measuring both sides is the only way to catch this.

 

[Nine_Iron]

I've seen so many threads about the float level. my haynes book looks as if you measure from the surface UNDER the gasket and it says 25mm +/-1 or 2. I've seen some on here say as low as 22mm. Mine are 24mm exactly on both sides of the float from the surface under the gasket. I hope that's good? And that's just resting on the spring loaded pin....not depressing it at all.