Can't quite get it to run right, '75 XS650


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Mosinee, WI
I bought this bike in Feb and now that it's nice out I'd like to get some riding in.

So here's where I'm at. Previous owner said he 'went through the carbs' whatever that means. It didn't run particularly good and was hard to start when I first got it. It also pissed fuel all over usually from one of the bowl overflows.

I took the carbs out, opened them up and checked things out myself. I'm fairly new to carbs, was raised on fuel injection. The jets seemed clean visually, poked around with a needle to make sure, and flowed air pretty freely when I blew through them with my mouth (lol). Nothing appeared amiss. Adjusted the floats to avoid fuel pissing issues. Choke valve, the slides, and the throttle plates all seem to move freely.

I reassembled and went to tune. Kinda difficult to start. The idle mix screws we out something like 2.5 turns on the carbs, and I turned them in all the way, then out about 3/4 on both and it seemed easier to start, and idled decent. I adjusted idle on the idle adjust screws (one per carb on the '75, no sync screw) and had it idling around 1200 rpm.

At that point the bike rode.. OK. Once you got going it ran good, wasn't backfiring and sputtering, etc. Howver taking off was always trouble. Somewhere between starting out and getting to 'cruising' speed in 1st gear there's a "dip" in power where the bike wants to choke off. It's bad enough that if I try to ride like I'm used to (I had a '78 XS750) it just chokes and dies, like I'm not giving it enough gas.
On the flip slide, to take off properly I either have to really slip the clutch (feel almost like trying to take off in 2nd gear on other bikes) or really give it quite a bit of throttle, or a mix of both. If I feel it trying to choke a lot of times I can give it more gas and keep it going but I need to be careful with the clutch at that point that I don't just launch the bike and lose it or something.

Today I went to try and get to the bottom of it and didn't really get anywhere.
Here's stuff I checked.
-Charging voltage. At idle, lights on is about 12.3V. Pretty weak but the bike should still run on that. At 3000rpm its about 12.8V or so. So I'll need to deal with that, but I'm not convinced that's what's causing my issues.
-Compression is about 130 per cylinder after around 2-3 kicks. Not ideal, and I'm fairly certain the head's leaking a little bit (damp colored where the head meets the block). I'm iffy on if this is causing any detriment. This was on a hot engine.
-Right exhaust feels "stronger" than the left, slightly. Something like Pa Pa Pa vs pu pu pu if that makes sense.
-I can't get the bike to run on one cylinder. I was going to try the dead cylinder syncing process and even around 1200 rpm one cylinder alone won't run the bike, it'll stall.

I tried the idle mix screws all over the place so far. I tried the 3/4 turn out that sounds like the 'starting' point. That's where I've been running it mostly.. where it runs.. so so.
I tried 3 turns out what a website said was the "sweet spot" for the XS650. Exhaust smelled pretty rich and wouldn't idle very well. I've been experimenting all over in the middle.

I've also been trying to balance the idle adjust screw on the carbs. I had them both at the same 'distance open' via measurment, I've tried opening up the carb slightly for the side where the exhaust felt a little weaker.

In the end I ended up with a bike that would idle OK ish around 1200 rpms, but still choked off sometimes, still didn't like to take off, and if I tried to open the idle screw even slightly more (1/4 turn) it'd hang around 3000 rpms and come down very slowly. Never got both exhausts feeling the same and the engine has a noticeable stumble from time to time as it idles, where it skips a beat here and there, and then of course sometimes just dies. If I let it drop down to 1000rpm idle or so it barely wants to stay, don't like that much at all.

So, I'm at a loss for the time being. I saw a few things that seemed to point to idle jet clogged, but since I a carb novice not totally sure what to be looking for in the carb (or otherwise) to verify it'd flowing properly.
Sorry to be long winded, wanted to give as much info as I could. Any thoughts?
Yes, it does sound as if your idle circuit is partially plugged. It could be the jet or the passageways the jet feeds it's fuel/air mix through into the main body. The pilot jet is mounted in the top of the float bowl so the bowl will have to come off to access it. Your carbs would use the BS30/96 type jet, as shown on the right below. The jet is the little brass thing with the screwdriver slot in it and screws out of the bowl .....

You'll need to remove it and insure that it's clean/clear. There's a tiny hole down through the center. Hold it up to a good light source and see if you can see light through it. If not, you'll need to blow it clear with compressed air, maybe use some carb cleaner too. The pilot jet pulls it's fuel supply through a passageway in the the side of the larger hole below it in the pic. If you blow compressed air into the pilot jet hole after it's been removed, it should come out that side hole in the larger hole.

Now, that just covers the jet and where it gets it's fuel supply from. Now we have to check the passageways it feeds through into the main body. On the bottom of the carb body, near the front edge, you'll see a small hole. This is where the pilot jet fuel mix goes in .....

From there it exits out 4 tiny holes into the main bore around the butterfly plate. 3 are clustered together in a group at the top, the 4th is off to the side. That 4th hole comes from your mix screw. Fuel comes in the bottom of the carb and goes directly out those 3 holes. It goes to the mix screw then out the 4th. You can adjust the flow on that one but not on the others .....

So, using compressed air again, and maybe some carb cleaner, you'll need to blow in the bottom hole and watch to see if it comes out the 4 little holes. You'll need to remove the mix screw for this then hold a finger over the mix screw hole as you blow the air through. You can also "back flush" the circuit. Blow air and cleaner in the mix scew hole while blocking the 4 holes in the bore and it will come out the feed hole in the bottom of the carb. By working back and forth like this, you can usually clear the passageways.
Awesome info, it helps a lot to know which passages I'm looking at and how the air/fuel circuit it working in general. I'll pull the carbs again and do some more cleaning. Appreciate the detailed pictures.

As far as the idle mixture screw.. I've read that I should hear some form of change in how the engine is running when adjusting it? I can go from all the way closed to 2.5 turns out on the screw when I have it idling ok and I don't hear any change. No change in the exhaust note, no rpm fluctuation, nothing. Is this something that would be consistent with needing to clean/clear the idle circuit?

After some tuning/adjustment last night I think I finally got the thing idling ok and fairly even between cylinders, but now I'm going to go and clean stuff and be back at square one, LOL! No big deal.
Yes, no response from adjusting the mix screw could indicate passageways are plugged or partially plugged. It could also indicate incorrect float levels. Usually, when everything's clean/set right, you can kneel next to the cylinder you're adjusting and easily hear it begin to run poorly as you go too far past the ideal setting in either direction. Your '75 carbs are pre-E.P.A. mandated so mix screw adjustments should really show. Later pollution mandated carbs don't show the adjustment effects as readily.
This has been very helpful. Today I opened the carbs up again for cleaning and with that diagram I see exactly where the fuel and air are flowing and that helps to know where to spray cleaner and then blast it with air.
I wanted to pull the jets to really make sure they were clean but they are stuck pretty well. It loos like someone tried, because a screwdriver just doesn't 'grab' in the slot very well, it's a little marred and thus seems impossible to remove them! But I'm fairly confident they're clean.. when spraying through with the idle mix closed the cleaner only comes out of the 3 holes, open the idle mix and it comes out the 4th, and when pushing through with compressed air they make a nice little spray pattern.

I did notice that the air hole for the idle circuit looks like it was drilled.. Is it like this from the factory or has someone opened this up larger?

Anywho, I kept going and went to set the float heights and noticed that one of the floats seemed to push down on the spring pin "harder" than the other. I thought the spring was shot, but noticed that the float also had been wet basically all day. I took it off and shook it and sure enough, it's got gas inside it!

So maybe I'm on the road to figuring this out. Going to work on rectifying that issue now.
From what I've read the '75 looks like it needs 25mm +/-1mm for the adjustment? Mine were between 26-27mm or so, so out of spec to begin with. This definitely explains why one of the carbs would leak out the overflow from time to time I think!

Here's what I'm working with so far;
I'm not sure if someone drilled out that air passageway or not, but it shouldn't matter. The amount of air going through it is determined by an air jet that is back in the main carb body. I think you found your problem, that bad float. Replace that, set the levels right, and you should be good to go.
Alrighty, new float is in and I set them to about 25mm according to the ruler (no float gauge sadly).

With choke, idle needles set to 3/4 out, the bike started right up and ran great, let the choke out, was sounding good. I adjusted idle set screws until exhausts felt "even" and it seemed to be idling pretty well about 1500rpm.
I went for a couple laps around the neighborhood and the boggy feel when taking off from a stop sign seemed much better.
I stopped to adjust idle and didn't have much luck, and the performance ranked from there. It seemed very "heat" related, like when the engine heats to full operating temp my idle goes to hell. From left cyl I hear a steady thump and when idle gets crummy right cyl exhaust you can hear misses.

Also, when it began to run poorly when I would give it throttle it would pop and backfire sometimes give a little poomf out the carb. Left cyl worked well throughout.

It got dark and I hung it up for the night. Left plug was black but not sooty currently, right plug was black with the electrode tan.

Wondering if I have a coil failing as it reaches operating temp, may try swapping those side to side.

For the gurus, would float height play a part in this? My on throttle response is still excellent and the bike rips when it's above 2000rpm or so.

One other thing I noticed, it seems to "hang" if I rev it and the idle adjust set screws have the plates open even a tiny crack basically. Normal or does this imply lean conditions?

Ah! And when it's idling poorly if I even barely touch the choke it dies.

Progress at least :)
The idle becoming worse after the bike warms up is really typical from what I hear. A lot of us set the idle screw after a good run so that the idle is set at operating temperatures, even more stick an hand down there and adjust the idle at stop lights here at there. Give that a try, it doesn't explain the black plugs and popping though.

Popping is usually from an exhaust leak or lean condition. Backfiring is usually a timing problem, timing problems can also lead to lean conditions.

You know what they say "It always the carbs, unless it's electrical"
The float spec for the '75 carbs is 24mm so @ 25, you're set a little lean. The floats on these are often tweaked or bent on the mounts, one side sitting higher or lower than the other. For that reason, you must measure both sides or float "bulbs" to make sure they're the same. If they're not, you can remove the float and gently twist it to straighten it out.

Using a ruler is fine. A 6" steel rule with MM markings on one side works best. Be sure to measure off the gasket surface on the carb body, not the outer lip or the gasket itself. Set the ruler on the far side and sight over the tops of both float "bulbs". Make sure the tops are in line or your reading will be off .....

Stick the floats in a container of gas, weigh them down into the gas for a few hours, take them out and shake them to see if they may still be leaking.
I was measuring off the lip, not the gasket surface. So there's a problem, lol. I'll verify the float itself isn't warped, as well.
Going to grab some new plugs just for the heck of it, too. Previous owner said they were new, but it'll be cheap insurance. It'll give me a nice base line to see what's going on in the engine since I'll know if they're black it's not from previous tuning issues, etc.

Might be time to learn how to check/adjust timing and valves just to 'make sure' as well.
Of course, if you want the bike to run as well as it should/could, everything needs to be set correctly.

If you were measuring off the lip on the carb body then your current setting is lean. It's probably closer to 27mm than 25. And like I said, even 25 would be lean for your set. Since you're measuring/setting the float levels with the carbs upside down, a larger measurement translates into a lower fuel level when the carbs are flipped right side up.
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