chasing gremlins - engine/carb/??? issues

Yes, with engine warm. A back fire through the carb would be a timing or intake valve issue.
 
A lean mixture can also cause a back fire. But it has to be real lean to get the plug or carbon deposits glowing and ignite the charge during the intake stroke.

All engine tuning should be done with the engine at full operating temp - the oil has to be hot! This takes about a 20 min ride that is fairly aggressive.
 
OK

I adjusted the valves first thing. Intakes were a bit loose, maybe .005 or a little more. Set them at .003. Exhaust was at .007 or so, reset to .006.

Started and rode until warm. As it warmed up ran rougher and rougher. My guess is expanding intake valves and carbs still set at 2 turns out.

Went home and set up for tuning. First thing was go another 1/2 turn. Started up, much better. 1/4 turn more (so now at 2 3/4 out) running strong, no misfire.

Hooked up manometer. Left is pulling harder than right. Synced everything up, running awesome.

Only thing I will do need to get another pilot air (idle) screw for the left carb as it is stripped out - crud - and my gut says if I could get to 3 turns I think I'd be even better (have a feeling tomorrow it'll hesitate just a bit on the longer portion of my ride when it's warm). No complaints though, 100% better than it was last week!

Thanks so very much h for the help. Now I can start the fun part of making it look cool...
 
On these carbs, it is NOT a Pilot Air screw, it is a Pilot Mixture screw. A subtle but important difference.

I'd suggest replacing both screws and their o-rings. They are inexpensive and having a matched pair is good. Once you have the new screws installed, be sure to adjust the idle mixture correctly. Most people find the lean drop method works well. Adjust pilot screws for strongest idle, then slowly lean the mixture (turn in) until the idle speed just starts to drop, turn a 1/4 turn out. Sync carbs and enjoy!
 
You are correct of course, it's fuel only - I've been doing that all weekend, saying pilot air... bad habit that stops now. Thanks...

Read my mind on the replacement of both screws, and when this is all done I certainly will enjoy! Spring is creeping in... :bike:

Thanks!
 
Closer, but not quite right! The Pilot screws are MIXTURE screws. They control the flow of a mixture (blend) of air and fuel. These terms can get so confusing, but they are so important to understand.

Because they are mixture screws, they can be very precise within their adjustment range (approx 1 - 4 turns out). If you find that you are above or below this range, the Pilot Jets will need to be changed.
 
OK so it's been running well the past week. Couple more gremlins have arrived though, and I think they brought luggage...:wink2:

1) I started it yesterday after replacing the clutch push rod seal to check for leaks (good news, no leak at push rod seal - bad news, that is what I suspected was leaking, and I still have a leak :banghead:)

Anyway, I started it and let it run for 3-5 min - I did not ride it, it was just idling. Then shut her off and garaged her. Well a few hours later I come down and there a gas spill. Tracked it down and it came out of my left air cleaner. I suspect that the larger pilot jet and freshly cleaned carbs are shooting gas into the air box when cold. I haven’t had this problem because every time I've started it I've ridden it. NOTE: I do NOT have pods on, I have stock new filters. The left one had gas in it, and so that cylinder didn’t fire until the air box was dryer. But this morning, it ran pretty crappy - suspect they are both a bit saturated with gas, which could be richening the mixture. ugh...:mad:

I don’t know if I should be changing to pods or not given the larger pilot jets. Not really wanting to, but whatever. Comments on this one.

2) Couple days ago I noticed my blinkers weren’t working all that well, and when I put the brake on they stopped. I figured I would look when I got home. Forgot of course... well when I was fussing with the stuff above, I noticed it again, and also noticed when I give it throttle lights got brighter and blinkers worked fine. Great, now I get to chase an electrical gremlin - my favorite kind. Charged the battery and bling! Lights and everything are great.

So, is this the stator giving me grief? Battery is 6 months old, and topped off with electrolyte...

Thanks
 
If you are getting fuel out of the air filter it could be that you set your floats wrong, or you left the petcock on or in prime if its still vacuum seal (if its still vac and was left on, you may want to check to make sure it isn't leaking fuel when the bike is off).

Re-jetting carbs is basically a game of best fit. You have to run the bike a while, notice problems, re-jet to a different size, run the bike a while and repeat. You can't just go up a couple sizes and call it quits, because you may need to be higher or lower for various reasons (air intake, and exhaust pressures mostly) so keep that in mind.

Generally people up the jets in pods based on more air flow the pods cause, or a free flowing exhaust would require more air intake, which requires larger jets. Look at what you have on there, and the more air you take in means you need to exhaust more air, and you need more fuel to mix. If you only have more air exhausting and more fuel going in but limited air intake you will get a strange mix and could have problems.
 
The gas issue is a petcock that is not shutting off like it should. Also if the needle valves in the carbs are bad and not sealing it will leak out the carbs. Has nothing to do with main jets.
 
Thanks - I suspect the vacuum petcock. It is not on prime or reserve. It's not leaking visibly, but has a bit of "crud" on the outside of it, so it may be a very small leak, like bad seal.

Looked in carb through the air box connection and it is a little wet with gas, but not like a steady leak. Hasn't done it since.

Are the petcock rebuild kits any good or should I just get a new vacuum petcock?
 
They do sell rebuild kit on ebay. Make sure no gas has gone in the engine oil.
 
I have never used one of his kit before but if you look on ebay you may find others that sell them and see what kind of feedback they get.
 
OK - after some time riding, and trying to figure out where gremlins are, I have some news.

The bike was running rich, just barely, but enough that every 3-4 days I had issues, hence all the problems. I needed to adjust the pilot screws, and one of them decided to get stuck, and stripped. Perfect. Now what? I tried everything and that thing is really in there, so my only option at this point is to try and drill it out, which yes, I know, is super risky, and more likely to end in failure than success...

Let's say then that this doesn’t work and I need new carb bodies/new carbs. I see them on EBay, and while I know EBay is great for a lot of things, I just don’t have the money and more specifically the time to screw around and end up with used carbs that have the same problems I already have. So I have a question before I make a move.

I know Mikuni has the famous VM series, and I believe that compared to a working set of BS carbs they aren’t as good. But I see these all over the place, ready to bolt on. If I were to put VM's on the XS will they work, after figuring out the jetting of course (assuming I can)? Anyone have experience with them?

If not the VM, is there another 34mm carb that would work, rejetting etc assumed?

I just figure if I can get something newer, that maybe even has a better aftermarket, it might long term be worth it, and since I may be stuck for some time it may be worth considering... as you all know several of the parts for the XS400 are become scarce, so it's a matter of time...

Ideas?

Meanwhile I will be trying to get a much abused 1976 CB550 running today so I can limp to work and back while I figure this out...

Thanks
 
If you are not confident on removing the screw you may be better off taking the carb to a shop with the proper tools to remove it.
 
Success! :D With about 3 hours and some new swear words I created, I got the old stuck pilot screws out. New screws are in and working great. I have them at about 2 1/2 turns right now, and will do some longer test rides and tuning tomorrow... otherwise running much better...

What a relief...
 
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