Won't start after carb clean


XS400 Enthusiast
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Oakland, CA
Hey y'all

I'm having a bit of trouble with my '82. The motor was dying at idle so I figured the carbs needed to be cleaned. The carb cleaning went smoothly and I replaced all the filters and changed tho oil too. Now that I have everything back together the bike won't start. I'm a bit puzzled since it started easy and ran well before this.

The float bowls are getting gas. I also charged the battery and checked the fuses. The spark plugs are firing but they seem a bit weak and they are fairly dirty. When checking the spark plugs I shocked myself a few times when the motor wasn't cranking. Is that normal?

If anyone could point me to a relevant thread or give my trouble shooting tips it would be much appreciated. I was really looking forward to getting back on the bike. :banghead:
How many turns out is your adjustment screw on the carb? After you remove the plugs when turning it over for awhile, are they wet with gas?
One of the adjustment screws was mauled so I couldn't move it. On the other I mistakenly counted the turns to remove it rather than running it all the way down to count the turns to bottom. Perhaps this is my problem and I set the screw up wrong. I'll give that a look, but I would think that I would at least get some fire from the one cylinder where the screw was not moved.
Idle screws are delicate and the tips may have gotten jammed in there and broke off. Do not tighten them more than a light seat.

There are a number of reasons why the bike would stall at idle. I always check the maintenance first before actually removing things.
I never tightened the idle screw down to the bottom at all and it was in good shape when I removed it from the carburetor body. I guess I am going to have to check it again.

What are some other reasons that the the bike would stall at idle? so i can start looking into those. Are electrical issues one of the possibilities?
Perform basic maintenance first. Check/new battery, check idle voltage and charging voltage, check ground wire, check positive wire, check for loose connections. Check fuse box, original fuse boxes are notorious for failure. Check timing. Check plug boots for resistance and check the end of the wires, the boots screw into the core winding and eventually breaks off. Cut a bit of the wires off if theres not a good solid connection, dont over tighten the boots. Check for vacuum leaks, check valves, sync carbs. Check compression. New plugs and plug chop.

I check compression with the carbs off and after the valves are matched and set.
Thanks for all those tips. I'm still learning all the ins and outs, so I'll definitely check those things out first next time.

I found that the battery is putting out over 3 amps when the kill switch is on and nothing is running. Disconnecting one of the DC leads from the rectifier/regulator eliminates the current draw. Still won't start when that lead is disconnected though. I don't think the regulator is necessary to start the bike if the battery is fully charged.

Maybe I accidentally touched the + battery terminal to the regulator ground when I was disconnecting everything and fried something. :umm: Do you know how to test the field winding circuit?

Idk I have to let it be for now . . .

EDIT: I looked up how to check the field winding and that's my next step. I'm still not sure how a alternator or regulator problem would stop it from starting.
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The coil will be energized when you turn the kill switch to on position. You can quickly verify the coil works by check the left side round cover for magnetism.

Excited coils need a battery to function properly, or the bike will not run. Our bikes need a good battery.

I installed a volt meter on my bike so I can tell when things start to go sour.
Update: after a troubleshooting saga I got her to start with some starter fluid. It seems that my kick only start just wasn't enough to pull fuel through the newly cleaned pilot passages, but that's only half the problem. The bike still doesn't really idle, which is the problem I was trying to fix in the first place. I'm focused in on the LHS pilot screw with the mauled head that I can't turn. The motor rolls just fine with the throttle at 5%, but just won't idle.

If anyone has insight on how to release that screw I would be very interested. I'm hoping I don't have to replace the carbs. I'll probably start a new thread once I learn more. Anyways, the saga continues . . . :shootme:
Photos would help. The idle screws are obviously a big part of idling and they need to be addressed before you move on.

If I pull the choke/enricher, and give my bike a kick or two before turning the power on, it starts every time.

After you take care of the idle screws, i would get the carbs synced and make sure theres no airleaks and the timing is good. Those are all critical to proper idle. Check voltage at idle too. Should be around 14v on a good new battery. Ive seen mine as high as 14.8v with the lights on cruising but I use a newer automotive regulator.

I have a set of carbs from my 78 Id be willing to pass on to you. PM me if youre interested.
That's awesome Mike. I appreciate the offer. These carbs aren't so easy to find out there.

I'll update with some pictures and such when I get some time to dig in again.
I decided to change the intake and exhaust pipes before taking the carbs off again. I now have the H-pipe adapted to 54mm pod filters and the mufflers that @NewHavenMike suggested to me.

The night that I got it all back together I ran it around for 20 min and it felt great, smooth and powerful. I had high hopes. I put some finishing touches on the work and took it out the next day. About 10 minutes into that ride it started dying in a strange way. Suddenly the power would cut out and the throttle would have zero response. The motor was still barely limping until I pulled the clutch or stopped, then it would die completely. I still get good power when it is running though? I'm thinking that its running too rich, but such a dramatic cut out is strange to me.

It's a bit hotter out today so my plan is to ride it around and see how the performance compares to get some clues. Next I'll take the carbs off and check all the settings thoroughly, test compression, go down 2 sizes on the main jet, then test performance again. After that, if there's still no improvement I'll go for other causes. My starting issues are persisting and I need to deal with my LHS pilot screw with a the jacked up head. Some reading suggests that carb sync would not cause such a dramatic problem unless it is WAY off, but I could be convinced otherwise.

Anyways . . . its a great day for a project and with any luck it will turn into a riding day!
Those problems sounds like its electrical or fuel is cutting off. Does fuel flow freely out of the line?

The pilot screw will insure that the bike will never idle properly. If you had a Colortune, youd be able to see if the mixture is rich or lean. You can get a temperature gun and check that way too.

What do the idle screws look like? Can you not remove one of them or it wont turn?
Yeah the carburetor internals are in really good shape and all the passages are clear. I realized that when It was dying the other day I had the headlight and brights on which may have been throwing one of the auto resetting breakers I have. My electrical system is kind of a mess. That's the next project.

I took her for a ride this morning and it ran pretty well. Afterwords I took the liberty of checking the carb settings anyways. Everything was to spec. Needle was at the second slot from the bottom so I raised it one slot. We have hotter weather out here in CA right now and the bike has been a bit flat off the start so I wanted to see how that felt. I didn't end up changing the jets just yet, mostly because I mistakenly ordered only one of each size (dumb mistake). Compression was normal. Plugs indicate the mix is a little rich. The butterflys where a little out of sync (visually) so I gave them a small tweak. Floats at 32mm which matches my manual.

Unfortunately I forgot to take a picture of the pilot screw before I re-assembled everything. Basically the head is torn up so I can't even try to back it out. I'm going to have to get some easy-outs and try to yank it out. It starts and idles about 60% of the time and the other times I can get it by cheating up the throttle screw.

Thanks for all your all your input @NewHavenMike, It has really helped my get my head around the problem. I'll check out that colortune stuff soon. Seems like a good way to get a feel for mixture. After I get these problems sorted I think I'm going to do a more comprehensive electronics project that I'll document more thoroughly.

While closing everything up I accidentally dropped a washer in the slot below the starter motor. So now I need to get the cover off and find it before I start the thing again. Another dumb mistake. I've been thinking about covering that slot for a long time. :banghead:
The Colortune is expensive compared to using a temperature gun. Having all these tools is worth it for me because I have a bunch of motorcycles and an old MGB. At least you can also use the temp gauge when you do some grilling.. The Colortune is around $90, not counting overseas shipping.

If you decide to spend money on a tool, get a carb sync tool. They run around $100, I think mine was $120. I bench sync my carbs with a thin wire, like one of my guitar strings, a B string. A bench/visual sync is not the final step and is not accurate enough.

You need to be able to adjust the idle screws while on the road, i carry a pocket screwdriver so i can do it quickly and easily.
22mm for foam floats the manual is incorrect for this. Pilot mix screws need to be able to move in order to be set to at least 3 turn out from seated. What was your compression numbers? I know we have said this a bunch of times before but sync the carbs with a manometer. It will set by vacuum not the amount of openings between plates. Big difference!! You can make one for a few bucks. There are vids and pics on the web.
My manual says 32mm for the 400, and that's where I'm at (edit: brass floats). I know I need to take care of that mix screw, but there's only so many hours in a day. A friend has some easy-outs and vacuum gauges that he's going to lend me, so I'll be doing this the next chance I get. I'ts running reasonably well now, not perfect but I'm happy with the progress, and I'll continue to smooth out the details.

My compression is pretty good, 165-170 hot and 180 cold. The manual says 145-175 at operating temp, so it looks like I'm in the green.

Thanks for all the tips guys. keep them rollin' in.
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I got her out for the first decent ride today after all the issues and she's rippin'. I've got a 5mph gain on my top speed at least! The bike used to struggle past 7k rpm and now can rail it past 8k no problem. Might just be a mental thing cause its just smoother and quieter but hey, I'm having fun. H-pipe and back-pressure man, I'm a believer.

Still have some minor idle issues and some details to iron out. All with time . . .
Had an awesome ride this morning then when I started it after work it idled super rough and had no throttle response from 0 to 1/4 throttle.

I guess it's back to square 1 . . .
You never left square 1. Synch your carbs.

Also confirm proper float height. It's written about all over this forum that the float height in some manuals is wrong. Listen to advice if you want to tune it proper