chasing gremlins - engine/carb/??? issues

subt3rra

SUBT3RRA
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Hey guys

I am waiting on a clutch push rod seal to fix a leak, and while waiting I decided to sync the carbs now that my pipes are all welded up.

Well I bench sync'd, reinstalled and fired her up. She ran, but kinda rough. Tweaked the mixture a bit more and things were about normal that day. I didn't have time to take it out for a long test and had to work the next day. Don’t have a manometer, so that’s as close as I could get it. Mind you, it ran fine prior to all this...

Rode it to work the next day, and here’s basically what happens: starts first kick like always (yea!), runs good for 5 minutes. Then Right cylinder wont fire or intermittently fires, then when a bit warmer runs good. :bike: A few miles more, then back fire (technically after fire, in the pipes), :wtf: and random right cylinder will ” fire – no fire” . Feels like it’s not getting enough gas...

So this weekend I checked plugs – left is looking good, right is looking lean. So I pull the carbs. Back story, I rejetted when I redid the pipes from stock 135 to 140, ran good until I got the pipes final welded (yes I had to run it ghetto for a bit). So up to this point I am running 140 mains. At first I thought of raising the needles, but surprise! Mine are not adjustable. :mad:

So I went up to 142.5 mains. Put ‘em in. Fired up, ran basically the same except less backfire (after fire). But same routine... runs, then no right cylinder, then runs (really well with new jets) then runs bad, some backfire. New twist now, it will NOT START first kick when warm, and sometimes takes 10 kicks or so, but eventually fires... checked plugs again last night: left looks great, right looks lean, same as before... on mixture I am 1 3/4 turns left and 2 turns right... I had right out to 3 turns while setting but really ran bad that far out so back down to 2.

Sorry for the long back-story. So I am now thinking it is either I need to sync my carbs with a manometer, but that doesn’t explain the randomness of things...

Or my right coil is bad – checked them last weekend tho. Using the red and orange leads in the plug they both test 3.4 and 3.3 ohms ea, within spec. Tested resistance in my plug wires. They are slightly different lengths, and about 11.9 and 9.9 ohms (lower number is the right). I do not know what they should be, but a new untrimmed plug wire was reading about 12 ohms...

Or I have a clogged jet, which I guess I could, even though I cleaned the carbs and ride it DAILY (it’s all I have). But it seems like that would be clogged or be unclogged and that’s that...

Or...? stuck or sticking valve? I don’t know...:banghead:

Timing is factory, haven’t touched it.

I am running stock air cleaners, could that be it? Seems possible, but long shot maybe...
The left cylinder runs fine and has for the whole time

Open to suggestions... thanks
 
How's your float height looking?

Do you have brass or plastic floats?

Do you have contact points ignition?

Did you adjust the valves yet?

Are you making or buying a manometer right now?

What size is your pilot jet?

What kind of exhaust?

Have you ever left the keys in the ignition for a long time?

Things can get clogged randomly. It only takes one tiny bit of debris.
 
How's your float height looking? = Didnt look last weekend, but I set them when I cleaned last...

Do you have brass or plastic floats? = brass

Do you have contact points ignition? - electric, 1980

Did you adjust the valves yet? = I did in December when I put the motor back in. Could use it I guess but seems soon...

Are you making or buying a manometer right now? = one or the other...

What size is your pilot jet? = stock. That could be it now that you mention it...

What kind of exhaust? = basically open pipes, small baffle off stock header...

Have you ever left the keys in the ignition for a long time? = on occasion, why do you ask?...

Things can get clogged randomly. It only takes one tiny bit of debris... = I know, this weekend looks like of with the carbs again... maybe even afterwork...
 
Are you making or buying a manometer right now? = one or the other...

What size is your pilot jet? = stock. That could be it now that you mention it..

Have you ever left the keys in the ignition for a long time? = on occasion, why do you ask?...

Make a manometer, they are easy and cheap.

On the street, you will do 95% of your riding on the pilot jets. 4% on the needle and 1% on the main jets. Get the full throttle high RPM fueling dialed in with the main jets, then worry about the needles. Pilot jets are last on the list to get dialed in, though chances are good you will have to increase them at least one size.

If you've left the key and kill switch on for an extended period, you may have overheated one or both of your coils. Check them for leakage and the correct resistance values - cold and hot.
 
I am making a manometer for sure - budget - and I'll check out the threads on here prior to doing so.

I briefly thought about pilot jet, but was confused because the problems have always been the right cylinder, not left. I will change them both out one size up.

What is the stock size pilot jet so I can have them in hand when I disassemble (may do this tonight even)? 1980 stock carbs...

I can’t adjust the needles (fixed position, no grooves but the one) so if they needed to be adjusted could they be and what would we do there? I suspect its pilots from your description...

The key has been left in the bike on occasion, in the off position of course. Can that cause the coil overheating? Kill switch is typically on too. Bad habit I picked up from owning a newer bike. Never had this problem come up (even in conversation) before; like to learn more.

I checked coils (cold) and they were 3.4 and 3.3 ohms (right was lower number); this was on the red and orange wire via the plug end. It also shows a way to test with the positive probe on the spark plug connector, negative on orange, but I got no reading doing it this way on either coil. It was about 55 degrees F that day. I saw in the manual we should check the "secondary coil is at 9.5 ohms” which I must admit I haven’t looked for yet if it’s not part of the coil pack (diagram on Haynes page 78). .

Also, if there's leakage, are the coils shot? I'd rather keep them if they can survive a bit longer, but I'll replace them if I have to of course...

Thanks 16VGTIDave and bcware for your help...
 
...found the stock pilot jet size: 42.5. Haynes page 58. I'll get the next size up tonight...

Something else I just saw that surprised me is the manual shows the carb for the xs 400 as the BS34, and it says the stock main jet size is 142.5 - I just put in 142.5, as the second change from 135's I pulled out stock, and the 140’s I just replaced trying to figure this out.

I also noticed the xs360 also used Mikuni BS34's but has a main jet of 135...

All the anti-tamper plugs were in place on this bike. But maybe the carbs are off an xs360...?

Either way, now I wonder if I will find 17.5 pilot jets, or 42.5's when I pull these carbs tonight...

Weird.
 
You will find the 42.5 in there. The 70s model bikes had 142.5 stock mains but the later versions ran 135s for environmental purposes. Terrible mistake and had problems from what I've read. They also didn't want the needle adjusted so they changed that on the later models also. I have an 82 Heritage special with the exact same set-up on my carbs. I am getting ready to change them to 142.5 as well because I can't get my carbs to run right either. XSChris told me to try 45 pilots too because I have a short exhaust pipe as well.
 
The xs360 carb is totally different carb than the xs400 style. They are bs34 but a earlier style and setup.
 
OK that's what I though too - 80's emissions issues... I'll get the 45 pilots to start.

I still feel like I could have a coil issue though. One issue at a time I guess...

Hopefully get rejet done tonight...
 
One step at a time. Start with adjustments and cleanings since they are free. This will help rule out possible causes as well.

If you have brass floats the float height should be about 26 mm by the way. Depending on the entry in the manual you may get a different value suggested. Sometimes the Hayne's doesn't do a good job covering all years.
 
Will do - I have the 45 pilot jets since they were cheap in any case, as I would like to be able to make the change if that sounds like the cause...

I saw mention of "boiling" the carbs to clean them. Also sounds like its a 50/50 those who like it or don't like it. Not sure if that is such a great idea but cleaning them thoroughly would be great. Sonic cleaner maybe? Thoughts? Thx
 
If you have an altered exhaust or intake bumping the pilot up to 45 is pretty much standard.

I boiled my carbs in a light vinegar solution and I don't think it did much. I ended up with a crystal powder all over them. If you have a sonic cleaner definitely use that. Otherwise compressed air and carb cleaner works amazingly well.
 
...and Canada is a bit far from my house in the short run anyway :)

This will all have to wait until the weekend now anyway. Family stuff tonight. On it first thing Saturday....
 
Success!... almost...

I started the day off pulling the carbs and cleaning them. The right carb was pretty clean, got a few spots of grime I missed when I cleaned it after I got the bike, otherwise no surprises. Changed out the 42.5 pilot jet, it was a bit crudded, so that could have been part of the problem.

Went to the left carb, and it looked good until I got to the needle jet – which I forgot to clean when originally cleaned the carbs.:doh: It was gunked like crazy. Weird thing is that is the cylinder that never had a problem. Weird. :shrug: Anyway...

All pilots are 45’s now. Mains are 142.5 (last weekend). Set floats to 26mm – they were at 32 which is what the Haynes says for the xs400. Reassembled the bike and set up for a sync. Started the bike and it ran great! :thumbsup: No more after fire and the right and left firing just fine now. Hooked up the manometer, and the left did have more vacuum, so adjusted where they were even at idle, set again where they are close as possible at about 3-4k RPM. I laid the homemade manometer out in inches and they are within 2-3 inches of each other. Best I could do, and from what I have read sounds like about average at that rpm. Love some feedback on that...

Anyway I put her back together and took her for a test... :bike: SPIT! :wtf:

What was that? Oh well... great even acceleration...:bike:

SPIT! ... great, backfire. :mad:

enough with the cartoons -

To be clear I am talking about backfire, meaning it’s coming from the carbs. I had afterfire before from the pipes, which is effectively gone now.

So now it runs great but has random backfire. Not bad, just a SPIT and usually on deceleration and some at idle.

I then realized I didn’t check the valves before I set everything. So now my question: I set the valves before I reinstalled the engine. Would they be out in that short a time? I am checking them regardless in the morning, but wanted to ask. Could that be the cause of the backfire?

Is there anything else that could be causing the backfire? I am 2 turns out on both carbs. Doesn't see too rich, could lean cause that? Doesn't seem like it, but I’ll check plugs in the morning as well.

A quick look on the forum didn’t find anything jumping out at me to check on this subject...

So close otherwise! I can see the light at the end of the tunnel... love some input.

thanks
 
You should sync the carbs at idle NOT 3-4k. Also the fluid should be even with each other. I would recheck the valves and then sync.
 
sorry, what I wrote was confusing. I synced them at idle, and they are even. At three or four thousand rpm they are off a bit. The left cylinder has more vacuum. I'm checking again tomorrow after I adjust the valves. One question, is it best to sync the carbs with the engine warm, I like after a short ride? That's what I did today. Thanks
 
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