Left Cylinder Fires Intermittently When Started From Cold

I found the following thread that definitely relates to the problem I'm having with my '76:

http://www.xs650.com/forum/showthread.php?t=30627&highlight=left+cylinder+cold

I'm posting this because my carbs are BS 38, while the above thread applies to BS 34 carbs. The intermittent firing from my left cylinder goes away after about 15-30 seconds, and does not appear the rest of the day after the initial cold start up. (I don't use any choke after the first start up... and extremely little at the first, if that means anything.) A proposed solution with an accompanying photo, seen at the bottom of the thread, is to drop off the float bowl and clear out the choke feed jet. The photo shows what I gather to be the bowl from a BS 34.

Does this solution pertain to the BS 38? I haven't had the carbs apart, but I'm thinking that photo might not exactly represent what I'll find in my carb. Unfortunately, the OP did not say whether this, or something else, fixed his problem.

I will say that since I've been putting more miles on the bike (only have added about 200 so far), the problem seems to have lessened. Perhaps I'm running in "self-cleaning" mode! Other than that cold start-up glitch, the bike seems to run very well.

As usual... Thanks!

If the start jet isn't completely blocked, I could see it slowly clearing itself. If Ohio has 10% Ethanol in their gas, your jet issue may well be 'self cleaning' if you are riding frequently.

DirtyErnie said:

If the start jet isn't completely blocked, I could see it slowly clearing itself. If Ohio has 10% Ethanol in their gas, your jet issue may well be 'self cleaning' if you are riding frequently.

Click to expand...

That's what I'm hoping! Yep... there's ethanol here. I am trying to get some miles on it. Plus, I'm running a little Seafoam in a few tankfulls. Self cleaning..... would be nice!

Thanks for the reply!

Best way is to go into the Carb and clean it out right. There is no substitute. How is the bike running?

The bike runs beautifully... better than I ever expected! It starts immediately from cold (but with the left cylinder problem). Idles perfectly at low rpm, even when not fully warmed. Accelerates very briskly with no flat spots. Cruises at any speed (within reason) without any complaint. Is very smooth at 60+ mph. The carbs are now well-synced. Braking and suspension are now very good. I'm very happy with the way it is turning out!

But back to the left misfiring on the first cold start of the day. It's not as bad as it was... but it's definitely still there. I use the choke minimally when starting. I generally pull the choke lever up within seconds after firing. It is in this first minute or less that the cam chain is rattling, etc. due to the misfiring (perhaps more like "non-firing") of that cylinder. The motor sounds plain junky until it stops. Then the motor smooths out and quiets down significantly. I'm thinking of starting the bike without the choke when I get home. I'll just try a little extra throttle instead and see if that makes a difference.

So it is running so well, I really hate to mess with taking the carbs apart. Perhaps I can pull the float chambers and do some cleaning there. In another similar thread, 5twins posted a pic of the idle circuit that is easily unplugged that could be causing a similar problem... but the pic is of a BS34. I'm not sure I'll find the same thing if I start disassembling the BS38. I'm hoping the miles will clear it up.

Thanks!

Perhaps the intermittent firing is related more to low rpm than whether or not the engine is warm?

weekendrider said:

Perhaps the intermittent firing is related more to low rpm than whether or not the engine is warm?

Click to expand...

No... I don't set it down to a low rpm idle until both cylinders are firing as they should and the motor smooths out. I actually gas it a bit to keep it going until the left cylinder fires consistently. After a minute or so, after a cold start, all the nonsense stops, the motor is firing smoothly and properly, and it can be settled down to a nice, smooth, low idle, even though it's not fully warmed up. The rest of the day, even hours later, there are no issues when starting. It's just that first start of the day. And on that first start, I've always used the choke just for a couple of seconds. Later today, I'm going to try it with no choke to see what happens.

SMG65, sounds like you have a good bike, there. The problem on start-up? well, it's time to read the carb guide and make sure the engine is getting the fuel as it should. It takes a while to really understand the carbs, jets, the various functions of the parts, but it's not difficult to remove the carbs, dismantle, clean, and put it back together.

You may have some immediate maladjustment like the mixer screw of the synchrinisation is out. I believe ethanol causes some glitches, and everyhting points to cleaning.

You might want to get you multimeter out and test the HT leads and spark caps, and of course the plugs - then... Oh! it goes on, start with cleaning the carbs.

ANLAF

Yep.... gonna get into the carbs. Tried starting with no choke.... no go. I only gave it a couple of whirls... didn't want to beat it up. It started right away with a touch of choke. Rode it for 20 miles and all was great as usual. You long term XS guys probably know the bike needs some choke on the day's first start.

I'll work on the carbs. No problems with that. Ain't skeered! LOL. I've done carbs before, matter of fact, just a couple of weeks ago I rebuilt the carbs on my old MG. I have looked at the carb guide, and even printed it out a while ago. There is so much useful info on this site... it's great! I was just avoiding taking 'em totally apart since it is running great. I was looking for an "easy out"!

Thank you for your reply... it's greatly appreciated.

double check that your fuel LEVELS are right in both carbs. setting the floats to spec only gets you close... or not. ;-)

Have fun!

The choke jet and idle circuit are pretty much the same on the BS38 and 34. Visually inspect the choke jet using the light trick to insure it's clear. I don't think it's the idle circuit or it would run bad all the time, not just when cold. But, it wouldn't hurt to flush it. Since it's probably OK, just use WD40 first to check it. If it is plugged, then switch to the carb cleaner. Remember to flush it again afterwards with WD40 to get all the remnants of the carb cleaner out.

Thanks for the tips, 5twins! I didn't get much done today as I was juggling a couple of different projects. I did check the float levels (by sight tube) and they seemed OK. When I emptied the bowl contents the first time, there was a little "dirt" that came out with the gas. But not much, considering the years of accumulation. The PO did install fuel filters, but I don't know how long they've been in place. I'm hoping to pull the carbs this coming week as I should have some free time.... my wife will be in Florida for 2 weeks and I'll be on my own!

This misfire on one cylinder really had me puzzled on my '78 Standard. I noticed when I was looking at areas for possible vacuum leaks that the copper (brass?) tubes used for the vacuum fuel shutoff valves, that are pressed into the inlet spigots, can work loose over time. I found the problem by spraying compressed air through a handheld nozzle at all the suspect areas. The rpm definitely changed when sprayed around the manifold and particularly at the pipe/manifold junction. One pipe pulled out with a small pair of needle nose pliers a lot easier than it should have and the other one was not much better in providing an airtight seal. A little Loctite and enough time to let the stuff cure and the misfire is now a memory.

bikesbybruce said:

A little Loctite and enough time to let the stuff cure and the misfire is now a memory.

Click to expand...

It is a good feeling when a nagging problem is fixed! My petcocks on the 76 are different than yours. But someone with that same problem will see your reply while searching the forum and the problem will be fixed.

Thank you, your reply is appreciated!

Before removing the carbs you can remove the mixture screw and spray carb cleaner inside. Get the little red tube right inside and blast. Wait a bit for any varnish to soften and blast again. Drain the float bowl through a paper towel to see any debris inside. Fill the bowl and see if your carb is fixed. Much quicker/easier than a tear down.

Tom