Stumbling/Popping

[Steve P]

I recently put a 1980 XS1100SG back on the street. It has stock exhaust. The aircleaners have been changed to single units and the main jets have been bumped up 2 sizes. The machine starts right up, runs right up to over 8000 rpm in all gears. Carbs were soaked and rebuilt, valve lash checked, all ok. The machine idles flawlessy at just over 1000 rpm. The problem is that if I try to take off slowly, I get a pinging sound (similar to pre-ignition) and the machine seems to want to stumble a bit. Give it a bit more gas and it works fine. The other problem is a popping sound through the right side exhaust, (it's not a backfire). All the exhaust pipes look normal except #4 which is ever so slightly blue up by the head. Spark plugs look normal, none fouled or appear lean. Since I've had it, the carburetors have not been synchronized. Checked the timing with a light and did a leak down on the vacuum advance, all checked out ok. Anybody got any ideas. Thanks in advance for the help.

[mro]

carburetors have not been synchronized

Will be a noticeable improvement, sync carbs.
Also sounds like it's a little lean.
No vacuum leaks ?
Once you stray from stock, it's trial and error to get it jetted right.


mro

[Steve P]

Thanks mro for the quick reply. Can you recommend a synchronization tool? Back in the 70's, I used something called Carb Sticks, (mercury filled tubes with surgical tubing). As I recall I wasn't too happy with them because it would appear you have the carbs synced properly until you reved the engine. Then when it returned to idle they would appear out of sync again. If you could recommend a tool and a retailer I'd appreciate it and thank you very much for the quick response and advice.

[mro]

Merc stick sync tool works for most and seems to be the easiest.
I do mine with a single vacuum gauge, is a PITA and takes longer but gets the job done.

If you do a search on this site will find everything from my single gauge to 4 gauge set ups, merc sticks, clear tubing and what ever else some of the "inventive" or XSive minds here have come up with

May be something in the tech tips too


mro

[Steve P]

Thank you very much for the expeditious and excellent help.

[TopCatGr58]

Re: Stumbling/Popping

Originally posted by Steve P
Carbs were soaked and rebuilt, valve lash checked, all ok. The machine idles flawlessy at just over 1000 rpm. The problem is that if I try to take off slowly, I get a pinging sound (similar to pre-ignition) and the machine seems to want to stumble a bit. Give it a bit more gas and it works fine. The other problem is a popping sound through the right side exhaust, (it's not a backfire).

Hey Steve,
Congrats on getting your machine up and running quite well already. The above bolded statement has me/us a little concerned though! Did you actually take the carbs apart from the rail, and soak the entire bodies in cleaner? IF so, then you could have damaged the butterfly shaft seals and have a vacuum leak there, can test for that with several techniques. I recently thought using an UNLIT bic charcoal grill lighter would provide similar type gaseous fuel like propane without a large tank! IF idle changes, then you got a leak!

Also, what grade of gas are you running? Might try stepping up to mid if you are using lowly regular! You said you checked the vacuum advance, did you connect it to the correct CARB BODY metal port, not to the INTAKE BOOT synch port? Has happened to many before!

Also, were you able to access the pilot screws on the top front of the carb bodies, or are yours still brass capped off?! If still capped, need to remove for proper cleaning of the pilot circuit!
Let us know what you find, and post a photo in the MEMBER'S LOUNGE, we love photos!
T.C.

[Jimbo11]

Hey TC

What Octane do you run? I have had the same
problem just recently with my 79. I have
been running 87 maybe it's time to ste up
to 89 or 91?

[TopCatGr58]

Hey Jimbo,

I run 91-92 stuff, but I've also got a little big bore that's supposed to have 10.25:1 comp vs. the stock 9.0:1 for the 81 model. I know that mine is still not considered really high comp, but after rebuilding it, I didn't want to take any chances with burning holes in my new pistons!

Modern cars can handle the lower stuff due to fancy computer controlled EFI and ignition systems that can vary the timing, etc., but our older machines don't or can't do all of that!

The Owner's manual IIRC, as well as the lable inside the tool cover, and someone posted about it, states to use "regular", but that was 1979 regular which I still think was a bit higher in octane than what we can get now. Of course folks also state that they(fuel companies/EPA-Gov't) has changed the way they measure Octane now vs. back then, so who knows for sure. All I know is that my bike likes the higher stuff.

It's also been discussed about since it(premium) may not sell as quickly as regular that it can sit around in the storage tanks, and gas looses about an Octane point a month in storage!? So...I might not be getting my full 92/$$'s worth, but I still think it's better than 87 that pings and could damage the engine!

YMMV! T.C.

[Jimbo11]

That's why your the Top Cat

Thank you for your help..

[fredintoon]

Hi Steve P,
as T.C. sez, if you "soaked" your carbs a prolonged immersion in carb cleaning solution may have damaged the butterfly shaft seals so they leak. Here's a test for that:-
Put a layer of heavy grease all around the shafts where they go into the carbs. (2 sides on 4 carbs = 8 places) The grease will act as a seal long enough for you to do a short test ride. If the symptoms go away you know the seals need replacement.