Hi; If you have a minute, I'd appreciate your ideas. I have a bike that runs fine in warm weather andruns okay in the cold. I think it could be made to run better; it's getting a little old, lumbering off on one cylinder in the morning (31 degrees here today!)

I picked up a '79 XLS seven or eight year ago. At that time, it had unrestricted drag pipes, worn valves, and a velocity stack of about 1" between the carb and intake. It ran, not great, but ran over the years while I made gradual improvements. Idiscovered the need forbackpressue and put restrictors in the pipes (great improvement in running noted). I discovered the intake o-rings were prone to coming loose and learned to keep those tight - again, more improved running. Ididn't understand whatthey were trying to accomplish with the velocity stack, and eliminated that, and restored the stock mounting of the air cleaner. It ran the same after that, but made installing the air cleaner a lot easier!

Last summer, the drag pipes were getting old and worn out, so I replaced them with Cycle Shack tapered pipes and mufflers. The bike still ran fine (mild, mild flat spot noted) and looked a heck of a lot better. Now that it's cold out, the mild flat spot has worsened some. Cylinder 2 doesn't kick in until I hit about 2500 RPM, or the motor gets warm.

I'm thinking that even a little more restriction in the pipe would be helpful. I cut a pair of wooden bungs with a 5/8" hole in the center to test with, stuck them in the pipe ends, and I think I noticed some improvement (they didn't stay in long enough to do some real road testing!)

I have a pair of copper reduction pipes, 1 1/2" to something like 3/4" I'm thinking aboutboltingin the end of the pipe. Think it will help (without killing my engine?)

Other factors: plugs are new HD-4's, properly gapped; compression was good, 125, on both jugs the last time I checked, though its been a while; pipes and intake o-rings are tight; pushrod tappets are in good adjustment, and valves and seats were replaced a year or two ago; I've had the electronic ignition out once or twice to replace springs - I marked the timing with a pencil and re-aligned to that mark, though I didn't re-time; timing "seems" okay to me (idle is good).

Thanks a bunch.
D.

 

Usually a flat spot that gets worse in cold weather indicates a lean condition in that carb circuit. I don't know what rpm this occurs at, but richening thatcircuit should cure the flat spot, and may get cyl #2 to kick in when you start the bike, especially if the idle screw is the culprit.

The problem with cyl #2 could also be the opposite - it's getting too much gas when the choke's closed and is fouling.

If it's what I'm thinking about, the short velocity stack "ring"should have been mounted inside the air cleaner.

 

Isn't 125 psi a little low for the compression reading? I wonder if it has dropped even more since the last test?

I'm pretty surethat if there's not enough compression the intake charge won't ignite.

When I was a kid I had a dirt bike with a tired engine and low compression. You could kick it forever and it would never start, but one day a friend and I figured out that if you towed it fast enough with another bike while in gear it would fire up.

Maybe you're experiencing something similiar with your bike. The cold air may be harder to light off and when you experimented with the pipes it may have caused the cylinder pressure to increasea bit causing the slight improvement.

 

Thanks for the ideas - the mix may be lean; I have never had the carburetor open or cleaned (and it's about thirty years old!) I wonder if there could be some buildup in the passages, restricting fuel flow, now that you mention it.

I think the 125 reading for compression has more to do with my guage than the bike - I'm using an ancient, decrepit Schauer guage my Dad passed along to me many years ago. The rubber is cracked, and probably lets some amount of gas escape.

Regarding the lean mix - something else I have never done is adjust the high-idle screw. I can't really use the second and third stop on the choke because the high-idle cam kicks in, and "idles" the motor around 2,000. I may try kicking that down to 1,200 on the third stop. If I could take off at 3/4 choke, that might help. The mc quickly warms up after a mile or so of riding, and works just fine from that point on.

D.