Too easy?

November 1, 2005 12:26 PM

mcyfilter: Does the choke on a motorcycle affect the compression ratio of a cylinder?

I have a 1976 Yamaha XS 600, I brought it into the shop because I thought that the carbs were out of tune. They called me today saying that they can't get the right cylinder to start, they swapped the right side with left, and tried to start it and the right side still won't start. The compression on the left was 170 and the right was 110. I know a little bit about bike maintenance, but not a whole lot. The right side has fire, gas and air (but less compression then the left side obviously) The mechanic is stumped. My bike has two chokes, one for each carb. The right side choke lever faces the middle of the bike and you have to pull it out by hand. I was thinking that maybe the mechanics only pulled out the left hand choke, missing the right one completely. I made this same mistake when I first got the bike. Could this be a possible explanation? Is the lower rate of a compression a possible sympton of one choke in and the other out? I don't want to suggest it them and come off like a moron.

posted by youthenrage to Travel & Transportation (8 answers total)

No. The choke limits air flow into the cylinder to give you a nice rich mixture when starting a cold engine. Bad rings or valves cause lack of compression.
posted by caddis at 12:37 PM on November 1, 2005

Short answer: no, the choke has nothing to do with cylinder compression. In trouble-shooting poor compression I would check, in this order, the piston rings' sealing ability, a burned valve, or (this is a real zebra) a flat spot on the cam lobe.
posted by mojohand at 12:38 PM on November 1, 2005

Oh, and if trouble-shooting poor compression is baffling your mechanic, you need a new mechanic.
posted by mojohand at 12:41 PM on November 1, 2005

Best answer: The choke is part of the carburetor, which is upstream from the combustion chamber; the combustion chamber should be completely isolated from the intake manifold during compression, so I can't see how the one would affect the other.

If the left has higher compression than the right even with the cylinders swapped, that suggests to me that there's some kind of leak somewhere else--either the cylinder walls on the right are a little bigger, there's a leak at the head gasket or valves, something like that.

If anything (and this is sort of a fringe explanation), it is conceivable that the intake valve would be open for a few degrees of the compression cycle (Atkinson cycle engine); if that were the case, having the choke on for that cylinder would create a tiny bit more back-pressure and raise the compression on that cylinder, not lower it, but even then, I wonder if you'd see a big difference between choked and unchoked.
posted by adamrice at 12:43 PM on November 1, 2005

At the risk of a thread derail, and of possibly completely misreading the situation, I'll caution you on trying to get an old Japanese bike running, especially if you aren't fixing it yourself. With the exception of Hondas, parts are a real bitch to find for old models and usually there's a lot more wrong with them than "it just needs the carbs cleaned out."

Unless you're wrenching it yourself, they can end up being a complete money pit, and when the dust settles you've got a 30-year old motorcycle. Bikes, like cars, have gotten a lot better in these past three decades. While the XS650 (it is a 650, no?) is a classic, so is a SV650, and a three or four year old SV is a lot safer and just as much fun.
posted by mojohand at 1:09 PM on November 1, 2005

caddis is right on - I have an 81 xs650; I take great comfort in it's "good vibrations." ;)
posted by AllesKlar at 1:19 PM on November 1, 2005

oh, and FWIW, in these yams, the choke is actually a "fuel enrichner" in that it adds more fuel to the mix. If you look at the carbs (apart), with the choke out, there is a small hole below where the diaphram sits, that permits a little more fuel to drizzle in. (I'm assuming these are Mikuni carbs)
posted by AllesKlar at 1:21 PM on November 1, 2005

For what it's worth I'd check to see that the gasket between the cylinder head and barrel isn't leaking, the valves are seating properly and pull the cylinder barrel to see if the piston rings need replacement.

And agree with mojohand, if the mechanic is stumped you need a new mechanic.
posted by squeak at 11:10 PM on November 1, 2005