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1 Cyl not firing at low RPM

  • Thread starter Thread starter thom
  • Start date Start date
T

thom

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I've got a 1982 GS450L with two problems. I've worked on it and solved many issues and fixed many things over the last two years, but after riding a few times and thinking I was all set there are some issues.

1) When I start it and have the choke on, RPMs are about 4000 and both cylinders are firing. When I take the choke off, I lose cylinder #2. Had it out on the highway yesterday and it started losing power intermittently and then #2 quit. I barely made it home on one cylinder.

2) Previous to the problem above, it has always had an attitude about idling at a low RPM once it is really warmed up. I'll pull up to a stop light and it won't go below 3000 RPM, even though my idle adjustment is as low as possible. It idles low, possibly on one cylinder? when it is still getting warmed up (with choke off). It has always done this. Sounds similar to ScottieGS700E's problem.

I know it is getting spark, so I'm thinking there is a carb problem.

---Ohhh, also there seems to be an intermittent popping coming from the carbs. It is more noticable as the RPM's come down while I am easing the choke off. Sounds like "ploock, ploock, ploock".

Any suggestions? Thanks.
 
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Could be a tight valve. If its popping back through the carbs that means that it is fireing through an open valve so check the valve clearance. Also make sure that your carbs are all clean. You could run a compression check to see what kind of shape the motor is in. Worse case it could be burnt valves from being ran tight for too long.
 
make sure the vacuum line of the #2 carb to the gast tank petcock is good, if it has a hole or a bad seal, it will cause it to be a bit lean.
 
I'm taking a look at it today. The intake boot o-rings sound like an easy thing to try, I'll order those today. I cleaned/rebuilt the carbs last year. There is only one vacuum hose to the tank, I believe it runs from a nipple near the choke next to the cylinder that is firing well. If I recall correclty, I did adjust the valves last year, but maybe I'll double check this issue. Can you tell me more about the popping, and what happens to the valve, and what to look for visually?

Thank you guys.
 
Here is the follow-up.

I replaced the intake boot o-rings- the boots are also in good condition.
I set the float from the cylinder thats not firing(at low RPM), so that it matched the height of the other float.

All back together, still same problem. The left cylinder is not firing at low RPM.

Plugs are good. I've tried numerous used and brand new on the bad cyclinder. They are getting spark because when I pull the wires while its running I get those pleasant little shocks from both. Pull the left one and the engine dies, pull the right wire (with left wire in place) and absolutely no change in engine idle- obviously the right side is a dud.

I swapped the coils - no change, still the left fires, right is a dud.

An electrical problem upstream from there, that delivers a week spark to the right cylinder?

A mixture problem? I do notice the plug is wet when I pull it out from the right cylinder.

Is it safe to separate the carbs, and run a single carb (swapping the two carbs) to the dud cylinder, while the other intake is just left open with no carb? This might tell me if it is a carb problem. If that makes no difference, then I would look for more electric trouble upstream?

I didn't take off the valve cover to check those yet.

I did replace the RR and stator last year.
Battery has plenty of juice.

Please keep the ideas coming. thanks
 
Could be a clogged pilot circuit. Clean all the small passages. Tight valve clearance can also cause this.
 
fuel valve?

fuel valve?

I had the same problem with my 550. The vacuum line goes from the #2 carb to the fuel valve. The bike would start, run ok for a moment then start missing, backfiring, etc. Number 2 plug was wet and/or carbon fouled. Thanks to posts on this forum, I checked the fuel valve and found that it had a hole in the diaphram allowing extra fuel into the #2 cylinder. If the 450 has the same kind of valve, remove the vacuum line from the valve and plug it with something. Set the valve to Pri, make sure the spark plug is good (or swap them) and try running it. If it cleans up and runs well, you probably need a new fuel valve. Be sure to turn the valve off of Pri when you're done. I didn't and ended up with a puddle of gas on my garage floor. Good luck.
 
thom said:
Here is the follow-up.




I didn't take off the valve cover to check those yet.

I did replace the RR and stator last year.
Battery has plenty of juice.

Please keep the ideas coming. thanks
Click to expand...
You haven't ran a compression check yet?
You haven't checked the valve clearance yet?

Fuel+Fire+Compression=The bike will run
 
follow-up

follow-up

Ok, so today I took off the valve cover. All the valves have proper clearance (between .03 and .08mm). The cam lobes and valve shims visually look in good condition, and all moved properly in and out of the stroke positions.

Next I took the carberator from the good cylinder and moved it over to the cylinder that doesn't fire properly. I left the good cylinder intake open and pulled the spark plug wire from it. I started her up, and slowly eased the choke away. Low and behold my trouble cyclinder stayed running!

By process of elimination I'm thinking the carb has a blocked passageway somewhere, or some other problem.

I'll let you know what i find. Any other suggestions or input? Which parts of the carberator are utilized on the low end of the power spectrum (this seems to be where it wont fire).

Thanks guys.
 
I found that the carburator had a blocked passageway. The brass jet that threads into the float bowl (it has a stright screwdriver slot) was plugged with debris. I cleaned it out and quickly cleaned checked the rest of both carbs just to be sure.

Got her all back together and she came to life with both cylinders running smoothly, even at low RPM! ALSO, the problem with the idle not dropping back properly was I'm sure fixed by the new o-rings I installed in the intake boots. (thanks for that suggestion, easy and inexpensive fix for the idle problem)

OK, so a couple hours of riding and again at higher speed I had a cylinder stop firing for a period of 5-10 seconds, and this happened in three instances. The next day my dad experienced more of the same trouble and I think the carb is plugged up again! DAMN! This weekend I'll have it appart to confirm my suspision, or become even more confused.

I do have a filter in the fuel line, the cheap style with a single layer of fine mesh steel screen encased in clear plastic.

I'll let you know what I find this time.

:?
 
Follow up.

So, again I cleaned the carb again jet that is down in the float bowl. (It wasn't completely plugged like last time, but did have some specs of debris in it) I put on a good filter and new fuel hose. And in the process of buttoning her all back up and letting is sit and run on idle, I bumped the wire connector that feeds the coil for the trouble cylinder and it started running a little funny. Even though I went through all the connections last summer, this may be an elusive electrical problem that I couldn't reproduce previously. So I soldered it up good and got her all back together.

Been out twice so far for a total of 60miles and haven't had any trouble. :) !!!!
 
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Excellent . . . . glad it all worked out
Our old school bikes can be a lesson in patience
 
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