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need a little carb tuning help (I think)

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I think I have a carb tuning problem on my 81 450 twin. When I'm starting up the bike, my right cylinder doesn't fire and it idles low enough that if I don't hold the throttle at about 1/8, it falls to 600 rpm and then dies. As the machine is warming up, it backfires through the right exhaust pipe. Once it is warm it fires on both cylinders and performs pretty well, except that when I go to 3/8 throttle it revs to 4700 rpm then lags for half a second before climbing up again. There seems to be a significant rattle during this lag, but it's hard to say.

I've got a couple theories you can comment on (or suggest your own):

1. The backfiring is caused by a lean condition during warmup. (I'm still not sure how to properly fix it.)
2. The lagging at 4700 could be caused by some worn valve harmonic or something.

I have read the following web resourses, but I'm having trouble applying them:

http://www.mikuni.com/fs-tuning_guide.html
http://www.motorcyclecarbs.com/jetting/carbs101.pdf

I'm sure somebody will recommend tearing the carbs apart and cleaning them, but I took the bowls and slides off recently and it looks pretty clean. The gasket is in good shape too. I think it was cleaned sometime recently before I bought it (this summer). So I'll save the carb cleaning for next to last. Tearing apart the engine is dead last.

Thanks,

Michael
 
I don't think you need to tear into the engine. Check the float level on the cylinder that fires erratically at start up. Make sure the choke is pulling on and pushing off properly on both cylinders. Could be synching too. Synching a two cylinder can be done without guages. You should have a throttle stop for overall idle an an individual adjuster on one of the carbs. Unplug the spark plug on the cylinder with the individual adjuster. Turn the throttle stop up so it will idle on the one firing cylinder. Set the idle with the throttle stop to an rpm and remember that rpm. Now stop the engine and plug the other cylinder back up. Now pull the wire off the plug of the cylinder you just set the idle on. Turn the individual adjuster up high enough to get the engine to run on one cylinder. Then use the adjuster to set the idle the same as the first cylinder. As you're doing this, if you have access to them, set the mixture screws for the smoothest idle you can get.
 
You'll need to have it warm for two reasons. You want to adjust the mixture screws first for the smoothest idle if you have access to them. And that should be done with a warm engine. Then as you start setting idles for each cylinder the engine will need to be warm so it will idle on one cylinder.
 
Alllrighty! I did the carb synch. My main idle cylinder, the right, was idling the bike at something like 600 rpm, which is to say that it was not idling at all. So I changed my main idle so that the right cylinder ran the bike at 1300 rpm, spec idling speed. Then I switched to the left. 2000 rpm. Pulled it down to 1300, using the small screw that offsets from the main idle. It was a tricky manuver, to be sure. I had to get somebody to hold the gas tank so I could keep the engine supplied yet access the carbs. Anyway, just a very small adjustment (less than a turn) brought the bike down to 1300. So I hooked it all back up and found that the motorcycle then preferred to run something like 5000 rpm with both cylinders firing. So I brought that down, tested it, adjusted, tested, etc.

Anyway, I took him for a spin around town via the local bypass highway. He runs like a champ. Well, not perfect, but definitely strong. And my initial weak cylinder and backfiring thing seems to be gone. Of course, I don't let it idle very long, but suffice it to say that Mr. Ricks was right on the money with the throttle synching thing.

I believe this will also be fixing a little problem I didn't mention earlier, which was that I was having trouble getting one idle setting to both run cold and not fly away with the tach when idling hot at a red light. I'm thinking that the one cylinder was dragging it down and killing the engine when cold, but when the one cylinder came to life it was too stong and revved all high. Of course, you know all this, but I'm just letting it be said. Ho hum.

Thanks so much for the help. I'll definitely remember this fix for next time. Good call. Thanks again!

Michael
 
change your plugs now to make sure that one that was running poor isn't fouled up too bad. :)
 
OK, so I got the bike running better, but I had the left exhaust off the other day, then went to warm it up (so I could put a clear coat on the left case cover), and it was spitting two-inch blue flames out of the left cylinder when the throttle was any above idle. What does that mean and how do I fix it?

Michael
 
Tight exhaust valve? Have the valves been checked in the not to distant past?
 
Jeckler said:
Tight exhaust valve? Have the valves been checked in the not to distant past?
Click to expand...

No, valves have not been checked. Tearing into the engine would be last on my list of options, would it be the jet needle?

Michael
 
mopolopo said:
Jeckler said:
Tight exhaust valve? Have the valves been checked in the not to distant past?
Click to expand...

No, valves have not been checked. Tearing into the engine would be last on my list of options, would it be the jet needle?

Michael
Click to expand...

Valve clearance checking is regular maintenance. Any idea when they were last adjusted? Spec is 3k, I believe. I don't think it's a carb problem.
 
mopolopo said:
OK, so I got the bike running better, but I had the left exhaust off the other day, then went to warm it up (so I could put a clear coat on the left case cover), and it was spitting two-inch blue flames out of the left cylinder when the throttle was any above idle. What does that mean and how do I fix it?

Michael
Click to expand...

I think that flames out of the exhaust port is normal when no exhaust is mounted. My GS1000S did that after replacing the headgasket and adjusting the valves and firing it up for the first time without the pipes on. But correct me if i'm wrong.
 
I can see where robinjo might be right. In a four stroke engine, the exhaust gases being expelled may not have been fully combusted during the ignition/combustion stroke. When those very hot gases are exposed to more oxygen (as they come out of the port with no exhaust pipe on it) maybe the combustion of the remaining non-combusted gas gets triggered.

By the way, I would guess that running the bike without the exhaust pipe on would be useless for tuning purposes because of the lack of appropriate back-pressure created by the pipe/muffler. All those folks who change their pipes end up having to re-tune the carbs and/or air filter, so having no pipe at all would certainly have an effect, right?
 
And by valve adjustment, you mean the deal with the tappets?

Michael
 
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