wont accelerate

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azcafe

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Hey All,

So I just finished my rebuild and I was riding the bike a little the other day and all of a sudden it wont accelerate very fast. I though maybe it was the clutch so I replaced the friction plates and took out the synthetic oil and used a deisle oil instead and that didn't make any difference. It starts and runs fine and revs just fine. Its when you shift into gear and start to take off it almost dies when going taking off in first and accelerates at a very slow rate. It will get up to speed it is just extremely slow at getting there. I have taken both spark plugs out and they both fire when kicking the engine over. I just cant tell if its an electrical issue or a carb issue. I have a 1971 xs 650 with pamco and HHB PMA and am running a sparx capacitor. Let me know if you guys have any suggestions.

Also I already checked the cam chain and valves and they are good.
 
I hadn't thought about that Ben but I could see that possibly being the issue. So even though it revs fine when the clutch is not engaged that can still be the case?

Is there anything else I should check before I tear down the motor?
 
azcafe said:
Is there anything else I should check before I tear down the motor?
Click to expand...

YES!
IMHO as an experiment wire a "real battery" in place of the sparx see what happens.

Check your timing.
look to the idle circuits in the carbs. Does application of choke change anything?
place a container under a float bowl remove the drain screw does it flow out a pint or so steadily? Check both carbs this way. Any crap in the container? If so correct the problem, clean the carbs (yes, again:doh:).
Do a compression test, just because.
Are the carbs synched?
If you don't have electronic advance, check that the advance rod is functioning smoothly.
 
Thanks for the info Gary.

I will try all those tonight and get back with you on what happens.

I have not synced the carbs yet because I was just a little nervous about trying the whole dead cylinder thing but I guess I better get it done.

So the way I understand it is to get the bike running and let it warm up for a bit. Then shut the bike off pull out one spark plug and ground it to the motor somehow. Plug the open spark hole with a spare spark plug and then start the engine on the one firing spark plug. Then I adjust the throttle cables until the motor is about to die and then do all that for the opposite cylinder?
 
What carbs you running? Simplify; remove plug wire, stick in a spare plug, ground to engine. Some bare copper wire wrapped around the base of a plug and any handy grounded metal makes this easy.
 
Retired Gentleman,

I check the snap back on the advance and it seems fine. I still might lube it though since I have never done that. What do you suggest using just some oil?
 
azcafe, what carbs do you have on your 71?

My BS38s have the diaphragms with jet needles that can be moved to different positions. Damage to a diaphragm is likely to affect acceleration. Have you had those carbs apart recently? Putting them back together incorrectly like I once did has the same loss of acceleration - I re-fitted the jet and c;ip on the wrong side of the plastic disc.

Anlaf

that would affect acceleration

Anlaf
 
I rebuilt the carbs a little while ago when I was building the bike. everything was good when i first ran the bike. I had maybe 50 miles on it when I was riding to work one day and then all of a sudden I lost acceleration. I then had a extremely difficult time getting in to first gear. Basically had to give it all my throttle. When I got to work I noticed one of the carbs had come off the manifold and was just hanging there. So I figure Ill check the carbs and see if somehow that messed it up.

My carbs are off a 72 i believe.
 
axcafe, has the difficulty getting into first gear resolved?

That's a symptom of the transmission maladjustment that can be resolved by replacing a couple of springs under the right-side cover. That's another something you can look forward to.

Anlaf
 
Hey All,

So I took apart the carbs tonight and cleaned them and adjusted the float bowls which were off. I put it all back together and started it up to see if it made any difference. It still started idled and reved fine but wont accelerate correctly. I also noticed I had my clutch rod in backwards so I switched that around and I could certainly tell my clutch felt better.

So I checked to see if the choke made any difference and it made a big difference. When the choke was up it idled great but when I put the clutch down it ran super rough and blew black smoke out the right side exhaust. When I put he choke back up it idled fine and no more smoke out the exhaust. I still haven't synced the carbs yet and haven't checked the timing would those be the things to do next?

I posted a video of what is happening so you guys can see if you can provide any suggestions.


Also I pulled each individual throttle on the carb seperatley and when I did it on the right carb you could here it rev when you do it on the left carb it doesn't really rev it just sounds like its sucking in air. I will try and get a video of that tomorrow and post it
 
Black smoke suggests combustion malfunction, so at this point I would go through the adjustment sequence of cam chain adjustment, valves and then any further carb adjustment. What size jets are you on, by the way?

Anlaf
 
Anlaf

I am running stock jets. I have made all those other adjustments already. I will go back a double check them.

GGGary,

When I am doing to carb test where you drain gas into a cup do I do that with the bike running or the bike off?
 
Drain the gas with the engine off. You are looking for good flow and debris in the gas. I cut up a quart oil bottle to make a convenient gas catcher that fits the tight space. One of the reasons I buy valvoline oil is because the bottles are white... I think someone said this, but check diaphragms for holes or tears. Check floats for leaks.
You mentioned you just did a rebuild. How long had it been since the bike had last run? Real common for ethanol gas to dissolve ancient varnish out of an old gas tank and dump it into your carbs.... Don't use old fuel lines, they rot internally and send chunks of rubber in to fuggle carbs.
 
GGGary,

I basically replaced everything inside the motor. I have no idea when the last time it ran was.

I put on a old Honda tank on there that I red koted. Also I used new fuel line when I put it all together. I just pulled the carbs apart last night and nothing seemed to be clogged.

One thing I did notice after I cleaned the carbs last night and put them back on the bike and started it up was the left carb spit out all the cleaning liquid out the air filter. What would that indicate?
 
An overflow usually means an issue with float /float valve.
are the little spring tabs on the float valves free with good spring tension. float valves/seats clean and shiny? replacing the drain plug with an adaptor (made from a drilled out 90 metric zirk) and clear plastic tubing to see actual fuel level sometimes reveals issues.
 

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