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Less power at low RPM??????? HMMMM

  • Thread starter Thread starter Anonymous
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Anonymous

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Ok I think I have a rat in my motor. I started noticing a slight diference in performance after a 4 hour ride yesterday. Today I took it out and no problem but a few minutes ago my wife sent me to the store and on the way home it was like riding a huge 200. The power just was not there and I could hear the carbs sucking like I was runing out of gas or somthing. once down the road and at about 4000 RPM it felt like 2 cylinders kicked in. The bike got louder and damn near brought the front end up at 45mph "Not fun in my bookI like both my tires on the grouns" I let off the throttle right away and power was once agan gone. Seems like its only running on 1-2 untill 4000RPM then 3-4 kick in. Oh by the way its a 79 GS850G. Thanks for any advice.
 
ok I checked and checked agan. Everything feels and sounds kind of normal untill the RPM gets up. As soon as I hit 4000 RPS it feels like pushing a nitrus button. Any ideas?
 
All the Gs's I've rode did the same thing. A guy I know just chopped a 750 out and I rode it. I rode great and when I opened it up I heard it kick in and I was gone. It was a great ride.
 
Inline fours in general tend to pick up power exponentially as the revs climb, but it sounds like you do have a problem. The GS motor should have a pretty smooth power delivery, it just comes on faster and faster as you climb. I imagine it to sort of look like half a bell, though I've never seen a dyno curve for a GS.

It sounds like you have something intermittent going on, either ignition or fuel related. Easiest is to go through your ignition first, make sure your connections are tight and clean, points in good shape and adjusted, plug leads sound, etc. If the bike consistently runs poorly below 4k it should be easy to find and ignition fault, check for spark on all four cylinders at idle.

If everything looks good there then look for fuel stuff. Maybe you have sticky float needles or something, see if any of your cylinders are running cold at idle (maybe touch a wet rag to each pipe right by the cylinders) and investigate along those lines. It doesn't sound like the problem is up in the tank but it can't hurt to check the screen and vacuum system, make sure you have a free flow of fuel from the tank. (you should only have this while vacuum is applied to the petcock unless it is in prime (Pri) position.

There may be other more specific things that some of the experts know, but that should get you going. If your 850 is behaving like a Kawasaki Mach IV then I'd tend to say there's something amiss for sure.
 
Flow form the tank is ok. I have an aftermarket shut off valve in it and have disconected and pluged the vacume line to the petcock. While cold it dose not seem to run on all 4 but as soon as it warms up all 4 run fine. and I am geting spark at all4. Maybe its time for a carb rebuld and compleat bike tune up
 
lots of ideas posted.....

The key thing I read in the original post was "different lately" or something to that effect.

So something is different than it used to be so something is going on......

Hmmm....low rpm - sometimes fouled plugs do this......or when compression gets a little low in a couple of cylinders.

I don't know if a sediment in a carb float bowl would behave like this. I would think it would only if the restriction were partial and the cylinders were running a little lean. A lean condition can miss at low speeds, but when the mixture flows faster at higher RPM, the engine mixes the fuel better and the engine will then "fire" the lean cylinders.

More comments from the P-nut gallery?
 
your carbs may be badly out of sync, with the needles stuck or something like that.

It could also be the ignition timing, depending on what model you have, you may have to adjust it yourself.
 
Best bet is to give the carbs a good cleaning first. Carb sync wouldn't be intermittant, same with timing It is a good idea to check them when doing a tune up. . The timing can change a little if you have points, other wise it is something that gets set and forget about it. Once you get to about 6K rpm you will notice a power surge it will pull all the way past redline. No need to run over 9K If you still have problems after cleaning I would consider replacing the coils
 
The 3,4 cylinders rules out it being electrical or ignition. Since it "come one" and runs well above 4k rpm, its probably not valve adjustment either.
At about 4k rpm, the engine will be drawing harder to get fuel. I would say you have a fuel starvation problem on the 3,4 carbs. Probably debris in the jets and partly clogged in the low speed end. Also, if the carbs are badly out of synch, the problem could occur. Best guess is it is a combination of the two, dirty carbs and poor synch.

Earl
 
I dont know if this will help, but I run a home made intake system and filter that gives me a big rush once on the cam and life is pretty miserable below 5000rpm. The carbs are balanced and perfect and when I put the stock box and filter in it runs nice off idle even..

That being said I would double check all the boots and trumpets for holes, cracks or seperation before ripping the carbs into peices on your kitchen table... CJ

Either that or pretend you have a 2stroke engine...
 
Thanks a million that answers a lot of questions. I know my carbs are badly out of synk but did not know that would do this as the bike was runing fine before all this started. Here comes the bad news. I kept riding the bike "Daily" with this problem and now have a bigger problem. Now2-3 dont want to fire at all. I was stupid and jumped on it the other day and at about 70mph it felt like hiting the breaks. No power at all and the bike just stoped wanting to go. Thismorning I took the airbox cover off and shot a bit of carb cleaner into the 2-3 carb intakes and the cylinders fired and everything worked fine with the exception of the bad hesitation most likly do to the lack of synk. My air filter is also pretty bad so I guess I should replace that as well. Also just a reminder if it helps this is a bone stock 79 GS850G shaftie
 
if your carbs are clean and synced check your plugs and coils....my bike had the same problem it would feel weak and only when it sped up the other cylinder would come on i switched the coils and the problem went to the opposite side
 
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