Problem - Bike Idles But Will Not Rev

[Arlof]

Anyone ever heard of this one, tried a lot of stuff everything is hooked up good and tight. I see the throttle cable pull the throttle even move it by hand. Any idea why my bike would idle but not accelerate?

'90 GS500

Regards,

Arlof

[scratch]

Your last post was April 9th, has your bike been sitting since then? If so, I would venture a guess that your carbs have gummed up with bad gas, or your fuel filter is clogged. Also, when was your last valve adjustment? If a valve isn't closing all the way, you are losing compression.

Also, since your previous post, did you synchronize your carburetors? That would explain why one carb would need a little throttle to start the bike, it's because the butterflies aren't opening at the same time.

[Arlof]

Hi, there is new gas, the plugs are covered in black soot.
I suspect that it is too rich so have gotten smaller main jets, and have gotten new needle jets a bit leaner than original. the pilot jets have been replaced also. the floats seem fine when we checked them with a hose, fuel comes up to the gasket line. The carbs are sparkling clean and we have had them apart so many times we have the parts memorized.
As far as i know the valves have never been adjusted and when i had the carbs off i could see inside and noticed bubbles coming from around one of the valves. I called up a dealer and they suggested i check the compression because if it has low compression it won't suck the fuel in when the trottle is cracked open. This was news to me so plan to buy a compression tester today and check it. How do you adjust the valves?. if compression is 80 per cylinder would that cause my problem? how about 100? what should the compression be?. To me it doesn't make sense that it will idle but not rev. Is there a separate circuit to idle, and one to run?

[Arlof]

130 on the one 135 on the other. So that is too low or just right or what Haynes manual doesnt' really make much sense. It says it should be min 114 psi. But then makes another takement about how one of them should be over 142.

*sigh* Overhaul doesn't sound that fun. I found something on another bike about backing pilot jets. Ours are snug to the carb right?

Any other ideas? :dunno:

[Kerry]

Yep, what you said:



At least one cylinder should have 142 psi or more.  The other can be as low as 114, as long as there is no more than 28 psi of difference between them.

But since "standard" is listed as 142 to 199 ... looks like you have a decision to make.  :dunno:

[cheesy]

130 on the one 135 on the other. So that is too low or just right or what Haynes manual doesnt' really make much sense. It says it should be min 114 psi. But then makes another takement about how one of them should be over 142.

*sigh* Overhaul doesn't sound that fun. I found something on another bike about backing pilot jets. Ours are snug to the carb right?

Any other ideas? :dunno:

make sure you are holding the throttle WIDE open while you check compression.  it's easy to get false readings if you don't do this.  my numbers were where yours were until I did it with the throttle open... then I got much higher... like 165-170 on both I think

[cernunos]

Arlof, hi, another thing to do before you go tearing into your engine: With engine idling and air filter box removed; Open throttle and then raise the slides with a screwdriver or reasonable facsimile. In other words, when you open the carb butterflies also raise the slides to see if the engine is revving. If the engine revs when you do this then you have a problem with the slides not being raised by engine vacuum. Love the little twin, Brazil (Sugarloaf Mountain is a wonder), and this forum.

C........

[Arlof]

Arlof, hi, another thing to do before you go tearing into your engine: With engine idling and air filter box removed; Open throttle and then raise the slides with a screwdriver or reasonable facsimile. In other words, when you open the carb butterflies also raise the slides to see if the engine is revving. If the engine revs when you do this then you have a problem with the slides not being raised by engine vacuum.

Hrm, No Difference on that front.

make sure you are holding the throttle WIDE open while you check compression. it's easy to get false readings if you don't do this. my numbers were where yours were until I did it with the throttle open... then I got much higher... like 165-170 on both I think

Cheesy I dont' really understand how that could be. Did you have the bike running on one plug or something? My pressure didn't change with the throttle open or not!

However if I covered the air intake the PSI dropped to a solid 60 PSI on both. Should it increase between throttle fully open and at idle? Can someone test it on there bike for me?  Also i'am i doing it right? I have both plugs out, the tester in one and the other open.

Thanks for writing!

Regards,

Arlof

[dgyver]

Sounds like an intermitant spark problem. Typical of a low rough idle (around 500-800 rpms) that will not rev much higher than 1500 and wet black plug(s).

While the bike is running....
Pull one wire off a plug. Does it die or keep running?
Reinstall it and do the other side. Does it die or keep running?

If the bike dies or changes its idle when either of the wires is pulled, change your plugs.

If the bike does not die or does not change its idle when one of the wires is pulled, check the wires from the ignition box to the coils for corrosion or a bad connection.

[terry1013]

Hey Arlof....
   When you rejetted the carb did you clean or make sure all passages were open? If you look into the carb opening where the boots hook on from the airbox there are 2 tiny holes at the bottom that allow air into the carb housing ports. I had to spray carb cleaner in them and take a piece of 16 gauge wire apart and use one of the fine strands to work out clogs in those ports.     My advise would be to get a can of carb cleaner with one of those plastic spray tubes so you can spray directly into the ports. I'd take carbs off again.  Remove the float bowls and floats. spray into every small port there and in the intake port and see if the spray comes out somewhere else in the carb. you may have to hold the butterfly open at times to open certain ports but in all cases I found even after my first cleaning, clogs and rust came out of a lot of places. Spray into the T fittingg where the fuel line hooks up. I got a lot out of there. I then blew out all the ports with air to remove the residual carb cleaner.
   I would highly recommend removing the plastic glide holders from the carb bodies and cleaning the emulsion tubes under them and replace that friggin O ring in there. Mine were mush and the root of a lot of my problems. The emulsion tubes have 4 pinsized holes on their shaft that
were clogged shut on one of my carbs. Rejetting only helps or matters if the rest of the carb is in working shape.
 I assume you removed the plugs over the air mix screws and they're out at least 3 turns to run initially?
 After I rejeeted mine I had a crappy running engine till I dialed in the air mix screws. One cylinder was cold other hot. Too much fuel for the plug to ignite from a weak spark plug. I almost thought the same as you, ignition problem, but reasoned that it did that from the result of something "I" did. Friggin carbs.....  Use fresh plugs!