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my carb delima

  • Thread starter Thread starter acp4me
  • Start date Start date
A

acp4me

Guest
ok 1978 gs1000

motor stock, dyna coils, dyna ign., V&H header, K&N filters, DYNOJET stage 3 kit., #138 mains D.J, needle 3 notches up from bottom, with spacers in the correct order. Air pilot - one and one half turns out. Fuel mixture under carb-one and three quaters turns out. Pilot jet #15. Float height .94. New O rings in carbs and manifolds. Carbs bench synched, vacuum synched. Valves ajusted. Timing set. Mechanical advance unit working and free... My problem is when I rev. the motor from 1000 rpm to 2500 and up, the bike does not return to set idle. It might stay at 2500 rpm until I blip the throttle, then it will return and stay rock hard at set idle speed. I looked and looked for a vacuum leak and found none. So I removed the throttle cables thinking something was hanging up. Same results. I re-synched carbs. Same results. I removed the carbs with gas in the bowls and removed bowls. The gas level in one carb bowl was lower then the other three. Does this sound like a float height problem? Calling all carb gurus. Thanks, Darrell
 
I'm not exactly sure how you're throttling it to make it stick. If a quick blip makes things good, but a slower/deliberate opening/return of the throttle allows the idle to hang up, then it appears to be throttle housing or cable related, though a slide could be sticking.
If the Dyna rotor/collar (with the magnet) isn't mated to the mechanical advancer correctly, you can have this problem but assembling those parts is pretty obvious if you did it wrong.
It's not an intake leak. A leak would not be effected by various throttle actions. It would be constant.
Just to mention, from my past experience, I think your mixture may be lean regarding the jet needle positions. 3rd position from the top is generally lean. 4th position from the top seems to be the sweet spot with your mods. If your bike is in otherwise good tune, I bet a solid 1/3 throttle chop test reveals whitish/light gray plug reads. Maybe a little spit back through the pods at lower speeds, especially on colder mornings?? Just my opinion. The DJ recommended "base settings" for the needles (position 2) is poor info.
Also, I suggest you remove the two floatbowl vent lines if you haven't already. Fuel starvation/incorrect bowl venting will result, especially on windy days or in crosswinds.
 
Hi Keith
Thanks for taking time to reply. I think the needle setting is third position from the bottom. I will check tonight. Is fourth from the top the same as third from bottom? Vent lines were removed, throttle cables are disconnected, problem still there. Today after work I'm going to make sure throttle arm bolts are not too tight and causing something to bind. The throttle return spring is very stout. I find it hard to beleive that a slide or slides could be hanging up. Definitely a slow deliberate increase in rpms causes a sticking or hanging condition, you would think a tap on the throttle arm might return it back to set idle but it does not. A blip or two of the throttle works every time. When rpms hang up I can turn down main idle adjustment to reset idle at 1000 rpm. Then a blip of the throttle will cause the bike to stall.

Thanks for your time. Darrell
 
Yeah, my error regarding the jet needle position. I read your post wrong. Third from the bottom or fourth from the top is usually good.
I'm off to work so if I think of something I'll reply tonight. The hanging up sure sounds mechanical to me though. Something not moving smoothly.
 
Sounds like a mechanical hang up to me as well. A fuel related problem would change with engine heat. I would look at the cable routing, and if ou have some new grips just check that they are not binding. I have also had a cracked throttle grip housing (still have it, I have just fixed it!)

It is interesting you are running 138# mains as with some 28mm carbs, ported head, cams, K&N twind pods and exhaust I run 120#. So the extra flow does suck harder! (Oh, I do have a 1080 high compression motor as well)
 
Darrell, I'm thinking the twin throttle cables aren't adjusted/slacked correctly.
I don't remember off the top of my head the exact factory play for the pull cable, but it's about 2 mm of slack. 3mm max.
As for the return cable, many don't realize that the return cable must have ZERO slack to operate correctly. Most bikes I check have obvious over slack on BOTH cables.
Hope it's that simple. If not, re-check cable routing and check the cables for wear spots or if they need a bit of lube. Since the rpm's will drop as they should if you blip or use quicker grip movement, I feel it's a simple mechanical thing that only presents itself if you're kind of gentle with the throttle.
As for true mixture related symptoms, typically, if the rpm's hang a bit and are slow to return, that's a lean mixture. If blipping the throttle results in the rpm's dropping significantly below the set idle rpm before returning to normal, that's a rich mixture.
 
saaz said:
Sounds like a mechanical hang up to me as well. A fuel related problem would change with engine heat. I would look at the cable routing, and if ou have some new grips just check that they are not binding. I have also had a cracked throttle grip housing (still have it, I have just fixed it!)

It is interesting you are running 138# mains as with some 28mm carbs, ported head, cams, K&N twind pods and exhaust I run 120#. So the extra flow does suck harder! (Oh, I do have a 1080 high compression motor as well)
Click to expand...
Saaz, I run 138 DJ mains as well. 1085, K&N's, V&H pipe. 120's are way too lean in my experience for your mods. Maybe you mean you have 120 MIKUNI mains? That would be a bit closer, since a 138 DJ is approx' the same as a 130 Mikuni jet, but if that's the case, I figure you're still too lean by about 2 full sizes. Just my opinion.
 
Couldn't be cable related if both throttle cables are disconnected as stated. I'd say squirt the linkages down with WD-40 and see if anything improves, if it does try to find the binding and fix it.
 
Darrell, sometimes the throttle pulley-to-throttle shaft isn't aligned right and it will rub a bit on one side or the other. Check for any clearance problem. The holding nuts for the synch/slide adjuster screws should be torqued no more than 3.5 ft/lb. Same for the throttle arm-to-throttle shaft bolts.
Also, a poor synch at one carb can cause your problem. I know you vacuum synched with a tool, but it's possible? I still lean towards a mechanical thing.
 
Crap what do you have to lose. Adjust the float and see. I'm thinking it may change something. Gotta' have them the same.
 
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