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CR carbs setting problem?

  • Thread starter Thread starter Addy Leung
  • Start date Start date
A

Addy Leung

Guest
Help! Need professional advice.
Recently install 37mm Keihin CR crabs on 81' GS1100EX with all stock air box.
The setting of the crabs is following.

Main jet - 120
Main air jet - 230 ( range: 80 Richer to 250 Leaner)
Slow jet - 70 ( range: 35 -82)
pilot air jet - 2 turn out
Jet needle - YY9 middle size (YY5 Richer to YY3 Leaner)
Exhaust pipe is modified 4-2-1 with stright through 54mm can & head pipe is 38mm.

It can keep idle at 1100 rpm and the bike go well with carbs open below 3/4 and it can reach 8000rpm (9000rpm red line)
But when carbs full open at range 3500rpm to 45rpm, it dose not go up until carbs close back to 3/4 or less.
Spark plug check is very dark with carbon dust.

Does it need replace a smaller main jets and slow jets and a richer main air jets? or something else? :?
 
you need to explain a little more. if you are higher in the revs say 6000 what happens when you give it full throttle what colour plugs in the higher revs. what is done to the motor. it does sound a little large in the carb dept unless you have done a lot of mods..ozman
 
With smoothbores, and especially big ones, you just overwhelm the motor when you whack the throttle open. You end up with too much volume and not enough velocity. You have to use more of a roll the throttle on approach. It's not like CV carbs where you peg the throttle and let the slides do their thing.
 
i agree with you billy but he also seems to be saying he cannot use full throttle at all. 3/4 throttle gets the bike to 8000rpm. dunno sounds too rich need more input ozman
 
Ozman & Billy,

Thank you very for your advice.
The carbs still can not full open at 6000rpm or any other rpm.
But the performance is better than the CV carbs as long as carbs only open up to 3/4. The motor is stock without any mod but it is only a K&N filter putting in the stock air box.
I did not check the plug color at rpm and will try few days after.
If the bike is at neutral gear and I suddenly open the carbs to 1/2, rpm go up and close the carbs immediatly and repeat it few times. slight back smoke will come off from the exhaust.

Dose the slow jets/70 and main air jets /230 too extreme?
I just think should the slow ject back to 45 or 50, main air jets go to 160 and main jet down to 110? Addy
 
If what I think you are saying is that they just won't pull the slides all the way up it's because of cable adjustment. With my RS flatslides there is a point of cable adjustment where they start opening less and less. Until you get the pull cable adjusted right this can happen. Adjust it a little at a time until you get the slides to raise all the way.
 
Billy,

Sorry for my poor expression, the carbs physically can open full and all the way up is smooth.
But the motor will not provide power when the engin is running at 6000rpm with full open carbs and the exhaust sound like - pop-lu-pop-lu. If it is return back to 3/4 open, the motor will go strightly with strong power.
Dose it seem too rich when full open?
Besides, dose also 70 slow jets are too big, the original standard size in the CV carbs is about 40.
Addy
 
Addy,
That does seem pretty big for the slow jets. Usually somewhere in the 35-45 range is all I've seen. Topend problems usually are too rich of a mixture.
 
if they're anything like VM type carbs (no diaphram) they are inefficient at low RPM and full throttle. it will bog until it hits a point where it can pull air in fast enough. the roll on throttle approach as suggested is what you have to do.
 
Billy,

It would like to learn more from you that how is the main air jet work? I only know it control the air amount go into the Nozzle Holder(which is a small cylinder enclose the jet needle and let the fuel mix with the air before go into smooth bore). Should the size be leaner or richer for general street purpose?

Thanks

Addy
 
Addy,
The 120 mains don't seem too far off. I think I'm running 115's or 120's in my RS's. The 70 slow jets are huge but really shouldn't be effecting the topend rush. Try some 115 mains and see if things get any better. Until you get the mains right you can't do much with the needle positions. At full throttle you are completely on the mains. From 1/4 to 3/4 throttle you are on the slide needles. Below 1/4 throttle you are mostly on the slow speed jets with the needles starting to take over as you open the throttle. There is a bit of overlap as you go from one circuit to the next as throttle position changes.

The nozzles you are talking about is the needle jet and the needle is called the jet needle. The taper of the needle is what determines fuel mixture and different throttle openings.
 
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