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Air screw doesn't raise idle???

  • Thread starter Thread starter Smellii
  • Start date Start date
S

Smellii

Guest
Valves adjusted
New battery
Airbox sealed
New tires
Calipers rebuilt
Exhaust sealed properly
Carbs rebuild (just this weekend, freshly installed, and syncing now)

Why would my idle speed not respond to an air screw adjustment? I synced my vacuum but when I adjust my air screws I can't get it to idle higher. My air screws were two turns out but I can turn them all the way out (up) with no change in idle speed. From the same position (two turns out) I can turn them in about one turn and get it to stall.

Is it possible to sync the vacuum too low? Could this be the reason my bike isn't responding to a mixture change? Higher the vacuum the more sensitive the bike is to fuel/air ratio? Is it even possible to change the vacuum pressure while keeping all four carbs synced?
 
Hi,

Your carbs do not have"air screws", they have "idle mixture adjustment screws". If the idle no longer responds then you have turned them out too far. Turn them back in until the idle starts to decrease, then turn them out 1/8th turn or so. The engine will tell you where they should be set.

Sometimes, if the screws are turned in too far, the tips will break and they will not adjust at all.

The idea behind a vavuum sync is to balance the carbs relative to each other with the #3 carb being the "master", since you can't adjust it. It would be difficult to vacuum sync them "too low".

At what rpm are you doing your sync?

Thank you for your indulgence,

BassCliff
 
Smellii said:
...From the same position (two turns out) I can turn them in about one turn and get it to stall
Click to expand...

You said it right there. Turn the mixture screw in till you hear it change, (idle drops) then back out 1/8th.
 
I'd set them at 2.5 turns and leave them alone.
 
I am with Ness on this, I don't know where the 1/8 comes from, but then I was not good at maths when I was at school. I would not go under 2 turns out.

Mine are at two and bike runs well, when they were at 2 1/2 turns out I had a slight stumble just off idle. Took them in to 2 turns out and stumble gone, idles nicely and plugs look the correct colour.

Adjust them so it feels right on the road, idles well, pulls well and has smooth transition through each stage. Every bike will be different, I know that 2 turns out on my motor is a bit less than is usual, but it works on my motor. The 2 1/2 turns out is more normal, but it depends on the condition of your motor and the carbs as well.
 
Ok. I set them to 2.5 turns out and carbs 2, 3, and 4 are perfect. Bike has never sounded better...except for carb 1. The threads in carb #1 where the idle mixture adjustment screw goes are all messed up. It's really really hard to turn but I thought I had it down far enough comparing to the depth of the screw from the other carbs using a micrometer. Plus there is a gouge, looks like from a drill bit or something from the previous owner, along the threads in the carb body. I'm going to fill in the gouge and clean up the threads with a tap. From a search I found someone had the same problem and was going to use a 6M X 0.50 or 6M X 0.75 tap. Can someone validate either of these being the correct size for a mixture screw? How do you measure these?

Thanks for your help, guys. My bike is almost finished. This is the last thing it needs to be road ready! Woot!
 
Last edited:
Smellii said:
Also found this web page with some tips for the CV carbs.

http://www.nightrider.com/biketech/hdcvcarbappendix.htm
Click to expand...
Those are pretty good tips for the KEIHN CV-type carbs, but that does not mean that they will also work for our MIKUNI cv-type carbs.
well1.gif


.
 
so they aren't the same???? ugh! ok. so do you happen to have thread specs for mikuni cv carb mixture screws?
 
Smellii said:
so they aren't the same???? ugh! ok. so do you happen to have thread specs for mikuni cv carb mixture screws?
Click to expand...
Yeah, they are about as much "the same" as Fords and Chevys. :p

Hey, they are both cars, they must be the same, right?? :rolleyes:

The "CV" in the name of the carb is a type of carb, not a model. It stands for "constant velocity", which means that it has a variable venturi (the area under the slide) that changes in response to changes in air flow, to maintain rather constant velocity (hey, there are those words again) in an effort to maintain consistent jetting. Several manufacturers had the same concept, but used different methods and parts to make their carbs.

Sorry, I don't have the specs for the threads.
shrug2.gif


.
 
got it. (learn something new every day I suppose.....well like 38 new things in my case.) I measured it the best I can with my micrometer and it seems to be 6M X 0.75. Going to buy a tap and see if it's the right size. Thanks, Steve.
 
I have a similar circumstance. My screws didnt change the idle. After I changed needle position, It changed on 3 out 0f 4 carbs and not a drastic change. But since it is running like a scalded dog, Ill just set them a t 2.5 and check the plugs after a while.
BTW Im telling everyon how im in love with this bike now (like tom cruise/katie holmes love)
 
Good advice. I did the same and now it's running great. Still have to fix the threads for air screw #1 so I can get it even with the other three and I'll be set. Woot!
 
Wow. I've spent 2 days looking for a 6M x .5 tap. Not even a specialty fastener store has one. None of the machine shops here even have one. Will cost me more to have one shipped than it's worth. Have to wait till I have more money. hehe. (yeah I'm poor) On my list though. Thanks for your help guys.
 
Smellii said:
Wow. I've spent 2 days looking for a 6M x .5 tap. Not even a specialty fastener store has one. None of the machine shops here even have one. Will cost me more to have one shipped than it's worth. Have to wait till I have more money. hehe. (yeah I'm poor) On my list though. Thanks for your help guys.
Click to expand...
How much is the tap + shipping compared to a new carburetor?
 
It's not the relative cost that's the problem. It's the fact that it costs money. :) I'll get it fixed when I have the money. No biggy.
 
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