Pilot Screw Question

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johnblaid

johnblaid

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I did not know what these were until I got a manual. The manual says they are preset. Does this mean that they should be screwed all the way in. I note that attempting to "adjust" them gives variable results but the left front carb sends the rpms through the roof when I close it completely. Closing a couple of the others makes the rpm drop. I probably need a carb rebuild but if someone could answer this I'd appreciate it. It appears from reading the forums that they come in different diameters.


I know this is a noob question but nobody else seems have mentioned a simple answer to this question.
 
Present means Yamaha preset at so many turns out from fully seated based on emissions testing.
 
I may be wrong....but isnt another name for the pilot screws....is the A/F mixture screws.

Here are the factory settings.

1-2.5
2-2.75
3-1.75
4-2.75

I have mine 3 turns out for all 4 carbs.
 
Last edited:
ah. wonder why that one makes the engine run so much better when its completely closed
 
Well the pilot jet is what gets clogged...and you have to do a shotgun, and clean them.

So here is a question....when you mess with the A/F screws....are you regulating air at idle....or is it fuel? Unfortunately, I dont know the answer...and thats probably a question for Mark, Danny, or Sean

EDIT*** found the answer....when you turn the screws in....you lean the mixture out. So...the A/F screws are controlling fuel.

EDIT AGAIN....Marks answer below is the better answer.
 
Last edited:
Traumahawk said:
Well the pilot jet is what gets clogged...and you have to do a shotgun, and clean them.

So here is a question....when you mess with the A/F screws....are you regulating air at idle....or is it fuel? Unfortunately, I dont know the answer...and thats probably a question for Mark, Danny, or Sean
Click to expand...

You are correct, they are mixture screws. THey control the mixture coming out of air from PAJ's and the PFJ. Some of the mixture goes out the bypass holes just below the throttle valve and used for off-idle mixture.
 
A good base setting would be 2.5 turns out from seated and adjust from there as/if needed.
 
Well here is another question....how long has it been since the carbs have been synced?
 
Keep in mind that just because the revs go up doesn't mean that it is running better. It just means that you have leaned the mixture a lot. It can actually be a very bad thing, be careful. have you ever noticed just before your lawn mower runs out of fuel that it revs up for about 5-10 seconds before it quits? It's running out of fuel but still gets the same amount of air, that is running a very lean mixture of fuel to air. There is no one perfect setting for those screws and almost all bikes will vary, otherwise they would not be adjustable.
 
Traumahawk said:
I may be wrong....but isnt another name for the pilot screws....is the A/F mixture screws.

Here are the factory settings.

1-2.5
2-2.75
3-1.75
4-2.75

I have mine 3 turns out for all 4 carbs.
Click to expand...

Just FYI, factory A/F screw settings are different on each bike. My understanding is that they are set according to exhaust gas temperature. There are plugs in the OEM exhaust to take the readings from.
 
[email protected] said:
Just FYI, factory A/F screw settings are different on each bike. My understanding is that they are set according to exhaust gas temperature. There are plugs in the OEM exhaust to take the readings from.
Click to expand...

True...but the page that I got those from was an old VMOA discussion, and just so happens that they matched mine as well.

Ive found that by setting them a bit richer, you dont get that lean pop thru the air box that will happen occasionally.
 
Carbs were perfectly synced at about 21 IOM (factory spec is 22.6...very close) before I fooled around with the screws. All are at 2 1/4 turns now.

Ran out of gas today. Got barely 70 miles on a very full tank. What I did caused it to get WORSE gas mileage. Changed oil and filter, installed new correctly gapped plugs, synced carbs and adjusted those needles. WFT???

Will shotgun it this weekend. If that doesn't help, on the block it goes. I think it's jetted rich because it has kerker 4 into 1's and that's probably part of the problem. But 70 miles per tank just won't cut it. I can run a gen 2 for that.
 
Your best results will be to setup the carbtune sync and with the engine at 1200 rpm's
Turn each a/f mixture screw until the rod reaches it's highest point it can on the carbtune.
Then start syncing #1 and #2 carb-Go to the other side and sync #3 and #4.
Then sync #1 & #2 to #3 & #4.:eusa_dance:
 
Last edited:
blaxmax said:
Your best results will be to setup the carbtune sync and with the engine at 1200 rpm's
Turh each a/f mixture screw until the rod reaches it's highest point it can on the carbtune.
Then start syncing #1 and #2 carb-Go to the other side and sync #3 and #4.
Then sync #1 & #2 to #3 & #4.:eusa_dance:
Click to expand...

I like that. Thats like the old fashioned way of setting the idle of a car, using a vacuum gauge.
 
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