Topic: Keyster/Aftermarket pilot air screws have this much impact?

[MetroRedneck]

Carb swap is not moving forward with tuning.  Put a set of 627B carbs on a cb550 (manifolds back).  Carbs are super clean, floats set, manifolds sealed up, synced bench and on bike (although rich), all the boxes checked.  Started with 45 pilots (genuine Keihin) and was real rich at idle - black smoke burn your eyes rich.  Moved to 40 Keihin which would be stock 627B on a 500, still real rich, but a little better.  The pilot air screws do not seem to impact fueling at all.  I have flushed this circuit 3 times to make sure it wasn't clogged/trash. They are definitely aftermarket screws.  Wondering if this is more of the problem before I drop pilot jets again.  I can only find Keihin to 38, and aftermarket down to 35.  I don't believe either of those will be lean enough, and doesn't seem right anyway - I expected to be lean when I was testing with an open header and no filters.  I was running great with a 42 pilot on PD carbs on the same bike.  Doing the swap for cosmetic purposes.  Is this a fault of crappy aftermarket pilot air screws or am I missing something else?

[TwoTired]

Have you checked the air jet(s)? 

Have you compared emulsion tube holes?

The stock 550 uses 38 pilot jets.  (FYI)

Cheers,

[MetroRedneck]

Have you checked the air jet(s)? 

You asking about the pilot air screws?  If so then yes those are what I suspect are the problem.  I've checked and cleaned the circuit several times.

Have you compared emulsion tube holes?

Nope.  Can't.  I have to roll with what I have right now which is whatever aftermarket kit the PO put in these carbs.  I suspect Keyster because the pilot jet was the little short one.  I'm not going past idle yet.  I'll get to mid-high range later.  The main needles are set to 4th notch.  I've considered lowering them.  I do have main jets from Sirius in there.

The stock 550 uses 38 pilot jets.  (FYI)

Thx -  yep I'm aware.  I do not think dropping to a 35 would even benefit.  It's super rich, not even in the ballpark rich.  I might adjust the needle height to see if it improves.

Cheers,

[TwoTired]

Have you checked the air jet(s)? 

You asking about the pilot air screws?  If so then yes those are what I suspect are the problem.  I've checked and cleaned the circuit several times.

No, at the carb mouth entrance are air jets which feed the emulsion tubes. One is on the side of the carb inlet ramp (pilot feed) and the other feeds the mains emulsion tube.  They are something like 150mm holes.

Have you compared emulsion tube holes?

Nope.  Can't.  I have to roll with what I have right now which is whatever aftermarket kit the PO put in these carbs.  I suspect Keyster because the pilot jet was the little short one.  I'm not going past idle yet.  I'll get to mid-high range later.  The main needles are set to 4th notch.  I've considered lowering them.  I do have main jets from Sirius in there.

If the pilot jet emulsion tubes don't match what stock parameters are, they aren't going to aerate the fuel in the same way.   This is probably the source of your troubles, I guess.  Best to find and you genuine Keihin jets.


Cheers,

[MetroRedneck]

No, at the carb mouth entrance are air jets which feed the emulsion tubes. One is on the side of the carb inlet ramp (pilot feed) and the other feeds the mains emulsion tube.  They are something like 150mm holes.

Yes all clean.

If the pilot jet emulsion tubes don't match what stock parameters are, they aren't going to aerate the fuel in the same way.   This is probably the source of your troubles, I guess.  Best to find and you genuine Keihin jets.

Pilot emulsion tube?  Wasn't aware of such as a serviceable part.  I have genuine Keihin pilot jets in there now.

[TwoTired]

The emulsion tube is part of the pilot jet at the exit end.

Sorry. You typed:

I have to roll with what I have right now which is whatever aftermarket kit the PO put in these carbs.  I suspect Keyster because the pilot jet was the little short one.

If you have Keihin then I guess I can't help you.

Best of luck!

[MetroRedneck]

The emulsion tube is part of the pilot jet at the exit end.

Sorry. You typed:

I have to roll with what I have right now which is whatever aftermarket kit the PO put in these carbs.  I suspect Keyster because the pilot jet was the little short one.

If you have Keihin then I guess I can't help you.

Best of luck!

Gotcha.  Yeah I swapped slow jet to Keihin away from the Keyster parts when I went through them.  The main emulsion tube is from whatever kit was put in.  I assumed this is Keyster because of the way the pilot jet looked.

[MetroRedneck]

Set your fuel level with the Clear Tube method. Install fresh plugs, and idle the bike without the CHOKE on. Pull the plugs after a 3 minute idle. Inspect them. If the carbs are stock, induction is stock, then needle and jets should be stock. Making presumptive changes makes no sense to me...

I set clear tube, plugs are new enough - not brand new, too rich to idle with choke on so it is always off.  Everything in the idle circuit is stock except the pilot air screws.  I will have factory air screws in there today.  If they don't respond, then it will be time to jet down or lower main needles (I don't believe the main needle is having that much impact though).  I also can't imagine aftermarket air jets having this much impact.  I also have sourced Keihin emulsion tubes, so those may also be a next step if the other adjustments do not work.

Does the pilot jet length have an impact?  I ask because the Keihin pilot jets are 32mm and I believe factory in the 70's was a 28mm length.

[MetroRedneck]

Air screw helped. Probably dropped exhaust soot by half.  Making progress. I’ll raise needle clips next. Lots of fuel coming through main jet opening. I’m guessing the aftermarket emulsion tubes are also incorrect. I’ll try to make these work with needle height first.

Anyway to the original question yes air screws make a big difference. The air screws were way off. Angle and openings were different.

[MetroRedneck]

Adding more information for anyone looking in the future.  Like is always stated, trust your stock brass and use it over anything in aftermarket Keyster/K&L kits.

After swapping air screws, dropping needle height, and putting in 100 mains (to try to close off the main jet)I still had very rich conditions at idle. I could see droplets forming around the needle jet and droplets bouncing in the throat so I knew it was pulling too much fuel.  I sourced a set of stock 73 emulsion tubes and put Keihin parts in and all is well.

Took some pics to document the differences. The air screws are above. Below you can see the emulsion tubes are larger bore and have a smaller aerator profile than the factory brass. Also note the slow jet differences. The Keihin parts are longer than the typical aftermarket stuff. I believe the original slow jets are a few mm shorter, but there seems to be no negative effects of newer Keihin parts. The only Keihin parts in these when I got them was the needles. The needles were sloppy in the aftermarket tubes. No amount of clip adjustment would have prevented the main jets from coming in at idle.

 I ended up at 115 mains and 40 slows on the middle needle position. Airscrews 1.5ish out. Still need to dial in a little but my original guess at jetting appears to have been correct.