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Pilot Circuit

  • Thread starter Thread starter J_C
  • Start date Start date
J

J_C

Guest
Hey folks.... I wanted to clarify my understanding of something.

My bike was tuned and synched at 4000 feet. It ran like a beast unleased. However, back home at sea level, there are a few tweaks I think I should make. My observations were the following:

1) Slightly difficult time warming up and idling. Lots of popping and one of the carbs makes a "chuff" sound while on choke. Good deal of popping on deceleration.

2) Surging at the 4k-5k RPM range.
With many thanks to Chef, I believe shimming the needle should fix the second of the two issues, as it appears to be some fuel starvation taking place.

The first issue appears to relate to the pilot circuit. At 4000 feet, the carbs were running excellent at 3 turns out. I'm now at 3.5 turns out and the problem seems to be less, but not completely fixed. At this many turns, does it make sense to look into my carbs to rejet to a smaller something or other? I believe my cv carbs have a pilot air jet or something? As this appears to be a lean condition, my thought is that its requiring more turns of the air screw mixture to allow more fuel to compensate for the increased air intake at sea level.


But I could be an idiot. Please let me know :-D
 
You are now running weak at sea level. The denser air at sea level has more oxygen per unit of volume than at altitude. So more fuel is needed to maintain the same mixture ratio you had at 4000 feet.

Footy.
 
Right, that makes sense. Before I had my screws out way further (like 8 turns or so) until I went to 4000 feet and it was set at 3 turns. That obviously seems excessive. I think I had it set up kind of like a bandaid on something that needed stitches. So my question: I must have rejetted something to make it overly lean when I originally rejetted my carbs. Is that an air screw of some sort? What piece regulates the pilot circuit air intake?
 
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There are actually two jets that control the pilot mixture. The pilot fuel jet is next to the main jet in the bottom of the float bowl. This is the one that is covered by the rubber plug. The other jet is the pilot air jet, which is located in the intake throat of the carb, in about the 8 o'clock position. Either one of these might have been changed from stock, but most people just change the air jets, as they are easier.

Your best bet is to look up to see what jets are stock for your bike, then open up the carbs to see what's in there.

If you are going to be remaining at or near sea level, you would be better off to re-tune for that altitude. If you occasionally go up in elevation, it's not as much of a problem, as it's better to run too rich than too lean.

.
 
Thanks Steve, I believe the air intake was the one I was referring to. I rejetted because i went from stock airbox/pipes to K&N pods and V&H 4-1 pipe. Luckily for me, it sounds like I'll be able to rejet this without even taking the carbs off. Unluckily for me, it sounds like I'll be back adjusting the mixture screws from scratch ;)
 
I would go up to 47.5 on the pilot jet and 170 on the pilot air jet. I have the 170 air jet but I might need my 47.5 after my new pilot air jets come in but my gut instinct is telling me the 50s are right. This is what I ran when I first did my DJ kit and it ran great.
 
So you think I should go up on my pilot jet instead of going down on my pilot air jet? Won't that just make the bike less efficient? Thanks for entertaining my dumb questions =]
 
With the increase need of fuel with the increase of air from the pods you need a larger pilot jet. If you tried to richen the pilot circuit by just putting in a smaller pilot air jet you would still have a problem with lean conditions because there still won't be enough fuel.
The pilot air jet draws in air and goes directly to the pilot jet and premixes air and fuel before it hits the idle mixture screws. If you lower the pilot air jet you will still have the same amount of fuel but less air in the premix.
With the increase to 47.5 pilot jet and the increase to 170 on the pilot air jet you have the right premix of fuel and air so you can have your idle mixture screws out around 2.5 to 3 turns out and you'll be good to go.
 
I've never heard of anyone running 8 turns out on the pilot screws. I thought after about 4 turns it didn't matter anymore - all the same.
 
Ahhh, I see. Thank you. Was I correct in assuming that the pods do, at least to some extent, decrease the gas efficiency of the bike?

I'll see if I can pull the bowl and pod off one of my carbs tonight to see if I can verify what's currently in there. Then I'll check my dynajet kit to see if I have what I need and will post up to confirm.
 
Nessism said:
I've never heard of anyone running 8 turns out on the pilot screws. I thought after about 4 turns it didn't matter anymore - all the same.
Click to expand...

This may well be the case, I somehow ended up at that many turns out using a colortune. It seemed to be making a difference at the time, but who knows! :confused: :-D
 
I will let you know. If I went from 160 to 170, and I'm already running 47.5s, could that be the cause of the lean condition because it's not drawing in enough fuel? (I think I'm getting the hang of this)

I thought I already had 170 / 47.5, but I will check and let you know. Thanks!!
 
J_C said:
I will let you know. If I went from 160 to 170, and I'm already running 47.5s, could that be the cause of the lean condition because it's not drawing in enough fuel? (I think I'm getting the hang of this)

I thought I already had 170 / 47.5, but I will check and let you know. Thanks!!
Click to expand...
I remember telling you about this but I'm not sure what you did.
 
Ermm... OK.. lol. I found this email. Its from 13 months ago. Apparantly this project's been taking a long ass time.

You
Did you put in the 180 air jets? Are you getting any hesitation on the needle circuit? How do your plugs look?
P.S. You will get some popping on decal with your 4-1 pipe. Mine did until the 1166 kit now it just "growls"
Click to expand...


Me
I did use the 180 air jets. I didn't try it with the 170s. I'm not sure which way that would go... the 170s would make it richer or leaner? I was extremely surprised to find that I was still lean all the way out to 5 turns with the 47.5s! Judging by what keith says I should go up another jet size since 4 turns out is the max. But that's ANOTHER 30 bucks!
Click to expand...

You
In my theory the 170 would create less suction and create a leaner condition.
Click to expand...



So it looks like 180 / 47.5 Where's this leave us now? lol
 
I'll swap you the 180s for the 170s. I was mistaken in that e-mail. That will set you up.
 
I think I have them, I'll check my car. If this is all it's going to take, I am one happy dude! \\:D/ (Except for needle shimming, thatll have to wait :lol: )
 
Nope! I've got two sets of 160s (one stock?) and 180s in the carbs, and a whole lot of little baggies of things I know are jets but no idea what kind. Anyway, no 170s. Where should I get these?
 
I'm not sure if this helps with the change in altitude; but I have re-jetted (DJ) all three of my GS's (2-750s & 1-1100) with K&Ns. Never have I had to alter the pilot circuit. In fact when I tried (changing the stock 160s for 170s & even 210s) it screwed the machine up.

I find that the stock pilot circuit should not be altered when re-jetting with pods. (I am riding at 3000ft.)

Don't know if my experience helps; but there it is.

Good Luck
 
David, thanks for your input. I rejetted the pilot circuit (if I recall correctly) because it wasn't running off choke below around 2000 rpms. But don't quote me on that; I originally rejetted this a little over a year ago. However, swapping the 180s for the 160s is easy enough, but I still have the 47.5 pilot main in there.
 
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