Pilot Jet vs Mixture Screw

I feel very bad about creating another carb-based discussion but after searching a bit I haven't been able to find an answer. Also, sorry about the long post!

After years of struggling with my carbs I finally took them off and did the Big Clean. Stripped them all down, boiled what I could in lemon juice, new throttle-shaft seals, mixture screws seals and washers, fuel line o-rings - the works! After reassembly I struggled with them for a few days, as it was racing while idling. I went down to the stock pilot jet (I have a K&N Filter and 4-into-1) and it was still too high. I was struggling to get them to sync and after taking them off for the 5th time in as many days I found out that the #1 butterfly valve was sitting a little funny and would not fully close. This had prevented me from ever getting to the point where they would idle with just the mixture screw-ports and after fiddling and tapping I managed to get them all closed. And what a difference! After YEARS my ride in to work this morning finally gave me a glimpse of how the bike is supposed to run! So peppy! No bogging! I had a smile on my face the whole time!

Except: the final tweak (he said naively!). The only thing I noticed was that the bike, even after being warm, had to have some choke applied to idle. It still performed well, but if I turned off the choke it would die. This is with the idle-adjustment screw turned off fully (as I understand from previous posts - I should be aiming to have it idle with only mixture-bypass port exposed, correct?). For a temp fix during my ride in I turned the idle adjustment screw so it would hold idle without the choke on, but it didn't perform nearly as well as it did with the idle turned down and a bit of choke. I assume this can be corrected by turning out the mixture screws a bit, but they are already 3 1/2 turns out. My question is, if I can get it idling and performing correctly with the mixture screws turned out 4 turns, is that "acceptable" or should I be going up a size in my pilot jet? What is the maximum amount one should turn out the mixture screws to achieve the proper mixture? I am even right in assuming that a larger pilot jet would work the same as opening up the mixture screws? All things being equal, I would prefer to just be able to adjust the screws and save having to remove the carbs AGAIN - but if there is a performance or fuel-economy benefit to going up a jet size, I would be willing.

The specifics:
1982 650 Seca - with a 1983 650 YICS engine.
K&N filter.
4-into-1 pipes.
#40 pilot jet.
#116 main jet.

Again - sorry about the long post! But I am so excited and feel so close to getting her finally running great!

Jimmytime

 

Off idle to about 20-25% the mixture is controlled by the pilot jet.

Sounds like you are lean off idle and compensating by leaving some enrichment on. That extra enrichment causes problems with the idle so you have to compensate that by turning down (off) the pilot screws.

Now, is your performance problem at the lower end of the throttle or more throttle with higher RPMs? If the problem is at the lower end you probably need to go up on the pilot jet. If it's when you're laying into it you need to look at the main jets and jet needles.

Since the pilot jet is always in play, the proper sequence to size the jets is:

Pilot jet. (low speed)
Main jet. (high speed, high load)
Needle jet. (mid-range)
Final pilot mixture screw setting.

Lots of fun. Without a dyno you can experiment and see what feels good or go through a ton of plugs doing chops at the drag strip. This is why I always tell my customers wanting to make intake/exhaust changes that someplace else is the place to get it done.

BTW, if your carburetors aren't in perfect condition and the float levels correct you'll be wasting your time.

 

It's hard for me to speak about the top-end performance. My commute in was purely city driving and didn't really get a chance to open it up! From what little I did it felt really good, so I'm not worrying about the top-end right now. Basically, it rode great, idling and low-end, as long as I had the choke slightly engaged. I'm already out 3 1/2 turns and wondering if I should try to go out further or just up the pilot jets. If I could get it to run the way it did when it had that little bit of choke, I would be super-happy!

 

Jimmytime:

I think you're in the Ball Park.

Bench Sync the Throttles using a THIN Strip of 3X5 Card.
That will make you dependent on PILOT Mixture for IDLE.

A ColorTune Plug will "Show" you where the Pilot Screw's will provide you with an exceptionally good Idle Mixture.

Without a Color Plug you have to find the IDLE by "EAR"
Turn the Pilot Screw OUT ... Very slowly ... and continue adding Pilot Mixture as long as the RPM's Increase.

Once you arrive at THE PLATEAU just before the Pilot Mixture becomes TOO Rich is within a FEW DEGREES of on the Money.
Keep the Bike running by Adjusting the IDLE Rod.

As the Bike needs LESS Open Throttles while you add Pilot Screw Mixture INCREASING the rpm's ... Back-off on the Idle Rod and let the Pilot System Idle the Bike.

Once you get the Bike IDLING quite nicely with the Idle Rod backed-off ... you'll need to ADD just a little-bit MORE Pilot Screw Richness to deal with Fueling the (( 2 or 3 eye-blinks)) of Fresh AIR preceding the Main FUEL Jet feed as the Throttles are moved OFF-Idle.

That FINAL Pilot Mixture Adjustment is a TWEAK.
A very very small movement of the Pilot Mixture Screws.
In some cases, ... less than the Width of a Nickel.

 

Thanks Rick! I think I got what you are saying. I'll ask super-quickly though: you don't think I should go up in pilot jet size correct?

Right now they are pretty well benched-synced, just completely closed. When you say sync them with the card - you are suggesting that I basically open up the throttle with the Idle rod right? (this is assuming that they are synced together). Then, I'm basically (and I'm totally just paraphrasing here!) turning up the mixture while I turn down the idle, with the goal being them idling with the Idle rod backed completely off. That's the concept, right?

Is there a point that my mixture could reach that I should be worried? If it does take 4 turns - is that a problem? That's my main worry regarding the pilot jet! I don't want to take them off again!!!!

 

If the Mixture gets too RICH ... The Cylinder will begin to Misfire.
So, you'd Tune away from the Misfire until the Cylinder is steady.

With an Air Cooled Aluminum Head and Pistons, ...
Being too RICH is NOT a condition that will cause you to suffer a catastrophic failure.

Being too LEAN!
That's when you have to worry about overheating and damaging the Plant.

 

Sorry - I guess that wasn't very clear! At roughly how many turns out on the mixture screw is it fully open? I'd like to know in case I get to that point!

 

I'm going to get around to cutting-up an old Carb Body and get a handle on that.

I don't know.

 

Jimmytime, going back to the start of your thread, how did you bench sync' the carbs if a butterfly wasn't seating right?

 

Before I tore down my carbs, it ran best w/#1 pilot screw 6 1/2 turns out!

 

Hey Wizard - yeah I couldn't get them synced at first and couldn't figure out why (using a vacuum gauge). When I got them sorta synced, the engine was just racing and racing (I think because basically the other 3 had to open up to match the one that wouldn't close). Nothing was making sense! It was when I had them off afterwards that I noticed the one butterfly out of whack. After fixing that it really started to come together.

6 1/2 turns out! Yikes! Okay, well now I don't feel as bad. On my ride home I cranked them out to 4 1/2 and it STILL wouldn't idle without choke. It runs really well with it though! I'll just keep at it! Thanks for all the help/suggestions!

 

padre, if you are 6.5 turns out you probably want to change the pilot jets, say if you are running #40 go up a size to #42 & bring the pilot mixture screws back into speck, i.e. 2 3/4 turns out from lightly bottomed.

 

Naw, there was solid debris in both the main jets and pilots, in fact the
#1pilot was so clogged I couldn't see daylight through it. I had to use the finest hobby wire home depot had then ream it out agian with a tiny tourch tip cleaner file after soaking all of the jets in carb cleaner. It ran well after. In fact I rode it from Vegas to just out side Ft. Sill in Okalahoma.