Yeah, I have the pilot circuit working quite well. Stock low speed jets, screws started at service manual setting, fine tuned with 50rpm drop method and balanced with a carb tune. The screws aren't that difficult to get to with a long slender screwdriver. Not ruling out needle position as the problem, will play with that and remeasure float level while I'm in there. I may have a chance to sort it tomorrow. Weather has been amazing here, dying to ride in the early spring sunshine.

 

It's still cold and snowing here. You're lucky to have some better weather. I'm chomping at the bit here...starting to hate winter.


What is "carb tune"?


If you feel everything is ok, is it possible something contaminated the fuel and got into the carbs? I had that happen on my ATV once. Cleaned the carbs, had everything set and running good and it went bad...turned out something in the tank let loose and the pilot circuit plugged up.

 

The Carbtune is a synchronizer, pretty slick-

www.carbtune.com

I was thinking it could be some crud, though I can't quite figure how it would affect just that narrow range and not idle. Once again got swamped with work today, so hope to get into it tomorrow. I think my approach will be to take it in stages - try to turn the pilot screws in a 1/2 turn first. Return to original setting if no improvement, then open up the bowls, check for blockages and fine tune float height. Button up and try again. If that doesn't help then try moving the needles down one notch more lean.

If that doesn't do it maybe I'll visit the local dyno guy, though I hate the idea of not nailing it on my own.

 

You're making the assumption that you are lean...remember plugs are hard to read with fuels we have today. I would test both ways at least on the pilot from the existing position...you may be surprised.


If you pull your plugs after the bike has idled (a normal stop of the engine after riding) they will almost always be black. Don't use that to guide you on the mixture. It's a common mistake.


A proper plug chop is hard to do...especially with a low RPM flat spot. Without a plug chop don't assume a rich or lean mixture...test both ways.

 

I am initially assuming it's rich in that 2-3.5K band based upon the bogging misfire coming into play when the engine is warmed up, and the way it clears as the Rs pick up in the transition to the needle metering circuit. Maybe I have the adjustment of the pilot screws backwards - thought turning in (clockwise) was more lean, out (counterclockwise) more rich.

I may be wrong about all of this as it has been some years since I raced cars and played with this stuff on a more regular basis. Thinking back on those experiences it seems like a rich bogging rather than a lean flat spot. Before I go back into the rack I will ride it a bit when cold and then give it a few warmed up 1/8 throttle and also WOT runs through that range, to see if I can better clarify what I am hearing and feeling and exactly what Rs it comes and goes at.

Looks like I may have to borrow that Carbtune again...

 

Doc,


My point is to just not assume anything, because it can lead you down the wrong path and the more you change the further away you get.


If it bogs due to too much fuel, usually there is a result...like a backfire. A very small amount might not do that tho.


If it were mine I would first play with the pilot screws and not do anything else because that seems to be the right RPM for them to come into play. If turning them in changes it either bad or good it will tell you which way to go. I struggle a little with the method for adjusting these in the manual. I know it's difficult with 4 carbs, but I was always taught to do it when the engine is running and try to get the highest RPM from each carb. This may mean they are not all exactly the same...


I think you are on the right track, just take your time and try and isolate it...if all else fails I can give you a link to a good deal on a sniffer

 

Well that was interesting, I had it all wrong. Three of four pilot jets had stuff in them again even after taking apart the rack and cleaning them out twice. I can't figure out how it was idling so well! All the filters and screens are fresh. There must have been some residual crud from my previous cleanings hiding somewhere.

Did a lot of digging for stock specs and what is recommended for changes when adding a slip on muffler. The general consensus for a starting point seems maybe go one main jet size up from stock, or probably just leave it alone. Looked like there was sort of a stage 2 type setup in these carbs, as the mains were 120s. Pilot jets are stock for a 1990 at 38, and I had a set of stock for main jets for 87-88 which happens to be the same size supplied by Dynojet for a stage-one-with-slip-on setup - 105. Cleaned up the pilot jets yet again, installed the 105 mains, checked the needles - e clip on third notch as recommended. In the minus column I dropped my nice gas tank and put a gnarly dent in it, argh. Anyway, the thing takes off like a scalded cat now. I was so used to it running like a 250cc cruiser off the line that I lifted the front just a bit coming out of the driveway. Woo hoo! At a much better starting point now for dialing things in.

 

You didn't have it all wrong...if you go back a few posts we talked about this being a possibility.


You were thinking that crud wouldn't do this, but the pilot circuit is very sensitive and the orifices are so small...doesn't take much to muck it up. It could be some crud from inside the lines or the fuel pump...I've had that happen before.


I feel pretty good because I was on the pilot circuit the whole time.


Great you have it running good now.

 

Yup, your call was a lot better then mine. I'm glad I didn't mess with any adjustments before I went inside. I can't say for sure since the clogging problem kind of clouds my memory from the last time the engine ran clean, but it seems like the smaller mains may have had a positive impact on throttle response. You read so much stuff about how these bikes are lacking torque below 5K. I wonder if a lot of bikes have bigger mains than they need just because it's stylish and in the process they are sacrificing response. It took me a few passes up the street to get myself to go really easy cracking the throttle open at low revs so I didn't scare myself.

Lay in bed this morning mentally devising a tool to fit in the gas tank filler to try bump the dent I made out without having to strip off the vinyl. Such a dumb move, broke the rule I preach to all my kit building customers and worked too long, to the point of getting careless. I swore loud enough when I did it that my wife came out from the house to the freestanding garage to see if I was OK. Maybe I'll just consider it wabi-sabi until I get around to fixing it.

 

Carb tuning is art and science...not as easy as many think. You see so many people asking other's what jets they have...but it really depends on where you are and what's on your bike to get it right.


With pilot circuit you can get by with a range of sizes but the ones that are "right" for your particular bike are the ones that end up 2-3 turns out. Smaller will be open more, which creates a problem with the screws falling out or moving because the spring tension is not enough (it also depends on whether the screw is adjusting fuel or air which direction they turn). Larger and you have to close them down too far and then you loose sensitivity on the adjustment. The important thing is to find the size that is right for your bike in your location (altitude in particular).


Mains are the same in the sense that you want a jet and a needle profile that gives fuel at the proper rate for your bike with more concern on pipe(s), muffler and air intake. Here knowing whether your running rich or lean is important. The needle profile comes into play with maintaining the proper mixture throughout the throttle range.


I shy away from reading too much into most of the information on the web. If your starting from scratch (different carbs than stock) then finding some example helps get you in the "range", but you still need to tune to your bike and location so the info is really just a baseline. I usually buy jets around the baseline, 4 or 5 of each size either way. Most times this will get you there.


So when you talk about what other's are doing or have done, I don't put much weight in that information...when you say it is running this way, then you can make a diagnosis based on what the engine is doing...I've done a lot of tuning and some very difficult ones where I changed the carb type completely from stock...like going from Amal to Mikuni TM Flat Slide on my Norton...it took a bit more to get it tuned in, but runs better now than new with an Amal. Starts with one kick, idles smoother and runs out with more power.


As far as when to quit, I know that one. I remember when I built my house years ago...I would quit when I hit my thumb with a hammer...way too late. Now I stop way before then...all of my painted stuff get's wrapped in moving blankets just in case I drop something. Takes a minute longer, but many times saves the day.