Carb Needles & Mileage

[Bob Falter]

First let me say that everything I have read about Mikuni carbs tells me that the needle and needle jet only affect the ¼ to ¾ throttle positions. I’m beginning to think at all this info is for the earlier Mikuni’s where the slide was pulled open by the throttle (pre constant velocity carbs). Here is why I say that.

My bike has been running fine and I have put over 10,000 miles on it this year (just retired). However I haven’t been getting very good gas mileage on the interstate (25 – 30 mpg) so I drop the jet needles one notch (from the 3rd (center) notch to the 2nd notch from the top). I had to back out the idle screws ½ turn to get it to idle properly so obviously the needles effect the idle some. I also checked the sync of the carbs but it was fine.

I took it for a test ride of a little over 100 miles on the interstate. Checked the mileage and got 33.27 mpg. Check the plugs and they were a lot lighter than they had been (had been running slightly rich even with K&N filter and 4 into 1 exhaust). While I was on the test ride I also checked WOT (wide open throttle) performance. Going to WOT at 2K rpm in third gear it would pull hard to 6K, between 6 and 7K it would start to sputter or miss and kept that up to almost red line when I shut down. So much for the needles not effecting WOT also. I replaced the plugs and tested WOT again and got the same results. Plugs still looked like new after a 30 plus mile run (maybe too much like new). Note that it ran fine at WOT before I moved the needles.

The needles are #5GZ6 which I think were stock in 79 and I think all other parts in the carbs are stock also. I want to make a 2K mile trip in a little over a week (Talimena) but not sure I should run it this lean even though it runs fine and will pull up to red line, with no sputter or miss, at ¾ throttle. I don’t have time to tear the carbs apart and try bigger main jets but I could pop the carb tops and move the needles back to where they were. What should I do?

Suggestions?
Bob

PS: Sorry, didn’t mean to write a book.

[jetmechmarty]

Bob,

If it were me, I would put the needles back. My bike gets similar fuel economy to yours. I think that's all you can expect.

I'd worry about a holed piston on a long trip with it as it is.

[psycoreefer]

some ideas:
1. a full clip position is ALOT of change, check around and you'll find some small/thin washers that are used for adjusting less then a full clip position.
2. If your needles and or emulsion tubes are worn at all then changes in needle position will effect idle mixture because gas will be leaking around the needle when it shouldn't
3. If your goal is to improve millage and the bike seems to run "good" the way it was then put the needles back and try adjusting your floats just a little bit leaner - the floats are the only adjustment that has an effect on the entire range. Adjusting the needles not only changes the mixture in the mid range but also effects the "transitions" from idle to mid and from mid to WOT.

[Crazcnuk]

Basically, you have to find out if your bike is lean or rich at the top end. ie what is causing the sputter, too much or too little fuel.

If the bike is rich from idle to WOT, then dropping your float levels will help.

My fuel economy also improved when I dropped my needles one notch, but I suspect this is compensating for wear in the emulsion tubes and my altitude (3500ft). This also got rid of a low end bog.

When I drop my floats, (needles at stock) I have a low end bog, and even less top end.

Psyco may be correct with trying a half notch drop.

Yours sounds lean at the top, if I read it right, so I would not drop the floats, but try one jet jump on the mains and leave your needles down for now. Going up one jet size should help the top end, and counter your full needle drop a little.

[Tusler]

Just a thought

Is it cheaper to replace spark plugs cause of running rich or pistons cause of running lean.
I don't mean to sound like a smart allek but, back in the 80's we use to run them a little rich for better cooling, I still do run mine rich.

[Crazcnuk]

Don't know. Haven't done either.

By 'rich' we are talking too rich. Running rich is the #1 reason that motors in the 50s-70s only lasted 70-80,000 miles before needing re-ringing.

That and poor fuel economy.

[Bob Falter]

Thank you all for your replies. It has never ran rich enough to be hard on spark plugs. I replaced them so I could read the color better and to make sure they weren’t causing the WOT miss since they had 15,000 miles on them. Crazcnuk, I tend to agree with you, putting in one size bigger main jets and leaving the needles where they are would probably get rid of my WOT lean condition, give me better gas mileage, and get me away from running a little rich over the entire range. On the other hand, the floats are set a little low so they are running a high fuel level and setting them differently would lean up things across the entire range and may improve mileage too. I don’t have time to pull the carbs, order and get new mains, and get everything back together in one week but I may have time to pull the carbs, change the float settings, and move the clips back down (put them back in the middle notch). If not, I’ll move the clips back and look at putting a thin washer in to keep the needle down slightly, which I can do without pulling the carbs.

Thanks again,
Bob

[Crazcnuk]

It's just a matter of experimentation. Each situation is slightly different and has it's own set of paramters.

Yesterday, I took my carbs off, again, and put the floats back to stock (23mm) dropped my needles back down the one notch and put in larger (115) mains.

Dropping the needles cures any bottom end issues, but I lose a little more top end, which I have very little already.

Changing the mains does nothing.

I can crack it off the start and it will hit redline in 1st and 2nd, then it takes about 10 seconds for 3rd to go from 6500-8000rpm, and 4h would take about 1/2 hr. 5th will not accellerate at all at anything over 5000rpm.

To pass people I have to drop at least one gear. It runs fine from 1500-5000rpm, and I have hit 43mpg (imp).

[Bob Falter]

Crazcnuk, when your in 3rd and between 6500 and 8000, does it run OK or is it missing, surging, sputtering, etc? There is a lot of leeway on a four stroke engine but if they are running way too rich, or lean, they normally don’t run smoothly. At least in my experiences. If it runs OK but doesn’t have any power it is strange to me. Have you checked compression? Are the carbs being pulled open all the way? Could there be something restricting the air flow to the carbs?

I pulled the carbs too. Checked the float heights, they were all close to the 0.95” that I had set them at 9 years ago. Adjusted all the floats to 1.030” (1.012” +/-0.040 or 25.7mm +/- 1mm is spec) and set the clips back to the center position on the jet needle. The 1.030” is in spec but higher than the standard target so they should run a little lean. Re-synced carbs and set the idle screws back to 1-1/2 turns out. Took it for a test run and it ran well, back to being its old self, no surging/missing at WOT and high rpm's, idles fine, no hesitation off idle, etc. Takes less than 2 seconds to go from 6500 to 8000 in 3rd gear. Pulled a couple of plugs and they look like new (just installed new plugs). The porcelain is white as new and there was very little black build up around the outer base. Defiantly not rich, probably too lean. They didn’t look much better than they did with the jet needles down one notch, but it runs a lot better at WOT. That 0.080” (2mm) you have to play with makes a lot of difference.

I had set them at 0.950 originally to compensate for the K&N air filer and the 4 into 1 exhaust. I think I was on the right track but just went a little too far. I suppose I’ll pull the carbs and reset the floats again. Just not sure what to set them to. The 1.012” (25.7mm) target may still be too lean because of the mods. Maybe I’ll splint the difference between the 0.95” and my present 1.03” settings and put them at .99”. Or I could set them at 0.972” (24.7 mm), the rich end of the range but that is not far from where they were and probably isn’t going to help my gas mileage. Marty is probably right and I’m not going to get much better gas mileage.

Bob

[Crazcnuk]

http://www.xs11.com/forum/showthread...threadid=18735

If you want to wade through and see everything I have done.

No, it does not surge, miss or anything else. It just doesn't have the power to pull itself. Compression is good, motor is completely stock except for a K&N air filter. It burns a fair amount of oil.

Plugs stayed clean from 110's to 120 mains. No difference in how it ran.

It runs smooth from 1500-~6000rpm. Passing is just like a car.

I've been getting 42-43mpg (impg, and Derwat is slightly better than that with his special.