Raising the carb needle w/pix

[oldchopperguy]

With a year of use on it, my stock carb went bad. After installing a new Mikuni 24mm carb on my GY6 150, I'm in the process of fine-tuning it. I'm an old-school Harley guy, and these Asian carbs don't even resemble the old Lenkert carbs found on vintage hogs. So... I'm learning a LOT about these things, some of which I haven't seen posts about, so I hope my experiences may help somebody else...

The carb came with a 42.5 pilot jet, and a 120 main jet. Engine ran great on the stand, but initial test ride showed very poor acceleration, and way too lean top end.

The needle was factory-set at the next to highest notch, so I raised it to the last (richest) one. Much better acceleration, but still boggy. Many of the Chinese carbs have no adjustment notches or circlips to move the needle, but the Mikuni does. (I've seen posts on raising the Chinese needles with spacing washers...) So I pulled the needle and cut two EXTRA adjustment notches on a small lathe, but, if you don't have a lathe, this could be done (carefully) by chucking the needle in a drill, and cutting with a tool ground from a hacksaw blade held in Vice-Grips.

Please see pix below




Here's the finished needle, and another shot showing the plastic stop installed...





After re-installing the needle, the scooter accelerates like a rocket... much better than new! However the 120 main jet was still too lean. I put in a 125, and STILL way lean. I have a couple spares to play with, so I'm simply drilling them out to 130, 140 and 150 orifices. The lathe makes it easy, but again, you could do this carefully with a drill and a padded-jaw vice.

Below, that's the original Chinese #102 jet getting a #150 (1.50 mm) makeover... The Mikuni jets do seem to run true to the number matching the millimeter designation (#125 = 1.25 mm, etc.)




I'm dealing with some family health issues, and won't have time to try the new jets for several days, but I'll add the results to the posting as soon as I can.

The main thing I'm finding here, is that there's NO such thing as an "average" jetting or needle setting on these. Individual scooters, engines and carbs vary ENORMOUSLY from one to another, and experimenting is not difficult. I believe there is another 5 mph and a LOT of enjoyment to be unlocked in some of these Chinese scooters if we aren't afraid to do some old-fashioned shade-tree mechanics.

Most importantly, and I've not seen posts on this... The carburetion seems to be EXTREMELY SENSITIVE to the load on the scooter. Mine will run fine on the stand with most any settings. Put a 120 lb rider on board, and it still will run fair. Put my 250 lb self on board and it goes south without SERIOUS richening of the jets and raising of the needle.

While free-flowing intakes and exhausts definitely call for some richening, I'm finding the load carried affects the tuning far more. The setup of the carburetor (at least on my Xingyue) has FAR GREATER impact on performance than many performance-oriented alterations.

These carburetors are not particularly complex, and can be worked on with common hand tools. I hope my experiences may be of help to other troopers in this fine scooter rebellion...

Keep the shiny side up, ya'll...

Leo in Texas

[Rebel]

Leo,

Not only can you draw, but you can use a lathe very well. Thanks for the post.

[oldchopperguy]

You're welcome, Rebel!

I appreciate the compliments, but trust me, I can draw a lot better than I can machine! Until this year, I did all work with drills and files... I won't suggest ANYTHING in posts that I don't think a novice could attempt without machine tools. I believe info here should be good for the average Joe (or Jane) scoot rebel, wrenching their first ride... Now, if I could just master these computer programs!

This scooter never did have good pull beyond 45 mph. The plug didn't show seriously lean running, but I'm now seeing it was VERY short of gas at top end. I'm curious to see just HOW rich a main jet must get before THIS little 150 is satisfied! (And I suspect "150" may be the key word here... There goes my 90 miles per gallon... (LOL!)

I'll update the thread as soon as I can resume testing.

YEE-HA!

Leo in Texas

[kuzikood]

i want a lathe soooooooooooooooooooooooooooooo bad

[oldchopperguy]

Kuzikood,

Yeah, I wanted one in the worst way since I was a kid in the fifties! They are a fantastic tool, but I made do with a drill in a vice, and freehand cutting with a Dremel Tool, files, hacksaw blades and abrasive paper for more than 45 years. It's incredible what you can do freehand with no real tools.

This lathe is a small Sherline, which will do most anything needed for scooter use, except making a crank or piston (LOL).

I can't afford such tools, and wouldn't have this one except for a VERY kind friend whom I made some model airplane and tether car parts for. He wanted me to have a lathe to see what I might be able to do with a real machine tool. VERY GENEROUS to be sure! I use it for all sorts of things.

In the mean time, put away the spare change, and keep checking out eBay... You never know when a bargain may come along.

Sincerely,

Leo in Texas

[Rebel]

This thread inspired me to check out the H.F. they have a 4"X5" "micro" lathe at $299.00 then it goes to a 7"X10" $450.00 dollar mini and a 7"X12" one for $499.00 and up to big bucks after that.

I once bought a stand that had a cradle to hold a drill and strap it down to make a wood lathe, it was a pain to work with but I turned a few things (out of wood) with it, good tools are nice to have.

here is one that is similar to what I had;

dugnorth.com/blog/2007/05/grizzly-drill-powered-hobby-lathe.html

I think that you might be better off, over the kit, getting the cheap drill press that H.F. sells to use to hold small pieces and fabricate a clamp to hold a cutting device. Sounds like a project to do some time.

[xs650]

Jun 9, 2009 11:42:02 GMT -5 Rebel said:

This thread inspired me to check out the H.F. they have a 4"X5" "micro" lathe at $299.00 then it goes to a 7"X10" $450.00 dollar mini and a 7"X12" one for $499.00 and up to big bucks after that.

Rebel, I've known a couple of people that had different reincarnations of that lathe and they weren't bad lathes. The do go on sale occasionally at HF. The 12 is definitely worth the extra $50 over a 10. The 10 is cramped if you are working on anything of any size.

There is a forum that covers them
www.7xlathes.com/

[phaze5]

all i have is a wood lathe lol and i rarely use that just bought it to play with lol

[kuzikood]

my graNdfather had a full machine shop in his garage he was going to give me when i had a place to put it unfortunatly he ran over some guy on a bicycle one night who was wearing all black on a black bike down a dark road with no reflectors so he had to sell the equip to cover what insurance diddnt bah

[phaze5]

thats how it would go for me too lol

[jct842]

great carb post leo, can't wait till I get a new scooter to try it on. I bought a cumins 7x12 maybe 3 years ago, 400 bucks almost the same lathe as harbor except is 7x12 instead of 7x10 and comes with steady r. face plate and a few other items. I got mine because I am into making and modifying pellet guns. Have worn out a belt and pully due to use/abuse. I use mine for all kinds of things. made a dozen rivets for the boat, custom length bolts for trailer and the list goes on and on. good investment but like any other tool it requires you keep buying more accessories. John

[kliff]

A man can never have enough lathes. If anyone has a chance to afford a lathe, I strongly suggest you go for it.
I've got 2 in the Shop @ Dad's, 1 in my storage shed.....I've even got on of the small chinese 7x12's in my spare room... I LUV that lathe.... I do mostly plastics in the house, but it has seen it's fair share of aluminum and brass too.....what a pain vacuming the carpet though...

Maybe my Doc is more right than I give him credit for....
Just be sure to budget for some tooling and accessories too.

[usfmarine]

Leo in Texas,

Thanks for the excellent post! You provided clear and helpful photos and they even have pleasantly blurred backgrounds. I did feel left out of the discussion because I do not know what a lathe is... I assume that it is a tool that lets you add needle clip positions though

What else is a lathe useful for in the scooter hobbyist's garage?

I have the 28mm version of your Mikuni flatslide and I also had my needle clip on the richest position. I'm noticing quite a few Mikuni flatslide threads as of late, I think it's starting to catch on over the massive 30mm CV carbs.

[oldchopperguy]

Usfmarine,

Yes, a lathe is a machine tool that allows pricise turning of parts. Mine is very small, but it will turn objects up to about 2 inches in diameter, and 10 inches long. Great for modifying pistol barrels, making model car/boat drive shafts, etc. I do some custom work on small model engines too.

You can alter scooter carb jets, needles, seats, etc. You could make any number of custom parts like adaptors to fit American-size petcocks to small Asian metric fuel lines, filters, etc. You can even make handy parts like alumiunm reducers to fit custom exhausts, or fabricate mounts. I'm considering making some nice handlebar ends from billet aluminum and color-anodizing them... Maybe fitting them with bar-end mirrors to replace those factory mirrors that you can't see much in.

Foot pegs, light mounts, decorative "bling" and all sorts of things are possible with a lathe.

And, boy, is Kliff ever right about vacuuming up the mess they make!!!

Hope this helps...

Leo in Texas

PS: My Mikuni carb isn't a flat-slide version. It's a direct replacement for the 24 mm stock Chinese carb. I understand flat-slides are the cat's meow, but I just wanted a better version of the OEM carb. Bolts right on in just minutes, same lines, throttle hookup, etc... It IS quite a bit nicer than the original though.

[oldchopperguy]

jr,

I had the unfortunate double-whammy of the old carb going bad, THEN having the CDI crap out right after installing the new Mikuni. I got 3 full sweet-running miles before meltdown...Cheesh!

I just received a new CDI off eBay (same one as the "Free CDI" post) and it is nice. FAST accelleration and NO rev limiter. BUT... Now my carb is all out of whack because for a week, I was mistakenly trying to cure an electrical problem with carburetor adjustments! Duh... C'mon now, we ALL do that stuff once in a while! Once I get the Mikuni jetted right and the needle set, it should be fine.

When you look for a Kehin carb, be careful/alert... More than one eBay seller offers them at low prices, but they are often NOT Japanese Kehin carbs, but are Chinese, branded "Kei-Hin" or "Ke-Hin" on the float bowl. (They even say "Standard of Japan" by the logo!) These are probably decent, but are NOT genuine "Kehin" carbs, but more like OEM Chinese carbs. They appear identical to the "De-Ni" carb that came on my Xingyue scooter, the carb that went bad... It even has the bogus Kehin "K" between sylables, just like the Chinese "Kei (K) Hin" carbs on eBay.

As a side note, my new Mikuni came jetted with a 42.5 pilot jet, which seems perfect on my engine, and a 120 main jet which seems a little rich now. Still have to mess with it some!

I'm an old go-kart guy, and am sorely tempted to adapt a good old Tillotson diaphragm "pumper" carb to this scoot! They were KILLER on any 2-stroke from 5 to 15 cubic inches. They have FULLY ADJUSTABLE low and high speed jets you can set "on the fly" to make ANY motor run right... I'm just not sure how to get the diaphragm to work right on a 4-cycle engine. I know it works, Harley Davidson used them a lot in the past... Probably doesn't even matter with gravity-feed tanks, and the carb on top of the engine. The pumper feature is really to allow the 2-cycle to run in any position including upside down... You could probably scrap the diaphragm and make up a remote float bowl for 4-stroke use...

Keep them needle files handy!

Leo

PS: Please check eBay for lathes... They are really starting to appear in numbers, and getting cheaper and cheaper! With all the work you do, you deserve one!