Carburator Tuning Help

[slonomo]

I would like to know what procedures you personally use to tune your carburetors. In this case we are talking about golf carts, but since I use a Motorcycle carb, I would really like to hear about tuning your motorcycle carb.

I fully understand the basics of jetting, plug reading, air fuel ratios etc. So let's not go that direction.

What I'm interested in is, specifically, how to get a good idle quality and air fuel ratio at idle.

If you use Mikuni, Keihin, or other brand, please specify.

If you know of any high quality YouTube videos, please post them here. Crappy audio/video and/or videos with boobs and hillbilly teeth need not be posted.

[slonomo]

Ok, well nevermind.

[sho305]

Most of the guides I read basically say go by the idle mix, so if you have to turn the idle mix all the way lean to get your fastest idle then the pilot jet is too rich. If you have to turn it full rich to get best idle you need larger pilot jet. Most carbs have a range they like to be in such as 1-2.5 turns out, if you are in that range your pilot is probably good. If you haven't done it lean it will idle faster too lean it cuts out, rich it idles slower and too rich it starts to putt putt (fires every other time) like the choke is on.

That said I like a rich idle for easy starting and no backfires so I set it best idle then a little richer. I just did my new carb it was cutting out at low speed and surging at high speed. I put larger main in and high speed worked much better, had to wait for pilot jets went larger on that and it was too rich it missed at idle with the mix all the way in. Ended up going two sizes smaller and it still may be too rich, though I also run it in cooler weather and its 80s here. It idles good but when I let off a little it can putt putt which is usually meaning its rich, like from 20 to 5% throttle let off. At steady speed its smooth. If it were lean it would rev smooth when I let off.

And usually any mix screw on the intake side of carb is an air screw which will lean it as you open it, a screw on the engine end of carb often a fuel screw that works opposite will lean as you close it. I prefer to read the plug for the main jetting. Another tell for low speed (low carb opening) jetting is it will usually be a little cold blooded with cold engine, if you can go out on 60F morning and it starts and runs fine the first 10 seconds like mine does its likely rich. Most well jetted engines need to run 30 seconds or something to not cut out/miss when cold when you gas on it to take off. This is a copy round slide carb.

WOT jetting I usually take some WOT runs and turn it off fast as possible, then read the plug. Most say do main jetting last as the small jets can affect the main mix but they don't affect it that much. Some even say take the main out and do the pilot, so it floods out if you throttle enough to get on the main and then you know you are only doing the pilot which only works to 10-20% throttle or so. The needle can fine tune the midrange you can raise it up or down, that pulls it out of the main hole or puts it in more (in relation to the air the slide is letting in), it should richen or lean the whole midrange assuming the taper is close to right. I rarely change a needle as the carb usually flows similar air on different engines...nothing is changed in the carb throat. But you can change taper and diameter.

Other signs can help tell you if you let off from a throttle level and it bucks a little and stops firing a moment that is usually rich, if lean it will become very smooth running but can cut out when you increase throttle that is when you see/hear lean condition the most. If you drive around at say 1/4 throttle and ease on and off the gas just a little if it wants to slow down faster letting off some (rich) or does it bog a little increasing throttle but smooth when you let off (lean). That can tell you what is going on in the 1/4 throttle opening range. It gets cold out and it wants to miss and cut out when you gas on it, that is lean or too lean for the weather, if doing this while fully warmed up. Lean main can cause surging at WOT, or cause it to take a moment to get full power when you hammer it. If rich main it goes instantly when you hammer it but can miss as you let back on throttle, black tailpipe and plug. The one difficulty with carts is the CVT, which has its own effect on throttle response you feel. If you get the main jetting better it should rev a little higher because it has more power. It can be hard to feel response when you let off or increase throttle a little because the cvt is shifting some to changing conditions. Where on a bike you can run around in one gear and study response to throttle easier.

[slonomo]

I got it figured out. I understand all the theory, but none of that was applying in my case. I just couldn't get it to respond properly. Come to find out the intake adapter gasket had a leak. Fixed that, all good. Now it responds.

With that said, I just couldn't find any info on the forums about tuning motorcycle carbs.

[slonomo]

Here is what I've learned from this Mikuni VM34 project:

1. Make sure all intake gaskets are "NEW" and in good condition
2. Make sure all intake gasket surfaces are perfectly flat and clean
3. Make sure fuel line is 1/4" and not 5/16", this will increase fuel pressure
4. Test run the carb and set idle to 2000 rpm
5. After the engine is warm, set idle air screw to 1.5 turns out
6. Make 1/4 turn adjustment to the idle air screw and wait 10 seconds
7. Find an adjustment point that gives the best idle sound (subjective I know)
8. Set idle to desired rpm (800-1100 rpm)
9. While engine is warm, take a drive, vary rpm and note any issues.
10. In the midrange, raise the needle a notch to get rid of popping (lean), or drop the needle one clip to get rid of a bog (rich)
11. Top end, above 4000 rpm, popping is lean, go up one jet size, bog is rich, go down one jet size.
12. The plug reading method is good too, but that's a whole other thread. I find that process complicated. I do my plug reading after I've gotten the jetting as good as I can get it.

With a motorcycle carb, jetting is never perfect. I can get an atv to run absolutely flawless and the plug readings will show rich. I've also tuned carbs to the light brown/tan plug reading stage and had intermittent operating quality.

Carbs are very finicky. However, most carbs can deal with a little bit of being out of tune. I remember some car carbs that I could never get to run right, that's about the time I would switch to a Holley lol. To me, Mikuni's are like Holley carbs, they seem to work no matter what. I think that's why most ATV's use Mikuni because of their forgiving nature, especially the CV type carbs.

[sliding]

The throttle slide cut away makes a big difference in the low speed performance of your engine.
https://www.howtomotorcyclerepair.co...valve-cutaway/

Motorcycles are easier to jet as you can mark the twist grip so that you know where the slide is in relation to your throttle opening. This tells you what part of the circuit the carb is using. A wide band O2 sensor makes life even easier.

[Shepard]

Can the vm34 carburetor use an electric enrichener? I like a carburetor that uses this style starting system - less likely to foul out the plug from over fueling.

[slonomo]

Originally Posted by Shepard

Can the vm34 carburetor use an electric enrichener? I like a carburetor that uses this style starting system - less likely to foul out the plug from over fueling.

I've never seen one with an electric enrichener. Doesn't mean it can't be done. Some of the chinese copies have them, wonder if they would swap? I actually have one. Next time I'm out in the shop I'll look at it and see if it looks like it would fit.

[sho305]

Originally Posted by slonomo

I got it figured out. I understand all the theory, but none of that was applying in my case. I just couldn't get it to respond properly. Come to find out the intake adapter gasket had a leak. Fixed that, all good. Now it responds.

With that said, I just couldn't find any info on the forums about tuning motorcycle carbs.

Dont feel bad once was on my sled and it was subzero it stuck a piston. Got it running again I had to choke it every time I slowed down. I'd just had it bored and new pistons I was not happy. Yep next day tore into it and way down under there the intake gasket had bowed out and was leaking a little. Pistons looked pretty good so I just ran it no problems after that I was lucky.

I ran the enrichener on my CV carb it worked fine, but could not get the carb to have good top end. It ran great otherwise, I think on a stock engine it would work well. IT was a 32mm maybe it was too large for a 420.

[Shepard]

On a vm34 -- When you remove the bowl drain -- is the main jet right there for easy changing?

Think it would be possible to install a needle valve in the bowl drain nut?