Info needed for Jetting 28mm Mikuni Flatslide Carb

#1
I was wondering if anyone has used a Mikuni 28mm Flat-slide carb on a highly modified Honda GX200 engine? Specifically, what sized main jet did you use burning Gas? I have one and with the stock main jet it runs lean as hell, way leaner than a carb usually runs out of the box. I've used a lot of Mikuni's, mostly on Harley engines but a few round slide ones on small engines, and as they come from the factory, the jetting is usually close enough that it will run at least decent, but this one runs like S**t! I haven't yet pulled it apart to check the float level etc, and I almost think it has a problem with it other than the jet size. I thought I'd ask you guys what your experiences have been before I tore it apart. The 22mm round slide ones I've used ran good out of the box and took one jet size bigger to run perfect. The 42mm flat slide ones I run on Harley engines usually are spot on with the 160 main that it comes with and usually need 2 or 3 sizes smaller pilot jets to get the slow range running correct. So can anyone help me? Thanks!

Eagle
 
#4
Hey TT, I used your suggestions as to the main and pilot jet. I get a good idle and low speed operation but the needle is way to lean even with the clip in the bottom groove and it flattens right out above about 1/2 throttle. I used a couple of needle shims and it helped a lot but because of how it's secured in the slide I could only fit two of them. According to the documentation, I have the correct stock needle and it's not damaged. I looked up needles and there are a ton of them for the carb and they run with all kinds of tapers, not just rich to lean. Did you have to change yours to make it work right? If you did do you know the needle number you used? Any suggestions anyone has would be appreciated. Thanks!

Eagle
 
#5
Hey TT, I used your suggestions as to the main and pilot jet. I get a good idle and low speed operation but the needle is way to lean even with the clip in the bottom groove and it flattens right out above about 1/2 throttle. I used a couple of needle shims and it helped a lot but because of how it's secured in the slide I could only fit two of them. According to the documentation, I have the correct stock needle and it's not damaged. I looked up needles and there are a ton of them for the carb and they run with all kinds of tapers, not just rich to lean. Did you have to change yours to make it work right? If you did do you know the needle number you used? Any suggestions anyone has would be appreciated. Thanks!

Eagle
Man this carb kicked my butt. I tried changing the needle jet. I also tried a wide range of jets. It was good at idle .took off good but wouldn't transition right.. it wouldn't turn high rpm. I just gave up and put on a tillotson.
 
#6
Old post but same thing, 28mm flatslide on a highly modded 212cc predator, idle good but had no high-speed rpm. Put on a 24mm nibbi and perfect out of the box.
 
#7
Old post but same thing, 28mm flatslide on a highly modded 212cc predator, idle good but had no high-speed rpm. Put on a 24mm nibbi and perfect out of the box.
I tried a 28mm Mikuni flat slide on a very heavily modded 212 Predator and the engine just did not like it. That was running a .308 cam with 1.3 roller rockers and a 32/28 head that was fully done up. I swapped a 24mm Mikuni flat slide on it and the engine was much, much happier.

I'm slowly working on a 236 stroker build that I'm planning on trying the 28mm carb on. I'll be running the same head on that engine, so I'm curious to see if that carb will work any better on it.
 
#8
I tried a 28mm Mikuni flat slide on a very heavily modded 212 Predator and the engine just did not like it. That was running a .308 cam with 1.3 roller rockers and a 32/28 head that was fully done up. I swapped a 24mm Mikuni flat slide on it and the engine was much, much happier.

I'm slowly working on a 236 stroker build that I'm planning on trying the 28mm carb on. I'll be running the same head on that engine, so I'm curious to see if that carb will work any better on it.
Everybody I spoke to said my build is radical enough to get it running. I did switch to a 24 nibbi and she's purring now. Thanks for the info...here are my build specs, block is stock.
 

Attachments

Minimichael

Well-Known Member
#12
I just run velocity stacks on my 24mm carbs. They're cheap on Amazon. My bikes are street only, so I'm not real worried about having a filter. Been running them like this for years with no issues.

https://www.amazon.com/gp/product/B089D18VHJ
@Strigoi, .. and they make these with screens as well, yes? I'd wondered what others thought about these. Whether a velocity stack with a single filter-type membrane might be enough for average suburban streets...
 

River Rat Ron

Well-Known Member
#13
As a motor pulls air into the carb on the intake stroke, that air gains momentum via a venturi effect.
a huge misconception is larger carbs = higher horse power. Quite the contrary. These carbs work on the volume of moving air…cfms

With too large a carb you don’t get a strong venturi effect ( vacuum ) with results in poor fuel metering ( atomizing )
of not only the main jet but also your idle circuits.

Thus one kinda works the other does not, so you adjust and get an opposite effect. You’re using the wrong carb.
Truly dialing in any motor depends on multiple factors, cam, weight, gear, elevation etc…and all of that depends on what you’re trying to do.

This old technology is a balance of give and take, finding your Yin Yang so to say

Wish I had a dollar for every small block Chevy I’ve seen with a tunnel ram and two 780 double pumpers with mechanical secondaries that couldn’t understand why his motor wouldn’t perform
 
Last edited:
#14
@Strigoi, .. and they make these with screens as well, yes? I'd wondered what others thought about these. Whether a velocity stack with a single filter-type membrane might be enough for average suburban streets...
I have one with a screen on it that I got from a scooter site, but it's not a fine enough mesh to stop anything small. You could cut up some very fine wire mesh, or maybe even pantyhose, and run that over the mouth of the carb to filter out smaller particles that may get flung up. I've gone years with running open carbs and have never had an issue because of it.

The one I have with a screen looks a lot like this: https://www.thebonnevilleshop.com/p...ong-velocity-stack-round-screen-pn-tbs-0389a/
 
Top