[atomix]

The short story – I think it works. The Honda carb fits on my Toro 20684 (Suzuki 47P 2-stroke) in place of the Mikuni with a few modifications to the Honda carb, but with the 5 minutes of run time I had on the mower I could only get it to run on choke. More about that later, I’m going to assume I can fix that & describe what I did.

Why this is potentially great: The Honda Carb costs $14, the Mikuni costs $140 and potentially this can be done on other Mikuni-carb’d engines (Kawi FC150/180, Wisconsin Robin, Lawn Boy M, ??). The Mikuni develop wear between their throttle shafts and the carb body, leak unfiltered air & run lean.

Got all the real info from PERR (http://www.perr.com/phpBB3/viewtopic.php?f=14&t=7952#p59550) and discussed it another thread here (http://www.mytractorforum.com/showthread.php?t=237483&page=2).

The swap:
Install went as described on PERR with one BIG difference. I did not have to change the carb mounting studs on the engine (I did this, and then the air box would not fit, easily a wasted hour.) Honda carb fits almost like the Mikuni, it is just slightly thicker.

As you can see in the pictures, the carbs are very similar in size and linkages. The process was roughly:
1. Remove mikuni & clean up old gaskets if you’re going to reuse. All gaskets on the engine side of the carb can be reused.
2. Tape up all the places on the Honda Carb you don’t want aluminum shavings, and enlarge the mounting holes towards the center of the carb body. I couldn’t get a dremel in the holes, so I used a regular round file. Probably took 10 minutes – not a big deal.
3. Maybe omit this step? I took the PERR guys’ advice and drilled the main jet to .78 mm or wire gauge 68. Duraforce folks should know this size well. Except I couldn’t get the jet to actually fall out of the carb body, it unscrewed and then hung up at the bottom of the ‘tube.’ Does anyone have Honda carb experience with this? I think this is the GCV mowers if that helps (I will search more too). Since I don’t have to mower running right, although it seems I’m fuel starved, I can’t say if I should have done this. I don’t know if this caused the issue, or if the mower wouldn’t have run without it at all. Anyhow…
4. Attach the stock linkages to the Honda carb, and mount the carb to the stock studs. Here is where everyone gets to play their games for getting the carb on/off the studs. I was able to get the carb on/off with linkages attached, but it was a squuuueeze.
5. Start practice fitting the airbox, you’ll have to do 2 things:

a. Cut the gasket listed below to be very close to the carb body, or the airbox won’t fit flush, and​

b. Grind away/cut away a lot of the plastic on the choke lever. Just lots of trial & error.​

The parts:
1 each Carb=16100-Z0Y-013 (now 16100-Z0Y-813)
1 each filter plate gasket=17228-ZL8-000

Problem as of now: (hopefully to be edited out once I fix it…)
Have only had few minutes to play with it…but I could only get it to run on full choke and when the mower was loaded with the BBC engaged. It does run, and sounds pretty good, given that it’s on choke! So, clearly there’s a problem. I completely forgot this carb has an adjustable idle circuit (looks like it at least), so maybe that’s my savior. I’ll try to get home while the sun is still up tomorrow and try—and search for a huge air leak that maybe I created with all my gasket pulling & stud pulling…I want this to work badly for me and everyone else…and all my Mikuni mowers!

Had one other odd problem, I think the float hung up on me while I was fitting the Honda Carb, so I had big puddles of gas on my first try. (my excuse for not working on this more) I disassembled the carb, saw no problems, and pressure tested the needle/seat (10psi). No problems on reassembly, just odd.

 

[atomix]

And since I wasted the time on it...pictures of the carb not fitting on the longer studs. Or maybe the stock studs are my problem...that's why it's a project :drunkie:

P.S. I don't advocate my removal method...

 

[Y Block]

That's awesome if it actually works! The Mikuni carbs more the engines you listed into ticking time bombs. I wonder how well it will work long-term, because the couple GCV's I've dealt with don't tolerate old gas.

Keep us posted!

 

[Ingersoll444]

That's awesome if it actually works! The Mikuni carbs more the engines you listed into ticking time bombs. I wonder how well it will work long-term, because the couple GCV's I've dealt with don't tolerate old gas.

Keep us posted!

You know though... if you get the swap down, the cost is so cheap, you can pop a new one on every season or two when it plugs up. Not a ton more then a spark plug!!

 

[Y Block]

Yeah, it's nice that the carb is no longer a big deal, just a bit of time. It just gets annoying pulling the cord numerous times.

To give an example, I have a Honda HRX with a GCV190 engine and a Toro with a Suzuki 4-stroke and the Mikuni. I let them both sit this winter for a good month and tried starting them. The results:

Suzuki: 2 pulls
Honda: 11 pulls

The new engines are just fussier because of the leaner carburetors for emissions. Still, if you use good, fresh gas, as you should, I think this could be a great solution.

 

[atomix]

Well, no good news on the carb swap. Once the carb runs on the main jet, i.e. engaging the blade, it seems OK, but the idle circuit requires choke and just surges like mad.
I cleaned out the carb to make sure my jetting operation hadn’t screwed up the idle circuit. No luck, but I did finally get the main jet to thread into the lower part of the emulsion tube area and drop out.
Only other thing left to try (unless I’m missing something) is to tamper with the tamper resistant idle knob and try to open up that circuit.

I’m discouraged to hear all the bad Honda carb stories now, although on this particular mower, it does start faster. My Wisconsin & Kawi were both 1 pull starters with the Mikuni, so I’d hate to lose that…

 

[Mantas]

Are the holes for the linkage rods in the same spots?....perhaps the governor need to be adjusted?

 

[atomix]

Maybe the governor needs tweaking, but it does find a set speed once the choke is engaged (higher than when the choke is off and it's surging). I'm actually surprised the governor hunts so much given how responsive the Suzuki's is with the Mikuni.

 

[Y Block]

I’m discouraged to hear all the bad Honda carb stories now, although on this particular mower, it does start faster. My Wisconsin & Kawi were both 1 pull starters with the Mikuni, so I’d hate to lose that…

Just use good gas...I've had better luck since switching to non-ethanol fuel. I'm curious to see how fuel consumption changes. My Honda's are very good on gas.

Do you know if this engine ran okay before switching the carb? Are you sure the crankcase has no leaks? I also wonder if the Honda's idle circuit is much leaner. Can you access the pilot jet?

Oh, one more thing. I noticed the link you gave discussed this mod for the 4-cycle Suzuki. They're both Mikuni's but different carbs...maybe the 2-stroke needs different mods.

 

[Mantas]

I noticed the link you gave discussed this mod for the 4-cycle Suzuki. They're both Mikuni's but different carbs...maybe the 2-stroke needs different mods.

Wow....with all the excitement, I didn't notice that. If you can't get any headway I think I would figure out the pilot size of the two stroke Mikuni and try that. And if its still lean I would start drilling the jets a little at a time to see if it can be dialed in to give it more fuel.

You might want to hold off action til one of our many resident 2-stroke suzuki experts chimes in like GTP

 

[atomix]

Wow....with all the excitement, I didn't notice that. If you can't get any headway I think I would figure out the pilot size of the two stroke Mikuni and try that. And if its still lean I would start drilling the jets a little at a time to see if it can be dialed in to give it more fuel.

You might want to hold off action til one of our many resident 2-stroke suzuki experts chimes in like GTP

Wow, I feel like a fool now. A real quick search on Partstree shows two different styles of Mikuni for the 2 and 4 stroke. Biggest difference I see is that the 4 stroke used a .825 mm main jet, and the 2 stroke used a .775 mm (I think I'm interpreting the units of measure correctly.) So I may have drilled out the main jet too rich if nothing else.

Other difference is that the 4 stroke had an adjustable idle circuit/pilot screw, but they both use the same idle jet.

Hmm, have to run, but more to think about!

 

[Ingersoll444]

I'm really thinking you need to fatten up the idle a bit. I think your just not getting the fuel you need without the choke on.

 

[Y Block]

Biggest difference I see is that the 4 stroke used a .825 mm main jet, and the 2 stroke used a .775 mm (I think I'm interpreting the units of measure correctly.) So I may have drilled out the main jet too rich if nothing else.

It's going to smoke and sputter a lot if the main jet is too rich. But maybe the lean idle circuit is balancing that out???:dunno: I agree with the previous post, richen the idle somehow and then worry about the main jet. When I have surging mowers, it's usually the pilot jet.

 

[viking427]

4 stroke has no oil leaning out the fuel mixture. Id think the 2 strokes would need richer jets.

 

[atomix]

The dremel and I spent some quality time removing the pilot jet limiting tab from the Honda carb tonight...so I should be able to run it tomorrow or Saturday. Appreciate the input on mixture, I maaay be a bit rich on the main jet, but we'll see! A new main jet is $6 or $7 -- so I'm not motivated yet to buy just that.

EPA should be here by Monday, but they probably won't get past the historic vehicles in the driveway to find the mower.

 

[Y Block]

Your main jet could be too rich as you said, but this symptom comes from the engine running too lean. I don't think it would act rich until the lean idle circuit is fixed. Let us know how it works tomorrow. If this is sucessful, it'd be interesting to see what mods are necessary for other engines.

 

[Ingersoll444]

Your main jet could be too rich as you said, but this symptom comes from the engine running too lean. I don't think it would act rich until the lean idle circuit is fixed. Let us know how it works tomorrow. If this is sucessful, it'd be interesting to see what mods are necessary for other engines.

Yes that will effect main jet running also.

Carb tuning is something you have to work up to in steps. every change, changes something else. Its a give and take till you meet somewere in the middle and have a nice running engine.

 

[atomix]

Preamble: I feel like I'm drawing out this thread, but I haven't had much run time on the Toro-Suzuki-Honda.

With that disclaimer, I have it running pretty well, seems rich overall.
Changes:
1. Cut notch in throttle arm to allow mower to throttle all the way down to a real idle. (more dremel fun).
2. 1/2 turn out on the idle screw
3. Loosened the airbox / carb mounting screws.

#3 was odd, running with no airbox, I'd often get no surging after richening up the idle. BUT as soon as I tightened up the airbox, I would have the surge back, as bad as before. I can see no place where the airbox interferes or hits a linkage. There's almost no way the studs could be exerting enough force on the huge rigid carb body to deflect anything, unless I'm somehow compressing the carb bowl.

I'm out of ideas on the cause, but I do have an easy to start, smooth running and easy idling Suzuki.

I'll see how reliable this ends up being, with all the quirks I've seen so far I can't recommend this as an "easy" swap. I should get some more run time on this before winter, but I'll be exercising other leaf-loving machines soon too.

 

[Ranger302j]

Atomix, great thread on this. I have lots of Mikuni experience as I have them on my wisconsin robins and kawasakis. Like was said in another post, surging, unless under a very heavy load, will always be something in the pilot and/or intermediate carb circuit. The Keihin carbs used on Honda engines seem to be much less sensitive to stale fuel issues. A thought for you: if you think the governor might be causing the surging, you could remove the carb and disconnect the linkage to the throttle and operate it manually to see if that smooths the engine out. If not, from what you said about tightening the airbox to the carb making it surge still sounds like a pilot circuit issue, either the jet or the passages. I would make sure the airbox isn't blocking flow to the pilot air inlet on the carb as this can cause this issue as well. Let us know anything new! :trink39:

 

[atomix]

I would make sure the airbox isn't blocking flow to the pilot air inlet on the carb as this can cause this issue as well.:

Ranger, glad you enjoyed it. Looks like you're the right audience- a Keihin and Mikuni expert!

You may be spot on about the airbox blocking the pilot inlet, that would make perfect sense.

As a somewhat related point, I bought my first WT1-125 ~10 days ago, fortunately it looks fairly lightly used besides showing signs of a leaking carb at one point. The important part the Mikuni looks quite a bit different if anyone is thinking about this swap. I think the jetting is similar, but the choke plate is enormous even though the throttle is about the same size. Linkages looked like they should work.