Mikuni 34 mm questions

[bob58o]

I lied 6DH4 is mid of range at d5
6DH7 is leaner at D5

 

[Denny]

Bobby, what did we teach you about lying? Now go to your room and think about it for a while. But say your sorry first to the nice man.

 

[bob58o]

Bobby, what did we teach you about lying? Now go to your room and think about it for a while. But say your sorry first to the nice man.

I was taught that lying leads to long noses, pants on fire, and wolf attacks. None of these were true. Who is lying now, teach?

Bobs are disappearing from the Earth. We are built different. We understand unspoken apologies. We are Bobs. Here is our Emergency.

 

[Denny]

The only thing I learned was Bobs are incredibly boring!!!

 

[bob58o]

The only thing I learned was Bobs are incredibly boring!!!

Boring? You know nothing of Bobs, Jon Snow.
What you don’t know about Bobs I could just about squeeze into the Grand Canyon.

 

[Denny]

Thank you, you made my point.

 

[bob58o]

Boredom is that peaceful place located between anxiety and depression where you choose to be comfortable existing or not.

(When you start to become friends with one particular voice in your head):

 

[Rat]

Hey guys, I got a 440 Duromax engine. Billet flywheel, Billet side cover, Billet connecting rod. 275 Dyno cam, 55 lb valve springs. NR Racing Honda 390 Head with 40 mm intake, 38 mm exhaust stainless valves, head shave .060, (this NR Racing head has the small intake port for low end power. The port was matched to my intake manifold) 1.2 ratio roller rocker arms, chrome moly cut to length push rods. + or - 34 BTC ignition timing. Mikuni 34 MM round slide carb with a 15 low jet 210 main jet. Float set at 22 mm. Auto lite plug. Engine fires right up. Runs strong. You hit the throttle and no faultering, it just takes off. But after a bit of running it mis-fires once in a while at cruise RPM. The plug gets black right away. So I say it's running too rich.

Any tuning advice? Should I go even smaller on jetting? It is a gravity fed fuel tank. The bleed screw or whatever it's called is 1 turn out from being seated. I can't read the number on the emulsion tube but it starts with a 159. The needle in the slide is set in the center slot.

Assuming an ACTUAL 440cc displacement, 34mm is a bit small according to the math which means it's going to pull a little richer than it should... it clearly is. Since the A/F bleed on a VM is behind the fuel inlet it regulates additional fuel to the pilot (or low speed) circuit, and by having it in 1 turn farther than default you are artificially leaning out the low side because inward is leaner, out is richer.

On a PWK where the A/F is after the fuel inlet it regulates additional air instead, so in would be richer and out would be leaner

The default for the A/F screw is 2 turns on any carb of this type. If it's dialing in at only 1 turn out, you need to drop the pilot jet by at least 1 size and readjust the A/F.

While the default is 2, the ideal operating zone is 1-3/4 to 2-1/4. With the drive wheel(s) off the ground you get the engine ran up to optimal operating temp, then set the idle a little higher than a fast idle, set the A/F to its default, then seek the highest rev it will give before falling off with a 1/4 turn at a time keeping track of how many total.

If found at only 1 or less turns out, reduce the pilot size one and start over at default.
If found at 3 or more turns out, increase the pilot size one and start over at default.

Food for though, round slides have sluggish throttle response in comparison to a flat or D slide as found in a Mikuni TM series, or Keihin PWK series which is pretty instantaneous once dialed in.

The sluggishness is caused by turbulence in the throat which is caused by the deep large pocket found under every round slide and the slide shape exacerbates it, which odly the D slides have the same pocket without the turbulence... I don't know HOW that works, but it's because of the shape and it does and very well.
(don't waste your time trying to tell me how sluggish your round slide isn't, get a D slide and find out for yourself just how right I am)

 

[Rat]

 

[Big Bob]

So I re-jetted the carb. I put a 20 jet in the idle circuit and a 190 main jet in it. I moved the clip on the needle up one notch. Put in a new Autolite plug. It fired right up. I adjusted the air screw and idle till my ear thought it sounded right. It sounds good. I need to get it out and run it around and check the plug.

 

[Big Bob]

Still running way too rich.

 

[Rat]

Still running way too rich.

I done told you the problem. You won't be able to lean it out to the ideal range without using obscenely small jet sized and it going to be a moody POS when you do get it

 

[Big Bob]

I done told you the problem. You won't be able to lean it out to the ideal range without using obscenely small jet sized and it going to be a moody POS when you do get it

Well, it runs pretty good. Idles okay, but rich. I have the AF at about 2 turns. Throttle response is good. If you just let it idle for a minute or so and mash down the go petal it hesitates. But if you rev it a couple times then put it in gear and hit the throttle away it goes. I may try a bit smaller idle jet. But here's a question. The float on these should be in the 22-24MM range. Mine is at 22. 22 is the top of the range and 24 is the lower setting. If I shut off the fuel supply after about 10 seconds the idle really smooths out. Could this mean I need to lower the float to the 24 mm range? There is zero over flow.
On a related subject. Ignition timing. I think I have it at the 32-34 degree range. A 6 degree offset key was used. I am thinking of getting a degree wheel so I can nail down a more precise timing range. Any advice on this.

 

[Rat]

Well, it runs pretty good. Idles okay, but rich. I have the AF at about 2 turns. Throttle response is good. If you just let it idle for a minute or so and mash down the go petal it hesitates. But if you rev it a couple times then put it in gear and hit the throttle away it goes. I may try a bit smaller idle jet. But here's a question. The float on these should be in the 22-24MM range. Mine is at 22. 22 is the top of the range and 24 is the lower setting. If I shut off the fuel supply after about 10 seconds the idle really smooths out. Could this mean I need to lower the float to the 24 mm range? There is zero over flow.
On a related subject. Ignition timing. I think I have it at the 32-34 degree range. A 6 degree offset key was used. I am thinking of getting a degree wheel so I can nail down a more precise timing range. Any advice on this.

No, float level isn't going to make it run that different. Timing could be an issue but again in the grand scheme the carb is too small

 

[Big Bob]

So I lowered the float from the high setting of 22 mm to 24 mm. Put in a 17.5 idle jet. Dropped from a 190 main to 180. Haven’t been able to drive it cause it’s raining. But I can tell, and smell!, it’s running better. Great throttle response, idles better. I’ll run it at higher rpm today and hopefully be able to run it around the block.

 

[Big Bob]

No, float level isn't going to make it run that different. Timing could be an issue but again in the grand scheme the carb is too small

So you suggest what? A 36mm? I understand the single cylinder vacuum stuff going on with these engines. I have higher than stock compression with a 40mm intake valve with a small intake port for low end power. I did machine the port at the intake manifold to match up.

 

[Rat]

So you suggest what? A 36mm?

Exactly, maybe a 38mm actually

√(440)×2 = 41.95 (round to 42)

The problem with that equation is it is over simplified for quick use, is assumes 100% volumetric flow efficiency which even the best built race engine can't accomplish. Because of this fact the rule of thumb is go a carb size or two smaller than the sum rounds off to, going 3 sizes under is the smallest you can get away with and still get relatively "normal" tuning responses.

I would absolutely not recommend going larger than 40mm, and I am fairly confident that a 38mm is going to give you the best results and least tuning drama

 

[Big Bob]

Exactly, maybe a 38mm actually

√(440)×2 = 41.95 (round to 42)

The problem with that equation is it is over simplified for quick use, is assumes 100% volumetric flow efficiency which even the best built race engine can't accomplish. Because of this fact the rule of thumb is go a carb size or two smaller than the sum rounds off to, going 3 sizes under is the smallest you can get away with and still get relatively "normal" tuning responses.

I would absolutely not recommend going larger than 40mm, and I am fairly confident that a 38mm is going to give you the best results and least tuning drama

So if I understand you I’m not getting enough air? I got too much fuel. So a 38 mm with smallish jets.

 

[Rat]

So if I understand you I’m not getting enough air? I got too much fuel. So a 38 mm with smallish jets.

A 38mm will allow more air, thus allowing a "normal" jetting range.

The problem isn't specifically too much or too little air, but rather too much negative intake pressure.
Too small of a carb will draw too much fuel because the the pressue will pull from every hole it can find, not just atmosphere...and a too big carb won't pull enough fuel due to the lack of sufficient negative intake pressure to pull much of anything from anywhere.

Think of it in terms of a garden hose... the water pours out (big carb) but no real pressure. Then you stick your thumb over it and have a lot of pressure (small carb). Though it's a different type of pressure in the hose the principal remains the same, the right size carb get the right pressure and runs as it should.

 

[Big Bob]

A 38mm will allow more air, thus allowing a "normal" jetting range.

The problem isn't specifically too much or too little air, but rather too much negative intake pressure.
Too small of a carb will draw too much fuel because the the pressue will pull from every hole it can find, not just atmosphere...and a too big carb won't pull enough fuel due to the lack of sufficient negative intake pressure to pull much of anything from anywhere.

Think of it in terms of a garden hose... the water pours out (big carb) but no real pressure. Then you stick your thumb over it and have a lot of pressure (small carb). Though it's a different type of pressure in the hose the principal remains the same, the right size carb get the right pressure and runs as it should.

Makes sense. It runs pretty good now. Lots leaner. I’ll try and drive it around a bit and check the plug.

So, in the event I do go to a bigger Mikuni 36 mm or 38mm I have questions. First is, I am assuming I can’t use my current manifold. Or can I just blend it to a new larger carb? And second is round or flat slide? Looks like they are close to each other in price.