Mikuni 34 mm questions

[Rat]

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.

It depends if the carb is flange mount or boot (34-38mm carbs tend to have the same butter diameter and use the same boot... not always the case with flange mounted) by all means feel free to try, I would.

Flat slide is ALWAYS more responsive than a round so no contest

 

[Big Bob]

It depends if the carb is flange mount or boot (34-38mm carbs tend to have the same butter diameter and use the same boot... not always the case with flange mounted) by all means feel free to try, I would.

Flat slide is ALWAYS more responsive than a round so no contest

Thanks. Mine is boot.

I’m an old man. 72. Back in the day as a young man I can remember listening to those inline 6 cylinder cars driving around and you could hear the carb sucking air! I drove a ‘64 Chevy pickup with a 292 6 and 4 speed. The carb on those were small. You could hear the carb sucking like crazy. Maybe 100-120 hp and 7-10 mpg. My buddy had a similar truck but newer, ‘67 or so. He put a “junkyard” 2 barrel carb on it. Sort of a Jerry rig manifold adapter. His truck had lots more power and got hugely improved fuel mileage.
That’s what’s going on with my 440 with cam, high compression, and big valves. It’d be a job but interesting experiment—- but I bet I could hook a vacuum gauge to the carb bowl vent and it’d read positive.

 

[Rat]

Thanks. Mine is boot.

I’m an old man. 72. Back in the day as a young man I can remember listening to those inline 6 cylinder cars driving around and you could hear the carb sucking air!

Same for my Husqvarna 208... I've built it up a fair bit, it was already a tourquer but with the PWK 24 [non genuine as Keihin doesn't make a PWK smaller than 28mm] with a velocity stack on it... anyway it has that distinct hungry hiss when I whip it open.

That’s what’s going on with my 440 with cam, high compression, and big valves. It’d be a job but interesting experiment—- but I bet I could hook a vacuum gauge to the carb bowl vent and it’d read positive.

It wouldn't run if that were accurate. The bowl remains at ambient atmospheric pressure.

If by some gross error it were to reach positive pressure somehow it would attempt to lift the float while blowing the inlet needle shut, as well as spray fuel through the jet like an injector head.

If by some error the bowl were to reach a negative pressure, it would pull fuel like a fuel pump and pour through the jet in a typical flood out fashion.

In either scenario the engine isn't even going to run, therefore the pressure would equalize quickly.
The vents are there for the specific purpose of maintaining a pressure equal to ambient, this can be proven by connecting them all together with tubing or otherwise sealing them... it won't take long at idle before it starts running wonky and stalls.

 

[bob58o]

Because I’m a nerd and I apply physics to everything….

Did we cover the idea of “signal” and “air pressure” (often incorrectly described as relative “vacuum”)
- physics never sucks, it blows. Or more accurate… it pushes. It applies a force.

Carburetors work on the same principle that make baseballs curve and airplanes fly.

Bournoulli’s Principe…

Atmispheric pressure 14.7 psi at sea level pushes down on the surface of the fuel in the bowl.

As the carb throat narrows to the Venturi diameter from the air horn diameter, the air speeds up. Faster moving air is a lower pressure.
The difference in pressure between the ambient atmosphere air pressure and the reduced pressure in the carb throat / circuitry forces the gasoline up.

if the Venturi is too small, the air speed will be fast, the pressure difference will be great, a smaller jet might be needed to reduce fuel flow because of the “greater” difference in pressure.

If a Venturi is too large, the air speed will be slower, the pressure difference will be smaller, a larger jet might be needed to increase fuel flow because the fuel is being pushed with less force.

Carburetors, airplanes, baseballs,…
Same - Same, but different.

 

[bob58o]

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.

I see it was addressed, but I said it in “Geek”.

 

[bob58o]

“Mr Einstein, what do you think about marrying one’s own cousin?”

A.E. :
“Beauty is relative to the perception of the observer, now get off my photon”

Just to help understand (or complicate) here’s why physics just absolutely cannot suck (in psi nor inHg).

 

[Thepartsguy]

What about a taper manifold?? Lets say 30mm at the carb mount flange.

But tapers down from just behind the carb mount flange all the way through the middle all the all the way till it matches a 5hp intake port size??

And seriously I don’t even know what I would have to google to find answers on this.. I’ll take a wild guess and say it doesn’t matter about the taper at all and would still have a weak signal?

 

[bob58o]

I believe the most important diameter is that of the carb Venturi itself. You might be able to mess around with air bleeds and stuff inside the carb, but maybe it’s not worth the fuss.

if it runs good, but studders at 1/2 throttle… apply more throttle. When in doubt, more throttle!
If full throttle is too fast, more gearing!

Bigger sprocket on the rear = more fun.
More throttle = more fun.

 

[Big Bob]

What about a taper manifold?? Lets say 30mm at the carb mount flange.

But tapers down from just behind the carb mount flange all the way through the middle all the all the way till it matches a 5hp intake port size??

And seriously I don’t even know what I would have to google to find answers on this.. I’ll take a wild guess and say it doesn’t matter about the taper at all and would still have a weak signal?View attachment 146286

I don’t have a flange carb.

 

[Rat]

What about a taper manifold?? Lets say 30mm at the carb mount flange.

But tapers down from just behind the carb mount flange all the way through the middle all the all the way till it matches a 5hp intake port size??

And seriously I don’t even know what I would have to google to find answers on this.. I’ll take a wild guess and say it doesn’t matter about the taper at all and would still have a weak signal?View attachment 146286

It works in terms of making a slightly larger carb fit, but at most it will weaken the signal prior to reaching the carb. The significance lies in the diameter difference between the opposing ends of the carb.
Funny enough there exsists a few straight bore carbs for smaller engines which rely almost exclusively on the negative intake pressure instead of Bernouli's principle.

 

[Big Bob]

I believe the most important diameter is that of the carb Venturi itself. You might be able to mess around with air bleeds and stuff inside the carb, but maybe it’s not worth the fuss.

if it runs good, but studders at 1/2 throttle… apply more throttle. When in doubt, more throttle!
If full throttle is too fast, more gearing!

Bigger sprocket on the rear = more fun.
More throttle = more fun.

Engines seems to be happy with the smaller jets. I don’t use sprockets.

 

[bob58o]

Engines seems to be happy with the smaller jets. I don’t use sprockets.

How about Cogswell cogs if you don’t like Spacely Spacely?

 

[Denny]

Because I’m a nerd and I apply physics to everything….

Did we cover the idea of “signal” and “air pressure” (often incorrectly described as relative “vacuum”)
- physics never sucks, it blows. Or more accurate… it pushes. It applies a force.

Carburetors work on the same principle that make baseballs curve and airplanes fly.

Bournoulli’s Principe…

Atmispheric pressure 14.7 psi at sea level pushes down on the surface of the fuel in the bowl.

As the carb throat narrows to the Venturi diameter from the air horn diameter, the air speeds up. Faster moving air is a lower pressure.
The difference in pressure between the ambient atmosphere air pressure and the reduced pressure in the carb throat / circuitry forces the gasoline up.

if the Venturi is too small, the air speed will be fast, the pressure difference will be great, a smaller jet might be needed to reduce fuel flow because of the “greater” difference in pressure.

If a Venturi is too large, the air speed will be slower, the pressure difference will be smaller, a larger jet might be needed to increase fuel flow because the fuel is being pushed with less force.

Carburetors, airplanes, baseballs,…
Same - Same, but different.

Yea, I’ll take Peter, Paul and Mary’s version. Or even John Denver’s ( he wrote the song).

 

[ezcome-ezgo]

 

[Denny]

I prefer Spacely Sprockets. They have a better heat treating process and don’t bend as easily.

 

[Big Bob]

LMAO! Figured it out. It was the choke. I looked in many places. Choke lever up to choke, choke lever down to run. That is true to the enrichers with a knob. My carb has a lever. Lever up enricher is off. Lever down enricher is on. Runs great now. Spark plug looks okay.

 

[bob58o]

 

[Big Bob]

View attachment 147332

LOL OtherBob, I was about to pull the trigger on a 38MM Flat Slide carb. To my dumass pulling up the choke lever to choke and down to run just made sense. Once I looked at a schematic of my particular unit I saw I was wrong. BTW, I have been wrenching on stuff since around 1964. I was a professional John Deere mechanic for a while till I got drafted. Back in the late 1970's early 1980's I built a diesel pickup out of a '74 Dodge 200 Club Cab. I used a 1964 Perkins engine from a British Bus. I used a Military 5 speed (overdrive) transmission. I think that project took less time than my Golf Cart. I ran that truck for about 150K miles issue free. Back then I could get diesel for $.14 a gallon. This truck averaged about 20 mpg or so. I had a 60 gallon tank in it. Sold it for $900! LOL


But, thanks for your input.

 

[Rat]

LOL OtherBob, I was about to pull the trigger on a 38MM Flat Slide carb. To my dumass pulling up the choke lever to choke and down to run just made sense.

It is a little annoying that the enrichment plugs with a rocker lever are sometimes hinged to lift the plunger, or direct pivot to lift the plunger... I prefer Keihin carbs, no confusion issue to deal with on those... either the knob is up or it isn't.