big valves carb size need help

I'm running 32mm Mikuni round slide with gx390. With 40mm intake and 36mm exhaust. I was told by a friend that with those size valves that i was suffocating my motor and that I should be running a 40mm carb. I was just wondering if its true. It sounds good running but if it is suffocating I guess I should switch or any suggestions will help thanks.
 
Most every formula used to calculate carburetor size on any engine involves RPM as a main part of the equation. 40mm and 36mm are some bigguns. While I strongly suspect a 32mm carb may not flow as much as you may need at 8000rpm for instance, I have no proof and I have not tested it. If you plan on turning huge rpms, then I would think a 38mm (or 40) would be beneficial. If your really lugging around using a wide rpm range then staying with a 32mm may be OK.
 
I don't really know much about this, but just looking at it, a 40 millimeter carb has the ability, it seems to me, to pass a lot more air than a 40 millimeter valve.

Going to a bigger carb is, I think, going to mean slipping the clutch more. To get the real benefit you'll need to turn more rpm. At high rpm, obviously, the big carb has the ability to flow more than a small carb.

But then again, I could be totally wrong. If it was me, I would be dyno testing the two and see which one is better. Of equal importance would be, "where, in the RPM range, is it better", that is, if it is better? I would be comparing the area under the usable curve.

The damnedest thing is you have to spend all that money to buy the carb, and, do the testing.

Convert Area Metric <> Inch
MM To> Inch
40 mm 1.5748 inch
32 mm 1.2598 Sq Inch Diff. in Area
Diff. in Dia. 0.315 0.7013

Convert Inch to Metric
Inch To> MM
1.5748 Inch 40.00 mm
1.2598 inch 32.00 Sq mm Diff in Area
Diff. in Dia. 8.00 17.8130
 

Kart43

Member
I am not sure of the carb flowing 1.5 times the head, I can guess where that comes from. Looking at your user name Flamingwheelhorse, I assume we are talking tractor. A carburetor sized that large will work at constant high RPM, but it will be a dog getting there. At low or accelerating RPM the fuel signal will be weak. Now if the carb was equipped with an accelerator pump you can get by that.
 

Kart43

Member
You will benefit from a larger carb, be aware some have much better low speed circuits and will be more responsive through the entire RPM range. Some of the Mikuni carbs that are on the big bore, 4 stroke, single street or trail bikes will work. Yamaha had some 36 and 38mm carbs that were well designed.
 

W5R

New member
If it were me, I would try to find a 38mm carb if you want to stick with slide carbs, but on a racing mower, a Tillotson would be much better suited for what your doing, in my opinion anyways. Listen to cubcadet (Tyler), he races pulling tractors and knows his stuff when it comes to those
 

level600a

New member
Simple rule is for max power Carb should be same size as intake valve or very close to it.......a smaller carbs will give you better throttle responds........I have the 33mm pumper on my kart engine with 40mm intake valves, cuts down on max power but I gain tons in throttle responds, helps
a lot going off corners
 
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Armilite

New member
I'm running 32mm Mikuni round slide with gx390. With 40mm intake and 36mm exhaust. I was told by a friend that with those size valves that i was suffocating my motor and that I should be running a 40mm carb. I was just wondering if its true. It sounds good running but if it is suffocating I guess I should switch or any suggestions will help thanks.
Stock GX390 (88mm x 64mm) 389.4cc. Stock 11.7hp@3600rpm using 8.2cr! 81% Volumetric efficient! Your 32mm Carb can Flow 68.1cfm so I would suspect your CAM.

So what CAM Lift/Dur are you using?

What CR are you using?

389cc at 90% Volumetric efficient at 3600rpm = 15hp and makes 22cfm.
389cc at 95% Volumetric efficient at 3600rpm = 16hp and makes 23cfm.
389cc at 100% Volumetric efficient at 3600rpm = 16hp and makes 25cfm.

389cc at 100%:
At 5000rpm = 23hp and makes 34cfm
At 6000rpm = 27hp and makes 41cfm
At 7000rpm = 32hp and makes 48cfm
At 8000rpm = 37hp and makes 55cfm
At 9000rpm = 41hp and makes 62cfm

389cc at 110%:
At 5000rpm = 25hp and makes 38cfm
At 6000rpm = 30hp and makes 45cfm
At 7000rpm = 35hp and makes 53cfm
At 8000rpm = 40hp and makes 60cfm
At 9000rpm = 45hp and makes 68cfm

So as your Volumetric Efficiency goes up, your cfm needs also go up!

Peak Flow/HP Calc.
https://www.mk5cortinaestate.co.uk/calculator3.php

Mukuni Carbs CFM.

Look at this 460 Dyno Sheet I found. Notice Max HP at 5000rpm, but it was losing HP after 3600rpm. Probably Stock GX390 CAM.
 

Attachments

Jimbo

If you talk the talk you should walk the walk
What valve spring seat pressure are you using with your 40mm intake valve?
 

Yakattack

Member
I always wondered why a GX390 uses such a small carb?

On my YZ144 I use a stock 38mm slide carb...
And on my YZ250f I use a 40mm slide carb...
I have even run a stock SBN44 on each engine with no issue...

But the gx390 is more than twice the CC of the 144 and almost twice the 250f...
 

Jimbo

If you talk the talk you should walk the walk
So noooo body knows:)
How about this one? What is the weight of the 40mm intake valve?
 

manyfunnies

New member
I always wondered why a GX390 uses such a small carb?

On my YZ144 I use a stock 38mm slide carb...
And on my YZ250f I use a 40mm slide carb...
I have even run a stock SBN44 on each engine with no issue...

But the gx390 is more than twice the CC of the 144 and almost twice the 250f...
Rpms and two stroke vs 4 stroke. The yz250f sucks in air every rev where the 390 is every other rev.
 

95 shaw

Premium User
The gx390 is a low rpm industrial engine. The small carb is responsive to governor change of throttle position.

Mc engines are typically double or more rpm, with the powerband much higher in the rpm range.

Flow velocity is what you are looking at for the intended purpose.
 
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