Big bore carb

[DaveMarles]

Anyone had any luck with big bore carbs on .90's? I use a 12mm bore OS 9B and have tried 13mm and 13.5mm bore but the motor always wants to run lean at high rpm even with pipe pressure. Anybody tried powerjets as used on Mikuni's etc?

Dave

 

[Don Ferrette]

I tried going big bore (.500 & over) on my CMB 90, no faster than with OS9b & low speed throttling was totally in the pooper. Best top end so far has been with 9b fitting directly to front plate, no spacer. B)

 

[DaveMarles]

My boat is definitely faster on the bigger carb and bottom end is fine. Its just inconsistent at max rpm and the symptons are that its going lean.

 

[Anders_Martinelle]

I have used 14mm on the K90 w/o problem.

That lean thing can be related to the pipe,i have seen this problem with a cuple of pipes i have also but that was at full and almost full throtle.

Not fun .

Anders

 

[Ian Inverarity]

Dave,

I have seen a MAC 14mm carb used on a CMB 100, as far as I know without difficulty.

What the heck are powerjets?? :huh:

Ian.

 

[DaveMarles]

It was the Mac carb that I tried at 14mm but too many lean moments causing detonation and plug blowing.

A powerjet is an extra jet that is in the bellmouth just upstream of the throttle slide which is only exposed to the airflow at full throttle.

 

[Terry Keeley]

Hey Dave:

Are you using large fuel line with fairly straight, short runs from the tank? Do you have a flow meter or some other way to tell what it flows? I found some carbs that just couldn't flow enough without some modification.

Also, with a 14mm single speed carb (no low end needle) you might have to be so fat on the stand that it'll hardly run, especially in a mono where the motor/pipe get quite warm. Any chance you're just not rich enough to start with?

 

[Don Ferrette]

Hmmm, Terry makes a good point. The MAC motors with those big bore carbs need to be super rich on the stand or they are too lean on the water. That theory didn't seem to help my CMB 90 though when I tried the big carb. Maybe I missed something... :huh:

 

[Brian MAhoney]

I wonder if the high end leanness could be due to poor atomisation of the fuel, could be the spray bar (more than one may help) would need to be redesigned to get sufficient attomisation of the fuel at a relatively low air velocity.

Brian

 

[AndyBrown]

There are many consideration here, but lets start at the point where the MAC 14 mm carb does work. That is on the Stock MAC 84. On that engine it does everything well (just ask Don since he's close by.)

Now if works well on the relativly smaller 84 but does'nt work on the bigger 100. Why?

Someone mentioned fuel delivery. Fuel line length? We many times put the needle valve behind the engine, giving at least 12" of fuel line. Right Don?. A big engine will draw fuel better than a small engine given the same carb bore.

Higher air velocity = greater fuel draw.

I have one question Dave. Did you alter the carb in any?

 

[AndyBrown]

I'll add one more thing.

Higher air velocity means better fuel atomization and mixing which helps fuel burn more complete and more quickly. Meaning there will be a change in ignition point. Among other things.

 

[Don Ferrette]

Very true Andy. The MAC 84 handles the big bore just fine, idles down no problems.

Typical needle placement in an SGX is in the rear (between end of flywheel & radio box wall) of engine well so that's not it either. Now the one thing I didn't try on my CMB 90 was advancing the ignition point like we do on the MAC's. Hmmmm............. :huh:

 

[DaveMarles]

Andy,

I've tried a few big carbs but always have problems I put up a couple of pics of 2 of them.

http://www.prestwich.ndirect.co.uk/fsrv/bigcarbs.htm

Top one is MAC 84 carb, modified with OS radio needle that I mixed with the throttle to lean off the needle as I throttle down. As you can see the fuel nipple is pretty big and the fuel lines are big too. The second carb is an OS9B where I spark eroded the barrel to a square shape approx 12.5 x 12.5mm so its a similar area to the 14mm MAC carb, the spraybar has been bored out on this carb to 1.9mm.

They both throttle no problem and run fine up to 7/8ths throttle open but at full throttle, surge and have lean moments in a straight line test.

 

[Don Ferrette]

Have you flowed the fuel side of these carbs? I'm sure the airflow is there but I'm wondering what the actual flow thru the needle is.

BTW- what do I have to do to talk you into modding a 9B barrel like that for me?

 

[Propjockey]

Another somethin' to ponder...

The Predator carb (high performance auto) has a single spraybar through the center of the venturi with multiple holes in it. If you use a remote needle, could you do the same for these smaller R/C carbs?

Several small holes would give better atomization than one large hole, right?

 

[Guest]

Oh MY ! Dave :unsure:

 

[AndyBrown]

That was me.

Newbie forever Brown.

 

[DaveMarles]

What , what ?

 

[Slideblues]

Andy, I am with ya on the larger bore, better flow, and atomisation, but at the higher end of the rev range even with good fuel flow to the needle, if the spray bar cant keep up there will be a bog or flat spot, usually where you least want it correct ? Or did I miss something?

Gene

Ok, Wait a minute, Just reread the post and Dave said he bored the spraybar out to 1.9mm.......... :wacko:

 

[AndyBrown]

For now, all I'm gonna say is: The Stock MAC 14 mm carb works on the MAC 84 and the MAC 67 and the K-90 using the CMB isolated needle or the BVM or the CMD/Kalistatov Needles.