CARB SIZE

[NOVALTD]

TO ALL ENGINE EXPERTS: I WOULD LIKE YOUR OPINION ON THE SMALLEST VENTURI SIZE YOU CAN RUN WITH ON 1/8 SCALE HYDRO AND NOT LOOSE TORQUE OR RPM OR OVERALL HP. THE SCALE WILL BE A PROVEN DESIGN THAT HAS ONE NATIONAL CHAMPIONSHIPS AND IS 14-15 LBS.

HAS A QUALITY ENGINE IN IT LIKE MAC,CMB,K 67, RUNS ON 50% NITRO AND USES A PROVEN PIPE LIKE A AB,CMB,OPS OR MUCK. THE COMPRESSION ON THE ENGINE IS EXCELLENT AND THE CARB WILL HAVE A LOW END NEEDLE. ITS USES A MCCOY 9 GLOW PLUG. IS A .390-.410-430-.450-.470 THE SMALLEST YOU CAN GET AWAY WITH AND NOT LOOSE PERFORMANCE LIKE A BIGGER CARB. HOW ABOUT IT ROD,ANDY,MARTY,MARK,DON AND NW BOYS. THANKS DAN MCCORMICK NAMBA 221

PS: THE ENGINE IS IN PEAK OPERATING CONDITION AND THE BOAT SETUP IS OPTIMAL.

PS: CHANCE FAVORS THE PREPARED MIND.

 

[SayMikey]

For my money the OS9b is the favorite East Coast or West.

 

[Greg Hahn]

For my money the OS9b is the favorite East Coast or West.

ditto!

 

[Hydro Junkie]

I don't know of anyone that doesn't use a 9B carb on the CMB 67 that runs scales. I have them on all of my engines as well

 

[Bob Morton]

I agree with Mike and Greg. The 9B has been a proven carb. Use a third channel. Start at forty with a Boris Meter.

Nails.

 

[Greg Hahn]

I agree with Mike and Greg. The 9B has been a proven carb. Use a third channel. Start at forty with a Boris Meter.

Nails.

yup...........in the water at 40, up to 45 at the start and at 48 by by plug!

 

[Ken Hildreth]

I dont even know how long I have used a 9b, I even used it when I ran picco drum rotors. To me, a scale has to do two things, go really slow, then really fast, with a good transition from low needle to high needle. A 9b will do that.

 

[Greg Hahn]

Sorry it took this long but to answer your original question the 9B carb is .475

gh

 

[John Seybold]

Do you use the 9b with the standard needle or remote? Or can you use either one? I'm going to be running a few scales this year and currently have the stock CMB carbs on them, I may have to order a 9b

Thanks

 

[PakMan]

TO ALL ENGINE EXPERTS: I WOULD LIKE YOUR OPINION ON THE SMALLEST VENTURI SIZE YOU CAN RUN WITH ON 1/8 SCALE HYDRO AND NOT LOOSE TORQUE OR RPM OR OVERALL HP. THE SCALE WILL BE A PROVEN DESIGN THAT HAS ONE NATIONAL CHAMPIONSHIPS AND IS 14-15 LBS.

HAS A QUALITY ENGINE IN IT LIKE MAC,CMB,K 67, RUNS ON 50% NITRO AND USES A PROVEN PIPE LIKE A AB,CMB,OPS OR MUCK. THE COMPRESSION ON THE ENGINE IS EXCELLENT AND THE CARB WILL HAVE A LOW END NEEDLE. ITS USES A MCCOY 9 GLOW PLUG. IS A .390-.410-430-.450-.470 THE SMALLEST YOU CAN GET AWAY WITH AND NOT LOOSE PERFORMANCE LIKE A BIGGER CARB. HOW ABOUT IT ROD,ANDY,MARTY,MARK,DON AND NW BOYS. THANKS DAN MCCORMICK NAMBA 221

PS: THE ENGINE IS IN PEAK OPERATING CONDITION AND THE BOAT SETUP IS OPTIMAL.

PS: CHANCE FAVORS THE PREPARED MIND.

To answer your question.... what is the 'smallest you can run'.... I've seen a 1/8th Scale go 77+ MPH with a K&B .340 bore Carb (OPS 65 Motor), in cold weather (30 something (Degrees F). That number matters, due to air density, pulling a 1667 Octura prop. It was a SAW event, but the hull was a 'normal' weight (16#) oval boat. So, with that said, that is as small a bore as I've seen be successful. Many successful 'racing' carbs in the 1/8th Scale arena (in the NW) have been around .390 bore. That means they would throttle well and have good top-end.

In the recent past, 1/8th scale guys have seen to migrate to the OS 9B... .470 bore, I think.. but the spray-bar and low speed assy do certainly take up some space in that 'bore'. It is a very good transitional carb. It allows you to drop the trigger, and troll at idle speed if your boat is set up correctly. Then pull the trigger, and you're off... quickly.

Another thing to think about is ... your tuned pipe stinger//exhaust dia. Flow in, and flow out... hmmm... think about that.

Best,

DJ

 

[SayMikey]

Do you use the 9b with the standard needle or remote? Or can you use either one? I'm going to be running a few scales this year and currently have the stock CMB carbs on them, I may have to order a 9b

Thanks

Either will work well,just dont hack the low end system<needle

 

[Greg Hahn]

Do you use the 9b with the standard needle or remote? Or can you use either one? I'm going to be running a few scales this year and currently have the stock CMB carbs on them, I may have to order a 9b

Thanks

Either will work well,just dont hack the low end system<needle

definately don't hack it as mikey says but u will want to back the low end needle out flush with the outer barrel. the 9B will bolt right up, no mod needed

 

[PakMan]

Do you use the 9b with the standard needle or remote? Or can you use either one? I'm going to be running a few scales this year and currently have the stock CMB carbs on them, I may have to order a 9b

Thanks

Either will work well,just dont hack the low end system<needle

definately don't hack it as mikey says but u will want to back the low end needle out flush with the outer barrel. the 9B will bolt right up, no mod needed

"Low speed out flush with the outer barrel"... NO, I don't think so... that makes it (the low speed needle) irrelevant....

I have not run in a while, but the value of the low-speed needle on the 9B is in its ability to help idle, mid-range, and top end. By design, it engages in the idle position (to lean), and it dis-engages upon opening to create a smooth power curve. If I remember correctly, you want to set the low-speed needle at about 3 1/2 turns out when the carb is in the closed position for a .67 Scale. That allows proper idle and very good transitional throttle response. Cut off, and or remove the high speed needle if you want to run remotely with a third channel (as long as it is 4-5 turns out if I remember correctly), but again, don't defeat the low-speed component of this carb. It is one of its greatest qualities. And, as far as it bolting right up.. for many I guess it does. For those of us that are more precise, .015" of an inch here and there to make the bolts align perfectly is important, and requires some attention. Bottom line, if the o-ring seats/seals properly you've got it bolted up correctly.

DJ

 

[NOVALTD]

THANKS EVERYONE FOR THE ANSWERS,BUT I AM CONFUSED. IF ALL WHAT YOU ARE SAYING IS TRUE WHY IS EVERYONE RUNNING A 9B WITH A LARGE BORE .470 BORE. IS THE DECISION YOU MAKE TO USE A OS 9B BASED ON LOGIC AND REASON OR A POPULARITY POLL. I AM NOT TRYING TO BE A SMART A-- BUT IF ITS BEEN PROVEN A 16LB SCALE CAN GO 77 MPH WITH A

0.340 BORE CARB IT SHOWS IT IS GETTING PLENTY OF AIR AT TOP END. AND WE ALL KNOW A SMALL DIAMETER CARB HAS BETTER LOW END AND BETTER PRESSURE DROP ACROSS THE

DUMP TUBE. MY EARLY PHYSICS DAYS TAUGHT ME THAT

STATIC PRESSURE + VELOCITY PRESSURE = TOTAL PRESSURE SO IF WE REDUCE THE DIAMETER OF THE BARREL 50 THOUSANDS THE VOL0CITY PRESSURE INCREASES AND THE

STATIC PRESSURE DECREASES LEAVING THE TOTAL PRESSURE THE SAME. SINCE A ENGINE WILL PRODUCE A GIVING MAX CFM THE CARBURETOR VENTURI DIAMETER CAN VERY ( WITHIN LIMITS) AND NOT SACRIFICE TOP END PERFORMANCE. THE VELOCITY OF THE AIR JUST COMPENSATES FOR THE REDUCTION IN AREA. ASK YOURSELF THE FOLLOWING QUESTION AND BE HONEST WITH THE FACTS. I BET MOST SCALE DRIVERS FIND ( WHEN THERE BOAT IS RUNNING RIGHT) THAT WHEN THEY MOVE THE TRIGGER ON THERE RADIO THE LAST 1/3,1/4 OR 1/8 OF THE CARB BARREL TURN WILL NOT YIELD ANT MORE GAIN IN SPEED? IF YOU AGREE THEN IS IT FAIR TO SAY THE CARB DIAMETER IS TO BIG AND A SMALLER DIAMETER WOULD BE ADEQUATE AND MAYBE BETTER.

I BOUGHT A BORIS MAZOR FLOW METER AND GOT A BOB VIOLET REMOTE NEEDLE AND SET THE FLOW THE SAME TO SIMULATE MY SCALE SETTINGS. I FOUND OUT THAT THE OS 9B STARTS OUT AT 35 AND DOES NOT MOVE THE NEEDLE THE TOTAL SPAN OF THE BARREL TURN UNTIL THE BARREL IS CLOSE TO THE IDLE POSITION. IF THE 0S9B IS TO DO ITS JOB

SHOULD NOT THE FUEL BE REGULATED IN PROPORTION TO THE AIR. I TESTED OTHER CARBS FROM 30 YEARS AGO AND THEY METER MUCH BETTER THRU A CARB BARREL ROTATION THEN THE OS9B. IE: OSMAX7E. OSMAX7F. RPM ,ROSSI . DO YOU GUYS GET THE SAME RESULTS? OR DID YOU MODIFY THE LOW END NEEDLE ON THE 9B? SO IS IT CONVENIENCE AND FOLLOW THE LEADER THAT TELLS US TO RUN A OSMAX9B OR IS IT THAT

NOBODY HAS CHALLENGED THE STATUS QUE THAT OTHER CARBS ARE JUST AS GOOD OVERALL TO USE LIKE SUPER TIGER,PICCO,ROSSI,RPM,OPS SLIDE VALVES , K&B AND CAN BE A SMALLER DIAMETER FOR OVERALL BETTER PERFORMANCE. LIKE TO HERE FROM ROD G,ANDY,MARTY,GRIM AND ANYBODY ELSE THAT HAS A LOT OF EXPERIENCE AND LOGIC TO SHARE. YOUR TIME IS ALWAYS APPRECIATED. ------------ DAN

PS: WHY IS NOBODY IN SCALE RUNNING THE PROVEN ZOOM CARBS ? YES I KNOW ABOUT THE LOW END NEEDLE ISSUE BUT HEAVY 25LB TWINS RUN AIR VALUES AND EXHAUST THROTTLES FOR YEARS.

 

[SayMikey]

I have run a Zoom with some success but had a small hesitation when coming to the start slow mill. I felt it could get me beat at the start line. I will say the Zoom had tons of top end,but I would rather have the punch off the corner. The Os9b seems to do all I need,I ran 12 races last year on one OD9 Put a new plug in for the final at Madison just because the race meant a lot to me to win and I did..... In the end it is what works for you and your setup

 

[Duck899]

I don't know of anyone that doesn't use a 9B carb on the CMB 67 that runs scales. I have them on all of my engines as well

Well, then you know one guy.....lol

I run the stock CMB carbs.....The Bardahl will idle up to the line and has instant throttle response like an OS 9B. The CMB carb has a low speed setting and just needs to be set up. Most guys run the OS 9B because it is proven to win. The OS is very user friendly. And, as DJ said, the spray bar takes up alot of area in the bore, even though it is .475". It is the overall flow of air that is the bottom line.

 

[NOVALTD]

THANKS GREG FOR YOUR REPLY . WOULD YOU KINDLY SHARE WITH THE IW AUDIENCE WHAT LOW AND MID RANGE ACCELERATION YOU GAINED ALONG WITH TOP END SPEED OVER YOUR COMPETITION WITH A .390 BORE THEN .340.

THANKS DAN

 

[Hydro Junkie]

I don't know of anyone that doesn't use a 9B carb on the CMB 67 that runs scales. I have them on all of my engines as well

Well, then you know one guy.....lol

I run the stock CMB carbs.....The Bardahl will idle up to the line and has instant throttle response like an OS 9B. The CMB carb has a low speed setting and just needs to be set up. Most guys run the OS 9B because it is proven to win. The OS is very user friendly. And, as DJ said, the spray bar takes up alot of area in the bore, even though it is .475". It is the overall flow of air that is the bottom line.

Okay Rob, I stand corrected B)

 

[Douglas P Shepherd]

If set up correctly, the 12 mm CMB carb will give you the complete package and save you $90.00. I do have some 9bs and they are a couple of weak point about them. The stock arm breaks and the fuel nipple is to small for the fuel line and needs to be mod. My two cents Doug Shepherd Madison, IN.

 

[Brian Schymik]

Have run a MAC carb adapted to the CMB 67 on my scale for 9 years. Its no slouch It has a bigger bore than any you are talking about. .510

Brian