carbs

[Nason]

Questions for the Guru's

I understand it takes the right air intake to fuel mixture to gain hp and you also have to be able to exhaust same. Why is it you can take a carb that should have a 62 jet and motor runs rich, install a 60 jet and the rpms stay the same but plugs are correct color?

This is different carbs ,same motor.Now you take a carb that has a 70 jet and allows more air flow than above carb and decrease it to a 62 or 64 jet and still turn the same rpms. The 62 runs lean but the 64 are the right color.

Any help in this matter would be greatly appricated.

[Steve Litzell]

Well, You are way off on your thinking . Fuel mixture, ie: jet size has more to do with where motor is run and the range the motor is run in. The very basics are , if you have a big carb, it does not mean you need a big jet. The thing here is the air pump underneath the carb and it's ability to pump and use more air/ Fuel. I think what you are tring to understand is the fuel curve. A given motor at a certain RPM requires a certain mixture. The higher the RPM the more fuel it will need to remain at a good mix. Carb size has little to do with jet size, but a motor from stock to well modified does. It is all about volumetric efficiency more really than size of carb and to a degree motor size. Steve

[Nason]

Well, You are way off on your thinking . Fuel mixture, ie: jet size has more to do with where motor is run and the range the motor is run in. The very basics are , if you have a big carb, it does not mean you need a big jet. The thing here is the air pump underneath the carb and it's ability to pump and use more air/ Fuel. I think what you are tring to understand is the fuel curve. A given motor at a certain RPM requires a certain mixture. The higher the RPM the more fuel it will need to remain at a good mix. Carb size has little to do with jet size, but a motor from stock to well modified does. It is all about volumetric efficiency more really than size of carb and to a degree motor size. Steve

The motor is not stock, had a little work done to it. So what you are saying is if you have carbs that flow more air you can run the same jet as the carb that flows less air? Just trying to find a little more out the motor.

[champ20B]

The motor is not stock, had a little work done to it. So what you are saying is if you have carbs that flow more air you can run the same jet as the ca rb that flows less air? Just trying to find a little more out the motor.

Well a good way to describe this is.....If you have a larger venture (allowing higher CFM) and its on the same size engine, then you will likely need better or easier fuel flow such as a larger jet. This is because your engine will only draw so much air or CFM (cubic feet per minute) as is and therefor will not have as strong of a airstream effect and or pressure drop beyond the larger venture in order to draw fuel up as effectively. Generally when, lets say, a 20cid mercury has its 15/16" carter carburetor replaced with a 1-9/32" Tillotson for B-Mod racing, other things need to be done to reduce air restriction for extra needed rpm such as bigger and easier bending reeds and bigger ports ect. Otherwise, the bigger carb will need less restricted fuel flow for the limited pull of air (CFM) through a larger opening and would only render slightly more power if at all.

[Nason]

thats what i was trying to figure out, the idle side was good but the top end was slower getting to max rpms. thanks for the reply.

[champ20B]

thats what i was trying to figure out, the idle side was good but the top end was slower getting to max rpms. thanks for the reply.

Glad to help, but an easier way to see this still is that the CFM capability of your carb should not really be as much as what the engine is capable of pulling. This makes for good efficiency. However when the CFM of a carburator is increased closer to the engines max intake capability, then a larger jet is needed. It doesn't guarantee more power either under this circumstance. But if the engine is further modified thus increasing its intake capability (which means higher rpms efficiently) then a larger jet might not be needed with a bigger carb because the critical air velocity will be met for the carburators efficiency with a normal jet.

In some cases though, a bigger carb works well without much engine mods such as the case with the Champion Hotrod outboard. This is because it has big rotary valve intake and has a lot of port area as it is. Other engines that have reed valves are not always this effective without a lot of modifying taken into account.

[Steve Litzell]

The motor is not stock, had a little work done to it. So what you are saying is if you have carbs that flow more air you can run the same jet as the carb that flows less air? Just trying to find a little more out the motor.

No, You need tto reread this again. If you have a carb that flows more, It does not mean that the motor underneath it will need or use this extra flow capability. The motor needs what it needs and this is determained through testing either on a dyno or the water,( Which in my opinion water testis best) If you are looking at gaining more RPM's through jets, it won't happen unless the mixture is way off. RPM's are from port events and sizes. What you have done here is that you have found the correct mix for that motor. That motor only has the ability to pump so much so you can put the big carb on and it only hurts throttle response and not help RPM or top speed.

[tyst67]

What about adding another carb? I have a twin cylinder merc that has the same bore and stroke as a 60hp 3cylinder merc. I have 1 carb with a Venturi size of 1.25 inches and the 60 has 3 carbs with 1.25 Venturi size! I put 2 1.25 bore carbs on my engine and tried the same jets as the 60hp(.060) and my motor now has no hole shot due to being too rich! Should I see more performance? Some help would be great

[Fastjeff57]

From my 'vast' years of experience (sound of clearing my throat) I have found that simply adding a larger carb rarely provides increased performance you can feel. Now, if you do other things to increase breathing at the same time, then THOSE improvements PLUS the larger carb might be noticeable.

Jeff

[champ20B]

What about adding another carb? I have a twin cylinder merc that has the same bore and stroke as a 60hp 3cylinder merc. I have 1 carb with a Venturi size of 1.25 inches and the 60 has 3 carbs with 1.25 Venturi size! I put 2 1.25 bore carbs on my engine and tried the same jets as the 60hp(.060) and my motor now has no hole shot due to being too rich! Should I see more performance? Some help would be great

With a single carb feeding "alternate acting" cylinders, there would actually be a constant flow through the carburator. Where one piston stops pulling air, the other piston pulls. But any time you have one carburetor per cylinder that is isolated to one cylinder in induction, then the air flow is only repetitive (on/off). Because of this, it may be necessary to have a bigger jet to ease flow feed of fuel under this condition to keep primed.

Interestingly, a twin cylinder alternate firing arrangement with a single carburator can be much better than a twin carb/ twin cylinder arrangement....unless the two carbs are feeding at once through an interconnected intake path as a progressive stage systematic throttle, which I never seen on an outboard. It might be a good idea though....Ive seen this on old cars with twin carbs.

[Steve Litzell]

With a single carb feeding "alternate acting" cylinders, there would actually be a constant flow through the carburator. Where one piston stops pulling air, the other piston pulls. But any time you have one carburetor per cylinder that is isolated to one cylinder in induction, then the air flow is only repetitive (on/off). Because of this, it may be necessary to have a bigger jet to ease flow feed of fuel under this condition to keep primed.

Interestingly, a twin cylinder alternate firing arrangement with a single carburator can be much better than a twin carb/ twin cylinder arrangement....unless the two carbs are feeding at once through an interconnected intake path as a progressive stage systematic throttle, which I never seen on an outboard. It might be a good idea though....Ive seen this on old cars with twin carbs.

Don't tell the hundreds of Konig's and Za Quincy motors that for they might not run! Steve

[champ20B]

Don't tell the hundreds of Konig's and Za Quincy motors that for they might not run! Steve

If you mean the "opposed cylinder" pro-outboards, that is reverting back to the one carb per cylinder scenario (even if you have only one carb feeding both cylinders). An opposed twin is like a single cylinder engine as to how it runs in a (on/off) intake cycle regardless, but having one carb per cylinder is likely more efficient here than just one double sized carb shared by both cylinders........ In fact, thinking about the exhaust side, I cant help but to wonder why they don't just use a centered single expansion chamber fed by both cylinders (for a 350 cc as an example). That is a situation where weight could be saved, is cheaper, more practical and would work exactly as well as having two expansion chambers (one per cylinder) on the opposed engines perhaps.

[Fastjeff57]

Another advantage of the two cylinders/ one carb setup is elimination of that standing wave of fuel droplets in FRONT of the carb at certain low rpms. On my triple project, the droplets formed at low idle and extended at least 4 inches in front of the carbs--scary stuff! (I cured that nonsense with a series of equalizing passages between the carbs and reeds.)

On running progressive opening carbs, believe it or not I tried that, on my first external reeds triple project. It worked, but was 'boggy' thanks to an embarrassing screw up on my part (don't ask!) But it did work.

Jeff

[Steve Litzell]

If you mean the "opposed cylinder" pro-outboards, that is reverting back to the one carb per cylinder scenario (even if you have only one carb feeding both cylinders). An opposed twin is like a single cylinder engine as to how it runs in a (on/off) intake cycle regardless, but having one carb per cylinder is likely more efficient here than just one double sized carb shared by both cylinders........ In fact, thinking about the exhaust side, I cant help but to wonder why they don't just use a centered single expansion chamber fed by both cylinders (for a 350 cc as an example). That is a situation where weight could be saved, is cheaper, more practical and would work exactly as well as having two expansion chambers (one per cylinder) on the opposed engines perhaps.


You have never seen a Konig twin nor a Z series Quincy motor. They were alternating fire not like modern motors opposed. As far as exhaust is concerned, You can not get the primary tubes short enough for your idea. You must remember motors run at a very high rate of speed. Even your lawn mower motor and this ON/OFF thing you refer to is not really noticed by the two stroke motor. This of course depends on the intake track from the back of carburetor to crankcase. The wave you refer to is in all engines that reciprocate and many hours are spent to reduce this reversion on Intakes and cam grinds for 4 cycle. In a two stroke the closer the back of the carb is to the transfers, the better to a limit. In a stock type fishing motor, idle quality is more important than performance. Three cylinder motors seem to have more problems with reversion than two's or sixes. That may because of the crank index and the pulses the carbs receive so a balance type valley is used on fishing engines. The three banger Quincy had no problem with this. Most reversion can be slowed down with smaller Carburetors as the velocity in the intake track is higher. BIGGER IS NOT BETTER! even in racing as low velocity intake makes for a doggy motor and very hard to drive.

[smittythewelder]

Steve, they don't need to have seen either of those two rather rare alky motors; any old A/B/C/D Mercury (now all turned into Mods, I guess) has the single-carb-feeding-paired-alternate-firing-cylinders layout.

[champ20B]

You have never seen a Konig twin nor a Z series Quincy motor. They were alternating fire not like modern motors opposed. As far as exhaust is concerned, You can not get the primary tubes short enough for your idea. You must remember motors run at a very high rate of speed. Even your lawn mower motor and this ON/OFF thing you refer to is not really noticed by the two stroke motor. This of course depends on the intake track from the back of carburetor to crankcase. The wave you refer to is in all engines that reciprocate and many hours are spent to reduce this reversion on Intakes and cam grinds for 4 cycle. In a two stroke the closer the back of the carb is to the transfers, the better to a limit. In a stock type fishing motor, idle quality is more important than performance. Three cylinder motors seem to have more problems with reversion than two's or sixes. That may because of the crank index and the pulses the carbs receive so a balance type valley is used on fishing engines. The three banger Quincy had no problem with this. Most reversion can be slowed down with smaller Carburetors as the velocity in the intake track is higher. BIGGER IS NOT BETTER! even in racing as low velocity intake makes for a doggy motor and very hard to drive.

Not to change the subject, but I was wondering if these alternate twin konig and "Z" quincy models were what was used in "B-restricted" later on till the 1980s? I know that a modified alky Yamato 80 set the final 90+mph record in that class in 1986 I think. BTW, you are right about those expansion pipes. I forgot about the leading pipe length as well as the sliding tuning action they need.

[Steve Litzell]

Yes they were the same motor, The 90 mPH Yamato was a kilo record, but the man that owned that thing had it haulin a$$ for quite some time till the Quincy motor came to the class. Steve