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Thread: carbs

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    Default carbs

    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.

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    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

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    Quote Originally Posted by Steve Litzell View Post
    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.

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    Quote Originally Posted by Nason View Post
    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.

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    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.

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    Quote Originally Posted by Nason View Post
    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.

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    Quote Originally Posted by Nason View Post
    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.

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    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

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    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
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Quote Originally Posted by tyst67 View Post
    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.

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