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Old 03-22-2015, 01:49 PM   #1
TheRadBaron OP
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All things equal, will larger carbs require leaner or richer jetting?

I have a '72 Honda CB350 that I put Mikuni VM28 carbs on and got them jetted really well. I just got another CB350 that the PO installed VM30 carbs on. I'm sure the 28s are more suited for the job but the 30s are brand new and the bike is set up for them so I don't want to change them.
The engines are basically identical so the only variable is the slightly larger carbs on the second bike. I know it's tough to give jetting advice over the internet but does anyone know if, generally speaking, larger carbs would require richer or leaner jetting than smaller carbs on an identical engine? I'd just like to get the jetting as close as possible right out of the gate.
Thanks.
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Old 03-22-2015, 01:55 PM   #2
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In general, you start the factory installed jets. They should be close.

A bigger carb uses a bigger main jet. More air, more fuel. If the carb is too big, it needs even a bigger main jet since you won't have as much vacuum to draw fuel in.
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Old 03-22-2015, 05:50 PM   #3
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Quote:
Originally Posted by TheRadBaron View Post
I have a '72 Honda CB350 that I put Mikuni VM28 carbs on and got them jetted really well. I just got another CB350 that the PO installed VM30 carbs on. I'm sure the 28s are more suited for the job but the 30s are brand new and the bike is set up for them so I don't want to change them.
The engines are basically identical so the only variable is the slightly larger carbs on the second bike. I know it's tough to give jetting advice over the internet but does anyone know if, generally speaking, larger carbs would require richer or leaner jetting than smaller carbs on an identical engine? I'd just like to get the jetting as close as possible right out of the gate.
Thanks.
I suck at jetting. Bigger carbs, more air, but maybe also a stronger venturi.

So smaller jets so less fuel being pulled in? Needs to be leaner?

That's why I suck at carb jetting?

Or maybe the velocity of the air is lower so the venturi is weaker, bigger jets.



Or maybe the venturi stays the same and you need more fuel for more air getting to the combustion chamber?
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Old 03-23-2015, 07:05 AM   #4
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Ask Kevin

Cycle World, Service, April 2015, p.p. 76. Ask Kevin.
Jetting Off?
Q. (In a nutshell) If carburetors same size on different motor sizes.
Should jetting be leaner or richer?
A. (In a nutshell) Smaller engine - bigger main jet, medium engine size - lesser jet size, larger engine - substantially smaller main jet.
Said the flow through carburetors is not steady and continuous but is a series of pulses.
My thought would be larger main jets and needle height and pilot jets, but not much from standard jetting, since pulse signals might be a bit weaker. Also might only be needed in a specific circuit, i.e. pilot, needle, main.
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Old 03-23-2015, 07:48 AM   #5
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The problem is that all things are not equal, you simply need to re-jet to what that specific bike/carb combination wants.

I'm not sure about the quote from "Kevin", while it seems logical I'm not sure about the practical application of his answer.

Racing carbs are commonly jetted on a flow bench.
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Old 03-23-2015, 09:04 AM   #6
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As they come is close enough on a four stroke for a starting point. Every engine is different even if exactly the same spec wise. Buy a box of plugs and get going.
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Old 03-23-2015, 10:33 AM   #7
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If every engine is different even if the same spec-wise, how is it possible that the factory jets every bike of the same model that comes off the assembly line exactly the same and they run just fine?
I'm not being a wise guy, but for this exercise all things ARE essentially equal, except the carb size (they are both Mikuni VMs and basically identical other than the size). This is at least equal enough that a very general jetting question would apply.
I agree that two motors of the same type will have slight differences that might cause the optimal jetting to vary slightly, but it will be slight.
I plan to go through the proper jetting procedure but I'm just curious about the effect carb size has on jetting. I'm confident that there is a general rule of thumb about it.
The Kevin Cameron info that upshifts posted got me thinking. I had known about this in the back of my head somewhere but it hadn't occurred to me. It's accepted that, generally speaking, a larger engine for a given carb size will require a smaller main jet. I believe that this is due to the stronger vacuum signal from the increased displacement. The same is probably true when the carb size varies and the engine displacement remains the same. For a given displacement, a larger carb will have a weaker vacuum signal and require richer jetting.
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Old 03-23-2015, 10:40 AM   #8
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Also, the jetting that comes in a Mikuni VM carb when you buy it is totally unsuitable for a 4-stroke. You can get them to run but they won't run well. The main thing that needs to be changed is the needle jet. They come with a "primary-type" NJ which is optimized for a 2-stroke. They need a "bleed-type" for use on a 4-stroke. Also, I had to reduce the size of the air jets in the carb mouth on my VM28s, plus all the usual changing of jets and needles. They need a lot of work to run well but they beat the hell outta the original CV carbs.
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Old 03-23-2015, 01:09 PM   #9
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Well if you already know it all why ask?
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Old 03-23-2015, 01:20 PM   #10
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I surely don't know it all, though now I know more than I did before I asked this question. It wasn't until upshifts posted the stuff from Ask Kevin that I started connecting the dots. That's why I asked. I figured someone might have some insight. Cheers.
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Old 03-23-2015, 05:19 PM   #11
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Many don't know how to properly read a plug for main jet tuning at wide open throttle. What's shown in service manuals is mostly for part throttle readings.

This is a good example for full throttle readings with a new plug for every jetting change .

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Old 03-23-2015, 07:56 PM   #12
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The engine is a basically a pump, the volume of air/fuel in and air exhaust out is dependent on much more than displacement alone, which is why flow benches can be used tune a carb.

Jetted good enough for the average person is one thing, jetted well for a race tuner is entirely different. New bikes typically come leaner than shit so the production run can pass emissions testing/requirements.
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Old 03-23-2015, 07:58 PM   #13
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Quote:
Originally Posted by Truckedup View Post
Many don't know how to properly read a plug for main jet tuning at wide open throttle. What's shown in service manuals is mostly for part throttle readings.

This is a good example for full throttle readings with a new plug for every jetting change .

Fine with race fuel, new pump gas has all kinds of crap that makes plug reading impossible, that and getting a rider that knows how to do a "plug chop" all but makes plug reading a lost art.
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Old 03-25-2015, 12:55 PM   #14
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I don't entirely agree with the statement that plug reading can no longer be done. Your plugs still tell you somehting, even if it is only at the extreme end of the spectrum.

Check you plugs now, check them after the new carb is installed. If your jetting is different, you will see a difference in your plugs. You can get the idle circuit right without plug chops and you can come pretty close to the pilot circuit without plug chops too.
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Old 03-25-2015, 02:42 PM   #15
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I don't entirely agree with the statement that plug reading can no longer be done. Your plugs still tell you somehting, even if it is only at the extreme end of the spectrum.

.
Yes, a basic idea if it's way rich or way lean. The good way to do main jets is on a chassis dyno.. The needle position can be done pretty accurately by road testing .The slide cutaway is generally ok when the engine is respone seems a bit lean at lower RPM's after a cold start and choke off. The pilot jet is usually correct when the engine idles good with the mixture needle is the "recommended" position, like 1-1/2 turns out for example.
Some set ups are fussy and some are more tolerate of jetting.
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