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Discussion Starter · #1 ·
A couple things I can't remember and can't seem to find the the manual. I have an 83 vt750c with stock carburetor and a few basic questions.

1 - When adjusting the set screws for the air/fuel mixture, does turning the screw in (clockwise) increase the air:fuel ratio ie a more lean mixture? Or vice versa?

2 - So I do remember that a lean mixture burns hotter than a rich mixture but maybe we should make sure I have that correct.

3 - And what about rpm's? If you lean out your mixture you should see a decrease in rpm's right? Since the choke makes the mixture rich and causes an increase in rpm's.

I find questions 2 and 3 to be counter intuitive (at least to me) because I would think that a hotter combustion would lead to higher rpm's.

Well thanks for the help!
 

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Leaning the mixture will increase RPS as well...To a point. LEaning the mixture to peak RPM then dialing it back a bit makes for a good setting but hard to check without a Tach. The choke increases RPM by making the mixture excessively rich. Remember the ration of air to fuel is 14.7:1 this should be the target for best efficiency vs power. since there is no easy way to check that we need to rely on the engineers that designed the carbs.

SO...
1) YES...it leans the mixture. but only for about the first 1/8 to 1/4 throttle setting after that it has no effect at all
2) Lean burns hotter and burns valves. you have that correct
3) I answered this above. Leaning increases RPMs to a point as does the choke to a point.

Hotter combustion leads to detonation and engine damage.
 

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screw in to lean--out to richen---Start at about 2 1/4 turns out from lightly seated--go ride and get bike to warm operating temp---start turning out 1/4 turn at a time and you should see rpm rise as it gets richer--do this to both carbs as a pair---you will finally hit a point where the rpm stops rising and may start to fall---you are looking for the point right when you get the highest rpm---this will be the correct setting--some folks like to have the rear cyl 1/2 richer than the front cause it runs hotter---readjust idle as needed----this is called the BEST RPM METHOD for setting fuel mixture screws. Good luck
if you have aftermarket exhaust and a jet kit installed and a different intake or something you will probably end up at like 3 or 3 1/2 turns out---dont go past 4 turns out or the screw may fall out because thats all the threads that there are
 

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3 is BOTH can increase or decrease idle rpm. In is lean, out is rich. More air (oxygen) equals more heat.
Any adjustment to the AF screw will change the idle RPM, up or down depends on you starting AF mixture and Idle setting. Steps are adjust idle, check sync, adjust idle, adjust AF screws, adjust idle, adjust AF screw, adjust idle, repeat till AF and idle are correct.
 

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Discussion Starter · #5 ·
Thanks everyone, that clears almost everything up for me. I do have aftermarket exhaust and right now I think I'm sitting around 3-3.5 turns on my set screws. So explain why the rear cylinder burns hotter? I guess it's related to muffler length but some further explanation would be great. Thanks again.
 

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Rear cylinders on V-twins tend to run hotter and often need to be tuned richer than the front cylinder. Exhaust pipe length, if they are not balanced, can be a factor. A correct exhaust system should be equal flow rate if not equal length, front to rear. Also because the rear cyl is behind the front is get less air flow causing it to run hotter. It is also farther from the radiator, so coolant get heated by the front cyl before reaching the rear. AF screw will do NOTHING for the running mixture as it only affect the idle circuit. If you are running rich above 1/8 throttle you will need to change the jets and the rear may need a richer jet than the front.

What are the symptoms you are trying to correct.
 

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Discussion Starter · #7 · (Edited)
Rear cylinders on V-twins tend to run hotter and often need to be tuned richer than the front cylinder. Exhaust pipe length, if they are not balanced, can be a factor. A correct exhaust system should be equal flow rate if not equal length, front to rear. Also because the rear cyl is behind the front is get less air flow causing it to run hotter. It is also farther from the radiator, so coolant get heated by the front cyl before reaching the rear. AF screw will do NOTHING for the running mixture as it only affect the idle circuit. If you are running rich above 1/8 throttle you will need to change the jets and the rear may need a richer jet than the front.

What are the symptoms you are trying to correct.
Wait, the AF set screw only affects the idle mixture and not the throttle mixture? I'm confused again.

Well I swapped out my engine and carb and need to retune because the pipes have changed. I'm getting a bit of popping in the rear cylinder on deceleration which seems to go away if I keep about 1/4 choke. I thought at first the popping was from being too rich but now I'm thinking it's because I'm too lean. I'm looking into a tachometer and sync gauge but figured I'd start asking some questions to get a better understanding of what the carb is actually doing and how the engine and pipes effect things.
 

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Yes the AF screws adjust the fuel mixture at idle to 1/8 throttle, beyond that mixture is controled by the main jet and the Needle, which is raised by vacuum.

Decel popping, through the exhaust is a LEAN condition.

What motor did you put in? What Jets are in the carbs? What exhaust are you running? Is the intake stock? Are floats correctly adjusted? Are the slide diaphrams good?

Technically the Shadow does NOT have a choke. It has an enricher system that is called the "choke" and serves the same function. A choke cuts off air flow thus enriching the AF mix, our "choke" enrichens the AF mix by increasing fuel flow.

DIY, Sync Tool for $2, $1.55 Carb Sync Tool by Marty Ignazito, if the carbs are out of sync you will never get it tuned correctly.
 

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Discussion Starter · #9 ·
Interesting, I naively assumed the idle screw was for the idle and the set screws were for the AF mixture at idle and throttle. Good to know the truth.

I put in another 83 vt750 engine with its matched carb. The pipes are non-stock straight pipes, don't know what type but here's a pic I found of the same pipes:

Recent image by clodhead on Photobucket

The intake is stock, I haven't done anything with the carb except play with the AF set screws so I don't know about the jets or floats. I doubt the carbs are sync'd well either. I don't think I'm going to pull carb quite yet. Maybe I'll wait for some rainy days but I'll update with post with future findings. Maybe I'll just run with a 1/4 enricher for now to keep the mixture rich until I can change to a bigger jet.

Oh, so what's the difference between the jet and needle?
 

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Jets are basical nozzels the fuel is sprayed and metered through, into the throat of the carb. The needle is tapered and moves up and down with the vacuum slide. The needle rests in the bore of the main jet, it does not completely block the flow of fuel. As the RPMs increase vacuum increases the more vacuum the hight the vacuum slide moves. Remember the needle is atatched to the bottom of the slide, and the Jets are nozzles?. As the slide raises it withdraws the needle from the bore of the jet, the needle is tapered, so the farther it is withdrawn the more fuel can be spray through the jet.

The AF screw is basically the same thing except it is a fixed needle on a screw.
 

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Discussion Starter · #11 ·
Yes the AF screws adjust the fuel mixture at idle to 1/8 throttle, beyond that mixture is controled by the main jet and the Needle, which is raised by vacuum.
So if you purposely bias the vacuum to the carb running too lean you could get a richer mixture without having to rejet? I'm just curious because I came across a thread where a guy sync'd his carbs and now one the cylinders is running too lean. To me this sounds like a dirty quick fix for mechanics to cheat you out of $$ by telling you they had to adjust the floats and rejet and reneedle and resync and bla bla bla but all they did is bias the vacuum a bit.
 

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Discussion Starter · #12 · (Edited)
I'm rebuilding my parts bike carb just to get some experience and if I accidentally break it, I don't really care. So for the jet sizes? My main jet says 118 and my slow jet says 40. I have no idea what those numbers correspond to or whether those are typical values or not. Any info on the subject would be great.

After pulling all the little bits off the carb, can I sink the whole thing in carb cleaner? There are a couple plastic washers where the two carbs couple together. I'm not sure how corrosive carb cleaner is on plastic and rubber. I have a pale of NAPA Macs 6402.

And my needle says 1CD if that is of any value.
 

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Discussion Starter · #13 ·
After a little more investigation, my carb and jets are keihin and aparently for keihin jets the number (closely but not exactly) corresponds to the mm diameter, ie 150=1.50mm. But I'm still not sure if a #118 main and #40 slow are big/small/normal for an 83 vt750.
 

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i have 40's as the small and then 106 and 104 in the mains, this is in a 2002 but i dont think so much changed as far as the carbs go. so it soulds liek some one rejeted it at some point.


when i did my carb (still working with it) i soaked the whole thing but i removed all the parts i could but did not separated the carbs because i did not want to resink them.
 

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Discussion Starter · #15 ·
thanks rangerbrown
Wouldn't surprise me if the previous owner had re-jetted them. Is your carb and jets Keihin as well? So what about the jet is actually being made smaller/bigger? My jets look like an open cylinder with threads on the other end. There are like 4 holes drilled through the cylinder, well 2 holes drilled radially through making a total of 4 holes. So is the sizing the diameter of the drilled holes through the cylinder or the diameter of the open end of the cylinder, or both? And do people replace the needle with a smaller/bigger diameter or a different tapering or anything?
 
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