Dual Mikuni VM34 Jetting

acotrel · Dec 9, 2020

I use Mk2 Amals which are pretty much Mikunis. From memory the screws adjust the fuel. I've had Mikunis on two-strokes, and I think they adjusted the air.. The old type TT and GP carbs - I cannot remember and it was never important. If you winds the screws in and don't get the miss, you wind them out to get it. In either case, if you don't get the miss, the fuel jet is too big. It is the same with needle jets. If you cannot induce the miss, the jet is too big. But if you are using Amal carbs with petrol, the one thou difference between a 0.106 and a 0.107 is very large. If 0.106 is too lean, a 0.107 might not be a good answer, if you cannot induce the miss.

Fast Eddie · Dec 9, 2020

Al, mk11 Amal’s are the same as mk1 in this regard.

It is an air screw Al. If you wind it in, you reduce the air and make it richer. If you wind it out you increase the air and make it weaker.

The optimum setting for air screws should be approx 1 - 1 1/2 turns out from fully home if the pilot jet is correct. The pilot jet controls the fuel.

If they need to be further in than that, the pilot jet is too small. If they need to be further out than that, the pilot jet is too big.

Hence the importance of knowing the effect of screwing in and out.

acotrel · Dec 9, 2020

My friend had the fastest 500 cc Manx Norton in Victoria in the early 1960s. He ran 14 to 1 compression with methanol fuel. He told me he always ran it as lean as he could right down the needle. If you keept he jetting just slightly rich, that is where you get best performance without destruction. The problem lies in getting the jetting close enough without doing damage. Racing a Norton is the same as racing a rwo-stroke. With road bikes it is probably not so critical, but every Japanese bike of the 1970s had different needles and needle jets.

Bernhard · Dec 9, 2020

Fast Eddie said:
Re; "Hence the importance of knowing the effect of screwing in and out."

Do u want to rephrase that last comment?

Schwany · Dec 9, 2020

Back when dinosaurs roamed the earth on motorcycles, this is what dinosaurs were lead to believe:

(Not the idle slide height position screws)

Adjustment screws on the air filter side of the slide are idle air metering adjusters. turn in = less air, turn out = more air (Mikuni VM for example)

Adjustment screws on the intake manifold side of the slide are idle fuel metering adjusters. turn in = less fuel, turn out = more fuel (Keihin FCR for example)

As far as I know, the only thing that has changed with regard to fuel delivery tuning is pump gas stoichiometric is closer to 13.8 than 14.7 like when the dinosaurs were here. I'm sure that could be argued about for an eternity. I've never tuned a carbureted motorcycle with an AFR gauge though, so have no idea where I generally end up when I think I've "got it".

seattle##gs · Dec 9, 2020

If the motor does not start quickly with the P-4, put in the #35 pilots.

Fast Eddie · Dec 9, 2020

Schwany said:
Back when dinosaurs roamed the earth on motorcycles, this is what dinosaurs were lead to believe:

(Not the idle slide height position screws)

Adjustment screws on the air filter side of the slide are idle air metering adjusters. turn in = less air, turn out = more air (Mikuni VM for example)

Adjustment screws on the intake manifold side of the slide are idle fuel metering adjusters. turn in = less fuel, turn out = more fuel (Keihin FCR for example)

Never heard that one! It might well be correct for many carbs. But it’s not correct for Amal concentrics...

SteveA · Dec 10, 2020

Fast Eddie said:
Never heard that one! It might well be correct for many carbs. But it’s not correct for Amal concentrics...

On a thread about Mikunis, it could be said, who cares

I have to admit in most cases my adjustment of the idle circuit mixture is limited to 'screw fully clockwise to light contact and turn one and a half turns back out' Works well enough for me on Amals and Mikunis!

chaztuna · Dec 10, 2020

nomadwarmachine said:
snipped

Has anybody experience with shortening the housing? I will have to remove the solder end, shorten the housing, and reinstall the ferule. I am comfortable with this, only I am not sure how to reattach the solder end properly. Can I modify this, or should I just look for a couple of new cables/housing to run from the junction? Motion Pro seems to carry these.

I prefer to simply remove some of the outer housing. Less chance of screwing things up. Pull the steel cap off the end of the outer cable. Trim off the desired amount from the plastic sheathing with a razor blade or Exacto knife. Using a pair of small needle nose pliers and standard pliers, carefully unwind the needed portion of the outer cable. Cut off the excess. Bend the end of the outer cable so that you can refit the steel end cap.

Schwany · Dec 10, 2020

Fast Eddie said:
Never heard that one! It might well be correct for many carbs. But it’s not correct for Amal concentrics...

Applies primarily to many Japanese designs. I forgot about Amals a long time ago. I should have checked where the darn idle air adjustment screw is on those, so I could have beat you to saying it is not correct for Amals. 15 points to Gryffindor

chaztuna · Dec 10, 2020

To avoid confusion, best to refer to them as the mixture screw [works no matter if it controls fuel or air] and the speed screw, as you use it to lift/lower the slide to adjust the speed, once the mixture is set.

Fast Eddie · Dec 10, 2020

SteveA said:
On a thread about Mikunis, it could be said, who cares

I have to admit in most cases my adjustment of the idle circuit mixture is limited to 'screw fully clockwise to light contact and turn one and a half turns back out' Works well enough for me on Amals and Mikunis!

Well... some poor sod struggling with carburation in future, and searching this forum for help, and being told the wrong thing... might care...!

Your description of what works for you is, of course, possible because you have the correct pilot jets fitted...

Winding the air screws in and / or out to establish the best position, and using this info to establish whether or not the pilot jet is correct / too small / too large is a normal process. But one that would cause a lot of unnecessary frustration if your understand of it was (incorrectly) reversed.

chaztuna · Dec 10, 2020

Do u want to rephrase that last comment?

I think you had better "lay off" those little blue pills!

nomadwarmachine · Dec 10, 2020

SteveA said:
Why not just go with your original cable set up until you have your jetting close to final setting?

For the cost of new cables? order some ready made with angle fittings!

Keep your existing cables as spares!

Sensible advice, Steve. I refitted the existing cables, bench-sync'd everything and reassembled last night. I can work on cutting/fitting new cables once the carbs are dialed in. Should have a chance to test run today or tomorrow and see how we are doing.

Current settings are 159-P4, 250 main, 22.5 idle, 6DH2 (3d pos). Idle air screw is the 2.0 that shipped, turned out to standard 1.5 turns. Will send an update when I have had a chance to start it up.

Schwany · Dec 10, 2020

Fast Eddie said:
Well... some poor sod struggling with carburation in future, and searching this forum for help, and being told the wrong thing... might care...!

Your description of what works for you is, of course, possible because you have the correct pilot jets fitted...

Winding the air screws in and / or out to establish the best position, and using this info to establish whether or not the pilot jet is correct / too small / too large is a normal process. But one that would cause a lot of unnecessary frustration if your understand of it was (incorrectly) reversed.

I would think somebody looking for information about tuning Amal carburetors wouldn't be looking in this thread.

Fast Eddie · Dec 10, 2020

Schwany said:
I would think somebody looking for information about tuning Amal carburetors wouldn't be looking in this thread.

Maybe. It has dragged on I agree. I’m not sure why, we’re dealing with quite clear, documented fact.

But when someone (and it wasn‘t you) states that the air screw controls the fuel, when it doesn’t. And that screwing it in makes it richer, when it doesn’t. I do not understand what is wrong in correcting such things, especially as they stay written, and accessible, forever.

Schwany · Dec 10, 2020

Fast Eddie said:
Maybe. It has dragged on I agree. I’m not sure why, we’re dealing with quite clear, documented fact.

But when someone (and it wasn‘t you) states that the air screw controls the fuel, when it doesn’t. And that screwing it in makes it richer, when it doesn’t. I do not understand what is wrong in correcting such things, especially as they stay written, and accessible, forever.

I get it. I didn't say you were wrong though. Sorry about that. I generally stay in my own lane, and quite possible traveling in the wrong direction. By the way I completely agree with this all being very well documented. I'm procrastinating on doing something else by participating in this thread.

Bernhard · Dec 10, 2020

chaztuna said:
Do u want to rephrase that last comment?

I think you had better "lay off" those little blue pills!

Don't need those little blue pills - FYI I'm driving around in a little woodie, which has some tuning mods, although the ash frame needs some of those woodworm pills for it . . . . .

Super Seven · Feb 2, 2021

I would really like to hear from the original poster here. Hopefully there is progress.

Super Seven · Feb 2, 2021

I'm in a bit of the same boat, and I intend to get them a lot closer before I even install them and try them out.

Dual Mikuni VM34 Jetting

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