Dual Mikuni VM34 Jetting

Update: 159-P4, 250 main, 22.5 idle, 6DH2 (3d pos) was an absolute bear to start and backfired horribly before dying. Could not get it to run right at idle, so I figured I had to step up the idle jets. Switched out for 30s, which made a world of difference — easy start, settles into an idle and snaps quickly into the midrange. In the higher revs, the bike is plainly happier with the narrower jet (down from the P8) and the plugs, though still dark are showing some shades of tan.

Unfortunately, I still have trouble with the engine dying when fully warm. Suspect that I might want to go BACK down to the old 25 idles while keeping the P4. If that supports the idle when hot, I can look into getting the needle jet exactly right - a single step down on to a P2 and a single step down on the main from 250 to 240 may do the trick.

More to come when work let’s upland I have a change to get back into the garage!
 
my guess is to keep the #30 or even a #35. cut the needle jet to P-2. Try the 6DH3 needles, they are used the most.
The air correction jet is usually #2 or eliminated all together.
 
seattle##gs said:
my guess is to keep the #30 or even a #35. cut the needle jet to P-2. Try the 6DH3 needles, they are used the most.
The air correction jet is usually #2 or eliminated all together.
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Your comment on air correction is not valid for a 4 stroke! You will get weak running at WOT high rpm!

It is usually #2 because that is what Mikuni fit as standard, and it works for 2 strokes, though some highly tuned 2 stroke engines may indeed benefit from removing it!

It is true to say that most people never change it from a standard #2, but similarly most people would benefit from making a change towards a #1

It may also be true to say that many people don't ride at the throttle setting and engine speed that really depends on getting the air correction right!

Great to see the OP has made progress.

I suggest dropping the needle to see if that goes in the right direction. 6HD3 needle is richer, so seems a step in the wrong direction.

I would also be inclined to go back to #25 idle jets. But do one change at a time.

Main jet most likely still too big but maybe doesn't have so much influence at the moment.

If you mean engine dying 'on tickover' when fully warm, weaken the mixture screws a half turn at a time to get highest rpm then adjust tickover to suit.
 
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Maybe:

Run hotter plugs. Put in 230 mains. Needle jet is probably still too rich at P4. Drop the needles to the top slot, and put a shim under the clip, then raise the needle and go richer from there if needed. Leave the 30 pilot in there. Enjoy the tuning journey.

Motor dying at idle is a crappy symptom to deal with, because it could be fuel or ignition (charging system) related, especially if using EI.
 
I just pulled the VM34 carbs off of of my brother's 1973 MKII. he bought from HPI back around '79 or '80. It always ran well, quite peppy in its day. I'm going to take them apart clean them, and catalog all of the parts. It has an HPI Cam, some head work, and significant added compression. Should be a reasonable baseline for me. It was always a peppy street Norton. It has sat in my back bedroom for a number of years.

I'll publish what I find, when I get to it. Not the first project in the cue.
 
Super Seven said:
I just pulled the VM34 carbs off of of my brother's 1973 MKII. he bought from HPI back around '79 or '80. It always ran well, quite peppy in its day. I'm going to take them apart clean them, and catalog all of the parts. It has an HPI Cam, some head work, and significant added compression. Should be a reasonable baseline for me. It was always a peppy street Norton. It has sat in my back bedroom for a number of years.

I'll publish what I find, when I get to it. Not the first project in the cue.
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Very usable jetting specs for VM34 carburetors have already been mentioned in this thread pages back. Chances are what your brother had is a lot closer to what Sudco puts in the kits. The OP's jetting is an experiment at best.
 
Put in the 25 mains and forget about them for a while...like a year. 250 is plenty rich so no worries.
As for the pilot I like to have a range of them trying to bracket the problem. What is too lean and what is too rich? #25 is the leanest I would go. #40 is the richest. In my very standard Atlas with Commando pistons I use 37.5 pilot, single mikuni, #3 slide. I think the happy spot is between #30 and #40
 
Revisiting this after a long hiatus but want to log my progress for the benefit of the community. The 850 Commando was running well enough with the following setup, though consumption remained high and I needed to feather the throttle at idle to avoid a stall when rolling off to a stop. Reminder that these are dual VM34s on a stock rebuilt 850 motor with K&N filters.

159-P4
#250 main
#30 idle
6DH2 (3d pos)

When my jets arrive from Wahl, I will follow SeattleGS’s advice, sticking with the #30 idle jet but dropping the main to a #230 and the needle jet to a P2. It seems like I am close on the mid- and top-range. There is no bogging but the plugs are still very dark so I think I can lean it out further. Will leave the idle jets on the higher end until I have the main/needle closer.

I realize this is the reverse of the Mikuni tuning guide recommendation, but I was so rich in the main passage and I suspect that has something to do with my stalling issue. The bike starts immediately with the choke on and warms to a good idle with the #30s. The stall only occurs upon roll-off to a stop. (It is also difficult to start when fully warm - which suggests too rich on the idle, I know. I have pairs of the #25 and #27.5 just in case my diagnosis is wrong.)

Will update when I have tried out the P2 with the #230 mains. Feeling optimistic.
 
nomadwarmachine said:
Revisiting this after a long hiatus but want to log my progress for the benefit of the community. The 850 Commando was running well enough with the following setup, though consumption remained high and I needed to feather the throttle at idle to avoid a stall when rolling off to a stop. Reminder that these are dual VM34s on a stock rebuilt 850 motor with K&N filters.

159-P4
#250 main
#30 idle
6DH2 (3d pos)

When my jets arrive from Wahl, I will follow SeattleGS’s advice, sticking with the #30 idle jet but dropping the main to a #230 and the needle jet to a P2. It seems like I am close on the mid- and top-range. There is no bogging but the plugs are still very dark so I think I can lean it out further. Will leave the idle jets on the higher end until I have the main/needle closer.

I realize this is the reverse of the Mikuni tuning guide recommendation, but I was so rich in the main passage and I suspect that has something to do with my stalling issue. The bike starts immediately with the choke on and warms to a good idle with the #30s. The stall only occurs upon roll-off to a stop. (It is also difficult to start when fully warm - which suggests too rich on the idle, I know. I have pairs of the #25 and #27.5 just in case my diagnosis is wrong.)

Will update when I have tried out the P2 with the #230 mains. Feeling optimistic.
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Unless you are running race gas/AVGAS, you are pissing up a rope. Unleaded pump gas does not color plugs until it's waytohellandgonetoorich

A slight oil consumption will do the dark.

My plugs are snow white, now that all the oil burning is done.
 
To fix your off idle stutter you need to call Thunder Products. They make something called a UFO. It is a complete Mikuni Game changer, but do not install until you get your tune down:
thunderproducts.com

UFO Ultimate Flow Optimizer - Thunder Products Carburetion Performance

UFO Ultimate Flow Optimizer for round slide VM Mikuni Carburetors. Gives you best throttle response and low end to mid range horsepower gain.
thunderproducts.com thunderproducts.com

I was so impressed with the install on a friends bike I picked up one for my dads single carb mikuni set up along with one of these:
thunderproducts.com

Quad Flow Torque Wing - Thunder Products Carburetion Performance

Quad Flow Torque Wing increase air flow, air speed and fuel atomization and performance. For carburetor or EFI on Motorcycles, ATV, Dirt Bikes
thunderproducts.com thunderproducts.com

I cannot believe how strong the bike pulls away from a stop light and how much improvement is made from these two products. I have hated mikunis for years as I have never had them transition off idle easily and have always had to set them up to idle at 1500 rpm + to bandaid this.

The big issue I see in your tune is that you do not know if your idle is good. To tune your idle, get a color tune spark plug, this will let you know where you are at each change you make. From there you can adjust your needles and needle jets.

We were running 20 idles, but are now running 15s with UFOs under the slides

We used a P0 Needle Jet and have a P2 that was close. We have been playing with 6F9, 6DH3, and 6DH2 needles. Settled on the 6DH2, the 6F9 works well on Triumphs, but was not quite right on the Norton.

220 main jet has not been touched.

Slide was a 2.5, but is now probably a 3.5 because of the UFO.

Air jet is a 2.0

Bike is an 850 with an SS cam and bit more compression, the carbs are brand new and aside from them the bike is a late 70s performance time capsule.
 
maaseyracer said:
To fix your off idle stutter you need to call Thunder Products. They make something called a UFO. It is a complete Mikuni Game changer, but do not install until you get your tune down:
thunderproducts.com

UFO Ultimate Flow Optimizer - Thunder Products Carburetion Performance

UFO Ultimate Flow Optimizer for round slide VM Mikuni Carburetors. Gives you best throttle response and low end to mid range horsepower gain.
thunderproducts.com thunderproducts.com

I was so impressed with the install on a friends bike I picked up one for my dads single carb mikuni set up along with one of these:
thunderproducts.com

Quad Flow Torque Wing - Thunder Products Carburetion Performance

Quad Flow Torque Wing increase air flow, air speed and fuel atomization and performance. For carburetor or EFI on Motorcycles, ATV, Dirt Bikes
thunderproducts.com thunderproducts.com

I cannot believe how strong the bike pulls away from a stop light and how much improvement is made from these two products. I have hated mikunis for years as I have never had them transition off idle easily and have always had to set them up to idle at 1500 rpm + to bandaid this.

The big issue I see in your tune is that you do not know if your idle is good. To tune your idle, get a color tune spark plug, this will let you know where you are at each change you make. From there you can adjust your needles and needle jets.

We were running 20 idles, but are now running 15s with UFOs under the slides

We used a P0 Needle Jet and have a P2 that was close. We have been playing with 6F9, 6DH3, and 6DH2 needles. Settled on the 6DH2, the 6F9 works well on Triumphs, but was not quite right on the Norton.

220 main jet has not been touched.

Slide was a 2.5, but is now probably a 3.5 because of the UFO.

Air jet is a 2.0

Bike is an 850 with an SS cam and bit more compression, the carbs are brand new and aside from them the bike is a late 70s performance time capsule.
Click to expand...
I am also a fan of Thunder products. Like you, I discovered that the 6F9 works well for Triumphs but not so well for Nortons. Interesting that we both
came to the same conclusion. The TORQUE WING is their best product. Easy to install and requires NO rejetting. It is a noticeable improvement in acceleration especially when snapping open the throttle. $100 sounds expensive for a bit of tin but it really works. Anytime you can get a noticeable improvement for only $100, grab it.

As for the UFO....also quite an improvement but it requires fiddling. If are on the freeway or long straight roads where you can keep the rpms up it's great..definitely more punch just off idle. It's really fun to ride with the UFO in place. My problem is that I use my Atlas for a lot of in town riding and errand running. The UFO really increases the suction across the pilot jet and the needle jet so there is a need to reduce the jet sizes. I went from a 37.5 pilot to a 17 pilot and I needed to lean it further. Thunder products does not recommend going any smaller because just about anything will clog such a tiny pilot jet. The UFO is supplied installed in a 3.5 slide...the leanest. So the only other thing to work with is the needle jet reduction. My problem was that I was fouling plugs often and I picked up a little bit of vibration which I believe because it was too rich. TOO BE FAIR..my slow speed and many traffic lights was most of the problem. I had it on the freeway a few times and was very impressed with the performance. As I was removing the UFO and returning the carb to standard specs I knew I would miss the UFO but practicality and spark plug life won out.

CONCLUSIONS....everybody should buy the TORQUE WING. Easy to install, no rejetting, and it really works. No muss, no fuss. $100 well spent.
The UFO also works very well but requires jetting and fiddling. Not recommended for slow speed in town use because of plug fouling. But if you are racing I would install it absolutely positively. If you are using it mostly on freeways and can keep the RPMs and airflow up..yes,yes. But carry new spark plugs with you (don't forget!) and check the plugs fairly often untill you can get the jetting fully sorted.
By the way, I tried to get Thunder Products to make a torque wing for amals, even sending him an old body but it was a no-go.

Another product I HIGHLY recommend is the DENSO iridium spark plugs IW22 5307. A performance gain you can feel right away. Also it runs cleaner for longer. The best source is SIXITYAUTO around $8.50 apiece. get a 4 or 8 pack.
 
seattle##gs said:
I am also a fan of Thunder products. Like you, I discovered that the 6F9 works well for Triumphs but not so well for Nortons. Interesting that we both
came to the same conclusion. The TORQUE WING is their best product. Easy to install and requires NO rejetting. It is a noticeable improvement in acceleration especially when snapping open the throttle. $100 sounds expensive for a bit of tin but it really works. Anytime you can get a noticeable improvement for only $100, grab it.

As for the UFO....also quite an improvement but it requires fiddling. If are on the freeway or long straight roads where you can keep the rpms up it's great..definitely more punch just off idle. It's really fun to ride with the UFO in place. My problem is that I use my Atlas for a lot of in town riding and errand running. The UFO really increases the suction across the pilot jet and the needle jet so there is a need to reduce the jet sizes. I went from a 37.5 pilot to a 17 pilot and I needed to lean it further. Thunder products does not recommend going any smaller because just about anything will clog such a tiny pilot jet. The UFO is supplied installed in a 3.5 slide...the leanest. So the only other thing to work with is the needle jet reduction. My problem was that I was fouling plugs often and I picked up a little bit of vibration which I believe because it was too rich. TOO BE FAIR..my slow speed and many traffic lights was most of the problem. I had it on the freeway a few times and was very impressed with the performance. As I was removing the UFO and returning the carb to standard specs I knew I would miss the UFO but practicality and spark plug life won out.

CONCLUSIONS....everybody should buy the TORQUE WING. Easy to install, no rejetting, and it really works. No muss, no fuss. $100 well spent.
The UFO also works very well but requires jetting and fiddling. Not recommended for slow speed in town use because of plug fouling. But if you are racing I would install it absolutely positively. If you are using it mostly on freeways and can keep the RPMs and airflow up..yes,yes. But carry new spark plugs with you (don't forget!) and check the plugs fairly often untill you can get the jetting fully sorted.
By the way, I tried to get Thunder Products to make a torque wing for amals, even sending him an old body but it was a no-go.

Another product I HIGHLY recommend is the DENSO iridium spark plugs IW22 5307. A performance gain you can feel right away. Also it runs cleaner for longer. The best source is SIXITYAUTO around $8.50 apiece. get a 4 or 8 pack.
Click to expand...

I started at a 17.5 pilot jet and went to a 15, air screw is way out. Anything smaller than a 15 will get clogged unless you have access to ethanol free gas all the time and your tank is pristine on the inside. Thunder products recommended the 15, but told me not to go smaller. I sanded/filed matched the UFO from whatever slides we were using prior. Idle is tough to get dialed in, minimum idle is 1000 or 1200 with the UFO.

You might want to phone thunder products about your idle issues, the owner there really helped me dial it in.
 
They sell the UFO already installed in a 3.5 slide, that's the easiest. Idle was around 1200. The exhaust note changed...I believe because it was running rich. It would foul the plugs in about 3 weeks but I needed more reliability as this is a daily driver. The only way to make it run leaner is to reduce the needle jet. I was using a P-0, still am. By reducing it to a O-8 I could lean it a bit at the very bottom but what happens higher up when the UFO is not so much in play? Questions, questions.
I have talked to the owner of Thunder Products and he is always helpful.
 
seattle##gs said:
Another product I HIGHLY recommend is the DENSO iridium spark plugs IW22 5307. A performance gain you can feel right away. Also it runs cleaner for longer. The best source is SIXITYAUTO around $8.50 apiece. get a 4 or 8 pack.
Click to expand...

Hmmm, are you sure about the grade? I am using IW27s in my race bike and have done since 2015. I also use an IW27 in my SR500 single racer, and I have IW24 short reach equivalents in my Suzuki GT500 race bike. IW22 seems one or two grades out to me, even considering it's a road bike!

These grades are supported by the recommendations of others also using them. Including Kenny at NYC for the Norton.

But, yes, they are great plugs.
 
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My opinion comes from the crossover book at Advance Auto. I put them in and everything improved...easier starting, a slight performance gain that I could actually feel, and they burned cleaner and lasted longer than than the NGK BP7ES or the Champion N7. All good stuff. So I have not tried another heat range. However, if you have been using the IW27 with no ill effects, I'm willing to give them a try. NYC is a good endorsement, can you name other people? Please supply.
Changing brands of spark plugs is a minefield. The heat ranges are not exact duplicates of one another and many times one company will claim their plug is good for 4 different heat ranges of another company. Go to Iridiums and it throws in a few more variables. I know too hot a plug can put a hole in a piston so I'm cautious. I have a theory that the iridiums burn a little hotter than the BP7ES and that's why there was an improvement all around. Just a theory. A friend gave me a pair of E3.34 spark plugs and they were better than the IW22 but they burned bright white and I was worried that they might be too hot so I went back to the IW22.
Does the IW27 hold the heat longer so the plug stays cleaner in operation?
 
seattle##gs said:
They sell the UFO already installed in a 3.5 slide, that's the easiest. Idle was around 1200. The exhaust note changed...I believe because it was running rich. It would foul the plugs in about 3 weeks but I needed more reliability as this is a daily driver. The only way to make it run leaner is to reduce the needle jet. I was using a P-0, still am. By reducing it to a O-8 I could lean it a bit at the very bottom but what happens higher up when the UFO is not so much in play? Questions, questions.
I have talked to the owner of Thunder Products and he is always helpful.
Click to expand...
I would try the O range and see what it gets, you can always go back to the P0 From talking with Thunder products you should have a slightly lighter color plug than before. Try adjusting the needle position clip first, then compensative with air jets and main jets if you are still concerned.
 
seattle##gs said:
They sell the UFO already installed in a 3.5 slide, that's the easiest. Idle was around 1200. The exhaust note changed...I believe because it was running rich. It would foul the plugs in about 3 weeks but I needed more reliability as this is a daily driver. The only way to make it run leaner is to reduce the needle jet. I was using a P-0, still am. By reducing it to a O-8 I could lean it a bit at the very bottom but what happens higher up when the UFO is not so much in play? Questions, questions.
I have talked to the owner of Thunder Products and he is always helpful.
Click to expand...

I ended up drilling a hole in the hood of the P0 jet and cutting a bit of the top of the hood off. Made a huge difference. I may even have to remove all of it. Not sure you need to drop needle jets.
 
seattle##gs said:
My opinion comes from the crossover book at Advance Auto. I put them in and everything improved...easier starting, a slight performance gain that I could actually feel, and they burned cleaner and lasted longer than than the NGK BP7ES or the Champion N7. All good stuff. So I have not tried another heat range. However, if you have been using the IW27 with no ill effects, I'm willing to give them a try. NYC is a good endorsement, can you name other people? Please supply.
Changing brands of spark plugs is a minefield. The heat ranges are not exact duplicates of one another and many times one company will claim their plug is good for 4 different heat ranges of another company. Go to Iridiums and it throws in a few more variables. I know too hot a plug can put a hole in a piston so I'm cautious. I have a theory that the iridiums burn a little hotter than the BP7ES and that's why there was an improvement all around. Just a theory. A friend gave me a pair of E3.34 spark plugs and they were better than the IW22 but they burned bright white and I was worried that they might be too hot so I went back to the IW22.
Does the IW27 hold the heat longer so the plug stays cleaner in operation?
Click to expand...
More or less the opposite, as I would describe it! Bear in mind, mine is a race application, where a 'colder' plug may be preferred. I don't expect you to find the same is 'correct', certainly it will be safe. It worries me to run a plug that may be too 'hot'. In general terms 'hotter' plugs for softer tune and light use (pottering around at modest rpm). Go racing, or high speed road, go 'colder'. That has served me well enough for over 50 years!

IW27s do not foul up in my bike. And they last well. I am just a bit surprised by how much fouling and the short life you are getting, seems wrong! This is why I question the grade being suitable for your use. I don't think there is anything intrinsically wrong with the Denso IW plug range, or the equivalent NGK. Iridium plugs are intended to be 'for life' in commuter car applications! I tend to choose Denso over NGK because sadly there are more NGK knock offs out there, buying online can be a risk. I use NGK in my '56 MG and AJS, but from a known source. I use Densos in my 3 race bikes.

The IW22/BP7/N7 is a 'hotter' running plug than IW24/BP8/N6 which itself is 'hotter' than IW27/BP9/N4.

Way back when UK racers were using 9 range NGKs and when the first Gold Palladium plugs arrived, Champion gave me some N4G which worked well and replaced some Champion plugs I got from BRM F1 engine bench testing! N61Ys I think, and the old N57R race plug which I had used but were 'cold' and fouled regularly, such that we used to start up on a road plug in colder weather.

Your IW22 is two grades hotter, than I am using, I probably expected you would use just one grade hotter, IW24. Though in road use I accept BP7 or N7Y would be typical, I think the Iridium IW range tolerates using a colder plug. Which is why I find your comments puzzling!

1704979863082.png
 
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It is the difference between in town with lots of stoplights...also starting up with the choke. It could be the choke is 50% of the problem. If I ran it on the freeway or for long stretches of road the plugs would last a long time. I never make jetting decisions based on in town riding. The hotter IW22 gives me longer plug life for in - town. It also performs very well on the freeway. Quite often, if the plug is not too brown, it will clean up after 10 miles on the freeway.

Memo to all spark plug manufacturers.....please co-ordinate your numbers with each other so they all go in the same direction...the bigger the number the colder, the smaller the number the hotter.

I haven't tried drilling the hood or shortening it yet. Thunder Products mentioned doing that several years ago but later suggested just installing a smaller needle jet. I will try it this summer. However, most people have no desire or skill to spend days or weeks fiddling with jets so I cannot recommend the UFO to those unwilling to work with it.
 
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