Topic: New build - Mikuni V29 smoothbore - and other help

[macvit7906]

After more riding, I'm confident the carbs are tuned correctly.

idles great, takes right off of idle, almost no sputtering up to the 5000 RPM range.

No I am having an issue a believe is related to the ignition. I'm running a Dyna S ignition, a new reg/rec from Rick's specific for a lithium ion battery.

The bike loses power at the 5000 RPM mark and it will not rev past 6000 RPM!

Any ideas?

[PeWe]

Really sure about main 115?
Full throttle on a remote road, kill engine and check the plugs. Should be enough to check 1,4.

Remove 2,3 on a hot engine means burnt hands, wrists.

I used over 120 (122.5-125) with pods on my VM29's. (Rather open 4-1)
115 should definitely have been too lean on my bike with similar engine setup.
If replacing KN pods with less restrictive stacks on that build back then, I should start at 130.

If this is caused by way too lean main jets, there is a risk to blow hole in the pistons.

My forged RC 836 got hole with richer VM29's than yours back in 1984.
Ignition setting on 2-3 involved too. It had no problem to pass 200kmh on speedo with 3.25/3.50 front tire. (100/90 rotates slower when bigger, lower speedo rpm)

- Exhaust not too restrictive?

First time I used my setup on a holiday touring to former Jugoslavia I used a silent baffle in my 4-1 that added restrictions . Bike did not run faster than 130-140 kmh on Autobahn. I had to pull the baffle and ride with a very load completely open system, a Yoshi look a like.

[macvit7906]

Really sure about main 115?
Full throttle on a remote road, kill engine and check the plugs. Should be enough to check 1,4.

Remove 2,3 on a hot engine means burnt hands, wrists.

I used over 120 (122.5-125) with pods on my VM29's. (Rather open 4-1)
115 should definitely have been too lean on my bike with similar engine setup.
If replacing KN pods with less restrictive stacks on that build back then, I should start at 130.

If this is caused by way too lean main jets, there is a risk to blow hole in the pistons.

My forged RC 836 got hole with richer VM29's than yours back in 1984.
Ignition setting on 2-3 involved too. It had no problem to pass 200kmh on speedo with 3.25/3.50 front tire. (100/90 rotates slower when bigger, lower speedo rpm)

- Exhaust not too restrictive?

First time I used my setup on a holiday touring to former Jugoslavia I used a silent baffle in my 4-1 that added restrictions . Bike did not run faster than 130-140 kmh on Autobahn. I had to pull the baffle and ride with a very load completely open system, a Yoshi look a like.

My main is 120 right now. MAybe move up to 122.5 or 125?

The reason I think it is ignition related is because power is lost around the 5500 RPM range in any gear and is very hard to get RPMs up to 6000 and beyond in any gear

[PeWe]

The exhaust is loud?
If not, remove baffle if possible and see if it can breath better.

[macvit7906]

The exhaust is loud?
If not, remove baffle if possible and see if it can breath better.

Apologies. Forgot to mention the exhaust is an SC-project race muffler. It is wide open

[macvit7906]

Im back!

installed and tested 122.5 main jets. Still the same issue, power loss at 5k + RPMS and will not go above 5500 RPM

Could it be timing? to far advanced?

[y2kc0wb0y]

Will it go past 5k in neutral?


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[macvit7906]

Will it go past 5k in neutral?


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It'll maybe get close to 6k in neutral but hard to go further.

When under load (driving) past the 5k mark, power is lost and the bike struggles to pull. up until that point, power is great and it pulls hard

[y2kc0wb0y]

I believe you’re fuel starved.


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[macvit7906]

I believe you’re fuel starved.


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I'll step it up to 125 main and report back!

[y2kc0wb0y]

Are you confident that the floats are set correctly? I'm asking because you saw no change even with moving the mains up. I'm going to guess that you're going to see the same with 125's. I think you're simply sucking the bowls dry.   

[PeWe]

My VM29 setup revved a lot with 120, 122.5 mains.
Ignition checked? Full advance not before 2500 rpm, not too late either. 2600-2700 on my bike.

The only situation where bike could not rev much was with too restricted 4-1. Very silent baffle installed.

The only problem with carb setup was uneven behavior under 4000 rpm. Throttle lift that give 2000-3500 rpm on 5th gear.
My carbs need the needle jets correct together with needle heights.  I think O-8 and middle.
These afffect eachother. Smaller needle jet might need richer needles (clip lower) and opposite depending on current situation.

If remember correctly, I found pilot 17.5 and main 122.5 to be OK. Air jet 0.9.
This with KN oval pods and loud 4-1

[macvit7906]

Are you confident that the floats are set correctly? I'm asking because you saw no change even with moving the mains up. I'm going to guess that you're going to see the same with 125's. I think you're simply sucking the bowls dry.

Thats a good thought but i did the clear tube test and the fuel height is maybe 2-3mm below the gasket surface. its possible i will recheck

[macvit7906]

My VM29 setup revved a lot with 120, 122.5 mains.
Ignition checked? Full advance not before 2500 rpm, not too late either. 2600-2700 on my bike.

The only situation where bike could not rev much was with too restricted 4-1. Very silent baffle installed.

The only problem with carb setup was uneven behavior under 4000 rpm. Throttle lift that give 2000-3500 rpm on 5th gear.
My carbs need the needle jets correct together with needle heights.  I think O-8 and middle.
These afffect eachother. Smaller needle jet might need richer needles (clip lower) and opposite depending on current situation.

If remember correctly, I found pilot 17.5 and main 122.5 to be OK. Air jet 0.9.
This with KN oval pods and loud 4-1

I have to get a strobe light to properly check ignition. I'm using dyna S electronic ignition.

I'm using O-6 needle jet with middle clip. Maybe go a clip lower on the needle?

I'm thinking because i have velocity stacks, I might need a larger main than your set up.

Do you remember your float height?

[PeWe]

Floats checked with clear tube and adjusted since levels were too high sooting plugs.
Around 2mm under the float bowl gasket. 1-4mm recommended.

[macvit7906]

Floats checked with clear tube and adjusted since levels were too high sooting plugs.
Around 2mm under the float bowl gasket. 1-4mm recommended.

since im using a O-6 needle jet, i feel like im leaning out at higher RPMS.

Would you recommend lower the needle clip one notch to richen up?

[PeWe]

Needle or needle jet should not affect the max rpm/ high lift. These jets from throttle lift just a little after idle. Acceleration to 6000 rpm, yes.
My VM29 had also O-6 in the 80's. Top speed on speedo 210kmh after a while, quick to 180, soon after 200, squeezing it further to 210kmh. No high revving issues.
That must be main jets and/or ignition, too restricted exhaust.  A modified bike can run worse than stock with restricted stock pipes. I do not know if a way too advanced cam can act like that.

Ignition can be seen on plugs.
A colored band of a fem mm on the ground strap bend when correct.  If wrong, more to the tip or the other way closer to plugs threads. There are lots of info about this on Internet

I understood later on that the need of more pilot jet could be the result of the needle jet.
Air screws, needle jet and needle heights affect each other to get good even running engine up to 4000 rpm.
After that easy to get with needle heights and main jets only. But the lower area is very important. So irritating with too rich idle sooting plugs at red light or hesitations at take offs and cruising in low speed. It can be too rich and lean at the same time, but different lifts.

I have done a lot of jet changes on my Mikuni TMR 32.
They had a too rich area at low lift (around 3000 rpm when riding) not possible to solve without causing other isssues like leaner needles. Reduced needle jets and played around with different needle heights.

A dyno with air fuel check a must to see where too lean and rich. Difficult to feel difference. I had to increase pilots several steps to match the correct needle jets.
Pilot affect needle jet and needle area.
Pilot 25, 27.5 worked fine with needles at 4th which became too rich with pilot 30. 5000-6000 rpm area on 5:th gear.
Now pilot 35 so fuel screws (causing too rich at idle) could be reduced to 3/4 turn, needles under 3rd. 2 + washer making 2 3/4. I'll see if thinner washer is needed.
This setup had to push  fuel lower of the throttle area. I did not need to change air jets. There are 2 of them in TMR 32.

Trust me, you will save time and money when taking bike to a shop with dyno and air/fuel check. Measure idle, slowly increase to fully open throttles and see where too lean and too rich.

You might get everything perfect except a narrow throttle lift area at low lift forcing you to change needle jets affecting the other jets too. More dynos.....

Velocity stacks for racing only. I'm sure racers do not care about the area around 3000 rpm and lower. The trickiest area to get right.
Try KN pods to protect engine from grinding sand. Carbs too.

[macvit7906]

Quick update

Here is where I am at with tuning as of now. Build specs for reference incase anyone is doing similar:

836 wiseco kit
stage 5 ported head
wide open race exhaust

Current Carb specs:
mikuni VM29mm smoothbores
Pilot jet = 17.5
Main = 122.5
Airjet = 0.9
Air screw is 3/4 turns out
O-6 needle jet
Needle in middle clip position

I switched out the velocity stacks with pod filters!
What a difference!

The bike runs much much better, no power loss at the 5k RPM range but starting to sputter at the 6k RPM range. Overall runs much smoother and cooler.

That makes me think i am leaning out in higher RPMs, putting the air restriction on noticeably helped. I think I'll bring the air mixture screw in a little bit and maybe even consider 125 mains at this point

[PeWe]

"starting to sputter at the 6k RPM range"
Is that too rich or lean?

If to lean, a little bit choke might help in that area. If going worse too rich.

You can also slowly enter the 6000rpm area, if sputter during slightly more throttle (lean) or only when slowly roll off (rich)?
Both definitely rich.

A little bit too rich is welcomed during acceleration which is revelead when slowly roll off.

If going worse at >7000 rpm main jet to change.

This is based on my many rides finding the jets on my carbs last year.

Correct mixture better for engine and more power

[MRieck]

 I remember Brent Waller have weird jetting problems with the VM-29's. He eventually traced it to the floats and after replacing them the race bike ran great.