CV carbs slow to pick up in high gears?

[motopsycho87]

Morning all! I've got a V12 MK2 with full system and ramair pods with the velocity stacks in.

 

Hopefully the bike is modified enough it's allowed on here 

 

I've currently got 27.5 pilot, 1.5 turns, dynojet stage 1 needles at 3rd from bottom, dynojet stage 1 slide springs and 155 mains. The bike runs amazingly when going through the gears, the issue I'm having is that when I'm at medium revs and roll on the throttle the pick up is very weak and slow until I've twisted the the throttle fully. But when running through gears the bike runs lovely and powerful. It almost feels like the slide is refusing to open or maybe at lower positions it is rich? I had the needles at bottom clip before this and the I had the same problem off idle as well.

 

Any suggestions? My next thought is to drop the needles another clip...

[Lachie04]

Have a read through this I have used it successfully and to good effect 

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

 

 

[MeanBean49]

Stop guessing and get it setup on a Dyno.

[wraith]

A stage 1 kit is more for bikes with standard air box etc you need stage 3 kit

[Gixer1460]

As he ^^^ says - its the quickest way! But the spec you describe would usually require DJ stage 3 kit - pipe and pods! And 155 mains are mahoosive for (what I assume) is a Bindit? Going large on the mains tends to indicate poor airflow not producing sufficient depression over the jet to pull fuel through. But dyno is best way to avoid guessing!

EDIT - wraith got inbetween just before me LOL

Edited February 17, 2021 by Gixer1460

[MeanBean49]

37 minutes ago, wraith said:

A stage 1 kit is more for bikes with standard air box etc you need stage 3 kit

Only difference in "stages" is the size of the mains supplied in the kit iirc. Stage 3 kit is exactly the same as a stage 1 it just has some bigger jets in as well.

The stage thing is just a tag to make boy racer types think they have somthing special to shout about. Its not really a thing. Particulalrly when most of the time you get a bike setup on a dyno you find you need a jet size not included in the kit but one in between, what "stage" have you got then?

I just see the whole as setup and jetted to suit your spec/components, there are no stages 

 

[motopsycho87]

From what I've done so far 

 

Rich - no power but smooth

Lean - cuts out, pops and bangs

So this is making me think leaner on the needle

 

With regards to the main jet, the range with pods seems to be 150-160 so I just went in the middle for now and it's a great improvement on standard

 

 

 

[MeanBean49]

11 minutes ago, motopsycho87 said:

From what I've done so far 

 

Rich - no power but smooth

Lean - cuts out, pops and bangs

So this is making me think leaner on the needle

 

With regards to the main jet, the range with pods seems to be 150-160 so I just went in the middle for now and it's a great improvement on standard

 

 

 

Normally find with a b12 on pods that work (k+n or ramair) the kit supplied jets are too big. Ive always found 140/145 to be where you end up, maybe smaller.

Needles its not really worth bothering with until main jet is right.

Which if you arent going to use dyno takes some WOT in higher gears at high rpm, followed by immediately killing motor and stopping so you can check your plug colour.

Give yourself about 97 days faffing and chopping and changing and you should get close enough to start thinking about mid range and needles 

[motopsycho87]

10 minutes ago, MeanBean49 said:

Normally find with a b12 on pods that work (k+n or ramair) the kit supplied jets are too big. Ive always found 140/145 to be where you end up, maybe smaller.

Needles its not really worth bothering with until main jet is right.

Which if you arent going to use dyno takes some WOT in higher gears at high rpm, followed by immediately killing motor and stopping so you can check your plug colour.

Give yourself about 97 days faffing and chopping and changing and you should get close enough to start thinking about mid range and needles 

The plan is to get it nicely close then take it for a dyno run. I've got some 150 mains also to try. And I'm getting carbs out, rejetted and back within an hour 

[MeanBean49]

4 minutes ago, motopsycho87 said:

The plan is to get it nicely close then take it for a dyno run. I've got some 150 mains also to try. And I'm getting carbs out, rejetted and back within an hour 

Its not just that though is it, its all the yest riding, being able to stop the instant youve done your high speed high rpm wot run and pulled your plug at side of the road.

And in all honesty it will save virtually no time on the dyno whether its a mile out or pretty close. Good dyno operator will know pretty much what jets are needed based on whats fitted and what the afr shows.

This is just my advice based on my experience, would rather pay for an extra hour on the dyno than needlesly waste hours and hours of my own time

[wraith]

3 hours ago, MeanBean49 said:

Only difference in "stages" is the size of the mains supplied in the kit iirc. Stage 3 kit is exactly the same as a stage 1 it just has some bigger jets in as well.

The stage thing is just a tag to make boy racer types think they have somthing special to shout about. Its not really a thing. Particulalrly when most of the time you get a bike setup on a dyno you find you need a jet size not included in the kit but one in between, what "stage" have you got then?

I just see the whole as setup and jetted to suit your spec/components, there are no stages 

 

My mistake,

I thought the stage 3 kits you had to drill the slides and put a plastic jet in them, and the stage 1 you didn't.

Putting on a Dyno is the best way some would say some not. 

[MeanBean49]

5 minutes ago, wraith said:

My mistake,

I thought the stage 3 kits you had to drill the slides and put a plastic jet in them, and the stage 1 you didn't.

Putting on a Dyno is the best way some would say some not. 

It all depends on what bike the kit is for to what stuff is in the kit.

Always found drillimg slides, adding jets, or sticking bits in the air correctors ultimately does do much other than mean more faffimg trying to get it to run as well as using standard needles and shimmimg/adjuating them to suit.

Ive always managed to get the same results using stock stuff and a £10 set of correctly sized main jets. Sometimes some bigger pilots.

[wraith]

That's what I like to hear, someone still doing it the old fashioned way. Good on you 

@MeanBean49

[motopsycho87]

Done 4 changes today

Raised 2 clips, way better but now it has a lean spot that nearly clears when warm

Tried original needles on bottom clips, too rich at idle

Tried original needles at 3rd clip, too lean in the mid

So now I've left the dynojet needles in as they were, but out the original springs back in in the hope that maybe the carb was lifting too high too soon and not able to pull enough fuel during roll on, is this thought process correct? Work in the morning so no test ride until tomorrow evening

Also, visor flew off my helmet :'(

If that doesn't work then the next step will be to drop 1 clip and see if that's the happy medium

 

 

Edited February 17, 2021 by motopsycho87

[MeanBean49]

11 hours ago, motopsycho87 said:

Done 4 changes today

Raised 2 clips, way better but now it has a lean spot that nearly clears when warm

Tried original needles on bottom clips, too rich at idle

Tried original needles at 3rd clip, too lean in the mid

So now I've left the dynojet needles in as they were, but out the original springs back in in the hope that maybe the carb was lifting too high too soon and not able to pull enough fuel during roll on, is this thought process correct? Work in the morning so no test ride until tomorrow evening

Also, visor flew off my helmet :'(

If that doesn't work then the next step will be to drop 1 clip and see if that's the happy medium

 

 

If your needles are adjustable they arent stock, b12 carbs have fixed needles as standard.

And whether your thought proces is correct is hard to say without knowing how your riding.

On WOT above about 4-6k rpm (depending on gear/load) needles and slides will have little effect on fueling as the slide will be fully open. Its purely down to main jet size.

The slides have more of an effect on low rpm range, smaller throttle openings and keeping the fueling correct at constant throttle

 

[motopsycho87]

58 minutes ago, MeanBean49 said:

If your needles are adjustable they arent stock, b12 carbs have fixed needles as standard.

And whether your thought proces is correct is hard to say without knowing how your riding.

On WOT above about 4-6k rpm (depending on gear/load) needles and slides will have little effect on fueling as the slide will be fully open. Its purely down to main jet size.

The slides have more of an effect on low rpm range, smaller throttle openings and keeping the fueling correct at constant throttle

 

The main issue is transitioning through the mid range slowly when in a high gear, there is a strong hesitation that is only overcome when opening the throttle quickly. Not noticeable in lower gears, bottom and and top end run perfectly

[motopsycho87]

11 hours ago, MeanBean49 said:

 

Took it out again, same on the flat spot but definitely a bit richer off idle. Going to do the needle another clip leaner and see if it's better or worse. Or should i maybe try doing to a 150 main? Annoying thing is again it only happens when rolling on in a high gear just after cruising, otherwise it goes like hell... Any ideas?

[MeanBean49]

11 hours ago, motopsycho87 said:

Took it out again, same on the flat spot but definitely a bit richer off idle. Going to do the needle another clip leaner and see if it's better or worse. Or should i maybe try doing to a 150 main? Annoying thing is again it only happens when rolling on in a high gear just after cruising, otherwise it goes like hell... Any ideas?

Idle shouldnt have changed unless you have messed with the mixture screw.

I would just stick it on a Dyno. Even just paying for a power run with afr on the printout will show you exactly what its doing so you know which way you need to go

[Ted M]

If I am reading this right you have a mix and match setup of Dynojet parts. The B12 Mk 2 carbs are different to the Mk1 carbs. 155 main jet is a Dynojet part for the Mk1 B12. They are too big for the Mk2 carbs. The mixture screw is also wrong for those carbs also it should be 2.5 turns out you also need to adjust the float hieght to 11 - 11.5mm. Google Dynojet B12 Instructions and you will get the details you need to set the carbs up.

[MeanBean49]

37 minutes ago, Ted M said:

If I am reading this right you have a mix and match setup of Dynojet parts. The B12 Mk 2 carbs are different to the Mk1 carbs. 155 main jet is a Dynojet part for the Mk1 B12. They are too big for the Mk2 carbs. The mixture screw is also wrong for those carbs also it should be 2.5 turns out you also need to adjust the float hieght to 11 - 11.5mm. Google Dynojet B12 Instructions and you will get the details you need to set the carbs up.

They are still only ballpark instructions. Following them still doesnt mean it will run right. 

[Ted M]

6 minutes ago, MeanBean49 said:

They are still only ballpark instructions. Following them still doesnt mean it will run right. 

I agree totally mate but iI was just pointing out that it seems like the jet sizes that OP has installed are for the wrong carbs

[bluedog59]

By the sound of it, you have a right mismatch of parts and could just end up going round in circles. Have you checked the emulsion tubes aren't worn ? B12's are renowned for it.

My advice is the same as others, get it on a Dyno. Find a Dyno shop you can trust, try to book an "off peak" time and ask if you can be in the Dyno room while they're running it. Stand out the way ( never go behind a running bike ) watch, listen and ask the operator to explain what the various readouts are showing and you will ride out more knowledgeable and wondering why you didn't do it earlier.