Alternative Carbs - Z1000 MKII

[njwmct]

I have a 1979 Z1000 MKII with what I believe are carb issues - Mikuni VM28SS on standard tune motor with a Harris exhaust.
I'm considering upgrading the carbs to something from a more modern bike.
I can't afford £800 for something like Keihin CR carbs.
So what would your recommedations be for suitable modern carbs that could be picked up for sensible money?

[Al]

Copy of an e-mail i received just before christmas from Gremlinracetech.
Dont know if it helps at all.

Hi,

Thanks for your enquiry.

Mikuni RS34D 21-K flatslide carb set of four.
UK rrp £796.77. We can supply for £670.95. These are in stock at the moment.

Mikuni RS36D 3-K flatslide carb set of four.
UK rrp £849.64. We can supply for £715.95. These are not in stock and would have to be ordered into UK. We would be unlikely to be able to deliver to you until mid January.

UK mainland delivery would cost £9.95 including insurance.

If you decide to go ahead with this purchase, we can send you a secure email invoice payable in GB pounds sterling by personal cheque, any major debit or credit card or a PayPal account.

Regards
Sue

[njwmct]

Thanks for your help but unfortunately that's out of my budget.
I'm thinking of something suitable from a modern bike that I could perhaps pick-up from a breakers or e-bay etc.

Copy of an e-mail i received just before christmas from Gremlinracetech.
Dont know if it helps at all.

Hi,

Thanks for your enquiry.

Mikuni RS34D 21-K flatslide carb set of four.
UK rrp £796.77. We can supply for £670.95. These are in stock at the moment.

Mikuni RS36D 3-K flatslide carb set of four.
UK rrp £849.64. We can supply for £715.95. These are not in stock and would have to be ordered into UK. We would be unlikely to be able to deliver to you until mid January.

UK mainland delivery would cost £9.95 including insurance.

If you decide to go ahead with this purchase, we can send you a secure email invoice payable in GB pounds sterling by personal cheque, any major debit or credit card or a PayPal account.

Regards
Sue

[zed1015]

On your std motor you would be best spending a couple of quid on your existing carbs.
A good strip, ultrasonic clean and rebuild with say, a keyster kit etc would be well worth the while unless they are extremely worn.
Most modern carbs are unlikely to fit your airbox and are also CV's which don't like pod filters , Also other issues such as spacing , float chamber angle etc will be a problem.
You may end up spending a lot more trying to get them sorted and still end up with fuelling issues.

[njwmct]

On your std motor you would be best spending a couple of quid on your existing carbs.
A good strip, ultrasonic clean and rebuild with say, a keyster kit etc would be well worth the while unless they are extremely worn.
Most modern carbs are unlikely to fit your airbox and are also CV's which don't like pod filters , Also other issues such as spacing , float chamber angle etc will be a problem.
You may end up spending a lot more trying to get them sorted and still end up with fuelling issues.

Yes - I know where you're coming from.
The Bike has only done 4900 miles & there is no appreciable wear in the carbs.
They have already been completely stripped (several times), ultrasonically cleaned & rebuilt with new jets / gaskets, Float heights set & accurately balanced, + a colourtune used to set the mixture.

[LTD Project]

Perhaps it's not carb issues...


What the the symptoms?

Could it be an air leak between head and carb holder rubber, etc

[njwmct]

Perhaps it's not carb issues...


What the the symptoms?

Could it be an air leak between head and carb holder rubber, etc

It's a stutter/stumble just off idle at approx 1250 rpm - tried many things to cure it but never had any luck.

The inlet rubbers are well sealed at the head & I've tested for leaks with WD 40 while it's running - all appear to be leak free.

[Pigford]

Stumble just off idle is a v. common thing with older Zed carbs...... The std carbs aren't very versitile so trying to tune this problem out can be virtually impossible.

It's often due to lean mix - may need bigger pilot jets or even different slide profile

If really serious about running performance get it DYNO'd - no other real way

Then get some RS34 Mikuni's

[zed1015]

Are you running open carbs or K and N type filters.
If so you need air corrector jets fitting, These will rectify the lean condition just off idle which is the usual cause of your symptoms.
Ledar used to sell them years ago but its easy enough to make some if you are handy.
You will need some 5mm threaded brass rod and some tiny drill bits.

[chrisu]

sounds interesting - can you provide a few more details of whats needed ?

[uk kev]

This whole problem can just as easily be related to a knackered (technical term) or blocked fuel tap, they need a good flow of fuel to run correctly, float hights are important, as is the correct jetting for pod filters and a 4/1 exhaust.

[zed1015]

sounds interesting - can you provide a few more details of whats needed ?

They go in the air bleed for the pilot circuit (in the mouth of the carb).
Ledar jets were 0.6mm bore if i remember right.
I make 'em out of 5mm threaded brass bar and cut a slot in the end so they can be screwed in (the carb needs to be tapped to take them)
The hardest part is drilling the brass without snapping the tiny drill bits.

Without getting too technical.
Fitting pods or bellmouths etc weakens the incoming charge (More air)
Bigger mains etc will sort out 3/4 to full throttle but not lower down, That hiccup just off idle is a weak spot (lack of fuel)
Different needle profiles would help bridge the off idle weakness (which is what dynojet kits now include for newer motors) as would raising the needle to some degree but the simpler and cheaper option on these old mikunis is to restrict the pilot air which results in richening up the idle circuit.

Ideally you want the largest size jet bore whilst still eliminating the off idle hiccup.

Hours of drill snapping fun can be had making different size jets for the best setup.

[njwmct]

Stumble just off idle is a v. common thing with older Zed carbs...... The std carbs aren't very versitile so trying to tune this problem out can be virtually impossible.

It's often due to lean mix - may need bigger pilot jets or even different slide profile

If really serious about running performance get it DYNO'd - no other real way

Then get some RS34 Mikuni's

If the cause could be a lean mix I might try going up on the pilot jets - currently stds (15) as using standard airbox & filter.
Would you go to 17.5 or 20 ?

[Garn 1]

They go in the air bleed for the pilot circuit (in the mouth of the carb)......
........cheaper option on these old mikunis is to restrict the pilot air which results in richening up the idle circuit
Ideally you want the largest size jet bore whilst still eliminating the off idle hiccup.

Zed1015,
!. I'm interested in how you get the existing air bleed jet out. Could you just drill it in-situ to the correct size?
2. Further, on the Z1 series carb Vm28's, wouldn't this be achieved by altering the air screws from, say, 1 & 1/2 turns to 2 turns? I do know the adjusting screws on the MKII probably don't have an air screw, per se, but rather a fuel adjusting screw!
RegardZ.

[zed1015]

Oop's! my typo, Sorry. Shouldn't reply to posts when i've had a drink ,did make sense when i was typing it though .
The jets go in the air bleed to the needle jet/emulsion tube NOT the pilot bleed which is metered by the air screw.
Looking at the carb mouths (26 and 28 mikunis) they are positioned at 4 o,clock on the left hand pair ( 1 and 2 ) and 8 o,clock on the right hand pair (3 and 4).
There are no existing jets to drill out, which would be pointless any how as the new jets reduce the existing air supply.
The needle jet/emulsion tube comes into play just off idle at around 1/8 throttle , it is this point ( just off idle ) that is richened up to smooth the transition.
Also as a result it may also be possible to go down a size or two on the main jet.
Hope that makes things clearer.