making CV carbs more responsive ?

jerry · 19-04-2018, 08:28 PM

has anyone found a way to make Mikuni CV carbs more responsive ??

i know HD tuners drill a bigger airslide hole to speed up the carb ,, my 750ss is always slow to return to idle even with well lubed oled external springs and cables and throttle body

Flip · 19-04-2018, 11:41 PM

Yes Jerry, my 900’s Dynojet kit came with a drill bit to open up the hole in the slide along with lighter springs to speed things up.

I’m not sure what the effects of doing either would be without altering the jetting as well though- it might make the motor bog down when you gave it a big throttle opening.

slob · 20-04-2018, 06:15 AM

I had a mate who had an FZR thou' in the early '90s, it was a regular contender at 'run what you brung' drag events.
He had to fit much stronger springs in his flatslide carbs since with a forward sloping motor, inertia was keeping the slides open under heavy braking regardless of shutting the twist grip. Not such a problem on the drag strip but pretty scary on the road.

Nasher · 20-04-2018, 07:44 AM

The lighter springs that come with the Dynojet kits are to allow the slide to open more quickly and quicken response when opening the throttle, and could cause the issue Jerry has.
If he wants the bike to go back to idle quicker a stronger spring is in theory required.

I’ve just rebuilt a set of carbs, fitting a Dynojet kit at the same time, and considered putting the standard springs back in with the Dynojet needles and jets, as even on the bench the slides were so much slower to drop back down. I stripped them again, cleaned them again, installed new diaphragms, and finally got them to close a bit quicker, but still not as quick as with the standard springs.

My kit didn't come with a drill to open up the holes, and it was not mentioned in the instructions.
Flip, do you know what size drill it was?

Nasher

utopia · 20-04-2018, 10:57 AM

This may be of some "tangential" interest.
I've recently been messing with the Keihin, 42.5mm CV carb on my Honda Dominator.
There is a widely accepted procedure for these motors when running a free-flowing filter and exhaust, known as "Dave's carb mods".
In addition to increasing jet sizes and raising the needle, it is recommended that the two holes in the bottom of the slide are drilled out to 5/32 inch (I think they're 1/8 inch as std, but not sure).
There is no mention of a softer spring.

For now though, I've just changed jets and needle positions but left the slide undrilled, so I can't comment personally .... yet.

350TSS · 20-04-2018, 08:26 PM

Presumably that larger hole allows more air underneath the diaphragm which enables the slide to shut. It is quite surprising it does shut given that there would be negative pressure there from the intake stroke.

19-04-2018, 08:28 PM	#1
jerry Old Git Join Date: May 2006 Location: Cricklade Bike: Multiple Monsters Posts: 2,803	making CV carbs more responsive ? has anyone found a way to make Mikuni CV carbs more responsive ?? i know HD tuners drill a bigger airslide hole to speed up the carb ,, my 750ss is always slow to return to idle even with well lubed oled external springs and cables and throttle body __________________ MONSTERMAN

19-04-2018, 11:41 PM	#2
Flip Registered User Join Date: Mar 2010 Location: Beachtown Bike: M900 Posts: 2,188	Yes Jerry, my 900’s Dynojet kit came with a drill bit to open up the hole in the slide along with lighter springs to speed things up. I’m not sure what the effects of doing either would be without altering the jetting as well though- it might make the motor bog down when you gave it a big throttle opening. __________________ You're perfect, yes, it's true- But without me you're only you!

20-04-2018, 06:15 AM	#3
slob . Join Date: Jul 2003 Location: East London Bike: Multiple Monsters Posts: 9,707	lighter springs I had a mate who had an FZR thou' in the early '90s, it was a regular contender at 'run what you brung' drag events. He had to fit much stronger springs in his flatslide carbs since with a forward sloping motor, inertia was keeping the slides open under heavy braking regardless of shutting the twist grip. Not such a problem on the drag strip but pretty scary on the road.

20-04-2018, 07:44 AM	#4
Nasher Registered User Join Date: Jul 2014 Location: Portsmouth Bike: M900 Posts: 1,510	The lighter springs that come with the Dynojet kits are to allow the slide to open more quickly and quicken response when opening the throttle, and could cause the issue Jerry has. If he wants the bike to go back to idle quicker a stronger spring is in theory required. I’ve just rebuilt a set of carbs, fitting a Dynojet kit at the same time, and considered putting the standard springs back in with the Dynojet needles and jets, as even on the bench the slides were so much slower to drop back down. I stripped them again, cleaned them again, installed new diaphragms, and finally got them to close a bit quicker, but still not as quick as with the standard springs. My kit didn't come with a drill to open up the holes, and it was not mentioned in the instructions. Flip, do you know what size drill it was? Nasher __________________ Heaven doesn't want me, and Hell is afraid I'll take over.

20-04-2018, 10:57 AM	#5
utopia No turn left unstoned Join Date: Jun 2010 Location: leicester Bike: M750 Posts: 4,545	This may be of some "tangential" interest. I've recently been messing with the Keihin, 42.5mm CV carb on my Honda Dominator. There is a widely accepted procedure for these motors when running a free-flowing filter and exhaust, known as "Dave's carb mods". In addition to increasing jet sizes and raising the needle, it is recommended that the two holes in the bottom of the slide are drilled out to 5/32 inch (I think they're 1/8 inch as std, but not sure). There is no mention of a softer spring. For now though, I've just changed jets and needle positions but left the slide undrilled, so I can't comment personally .... yet.

20-04-2018, 08:26 PM	#6
350TSS Too much time on my hands member Join Date: Jan 2013 Location: Shipbourne Bike: M900 Posts: 1,419	Presumably that larger hole allows more air underneath the diaphragm which enables the slide to shut. It is quite surprising it does shut given that there would be negative pressure there from the intake stroke.