OEM Carb Mods While You Are Saving For FCR Flatslides.

[koko64]

While you are saving up the big $$$ for FCR flatslide carbs, there are mods to deal with some problems associated with the oem Mikuni CV carbs and improve their performance. For those that like to tinker, tune and refine things, you may find this useful.
I will outline a few tricks the Harley guys have shown me over the years. Since for awhile Harleys had a CV carb (40mm Keihin), it was reasonably affordable for performance shops to do extensive mods to improve performance. Since us guys have two carbs to modify and a jet kit to buy, its probably cost prohibitive to go as far as the Harley guys. After all you need to save for those FCRs .
There are some mods you can do yourself however that require few bucks and some careful time.
I'll go through all the mods and then the cheap DIY mods. I'll have a mate help me with pics in my next post.
Cheers.

[koko64]

Besides a jet kit the modifications that I know of, heard of, read about, attempted or have seen are as follows. You may have more info and if you do please share.

Unfortunately the BDST 38 CV carbs were built down to a price.

1. Boring the carbs and fitting a larger throttle butterfly. Basically making a bigger carb.
2. Radiusing or rounding the butterfly shaft if it is angular.
3. Knife edging or radiusing the edges of the butterfly itself.
4. Taper boring the carb.
5. Reorganizing the main fuel circuit to be more "linear". It looks like ducatigirl 100 did this.  Hey 100, you are welcome to share how this was done.
6. Radiusing the slide leading edge.
7. Radiusing/porting the transition ridge from the bellmouth casting to the bored throat area in front of the slide.
8. Polishing the bellmouth area.

Some of these mods require special equipment and would be costly to pay someone to do, or if you stuff them up DIY, destroy your carb.
I will focus on the last three modifications you can do at home. I will address these with relevant pics tomorrow.
Cheers.

[MotoPsycho]

But...What about riding to work everyday instead of driving ? 50mpg vs 18mpg will save me money faster for FCR's than anything.

I keeding you  I thought about taking a dremel to the throats of my carbs last time I had them apart. Smoothing out the casting ridges might help but by how much ? Is it worth it with a 750 ?

[ducatiz]

But...What about riding to work everyday instead of driving ? 50mpg vs 18mpg will save me money faster for FCR's than anything.

I keeding you  I thought about taking a dremel to the throats of my carbs last time I had them apart. Smoothing out the casting ridges might help but by how much ? Is it worth it with a 750 ?

in conjunction with other performance mods, it can help some.  there is a limit to what you can do with a street tuned 750 Ltwin though.

Think of polishing as lubricating the path that the air or air/fuel mixture takes.  It allows it to flow faster which can contribute to smoother running and some hp gains.

[koko64]

Most wouldn't bother, but if you're down on $$ but handy and like to tinker, then you may be interested. Its like Tiz said, incremental improvements.
I put my money into hi comp pistons as a priority on my old M900, so the FCRs had to wait. Years later, my wife bought me my FCRs, probably out of pity.  I think the FCRs tie together the whole performance package when you have had your motor worked. Or another way of putting it, the oem carbs just hold it back.
The labour intensive nature of carb work makes it cost effective to just save up for the FCRs, unless you can do it yourself without wrecking them. I'm thinking put the gas money towards the FCRs.

[Howie]

Some time ago Doug Lofgren  was doing some creative stuff with  BDST 38s, taper bore and other good stuff.  Rumor was they worked well plus you had chokes.

[koko64]

I have always been impressed with what he did with the CV carbs. I was going to send him a set to modify! He moved onto EFI.

[koko64]

The pics will show a job 90% done on a set of CV carbs.
#I have radiused the bellmouth to carb throat transition and removed the hard angle there. This helps improve flow by reducing turbulence. You have effectively ported the area. The cast angle of the bellmouth meets the straight bored angle of the throat with the edge restricting air flow and causing turbulance.
#As Tiz said the polishing of the bellmouth reduces friction and increases air speed. Zipper Performance and other tuning houses highly polish the carb parts that meet air, as distinct from air/fuel. S&S do this and Mikuni do this on their high end carbs. You pay extra for Mikuni Pro Series carbs that are hand finished.
The dremel is your friend. Use fine sand mops as you would for porting or equivalent tools. I finished with finer and finer polishing using very fine paint finishing material on a foam backing and then metal polish. Overdo it and you'll be calling SUDCO. You've been warned.
# With great care radius only the leading edge of the vacuum slides. Again to promote flow and reduce turbulence. Metal slides give more room for error. These Mikuni slides are plastic, so go slow and careful. The slide leading edge is a hard square angle and tuners deal with this. Dynojet are now selling nicely finished carb slides in their kits for Harley carbs. Again overdo this and you'll be calling Powerbarn or SUDCO. The straight edge is gone and some say the straight edge contributes to slide flutter which gets worse with soft springs. Lofgren uses stock springs.

You'll need to give the carbs a bloody good clean and blow air through them.

[koko64]

See below more like how they should come from the factory compared to stockers which show the bored ridge, sharp edged slide and unfinished bellmouth casting.

[ducpainter]

Any before pics?

[koko64]

As you can see, still more polishing to go. I put them together for pics and to not lose anything. I will do some more polishing of the bellmouths to promote air velocity.

The pics of the slides show the radiused leading edge. Despite the discolouration they are smooth as a baby's bum. I did not touch the actual cutaway size itself,  just the bottom leading edge. Dont be fooled by the phone pics, each cutaway is symmetrical.

They need a little more work but there is a gentle radius to the throat from the bellmouth with the angled edge eliminated. It gives a nice taper which will help airflow.

All this should help reduce turbulence and improve airflow in volume and velocity.

[ducpainter]

Sorry about no before pics. The unmolested carbs are on the bike. You should have a set to photograph soon. This thread was an afterthought.

As you can see, still more polishing to go. I put them together for pics and to not lose anything. I will do some more polishing of the bellmouths to promote air velocity.

The pics of the slides show  the radiused leading edge. Despite the discolouration they are smooth as a baby's bum. I did not touch the actual cutaway size itself,  just the bottom leading edge. Dont be fooled by the phone pics, each cutaway is symmetrical.

They need a little more work but there is a gentle radius to the throat from the bellmouth with the angled edge eliminated. It gives a nice taper which will help airflow.

All this should help reduce turbulence and improve airflow in volume and velocity.

Actually...my stock carbs were gifted to another member that had a set on his SS that were acting up.

I'll never have to look at the inside of them again.

[koko64]

I was surprised how bad the transition to the throat was and how "grippy" the bellmouth casting was to the touch. With the square edged slide and throttle butterfly, and bad transition from bellmouth to carb throat, it's no wonder the carbs have slide fluttering turbulence.

[ducpainter]

Surface finish alone is a complicated enough subject when discussing flow and turbulence, never mind shape. It can all get very counter intuitive.

Hurts my brain.

[koko64]

Surface finish alone is a complicated enough subject when discussing flow and turbulence, never mind shape. It can all get very counter intuitive.

Hurts my brain.

I hear you.
Mikuni use coatings on some TM carbs,  polish RS slide carb bellmouths and hand finish the bellmouths on TM Pro carbs. WASP use golf ball like dimples on their velocity stacks and CD/TPO have smooth anodized velocity stacks. S&S leave a mirror finish on their bellmouths.
 
I went with polished on the air side and textured on the air/fuel side (ports and manifolds).
When I fit these carbs Im curious how soft a slide spring I can use without slide flutter.