Tuning bike carbs- on a car

Did a thread search, didn't find what I think I need adapting bike carbs to car engines. Found an old thread from 10 years back, but it didn't really address my questions.

My head porter doesn't like my choice of carburetors. Fair enough, I don't like his choice of carbs, either (I have a thing against DCOE's. Something about spending the amount of the national debt of a third world county on jets, emulsion tubes, and other parts.) After some discussion, I think we agreed maybe bike carbs may be the answer.

So, does anyone have information on tuning CV bike carbs? Are alternate needles available? Is there any information or guides for setting these up? Or tuning information? Any help or pointers are appreciated.

I wonder if the editors of GRM ever considered an article on this?

how about bike ITB EFI?

I assume you've seen this?

https://www.classicfordmag.co.uk/wp-content/uploads/2017/12/classic-ford-ultimate-guide-to-bike-carbs.pdf

It suggests that basically you need to enlarge the main jets some but otherwise just make sure the carb is in good running order. 

I thought there was something you had to do to them so they could handle side loads.

Pete. (l33t FS) said:

I thought there was something you had to do to them so they could handle side loads.

I thought about this a lot, then remembered stunt bikes. I think bike carbs are pretty well buffered against odd gravity angles. I wouldn't worry about it until I started encountering a specific problem. 

But I would also probably do a bike ITB EFI conversion before trying bike carbs. The literbikes all went to EFI over 20 years ago so excellent EFI stuff is insanely cheap and carbs are starting to become 'old'.

air cooled 911 guys have been putting on  bike ITB EFI conversions , not sure what they use for the "brain"

I'm a big proponent of carbs, but if you have to ask, EFI is probably a better option. If you're not fully prepared to jump in head first and do a lot of trial and error, you're not the type I'd recommend carbs to. I'd go with Webers strictly because they're such a known quantity.  I will happily spend money on parts that are easy to tune with good documentation and a large base of experts to help.

There are companies that sell manifolds (plates w/ tubes welded on) that mate with certain commonly used bike efi throttle bodies. What's the head you're using?

I have a set of MC carbs that were set up to work on a Spitfire engine. My understanding is they were set up for a 1300 "race" engine, so I'm hoping they'll be adequate for a 1500 "street" spec engine.  These carbs were supplied by a vendor (no longer in business) who came from MC racing and knew the carbs well and how to tune them.  Apparently, getting the carbs to run at higher engine speeds isn't too hard; it's the cold-start and low-RPM areas of drivability where the tuning gets tricky.  Hopefully I'll find out before too long when I can start on swapping the new engine in.

In reply to twentyover :

I can't help you on motorcycle carbs.  I looked into it and realized there were other carbs better suited for the task.  The chief problem is motor cycles lean when cornering. So instead of dealing with side loads in cornering they deal with different pressures.  
 Second problem, most bike carbs are gravity fed.   Dealing with the pressure required to pull fuel from below and a long way behind is going to cause trouble. 
   Simple carbs like SU already have solved both of those problems in their basic design. Once you understand how easy they are your life gets extremely simple.  
       The really wonderful thing about SU's is how big they can be compared to  the engine size.  
    For example MGB uses a 1&1/2 while other engines that size can use an 1&3/4  Yet all out racers use 2 inch SU's on their MGB's. 
     Tuning is a matter of  of using well known needle sizes. Then moving the jet up or down either with a screwdriver or a wrench.   If you find yourself a little lean at some RPM  make note of where the needle is, pull the needle chuck it in a drill and polish that area with some fine sandpaper. 

My Datsun has Keihin FCR Flat carbs on it. They've been on the car for 12 years, I vintage race the car. They work exceptionally well and in general are of a much higher quality than automotive carbs.

Ignore the people who tell you they are somehow mystical and can never be made to work on a car. The carbs have no idea what they are bolted to and work exactly the same as they do on a bike.

I prefer the flat slide carbs to the CV carbs as the flat slides flow considerably more (no throttle shaft impeding the flow) As for the needles on CV carbs it's dependent on the carbs but needles are available for some. You can also shim the needles to change the height.

The main thing you need a fuel pressure regulator as you only want about 1.5psi of fuel pressure, more than that and the float needles won't seat.

I have no problems with fuel starvation in corners; remember the carbs are turned 90 degrees form their normal orientation so the fuel in the float bowl is moving fore and aft not side to side.

Here is a picture of the flat slides on my Datsun (ignore the shredded fan).

I think your title would be just as accurate without the "on a car " part. 
Just get a set of mikuni flat sides and start the motor. Maybe shoot for a bike with similar hp as the motor your putting them on. Lots of bikes had small fuel pumps, just don't overfeed with too much pressure. 
You're over thinking this! 
 

Edit: Picking up a wide band sure can help things once you get running and working the kinks out.

My Zetec powered Europa (and soon my Zetec powered MGB GT) is running Kehin carbs off of a CBR900.  I had to change the main jet size and the idle jet size, but other than that they were fairly straight forward to fit.  The mains were drilled out to 1.6 mm, while the idle (slow running) were swapped for 50's (I think).  The B is getting proper 1.6 mains instead of drilling, as you can buy the jets online in various sizes.  One day I might hook up an 02 senor and see what the mixture is actually like, but currently it runs very well.

RoddyMac17 said:

My Zetec powered Europa (and soon my Zetec powered MGB GT) is running Kehin carbs off of a CBR900.  I had to change the main jet size and the idle jet size, but other than that they were fairly straight forward to fit.  The mains were drilled out to 1.6 mm, while the idle (slow running) were swapped for 50's (I think).  The B is getting proper 1.6 mains instead of drilling, as you can buy the jets online in various sizes.  One day I might hook up an 02 senor and see what the mixture is actually like, but currently it runs very well.

I mentioned mikunis above but you're right on bikes you'll likely find more kehins, I'm up where there's snow and the snowmobile guys love mikunis.

GSXR ITB's and Megasquirt on my 4AGE Locost for y-e-a-r-s.

I'd do it again in a heartbeat.

And I don't have to have a collection of ANY jets, and I never smell like fuel.

Tom1200 said:

My Datsun has Keihin FCR Flat carbs on it. They've been on the car for 12 years, I vintage race the car. They work exceptionally well and in general are of a much higher quality than automotive carbs.

Ignore the people who tell you they are somehow mystical and can never be made to work on a car. The carbs have no idea what they are bolted to and work exactly the same as they do on a bike.

I prefer the flat slide carbs to the CV carbs as the flat slides flow considerably more (no throttle shaft impeding the flow) As for the needles on CV carbs it's dependent on the carbs but needles are available for some. You can also shim the needles to change the height.

The main thing you need a fuel pressure regulator as you only want about 1.5psi of fuel pressure, more than that and the float needles won't seat.

I have no problems with fuel starvation in corners; remember the carbs are turned 90 degrees form their normal orientation so the fuel in the float bowl is moving fore and aft not side to side.

Here is a picture of the flat slides on my Datsun (ignore the shredded fan).

 

 

 

Exactly,no problem either with my turbo blow thru cv carbs on my 4ag 1600 AE86 on slicks.

You might find snowmobile carbs even cheaper than motorcycle carbs.

I've never had an issue with snowmobiles being affected by lean, climb/descent angles, or acceleration.

I think they might be even simpler than CV carbs, and jets are readily available for tuning. You have the pilot jet (with screw), main jet, and needle in the slide (clip can be raised and lowered).

They are fed through a vacuum operated fuel pump, but I don't know the pressure that the pump puts out. They use two floats the ride on guide pins that move vertically to actuate the needle instead of a float that rides on a pivot. There are also option to replace the needle and seat with a ball type valve.

What you have to ask yourself with CV carbs and side loads on a car is what angle is the carburetor. CV carbs on bikes hate rough terrain and whoop-de-doos especially since they don't have much of a method to keep the slide from moving up and down on its own when jarred on the vertical plane. When mounted on a bike carbs are nearly vertical. The closer to horizontal the carb ends up being on a car the more side loads are going to affect it due to g-forces. When bikes lean on asphalt it is usually very deliberate and the down lean takes a decent amount of time. Cars on the other hand pull higher Gs in corners (bikes suck at turning -- they just have to turn less than cars) and the carb might be starting at an angle already. Rapid transitions especially could make the slides move on their own. Still probably not as jarring as a set of whoop-de-doos.

In contrast, with conventional bike carbs the cable is connected directly to the slide and locks it into the position selected by the controls.

In reply to GCrites80s :

My old D-sports racer used a Yamaha 1000 engine with CV carbs. That car pulled over 2 lateral Gs and 3Gs on the brakes. Never had an issue.

Crap-

Have about 30 minutes into a response and it disappeared. Rather than retype- I'll just say thanks to all respondents, your comments and advice are appreciated even if I ultimately do not take it

SkinnyG (Forum Supporter) said:

GSXR ITB's and Megasquirt on my 4AGE Locost for y-e-a-r-s.

I'd do it again in a heartbeat.

And I don't have to have a collection of ANY jets, and I never smell like fuel.

I was clued in that ZX12 throttle bodies are built on 90mm centers.

This means they should line up perfectly with any dual DCOE manifold.

Tom1200 said:

In reply to GCrites80s :

My old D-sports racer used a Yamaha 1000 engine with CV carbs. That car pulled over 2 lateral Gs and 3Gs on the brakes. Never had an issue.

I kinda figured it would be hard to recreate the effect that whoop-de-doos and rough terrain have on CV carbs on the asphalt. The moves aren't jostling enough.

In reply to GCrites80s :

The Datsun originally had Hitachi SU style carbs; I once tried to rally cross it, the venue had heavy duty wash board surfaces. The car wouldn't carburete at all becuase of the bouncing.

Learn me - when running bike carbs in an ITB configuration (one carb per cylinder) on an automotive engine, are there no issues with the lack of acceleration enrichment and lack of power valve? 

We experimented with a pair of huge snowmobile Mikunis on our 1UZ Volvo, and it was a resounding failure due to the lack of accelerator pumps. But, this was two carbs on an 8 cylinder with more of a conventional plenum intake, not ITB layout.