Tuning bike carbs- on a car

Many bike carbs do NOT feature acclerator pump enrichment.
Lighter vehicles like bikes can get away with this, but heavier vehicles like cars will 'bog' when you snap
the throttle open from idle (like at a stoplight).
The extra fuel is required to generate additional energy to accelerate the greater mass.
You have to 'roll on the throttle', rather than just flooring it.
You can run the idle mixture very rich, reduce (but not eliminate) the issue, but that creates its own
issues with plug fouling.
For this reason, many newer and larger bike carbs nowdays do feature accel pumps.

So choose wisely.

In reply to newrider3 :

That is just how CV carbs work. Every SU equipped car lacks an accelerator pump. Even the huge Rolls Royces work fine without them. 

Tuning the throttle tip in fueling is done with damper springs and in extreme cases heavier oil in the damper. 

What you want is the damper to stay down a fraction of a second longer so the vacuum pulls more fuel from the jet. Sounds hokey, but it is just part of the design. There are 3 or 4 springs of varying stiffness labeled "weak" to "strong" for most  SU's

Yes, after 50+ years having owned, raced, and modified LBC, I'm fully aware of how SU (and  ZS) carbs work.
The DAMPER is what makes this occur, by delaying the rise of the piston, thereby enriching the mixture by creating
a temporarily higher air velocity and thus a larger vacuum signal at the bridge which draws more fuel from the jet,
enriching the mixture until the piston eventually rises and reaches equilibrium

In my experience, unlike SU & ZS carbs, the M/C carbs I cited LACK that critical damper function that accomplishes this,
do you not agree?

Springs do NOT implement a damper function, they provide the force that determines the rise of the piston vs the vacuum signal developed at the bridge that is connected to the suction chamber.

That is why SU & ZS carbs must feature a seperate damper device to provide accel enrichment.

(there is a cV carb diagram posted above, and no damper is apparent)

As I stated, lighter vehicles do not usually require this additional mixture enrichment for accelleration.

Cars do.

What's your time frame on needing carbs? I intend to pull the L24 out my 240Z this winter and could make the stock SUs available to you 

Well, Oldskewltoy has my head, pistons are somewhere between an ingot and a finished part at the manufacturers (and the retailer is telling me they don't want to hear from me before November), and my hobby budget says I'm broke, having bought all the parts I have (so far) on my retiree income. Assuming I get the pistons in November, I maybe can start on the engine machine work in early December. What with the holidays and the typical stackup of builds at machine shops I've experienced in the past, we're probably looking at March before machine work is done.

So yes, I am interetsed in the SU's off the 240, assuming they are the round tops and not the tuna cans.

Think I'm planning to start w/ 1.75's, and if it doesn't get me to where I want to be, I'll look at other options.

3 bolt round tops, yessir. 

In reply to clshore :

There are a couple dozen of us Datsun guys running bike carbs. Several of them are using CV carbs without issue on both street cars and race cars.  

newrider3 said:

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? 

The carbs I use, Kehein FCR flat slides have an accelerator pump so it's not an issue. The down side of these is there is no choke, so you have to pump the gas pedal several times when starting them up and wait for the car to warm up for 2-3 minutes. The are meant to be racing carbs.

As an experiment I did fit a set of CV carbs. I had a set lying around and they slid right on. They seemed to work fine, I didn't spend a lot of time on them as I was more curious than anything else. The Flat Slides flow more so I use them instead.

Dellorto bike carbs have accelerator pumps. Have had for years.

Nice thing is, I've never ever seen a Dellorto that ran lean.

Tom1200 said:

In reply to clshore :

There are a couple dozen of us Datsun guys running bike carbs. Several of them are using CV carbs without issue on both street cars and race cars.... 

.... As an experiment I did fit a set of CV carbs. I had a set lying around and they slid right on. They seemed to work fine, I didn't spend a lot of time on them as I was more curious than anything else. The Flat Slides flow more so I use them instead.

Interested in what jetting changes were required. Did (Do) they have an accel pump?  Remember what bike to CV carbs were from?

In reply to Tom1200 :

The issue is not all CV carbs, it's whether or not the ones selected provide accel enrichment.
Some do, and some do not.
If yours do, then great.
Carb manufacturers didn't just decide to spend the extra money to put accelerator pumps or
CV piston dampers on their products on a whim, they did it because the physics and engineering
REQUIRED them in order to deliver the needed throttle response on heavier vehicles. 

Honestly, this stuff was known and has been worked out for more than 100 years.

Having tuned a lot of bike ITB on cars I can't imagine how horrible it would be without accel enrichment  that's about 50% of the tuning and every setup is quite different even with similar engine setups.

clshore said:

Yes, after 50+ years having owned, raced, and modified LBC, I'm fully aware of how SU (and  ZS) carbs work.
The DAMPER is what makes this occur, by delaying the rise of the piston, thereby enriching the mixture by creating
a temporarily higher air velocity and thus a larger vacuum signal at the bridge which draws more fuel from the jet,
enriching the mixture until the piston eventually rises and reaches equilibrium

In my experience, unlike SU & ZS carbs, the M/C carbs I cited LACK that critical damper function that accomplishes this,
do you not agree?

Springs do NOT implement a damper function, they provide the force that determines the rise of the piston vs the vacuum signal developed at the bridge that is connected to the suction chamber.

That is why SU & ZS carbs must feature a seperate damper device to provide accel enrichment.

(there is a cV carb diagram posted above, and no damper is apparent)

As I stated, lighter vehicles do not usually require this additional mixture enrichment for accelleration.

Cars do.

Actually, acceleration enrichment on a motorcycle CV carb works exactly the same as it does on an SU carb which is also, technically a CV carb.  The dampening on a motorcycle CV carb is controlled by the vacuum port in the bottom of the slide.  A smaller hole slows the opening where as a larger one allows it to open faster.  The dampener on an SU is more easily tuned but the principal is the same.  

Motorcycle carbs can work just fine on a car but the further the car application gets from the original motorcycle application in terms of airflow range the more difficult it's going to be to tune the carbs.  You may have to start playing with things like emulsion tubes, air jets, needles and yes, vacuum port size to make them happy.

The slide air bleed jet is not replaceable in all motorcycle CV carbs, so that's something to be a bit careful of. In some cases, the slides also have additional mass added to them to create a delay in opening. No, it isn't a damper, but the effect seems to be similar.

One of my bikes has CV carbs with weighted slides and an accelerator pump. I'm not quite sure why they needed both solutions, but they did.

Motorcycle FCRs typically do have an enrichener circuit, but I'm not sure how you would set it up to work in a car. It would be a pretty odd linkage.

Personally, I find it much easier to tune direct-acting carbs for inappropriate applications because the throttle position always is what it is, where on a CV carb it can be challenging to know what part of the needle you're operating on for driveability tuning. I spent some time trying to get this right by removing filters and using a camera to film the slide position, only to discover that the loss of restriction from the air filter made a pretty big difference in fueling and slide operation. I never did find a good way around that and went back to trial and error. That thing had dual main jets and a goofy air bleed system to control them, too. It never did quite work the way I expected. Yes, CV carbs can work great, but for me, I found I was able to get closer to "perfect" when I didn't have quite as many interrelated variables to play with.

In reply to twentyover :

The 39mm Kehein carbs were set up for a sports racing car that used the same Nissan engine that is in my Datsun but they are intended for a GSXR 750. I had to drop the mains way down from something like 180s to 145s. I actually had to fit a slightly bigger pilot jet, they more highly tuned motor ran a cam with more overlap. My A12 engine runs a street cam (73whp) & the A15 (99 whp) motor runs a rally cam. When switching from the A12 to A15 I have to go up a couple of sizes on the main jet and down one size on the pilot (cam overlap). I did go to a shorter needle initially as that seemed to help the transition.  I got the car about 95% and then took it to dyno place. All that needed doing was going up one size on the main to get the air fuel where it needed to be.

I used this site as a guide to tune the Kehein carbs.

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_FCR_Burns,pat.html

The CV carbs were off an FZR1000. I didn't really mess with the jetting and it seemed to run OK.

In reply to gearheadE30 :

I have a tuner friend that played with CV carbs back in the day. He add/change springs and or add weights to the slides and sometimes turn the needles on a mini lathe to get where he wanted to be. The bikes he tuned always ran flawlessly.

gearheadE30 said:

The slide air bleed jet is not replaceable in all motorcycle CV carbs, so that's something to be a bit careful of. In some cases, the slides also have additional mass added to them to create a delay in opening. No, it isn't a damper, but the effect seems to be similar...

Good points.  The spring also effects opening rate.  Those are all things that can need adjustment when you try and run a carb on something that's different from the original application.