Topic: Keihin Float Bowl Baffles

[ealanm]

I'm using a stock (near as I can tell) CB750K2 engine in a vintage racing car.  Among the many things I don't know about this engine is whether or not the carbs are original.  I can't find any identifying marks on them other than the Keihin name.  Can anybody identify them?



As I'm running this in a car it's exposed to lateral G and, yes, I'm having starvation problems in the turns.  I'm planning to put baffles in the float bowls but, before I go to all the trouble to make my own, I'm trying to see if there are aftermarket baffles or aftermarket bowls for this carb.  (Hence the need to identify it.)  If it's any help, this is what the float bowls look like.

[Tintop]

From your avatar it looks like you are using this in an F4.  Have you tried contacting the 3/4 Litre guys - http://www.formulafour.com/   The Honda 4 was/is very popular, and has been used for a long time (gas & methanol).  They will have an answer for the starvation problem.

[ealanm]

Have you tried contacting the 3/4 Litre guys...

Nice to run into someone who knows what F4 is!

I've spoken to a few F4 guys but, unfortunately, they all run fuel-injected methanol, which is what I'll be running when I get the good engine together.  This stock engine is kind of a stop-gap, but I still need to make it work for at least the rest of this season.  I know that prior to running Hilborn a common method in F4 was to convert the engine to Webers.  But that's too radical a solution for me; I'd rather put that money and effort into the injected engine.

My dream solution would be to find aftermarket baffles, which I know exist for some bike carbs.  I don't mind making my own, but something I could just drop in and have work would save a lot of time and effort.

[ealanm]

Another thought has just occurred to me about the fuel starvation problem.  It's much more noticeable in turns where the RPM is lower.  When I'm exiting a turn at high RPM I can work around it fairly well by not opening the throttle too fast.  Not ideal, but it works okay.  But where the RPM is lower the starvation is bad.  I'm wondering if I can partially fix it by opening up the needle jet?  I have a tuning guide for Mikuni carbs that says the needle jet is effective between about 1/3 throttle and a bit less than WOT.

[Tintop]

I've been a fan of F4 ever since I saw a Walker (Lotus GP paint job & Bonny 650) at a Volvo dealer's.  Some  friends from back then raced DSR with a Kawi H2.

Not sure that opening the needle jet will do any good (you can do the same thing by lifting the needle), and could make things worse.  If the problem is bowl fuel starvation, there's either not enough fuel there now; or its in the wrong place, so asking for more is not likely to help.

What kind of fuel pressure are you running?

[ealanm]

I've been a fan of F4 ever since I saw a Walker (Lotus GP paint job & Bonny 650) at a Volvo dealer's.  Some  friends from back then raced DSR with a Kawi H2.

They are great classes.  I can't understand why they're not more popular.

Thanks for the advice on the needle jet.  I think you're right that I should stick to solving the actual problem, which is with the float bowl.

What kind of fuel pressure are you running?

I use a Facet pump run through a regulator that takes the pressure down to somewhere around 2 psi.  I can't be too sure of the pressure because my gauge isn't accurate down that low, but it's in that ballpark.  I tried to simulate the head you would get from the tank on a CB750.

My brother-in-law showed me a photo of a horizontal baffle from a Keihin on an ATV.  It would be easy to make, and it looks promising, so I'm going to make a set of those for the next race.

Another idea I had was to put a 5-6 mm thick layer of fuel-cell foam in the bottom of each float bowl.  But the horizontal baffle is easier and maybe more promising, so I'll try that first.

[TwoTired]

Another thought has just occurred to me about the fuel starvation problem.  It's much more noticeable in turns where the RPM is lower.  When I'm exiting a turn at high RPM I can work around it fairly well by not opening the throttle too fast.  Not ideal, but it works okay.  But where the RPM is lower the starvation is bad.  I'm wondering if I can partially fix it by opening up the needle jet?  I have a tuning guide for Mikuni carbs that says the needle jet is effective between about 1/3 throttle and a bit less than WOT.

You will find the carb's pilot jet tube is shorter and has a higher pick up point in the bowl than the main jet has.  I suggest putting an extension on the pilot jet tube so it picks up from near the bottom of the float bowl.  Additionally, your extension should have a narrow inner diameter to minimize the fuel column weight being drawn up to the carb bore.  As long and the inner diameter is slightly greater than the jet orifice diameter, it should not restrict fuel flow.

The pilot jet is normally shorter because the tiny orifice is more prone or sensitive to blockage than the main jet system.  Moving it closer to the bottom will allow it to pick up up sediment.  I recommend pre-filtering the fuel and also the air vents for the carb bowls.  Otherwise, a particle the size of .017 inch will stop pilot circuit flow for whatever carb it finds.

Note that the pilot jets are outboard for the right and left carbs.  Which means half the pilots will starve in either right or left hand turns.  But, it will alternate which side starves depending on turn direction.  If you only turned right it would be the left two cylinders.

What is the duration of sustained turns?  I wonder of the floats are being held up too long and the bowls emptying before they are allowed to fall in straightaways, and refill the bowls.    I might be over thinking this.  Get the pilot jet fuel pick up lowered first.  If the problem persists, you may have to address the float shape withing the bowls, giving a larger reservoir size and and less float buoyancy when the fuel is pulled sideways.
The picture below shows the carbs tilted.  But, the effect is the same when lateral G loads are applied to these carbs.

Cheers,

[TwoTired]

One more thing.  The picture you posted shows the carbs tilted forward, as opposed to the upright position they are found in normal installations.

Have you checked the actual fuel level in the carb bowls rather the just the mechanical float height?

[Tintop]

I would invest in a fuel pressure gauge that is accurate between 0 & 5psi.  An oil filled one is the best.  2psi (?) is very low, Webers run 3.5 to 4 generally.  You want the pressure set just below the point were it pushes the float needle off the seat.  Which regulator are you running?  The worst ones are those chrome ones with a dial on top, very inaccurate.  I found the Holley reg (red) to be the best for setting an accurate low pressure for Webers.

Thanks for chiming in TT.  Not sure his issue will be slow speed jet, as he shouldn't be getting below 4/5K rpm even in the long slow corners.

[bear]

We run two facet pumps on seperat switches in parallel to a small header tank that feed the carbs. The header tank has a return line back to the fuel tank. the pick up for the return line is mounted in the header tank 3/4's of the depth of the tank so the header tank has to be 3/4's full before fuel flows back to the tank. We also have a tap on the return line to adjust fuel pressure during the race. It all works very well no reg.

Cheers,
Brian

[TwoTired]

Not sure his issue will be slow speed jet, as he shouldn't be getting below 4/5K rpm even in the long slow corners.

I'm switching over to "Pilot Jet" terminology instead of the term "Slow Jet" or "Idle jet" for technical reasons.
In the SOHC4 carbs, there is no mechanical linkage applied to control pilot circuit output.
It's fuel flow depends entirely on the the carb throat vacuum or the differential pressure between carb throat and pressure at the carb bowl vent.
Therefore, the pilot jet doesn't only supply fuel when the throttle is at idle position or the engine is operating in the "slow" regime. 
It makes a contribution at all throttle positions and engine RPMs.  Certainly, the ratio of dominant factors is different over the throttle and RPM range. 
But even at high RPM/WOT, the pilot circuit is still providing some fuel.

Cheers,

[Tintop]

Interesting setup Brian, thanks for the description.  Have you or any of your mates experienced a similar issue while cornering?  I would think the lateral loading would be very similar.

TT, I know you want hard facts, and at the moment the best I can offer is from my somewhat age disadvantaged (among other things ) memory.  When I dyno the 550 I will be able the get a definitive answer for everyone.  It has PD's with clear overflow tubes which I can hold up and open the drains to check levels.  I'll take a picture - stopped / idle / 2500~ / and what ever we use above that.

There is a carb diagram that showed the fuel level (term used was 'wet area') at various throttle openings.  It clearly referred to the fuel level dropping until a point were the only 'wet area' is the main jet (WOT), and most its emulsion tube is dry.

To me this makes sense, because of how float height is set.  At the correct height the float tang is just touching the spring'd pin in the needle, with the needle resting in its seat.  When you add fuel, the float rises, compressing the spring'd pin completely, closing the valve.  Before the needle can unseat the float has to sink (fuel level drop).  It is this drop that uncovers the slow/pilot/idle jet, and effectively removes it from further influence.

And how does this relate to the current issue you may ask.   It means any baffle, has to be set low enough to allow full float travel, and this means it will likely be about the level of the S/P/I jet.  If a baffle is the solution, rather than a plate, a perimeter ring that leaves the centre more open might be worth looking at.  Something as small as a 1/4" lip at each end on the side, would probably be all that is needed to keep sufficient fuel centred around the main.

Do think checking out the fuel system up to the carbs should be the 1st thing.  A lot easier, and cheaper to fix a 'fuel system' starvation issue, than having to mod float bowls.

[TwoTired]

So, if I understand what you are saying, it is basically that the pilot jet is intended to starve, because the fuel inlet is flow restricted relative to what the engine consumes whenever the throttle is opened?

Assuming what you propose is actually true...
I don't understand what benefit such a design would have over simply providing a base line fuel amount necessary for throttle closed(idle) and then simply augmenting whatever the pilot provides (mixture correction) for all throttle settings and speeds above the idle position.
The pilot circuit contribution is going to be small as it is down stream from the main venturi where max pressure drop should occur at high velocities.

I still have a hard time believing the reservoir design relies on a fill time vs siphon time dynamic just to control fuel supply to a pilot jet.

Also, what you propose starves the mains emulsion tube of fuel as well, adding yet another dynamic of how easy it is to lift the emulsion up to the carb throat.  The changing fuel level would change the emulsion density.  So, it would also be a mains starvation action, as well as a pilot circuit starvation.

Hard to accept (on my part) that the carbs were quite that complex in the 60s/70s.

Cheers,

[ealanm]

Wow, lots of great comments!  Thanks, everyone.

So everyone can have the big picture, you can see the track here:

http://tinyurl.com/3rvtxgn

It's a road course wrapped around a drag strip.  The problem corners are turns One and Two, which are the two 180+ degree corners on the north side of the drag strip.  Turn Three, which is the tight left-hander at the extreme north-west end of the track (following Two, obviously), is also a problem.  Other turns are not a problem, presumably because I'm not under G for long enough for the problem to manifest itself.

I have checked the actual fuel level in the bowls, by removing the bowl and measuring the depth.  It's consistently about 11-12 mm below the rim of the bowl on each carb.  But I don't know if that's correct, as the engine manual I have only talks about the static position of the float itself.  But keep in mind that the car doesn't exhibit any problems whatsoever in a straight line, so I don't think the float level or the tilt of the carbs is a problem per se.  The tilt may exacerbate the sloshing problem under lateral G, though.

TwoTired, your comment about the duration of the turns and the floats being held up by the sloshing fuel really caught my attention.  I had been thinking along those lines, but your diagram of the tilted carbs really brought it home.  The fuel is going to slosh to the outside and lift the float, possibly enough to close the needle valve completely.  That suggests that my idea of a horizontal baffle, as I've seen on some off-road bikes, won't help much, if at all.  The only thing I can think of that would reduce that effect is two longitudinal baffles, one between the main jet and each float, so that the bowl is divided into three narrow chambers instead of one wide one.  I had drifted away from that idea because it was looking too hard to fabricate, but maybe I should revisit it.

I suppose it might help to run more fuel pressure, too, as Tintop suggested.  It may be that the needle valve is closing completely when the sloshing fuel lifts it, whereas at higher pressure it would remain partly open, allowing fuel to continue to flow into the bowl.

But, to get back to my original question from the first post, can anybody tell me exactly what those carbs are, or how I can determine that?  I still hold out hope that somebody has already solved this problem and has a baffle available.

[bear]

Interesting setup Brian, thanks for the description.  Have you or any of your mates experienced a similar issue while cornering?  I would think the lateral loading would be very similar.

Yes.
We run this system to fix the problem mentioned.
Try larger needle and seats as well.
Pussy footing around with baffels will not fix the problem, the fuel bowel on these types of carbs are just to small.
The carbs look like std honda carbs.

Cheers,
Brian

[scottly]

One more thing.  The picture you posted shows the carbs tilted forward, as opposed to the upright position they are found in normal installations.

Have you checked the actual fuel level in the carb bowls rather the just the mechanical float height?

1+ Check the fuel level in the bowls while they are mounted by using the "clear tube" method: unscrew the brass drain-plugs at the bottom of the bowls, jam/screw in pieces of clear vinyl tubing, and hold the tubes up along-side the carb body. The level in the tube should be 2-3 mm below where the top of the bowl seats against the carb body. 

[bear]

One more thing.  The picture you posted shows the carbs tilted forward, as opposed to the upright position they are found in normal installations.

Have you checked the actual fuel level in the carb bowls rather the just the mechanical float height?

1+ Check the fuel level in the bowls while they are mounted by using the "clear tube" method: unscrew the brass drain-plugs at the bottom of the bowls, jam/screw in pieces of clear vinyl tubing, and hold the tubes up along-side the carb body. The level in the tube should be 2-3 mm below where the top of the bowl seats against the carb body. 

Good point.

Cheers,
Brian

[ealanm]

1+ Check the fuel level in the bowls while they are mounted by using the "clear tube" method: unscrew the brass drain-plugs at the bottom of the bowls, jam/screw in pieces of clear vinyl tubing, and hold the tubes up along-side the carb body. The level in the tube should be 2-3 mm below where the top of the bowl seats against the carb body. 

Thanks!  Just tried that and it checks out.  With the tube on the vertical centerline of the bowl (to allow for the forward tilt in my installation) the fuel level is 3 mm below the bottom face of the carb body.

I've also raised the fuel pressure to around 4-5 psi (need to check it with a more accurate gauge to be sure).

[scottly]

Try to level the carbs, front to rear; you will probably have to rotate the rubbers between the carbs and the head to do so. Then re-check fuel levels. Also, these carbs do not have accelerator pumps, so sudden throttle openings often result in a lean bog. BTW, the later 750 carbs (K7/8) do have accel pumps...
How about more pics and info about your racer? It sounds like a go-cart on steroids!

[ealanm]

How about more pics and info about your racer? It sounds like a go-cart on steroids!

I keep meaning to put together some kind of web page on it, but actual racing keeps getting in the way.

The class is Formula Four.  There are now several different classes called F4, but this particular class is based on 750 cc engines (850 cc for two-valve engines).  The rules are very simple:  four wheels, 750/850 cc, open wheel, open cockpit, and some safety regs.  A lot of the cars are one-offs, as the class tends to attract people who build their own cars.

My car is a 1974 Xpit (pronounced Speet).  The original design was a Ferret Mk. III Formula Ford, Ferret being a company in Ontario, Canada,
that made formula cars in the 60s and 70s.  It was designed by Eric Seigrist, who left Ferret shortly afterward to form Xpit, and converted the design
from Formula Ford to F4 by fitting the CB750 engine.  (At the time, that was the hottest four-stroke around.)  It raced with several different engines over the years, including a Suziki water buffalo and a GSXR 750.  But it has been converted back to the CB750 and non-wing form for vintage racing.

I also have another CB750 engine that's set up to run methanol, with Hilborn FI and a very high compression ratio.  The long-term plan is to put that in the car, which will take it back to it's peak form from "back in the day."  But, for now, I'm just running it on a stock CB750 while I sort out the chassis and try to remember how to drive.  (I haven't been on the track for 25 years.)

[ealanm]

By the way, my avatar shows the car as it looked with the GSXR 750 engine.  Note the side radiators and wings.  With the CB750 engine the side radiators are
replaced with simple scoops that duct air around the back of the fuel tank and onto the front of the engine, and I don't run wings.

Here's a photo showing the engine installation, from last winter when I was straightening the back of the chassis (note the jig bolted on where the rear
bulkhead normally goes).

[scottly]

Had a friend with a formula Ford(?) back in the '70's. I think it was a Ford Cortina 1600cc motor, maybe
smaller?? It looked like a '60's Lotus Indy car, only smaller, best as I can recall.

[ealanm]

Had a friend with a formula Ford(?) back in the '70's. I think it was a Ford Cortina 1600cc motor, maybe
smaller?? It looked like a '60's Lotus Indy car, only smaller, best as I can recall.

That's exactly what my first race car was like.  The original FF engine was the Cortina single-cam.  Then they switched to the Kent engine, which was the original Pinto 1600.  It was a very similar engine although, as I recall, there weren't actually all that many interchangeable parts.  The Kent engine lasted for quite a few years, and there are still many FFs running them.  But I think they use a Focus engine, or something like that, in the newest ones.

My car is substantially lighter than an FF, mainly because of the Honda drivetrain.  An FF weights about 925 lb empty; mine is 632 lb.