Bike Carb Manifold

[gazzz]

Hi all,
I,m new to this forum but need some advise off any one who can help, I am thinking of running four bike carbs on a four cylinder engine but because the ports are spaced unusual(two ports next to each other then big gap to other two ports) I was going to use a std injection manifold (off different modal but fits) cut the centre throttle body off and attach carbs to plenum chamber, my problem is two runners go to the outside at end of chamber on each side with big gap in the centre but the carbs are spaced evenly so the two middle carbs would be far away from the runners but the two outside carbs would be inline with outside runners, So finally the question would the fueling be uneven?
regards
gazzz

 

[patprimmer]

It will depend on a few things not apparent in your sketch, like plenum size and distance from carby base on plenum to runner inlet from plenum but it could work, but far from perfect. Stagger jetting could correct fuel distribution.

Why not curve the runners to line up with the carby chokes or move the chokes to align better with runners or a bit of both. Maybe put the carbies on a spacer that has a curved bore to aim air at the appropriate runner.

Regards
Pat
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[gazzz]

Hi pat,
Thanks for reply, the reason for me wanting to do this is price first and second I could do this myself but any angles or bending I would have to pay some one, The runners are 7" long and angle up to the bottom of plen, The plen is 12" L by 2"D by 3"W nearly flat on top so would be easy to weld 4 tubes on to mount carbs(given to me from a friend) because the way the angle on manifold is and the angle the carbs sit it would leave carbs facing straight to runners inside plen, I do not know anything about plenums just thought it might work but as said dont know how fuel distribution would affect it, I have no means of checking fuel on each cylinder I do have a AFR gauge but I,m after performance so do you think this is a project worth trying given my lack of knowledge and skills and engine damage that could happen? I do know there are a lot of clever people on this forum which is why I posted thread to get honest reliable answers
thanks again
gaz

 

[patprimmer]

You have spark plugs don't you.

You can make a 2" spacer out of a block of plywood if you like. Just use layers of good hard plywood stuck together or doweled for alignment, then bore, chisel, cut, file or whatever takes your fancy to make the runners then paint with a good floor varnish that penetrates the wood a bit and sets hard. Heck you could even use medium density fibre board so long as your a bit gentle tightening it down.

Regards
Pat
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[DuncanM]

What kind of engine?
What bike did the carbs come from?

Looks rough and nasty but how about something like the pic below? R6 carbs on a Peugeot 205 1.6 litre engine according to the forum thread it was posted in.

Forum thread (external)

 

[gazzz]

Hi,
There honda blackbird carbs Fiat x19 engine, Think its probably best to get manifold made just very hard with the spacing
regards
gaz

 

[winfieldblue]

An interesting idea gazzz, here's a link to another similar experiment- motorcycle carbs mounted onto a ford v8 manifold. Unfortunately despite his persistence there wasn't a gain in power,but lots of headaches with jetting etc.-

http://www.fordmuscleforums.com/makin-progress/506905-motorcycle-carbs-tunnel-ram.html

 

[Tmoose]

On the bike each individual carb gets the sharp individual intake pulse.
With a plenum I envision some blending and softening of the pulses, and they will be shared by 4 carbs. (or 8 in the Ford story) .
CV carbs are pretty smart, but I guess I'm not very surprised the bike jetting was way off.
Perhaps that is another reason to maintain individual runners.

Jaguar XKE intakes (1 carb servers 2 ports) swerve around a bit to line things up.

http://www.paltech1.com/sitebuildercontent/sitebuilderpictures/11-30-04004.jpg

http://jag4jag.com/e_manifolds.jpg

The "sailor leg" end manifolds look like an effort to keep lengths equal at the expense of a little theoretical pressure drop. Probably more important in a 2>1 manifold

Small curvy Steel exhaust tubing harvested out of MIDAS' or SEARS' auto repair dumpster might be used to make a manifold fabricate-able (weldable) by mortals. Could borrow a uniformed cub scout cousin to legitimize the request for permission to dig thru the dumpster since it is for a "Scout project."
(As an automotively-active teenager a friend lived pretty close to a big shopping mall. In those prehistoric days stores were closed on Sundays, providing privacy and daylight while scavenging choice bits for the sometimes pretty sophisticated exhausts sported by His various car projects. And allowing him to skip the "permission" part.
It was probably inevitable he would become one of the most capable fabricators I know )

If I was going to go the epoxy/wood/fiberglass route I'd want a metallic skeleton in there somewhere, and a sloping strut or 2 to support the manifold near the carb faces off the engine. An inline 4's secondary vibration can be brutal.

 

[patprimmer]

I have made inlet manifolds that worked quite well from copper pipe from plumbing stores, including preformed bends with slip joints. They need support to stop cracks but they performed well.

Regards
Pat
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[crerus75]

If you make the intake out of fiberglass, adding the curves is doable at home. Here's an interesting, although time and labor-intensive approach:

http://bellsouthpwp.net/j/o/johngd/files/car/fiberglass_manifold/

A fiberglass intake manifold design created using a lost-wax form. Not for the faint of heart but it can be done.

 

[gazzz]

Thanks for the thread on the v8 was very interesting and give me my answer FOR GET IT I was surprised he had to put stronger springs in the carbs? when we put the AFR gauge on my friends conversion it was weak when throttle was opened flat spot untill we fitted the dyno softer springs it then richened up but as you said it was a headache trying to get the fueling right which is really what I wanted to know
regards
gaz

 

[Tmoose]

MC CV carbs sometimes vary the size of the orifice leading the venturi vacuum to the top side of the piston diaphragm. Kevin Cameron says the result of a smaller hole is to delay vaccuum above the piston, preventing the piston from rising when the throttle is opened, which would seem cause a lean condition, except that the increased amount of air whooshing thru the smaller space creates a lot more vacuum right where the fuel discharges and generally speaking enrichening. The result when when accelerating is similar to increasing the accelerator pump shot on normal carbs. The piston delay = acceleration enrichening instead of lean-ness as would happen with a fast slide due to ease of big air increase outpacing main circuit fuel delivery is what has been said in SU theory for decades

This guy seems to say the opposite.

http://www.cyberranch.org/bike2.html

I's expect softer springs to let the slide rise faster (leaner?) and to have some effect on steady state slide height (and jetting), but have found no reference for that yet.

 

[gazzz]

Yes not sure why, is it letting slide up a bit higher than it was before when air flows in like lifting needle? I know with SU,s we used to put thicker oil in dash pot to make it richer on acceleration, I remember years ago on the rollers having a Fiat with a long duration cam tuned(autograss racing) as it was coming on cam it was running lean so the guy put jets in to make it richer but it went weaker? He said the cam was not timed in right so we took it away and checked timing it was perfect, We then took it to a different place and this guy was good he explained it was standoff with gases trying to reverse flow and showed us(as it was being tuned on rollers)fuel coming out of carbs if engine was held at coming on cam revs(if that makes sense) and said with race cam you need to be cam all the time so holding at standoff was pointless, He was an old guy with years of experience and the car went really good after this R/R but always amazes me how things can do the opposite of what you expect
regards
gaz

 

[patprimmer]

The plenum should pretty well dampen the pulsations that cause stand off

Regards
Pat
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[Tmoose]

If fuel could leap out of the fuel discharge point as easily as the air passing by speeds up then no-one would need accelerator pumps.
Typical Slide carbs (CV or not CV)_don't have pumps (one exception is Smaller/older BMW boxers that used smallish Slide type Bings. They have an upwardly spring loaded plunger that contacts the tip of the needle, so when the slide and needle are raised, the plunger rises and pumps extra fuel out the needle jet).
So pumpless slide type carbs need some other mechanism of enrichening to compensate the lean bog that is inherent when airflow increases suddenly. Slide delay can accomplish that enrichening.

 

[gazzz]

Hi all,
Well finally got manifold made(got some different carbs with better spacing)Don,t know when i,ll get around to trying them but will post when I do
les