SU carburator on jeep

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Re: SU carburator on jeep

Post by Wolfman » Sun Jan 28, 2018 4:57 am

OK I give up, Luc.
Been thinking about this for a while.
What is an SU constant vacuum carburetor ??
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Re: SU carburator on jeep

Post by GI. » Mon Jan 29, 2018 10:33 am

Why would you need a larger carb. ? are you building a hot rod ? :wink:
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Re: SU carburator on jeep

Post by Fabrizio » Mon Jan 29, 2018 11:40 pm

I have a Mini at home; I broke some parts by staring at them with a little too much force!

Wouldn't it be wiser to pick a little nice Weber, single or twin body?
Or to look at what was fitted on Citroëns tractions, ID and dS? I think the m201 Solex came from there
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Re: SU carburator on jeep

Post by Scoutpilot » Tue Jan 30, 2018 1:28 am

God bless you my friend. I'm sure this is all in fun and a need to satisfy one's curiosity. Many designs, and just about every concept have been tried. Most worked, more or less. The best achievement? A gain of about one half horsepower. A proper build and kept in proper tune and you will be able to go scary fast.
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Re: SU carburator on jeep

Post by Wolfman » Tue Jan 30, 2018 8:16 am

I have built several performance engines in my life and one big misconception is you can bolt a larger carb. on an engine and give it more horsepower. What Scoutpilot said. A little.
Shaving the head to increase the horsepower is a good start. Along with the larger carb, porting. Different manifold design for better flow. Valve lift, timing and over lap come into play. Piston head design. Distributor advance curve and maximum advance ( for higher RPM apps. ). And then when you get all that fuel into a cylinder and get it burn better and produce more power, you have to get rid of the increased exhaust. Headers is the common choice. On a jeep, that creates another problem to deal with.
My point, major power improvement is a package deal. Not just a single bolt on.
Got it on the carb. type. Most of my experience with those were on motorcycles.
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Re: SU carburator on jeep

Post by Scoutpilot » Tue Jan 30, 2018 9:12 am

What he said. In brief. Many man-hours in labor (tick$tock$tick$tock$). Large$$$ parts$$$ costs$$$. Then, of course, once you gain all of this power, you will need to address the braking issue. You've got to be able to stop when and where you need to. Nine inch stock brakes will only extend that brief time before the wreck. Don't get me wrong. It's your vehicle. Have fun.
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Re: SU carburator on jeep

Post by eaw46 » Tue Jan 30, 2018 9:37 am

Way better off to buy a newer CJ with much more performance than to try to boost a GPWs HP. My opinion and worth what you paid for it

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Re: SU carburator on jeep

Post by Wolfman » Tue Jan 30, 2018 11:47 am

:D Good choice ,Luc ! :D
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Re: SU carburator on jeep

Post by Fabrizio » Tue Jan 30, 2018 8:17 pm

I agree with all the above statements about the engine being a system that needs to be "upgraded" at all levels at the same time etc; BUT! In some cases a carb modification alone can change a lot of things.

A lot of Alfa Romeo's DOHC were offered with both twin barrel carb OR two twin Weber dcoe 40; with sometimes around 20% more hp; with identical camshafts etc.
But in these cases the engine had probably been developped with the bigger (expensive) carbs, and downgraded afterwards.

Exactly like on our Jeeps; I can't remember where, but the french army did specify very clearly that the new replacement Solex carb had to cut fuel consumption by 10 or 15%. The power/torque difference between Solex and Carter/Zenit on the same engine appears to be noticeable.
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Re: SU carburator on jeep

Post by tamnalan » Fri Feb 02, 2018 5:33 am

It's hard to beat the WO for all around performance and easy road-side repair capability on these motors.

I'm guessing that's a 25% reduction in your overdrive? I have a front-shift ATV unit too, which I like.

Agree on better brakes, particularly if towing a trailer. I'm going to Bubba my way into front disc brakes for my MB. Too many near-misses on the convoys!
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Re: SU carburator on jeep

Post by Marty, SoCal » Sun Feb 04, 2018 6:56 pm

SU's are variable venturi carbs and even if the throttle blade is larger than the old Carter's throttle blade, it won't open more than the engine can take if it's tuned properly.

A good fabricator should be able to make an intake manifold to fit the engine with a single or dual SUs and by also using a Willys F head exhaust manifold it would eliminate the heat stove function of the original for a cleaner look and will bolt right to the stock head pipe. A couple SU's for 1.0l engines or a single SU originally for something like a 1800 CC engine should work without being too large to be efficient.

If done properly you should be able to replace any modified parts with the originals in a couple hours, so I say have some fun and go for it!
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Re: SU carburator on jeep

Post by Wolfman » Mon Feb 05, 2018 5:18 am

Just curious ?
Been studying this design.
The SU carb. has a throttle plate on the intake side of the carb. and a slide with a metering rod, connected to a piston, that is held closed by a spring, in the center of the carb.
When the throttle is opened, vacuum is created between the throttle plate and slide and a passage from this intermidiate area applies vacuum to the top of the piston, raising the piston against the spring and as the piston rises, it opens the slide and raises the metering rod.
No doubt, this would give a smooth throttle response since the slide and metering rod are working in unison.
What happens at full throttle ??
The engine vacuum drops and the piston and slide are being held up against spring pressure by vacuum.
???
So, as the vacuum peeks and begins to drop off, does the piston and slide lower back down until there is some balance point between the vacuum and spring pressure ???
Looks like this would put a damper on " full throttle " operation.
Maybe some mechanical means of holding the slide open ???
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Re: SU carburator on jeep

Post by Marty, SoCal » Mon Feb 05, 2018 6:11 am

There still is a bit of a venturi rise in the bore below the slide so there will be a bit more vacuum on the rear of the piston to keep it open fully at full throttle/higher rpm as long as the carb is sized appropriately for the engine.

Here's a link to a MG site with some good videos on the SU:
http://www.mgexp.com/article/video-secr ... retor.html

SU Wiki:
https://en.wikipedia.org/wiki/SU_Carburettor
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Re: SU carburator on jeep

Post by tamnalan » Tue Feb 06, 2018 4:30 am

That's pretty cool, Luc!

Wrapping the runners with fabric exhaust pipe tape would help the icing problem.

So the next step would be to move the injectors to the head? That does sound expensive.
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Re: SU carburator on jeep

Post by Wolfman » Tue Feb 06, 2018 6:58 am

My take, Luc.
I think the problem you had is because, as you said, the firing order and the way the fuel is drawn in to the cylinders.
The original jeep manifold center is a plenum chamber. The air and fuel mix is pulled to the center plenum, from the carb. and then drawn to the cylinders from there.
With your design, there is no plenum. With the long tubes, the inertia of the fuel moving through one tube to cyl. 3 followed by cyl. 4 has to be stopped and then the process started again to get the fuel moving down the tube to cyl. 2 then cyl. 1, down the other tube, from a single carb. This causes a lean condition and back firing. Two carbs. One to each port might help the flow reversing problem. Maybe ?? The valves opening and closing as well as the air/fuel flow stopping and starting generate pulses in the intake tubes. The intake tube length to control these pulses would also come into play.
Think I found my own answer to the vacuum to the diaphragm piston on the SU type carb.
The vacuum does not come from a port on the engine intake side of the carb, but the carb. venturi. The top of the diaphragm is ported to the venture. Not the intake side of the carb. On initial throttle plate opening, intake vacuum ( I know. Barometric pressure under the diaphragm pushing. Trying to keep this simple. ), pulls the diaphragm up but once engine RPM increases, the velocity of the air moving through the venturi and the low pressure area created there, supplies the necessary vacuum to hold slide open.
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