Hints and Tips - Carb Tuning

From: Malcolm Whitehouse
Date: Wed, 5 Jun 96
Subject: RE: Vertical SU's
To: british-cars-pre-war

Sergio Montes asked if anyone had experience of mounting SU's horizontally.

I have a 4400 cc 1930 Lanchester which originally had a twin barrel updraft Zenith 105DC carburetor made in USA. The Zenith is largely made of Mazak and was disintegrating. I tried to get another Zenith without success so I replaced it with a single modern 2in SU fitted to the existing updraft manifold. Necessity is they say the mother of invention.

I made an adaptor to fit between the manifold and carb. I then manufactured a float chamber bracket and some new operating levers. A modern SU has lots of drillings for breathers and balance pipes that need blocking up as well. If you don't make a good job of sealing these drillings then the carb may well whistle as air is drawn across an open end of a blocked drilling in the body of the carb.

Orientation of the carb is important, the throttle spindle should lie in the same plane as the cylinders and ideally the float chamber should be fitted so that when the car goes up or down a hill the mixture is unaffected. This means the float chamber is fitted at the end of the jet housing. If you need to fit the float any where else fit it towards the front of the car but keep it as close as possible to the carb body. (The mixture will tend to get richer up hills in this config).

Adjust the fuel height so that it is about 1/8 to 3/16 below the jet outlet and then adjust the needle as normal. You will probably need a 12oz or more spring in the dashpot. I am still experimenting with the spring and the needle to get the best economy and performance.

Its not too difficult to accomplish a successful installation, personally I would select a modern SU which has the non ball bearing dashpot as a starting point.

Incidentally I have seen some down draught SU's at auto jumbles, however the biggest that SU made as a down draught was 1 5/8 bore and fitted to Daimlers and Wolseleys among others.

Malcolm Whitehouse


Date: 18 Jul 96
From: John Hardy
To: BRITISH-CARS-PRE-WAR
Subject: SU Carbs and Rolling Road

Jackson,

you wrote...................

"Rolling Road dynos are relatively hard things to find these days, especially any that are operated by someone knowledgeable about SU carbs. What exactly are you trying to do that can only be done on a dyno? Other than to know the actual rear wheel hp, most everything else can be accomplished using relatively inexpensive stuff on the road... I would assume that the original poster of this message wants to make sure that he has selected the correct needle profile for a particular engine that has been modified."

It's all very well to try to set up carbs statically but you need to know what is happening at full throttle under load to get a full picture of a needle profile. Believe me; I got my carbs set a treat at tickover and revving to 3000rpm, but after a session on the rolling road the needles we finally selected bore no relation whatsoever to the ones we started with and I ended up with an extra 5bhp which is a lot on an 1100cc vintage engine! What is more the original needles were weak at full load and that can be very damaging. Remember we're talking about modified engines here with no needle starting point. A rolling road can be a great investment. And this is before we start on timing.

I'm not one to spend money unnecessarily. This is my experience not theory but many thanks for your letter only a part of which I have reproduced.

Thanks, JH


Date: Fri, 19 Jul 1996
From: Dr G W Owen
To: BRITISH-CARS
Subject: Rolling roads, modern fuels and ignition timing

My experience of Rolling Roads is similar to Johns. I arrived at the rolling road with about 38 bhp at the wheels for an 1100cc vintage engine and left with 48!!! It was also more driveable. More significant than being able to properly jet the carbs (AMALs) was the effect of getting the correct ignition timing. Various books outline the settings for a JAP engine of the period from 38 to 42 degrees before, I found using the advanced retard lever that my engine was smooth at about 42 degrees - I have a higher compression ratio than standard (which means less advance because the flame travels quicker) BUT MOST IMPORTANTLY, I LIKE EVERY ONE ELSE, HAVE TO USE MODERN FUELS.

Modern 4 star (leaded gas to our colonial friends) has a significantly higher octane rating than the pre war fuels - this allows us to put up the compression (and when you do, one assumes the timing stays the same - but it doesn't). But the burn of higher octane fuels is of course slower at lower compressions (more advance needed). This is why engines today always seem to go better with a few extra degrees of advance.

To contradict all this debate, we put the car on the road and found that peak torque is at 33 degrees! This is due to the increase in the compression ratio from 6:1 to 8.5:1. I now use 36 degrees as I have raised the compression further (9.5:1), but to do this I have used domed top piston which give a crescent shaped burn path (much longer - hence more advance)

BASICALLY IF YOU USE MODERN FUELS YOU HAVE NO IDEA WHAT NEEDLES AND JETS OR IGNITION TIMING TO USE UNLESS YOU SET THE CAR UP ON A ROLLING ROAD. And remember where else do you get an extra 10-20% power for less than 100 pounds.

Final point if you do go to a road, make lots of notes for future reference, if you are worried about the cost then trade advice in the bar for a beer or two and make a few quid back

Geraint Owen


Date: Fri, 30 May 1997
From: Dr G W Owen
To: british-cars-pre-war
Subject: SU carb swirl pots and supply pressure

Two technical questions on SU carbs

1. What sort of fuel pressure and flow rate do you need for a pair of 2" SUs?

2. What experience has anyone had with fuel surge from or to the float bowl during braking/cornering/acceleration? I have seen float bowls with surge/swirl pots. Are they really required?

Thanks

Geraint Owen


From: J Harkness
Date: Sat, 31 May 1997
To: british-cars-pre-war
Subject: Re: SU carb swirl pots and supply pressure

I run 1.75 SUs on my TR3 vintage racer. Run pressure control at 3 psi. Have no idea about flow rate. One thing I have found necessary is a vibration damper mount. I made my own from aluminum stock and a 2" ID O-ring. Similar in nature to the ones available commercially for Webers. Replace the pressed composite spacer with the aluminum/O-ring. Also need longer studs with springs under the nuts. Found shortened sections of TR overdrive springs work very well. Otherwise, vibration would keep fuel shutoff needle valve open and fuel would pour through into my overflows. Could feel it in performance loss also. If you need more detail, give me a call or e-mail.

John Harkness.


From: Hugh Barnes
To: british-cars-pre-war
Date: Wed, 27 Aug 1997
Subject: Re: SU Carb sought

David Laver wrote:-

>I got my SU from the local scrap yard off a fairly recent mini. It has a black
>plastic dash pot 'threaded bung thingy with the damping piston on' but I'm told
>the early brass ones fit and you could transfer it from your old one. The only
>modifications necessary are to block the vacuum advance holes. I have a blue
>spring and a number 6 needle (having made a survey of moderately hot sevens).

I didn't realise that mini ones were of the correct mounting flange. The horizontal mounted ones seem much rarer than the vertical mounts (which always seem to turn up). As far as needle is concerned, I have no idea. When the car was last on the road (too long ago - don't ask!) I remember it seemed to perform quite adequately and returned 40 mpg or so. Even in the incredibly knackered state it appears to be in, it last saw active service on a Ruby that did a sub 24 hour run Lands End - John O'Groats, so it cant be too bad!

Hugh


From: David Laver
Date: Wed, 27 Aug 1997
To: Hugh Barnes
Subject: Re: SU Carb sought

> I didn't realise that mini ones were of the correct mounting flange. The
> horizontal mounted ones seem much rarer than the vertical mounts (which
> always seem to turn up).

These carbs are mounted at a 45 degree angle. You can buy adaptors for downdraft manifolds and complete manifolds. The 'complete' type are not suitable for an exhaust of the 'straight out the side' type - it fits but a bit too close. Contact Lois Raeburn 01342-850-422 , High Buckhurst, Mark Beech, Edendbridge, Kent. TN8 5NS. Manifold 22, adaptor 12, 'all in one' manifolds 32, 10% p&p. They do other bits and will send a list in return for an SAE.

> As far as needle is concerned, I have no idea.
> When the car was last on the road (too long ago - dont ask!) I remember it
> seemed to perform quite adequately and returned 40 mpg or so. Even in the
> incredibly knackered state it appears to be in, it last saw active service
> on a Ruby that did a sub 24 hour run Lands End - John O'Groats, so it cant
> be too bad!

I get about 20 mpg but it was faster than some modified minis at Curborough. 24 hour run would be beyond it - and me !

David


From:John Hardy
To: Mark Sherman
Subject: Re: Jetting SU Carbs for Supercharger
Date: Thu, 5 Feb 1998

Mark Writes a long time ago............................

> Most of what I have read on blowers discuss a 1.5" carb with a 0.100 jet
> and an RA needle. I still have not found any recommendation for jet and
> needle for the 1.25" carb. Any information you can send my way would be
> greatly appreciated.
>
> Mark Sherman

Mark,

Might I repeat a suggestion I made some time ago on the same subject. In my experience different head modifications and camshafts give rise to different engine characteristics and therefore different needle requirements in the carbs. The best you could do is go to a rolling road with a variety of needles and sort it out UNDER LOAD. This will ensure that the needle performance is correct across the rev range with the engine AT WORK. Burlen do a needle chart quite cheaply. If you're in the US Bill Hoskins says he knows someone with a mobile Rolling Road. Possibly it's a franchise???

Yours,

JH


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