How to Build & Power Tune SU Carburetors by Des Hammill

[Sprintinbits]

Is it worth a read or only worthy of a few tips?

I'm toying with the idea of buying a copy before I rebuild my twin SU's on the Green Sprint.
Well that's not strictly true, they are on the bench, stripped, as we speak
Not much tuning to be done on this car's engine, just a Sports Exhaust, K+N's and probably a megajolt kit.

If there are any simple mods that could be done with a Dremel. Or any other tips other than clean, check wear and replace then re-adjust. I might just be tempted

All thoughts appreciated (other than swap to £300 worth of 45's -that's for the red car )

[triple tango]

just make sure they're in good order, especially throttle spindle wear. Get rid of the poppit valves either by soldering up or using solid butterflies. Keep the standard airbox.

[JonyPI]

just get solid valves, not much and then u dont have o worry about your solder egttin into the engine! plus i hought the standard airbox was crap? seems way to restrictive to me?

[Paul Geering]

But how much will you loose with K&Ns drawing all that warm air, as opposed to cooler ambient air?

Can one of you techno buffs explain why which is best?

[Neil907]

A freer flowing version of the air box is the ideal solution, I think sprint parts bo one.

[adasdad]

I am certainly not one of the sprint techno buffs, but I have tried several options for induction to the carbs. One of the problems for a proper scientific test of the effectiveness of the various filter's, boxes and hoses available is that on a rolling road you dont get anything like the amount of air forced into the engine bay that you would at 30 to 100 mph, so the engine runs hot and lean in comparison to driving conditions. I have tried std airbox with and without hoses to the grill, the Sprintparts cold air induction kit and K&N's. On the road there was little or no difference between std airbox with or without hoses to the grill, no changes in engine temp or plug or exhuast colouration. With the Sprintparts cold air induction kit fitted the engine ran slightly hotter under load and at high rpm and this showed on plug colouration too. Adjusting the mixture to run a little richer cured both and the car ran very well, better idle and more stable temp when caning it in hot weather. Then I tried K&N's, the larger ones with the internal trumpets and the mixture went weaker again and could only be made to run well on richer needles. It ran hotter and had flat spots on std needles but once the richer ones were fitted it ran very well.
I've stuck with the K&N's because to my mind if you need to go to richer needles you're getting more air in and as it's running well and staying cool I'm happy. If I wanted a standard look I would use the airbox with the Sprintparts cool air kit as my car ran better on that set up than with the std airbox alone.

Andy.

[david in South Oz]

I've found that Des Hamill's book has only limited useful info for a road-going Sprint.
There's lots about how to polish needles to alter their profiles for modified engines when you can't buy a suitable needle, and setting up the correct size SUs on highly modified engines that may or may not have had SU's fitted as standard.

One thing I've found is that the the front carby in Sprints can often flood very slightly even though the float levels are identically set. Definitely no good for that performance edge. This is because the Sprint engine isn't level, it slopes downhill from the front to the back, and the float chamber on the front carby is actually higher than the carby body, upsetting the petrol level in the main jet slightly. The trick is to adjust the float levels in each float bowl so that the petrol level in each carby jet is exactly the same. Thus you start with exactly the same mixture in each carby and its much easier to tune from there. This means that the front float level often needs to be set very slightly lower (ie the gap between the float and the lid is slightly larger) on the front carby.

David

[JonyPI]

tbh i think the whole cool air induction is abit silly, becuase the standard tubes are just plain rubbish, a nice big one maybe, but 2 littlens isnt doin much good, plus, once you get goin your having loads of nice cool air forced under the bonnet! K&Ns or ram pipes pretty much always increase performance which says it all really! although i am probably going to go the whole hog on the spit and have ram pipes and a cool air intake feeding them but it will be a BIG intake!

[tinweevil]

just a Sports Exhaust, K+N's and probably a megajolt kit

Are you sure about this on your 'nice' car? You'll get a Big increase in noise inside the car from the exhaust, another from the K&Ns assuming you mean pancakes.

The sprint I've just bought is a massively nice place to be inside, just incredibly quiet. Most of this I put down to it having a standard SS exhaust. It's pretty much standard throughout. The contrast with my rustomite is phenomenol. This is low on soundproofing, stiff on suspension, fat on the pipes and requires ear plugs for long journeys.

Why not standard pipes, K&N panel filter, magnacores & jodsport ignotion for the 'nice' car and throw the kitchen sink (tuning wise) at the red one?

Just my £0.02.

Tin

[Sprintinbits]

All interesting comments, especially the one about the fuel in the float chambers. I didn't think it would make a difference but I can see it might. Therefore, I will set the front float a little lower to compensate.

Good point Tin, the car already had a s/s sports exhaust when I bought it. The K+N's? yeah fair comment that was me
You are right, above about 3,000 RPM it gets a little loud but I don't really hold it at those revs unless I'm gunning it.
I must admit, once the red car is ready I think I'll take the K+N's off to calm down the induction noise
The megajolt upgrade was to improve driveability and fuel consumption really.
Plus I think the dizzy may not be advancing correctly. Well either that or a partially blocked jet.
Hence the carb rebuild.

I'll keep ya'll posted on my refurb, I wondered if there was anything worthwhile doing to the carbs themselves away from 'as designed' to improve flow or something.
I'd be more inclined to think that port matching and alternative air filters/boxes would make the most difference anyway.
Just going back to the shed to carry on...
Cheers

[Sprintinbits]

One thing I've found is that the the front carby in Sprints can often flood very slightly even though the float levels are identically set. Definitely no good for that performance edge. This is because the Sprint engine isn't level, it slopes downhill from the front to the back, and the float chamber on the front carby is actually higher than the carby body, upsetting the petrol level in the main jet slightly. The trick is to adjust the float levels in each float bowl so that the petrol level in each carby jet is exactly the same. Thus you start with exactly the same mixture in each carby and its much easier to tune from there. This means that the front float level often needs to be set very slightly lower (ie the gap between the float and the lid is slightly larger) on the front carby.
David

Ahh, I've worked it out whilst cleaning the carbs out
Whatever the level of fuel in the float chamber, it will be mimicked in the jet. Hence the tendancy of slightly rich or lean .Of course, how obvious
Yes, yes, yes. I get it now!
Mind you, saying that, wouldn't the fuel surge to make the mix slightly lean or rich in opposite carbs under heavy braking or acceleration?
I guess that's just a small problem on the scale of things and probably another reason fuel injection is more refined!

[david in South Oz]

Yep, you've got it. The important thing is to get the level of petrol in both carby jets identical. Here's a few other things to look for:
roll the needles around on a flat piece of glass to make sure they aren't warped or bent.
Check for any wear marks or scuff marks on the needles too, any problems buy new ones

Make sure the dashpot springs are the right ones and haven't been stretched or cut or squashed. If they have been mucked around with, or are different lengths, buy new ones. Don't try to fix them, I've spent hours and it don't work.

To check them (assuming you have SU6 carbies) cut a stiff cardboard strip that fits nicely inside the coil without touching, about 3.5 - 4 inches long. Make a line across the strip at exactly 2.75 inches. Place the spring with the piece of cardboard slipped in the middle of the coil on a set of electronic kitchen scales. Press the spring gently down until its exactly 2.75 inches long (that's the reason for the piece of cardboard marked at 2.75 inches). The scale should read 8 ounces. Now do the other spring and see if they are the same. If they are different, or they read less than say 7ounces or more than say 9ounces, get new springs. The SU part number is AU1167.

When fully assembled and back on the car, use engine oil or 20 grade SU oil in the carby dampers. Fit the damper rod and very gently raise the piston right up. Any excess oil will squirt out of the air holes in the top of the cap, so have a small rag handy.

Don't experiment with really light oil or with light springs until the carbies are set up properly. To be honest, there's a lot of myth and magic about springs and oil and increase in performance, and there probably is for the race track where the revs are constantly high and in a narrow band - but for road use I've found all it does is make the engine less responsive and idle rough.

[tinweevil]

Now thats a good tip. Presumably you zero the scale after putting the spring and card on?

Anybody know the correct figure for 1.5 SUs?

Tin

[sprint95m]

AUD4355 is spring in HS4.
(These are painted red in colour on one end.)
The load at length is 4.5oz at 1.530".

This is from the SU catalogue.

[david in South Oz]

Yep, zero the scale after putting the spring and card on.