3.5 SU carbs... a bit flat...

evening All, finally got round to firing up the old beast (being in storage for about 4 months), fired up only after a few cranks once the fuel had drawn through, under no load, seemed to run ok but on a run it ran like a bag o ****e.

put some fresh fuel in it, but still backfired under load. eventually pulled over and did a few checks (checked for damp on dizzy cap, rotor arm, HT leads etc.). Upon checking the SU's noticed there was no oil in the dampers (So this will be topped up at next visit.....) but once I'd checked them the backfiring stopped on the way home but it did feel "gutless", its being like this before I put it away in storage, I remember last year in the winter when I was playing in the snow, it seemed to be "ok" when you floored it but when you pulled the choke out, it really picked up....drawing in air??

so a carb overhaul and tune up is probably long overdue. I've no problem in doing the overhaul of the SU's myself but wondered if anyone knew of anywhere that could do carb balancing on 4x4's (rolling road etc.??) near Bradford, west Yorkshire?

oh, one more thing, when I got it a few years ago the MOT man put it on the emissions test and "Hydrocarbons were thru the roof and an instant fail" (his words), luckily its a 1971 on the log book so he said "ah in that case it doesn't matter...."

cheers

p.s. sorry for any spelling mistakes am currently enjoying a nice red wine with Led Zepplin banging out

SU's NEED oil in the dampers, to stop abrupt movement of the pistons which causes backfiring.....

The pistons can move much faster than the fuel coming up the jet, so if the pistons suddenly jerk open, there is a large volume of air being sucked into the engine that has the wrong petrol/air mixture.

Peter

Yes, no oil in the dashpot will make it run weak, so do that first. Have you got none std air cleaners fitted as higher flow aircleaners will result in the engine running weak when the engine is underload?. Make sure that you have the correct needles fitted (they have 3 letters stamped on them at the top of the needle). Not sure what they should be but they should be identified in a workshop manual.Clean out the carbs as you suggested.
Don't waste your money on a rolling rd trying to balance the carbs. They are balanced at idle with the air intakes removed. Balancing is only trying to get the airflow the same through the carbs at idle. Either buy a cheap airflow measuring tool or stick you ear close to each carb intake and adjust the screw on the linkage until airflow rush noise is same for each carb. If you set the carbs up with engine warm so at idle the CO at the tailpipe is ~3% , then fueling should be correct at full throttle as well. Make sure that the main jets in the carbs are always at the same height for the 2 carbs and adjust the carbs at same time by same amount when adjusting the CO level.

Cheers guys, much appreciated

I've got FZX1381 L & R with CUD1161 needles on my SU/Butec Master Catalogue, there is also FZX1382 L & R with the same needles.

Stamped 'BCP' on the needles.

Both are HIF6 carbs.

Peter

The're not difficult to set up

I have an old Crypton Syncro check which has been in the tool box for years and still does an excellent job of balancing the carbs up but other vacuum gauges do a similar job.

Next test your mixture - remove the air filter so you can see the piston inside the carb. Get your engine warm and using a small flat blade screw driver lift a piston a small amount - I mean small as well - 1/32 inch - 0.75mm.

If your engine speed rises slightly and remains there it's running a little rich
If it drops then it's a little weak
When it rises a little then drops back to the original speed it's just about right.

This method takes some practice and you have to listen very carefully especially on a V8 as the rise/fall can be almost imperceptible. I've used it for years on all types of cars. If it doesn't work, that can point to something else causing the trouble

Forgot to say - adjusting the mixture will alter the idle speed slightly so do idle speed then the mixture, then the idle, then the mixture until you're right.

SUs do go 'off' so don't just do it once and forget.

BTW - gutless could also be a stuck advance/retard or even wrong timing!

dont they have a plunger under carb to do just as you describe???(without the screwdriver)

MattMatt said:

BTW - gutless could also be a stuck advance/retard or even wrong timing!

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ah, glad you said that...... You may be right with this...... I have noticed it "pinking" under load in the top gears :doh:

zen said:

dont they have a plunger under carb to do just as you describe???(without the screwdriver)

Click to expand...

Yep correct - I can never get it to move just that little bit with those so I stick to the screwdriver. Old habits......