Problem with SU No.2 !

[genneton4]

I have an SU conundrum. Can anyone identify my problem or indeed has anybody suffered the same.

I have swapped out my stranglebergs for triple rebuilt SU's (new bushes, spindles etc etc) on a brand new manifold.

Carbs one and three are acting normally but Carb two has way too much air. Carb's one and three are at 1.5 on my meter and two is nearly off the scale !

I had the problem initially and thought it was the slight play in the butterfly spindles. So I swapped out the body for another that has been reamed and re-bushed. Have just put everything back together and the problem is still there.
Even if I screw the slow running jet right in on Carb two it makes very little difference.

Does this strike a cord with anybody ?

Thanks - Keith

[Simonpfhc]

Hi Keith,

If the slow running screw is screwed right in and it’s still taking in air, the butterlfy can’t be closed. That’s the only source of vacuum.

[tinworm]

If you can screw the air screws down to the bottom and the engine still runs on the affected cylinders you have a massive air leak from the manifold somewhere - first try blocking off the brake vacuum circuit and see if that changes anything.

Barrie

[ALAN COCHRANE]

Keith

Try this-screw all the air screws in fully. Check the relative heights-they should be the same.
During the carb rebuilds I discovered that my rearmost carb screw sat way higher compared to the other two. The reason being the screw housing thread did not continue all the way down to the seat. Unbelievably this had almost certainly been the case since 1961! The screw recessed down fully once the thread had been cut all the way to the seat. I had carb tuning issues for years because of this.
This may not be your particular problem but it's worth checking and easily fixed.

[genneton4]

Have checked a few things this morning.
All slow running screws are the same height. Seats all look good (slightly scored but nothing nasty).
Butterflies look really tight and are actually holding fuel at the bottom. Butterfly bevel is toward engine.
On the USA site someone did have this problem and tried swapping domes around etc so isolate the problem. Could there be any mileage in this ?
I am at a loss !

Keith

[mgcjag]

Hi Keith...a friend had a similar problem on an MG ......turned out to be a very weak dashpot/piston spring....you could try swaping around....Steve

[genneton4]

Hi, I swapped dashpots, springs and needles over from carb 3 to carb 2 and problem remains. Carb 2 sucking like billyo.

Keith

[MSM]

Hi Keith

Instead of trying to close down No 2 to match the airflows of 1 and 3 is there any mileage in trying to open up 1 and 3 to try and match No 2? You will probably end up with a high tick over but it may be more even and possibly a better starting point.

Presumably at the moment the tick over is appalling due to the mismatched airflows. You haven't actually said.

[rswaffie]

If you block the vent in the dashpot on no.2, does the airflow decrease?

[MarkRado]

Did the car run well with the strombergs?
Start trying to get a good idle. I assume that valve clearence is ok and there are no manifold leaks (if you are not sure about brake servo, blank off). Make sure that ignition timing is around 10 btc static. Jets approx 1,6mm down from the bridge, float levels ok, needle valves working, standard fuel pump (pressure). All mixture / idle screws set equal. If everything is fine up to now and the car running well, and you use this sync tool
,
you should get values of 5 to 7 on all carbs for an idle of around 700.

[genneton4]

Thanks for all your input. Problem now solved.

It is silly.

Looking very, very closely at the supposedly new butterfly flap/disc, it had a slight graduated bulge on the chamfer. Manufacture/chinese copy ?

I had even tried another HD8 body and the problem was the same. Should have twigged really.

By the way. I had the body re-bushed as a matter of elimination by 'Carburettor Exchange' Leighton Buzzard. Very pleased.

Thanks again.

Now I can enjoy my summer ... oh it's raining again ....

Keith

[politeperson]

I has this problem once.

It turned out that the throttle-plate was not fully seating closed. You could see light from a torch shone behind it at the top and at the bottom of the plate.

I assume the plate had been tightened on the spindle when the throttle was open and was not centered on the spindle in relation to the carb body.

Unfortunately the screw heads are the engine side of the spindle.

So to fix it I had to remove the carb body, slacken the two screws, jiggle the throttle-plate so it seated against the carb then re-tighten and reassemble it all.

After that it was straightforward to get an even air flow through all 3 at idle.