38 replies to this topic

[bigcaddy]

Ok SU gurus.

Idles great accelerates great through all the gears but once you ease off the throttle and cruise at any speed it seems to hesitate and miss and run like ass.

Engine is a 1275 bored 20*. Stock cam

just about everything is new on this engine.

Tried 8* advance, 15*advance and no change
Readjusted the carbs many times.

Used a synchrometer to sync the carbs
Dual 11/4 SUs

I have added a Pertronix to the distributor thinking the points may have been acting up.
Took the distributor apart and regreased everything

Any ideas?

[oltonlad]

what needles are fitted in the carburettors?

it sounds like the engine is running far too lean at part throttle, you really need to get the car to a dyno before you damage anything.

[bigcaddy]

They are dual 1.25 carbs
Needles are AAC spring loaded

colortune shows blueish on cylinders 1 and 4

[bmcecosse]

Colour tune is a waste of time - can only show the idle mix - and you can set that with the lift pin...... What colour are the plugs - that's a much better indication of running mixture.

[oltonlad]

spring loaded needles in hs2's??...........i thought that hs2's had fixed needles.

[jaydee]

Infact early twins like those fitted to the Coopers and the midgets had fixed neeldes, then SU started producing bias needle HS2s, so it could be a pair of later carbs or they've been converted to bias needles.

Try richening mixture a flat and see if it goes better, it sounds like mixture is too lean, i'd try different needles richer between station 3 and 6 .

[oltonlad]

my engine struggles on the twin hs2's around 2500rpm on part throttle now that i've put the new engine in with a different cam and head, i'm not bothered though as its going on the rollers on tuesday.

[bigcaddy]

here is a picture of my carbs.

[bigcaddy]

number 1 plug looked great. number for was blackish.

[jaydee]

Carbs are unbalanced or one of the carbs runs rich.
Those carbs look in mint conditions, though its impossible to know the conditions of the internals by just a pic..but i dont think you have a worn spindle on one carb leaning it off.

[bigcaddy]

I sprayed carb cleaner around the shafts while it was running and there was no change in idle speed. Looking for vacuum leaks.


Do I need to go to a different needle? AAC is what is in them


I pulled the floats yesterday and they look fine. they are the all plastic ones that require shims on the valves for adjustment.
Also pulled and cleaned the needles again. The look great.

[jaydee]

You need to spend some time on the balancing, it will require a bit of time if you never done before.
At a guess your AAC are too lean between station 3 and 6, something you can cure by polishing a little with some 600 grit. Thats a guess because not all engines perform the same and in pennsylvania you have different humidty/atmospheric pressure.
Spark plugs can give great infos, ie you run at 2500rpm in 3rd gear for about 30 sec, then switch the engine off (please be carefull that when you switch ignition off to put the key in pos 2 again or sterring lock could engage!!) and stop the car, take plugs out and inspect them.

[bigcaddy]

I have them sync'd using one pictured below. both carbs are at 5 at idle.

[bigcaddy]

I'll get it back together and do the 30 second runs.

I'll tune that at idle first again first.

[carbon]

Do you know what type of piston springs are fitted?

The lighter pressure Blue springs can sometimes give the effect you describe in a 1275 but it also depends on the needle profile and type of damper oil used.

For twin SUs to balance the springs also need to be closely matched.