High idle, a bit hesitant on acceleration

[hanvyj]

Hi,

Just finished a lot of work: 1275 engine, HIF44 & metro inlet, large bore LCB. It pulls really well at higher revs but just seems a little bit hesitant when you put your foot down, especially at lower revs (you can feel the acceleration noticeably increase as the revs go up). I've also noticed it doesn't like the low-end of the gears ie, 22 in 4th was ok with the 1089 but with the 1275 it is a little bit juddery and I have to drop to third, low torque?

The idle isn't dropping - it did with the hif38 I had fitted for a few days so I suspect I need to tune the carb. The throttle cable isn't too tight, but I can't lower the throttle any more using the adjustment screw.

Its also a little more reluctant to start, turning over for quite a while.

It feels like something is off, any ideas on what it could be?

[bmcecosse]

What needle in the carb? Have you tried heavier oil in the carb damper - 20w50? A standard 1275 should romp away at 22 mph in top gear......

[hanvyj]

BFM which according to the SU Needle Compare-o-rama is very similar to BDL which I understood to be pretty standard. Currently has 20W50 engine oil in the dashpot.

The throttle butterfly has one of those poppet valves on - wasn't that an economy thing? Might be the cause? (never had one before)

[hanvyj]

Could there be an air leak? There aren't any obvious ones - is there some way to diagnose air getting in where it shouldn't be? (Spray it with WD40 or something to try temporarily block it up and check for a drop in revs?)

[bmcecosse]

Oh yes - you should have removed the valve and soldered over the hole.......it was an emissions thing - not economy - to ensure a good inrush of air when the throttle is closed down from an open position.....

[MarkyB]

Is there a fast idle adjustment on the carb, I can't remember, if there is maybe it's keeping the tick over high?

[hanvyj]

If there is a fast idle I'm not aware of it! How does it work if not by opening the butterfly a little like the normal (?) idle screw?

[hanvyj]

Unless the fast idle is the idle when the choke is on?

[bmcecosse]

Like any other SU - there is an idle screw - and a fast idle that lifts with the 'choke' lever.

[hanvyj]

Annotated an image grabbed off the internet, but mine is in the same configuration (idle screw on 'minimum' the screw against the choke cam is screwed out enough not to be keeping it open. The linkage is resting on the body of the carb - so unless the fast idle screw isn't what I think it is, it must let air in through a different area (around the butterfly?)

[bmcecosse]

It draws air through the little valve on the throttle plate - remove it. And through the breather tube to the crankcase. Maybe the throttle plate is not seating properly - if you hold it up to a bright light and look in - what do you see?