Tim,
Did you manage to fix this?
I finally got round to looking at the problem on my car this weekend, and this
is what I found. (Apologies if I use some incorrect terminology, I don't really
know my way around carbs.)
Attached to the throttle disk (is that what it's called?) on each carb is a
fork, which is acted on by the throttle linkage when you apply pressure to the
pedal. When pedal pressure is released, this fork is moved back into position
by a spring (one for each carb) attached to a tab bolted onto the inlet
manifold, until it reaches the idle-screw (or part of the choke mechanism,
although I was working with the engine hot and the choke in, so this didn't come
into play). The linkage plays no part in returning these forks to their "home"
position. The forks/disks on each of the two carbs move completely
independantly of each other.
I played with the accelerator until the engine was revving a little high, then
took a look at the carbs, and found that the forks on both carbs had not quite
moved back all the way to the idle-screws. The engine was idling at around
1500rpm. Using my fingers, I could push the fork back about another 1/32 inch,
or maybe even less, on one carb, and the idle dropped to 1200rpm. Repeat on the
other carb, and the idle drops to a perfect 900rpm.
I cleaned the entire area using a liberal amount of carb cleaner, and also bent
back the tabs the springs attach to on the manifold to get a bit more tension in
the springs, but this didn't help.
I'm now open to suggestions from the list as to what to do next. Hopefully this
will help Tim fix his almost identical problem, too.
Cheers,
Dean
'77 Spit 1500 with twin HS4s
-----------
Date: Mon, 17 Apr 2000 08:30:07 -0400
From: "Dean Dashwood"
Subject: RE: Throttle control problems
Tim,
I have an identical problem to this. I also have twin SUs (HS4s) and until this
weekend the car would often idle at around 1500rpm, with a little blip on the
throttle settling it back down to 750rpm. This didn't bother me (1500rpm is not
too much to handle) but I put it down to the frayed throttle cable. Eventually
got round to replacing the cable, but this made no difference, so I forgot about
it.
Since getting a little too close to the tyre-wall this weekend, though, the idle
speed has gone up to about 2500rpm, with a blip on the throttle usually putting
it back to normal, but sometimes making it worse, so I was planning on looking
at it in more detail this weekend. My first plan was to check the pistons in
the carbs.
If you fix this, I'd be interested to know what it was. Likewise, I'll let you
know if I manage to fix mine.
Dean
---------------
Date: Mon, 17 Apr 2000 08:30:07 -0400
From: "Bowen, Patrick A RP2" <PABowen@sar.med.navy.mil>
Subject: RE: Throttle control problems
Tim, my answer would be you have a frayed cable, but you say it is new. I
would take a second look at it anyhow.
Patrick Bowen
- -----Original Message-----
From: T. .R. Dafforn [mailto:td214@cam.ac.uk]
Sent: Monday, April 17, 2000 5:44 AM
To: spitfires
Subject: Throttle control problems
Hi Listers,
Finally after 2 1/2 years my car is legal again...
However I have just picked up a problem...
While driving back from the test station the throttle seems to stick
open causing the car to idle at 3K rpm...
Quick blip on the throttle and it dropped back to idle at 750 rpm...
Then it happened again 5K rpm...
blip and back down..
Got back and had a look at the throttle linkages, they seem fine
although the car has never had the return spring which attaches to the
header are these required? It has always relied on the return springs on
the carb...
Also noticed that the rear float bowl was leaking where the tube from
the jet screws in....
Does any one have any ideas whether the problem is mixture or sticking
throttle cable (which btw is brand new!!)
hope some one can help
cheers
Tim
PS the car is a 1500 twin SU carb...
- --
Tim Dafforn
Structural Medicine
Department of Haematology
CIMR
University of Cambridge
Wellcome-MRC Building (Level6)
Hills Road
Cambridge
CB2 2XY
Tel. (01223) 336829
Fax. (01223) 336827
http://smokeroom.cimr.cam.ac.uk/
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