Jim: I am impressed with the details of your post. My situation is similar
in that I purchased a TR6 with TR4 carbs(HS6's),GT6 Distributor, and a PI
Cam. I won't waste anymore Broadband on the details, but you should be able
to tell something about the cam by the amount of vacuum, around 17"---18"HG
being pulled at your best idle @ the 700 or 800rpm if that dosen't look good
check the lift and duration of I and E valves.
Sounds like you have good understanding of the SU needles,Jets, and floats,
The TR6 engine does not sit level either, the front carb's bowl is mounted
forward of it, making a higher level of fuel in the jet and the rear carb's
bowl sits aft of it causing a lower level, I too don't like the level of the
front carb as it is to close to the top of the Jet it would cause the
forward carb to go more rich upon accelleration or up a steep incline, and
the rear carb would go lean but I don't notice this in driveability.
Timing is critical with regard to your cold nature engine as well. The lack
of advancement or too late of timing will also cause the cold nature that
you describe.
I am guessing that if you have the correct Jet and needles your problem
would be corrected.The Triumph 2000 came with HS4's fitted with .090" jets
and needles coded ABL, I would expect that profile would work for you.If
there is a wild cam and you don't know the tappet clearance you need to be
up for a serious challenge. Joe Curto in NY http://www.joecurto.com/ or
Burlen Fuel Sytems in the UK http://www.burlen.co.uk/ should be able to set
you up.
I am not a GT6 expert and this is my .02 so any corrections please chime in.
Enjoy the Challenge
Brooks
76 Spit
74 & 76 Six
-----Original Message-----
From: owner-triumphs@autox.team.net
[mailto:owner-triumphs@autox.team.net]On Behalf Of Jim Muller
Sent: Tuesday, October 15, 2002 8:16 PM
To: triumphs@autox.team.net
Subject: SU query please (longish)
Some help/info, please.
I've been chasing an issue on my GT6 since I purchased it. Some
background: A PPO (Previous Previous Owner) installed HS4's. When I
took delivery the PO admitted that an LBC "specialty shop" had
screwed it up in preparation for his sale, and at that time it
wouldn't rev above 3000 rpms. He knew very little about cars and
couldn't tell me much technical.
I've gotten it to run pretty well, but it still has a problem. Idle
is okay, not superb, and no amount of fiddling seems to make much
difference. At full-throttle it runs like an escaped pig at a pig
roast, but at part-throttle (which mostly means cruising) it acts
like it is too lean. I can feel it miss frequently, and the exhaust
always smells like incomplete combustion. It also takes more time
than I would expect before I can push the choke in. Now that the
weather has gotten colder the problem seems worse. And it has always
been a bit tough to launch, requiring rather more starting rpms than
I would expect, especially when cold. (I'm beginning to wonder if
the previous PPO also installed a non-stock cam. It sure had a non-
stock, noisy muffler.)
I've checked for air leaks (can't find any but I'll check again,
maybe re-seat the carb/manifold spacers with some appropriate glue-
like stuff), replaced some stuff like throttle shafts, jets (one was
leaking), etc. Incremental enrichment hasn't helped. A
knowledgeable LBC person once observed that the needles weren't
matched and that at least one was for an emissions-controlled MGB.
So one possible solution is that the needles be replaced. I've seen
tables of SU needle numbers but I would have no idea how to select
good ones for this application. Any advice would be appreciated.
Think of it as a displacement-challenged TR6 engine. :-)
Another possible issue is the float level. I know the procedure,
tweak the float arm until such and such a drill bit fits under the
arm, etc. But a GT6 engine points uphill at the front. Unless the
carb mounts accomodate this, this means that on the front carb, which
has the float bowl in front of the jet, the fuel level would be
slightly high w.r.t. the jet. On the rear carb, the fuel level would
be slightly low. I don't recall how or even if I ended up dealing
with this when I last looked at the float levels. The manifold mod
(drilling and installing new mounting studs to hold the SU's) was not
done with super alignment. This brings me to my SU question.
How does float level affect the behavior of an SU- or ZS-type carb?
On an SU, both the choke and the mixture adjustment simply pull the
jet down. The needle is tapered but the jet itself is a constant-
bore cylinder, no? So the effective opening of the jet is increased
only over that portion of the jet near the top, specifically over the
portion within the range of motion of the jet's top surface. The
fuel level doesn't change, so the needle width at the top of the fuel
level stays the same, and this is where the gas is drawn from. Thus
the choke position and the mixture nut really make only a small
difference in how easily the gas is drawn up. The only thing I can
think of that modifies this explanation is that when the engine is
running, the effective fuel level is at a dynamic position above the
top of the jet. Here's an ASCII picture.
_\ /_ <- position of jet top w/lean mixture setting
|__\ /__| <- position of jet top w/rich mixture setting
| \ / |
|~~~~\ /~~~~| <- fuel level
| \/ |
| |
Now consider the float level. The gas has to be drawn up against
gravity, so the height between the at-rest fuel level and the venturi
would seem very important. If one carb's float level was too low,
that would make drawing its gas difficult, and no amount of mixture-
nut tweaking would fix it. So a low float level is a possible cause
of a carb seeming too lean. I do know that when I look down into the
jets, one of the fuel levels is noticeably lower than the other. But
I have no idea how this translates into actual running, whether the
various balance in pressures makes this show up more at cruising or
accelerating or what. Maybe even one level is too high and the
mixture nut is compensating for it the opposite direction!
My only data is how the car behaves. An air leak would show up more
at part-throttle than full, but I can't find any. So I'm thrashing
around for more knowledge. And I don't believe this carb problem is
electrical!
If anyone can give me insight into the voodoo of SU needle-ism or
explain the practical results of float-level error, please do so!
Tanks,
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
/// triumphs@autox.team.net mailing list
/// or try http://www.team.net/cgi-bin/majorcool
/// Archives at http://www.team.net/archive
|