I'm thinking fuel supply- fuel pump. Could be at lower speeds, supplying
enough fuel despite a problem, and above that RPM supplying enough by sheer
speed of the pump, and at 3k it balances and starves it.
Scott
----- Original Message -----
From: "Terry Thompson" <firespiter@yahoo.com>
To: <spitfires@autox.team.net>
Sent: Sunday, March 11, 2001 2:27 PM
Subject: Still sputtering @ 3k rpms
>
> I've actually been driving my spitfire a good deal the
> past month. But with the colder weather coming and
> going (~ 2 weeks ago we were up to high 60's one day
> and 30's as a high the next) I'd been using the choke
> and leaving it on. Eventually fouling the plugs.
>
> I swapped out the plugs about a week ago, balanced
> re-adjusted the carbs, checked the points gap etc. And
> the car ran smooth (In fact, it's so smooth now that
> when it's still cold, I let the choke out after a few
> seconds from start-up and the car will idle smoothly
> at <= 500 rpm until it's warmed-up and will idle at
> around 900 when fully warm.) No sputtering or hunting
> idle.
>
> But I still get a sputtering and loss of power at
> 3,000 rpms. If I accelerate past it, it goes away. But
> if you sit cruising at that RPM, the car sputters more
> and more and eventually sounds like the engine is
> going to drop off from flooding or air-starvation.
> I would just deal with it by not sitting at that
> speed, but 1) it's difficult to power past it up hills
> (you really don't get good pushed into your seat
> acceleration until you hit 3,500 rpm) and 2) traffic
> on the road seems to want to keep me doing 45-55 mph
> (which is in the 3,000 range) a great deal of the
> time.
>
> I have the static advance set to 10 deg. BTDC. on the
> low-compression '77 engine that's in the car.
> I have the mixture set to about average - 4-5% CO at
> idle. I'm using ABT needles in the dual 1.5" SU carbs.
> And the stock cam. I have an electric fuel pump and
> it's set to around 4-5 psi. And I have a 25D lucas
> points dizzy (I think it's from a Midget or early
> spit). I've tried using AAU (rich needles) and using
> the choke at speed, just to see if it's a mixture
> issue, but it doesn't seem to help (it does help
> enrich the mixture to help power past the 3,000 rpm
> mark though if I give the choke a small tug).
>
> I'm wondering if I'm running out of advance on the
> dizzy's vacuum or if the static timing needs to be
> adjusted, since I'm not using a stock distributor.
>
> I've used my mity-vac to make sure that the advance
> moves with vacuum, but the distributor advance marks
> in the books (start/final/range) are gauged in HG
> (inches of mercury) not air pressure. And I'm not sure
> how you measure the degrees of advance that a vacuum
> is giving you on the body of the dizzy(use a
> protractor?).
>
> Does anyone on the list in MD have a Mercury vacuum
> gauge I can borrow and maybe a foot to depress the
> accelerator while I'm over the engine checking the
> vacuum readings?
>
> -Terry
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