Tim
That is fantastic; thanks for spending the time to take a novice thru it
all. More twiddling than I ever dreamed possible!
Its a daily driver so could be the cold weather has upset it.
Cheers
Rob
-----Original Message-----
From: Tim Engel [mailto:***@***.***
Sent: 18 February 2005 19:17
To: ***@***.***
Subject: Re: [LotusElan.net] Too fast!
From: "Rob Thornton" <***@***.***>
Sent: Friday, February 18, 2005 12:17 PM
Subject: [LotusElan.net] Too fast!
Rob.
Assuming the basic carbs and their settings are correct, there are a couple
of bolt-on items that can cause a rich condition if they fail.
On each carb, there's a bolt-on Throttle Bypass Valve on the front side,
near the mounting flange, low. That may be stuck open. It's normally
closed and vacuum controlled so it only opens on high-vacuum,
closed-throttle over-run. The idea is to allow a little mixture to do an
end-run around the closed throttle butterfly, feeding the engine just
enough to slow the rate at which the rpm bleeds off. This is done in order
to minimize the emissions spike that occurs on strong-vacuum over-run. If
the valve sticks open, the effect is similar to having the throttle stick
open a bit.
If the part is defective, it's normally considered not serviceable... just
replace it. However, it's worth a try to clean it out with carb cleaner
and lube it with a light oil like WD-40 or CRC 3.36... nothing viscous.
Or just blank it off with a hand-cut solid gasket. The engine will run
better and respond more crisply when the throttle is closed without the
valve.
An easy way to diagnose it is to blank it off with a solid gasket. If the
problem goes away and the engine generally runs better, then the Bypass
Valve on one or both carbs was bad. At least contributing to the problem.
Then the decision is whether to replace the valves or just stick with the
solid gaskets.
There's also a Temperature Compensator mounted on the front side of each
carb. Look for the sorta rectangular plastic cover that runs at an
angle... high near the airbox to low near the throttle. A bi-metallic
strip inside flexes with temperature changes, moving a plunger/ plug to
open and close an air passage. When warm, the plug opens the airway to
allow some un-metered, raw air to step around the air valve (piston),
leaning-out the mixture to it's normal running setting. The carb is jetted
to produce a proper running mixture with this air passage open.
When cold, the plug closes the passage and shuts off the extra air to
provide a slightly richer mixture... sort of an automatic, seasonal mixture
adjustment supplement to the cold start enrichment device (choke). The
temp at which the valve opens varies by engine application, is set at the
factory and is not published... but roughly, more-or-less generically
~140? F).
If the plug sticks and fails to open when warm, then the mixture will be
richer than normal.
This part is also considered to be not serviceable... just replace it.
However, it can respond very well to a gentle cleaning. The plug is a
sliding fit in a bore and gets dirty. Clean it with carb cleaner and
spritz it with a light lubricant until it moves freely. The trick is to do
that without removing the bi-metallic strip itself, disturbing it's setting
or bending it.
Do not turn the screw that retains the strip or the nut that adjusts it's
pre-load. If you do, or if you suspect someone has been in there before
you, then there is no official procedure for calibrating it. However it
can be done. There's a good article posted on the Vintage Triumph Registry
website at
www.vtr.org.
You can also blank-off this device. Just turn the adjusting nut on the
bi-metallic strip... or bend the strip... so the plug is out so far it
will never close. That will solve the rich-when-warm failure mode, but
you give up the cold weather enrichment feature. But then, how often do
you drive the Elan in the Winter? In a northern climate, probably never.
But if you're in a more temperate southern climate where it gets cold but
not "wintry", then perhaps the Compensator is a feature you will want to
preserve.
Either device can result in properly tuned carbs suddenly running rich.
Together, they can really mess up the mixture. Neither device lasts
forever and given their age, they're due. The running problem you're
experiencing may be due to failure of one or both of these devices on one or
both carbs... four possibilities. I suggest you look at them all. And
whatever you do on one carb, do on the other carb as well.
Good luck,
Tim Engel
Lotus Owners Oftha North