Chris wrote,
>So, my car is back from the shop, with little improvement. This problem
>is not new, but I thought that it could eventually be tuned away. Guess
>not. My mechanic tried various different things (including changing the
>accelerator pump position), checked for leaks and such, and found no
>apparent problems with the carbs (Zenith downdraught, 36 WIP3),
>ignition, timing, or motor in general. This is a real head scratcher.
>Here's what he thinks might be contributing to the problem: as the
>carb's come from the factory, they are set so that the initial throttle
>opening (which is a function of the accel. pump, right?), is very lean.
>Upon an engine rebuild (which I think mine has had done), the cylinder
>size is typically increased, so that if the same jet as before is used,
>the mixture is now too lean, causing the 'burp' that happens with the
>initial throttle application. Does this make sense? Or is it possible
>that somewhere along the way the jets in the carb were changed? Short of
>getting a box of various sized jets and spending a few days on it, is
>there some 'easy' way to check this? As I said before, other than that
>little 'burp,' it runs great the rest of the way through the RPM range.
>Idle is fine, too. Makes a good case for fuel injection...
>
>Thanks for all the input!
>Chris
>'62 SII
You dont say whether your burp is a transitional problem that soon goes
away, or if
the problem is a hole, that exists any time you are at that RPM/throttle
position.
Here are some things to look out for.
1) With a quick opening of the throttle, do you get a consistant and steady
stream
of fuel from the accel pump jet into both carbs (consistant and equal)?
2) Do both accel pump cams contact the accel pump level at the same throttle
position?
3) Are the carbs syncronised?
There are two possible likely problems with the symptoms you describe.
One "the obvious" is that you have an accel pump prob.
This circuit provides a throttle rate dependant enrichment of fuel for a
sudden
loss in manifold vacuum when the throttles are opened. The accel pump
system
is a cover up for caburettors in that is provides instant fuel in response
to
a throttle change where the main and pilot jet would not allow fuel to flow
fast enough to satisfy the changing requirements.
Problems here usually only last for half a second or so, then revert to the
main
and pilot jetting for the engines fuel source.
Likely sources of grief here is a worn pump piston, stuck piston
bad or missing pump check ball/valve, plugged of leaking accel pump nozzle.
The second possible problem lies in the pilot/progression system.
This system provides idle "pilot" fuel for the engine to run with no load.
It also acts as a throttle position based metering system, that uses several
small holes drilled beneath the throttle plates to add progressively more
fuel from the idle jet at larger throttle angles.
Problems here manifest in reproducible holes or bogs that dont go away,
and occur across a range of throttle opperation.
Likely sources of problems here are pluged progression holes, plugged pilot
jet, plugged pilot air bleed, loose pilot jets.
I would put a new set of plugs in the engine, produce the problem as many
times a practical in a shor time, then inspect the plugs.
This may alow you to find the side of the engine that is causing the problem
if the above advice doesnt pan out.
Good luck, and oh, Contact me when you are serious about injection.
Jarrid Gross
This archive was generated by hypermail 2b30 : Tue Sep 05 2000 - 08:59:39 CDT