I’ve adjusted my SU carbs to 800 rev while at idle. As I drive the
car the rev’s creep up to between 1200 and 1500 when I bring the
car to idle. Even when I hit the gas pedal the rev won’t drop down.
Any suggestions?–
herb s
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
Was the car fully warmed up when you set the idle mixture?
Eric
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from herb s sent Wed 11 May 2005:
I have a few thoughts …
Adjust your idle after you’ve run it around a bit and it’s creeped
up to 1500 or whatever. Then don’t touch it again. When you start
the car up from cold use the choke just to get it started and then
push it off and hold the RPM’s up with your foot for a minute or
two, then either drive off, or turn it off, but try to avoid
sitting at idle in the driveway any longer than you would at a stop
light.
My theory is that the car idles up after you burn some of the
carbon out of the cylinders and off of the plugs and everything
gets good and warm. Idling produces more carbon deposits than
driving and that causes fouling and that leads to more fouling,
etc.
I would also think about putting in some new plugs (Champions) and
having the air/fuel mixture set with an analyzer if you have SU’s.
If your plugs are carboned up there is something causing it, if you
do what I say and don’t idle it, but just drive it around for the
day then pull the plugs and they’re clean, then your mixture is
probably OK, if they’re black then you mixture is likely off.
If you have Strombergs check to see that your diaphragms are in
good shape, they can weaken and then they balloon up when they go
to lift the pistons and that can cause a piston delay that makes
the mixture to go rich.–
The original message included these comments:
I’ve adjusted my SU carbs to 800 rev while at idle. As I drive the
car the rev’s creep up to between 1200 and 1500 when I bring the
car to idle. Even when I hit the gas pedal the rev won’t drop down.
Any suggestions?
–
Doug 68 OTS, 72 240Z, 91 Miata
WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
First thing to do is to clean and lube all the connections in the linkage.
Check spring tension, replace any spring that’s weak, make sure none of
them are fouling.
Finally, you may need to synchronize the throttle plates, meaning you have
to make sure that they close tightly at idle. The procedure is in Bentley,
but in a nutshell: With the throttle in idle position, undo the throttle
bellcranks by loosening the clamping nuts. Press the throttle plate tightly
closed, and retighten the clamping nut. Repeat for all three carbs.
Mike Frank
At 07:40 PM 5/10/2005, you wrote:
I’ve adjusted my SU carbs to 800 rev while at idle. As I drive the
car the rev’s creep up to between 1200 and 1500 when I bring the
car to idle.
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from herb s sent Wed 11 May 2005:
Herb
The bells are dirty.
Tiny bits of grit jam the piston slightly.Tapping the gas jiggles
them loose & the idle returns to 800.
Clean the insides of the bells(you can look down the throat if you
want to see which one it is) & the rim of the pistons.
Mike B–
The original message included these comments:
I’ve adjusted my SU carbs to 800 rev while at idle. As I drive the
car the rev’s creep up to between 1200 and 1500 when I bring the
car to idle. Even when I hit the gas pedal the rev won’t drop down.
–
E type Mike
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from herb s sent Wed 11 May 2005:
Another possibility:
I am experiencing a similar type of problem which may be associated
with the tach itself. Form research on previous forums, it may be a
faulty diode.
Cold, my tach appears to run fine. Hot, it reads high: idle around
1100 rpm, at speed some 1500 rpm higher than normal.
I will address this issue within the next two months, and post my
findings.
Cheers,
Bruce–
The original message included these comments:
car the rev’s creep up to between 1200 and 1500 when I bring the
–
RB '69 ots
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from herb s sent Wed 11 May 2005:
There have been some good ideas here but I will add
another…keeping in mind that 90% of carb problems are in the
ignition (although the triple SU set up may put paid to this
saying), it is possible that the mechanical advance is worn and
hanging up. It is a very common problem in my BMW R69S from 1965.
When the advance jams due to weak springs or jamming pins, it will
eventually not return to the idle settings while the engine is
running and causes the engine to idle fast. After the engine stops
spinning, the weak parts are able to pull it back into position and
it will idle upon start until it runs up again. Another possibility
is a vacuum leak.–
The original message included these comments:
I’ve adjusted my SU carbs to 800 rev while at idle. As I drive the
car the rev’s creep up to between 1200 and 1500 when I bring the
car to idle. Even when I hit the gas pedal the rev won’t drop down.
–
Brian Caro
Newport News, VA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
Mine used to do that. I assumed some part of the linkage was expanding as the engine heated up and was causing one of the throttle plates not to close fully. When mine did that I could drop the idle to spec by very quickly blipping the throttle. It still does it occasionally but not very often, not often enough that I am going to try to fix it.
As was mentioned make sure the throttle plates are all opening and closing together and that all are closing completely when the accelerator linkage is at idle. I’ve set mine up with just a slight amount of play in the bell cranks at idle, that is when you step on the gas the linkage moves ever so slightly before the slack is taken up and the throttle plate spindles begin to rotate.
Things to check for (some more likely than others):
Make sure there is no binding anywhere in the linkage, from the accelerator pedal to the spindles.
Check to make sure the linkage is not contacting the frame rail under the carbs. Can happen if the motor mounts are compressed from age. Usually this happens at wide open throttle and causes much excitement when it happens in traffic.
Could be grit or some kind of deposits on the carb piston rim or inside the vacuum bells. Clean with carb cleaner and be sure to mate the same piston and bell when reassembling. DO NOT use an abrasive (like emery paper) here.
Make sure the jet is centered and the piston falls freely. An off center jet will cause the needle to bind when the piston drop.
Make sure when the engine is hot and the choke is off that you have a little clearance between the fast idle lever and the adjusting screw.
Make sure each piston damper that screws into the top of the bell is in good order, check for a bent shaft.
With the engine hot and the engine not running open the throttles by hand by moving the linkage that controls all three carbs. Watch the individual linkages at each carb. You want the spindles to all start rotating at exactly the same instant.
When the engine is hot and the idle is high try closing each throttle plate individually by pushing on the bell crank attached to that carbs spindle and see if the idle drops. Seems like that would tell you if the problem is in a single carb.
There are probably other things to check as well but these few come immediately to mind.
John
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from emalossi@austin.rr.com sent Wed 11 May 2005:
In responce to Eric question the car was warm when the carbs were
adadjusted.
Herb–
herb s
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php
In reply to a message from Walker, John JR SITI-ITIROA sent Wed 11 May 2005:
My father & I were experiencing the same problem…
Turns out that it was a combination of 2 things:
We had needles of different sizes in the carbs. He
neglected to tell me that he broke one of the needles and
replaced it with a new one from a rebuild kit, which
happened to be a different size than the ones in the other 2
carbs.
the jets weren’t centered, so it was causing the needle
to bind ever so slightly and the piston wouldn’t drop to the
full bottom travel amount.–
The original message included these comments:
Make sure the jet is centered and the piston falls freely. An off center jet will cause the needle to bind when the piston drop.
–
Doug Abel
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php