[E-Type] Carb adjusting foibles

I had a bad day! In the interest of setting a how-not-to-do-it example for
other amateurs, and provide amusement for the more experienced hands here are
a few of the problems I experience setting my SU carbs:

  1. When you’re reading the idle speed on a 3.8 from an electronic tach-dwell
    meter, its better to have it set to the 6 cylinder scale versus the 8
    cylinder scale.

  2. If you’re setting the mixture using a vacuum gauge, and the vacuum gauge
    hose gets across a hot part of the engine unnoticed, and slowly melts until
    it seals off the hose, you will get astonishing results as you richen and
    weaken the mixture and the guage becomes increasingly less ensitive with
    greatly increasing amounts of mixture enrichment!

  3. It sure is easy to mistake the mixture screw for the fast idle screw.

Mike Moore
63 OTS

Mike,

I feel your pain!

Jerry

Jerry Mouton “Laissez les bons temps rouler!”

I had a bad day! In the interest of setting a how-not-to-do-it example
for
other amateurs, and provide amusement for the more experienced hands here
are----- Original Message -----
From: MMoore8425@aol.com
a few of the problems I experience setting my SU carbs:

Whoops.

So did you end up having to convert to -ground to get your tach meter to
work? Mine won’t with +ground.

Eric

…reminds me of something my flight instructor told me as I porpoused
the plane down the runway.
“There are those who have done it, and there are those who will do
it.”
LLoyd

MMoore8425@aol.com wrote:>

I had a bad day! In the interest of setting a how-not-to-do-it example for
other amateurs, and provide amusement for the more experienced hands here are
a few of the problems I …

Out of curiosity, could you explain how you connect the tach? I have one
of those Sears dwell tach meter things, probably from the 70’s, but no
instructions. I’ve never used it because I never knew how to connect it
to the engine.

Also could you elaborate on the vacuum gauge setting method? I think my
mixture needs fine tuning, and I’m always up for a different method.

Craig
64 OTS

MMoore8425@aol.com wrote:>

I had a bad day! In the interest of setting a how-not-to-do-it example for
other amateurs, and provide amusement for the more experienced hands here are
a few of the problems I experience setting my SU carbs:

Mike:

Don't feel bad. At least you didn't forget to put the springs back in 

after you’ve cleaned the pistons and bells. Or forget to tighten the
bellcranks after adjusting the throttle plates.

Mike Frank

This is a very timely post. I was in the garage early today with my 15-year
old son to start up the Jag for the first time with it’s new-to-the-car
triple SU’s.

After double checking all the hose connections, and giving the engine bay a
once-over to see if anything was obviously missing, and having charged the
battery for an hour previously, I turned on the key while my son watched for
leaks with his keen eyesight. The fuel pumped into the bowls and…LEAK at
the rear banjo bolt! Quickly tightening, and wiping off the fuel, we tried
again. After about 5 seconds the engine started. We ran if for a while
with the choke on, gradually reducing it until it ran without out the choke.
Tightened the clamp on the top radiator hose…leaked stopped. Here’s
where things got weird.

We pulled out the Uni-syn gauge and tried each carb. The front and rear
carbs were identical, with the ball near the same place on the gauge. But,
the middle snapped the ball right to the top of the gauge…way too much
suction. We tried to adjust the slow running screw on the middle carb but
it didn’t make a difference, even after it was closed all the way down.

Then, after a quick consultation with a friend who used to tune cars for a
living, we loosened the throttle linkage! DUH!? I felt sure this was the
cure. But, it didn’t seem to make a difference, either.

Finally, were able to get the Uni-syn to read the same on all three carbs by
adjusting the slow idle screws on the front and rear carbs out many turns.
This raised the idle speed of the engine up quite a bit. And it didn’t seem
right that the slow idle screws would not all be in the same relative range
as far as turns out from bottom (two full turns).

Then, I tightened the throttle linkage. I didn’t separate the choke linkage
between the carbs. (Should I have?) Then I tried to adjust the mixture,
first on the front carb. Whenever I lifted the piston the the engine would
slow down. So, I kept lowering the screw to enrich the mixture, with no
noticeable change after lots of turns, maybe six total. I removed the
dashpot and reset the jet flush with the saddle, and backed off 2.5 turns to
start over.

Next, I tried the same mixture adjustment on the middle and rear carbs, and
the same thing happened. That is, when the piston was lifted slightly
(1/32") the engine would slow, so I screwed the mixture screw down to
enrichen it and nothing happened, even though I was waiting about 30 seconds
between adjustments.

One possible concern - I felt that the center carb’s piston was not dropping
with the same speed and “clunk” at the bottom as the front and rear carbs.
So I took the piston out, looked it over, tried it a few times, and
performed a suction test by closing off the vent holes and inverting it to
see how long it took for the dashpot to drop off. It took about 8 seconds
which I felt was OK. After putting the middle carb back together it still
didn’t seem the same as the other two. It drops faster, and there’s no
“clunk” when it reaches bottom.

That’s where I am at this point. HUGE success getting the car started.
Less so with the fine tuning. So…any suggestions?

Regards,

Randy Olson

1969 2+2_________________________________________________________________
Help STOP SPAM with the new MSN 8 and get 2 months FREE*
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Randy,

Forget about balancing the carbs and any other adjustments.
First thing it to deal with the center carb.

No clunk = jet not centered. If bad, this can hold the piston
above its rest, and mess everything up.

So first, center all the jets again, just to be sure.

Adjust the mixtures full lean, i.e. jets at the very top of their
travel. Lift each piston and let it fall. Any that do not drop
with perfect ease and smoothness have to be changed.
Do the whole jet centering thing from the book over again
until the piston falls smoothly and clunks. If you are finished
adjusting and no clunk, start again.

If you can’t fix it to clunk, try removing the needle entirely and
dropping the piston. If still no clunk, your dashpot damper is
interfering either through bend rod or other – alternatively, the
bearing mechanism for the piston has some interference from
corrosion or damage. Get that fixed.

Once you get full rich clunks everywhere, rebalance the
carbs. I find visually works super. Just watch the fast idle
screws very carefully to detect when they just begin to budge
and insure that happens at the EXACT same time. If this
procedure does not result in evenly balanced carbs using
the Unisyn or other tests, then you have other problems.
Verify nut tightness, vacuum hose tightness and integrity,
and gasket integrity downstream of the carb. Any of those
could be letting more air in beyond the carb’s control.
If all that is perfect, your butterflies may not be installed
properly.

To fix the butterflies you have to remove the carb
and loosen the two screws holding them in, just a little till they
can move around in their shaft. Close the carb and make the
butterflies move until they completely close up the throat. At best,
little or no water will pass through when you hold them closed
and put water in the throat. Then reassemble and balance
using whatever method you wish.

While you are in there check the shaft for clearance – there
should be seals in there that eliminate any bleeding of air in.
If there is any slop, get them reworked by Joe Curto or another
experienced person. Or try doing it yourself.

Make sure the Choke linkage is not applying any force to the
jets or the fast idle screws. In fact, you can just unscrew the fast
idle screws way out of the way until the basic carbs are well
adjusted.

Then you have to get the idle down to about 500 RPM before
you adjust the mixture. Maybe 800 will work, but you should be
able to eventually get a nice smooth idle at 500 after doing some
preliminary adjustment. Then do the book procedure. You
have to observe the RPM very closely to detect the rise nd fall
that the book prescribes. You can go back in the archives and find
a lot of tips from many people about alternative ways to adjust.

But you will never get anywhere trying to work around some
problem, and adjusting from there. Everything has to be perfect
(including points, plugs, timing, valve settings, etc.) But the
payoff is you’ll never have to fool with them again. BTW, after
they are perfect, never adjust them again – the problem is
elsewhere unless you see fuel pouring out.

Jerry

Jerry Mouton “Laissez les bons temps rouler!”

HA!! (I’ve done that too)
LLoyd

Michael Frank wrote:> … At least you didn’t forget to put the springs back in

after you’ve cleaned the pistons and bells.

Mike, I had to laugh at your list – especially #1.

Been there, done that :slight_smile:

Ray Sharp
'61FHC
'71s2OTS

Date: Sat, 14 Dec 2002 12:27:47 EST
From: MMoore8425@aol.com
Subject: [E-Type] Carb adjusting foibles

I had a bad day! In the interest of setting a
how-not-to-do-it example for
other amateurs, and provide amusement for the more
experienced hands here are
a few of the problems I experience setting my SU carbs:

  1. When you’re reading the idle speed on a 3.8 from an
    electronic tach-dwell
    meter, its better to have it set to the 6 cylinder scale
    versus the 8
    cylinder scale.

  2. If you’re setting the mixture using a vacuum gauge, and
    the vacuum gauge
    hose gets across a hot part of the engine unnoticed, and
    slowly melts until
    it seals off the hose, you will get astonishing results as
    you richen and
    weaken the mixture and the guage becomes increasingly less
    ensitive with
    greatly increasing amounts of mixture enrichment!

  3. It sure is easy to mistake the mixture screw for the
    fast idle screw.>
    Mike Moore
    63 OTS

In reply to a message from Jerry Mouton sent Sat 14 Dec 2002:

So…I’ve rebuilt carbs before, but reading Jerry’s last post has
me convinced that might REALLY be in over my head when I attempt
rebuilding these SU’s of mine next week. On everyone’s
recommendation, I bought the Moss video that should be here on
Monday. I hope it helps to make Jerry’s last post NOT seem like it
is in a different language. Don’t you just hate that feeling?? I
must admit, that this isn’t the first time I’ve been quite
intimidated by technical jargon, only to find out later that it
sounded MUCH more complicated than it actually was. I’m hoping to
find the same thing with these SU’s…

fingers crossed, Alan–
New E-type owner who’s learning…
Austin, Texas, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Jerry,
Does the carb need to be removed to center the jet, or can I simply try to
do it by removing the dashpot and adjusting the needle in the piston? I
believe only the middle carb is not working right, so I don’t
Randy>"Forget about balancing the carbs and any other adjustments.

First thing it to deal with the center carb.

No clunk = jet not centered. If bad, this can hold the piston
above its rest, and mess everything up.

So first, center all the jets again, just to be sure.


Add photos to your messages with MSN 8. Get 2 months FREE*.
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Alan:

     Relax, there's no carburetor that's easier to rebuild than an SU. 

If you want a real challenge, try a Carter or Rochester four barrel…

Mike Frakn

At 03:41 PM 12/14/02, you wrote:> only to find out later that it

sounded MUCH more complicated than it actually was. I’m hoping to
find the same thing with these SU’s…

In reply to a message from Michael Frank sent Sat 14 Dec 2002:

really?? I’ve rebuilt rochester four barrels before and seem to
remember them not being all that bad, so that makes me feel better.
I guess there being 3 of them makes it seem worse and these SU’s
seem to operate on a different principle than the rochester or
Holley’s do. The old ‘‘Quadrajets’’ seemed pretty simple, so I’m
hoping what you say is true.

feeling a bit better, Alan–
New E-type owner who’s learning…
Austin, Texas, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Hello Randy,

The problem you are describing is where the throttle plate of the
center carburetor is not closing fully. That gives rise to high flow
and ineffective idle air screw.

When you rebuild these carburetors you must follow the following
procedure for centering the throttle plates to their individual bores.

Leave the two screws that clamp the spindle on the plate initially
a bit loose. Then force the spindle fully closed. The plate now
slides about until it finds its best seating position. The holes in
the plate are purposely made oversize for this. Then tighten the
clamp screws fully. Then check with a strong light that no light
passes anywhere along the circumference of the plate while holding
the spindle fully closed. Then stake the clamping screws and make
a final light check.

Out-of-round throttle bodies pop up from time to time. While the bore
was precision machined at the factory, excessive heat, accidents and
uneven torqueing can take their toll during its service life. It you see
the plate closing on one or two points, additional adjustment is required.
If it closes on three or more points but still leaks light, you are out of
luck. Overboring is not an option unless you are prepared to machine
a matching throttle plate and live with inherent imbalance with the
remaining carburetors. Given the thin wall thickness of the casting,
overboring, sleeving and reaming to original would be a real challenge.
At current prices a new body would be far cheaper. Burlen Fuel still
sells them.

Another problem can crop up if the throttle shafts have been replaced
in the past. They come unbored for the pins of the external stops.
Sometimes the hole is not put in right place and the spindle stops on
the external stop instead of the plate in the bore. That gives rise to
the same problem. It is easily recognized, as on fully closed throttle,
light passes from top and bottom of the plate.

This problem affects everything else, you cannot get proper tuning
until the idle air screws are in full control at closed throttle.

Dick Vandermeyden
San Carlos, California

Think about a typical 'murican 4 bbl. Deduct the accelerator pump, power
valve, the ball bearing check valves, emulsion tubes, even the jet
assemblies. Add a piston and a needle, and a rubber mounted bushing that
serves as the jet. Divide by four. That’s an SU. A week from now, you’ll be
wondering what you were concerned about.

Mike Frank

At 04:01 PM 12/14/02, you wrote:>In reply to a message from Michael Frank sent Sat 14 Dec 2002:

really?? I’ve rebuilt rochester four barrels before and seem to
remember them not being all that bad,

Sounds to me like those “strong bergs” are easier. They take only
about an hour each (not counting the 20 minutes you let them sit in the
carb cleaner basket), and adjusting them is a snap (the dual version).
LLoyd

Michael Frank wrote:>

Alan:

     Relax, there's no carburetor that's easier to rebuild than an SU.

If you want a real challenge, try a Carter or Rochester four barrel…

Mike Frakn

In a message dated 12/14/02 10:01:51 AM Pacific Standard Time,
emalossi@austin.rr.com writes:

<< So did you end up having to convert to -ground to get your tach meter to
work? Mine won’t with +ground.

Ni, I just reverse the leads which connect to the coil.

Best Regards, Mike

In a message dated 12/14/02 10:50:34 AM Pacific Standard Time,
talbotc@telus.net writes:

<< Out of curiosity, could you explain how you connect the tach? I have one
of those Sears dwell tach meter things, probably from the 70’s, but no
instructions. I’ve never used it because I never knew how to connect it
to the engine.

Also could you elaborate on the vacuum gauge setting method? I think my
mixture needs fine tuning, and I’m always up for a different method.

Craig
64 OTS

Hi Craig,
My 1968 model Heathkit dwell tach has two leads-one black and one white. The
tewo leads go to the low voltage side of the coil-red to pos, black to
negative. I have several switch positions (hi tach range, low tach range,
dwell no cylinders).

The vacuum method as I understand it is to connect the vacuum gauge to the
vacuum line going to the booster reservoir. Adjust each idle mixture screw
for optimum vacuum. It seems to be an iterative process and it seems like you
can be off probably a couple of turns one way or the other, but I’m no expert
and I’m still trying to optimize my carbs. Of corse its necessary to
periodically check balance and speed. It seems like once the speed gets to
around 850, it goes through some instability probably due to the mech advance
coming in.

Best Regards, Mike Moore

Alan,

Sorry, not to worry!

Most of the stuff there is only a problem if something
has badly worn or has been disassembled. This does not
have to be done normally. Read through the Service
Manual’s description of the carbs, you’ll see the terminology!

The butterflies were assembled properly from the factory,
and they don’t go bad (except wear if the carb has been
abused somehow) and you never have to deal with them –
unless somebody has taken the carb apart to see how they work.
Never loosen those screws!

The jets DO have to be properly centered, this is critical.
But it’s no big deal if you follow the instructions. Better,
if you get a jet centering tool, you do them once and
future replacement jets remain centered – assuming
they are not trash. And chesk them first – don’t center
if they are already centered.

Synchronization DOES have to be done exactly, too.
That is the other challenge. But yours are probably
synchronized already, so check them before you
loosen them…

The sticking part is from abuse – not having been lubed
for years and corroding, or having been dropped,
or somebody putting the wrong piston with the wrong
chamber.

No, just do as in the manual, following always the rule
“it’s probably not the carbs” whenever anything goes
wrong. Get ignition points, timing, plugs, valve gaps,
everything else perfect – then see whether the carbs
are set right – probably they are if you have not messed
with them.

Be bold! Initiation into the Rites of the Skinner’s Union
awaits you!

Jerry

Jerry Mouton “Laissez les bons temps rouler!”----- Original Message -----
From: “Thrifty” astulberg@hotmail.com

In reply to a message from Jerry Mouton sent Sat 14 Dec 2002:

So…I’ve rebuilt carbs before, but reading Jerry’s last post has
me convinced that might REALLY be in over my head when I attempt
rebuilding these SU’s of mine next week. On everyone’s
recommendation, I bought the Moss video that should be here on
Monday. I hope it helps to make Jerry’s last post NOT seem like it
is in a different language. Don’t you just hate that feeling??