David McCoy seemed to think that this might be of some help to some of the +4
owners. So here it is. Some general information on setting up the SU carbs.
On Mon, 25 Mar 96 07:35:31 Kendall writes:
> I was concentrating on getting my MGB running properly this weekend
> and ran into a bit of a problem with the carbs. Hopefully some of you
> with more experience than myself can help shed some light.
>
> The problem I'm having is as follows:
> When I set the mixture correctly on the back carb, according to my >
> ColourTune,
Hold the presses - ColourTune? What's that. I must admitt that I'm getting
old but I've never heard of that. All I've ever used to set the SU on any
of my cars is a unisyn, a tach and the obligotory carb wrench.
> the engine speed drops off dramatically when I try the "lift the carb >
> piston" test.
This is correct at least with the old unisyn. The idle speed should drop
since you are trying to fire all 4 cyl (or 6 in the case of the Jags or
Healeys) from only on carb (or 2 if a 3 carb manifold is used). But when the
carbs test carb is released, it richens the mixture back to normal.
> Before trying to set the mixture on both carbs, I was having a problem
> with the piston in the back carb sticking a bit. After some head
> scratching, I determined (I think) that the needle wasn't seating
> properly. It seemed to prevent the piston from dropping all the way,
> and it seemed to cause the piston to stick once it was seated all the
> way. I'm not exactly sure how to correct this problem, or exactly what
> causes this problem.
This is a very common problem with the SUs. You have to perform a process
called "centering the jet". You must back off the Large nut that buts up
against the carb housing and through which the jet passes. With the Nut
loose, wiggle the jet and tighten the nut. This lets the jet center on
the needle.
Also, you should check that the needle is correctly positioned in the piston.
There is a locking screw in the side of the pistion which holds the needle in
place. The needle is tappered from the point back to the base or seat, where
it fits into the piston. At this point the needle necks out - approx. 2 times
the dia. of the needle. This lip should be exactly even with the bottom of the
piston. The way this is set is to loosen the set screw and pull the needle
outwards a little to bring the lip beyond the piston. Now using a streight
edge, like a thick feeler guage, gently press the needle back until the
streight edge touches the piston and tighten the set screw. You might want to
tighten it slightly before starting this process to put a little drag on the
needle.
> One final symptom is that I can't synchronize the carbs without ending
> up with an absurdly high idle. The back carb sucks more air than the
> front one, even with the throttle stop backed all the out.
A couple of things to check.
a. Do you have some type of oil in the dash pots - access is gained by
unscrewing the hex nut on top of the carb (the vaccum chanber actually).
If not, do so. Almost any type oil can be used. I usually use 10W30,
However, I've heard of people using Automatic Transmission Fluid (ATF).
The thicker the fluid, the slower the responce will be to acceleration.
b. When you tried to adjust the carbs, you need to undo the set screw that
holds the choke (I'm sorry - the strangler).
c. Also you must losen at least one of the two screws on the cross shaft
that ties the 2 (or 3) carbs together. Otherwise you will also be
adjusting the other carb. Now each carb can be adjusted seperately.
Once the process is complete reconnect the cross over or connecting
linkage.
d. With the carbs seperated, the unisyn is used to sync the 2 carbs for
airflow. Once sync'ed, check the idle speed. The idle screws on both
carbs will have to be adjusted to set the idle to the desired speed.
Most books call for about a 750 rpm setting. However, I prefer 1,000
rpm. It allows the engine to pull easier when starting from a stop
and that is usually high enough that the generator will output enough
electricty to not drain from the battery.
e. I haven't discussed setting the choke. It is basically similar to the
above procedure but different. Again, both will have to be disconnected
from each other.
I hope this helps. Sorry I didn't answer before, but there appears to have
been some problem with the mail. I've only been getting 1 to 6 pieces of
mail a day last week. Untill Friday night, I got 157 pieces. Its taken
me a while to sort through it all.
I didn't see any answer so I thought I'd see if this helps.
John T. Blair WA4OHZ email: jblair@nhr.com
Va. Beach, Va Phone: 495-8229
48 TR1800 48 #4 Midget 65 Morgan 4/4 Series V
75 Bricklin SV1 77 Spitfire
John T. Blair WA4OHZ email: jblair@nhr.com
Va. Beach, Va Phone: 495-8229
48 TR1800 48 #4 Midget 65 Morgan 4/4 Series V
75 Bricklin SV1 77 Spitfire
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