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Carter Thermoquad Rebuild
By Bill Cooke
If
you have ever had to work on one of these carbs you know why they are not
exactly well loved. If the metering rod adjustment screw isn't enough to
scare the daylights out of you there is always that plastic
body. Most people will opt for a different carb altogether but this
isn't an option for some people. Besides with a little hard work you can
make a fairly decent carb out of it. All in all it's not the worst carb
out there.
Once you've
determined you're sticking with the Thermoquad, a rebuild is the next
logical step. However, this thing has a few items that will make the
rebuild a little trickier than most. If you are unfamiliar with the
adjustment procedure, working on the Thermo-Quad can become a very long
day indeed. Follow along and we'll take you through it step by
step.
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 From the outside, it appears to be just another carb. Well,
that's about right. It is very similar to the Quadra-jet in many
ways. The big difference is the two floats, the lack of secondary
metering rods and the primary metering rod adjusting screw. Oh, and
the PLASTIC BODY!
Remove all the screws and linkages and
remove the air horn assembly from the carb. Be very careful not to
bend the metering rods or anything else for that matter. Check the
body of the carb to make sure it's not warped. Any little problems
can be fixed with a sheet of 100 grit sandpaper and a flat surface.
Just rub the top of the plastic body of the carb until it's flat
again. If it's really bad, maybe it's time for a new carb. Pick up
the paper and you'll find these little gems all over the place,
usually 10 for a dollar! Those poor saps don't know what they
have! |
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Here is the number one cause for
problems with the Thermoquad. It's this little seal at the bottom of
the venturi tube pick-up. When this starts to leak it lets fuel go
straight down the carb and floods the engine. A lot of people have
thrown this carb away thinking it had gone bad when all it needed
was a stupid little seal. |
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Here's a good shot of where this
little bugger lives. It can be a bit tricky to get it in right and
it has to be in right. Do not move or shake the body after you have
the seal in place before you install the airhorn (top of the
carb). |
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This is one of the metering jets.
There is an identical one on the other side of the carb. I fact
almost every part on this carb has a twin on the other side. The
exception in the accelerator pump, only one of those. Changing the
jet size will effect the air/fuel ratio. Larger jets let more fuel
pass and smaller ones the opposite. Now it's backwards with the
metering rods. Larger rods lean out the carb and smaller ones richen
the mix. This carb is also adjustable so you can fine tune the
mixture later. That fact alone makes this a worthy carb. In high
altitudes you can just lower the metering rods to thin the mix. More
on that later... |
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Here's a good picture of the upside
down airhorn. You can see all the guts from here. Almost all the
parts in the Thermoquad are right in the airhorn and that can make
it pretty easy to work on. The circles are pointing out the whole
fuel delivery system of the carb. The lower tubes are the primary
venturis and the upper are the secondary pickups. That little
itty-bitty rod in the middle is the metering rod. Be very careful
with those. They are hard to come by and a little too easy to bend.
In fact, getting them back in the carb is the hardest part of the
whole job. More on that later too... |
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If you mess this one up, you're in
big trouble. This is the transfer tube for the accelerator pump. If
you get the carb back together and it seems to idle fine but stalls
every time you hit the gas, this is most likely the culprit. Do not
crimp, cut, or otherwise damage this tube! Make sure it's on all the
way and has no obvious leaks. Other than that it's no big
deal. |
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Here's the baseplate-throttle body.
Check to make sure your throttle shafts aren't too worn out and get
every nook and cranny clean! I don't care what the outside looks
like, get the inside clean! You can also see the ramp that actuates
the metering rods. Don't lose it or let it get jammed when you put
the body back on the base. Make sure everything is still moving
freely before you put the carb back on the engine. Once is enough
for this job. |
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This is a no-brainer but I thought
I'd show it to you anyways. This is the needle and seat. It's in the
back of the airhorn and has a twin on the other side. CHANGE THEM!
Also blow out the tubes leading up to them to make sure there isn't
any junk in there that will cause problems later. Set your float
height according to the instructions in your rebuild kit. Most
require that the gasket be in place to get the measurement and there
is a difference between the plastic and brass floats. Brass floats
better so the level is higher. For off-roading you can set the
levels 1/32" or more lower (higher number) to keep from having fuel
slosh. That isn't too big a problem for this carb but a little
insurance is nice. |
 Getting the metering rods back
in is a pain. I found it easier to remove them from the carb before
reinstalling the airhorn. That way you can't bend them accidentally.
Remove the little covers on each side of the carb and slide out the
rods. After you have the top back on your cab, remove the hold down
from the top of the metering rod assembly and lift it up. This gives
you room to get the rods back in easily. Reinstall the hold down and
the side covers and bolt on the carb.
Now on to adjustments, here's where
you will be spending all you weekends from now on. Once you get used
to setting the metering rod height you won't be able to stop. It's
just so much fun and easy as can be. Just don't get too carried
away. If you have rebuilt the carb correctly then small adjustments
are going to make a big difference. Clockwise richens the mix and
counter-clockwise leans. This raises and lowers the metering rods in
the jets and increases or decreases fuel to the engine. In higher
altitudes you should turn it about a 1/4 turn for ever 1,000 feet or
so. This is not an exact science as all engines are a little
different and will need more or less fuel at different points. If
you have been turning this screw with little or no effect and you're
sure the carb has been built right, it's time to swap jets or rods.
Rods are easier as they can be done with the carb on the engine. Go
with smaller rods to richen the mix and larger rods to
lean. |
I hope this will help you with
your carb problems. As with most parts on a Dodge, it can be a bit
frustrating trying to get good info about them but once you understand how
they work, you're set! These are good everyday carbs and offer a few great
feature we off-roaders can really use. Now get out there and rebuild that
thing!
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