Best current out of the box spreadbore carb for my 318?

[8v-of-fury]

Not really looking for the one that is gonna give me the most hp, as that is not the aim of this car.

Drive-ability is #1. If I can't drive it, doesn't matter how much hp it makes right ? lol.. I need one one that is gonna start properly every time between -10 to 35 Celsius. One that will have cold drive-ability without having to idle a bunch of heat in to the engine. One that is gonna be able to creep through hot traffic the odd time without starting to stumble.

Now here's the thing. The boats at work (if carb'd) utilize a Rochester 2 or 4 jet. I run the same real gasoline they do, being 89oct with absolutely no ethanol to speak of. They can sit after running and start up with two-three pumps a month later. My Thermoquad can't sit 24hrs without needing the float bowls FULLY re-primed. 20-30 seconds of cranking before it will even fire a little bit. The bowls are fed from the rear as you likely know, from the top. It is impossible that they are being drained back to the tank. HOWEVER, I do suppose it could be the fuel line from the carb back to the tank could be draining back. Although it should run on the huge bowls at idle long enough to replenish the fuel pump, no?

So either it isn't my TQ causing all my issues, or it completely is my TQ causing all my issues..

 

[89.Fifth]

This is supposed to be the best new carb you can get for a daily driver.

Demon Carburetion Street Demon Carburetors 1901

 

[slant6billy]

625 is a nice good cfm for a 318. Should not bogg at all

 

[Aspen500]

The Thermoquad's can have the nasty habit of the fuel bowls draining. There's 2 pieces epoxied to the bottom of the plastic body that come loose and then the fuel will seep out of the bowls and into the intake. Not fast enough to cause driveability or flooding but enough to go dry over night. It's similar to a Quadrajet having the bowl plugs leak. We've got a '74 Plymouth motorhome (cutaway van style) at work with that exact problem. A reman carb is coming,,,,,,,eventually, hopefully, so Advance says....... (lol).

 

[8v-of-fury]

The Thermoquad's can have the nasty habit of the fuel bowls draining. There's 2 pieces epoxied to the bottom of the plastic body that come loose and then the fuel will seep out of the bowls and into the intake. Not fast enough to cause driveability or flooding but enough to go dry over night. It's similar to a Quadrajet having the bowl plugs leak. We've got a '74 Plymouth motorhome (cutaway van style) at work with that exact problem. A reman carb is coming,,,,,,,eventually, hopefully, so Advance says....... (lol).

I've had the carb to inspect those well plugs before. And they were just fine then so I am unsure as to whether or not it is them causing the problem. I even cleaned them and applied JB or some fue safe epoxy.

 

[BudW]

If those two bottom parts are in place (ie: not fallen off) and no signs of missing epoxy – then that is not the problem.

The second most common leak source is the pair of X-rings or X shaped O-rings. Very common for these not to be in place after a rebuild – or to leak (siphons off the gas).

Next in line is low fuel level (which I don’t think is your problem) – but compounds with the next issue:

Heat soak is evaporating the remaining fuel off, after turn off – which I think is the most likely possibility. Using a thicker gasket and/or some kind of heat shield underneath should help.
An electric fuel pump can also fix this issue.


I love TQ’s, especially the early ones.
The later ones (like ones used in all FMJ’s) – not so much.
That said I’m going to go with Fuel Injection when the time comes to go 4-bbl. That way Ethanol fuel will not be as much of an issue and drivability will always be there.


If a person is looking for a TQ that should work great out of the box – then look for model numbers: (Smaller size) 6138s 6139s 6318s 6319s 6339s 6340s 6394s 6614s 9000s
(Larger size) 6090s 6140s 6144s 6165s 6166s 6320s 6321s 6322s 6324s 6341s 6342s 6410s 6411s 6446s 6452s 6453s 6454s 6455s 6456s 6457s 6459s 6460s 6461s 6462s 6463s 6488s 6489s 6496s 6503s 6518s 6545s 6615s 6616s 9013s 9014s 9015s 9016s 9017s 9019s 9020s 9023s 9024s 9025s 9035s 9036s 9096s
Note: the model numbers in red are California versions (not that it really matters, on the above numbers).
Note: I not have any issue on installing a larger size TQ on any Mopar - for the Secondary air valve is adjustable and (when adjusted correctly) will not cause any bog.

Any other TQ number can be made to work well after a lot of work, but not “right out of the box”.
BudW

 

[8v-of-fury]

My TQ is a 9046s with the larger primaries. It works quite well, but being so old and used.. I don't trust that it is not causing allll of my problems lol.

The bottoms of the wells were epoxied last year when i had the carb off. Also the gaskets under the secondary jet areas were present as I made sure of it.

I can't see that it could possibly be boiling the entire bowl off, phenolic spacer and phenolic bowl? Yet other people can keep fuel in a metal carb with a normal spacer?lol Do i have super bad luck or what haha. Low fuel level in the tank? or the bowls? The tank can be full or dead empty, doesnt matter. The floats are set correctly as well.

 

[kkritsilas]

The Street Demon carbs are spreadbores, if you look at the bottom of them, or if you take the airflow of the secondaries vs. the primaries. The secondaries are a little weird looking from the bottom, in that the one throttle plate covers both secondary barrels (ie. Goggle Valve, and that is not a misspelling). There is a version that uses a phenolic body, just like the TQ.

Also, the Demon carbs are being made by Holley.

The Street Demon carbs are the newest version of the TQ, in my mind, with some improvements to them in the form of leak prevention (there are no gaskets below fuel level), and easier adjustments for the idle speed, and secondaries.

 

[BudW]

The 9046s is an early ’75 49-state 400ci automatic B or C-Body, with large primaries and has Idle Enrichment. I rate it a 2 (out of 3) (my opinion only).

There should be a ¼” thick gasket between it and intake, usually light grey in color. Most carburetor kits come with it as well as a paper thick gasket that is black in color. The paper thick gasket is notorious for heat transfer (ie: baking fuel out of the bowls). The thick one does a much better job of heat insulation – but it is also, not perfect.

If heat soak is the problem – then the paper thick gasket along with a fashioned aluminum (heat) shield placed above the 1/4” thick gasket - usually does the trick

In a perfect world, the carburetor body would have a big sight glass on it – to check fuel level both when running, and after a heat soak.

To check a suspected heat soak fuel level – means to remove carburetor from engine and turn upside down to drain existing fuel out as much as possible. If not much comes out of carburetor – then you will know your answer. Note: this is done when engine is cold the morning after a long drive.

Disconnecting the fuel line also helps to find out – but sense a TQ has a lengthy passageways to both needle/seats and both drain downhill – that method is not reliable on TQ’s.

The next test is to see how long it takes to fill a measuring cup (or something fuel will not eat up, like Styrofoam) – after a heat soak.


Summery: after a possible heat soak (morning after, for example), and carburetor holds a reasonable amount of fuel and if fuel pump/lines fill a measuring cup to specifications – then it is time to look elsewhere.

I have chased down my share of these problems over the years and fuel evaporation after hot shutdown – is a big part of those problems.

The problem is it is not easy to diagnosis.
BudW

 

[8v-of-fury]

Kkritsilas, that seems to be some pretty neat stuff on that Demon TQ. I think I will save up some coin and buy two! One for the 5th and one for my 302 ford . I love the look.

BudW, I do have the 1/4 thick gasket and i'd run a larger wood or phenolic spacer if i could.. but the air cleaner is already touching the hood insulation eh.

It definitely is the carb going empty.. either through boiling off or leaking through the bowl somewhere. I suspect a leak over boiling.. I got it running yesterday after work to only move it in the driveway and today after work it was nearly bone dry. I removed the air cover and then watched how many pump-shots i got. 3-4 full ones, and then the 5th was half air and the 6th was nothing. Carb was bone dry.

 

[BudW]

You will have about 3-4 accelerator pump shots available on a dry bowl – just because of the accelerator pump hose and circuit length is so long – and there, the fuel can’t boil out (can’t go anywhere). After shot number 5 and nothing – then that can also answer your bowl level question – but then you have too much fuel in intake for car to start.

I like to leave the accelerator pump circuit alone for even if carburetor is turned upside down, so that circuit will still be fully primed.
Could just be a personal thing, though.

That said, and re-reading your post – you came to same conclusion.


A simple fix could be to either:
- Add an electric fuel pump parallel to mechanical fuel pump. Just apply current to it for a few seconds to fill fuel lines/carburetor, then turn it off to let mechanical pump do its normal job. Or
- Replace mechanical fuel pump with an electric version.

_ _ _
If you do install an electric pump – almost anyone out there will work – just as long as it has 5/16” nipples and output pump pressure is between 5 to 7.25 psi. Any stronger than 7.5 psi and the fuel pressure will override the floats and cause flooding (not good).

It can be mounted by frame close to mechanical fuel pump.
If replacing the mechanical pump, it could be bolted there.
That said, electric pumps work happier as close to (and slightly lower than) the fuel tank as you can get.

If using a parallel pump, then there is nothing wrong with it in engine compartment.

JUST BE SURE FUEL LINES (metal or rubber) ARE MOUNTED WELL AWAY FROM ANYTHING EXHAUST!