Goodbye Thermoquad

[jasperjacko]

Well I finally said goodbye to an old friend. I love the old tq, but finding parts and endless rebuilds, have made me get a new carb.
I purchased a 625 street demon after much consideration. After modding the fuel line and the throttle, Modoba fired up quickly and seemed to run fine. Went for a test drive and ran good but had some pinging, so came back to lessen the vac. advance. Went for another drive pushed it harder, then the car starts bucking and missing real bad. I came back home, opened the hood, and found fuel dripping from the.....vac. canister on the distributor! The vapors were lighting of in the cap! Could've burnt my car to the ground. Holley has some 'splainin' to do. Anyone heard of this before? I haven't.

 

[AJ/FormS]

no, I haven't ever seen it.

 

[BudW]

I hadn’t seen that before, either.
A new vacuum advance should fix that concern – for I have seen them go bad before (more often, than not).

There are two things to look for on a new vacuum advance:
- Stamped on the arm is how much advance it allows, so if stamped “12” means it will allow 12’ advance when vacuum is applied. You want to keep that the same, if possible – or look for correct one if that figure needs to change.

- Not stamped (typically) is spring tension inside of the pod. There is a tug-of-war for engine vacuum vs. spring pressure that indicates when vacuum advance starts to work. There are adjustable vacuum advances out there which is the way to go, if you can find them. To adjust the spring pressure, you insert an Allen wrench into the vacuum nipple and give it a spin. This is a LOT of trial and error – but a person can get one dialed in with some work.

The leaking vacuum advance has nothing to do with your new carburetor. It is more coincidence than anything else.
BudW

 

[jasperjacko]

My bigger concern is why or how did fuel get in there from a ported vac sourve?

 

[BudW]

The big block vacuum advance pods look very similar to small block – but are not interchangeable. Sense the distributor rotates CCW (small block and /6 rotate CW), they point in wrong direction and the arm is made differently.

I had once thought a person could take the vacuum advance off, flip it upside down and use it for the other engine (big vs. small block). My theory sounded good – but it doesn’t work that way for several reasons.

I’m not sure about small block and /6 vacuum advance interchangeability (not investigated it).

The vacuum advance unit can be changed in-car – but I don’t recommend it.
Take a good picture straight up from distributor showing rotor direction and vacuum advance angle (to assist on distributor reinstall).

Take distributor to a bench and remove the two Philips screws. The vacuum advance just pulls out. There is a lever the rod connects to. Just reinstall the new rod to same lever (but give that rod a touch of wheel bearing grease first). Push it in, attach screws. Before install, attach a vacuum pump (or your lips, if you want) to vacuum advance pod to make sure everything works like it should, then reinstall.
Adjust ignition timing to complete the operation.

My bigger concern is why or how did fuel get in there from a ported vac sourve?

I can see fuel vapor getting in there.
When there is no vacuum applied – or vacuum that is applied is less than spring pressure, the pod relaxes and thereby sucking in air from the port on carburetor. In some cases, that process occur several times in a short distance.
Any fuel vapor sucked in will eventually get sucked back into carburetor over time – unless you have a vacuum advance diaphragm rupture (like as mentioned) and in that case, the vapors will not be in a good location (brings a new definition to “hot spark”).
BudW

 

[AJ/FormS]

Ima thinking you might have other fueling and or tuning issues.
Specifically the primary throttle blades may be too far closed.

 

[Oldiron440]

It's to bad you have given up on the thermoquad, I've had several that have worked very well. I'd like to put a pair of them on a tunnel ram on one of my 440s.....just for fun, I've heard it works.

 

[jasperjacko]

I love the tq. Modoba ran great with it. The million dollar ? still remains.....why is fuel getting to the vac pot?

 

[Oldiron440]

So have you pulled plugs to see if it's running fat?
If it has an electric choke have you verified its opening fully?
I would plug vac. port for the distributor and readjust carburetor.
Obviously you have a vacuum leak at your distributor vacuum advance so it must be replaced.

 

[BudW]

The vacuum advance gets used often and forth displacing air back and forth on every cycle. Being the carburetor port is close to the venturi's (the venturi fuel flow, that is) – it will suck in fuel vapor.
I have no question about vapor getting in.
If the vacuum diaphragm wears out, a small amount of the vapor can get into the distributor.


I have a question about the old TQ. What is its model number and date code?
BudW

 

[M_Body_Coupe]

I'm with you, that sounds like a crazy amount of fuel to get sucked into the vac. pot purely due to the normal 'back and forth' diaphram pulsation.

Sounds to me more like there is excessive fuel being dumped into this ported vacum source...not familiar with this carb (I'm a TQ die-hard, sporting a 9800 series with the electric choke on my coupe), but can you tell whether you have fuel dumping into the primary venturis, even at idle?

Here is what I've seen once in my TQ...I got a bug (no joke, a freaking mosquito) stuck between the fuel needle and the inlet valve, that in turn caused excess flow into the fuel bowl which then spilled right out of the primary venturis...talk about flooding...the engine managed, it would load up a tad, but it was only noticable at idle. Somehow that bug got in there, and how it manged to get stuck is beyond me, but when I found it there having pulled the carb apart I was floored!

 

[jasperjacko]

Well, I may not be done with the tq after all. The new street demon accell. pump quit working also. It will be going back to Summit.