Drop Top GTX
Member
A properly operating/prepared Thermoquad is very tough to beat.
Thermoquad replacement suggestions
- Thread starter Charlied
- Start date
- Local time
- 3:15 PM
- Joined
- Sep 6, 2009
- Messages
- 1,336
- Reaction score
- 1,071
- Location
- Boulder City, NV.
Agreed, but most guys do not want to spend the
time to understand them.
time to understand them.
Gunnar Schroder
Member
When I bought my Dodge Diplomat G22 it was fitted with a Thermoquad carb. All well but it soon proved to be a troublesome carburetter. She did not idle correct and was a petrol gussler. I took her down to the importer here in Norway and the workshop checked her over, but in vain. The boss said my manifold gasket was leaking and caused the trouble. "No way" I said as I had myself swopped the manifold gasket and done everything by the book. Inserted "Umbrellas" over the valves to minimize oil comsuption, if any. "Look here" he said and with the air cleaner off sprayed Ether around. Of course she reved up. I took her out from the workshop and drove down to a former workshop manager who had a garage a few miles away. Left her there and called in after I finished work. "Sorry, I cannot make her run properly" he said so you pay nothing.
I took her home, in the garage and took the carb out and in my own "hobby workshop" i found the fault more or less straight away.
The return spring on the front two ports showed a lot of wear. Spring is a strong one so it wobbled. No repair set for the carb I was told by the importer so what to do?
As I have had experience with worn out motorcycle carbs the solution was simple:
Dismantle the front axel and using a reamer I line reamed the bores for the axel to oversize. Tinned the axle and in the lathe I made it perfectly "as new". Made new bushings for the axel in the carb of self-lube Nylatron and fitted an O-rong on both sides to make sure no false air.
Same thing with the power steering pump. No overhaul set available so I made a new bushing in manganeze bronze, reamed to fit after being pressed in and locked with Locktite. New oil seals no problem and oiltight since late 1990. Where there is a will there is always a solution! Happy driving all of you wonderful people
I took her home, in the garage and took the carb out and in my own "hobby workshop" i found the fault more or less straight away.
The return spring on the front two ports showed a lot of wear. Spring is a strong one so it wobbled. No repair set for the carb I was told by the importer so what to do?
As I have had experience with worn out motorcycle carbs the solution was simple:
Dismantle the front axel and using a reamer I line reamed the bores for the axel to oversize. Tinned the axle and in the lathe I made it perfectly "as new". Made new bushings for the axel in the carb of self-lube Nylatron and fitted an O-rong on both sides to make sure no false air.
Same thing with the power steering pump. No overhaul set available so I made a new bushing in manganeze bronze, reamed to fit after being pressed in and locked with Locktite. New oil seals no problem and oiltight since late 1990. Where there is a will there is always a solution! Happy driving all of you wonderful people
Geoff 2
Well-Known Member
Gunnar,
Your 'gas guzzling' TQ probably had a very simple problem: the internal o rings were leaking, which would cause all of your symptoms. All carbs have slop in the t/shafts, important for safety so that they do not bind & return the blades to the closed position. Spraying accelerant around the shafts will cause the rpm to increase, even on a new carb.
Your 'gas guzzling' TQ probably had a very simple problem: the internal o rings were leaking, which would cause all of your symptoms. All carbs have slop in the t/shafts, important for safety so that they do not bind & return the blades to the closed position. Spraying accelerant around the shafts will cause the rpm to increase, even on a new carb.
Gunnar Schroder
Member
Thanks Geoff for your comments. The O rings was changed but she still had erratic running. I could move the t/shaft up and down at least 1 mm if not more so the problem was there. Spraying accelerant around the carb with the aircleaner off is no good idea
Geoff so the workshop manager should know better. Anyway after that she ran perfect and of course I had calculated a little tolerance between the bushings and the axel. The return spring was indeed very powerful. Later I swapped that carb to a Edelbrock and found that easier to tune. After 22 years of storage she starts easily and runs well. The only snag over here in Norway and Sweden is they mix the petrol with Etanol (Biomass) up to 35 %. So far only 10% and that is enough for an old 3.18 engine.
Geoff so the workshop manager should know better. Anyway after that she ran perfect and of course I had calculated a little tolerance between the bushings and the axel. The return spring was indeed very powerful. Later I swapped that carb to a Edelbrock and found that easier to tune. After 22 years of storage she starts easily and runs well. The only snag over here in Norway and Sweden is they mix the petrol with Etanol (Biomass) up to 35 %. So far only 10% and that is enough for an old 3.18 engine.
Jonas Nordstrom
Well-Known Member
Interested, do you have a link to the thread?
Geoff 2
Well-Known Member
Jonas,
Sorry do not how to do direct link. But if you go abodies, Fuel & Air systems, you will see it.
forabodiesonly.com
Sorry do not how to do direct link. But if you go abodies, Fuel & Air systems, you will see it.
forabodiesonly.com
Thermoquad mods that work.
A few comments first on the earlier thread on the forum by Brook James: - there is no need for additional atmospheric vents on a TQ. It has 3 already. A 1050 Dominator gets by with two 5/16" vent tubes. - enlarging n/seats. Going too large [ or just larger ] does 2 things: it raises the fuel...
www.forabodiesonly.com
1KoolBee
Well-Known Member
I ran the Thermoquad/Qjet replacement Holley spreadbore, 650 double pumper on my slightly modified Bee’s 383 ( intake, headers, mild cam) back in the late 70’s. Best running carb I ever ran on that car. Great off-idle response and pulled hard when the secondaries opened.
mmissile
Well-Known Member
My carb builder is a good friend. He also raced bowtie NHRA S/S. He rebuilds lots of thermoquads for NHRA class racers, and loves them. He says they are a far superior design to the quadrajet. The one thing he points out, is the capacity of float bowl.
rumblefish360
Well-Known Member
This is not exactly the case since the large TQ flows easily 800+ cfm. The trick is the small primary and large secondary. The primary is about equal to the 750 primary side of a Holley
Various publications have stated that the primary is at 250 cfm on the large TQ. While I wouldn’t normally suggest a approx flow of a 400 cfm primary, it’s easily tunable for his engine. I using an 800 AVS II right now on a 340. Runs and drives excellent with very good mileage and driving manors. It’s all in the tune.
It really doesn’t matter if the little 340 only consumes 600 cfm on the dyno, simple fact of the matter, the 800 provides more performance and works very well.
rumblefish360
Well-Known Member
Good mention
- Local time
- 3:15 PM
- Joined
- Jul 1, 2015
- Messages
- 4,621
- Reaction score
- 5,590
A classic example of over carburating a small displacement engine giving up crisp throttle response for the potential of peak RPM/HP. But where is the engine/car going to operate 90% of the time?...not at WOT but at mid rpm. Another consideration is what cam us used, rear gear, trans, compression ratio. Too many variables to say an 800 CFM carb is the best solution.....but to each his own volition.....
BOB RENTON
- Local time
- 1:15 PM
- Joined
- Mar 14, 2012
- Messages
- 17,063
- Reaction score
- 28,640
- Location
- Surrey, B.C. Canada
One of the classic arguments over using larger carbs is that the standard speed shop formula (Cubic Inches x RPM x Volumetric Efficiency ÷ 3456) gives a smaller number than people had expected, and the larger carb seems to work out just fine. But that formula is flawed; it would theoretically work alright with a constant flow engine, such as a jet turbine, but your four cycle engine is not that.
Examine a single cylinder - it requires, for argument's sake in this discussion, 100 cfm. That is how much air it will use at peak power and rpm. But, the engine is not constantly drawing that air in. For one quarter of the time, the engine is operating in intake mode, the other three quarters of the time it is compressing, powering and expelling while the carb sits idle. So that 100 cfm flow is actually divided into pulses, where one quarter of the time the cylinder is ingesting the full flow, while the flow is idle for the remainder of the time. This is why independent runner designs, such as running four 2bbl Webber carbs on a V8 require around four times the cfm values as compared to a four barrel with a large plenum.
So yes, overlapping pulses and the plenum on a multi cylinder engine will help mitigate the increased flow demand during the short intake cycle but the carb is still going to see signals of rapid high and low pressure pulses, not a smooth flow, and the total CFM flow has to be enough to meet the high pulses. Answering the mystery of why larger carbs can still work well.
Examine a single cylinder - it requires, for argument's sake in this discussion, 100 cfm. That is how much air it will use at peak power and rpm. But, the engine is not constantly drawing that air in. For one quarter of the time, the engine is operating in intake mode, the other three quarters of the time it is compressing, powering and expelling while the carb sits idle. So that 100 cfm flow is actually divided into pulses, where one quarter of the time the cylinder is ingesting the full flow, while the flow is idle for the remainder of the time. This is why independent runner designs, such as running four 2bbl Webber carbs on a V8 require around four times the cfm values as compared to a four barrel with a large plenum.
So yes, overlapping pulses and the plenum on a multi cylinder engine will help mitigate the increased flow demand during the short intake cycle but the carb is still going to see signals of rapid high and low pressure pulses, not a smooth flow, and the total CFM flow has to be enough to meet the high pulses. Answering the mystery of why larger carbs can still work well.
Geoff 2
Well-Known Member
Where this gets confusing is terms like cfm & VE. What is being measured is not volume at all, rather the mass of air inhaled by the engine. However the term VE has been in use for so long that it is just accepted.....even though it is incorrect.
The CFM ingested is the same for any engine speed because the displacement doesn't change. The density does...
As for statement in post #35 about IR Weber carbs needing 4 times the cfm rating of a 4bbl. There is no way of really equating the reqd carb flow because of the difference in the manifold design.
A D. Vizard test of four 48 IDA Webers produced about 40 hp more than a 850 Holley up to 6500 rpm, & made more tq everywhere.
The CFM ingested is the same for any engine speed because the displacement doesn't change. The density does...
As for statement in post #35 about IR Weber carbs needing 4 times the cfm rating of a 4bbl. There is no way of really equating the reqd carb flow because of the difference in the manifold design.
A D. Vizard test of four 48 IDA Webers produced about 40 hp more than a 850 Holley up to 6500 rpm, & made more tq everywhere.
- Local time
- 1:15 PM
- Joined
- Mar 14, 2012
- Messages
- 17,063
- Reaction score
- 28,640
- Location
- Surrey, B.C. Canada
VE is still a useful measurement for showing the efficiency of the flow of an engine, so knowing that a 400 c.i. engine has an 80% VE, then you can figure on it being able to ingest 360 c.i. of air with every two rotations. Mass is also useful to know, but it changes with altitude, temperature and water content.
While technically correct in saying that displacement doesn't change, but the density does, that part is inside the cylinder where restrictions cause a partial vacuum in most cases. What people really need to know is how many CFM of atmospheric pressure air above the carb are going to be breathed in. Knowing the VE at different operating parameters then allows calculations for mass to be made.
While technically correct in saying that displacement doesn't change, but the density does, that part is inside the cylinder where restrictions cause a partial vacuum in most cases. What people really need to know is how many CFM of atmospheric pressure air above the carb are going to be breathed in. Knowing the VE at different operating parameters then allows calculations for mass to be made.
On a scale of " it run good!" All the way to, " it run REAL good! "
where does your setup rate.
Lol fun threads for sure.
where does your setup rate.
Lol fun threads for sure.
Jonas Nordstrom
Well-Known Member
Remember that the size-calculators does not matter if you run spreadbore.
For throttle-response, economy etc it is about the primary venturi size.
Check my carburetor thread about real tests.
For throttle-response, economy etc it is about the primary venturi size.
Check my carburetor thread about real tests.
It comes down to how many oxygen atoms are in the cylinder.
Similar threads
