Do I need to re-jet my Thermoquad?

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Bill Crowell

Bill Crowell

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Do you think I should re-jet and re-rod my Thermoquad 6518 (an industrial model with the 1-1/2" primary bores)? Here are the results of two A/F ratio tests.

The engine sure seems like it's running too lean, and I'm getting quite a bit of detonation on 91 octane under light-ish acceleration. It's an LA 360 with "J" heads (ported), 10.5:1 compression, 2.02" intake valves, Edelbrock Performer intake (match ported), Comp Cams 20-243-4 cam (244 degrees at .050; .501" lift), and a Mallory YH distributor.

The air/fuel ratio seems too high, but some mechanics are telling me that I shouldn't re-jet because the CO numbers are low. I don't understand that because I'm a complete tuning newbie!

If the consensus seem to be that I should re-jet, then I'm going to have to disassemble the carburetor to see what jets and rods I've got now (I forget what they are), and will then probably bug you guys for specific recommendations on what jets and rods to buy. (I haven't disassembled it yet because the car is a daily driver.) I figured I should try to get the mixture right before trying to do a custom advance curve.

Is anybody aware of a video or article about how to choose the correct jets and rods on a TQ?
Thanks for now.

AF ratio test 1.jpg


AF ratio test 2.jpg
 
It was probably an ad, I just clicked on it again and it works for me.
You can also search( mopar air fuel ratio for power ) and it should come up
Let me know.
 
coloradohill said:
It was probably an ad, I just clicked on it again and it works for me.
You can also search( mopar air fuel ratio for power ) and it should come up
Let me know.
Click to expand...

No, it's smog-exempt due to age.

What I meant was, the author of that article went into about zero details concerning how to tune your A/F ratio.

My Wild A** Guess is that my engine should be running at about a 13:1 A/F ratio, so I want to figure out what jets and rods I will need to get there. (Of course I've got to determine what jets and rods I've got now.)
 
Oh, ok. Well I think that’s because there are so many carb combinations out there. I thought you were wanting the best ratio for drive ability Ect. I run Holleys. I wish you the best.
 
I myself would richen it up just a smidge. I can’t recommend a exact rod and/or jet to use. I can suggest only trying a set of rods that are thinner. As such it would richen up the mixture. You are a little lean. Not really a bad thing. When it gets cool out, at night, winter, the air molecules condense leaning out the mixture even more. Post up what rods and jets you have in there now.

If it were myself, a O2 sensor would be an excellent additional tool to help tune it up. Id try a rod that is .002 thinner. Or a jet size .002 bigger. And see where I’m at.

Now there a bit pricey to get. So let me know what you have. I MAY have a double set of something here that could be worth the try.
 
Always went a step up from stock on my 340 Duster.Now that was back in the days when you drilled your own jets and before air fuel gauges. Back then we read the plugs. I still have the # drill bit kit for hand drilling jets from 46 years ago..
 
You mentioned detonation. What are your timing specs? May be nothing wrong with your carb.....
 
Congrats on using an exc carb, the TQ!
More info needed.
- is it lean on cruise off idle? If so, then it is running off the idle cct & IFR size would need to be increased. But don't do anything yet...
- not surprised you are getting det with 10.5:1 & iron heads.
- that cam will need a HEAP of initial timing
- the pri blades are probably open too far because of the big cam.
- if your initial timing is under 20* BTDC, I would do this 5 min test & report back. Engine idling, advance dist to about 35* [ exact number not critical ]. I haven't seen you car, but I know this: rpms will increase & idle will be smoother.
 
Bewy said:
Congrats on using an exc carb, the TQ!
More info needed.
- is it lean on cruise off idle? If so, then it is running off the idle cct & IFR size would need to be increased. But don't do anything yet...
- not surprised you are getting det with 10.5:1 & iron heads.
- that cam will need a HEAP of initial timing
- the pri blades are probably open too far because of the big cam.
- if your initial timing is under 20* BTDC, I would do this 5 min test & report back. Engine idling, advance dist to about 35* [ exact number not critical ]. I haven't seen you car, but I know this: rpms will increase & idle will be smoother.
Click to expand...

Thanks for the kind reply, Bewy. So advance the timing to 35 degrees BTDC at idle and see how it idles? Now if I were to rev it up with that much initial advance I would have WAY TOO MUCH total advance, so I should avoid that, right?

Just checkin'. I don't want to break my engine.
 
I think without re reading he means 35*’s total.
If this is a hot rod, I’ll tell you to run as much advance as you can and limit it to 35*’s (iron head) total until the dyno or track tells you differently.
 
Bill,
Post #12 is correct. Yes, don't drive it, just do the idle test.
This is a quote from D.Vizard. It is about idle timing, but in a carb book, which is something to think about....
' The optimum idle advance is typically about 35-40* for a short cammed street engine & though not commonly realized as much as 50* for a street/strip engine.'
 
What would be a good air/fuel ratio to shoot for under load with this engine?

If you were to go from an .092" main jet to an .098", how much do you think this would change the A/F ratio under load for my engine?

Thanks again!
 
Bill, that’s a hard question to answer as it’s a per build thing coupled with elevation. Generally it can be answered with more info but not exactly. Though a seasoned carb tuner could probably call it really close. Unfortunately, I’m not that guy.

Last time (recently) I swapped rods out on my son’s 360 it was (numbers not exact) a .73 a .78 rod with a fuel ratio change of 12.2 - 11.4. This much or little swing will probably NOT be the same for you.

10.5-1 - .030-360, Edelbrock top end/ heads, intake, 600 carb, Hedman headers, summit split duration cam..
Damn it, can’t find the cam card. It’s a small duration… I can’t remember exactly. Something like 216 on the intake 228 exhaust… dang it! 114 LSA for sure.

But that’s what I got on the rod only changes.
 
The large difference in MJ sizes is accompanied by changes to the air bleed size, to keep the a/f ratio correct.
I am preparing a Ford 800 TQ for an engine, & it came with 101 MJs. Sounds big but so are the air bleeds.
Not sure why the change to bigger jets was made over the years. If I had to guess: bigger air bleeds, adds more air, creates a better atomised mixture for reduced emissions; bigger ABs need bigger jets.
Going from 092 to 098 is a HUGE increase. Try a 095.
 
Thanks a lot for all the advice, everyone. I'm going to watch Rocky Roadshow's videos again in order to learn how to work on the air bleeds. I've got a jet assortment and a pin gauge set ordered.

I sure would like to take this car to the strip some day, if I ever get it sorted out.
 
Bill Crowell said:
Thanks a lot for all the advice, everyone. I'm going to watch Rocky Roadshow's videos again in order to learn how to work on the air bleeds. I've got a jet assortment and a pin gauge set ordered.

I sure would like to take this car to the strip some day, if I ever get it sorted out.
Click to expand...
Can you put up a link please.


Bewy said:
The large difference in MJ sizes is accompanied by changes to the air bleed size, to keep the a/f ratio correct.
I am preparing a Ford 800 TQ for an engine, & it came with 101 MJs. Sounds big but so are the air bleeds.
Not sure why the change to bigger jets was made over the years. If I had to guess: bigger air bleeds, adds more air, creates a better atomised mixture for reduced emissions; bigger ABs need bigger jets.
Going from 092 to 098 is a HUGE increase. Try a 095.
Click to expand...

It would be nice if the air bleeds were easy to change out on TQ’s.
 
rumblefish360 said:
Can you put up a link please.
Click to expand...

RockysRoadshow - YouTube

Rocky knows a lot about Thermoquads, and his videos are almost an embarrassment of riches because he goes into so much detail. Videos 1 - 10 or so are about the TQ's theory of operation, and videos A - H or so are how to work on the carb.

The only problem is that it can be hard to find the video that covers the desired subject. I am going to have to go through all of his videos in both the "theory" series and the "how to" series in order to find where he discusses the air bleeds.
 
Last edited:
Rumble,
I have always thought of air bleeds like this: it is getting into carburetor engineering. Not to be messed with by amateurs....
Only reason I can think that H [ & clones ] brought out carbs with removable ABs is so that all the 'experts' could tinker with them. I think H had to play 'catch up' to Barry Grant...
 
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