How to tune ThermoQuad secondaries?

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MopaR&D

MopaR&D

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I'm trying to dial in the secondaries on my ThermoQuad but I'm afraid the jetting is just way too rich. When I open them up there's a bit of a stumble then the engine will rev to about 3500 RPM at which it starts to misfire and kind of "flutters". Would this be an over-rich condition or over-lean? I have the air door spring set to about 2 turns and the tang is bent a tiny bit to reduce the travel from stock but it didn't seem to do much.
 
Your determined to use this carb aintcha`kid8) Sounds like a lean bog. Do you have the tool for sec. lock plug? if not, its not too hard to make one, and will make it easier to dial in the sec. air door. Put it back where it was and dial from there.. 1/4 turn@ a time. I hope you have a good grasp on how these carbs work. If someone really jacked with it, their are 15 adjustments that can affect it. In your earlier post you said you had it apart, hope you used a rebuild kit for gaskets etc.? And a good kit has good instructions on adjustments, and a blown view to let you see if any parts are missing. 30 bux well spent for specs, and new important parts. good luck
 
Yeah I made sure I cleaned it and replaced as many parts as I could that came in the kit. It runs great on the primaries (I think a bit rich though, only an O2 sensor will tell) but I need to adjust the secondaries a bit. I have a special pair of pliers that have small pointed tips to loosen the lock-screw too. So I should tighten up the air door some more if it's lean, right? What about the overall travel of the door; when do I know I have to adjust that?
 
A bit rich won't cause that much of a stumble. Someone driving behind would see a cloud of black smoke.

Sorry I can't help you with the TQ. I'm a Holley guy.
 
Start at 2-1/2 turns and go tighter. This carb must accel on the primarys before the secondarys kick in at the slightest. The pump shot can not cover a pair of 2-1/2 inch barrels opening up early, the system must be moving along , not just starting out.

Remember, the butterflys are mechanical in nature and open up with the press of the throttle. The tunable air door must have decent tension on it.
 
The 'tune-ability' of the 2ndaries depends on the carb model # and the top casting #. You might simply have too small a jet size for 2ndary jetting. You likely have lean 2ndary jetting causing the problem, David.
 
I finally got the correct crank pulley on my engine and put my hood back on; I tried to tune the secondaries again but it seemed like no matter how loose or tight I made the air door it would bog a little while after I opened up the secondaries. I think I told you before DemonSizzler but the # on the baseplate is 9379S and on the top is 2652. It was originally on a 360 with EGR and I'm at 7000 ft. elevation so I'm thinking it's jetted too rich. Is the air door tension a very "sensitive" adjustment, or will it still work if it's a little off?
 
Okay, I still can't get it. Is there some other adjustment I can make besides the air door tension? I bent the tang on the door a bit but should I perhaps bend it to give the door a specific max travel?
 
MopeKid - You probably shoud try to explain again exactly what the car is doing....

Make sure you cover off what happens when you nail it, and what happens when it dies on you.

It sounds like two issues.

Lets look at the "bog"

Does it do it "hesitate" at all rpm or only from a stand still?

Does it backfire through the carb?

Have you modified anything on the carb?

Have you checked to see if the choke pull off is allowing the air door to open when you stomp it?

Is your pump shooter on the nearest hole to the top plate?

The air door should neither "flop" open too easily, but it shouldn't be at 5 turns either!

REmember - if the door is too tight, it doesn't let air through, so you get no fuel draw - and it backfires.

If its too loose, it flops open and you get a lean bog - the air rushes in and the AF mixture gets diluted.

BOTH are lean conditions, but they show up differently.


THe "flutter"..

This sounds like a fuel supply problem, it sounds like its falling on its nose because its running out of fuel.

Checked you float settings?
Is your car running a big cam and hi flowing heads?
Are your needles and seats sticking?
Does it suddenly sound like a Kirby cleaner? This is a sure fire way of telling its lost fuel supply.
i
 
It's a stock 318 short-block with stock reman'd 302 heads, a Lunati Voodoo 256/262 cam, Comp 901 springs, and stock single exhaust (which I think might be contributing to the problem). Anyway, when I tip into the secondaries (no matter what RPM I do it at) it hesitates for a split-second, then accelerates a bit harder, then does the "flutter" thing like it's misfiring a whole bunch and will no longer accelerate at all (stays at the same RPM until I let off). The interesting thing is when I let off just enough to close the secondaries (but keep the primaries open) it will lunge forward again and pulls pretty damn hard for a second or two, which makes me think it's not a fuel supply issue (and yes the floats are adjusted and the needles and seats are new). Oh and BTW the carb has no choke, I broke a piece of the linkage and just removed all of the choke-related stuff for now (works OK as it's pretty warm this time of year).
 
Put the "bog" aside for the moment, because as you've discovered, nothings working and the air door timing is only a small part of the problem - its mainly a fuel supply issue - guranteed.

What you describe as a "foot off lunge" is the CLASSIC symptom of fuel starvation/lean running TQ.

In fact, Dave Cheves and I were discussing this the other day when I called him. (Mine was a different supply problem - but the foot off lunge conformed it was fuel supply)

The fact that it "flat lines" even well after the air door opens tells me this as well.

So, you are either :

1. - Not getting fuel to the spray bars (fuel distribution tubes) , or

2. - Opening the secs is causing a huge ongoing disturbance to the air flow, so no fuel is being drawn into the secondary venturis.(This is unlikely but can happen)

3. - Theres not enough fuel in the bowl to feed the secs.

Treaments

1. - Disassemble the carb and blow out the secondry circuit with compressed air.

2. - This is MOST important - What secondary jets are you running? - for your combo start at .134 and work up. I see Demon also noted this..I agree and reckon this is the likely cause....

3.- How far do the secondary butterflies open? - they should only open 80% of fully vertical - Demon Sizzler will confirm this.

A lot of Holley guys mess with them so they open all the way - this causes a HUGE air bog due to a lean condition.

4. - Check your float level - 1" from the gasket to the top corner of the float with the plate upside down.

My monies on what Dave said - on way lean jetting.

If not, maybe poor float setting or a "messed with" secondary butterfly opening too far. But its DEFINITELY a fuel supply issue causing most of your problems.
 
OK, thanks a bunch for the help.

1) I can just remove the top part of the carb with it on the engine, right?

2)The secondary jets have the number 5125 on them. According to the Vaanth ThermoQuad guide that's a .125 jet.

3)I'll check the air door travel; exactly how much distance should there be though from the carb body to the top of the air door when it's fully open?

4)The float level should be fine, I checked it multiple times when rebuilding the carb. I'll check it again though when I go to clean the secondary circuits.

Something interesting about tuning at this altitude, I brought my Traxxas T-Maxx (nitro-powered RC truck) here from central PA and tried to tune it to this air. Everything I read said that it should be leaned out, but whenever I'd run it it would act like it was running super lean (even with the screws set the same as back in PA). Turns out it was lean, and when I finally tried richening the mixture it started working better. Unfortunately the engine was toast by then so it didn't matter but it was interesting. BTW those run on a 20% nitromethane/methanol/oil 2-cycle mix. This might relate to my current problem.
 
1) I can just remove the top part of the carb with it on the engine, right?
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Dont!! Nothing more annoying than losing a fuel well seal down a venturi - do it correctly, remove the carb.:read2:

2)The secondary jets have the number 5125 on them. According to the Vaanth ThermoQuad guide that's a .125 jet.
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Id bet the Sydney Bridge that this is your problem -

You are so lean its mental! - Wanna know something? The factory 340 came with .134th secondary jets!!!

- either buy some .134s or drill yours out - starting at .134 won't be too fat, but it might still be a little lean - depends on what primary rods you are running.

3)I'll check the air door travel; exactly how much distance should there be though from the carb body to the top of the air door when it's fully open?
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No - I wasn't talking about air door travel - I was talking about the Secondary mechanical butterflies - they should only open 80% at WOT...some guys muck around with the linkage and the stop - just check them and make sure they aren't completely wide open at WOT.


)The float level should be fine, I checked it multiple times when rebuilding the carb. I'll check it again though when I go to clean the secondary circuits.
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May as well clean them - but I wouldn't worry too much - your secondary jetting is 95% of the issue.

Do the checks
Start with you air door at 1.5 turns,
your primary rods at around 2.5 turns to be safe
- and swap the sec jets - I bet she'll scream! :toothy10:
 
Well, I drilled out the jets with the next bigger size drill bit I have (I believe it's 0.140", I know, probably a bit too big) and it seemed to help a little bit but it still doesn't have any extra pull to it when the secondaries are open (tuning the air door doesn't help either). I'm going to give it a couple more tries on tuning the air door but I think I'm going to end up ditching this carb for a decent used Edelbrock 600.
 
The best way to tune a "thermo-bog" is to throw it in the trash...:-D:-D sorry,i don't trust "plastic" carbs. buy yourself a nice Holley you'll have a lot fewer headaches...
 
Before you give up on the carb, you need to do one last mod:
there are two #2 pencil lead holes to the inside of the choke tower.
You will see these at the front choke tower wall, again to the inside.
Take a dab of epoxy and seal each hole. To test the carb before final sealing,
take a small piece of tape and completely cover each hole and go test drive the car. This should stop the lean condition, especially if you have at least a .095" id primary jet, David.

**pic of rt side hole(red pointer)and lt side visible.**

DSCN1955.JPG
 
stroked 340 said:
The best way to tune a "thermo-bog" is to throw it in the trash...:-D:-D sorry,i don't trust "plastic" carbs. buy yourself a nice Holley you'll have a lot fewer headaches...
Click to expand...

Definitely NOT going with a Holley for this car. IMO they are just too simple; it's either power or gas mileage with them, and I want to try to get as much of both as I can.

DemonSizzler, what exactly do those two holes do? I'll do what you suggested the next chance I get but I'm just wondering what exactly those holes are there for.

EDIT: My little bro just got a new digital camera so next time I take the car for a test-drive I'll record exactly what happens.
 
The two holes are for emissions purposes; starting in 1975(except for '75-'77 440 trucks or IH engines the same years), the cabs 3 main circuits were 'leaned' this way. So generally speaking, even re-jetting will not correct metering since jetting does not directly affect the idle circuit but the two holes do. So cover those holes and see what happens:burnout:, David.
 
You go, Kid! NHRA stock and Super Stock guys have them running reeeeal good.
A thought: Is the distributor timing curve set up? Vacuum and mech?
 
MopeKid - Don't give up on it. Sorry the re-jetting didn't work out...but richer is better than leaner - and it'll end up closer to .140 than .125 by the time your done.

I guess I owe you a bridge! LOL!!

Do what Dave suggests, he's the guru and then you should get back to us.

Make sure to tell us what the cars doing exactly so we can tell if things have changed ....

I remember asking if you checked the secpndary valves - Did you check to see how far the secondaries are actually opening? Are the butterfly valves vertical, or slightly angled?

FWIW there's still some other things to check as well, but get back to us on Dave's suggestions.
 
I think it's time to give up on this carb. I epoxy'd those two holes in the front of the primary air horn and although it idles a bit better now, it still does the same exact thing when I open the secondaries. I took it out for the umpteenth time to try to tune the air-door tension and again, no matter how tight or loose the spring is set it bogs (just like before) as soon as I tip into the secondaries, then pulls hard for a split second when I let off (followed by a faint puff of black smoke out the tailpipe that I can see in my mirror).
 
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