Thermoquad question

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midnight340

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For anyone who might enjoy thinking about TQ's a bit...

I have everything pretty dialed in, and TQ sorted to the point of good looking plugs and good numbers in general on my A/F meter. (Timing on the 340 is at 18/38 which gives best 1/4 MPH and the ignition curve is pretty worked out as well.) Mostly it's running clean, strong, no bog, etc. Jets .101/.149 (need a bit smaller for WOT)

Now the air door spring is at 2-1/2, the rods up 1-1/2 turns, idles screws now out 1-1/2 with the fall air.

Here are the symptons:
1) I get a new very slight stumble/falter with snapping the throttle at idle or very low rpm.
2) Idle mix shows mid 14s A/F BUT occasionally. all of a sudden, shows 16-17 on the meter! (I'm OK with leaner idle, but not sure why it would jump back and forth.)
3) It was acting very good for months, now with the slightest throttle (added vacuum) the mix drops to 10-11:1, vs the previous 15.5 on light acceleration. (13.5 on decceleration)
4) low rpm cruise is a bit rich at 13.5 when it was at 14-15.

What has changed???
To be clear, the main issues are the 10-11 on light throttle and that initial falter/stumble.
My first response would be be to lean out the idle more, or to drop the rods more, but either of these makes that initial stumble worse.
I had for a while removed the spring under the rod piston as some had suggested but it would get too lean with part throttle so put the stock spring back in and was working fine. What has changed????? I could remove the spring again, or use a lighter on, but that seems like it would make the stumble worse.

At this point my brain gets scrambled, so I thought I'd ask you guys.
 
Check the bore the metering rod piston rides in. Piston too.
I had similar problems, back in the day, when I was using a TQ. They were easy to find in the wrecking yards back then, so it wasn't a big deal.
 
You can use a larger diameter accelerator pump nozzle and/or adjust for a longer shot.
The nozzles I’m using are a little bigger than stock, but I believe I also drilled one another step larger so will try that.
(And I believe moving to the top hole could bring the shot in quicker too).

Check the bore the metering rod piston rides in. Piston too.
I had similar problems, back in the day, when I was using a TQ. They were easy to find in the wrecking yards back then, so it wasn't a big deal.

Never thought of that!! I will check it, and I do have a couple of parts carbs so if if I need another piston I might have one.

thank you both!
 
Never thought of that!! I will check it, and I do have a couple of parts carbs so if if I need another piston I might have one.

All too glad to help :) It's hard to see any wear in the bore, but the piston tends to be pretty obvious.
Good luck with it !
 
Fuel leaking by the epoxy on the bowl wells?
Hmmm Shouldn’t be, as the bowls appeared quite well attached and the I did coat of sealer (Seal-All? I think) over the seam to be sure. But...? I’ll also keep this in mind.

May be a few days, maybe after a fall road trip planned, but I will check back with update.
 
OK, after a vacation camping trip with wife and dogs, I am back on this.

I enriched the rods screw a bit. The slight stumble when cracking throttle seems to be gone for now. The part throttle reading of 10's A/F is gone.
But I still have some really weird stuff going on!!!

I get a good A/F reading for a bit, 14.5 idle and had cruise at 15 A/F, 12's WOT from cruise, WOT from standstill goes 10.X then climbs to 11's/12. All this is really fine. ...But then, I blip the throttle or accelerate and the reading goes really lean. (17 to off scale.)
I've had several incidents of this. I get the tuning good, then all of a sudden I'm lean, then OK. It runs fine through all this. Plugs are perfect. Just went for a warmed up neighborhood drive and it's back to "off the scale" lean to richest of 17+!!i!

Could this be an O2 sensor failing, or the meter??
 
Are you using the stock spring under the tree that hangs the rods?
How much vacuum are you pulling?
What are the cams specs and lifter used?
Is your engine build on here @ FABO?
 
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Must be just in the readings, I'm thinking... What are the symptoms if an O2 sensor is bad? Does it just quit? Mine isn't that old miles-wise. AEM A/F meter seems to be working fine, other than when it reads way lean part of the time.

Engine build is on here. (freshen up) ...some basic info in first post here.
Yes right now running stock spring under rods. Tried none, and a lighter hardware store spring but (best I can remember!) seemed to go too lean on part throttle around town. Put stock back in and was doing fine till lately.
vacuum is 10-11" idle, around 15-17" at cruise.
Solid lifter Lunati with EDMs, 235/243 at .050 and lift of .526/.546.

Big TQ. Most all jetting and settings according to the old Mopar bible and/or old posts here from "demonsizzler." Everything dialed in carefully. NGK BP5ES plugs really are spot on, and like I said it's running great. But something is haywire!!

Either fuel supply (which I doubt, or it would show up in performance,) or in the Bosch O2 sensor and/or gauge???? Not sure what else it could be. Ideas???
 
Is the lean condition momentary?
Or longer than a second or two?

I’ve been reading. Trying to figure out the issue.
The only idea I can come up with so far is the actual amount the butterflies close on each throttle blip. These are old carbs and the linkage could be a bit loose allowing the throttle blades a ever so small amount of resting positions however small & minute. There is a lot of external parts on a TQ that can be effected. Worn, bent, out of wack/adjustment.
 
More than a second for sure. Go for a short drive and all looks good, then all of a sudden the A/F #s read very lean for the rest of the way back. But I don’t notice a difference in how it runs.
I’ll do some close inspection of the external parts.

...it seems to be true that the numbers are consistent and normal at WOT. But I’d think the same would be true at cruise and part throttle if it was how much the the butterflies were open.

When I blip the throttle and watch the the rods they seem to operate properly.
 
That's a pretty big bumpstick for a metering rod carburetor. Sometimes, the metering rod piston gets dry and tries to hang up in the bore. This is especially true when vacuum drops and they tend to bounce around as with a large camshaft. Have you inspected the metering rod piston and bore closely to make sure they are clean and smooth? Also, use a light oil to lube the piston up in the bore so it will operate more smoothly. I wouldn't use anything heavier than Ford Type F ATF.
 
That's a pretty big bumpstick for a metering rod carburetor. Sometimes, the metering rod piston gets dry and tries to hang up in the bore. This is especially true when vacuum drops and they tend to bounce around as with a large camshaft. Have you inspected the metering rod piston and bore closely to make sure they are clean and smooth? Also, use a light oil to lube the piston up in the bore so it will operate more smoothly. I wouldn't use anything heavier than Ford Type F ATF.

I checked the piston and bore, did not seem rough or scarred and seemed to work OK, but I will try an ATF lube and see if that makes a difference.
 
The tree is for enrichment. If it gets stuck up top, you’ll run rich at low/idle rpm. The reverse happens if it gets stuck low. Fine at idle and cruise but lean under acceleration & WOT.

Idle trim screws exhibit force in the screws?
 
I know it’s and expense but try a new/different O2 sensor and see if it repeats.
When the Bosch O2 sensor failed on my AEM unit,, it would go to full lean immediately after its warm up cycle.
My failed sensor did not have a lot of use on it. Was maybe 3 years old, but in a drag race only car with probably 60 runs on it. I called the AEM Tech line and they had me do the ‘Gas Rag Test’ to confirm that I had a failed sensor. Prior to spending the $$ on new parts you may want to call the AEM Tech line and get their input to resolving if you have a sensor or gauge issue.
 
When the Bosch O2 sensor failed on my AEM unit,, it would go to full lean immediately after its warm up cycle.
My failed sensor did not have a lot of use on it. I called the AEM Tech line and they had me do the ‘Gas Rag Test’ to confirm that I had a failed sensor. Prior to spending the $$ on new parts you may want to call the AEM Tech line and get their input to resolving if you have a sensor or gauge issue.

Ahhh, that’s pretty similar to my symptoms!! I’ll call them and see what they say. Thanks.
 
Ahhh, that’s pretty similar to my symptoms!! I’ll call them and see what they say. Thanks.
Yes, they were very helpful with me. FWIIW, I have an AEM AF gauge in another vehicle and it is a daily driver type of vehicle, have over 40,000 miles on it with no sensor or meter issues. Go figure why the one with very light use failed first.
 
AEM Tech confirmed that the erratic readings indicate a need for a replacement Bosch sensor. Ordered one from Summit.

I always appreciate the variety of suggestion here that help track down a solution. Often the info is helpful even if it turns out to not be the answer to the issue of the moment. :)
 
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It's a 6518 which my info lists as "industrial" so maybe motorhome???
340 motor is 9:1 (actual) LD30, 1-5/8 headers for now, ported X heads, 273 adjustable rockers with the solid cam and EDM lifters as listed above. Timing is set at 18* with 38* total. (tested quickest in the 1/4) MSD distributor vac adv. is limited to 15*
Plugs are NGK BPR5ES and are FINALLY looking good! To the best of my knowledge. (light grey at very base of porcelain, an even light black around base of plug, timing mark right at bend, blue line just above the weld, etc.)

In case it's helpful to anyone still running a Thermoquad here's the full set-up:

No choke (starts great, warms after a couple of minutes)
Jets .101/.149 ( likely may require a change when I go to bigger headers)
Rods are 68 x 57 x 47 (I made these from Edelbrock rods, but stock 340 carb rods work well #1966 - 67 x 52 x 45 )
Accelerator pump nozzles I drilled out to .138"
I have my pump set at 1/2" up
Idle screws 1-1/2 to 2 open
Air door spring at 2-1/2 turns
Rods adjustment screw 1-1/2 to 2 turns up
The air door opening at 17/32 from choke tower
The air door drop is set at .010"
No restrictor in the pull-off vac line (if there is a bog problem this is something to look into, maybe try first)

After a lot of tweaking, (when my meter is working) I get a pretty consistent 14.7-ish warmed up open highway number, drops to 15-ish under light throttle.
I try to keep the idle A/F at 14.5 to 15
WOT now launches at 10-ish but quickly climbs to around 11.5 or so.

It's pretty late for me to be learning what I wished I know when I was young, but never give up!!! (Hahahaha) A lot of everything I know about any of this I've learned from this site. It's been an adventure, but that's where I am now. :D
 
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