850 Thermoquad Competition Series rebuild

[gzig5]

I'm getting into a rebuild of this 850 Competition Thermoquad 4846s I picked up locally, and this one being something of a unicorn, I've got a few questions as I get going. I found a rebuild kit at The Carburetor Doctor, but if there is a better source, I'm all ears. This carb has some issues but seems to be in pretty decent overall condition. I'm looking forward to rebuilding and putting into use on my fresh 340 later this year (fingers crossed). I'm starting out with a Holley 750 but plan on trying several intake and carb combos.

So these were the first Thermoquads made back in 1969 time frame and they used press in jets for the first year before changing to a threaded version from 1970 on. Press in jets aren't available and I'm OK with that as I can make my own jets or more likely convert it to accept a threaded jet of some sort. But, I'm wondering how to get the pressed in jet out. It has a screwdriver slot, but that just spins it in the bore. Access to the well the jet rod goes in is blocked at the top by a riveted plate, so I can't push it out. I was going to modify a pliers for that specific diameter to get hold of it and then pry off the top of the stand. If there is a better way, let me know.

The secondary discharge nozzles are missing. I haven't measured but they look similar diameter to the rods in another newer carb I have on hand. Does anyone know if these are interchangeable? I'm sure I can probably get a set from a parts carb somewhere but need to know what I'm looking for.

I played hell getting the carb apart. Used the sticky that @RustyRatRod made which was very helpful, even though mine is quite different inside. The gasket between the bowl and top did not want to yield but some penetrating oil and patience helped get it apart without breaking anything. Still trying to get the gasket off the surfaces and have the parts soaking in straight Simple Green for a couple hours. I think that stuff will not hurt the metal or plating too bad. I've read where Pine Sol works too but I don't have any on hand. If there is a better way to get forty year old gasket off without too much scraping I'd like to hear it. I don't want to butcher the sealing surfaces with hard tools and so far the plastic scrapers are ineffectual.

In the RRR sticky, he replaced the nitrophyl floats with brass ones. Is that always needed or just a good idea? These floats look new, or at least the metal arms do, like someone put a kit in and it didn't run much or all after that. Not even sure if I can get them for this model but I'll look.

I need to check the main shaft clearance to the plate. It feels like there is a little slop but I don't have a new one to reference it. I haven't found a procedure for removing the butterflies, springs, linkage, etc required to pull the shafts out. No sense putting a kit in it only to find vacuum leaks on the shaft, so I want to inspect it first. Any guidance or shaft/bore tolerance spec would be helpful.

 

[RustyRatRod]

If you use the brass floats, make SURE they are not soldered to the brackets upside down. There was a run of them for a long time that were like that. That made me go back to the nitrophyl floats. I would still replace them with new.

 

[Kendog 170]

My dad a mechanic from the 40's thru 70's use to boil carburetors before cleaning and rebuilding. Old school guy. I have never tried but wondering if it would remove the gaskets. Someday I'll throw an old junk carb. in a pot and see what happens. Good luck with your build. Way over my skills to attempt. I have put a kit in a Thermoquad years ago. Their was a member on here that had a ton of carbs. Maybe you can do a wanted for a parts carb?

 

[ch1ll]

Cleaning any fuel mixing setup these days is a real chore. Nothing seems to work as good as the pre EPA carb cleaner containing dicloromethane. I would soak the plastic fuel body in luke warm water to soften it up before I would use any chemicals or cleaners. Remember you are rebuilding a relatively rare carburetor.

 

[ch1ll]

Also these comp series carbs are much different than the other thermoquads, even the 71.

 

[rumblefish360]

@gzig5 Do you have spare TQ’s?

 

[gzig5]

@gzig5 Do you have spare TQ’s?

I do. Have a 9xxx and a 6xxx model. Just haven't dug them out of the back corner yet.

Also these comp series carbs are much different than the other thermoquads, even the 71.

No joke there. It's pretty interesting how much more crap is hanging off the two later models that I have. Most of it is probably emissions related. Not much going on inside the 850 Comp.

 

[RustyRatRod]

Yeah, those early 850 and 1000 carburetors were bare bone flow monsters.

 

[Valman]

I understand that the primary jets are push in, but I seem to remember someone converting the secondary jets to use standard Holley jets. Not sure who or where it was.

I think some brass tube was used, cut to size, then threaded. Are AFB primary jets a similar size to the original push in TQ C.S jets ?

 

[Bewy]

[1] The press in jets are removed by gripping them with bull-nosed pliers, gently wiggling side to side while pulling on them. Use WD-40 first.
[2] Original floats were brass. Ok to use nitrophyl but float dimension should be reduced about 0.100".
[3] CSTQ sec discharge tubes wwere unique, cut off at an angle. If you can find another set to install, make sure they are tight in the casting, use Loctite. Later TQs had holes drilled in the side of them. You will need to solder [ soft ] these up if you intend to use these in the 4846.

 

[Bewy]

A couple more tips:
- it was claimed when these carbs came out that the press in jets could fall out. Very simple fix. I use AFB/AVS piston springs. Push over the sec jets & the springs will rest on the bottom of the float bowl & prevent the jets from dropping. The pri jets are bigger. So using long nose pliers, open the end of the spring slightly so that it will fit over the jet.
- if you use 6xxx or 9xxx sec discharge tubes & solder the holes shut, run a drill up the tube to make sure the solder has not run inside & blocked the tube.
- not 100% sure about this, but I think it is possible to push the discharge tubes too far in....& block flow from the jet.

 

[rumblefish360]

[3] CSTQ sec discharge tubes wwere unique, cut off at an angle. If you can find another set to install, make sure they are tight in the casting, use Loctite. Later TQs had holes drilled in the side of them. You will need to solder [ soft ] these up if you intend to use these in the 4846.

A couple more tips:

- if you use 6xxx or 9xxx sec discharge tubes & solder the holes shut, run a drill up the tube to make sure the solder has not run inside & blocked the tube.
- not 100% sure about this, but I think it is possible to push the discharge tubes too far in....& block flow from the jet.

Exactly where I was going. Bingo!
That’s why I asked gzig5 if he had extra TQ’s laying around.

 

[gzig5]

1] The press in jets are removed by gripping them with bull-nosed pliers, gently wiggling side to side while pulling on them. Use WD-40 first.

Do you have a picture of these magical bull-nosed pliers or of the removal process? I've been unsuccessful in removing the jets and I'm trying to be very careful not to gouge them up. The primary jets have a slot and will spin with a screwdriver or pliers, but they are short and its difficult to grip them. I 3d printed some bushings that are on size but the plastic isn't strong enough. Way I'm looking at this, I'm going to need to make some brass jaw adapters that are the same diameter as the jet. Maybe a little bigger and use some crocus cloth to help get a grip. It's one thing to grip it so it will turn, but another to pull them out axially. No wonder they changed to threaded after a year of this. I'm sure the user feedback was not positive.

I haven't been able to get the secondary jets to budge yet. Might need some special jaws there too. Trying not to heat it up and ruin the o-rings but I imagine they need to get changed too, if available.

 

[Bewy]

I think you will need new o rings. I wouldn't trust o rings that have been in fuel for years & have since dried out. Look at the acc pump plungers.....the rubber goes hard.

I have never had a problem removing the jets, especially if they rotate. Try heating the alum with a heat gun to expand the opening & dribble in auto trans fluid.
Vice grips [ the gadget with the screw adjuster ] might be better than bull nose pliers on the pri jets, but you would need to be careful not to squash the sec jets,

 

[gzig5]

Finally won. Made inner and outer bushings to support the jets and a little heat. One of the primaries was a real ***** but it eventually yielded.
Got a couple ideas on how to convert to a screw in jet but need to get it on a motor to verify function.

 

[ch1ll]

You did well. Keep posting your progress please.

 

[RustyRatRod]

Finally won. Made inner and outer bushings to support the jets and a little heat. One of the primaries was a real ***** but it eventually yielded.
Got a couple ideas on how to convert to a screw in jet but need to get it on a motor to verify function.

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Smart feller! What's the plan? Are you going to tap for screw in jets? I just got done rebuilding a 625 Street Demon for a friend and the similarities to the TQ are astonishing.

 

[Bewy]

Couple of comments:
- having removed the jets, the hard part is now done. Just get new O rings for the jets.
- trying to modify the carb to use primary jets from another carb brand is fraught with danger. The position [ up & down ] of the met rod is critical to the cruise/power mode & trying to adapt another jet....& get it correct....is a lot of work for nothing.
- in this forum section, in Oct 2023, I posted pics of mods I made to 6000 & 9000 series TQs. After doing these mods, the butt meter could not feel any difference in performance between a 6XXX & 850 CSTQ. Both carbs had the choke blade/assy/linkages removed.

 

[gzig5]

I understand that the primary jets are push in, but I seem to remember someone converting the secondary jets to use standard Holley jets. Not sure who or where it was.

I think some brass tube was used, cut to size, then threaded. Are AFB primary jets a similar size to the original push in TQ C.S jets ?

I can't comment on the size of the AFB jets as I've never had one apart. These primary push in jets look close to the later TQ thread in jets, at least in the pictures. I need to pull my other carbs apart and compare.

Smart feller! What's the plan? Are you going to tap for screw in jets? I just got done rebuilding a 625 Street Demon for a friend and the similarities to the TQ are astonishing.

I am not sure yet what I will do for the secondary jets. The hole in these is pretty big so I'm not sure how much flow changing the diameter a couple thousands is going to make. There is no rod in the secondary obviously, so it's pretty much just a big tube that regulates flow. I don't know much about these carbs yet, but I imagine the air bleed emulsion tube that is in there working with the secondary air door help fine tune the secondary needs. I would like to hear some feedback from folks that use a strip kit to tune their TQ. I am also under the impression that these early carbs work on a different fuel regulation theory than the later ones?

For the primary, there isn't a lot of wall material in the casting to thread. It's pretty thin. So I think what I'm going to try is to create a press-in jet carrier that will accept screw in jets. The carrier will stay in place and using a tool to hold it in place, the jet can be screwed in or out in a few seconds without all the cussing I was doing trying to get these mother-&^%&**#'s out. The jet bores will need to be similar length to the original and end up in the same position relative to the rods so the timing stays consistent to original design. I don't know yet if a standard Holley jet will accomplish that, but it looks like they will at least fit in place to have a try. Would be great to be able to leverage off the shelf jets. If successful I'd make conversion kits for those interested but I don't want to spend all my time in retirement bent over the lathe threading and drilling jets. I should have a proof of concept in a few weeks when I get back from vacation and picking up my fresh 340 from member NC Engine Builder. But I won't be able to test on the running engine until much later this year.

 

[gzig5]

One other thing I wanted to mention about cleaning the gasket material off. I don't know what they used but I think it was shellac or something to glue the gasket to the upper casting and bowl. The lower one came right off. The upper one wasn't coming for love or money. Soaked in penetrating fluid and then simple green for a while softened it up some. Was fresh out of carb cleaner which may have worked better, though the stuff we can get now is pretty weak. Anyway, to avoid scratching and galling the flanges up with sharp metal tools like knives or chisels, I "sharpened" a 3" x 1/2" x 1/4" hunk of plexiglass on the belt sander to give it sharp 90 degree corners all the way around. These sharp, blunt edges worked great to push the gasket material off bit by bit. It was still a lot of work but as that last pic shows, it came off and didn't ruin that little casting line down the middle of the flange that helps the gasket seal. When all the sides got rounded off, just threw it on the belt sander again and squared them up. A good substitute if you are fresh of plexiglass would be a 6" Starrett or Lufkin ruler. They are steel but the edges don't grab like a chisel will. Use it like a cabinet scraper.

 

[volaredon]

Smart feller! What's the plan? Are you going to tap for screw in jets? I just got done rebuilding a 625 Street Demon for a friend and the similarities to the TQ are astonishing.

huh. thats interesting.. might have to look into one of those.
Whats the difference between the OPs carb and the 9801/9811? I have (I think) a 9811 here I was getting all ready to plop on an engine this spring/// I thought this was just a reissue of the CS series carbs?

 

[RustyRatRod]

One other thing I wanted to mention about cleaning the gasket material off. I don't know what they used but I think it was shellac or something to glue the gasket to the upper casting and bowl. The lower one came right off. The upper one wasn't coming for love or money. Soaked in penetrating fluid and then simple green for a while softened it up some. Was fresh out of carb cleaner which may have worked better, though the stuff we can get now is pretty weak. Anyway, to avoid scratching and galling the flanges up with sharp metal tools like knives or chisels, I "sharpened" a 3" x 1/2" x 1/4" hunk of plexiglass on the belt sander to give it sharp 90 degree corners all the way around. These sharp, blunt edges worked great to push the gasket material off bit by bit. It was still a lot of work but as that last pic shows, it came off and didn't ruin that little casting line down the middle of the flange that helps the gasket seal. When all the sides got rounded off, just threw it on the belt sander again and squared them up. A good substitute if you are fresh of plexiglass would be a 6" Starrett or Lufkin ruler. They are steel but the edges don't grab like a chisel will. Use it like a cabinet scraper.

Try soda blasting. That's how I clean them now. It works pretty good.

 

[RustyRatRod]

huh. thats interesting.. might have to look into one of those.
Whats the difference between the OPs carb and the 9801/9811? I have (I think) a 9811 here I was getting all ready to plop on an engine this spring/// I thought this was just a reissue of the CS series carbs?

A lot. The PO's carburetor is a born and bred race piece. The 9800 series are all aftermarket street versions that came with electric chokes. They are nice carburetors. I have one I am currently working on. I don't really know what I am gonna do with it. lol