Technical Apparently a rare carb... help

Well... A carburettor works by using the air speed through it create a (dynamic) vacuum there, sucking in the correct amount of fuel through the jets from the fuel bowl. This works well as long as the air speed is high, but if you for example have a big carb on a small low power engine and give it full throttle at low rpm air speed would be low, the carb wouldn't be able to accurately do it's job, and the engine will give less power (perhaps even stall) than it would have at perhaps quarter throttle - the smaller throttle opening keeps air speed up.

With a mechanical secondary you have complete control over how much the throttle plates in the carburettor opens, menaing that you yourself have to get a feeling for when the engine actually can use full throttle - you might get a better start from idling at the red light if you push the pedal down "slowly" as rpm increases, rather than going pedal to the metal straight away.

With vacuum secondaries the carb uses the (static) vaccum in the intake to sense if the engine "needs" more air & fuel, when the engine can suck more air the vacuum increases, the secondaries open proportionally to that vacuum to let more air in, meaning that you can go pedal to the metal and let the carb do all the "thinking". Assuming the carb is working correctly, is properly adjusted, and so on.

 

Wow, thank you, seriously cleared up several things I had been going to look into. by that thinking, it seems if an engine is making enough power, and you get the RPM up before launch (like one would), then a mechanical secondary should work just fine, no?


Sent from my iPhone using The H.A.M.B. mobile app

 

Generally, most enthusiasts avoid using a mechanical secondary carb on the street because of drivability, even the square-bore carbs. A few enthusiasts with radical engines, manual transmissions, and high ratio (low-speed) rear gears do use them on the street. I was unaware that the Holley spread-bore was available in a mechanical secondary.

A really good analogy would be a quadrajet with a fatigued air-valve spring; which many, rather than replace the spring have labeled as a quadrabog.

But since you have the carb, try it to see what you can make it do.

Jon.

 

Well, both types will work, with their own pros and cons (carbking mentioned fuel economy for example) - neither is a bad type of carburettor, when adjusted and used correctly. As long as you are aware that pedal to the metal at any rpm probably isn't a good idea I'm sure you'll be able to figure out how your carb & engine likes to be throttled after driving it enough.

 

I can't thank you all enough, many questions I didn't know I had are now answered, I'll run it and see what I can do with it
It will be interesting either way


Sent from my iPhone using H.A.M.B.

 

Jon...the vast majority of those spreadbore carbs I run across at swaps here in Socal...list 6210....and I currently have a 6212...are double pump, mechanical secondaries. I would say at the swaps it's easily 15 or 20 to 1..double pump over a vacuum secondary Holley spreadbore. Holley probably sold few vacuum secondary spreadbores it seems. I agree with someone who said it might work out best with manual transmissions and lower rear gears. My Corvette example was a 4 speed and had a 4.11 posi. Not the right carb for everyone...just wanting people to give it the shot it deserves in oddball situations.

 

Squirrel...the quadrajet has similar plugs...two of them...under the fuel bowl, underneath the carb. Every QJ ive ever had rebuilt or rebuilt myself got those stupid things epoxied over. When those leak...it drips gas into the motor at inopportune times....like when your out to dinner...or your battery is weak. A QJ is best for all around driveability and economy for sure....but you can't beat a double pump carb if the rest of your drivetrain was setup for speed.

 

The Qjet leaks down inside, when the plugs leak. The 6210 leaks fuel all over the place, and there are 6 of those plugs, I think. Just beware.

I'm surprised carbking doesn't remember the spreadbore double pumpers. pretty common in the 70s to replace the "evil" quadrajets.

Good luck with it, don't waste too much time on it...

 

get a run-of-the-mill vac secondary 600 holley and be happy!

 

Can sorry! To busy makin' life hard on myself!


Sent from my iPhone using The H.A.M.B. mobile app

 

I expect you could learn a lot about carbs, with this one.

 

That is the hope, might s well learn on a cheaper one, than brake a nice one


Sent from my iPhone using The H.A.M.B. mobile app

 

Jim - we never considered the Q-jets "evil", well at least after 1966. The 1965's and 1966's did leave a lot to be desired. And when we couldn't talk the owner into fixing a later Q-Jet (often, a secondary air-valve spring was the only issue), we did (and still do) replaced the Q-Jet with the Carter TQ.

Jon.

 

I know you understand how Qjets work, but many guys didn't want to learn, so when they had problems that they didn't want to take the time to learn how to fix, they badmouthed the carbs, and replaced them with the familiar Holley.

 

Friend of mine gave me a vacuum secondary spread bore that he had run on a dirt track car. Class he was in required a stock cast iron manifold and stock style carb, the Holley was accepted as a stock style replacement. He had the rear bores linked mechanically so they came in at about half throttle, of course he was running flat out on a track most of the time. He said it worked good for him. It's still on a shelf in the garage, I'll never use it, but you never turn down a friend when he's giving you something.

 

good luck and congrats on tackling a carb like that

 

I think many carburettors and other things meet that fate. People fear what they don't understand.

The (OT) small twostroke bikes I mostly work on usually comes with an oil pump for automatic lubrication, rather than mixing the oil in the fuel. Supposedly, if you trust what everybody "knows", these pumps are unreliable and you shouldn't use them, but in 20 years I still haven't seen a pump that has failed. I HAVE seen the oil hoses cracking from age and incorrectly adjusted control wires, but that comes down to pure lack of maintainance - people seem to think things should work w/o proper maintainance for decades, and when something fails it isn't their own fault...

 

I have a rare one which has the square bolt pattern... The primary throttle bores check at 1-9/16" like a 600 and the secondaries check out at 1-3/4" like a 750....
Last time I did a search on it, sez it came off a 1970 Chrysler car..
It also has vacuum secondaries...

 

If it's the one made like this they fall flat over 5,000 RPM this version of the Streetmaster came out during the Jimmy Carter "gas crisis" it was made for fuel economy and low RPM torque. You should use a Camshaft that goes along with it for your truck. And this: https://www.summitracing.com/parts/sum-g1412-q/overview/ With your set up you may find the need to lean the primaries and do a vacuum check to get the correct power valve.

 

"307 in a 68 Corvette" Must have been a really rare option.

 

Lol....all Corvettes always have their original engines...right? In my case it was a 1968 427-435 hp roadster that someone stuck a 307 in to keep the car running. That double pumper spreadbore made that motor seem more like a 350..

 

Gavin -

One thing it seems no one has mentioned...
It's OLD...before you spend any more money on it, make SURE the throttle shafts are tight, or at least snug.
If you can wiggle either the primary OR the secondary shaft back and forth...hang it on the wall..!
If either shaft is loose, you'll NEVER get it to idle properly.
A couple problems - 1. air will leak between the shaft(s) and the base plate. 2. the butterfly's will NOT seal properly. 3. the butterfly's will not stop at the same position within the throttle bore every time you take your foot off the throttle.
None of the above is a good thing..!

Mike

 

No way to fix that.........sure is a way. JB Weld or epoxy works well.