The Stovebolt Site Forums The Shop Area The HiPo Shop Vacuum/ Carburetor Selection

A lit of information has been shared in this discussion; I’m glad it got legs.

We know that I have low vacuum at low rpm, and it is likely that my camshaft is contributing to that.
We know that the primaries on my Edelbrock 1405 600 cfm carburetor are smaller than the secondaries, but the difference is not great.
I agree that a full review of timing is in order, in the interest of optimizing vacuum.

I would now like to pose a simple, specific question. A premise of the question is that the issue I’m dealing with is not eliminated through timing adjustment.
Q: What are the chances that a QuadraJet , with it’s smaller primaries , will yield a significant benefit to my problem? My simple reasoning suggests that the smaller primary venturies would function better at a lower vacuum than the larger ones I now have. I’m not sure if that reasoning is sound…?

I know I posed this question early in the thread , but the discussion has swung in several directions since, so I’m asking again.

I’d like to add - my guy Rick is a strong proponent of the Q-Jet and he considers himself an expert in tuning them.
He has a) several Q-Jets on a shelf awaiting a new life , and b) my complete confidence.
I’m pointing that out so we know that I have access to that option.

The interweb say the primary bores on the Edelbrock are 1.44 inches, the secondary bores are 1.69 inches. The Q-Jet primaries are 1.375 inches, probably not enough difference to notice if that were the only thing we are concerned about. The Q-Jet probably will have better idle fuel control and atomization due to the triple venturi design. If Rick has a variety of main jets, along with primary metering rods, he could use them to fine tune idle/off idle/low rpm characteristics. It will be a labor of love and I know you don't want to take advantage of him. We are still guessing about the camshaft, having no idea what it is. We know that whoever put the engine together thought they were going to stick a big hog in the butt with the single plane manifold and 600 cfm carb, so the cam might be just as much over-kill as the rest of it. Hot Rod Lincoln nailed it when he said the two most common problems when someone makes modifications is too much carb and too much cam. If I was working on your truck, I would play with the ignition timing first, then if it shows some improvement I would swap to a Q-jet and see if you can dial it in a little more. Thats my story and I'm stickin' to it!

Buckshot - with all due respect, there is an error in the content of your above post.

While throttle bore diameter has meaning, the airflow is determined NOT by the throttle bore diameter, rather the internal venturi diameter. This error is very wide-spread.

For better understanding, even though this article is a Stromberg single barrel updraft, I think it will add understanding:

Example of different air flow, same throttle bore

If the imitation were a genuine Carter of the same size, the air flow on the primary side would be 250 CFM.

Most Q-Jets prior to 1976 have a primary size of 150 CFM.

I'm too lazy to figure the relative air velocity difference, but it is SIGNIFICANT!

Waveski - you may find an unintended result of higher vacuum!

Jon

Please explain why higher vacuum would be unintended and / or undesirable.

I need to update the article, but technology updates (I cannot modify my website at present) prevent me from doing so. Isn't it wonderful when something that worked before a software update, no longer works.

Since I wrote the article many years ago, I have since found a dusty Stromberg chart in all my miscellaneous Stromberg literature where Stromberg DID publish air flow.

The SF-2 Stromberg, which is the roughly the same throttle bore as the imitation AFB in question, can flow from 94 CFM to 187 CFM depending on the internal venturi size.

These numbers are measured at 3 inches Hg (standard convention for one and two barrel carbs), whereas the 4-barrels are measured at 1.5 inches Hg.

Converting: the 94 becomes 66, and the 187 becomes 132.

The SF-2 is a single barrel carb, so to get an approximate range, multiply the above by 2.

So the low range for the imitation would be 132 and the high range would be 264. The 250 I mentioned if it were a genuine Carter, is well between the two.

Jon

QUOTE Waveski "Please explain why higher vacuum would be unintended and / or undesirable." END QUOTE

Unintended, because you were not expecting this result, assuming the low vacuum was only a function of the cam.

NOT undesirable, VERY desirable.

Jon

Music to my ears , thank you.

One of the real advantages of the QJ carb is the fact that the primary barrels have not one, but THREE venturis. That allows gasoline to begin flowing into the high speed circuit just above idle speed, and as the air velocity increases, the second and third stages of the venturi stack start adding fuel to the airflow. In simple terms, each primary barrel becomes not one, but three carburetors, operating at different airflows. There's not a single venturi in the world that can offer such an accurate level of fuel metering. Then the secondary barrels come into play once the airflow in the primaries maxes out- - - -and those barn doors are on a mechanical linkage- - - -no need for vacuum to open them up. The airflow (and fuel metering) in the secondaries is controlled by a spring loaded flap that resembles a choke plate at the top of the carb- - - -preventing any possibility of over-carburetion. BTW- - - -that air valve has an adjustable torsion spring, so its rate of opening can be fine tuned for a specific engine's needs- - -like camshaft changes or other non=stock equipment like a single plane intake manifold, etc. The Carter AVS and thermo-quad carbs have a similar, but not identical design.
Jerry

The link below is to an illustration from my Carter Carburetor School workbook explaining the triple venturi mentioned by Jerry above.

The experiment was conducted using several OPEN half-full glasses of water, with transparant tubes connecting one end to a vacuum take-off at a specific location in the venturi, and the other end submerged in the water.

The water head differences in the tubes demonstrates the differences of internal carburetor vacuum at various parts of the venturi.

Carter triple venturi effect

Jon

Mr Lincoln,
Thank you for describing the elaborate design of the Q-Jet primary venturi. I was nor aware if that significant design feature.

Please clarify a point about the secondaries :
You stated that they are controlled by mechanical linkage, not vacuum dependent. You also referenced a spring in that linkage that can be swapped/tuned to a particular engine. If the spring is not vacuum reactive, what is it’s roll?

Waveski, the Q-Jet secondaries have a two step actuation. When the primary throttle valve (this is directed by your throttle peddle) reaches approximately 3/4-7/8 of its full opening, the secondary throttle valve is actuated and linked to the primaries and will open progressively. Fuel will not be pulled into the secondary bores until the engine can produce enough vacuum to overcome a second "air valve" which is above the throttle plate. The air valve is spring loaded to stay closed until the vacuum level is sufficient to open it, when it opens, fuel is pulled into the secondary bores and ingested by the engine. The secondary air valve spring is adjustable and can be tailored to the demand of the engine. The Q-jet has massive secondary bores, it also has metering rods that can be changed and/or adjusted to meet the engine demand, the spec for the more common Q-Jet is 750 CFM, but this is directly dependent on the volume of air that the engine can actually handle. As Carbking has stated, the primary side of the carb is rated at 150 CFM and that is where you spend 99% of the time, when you need horsepower for going up a steep hill or passing another vehicle, you push the "GO" peddle to the floor and the secondaries will feed the engine with whatever it can handle.

For anyone interested in migrating a Q-Jet to a different application OR using an original Q-Jet on a modified original application:

A gentleman by the name of Doug Roe wrote a book on various Rochester carburetors, including the Q-Jet.
A gentleman by the name of Cliff Ruggles wrote a book on modifying Q-jets.

Mr. Roe was the design engineer on the Q-Jet in the early 1960's. I think his book was written around 1970.

Mr. Ruggles is "Mr. Quadrajet" to the current car hobby. I think the first issue of his book came out about 2010. There has been a lot of changes from 1970 to 2010.

I started reading carburetor literature and working on carburetors in 1959. Since then, I have acquired, and read (mostly) hundreds of books on carbs. Of them all, Cliff's is absolutely number 1 when it comes to being "user friendly". It is not as technical as Doug's book, but it does not require a degree in higher mathematics to understand everything.

The book is paperback, and not expensive; anyone working on one of these should own the book!

As to the secondary operation, Buckshot is spot on; however, the secondaries may also be tuned with choke pull-offs with a different vacuum orifice. And while for exceptionally radical engines, the spring tension MAY be adjusted, we have found for less than full race applications the original setting on the tension works best. Tuning of the secondaries is better accomplished by: (A) change in secondary rods, (B) change in secondary hanger, (C) change in CPO vacuum orifice.

Also remember when rebuilding a Q-Jet ALWAYS replace both the secondary cam and the secondary spring!

Jon

One more bit of carburetor trivia- - - - -having an airflow-related volume control valve in the secondaries is really nothing new. At least some (maybe all) of the Rochester 4G square footprint 4 barrels in the 1950s used spring loaded air valves in the secondary bores. Those carbs also had two floats- - - -one each for primary and secondary circuits, similar to how Holley 4 barrel carbs work. The 4G had a tiny float chamber volume- - - -under full throttle operation it was possible for a slightly low float level to cause the float bowl to run dry and cause a nasty lean surge. I believe some of the Carter AFB series had a weighted air valve that served a similar purpose.
Jerry

Jerry - you aren't old enough to remember LOL

Schebler (and others) had the air valve in the 'teens (the 19 teens).

The early 4GC's did not have the air valve; from memory, 1956 was the first year, and too lazy to look it up.

Jon

Hmmmm- - - -I'll have to check the updraft carb on my 1936 Allis-Chalmers WC- - - - -I'm pretty sure it has a Marvel-Schebler carb on it!

LOL!
Jerry

Well Waveski, hopefully we don't have you completely scared and confused. I was thinking (not common for me) that if your buddy Rick has the tools, he could figure out what the camshaft specs are without pulling it from the engine. He would need a degree wheel and dial indicator, those items can be affixed to the engine and by turning the crankshaft by hand, the lobe lift, lobe separation, valve overlap and duration can be gleaned from reading the lift of the pushrod and amount of rotation in degrees. Those numbers would tell you if that camshaft is too aggressive for your needs. If the cam is mild enough, the Edelbrock manifold and a Q-Jet can make the engine happy, if the cam is deemed too hot, you can decide on the next step.

Couple thoughts. That carb should do well with that engine out of the box.
There are some adjustments to that to help with the quitting into gear.
What Jerry said 2nd post, the timing set is crucial and not an easy thing to do alone. Engine needs to be at 3100rpm.
I can say that once I had my 327`s timing set proper, It starts much better now. Idles smoother.

Buckshot —-
Am I scared? No , and you’ll be glad to hear that I am much less confused now than I was at the start of this thread.
I have taken into consideration the information offered here, I’ve had several discussions with Rick aka Auto Doctor, and factored in my willingness to invest time and money - and arrived at a logical sequence by which to deal with my engine issue—-
1) Pull manifold AGAIN and install special gasket that blocks exhaust port at the upstream end. This should lower my carb temp a bit further, and it makes sense to mediate the heat issue as much as is practical as a first step.(note: the carb body was approx 170 prior to spacer , now is approx 150 , better but still above the 135 bubble threshold)
2) If problem persists after step 1 , full re-analysis of timing in an effort to improve vacuum at idle.
3) If necessary, analyze camshaft as you described. Auto Dr does have necessary tools and knowledge; discussion of that process has already taken place.
4) Quadrajet.
If it comes to carb swap , I’ve narrowed my search to 2 sources. Both are small American companies that rebuild Rochester carbs (and others) to specific applications. That seems to me like a much better option than buying a new one on Amazon for $200 - who knows what 3rd world country they come from. (Those cheapies are referred to as “QuadraJet” , but not Rochester…)

A distant 5) is pull the camshaft.
It’s doable , but I’m trying to avoid it. In fact , I’m trying to avoid thinking about it.

I went through, and and am continuing, a similar thought process regarding my problematic heart over the last 3 years. The nuclear option was transplant. After much research and consultation I chose a course of action that has - so far anyway- bought me time and avoided the risks and other downsides of the nuclear option. 2 more procedures planned between now and summer, in stages somewhat similar to my plan for my 283.
Trying to avoid a camshaft swap , trying to avoid a heart swap. Willing to accept dome degree if comprised function in both cases.

Am I making sense?

I like your sequence, I totally agree with not buying a carb of an unknown source, a $200 QuadraJet is not realistic. If Rick doesn't want to part with one, Carbking would be my next "go to" and could probably set one up real close to the application. I just had my 3rd appointment with the head of Urology at The University of Michigan prostate cancer center, already had MRI and biopsy, just need to decide between surgery or radiation. GOD has this one under his care and HE will lead me through it, I'm leaning towards radiation.

I know the Edelbrock carbs are frowned upon here, but if I were you, I’d research the latest AVS 2 . See if it’s available in a 500 or less cfm. I’d been reading about how good they were at idle & off idle response that I started reading user reviews, which were all excellent. Once I got my C10 on the road I installed a 800 AVS 2 with mechanical choke on my 454. I don’t know how truck could respond any better than it does now. It’s at least worth looking into, imo. Check out the reviews & see what others are saying

Unless this engine will be in a trailered racecar, I would suggest at least reconsidering that option.

I started working on automobiles in 1959 (my own, Dad's before that). In that time I have made many mistakes, and will probably make a few more.

But blocking the heat cross-over on my 350 was one of the top three DUMBEST mistakes I have EVER made on things mechanical!

Consider the Law of Unintended Consequences and how it might relate to idle quality. (Or, maybe your heel/toe technique is better than mine) ! 50 years ago, it was pretty good, now the knees don't work as well LOL

Jon

I haven't seen a good way to block a heat riser passage in the past 50 years- - - -including that tiny piece of stainless steel sheet metal that some of the hotrod places sell- - - - -it will burn out in a matter of weeks at the longest. I've even seen people pour the heat riser passages completely full of molten aluminum- - - - -it disappeared in less than 3 months of street driving. If you really want to eliminate that passage, buy an intake manifold without one- - - - -and trailer your car to the race track!
Jerry

Just when I thought my path was clear …

I do not understand how blocking that port could have a detrimental effect on idling. That heat seems useless to me - especially since I have manual choke. I’m sure that the manifold will warm up sufficiently without it; I doubt the heat reduction will be drastic. And - I’m sure that my carburetor is hotter than I want it to be. It doesn’t need to be 150+ degrees.

What is wrong with using a gasket specifically made to block the port? I’m not planning on using steel wool or some other bubba fix. I’ve spent 3 years un-bubba-ing, not planning on re-bubba-ing.

Engines NEED that exhaust heat to keep the mixture uniform- - - -all the time- - - - -it's not just a warm-up thing. Once the heat riser valve opens up the flow is reduced drastically, but there still needs to be some manifold heat, even at full coolant temperature. If all you're doing is racing, and the carb is jetted rich enough that uniform mixture distribution between cylinders is a moot point- - - -then disable the heat riser. Street engines need at least some manifold heat all the time. Like I've claimed quite often- - - -doing race engine stuff on a street machine is going to be frustrating at best, and downright dumb at worst! Put a wireless BBQ grill thermometer sensor under the hood sometime to see what the whole engine compartment temp is- - - -you'll get a real education.
Jerry

I packed an exhaust crossover / heat riser on a big block V8 with refractory cement once to stop an overheating carburetor problem. Refractory cement is good to 2000 degrees for repairing fire places, Franklin Stoves, etc.

Otto -
Did you happen to get a heat reading on that manifold after the modification?
The reason I ask is because I am inclined to think that the manifold will get sufficiently warm for the purpose of keeping the carb warm even with the port blocked.

When I replaced intake gaskets on my stock 1976 K20, 350, 4 bbl, the gasket set came with a selection of restrictor plates, the original restrictor plates were still intact, the instructions with the new gaskets had a chart for different applications depending on the duty type of truck, light duty called for no restrictors, medium duty use may call for a restrictor in one side only, heavy duty use would usually call for a restrictor in both sides, they were not intended to completely block the heat, they helped moderate the heat under the carburetor. Waveski is using an aluminum manifold, it will absorb and radiate heat much quicker than cast iron. FelPro, Victor, Mahle, and others will have those restrictor plates in specific gaskets sets. We know it is a balancing act to get a modified engine running correctly, especially if we don't know exactly what the internals are, in this case we don't know compression ratio and camshaft specs, we know the single plane manifold was not a good choice for street, the carb may not be perfect, etc. hopefully we can help Waveski get this thing in better shape.

\
It's been a while. The base of the carburetor was getting so hot that the gasoline was boiling in the carburetor. It was 400-500 degrees, if I remember.

Afterward it was much cooler but plenty hot enough for the carburetor to work properly and not boil the fuel out of it.

RLB,
Thanks for the interesting input.
At this point I am not inclined to bark up a new tree.
My issue is apparently based on a combination of excessive carb heat and low vacuum. I have no way of knowing if the new Edelbrock carb will address those factors.

Those restrictors are there to control the volume of exhaust gas that is available to the EGR valve- - - - -they have nothing to do with carburetor heat.
Jerry

My '73 C30, '74 K1500, '76 K20, and '84 C20 were all non-catalyst, non-EGR engines due to the GVW per year. The chart would spec restrictors based on the type of vehicle/duty and whether it was a 2 bbl or 4 bbl. My point being that if Waveski's intake and carburetor are too hot, the restrictors should help reduce the amount of exhaust cross-over heat being absorbed by the manifold.

QUOTE Waveski "I do not understand how blocking that port could have a detrimental effect on idling. That heat seems useless to me - especially since I have manual choke. I’m sure that the manifold will warm up sufficiently without it; I doubt the heat reduction will be drastic." END QUOTE

It probably will, EVENTUALLY.

I personally built a very high performance 350, crossover is NOT blocked, rather the intake manifold I am using does not have the passage.

Results of cold start:

Drive around town, and with the carb I chose (electric choke) the engine would stall at EVERY stop sign or light for 30~45 minutes! Even when pushing in the clutch!
Baby it to the highway, and after 15 minutes at 70 MPH, then engine would idle normally.

In town, I used to be pretty good at heel / toe; but those days are now a figment of my memory. The solution was to change out the carb for one with a manual choke, and pull the choke about half way out at each stop for the first 45 minutes. Eventually, the engine will warm to the point where it will idle without choke.

When I built the engine some 25 years ago, the internet said block the cross-over; at WOT, the missing cross-over is good for about 1.5 percent increase in HP (so I went from about 450 to 457, yay!). By the time I finished the engine, I was too old to take it to the strip. With the manual choke, and pulling the choke at EVERY stop, the vehicle is as much fun to drive as I anticipated. I listened to the internet at the time. As posted earlier, one of the three dumbest mistakes I have made with cars in since 1959.

I guess my question in your case is why? I thought from earlier posts that you had decided to remove the imitation AFB and install a Q-Jet ?????

Jon

Jon, I have a feeling that we're both flogging a room temperature equine quadra-ped here!
LOL!
Jerry

CarbKing,
In answer to your question of “why bother” with exhaust restrictors in my manifold, I refer back to my post of 2/20.
In it I outlined the 5 step process I have decided to undertake in an effort to correct my problem of low vacuum/ hot carb/ engine dying at idle.
Step 1 is restricting exhaust heat to manifold.
Prior to installing the heat spacer my carb was 170+ degrees. With spacer in place I knocked off about 20 , now at 150+.
I don’t need my carburetor to be that hot. Gas gets bubbley at around 135. My 283 runs just great when cold , prior to reaching normal operating temperatures. If I lower carb body temp another 20 degrees by restricting exhaust in the intake manifold it will not harm performance- not if it now runs great when stone cold or luke warm.
I have a manual choke that I don’t even use unless it’s really cold out, and that choke is cleared by the time I leave the driveway.

Now - if cooling that carb via Step 1 solves my problem, I don’t need to go on to Steps 2,3,4 or 5. That would be nice.

In summary, I may be swapping Edelbrock for Q Jet in foreseeable future, but only if I get to Step 4.

If you are happy, then I am happy for you.

Jon

Try 2 or 4 spacers....Two heat spacers on a carburetor" refers to installing two carburetor spacers stacked on top of each other, essentially creating a thicker layer of insulation between the hot intake manifold and the carburetor, further reducing heat transfer and potentially improving engine performance by keeping the fuel mixture cooler; this is often done when trying to significantly cool down a carburetor in high-heat situations, like in racing applications
Might have to put in longer bolts. I know this is not the complicated fix but it might work.

Extra insulators may help but you need to be aware of the type, if they are open center (one large hole), the velocity through the individual primary venturis will be reduced, this would be opposite of what we think is needed. Also, the more you raise the carburetor, you are effectively making the intake runners longer, this would be ok in high RPM applications but not so good for streetability.

Edelbrock 9266 Heat Insulator Gasket . if raising the carb .320 inches affects it. I don`t know what to say.
Intake runners? Your engine could be running hotter than it should cause the timing is off.

This is what I used. The carb temp reduction was about 20 degrees.
Next I will deal with the heat at it’s source, I will put the exhaust restrictors in place.