[firechicken]

How far should the flaps open for max flow?

[Cliff R]

If you have my book the maximum open point is discussed in the Chapter on Edelbrock carburetors.

When we did cfm testing the entire stop was ground off and an adjustment screw installed so we could continue to adjust the flaps past 90 degrees to find the optimum open point, and when going any further made no improvement or diminishing returns.

I backed up the cfm testing with dyno testing using the adjustable "stop" and it confirmed what the air flow testing did exactly. Going too far open not only doesn't improve airflow, it starts to impede/redirect fuel flow from the secondary nozzles as well. This testing was done over 40 years after the 1973 information was printed.

It would be unlikely that any of that sort of testing was done way back in 1973 and why the cfm ratings are less than what we came up with.......IMHO......

[glenn911]

SDBob, your right down the road from me.

[70GS455]

Originally Posted by GTO-relic

Cliff,

It would be great if those Qjets flowed 850 cfm, but they've been called 800 cfm by some very reputable sources for a very long time- such as Doug Roe, back in the day when they were still being put on cars new, i.e. 1973.

If we're going to now second guess these sources, then we can second guess all the carburetors made out there, and add 50 cfm at will. You know as well as I do, the people who started using the 850cfm number, were Edelbrock, when they copied a truck carburetor exactly. Opening the air valve or throttle a hair more, doesn't add 50 cfm. I'd have to see it on a flow bench at 20" to believe it.

I remember specifically seeing from several sources, information stating the throttles should NOT be opened into a vertical position, they should be in line with the baffle- and it was designed that way for a very good reason- not to open all the way.

If you have enough motor and rpm, yes indeed it could flow 850 cfm. But that's not what it would flow at 1.5" HG, or 20" water, the traditional testing standard. The guy at Stealth Carburetors explained it this way, when I asked him what their carbs flowed:



"it is dependant on the engine



a 1.250 venturi carb (650) on a 350 engine at 4000 rpm flows 400cfm

a 1.250 venturi carb (650) on a 430 engine at 8000 rpm flows 880 cfm



the actual "flow" is dictated by engine size and rpm not carburetor size



the true CFM is not determined by the carburetor ---it is determined by the engine, but in the retail world this is how people determine CFM with a carburetor:



1.6875 throttle blade--1.250 venturi =650

1.6875 throttle blade---1.300 venturi=700

1.6875 throttle blade--1.345 venturi = 725

1.6875 throttle blade--1.375 venturi =750"



so...can you add 50 cfm to a Qjet ? sure, if you have a monster motor, and rev the piss out of it, until it pulls that much through the venturis by sheer force.

but the standard of the industry still is 1.5" HG/20" water. not what the biggest engine made can pull through it.

When i dynoed my 482 ci with a 7043240 Qjet and ended up at 586 hp, the vacuum at the end of the pull was 2". So we were some 15% over the rating at 1.5", whatever that number really is

Sent from my SM-G991U using Tapatalk

[Ram Air IV Jack]

Originally Posted by mchell

I am enjoying the information being presented here…..

I also enjoy a variety of publications on the Perfomace modifications of qjets and Pontiac’s in general ….

McCarthy, Roe, Ruggles, Hand….. they ALL offer something. Do they cover every “trick in the book”? No…. You still have to get out and try some things for yourself….

Thanks to those willing to offer their experience and wisdom….

You have a good point here and there is some logic in what you say. However, Cliff has been an expert with these Qjets for years and his data is solid and recent. One thing about the net, the BS out there is incredible. I'm a data person and knowing Cliff and having work done by him makes me trust his judgement. Experience is the key, IMHO. I have a 69 273 Qjet that was rebuilt by a renowned carb shop in California back in the 90's. Highly recommended. That shop is now defuncted BTW. I needed service for it 20 yrs later and Cliff took it in. Cliff found a lot of problems with the internals to my surprise and brought it back to factory specs. The RAIV engine has never run better. Now everyone is entitled to their opinion. But Cliff's research and knowledge with Qjets is golden from my experience with him. There is and always will be people claiming to be "experts" with anything. Do your research so you don't get burned!!!

[Cliff R]

Tks for the kind words.

As far as cfm ratings I'll add this. Edelbrock made their 1904, 1905 and 1906 from the late model castings with the large primary bores. They rated them at "795" cfm. Interesting that the secondary air flaps open EXACTLY to 90 degrees same as the testing done in 1973.

The Edelbrock 1910 uses the same castings, but has the airdoors open considerably past 90 degrees with a much shorter stop. Their is a good pic of it in my book in the chapter on Edelbrock Q-jets. Assuming Edelbrock tested their Q-jets prior to selling them their information is exactly what we found here flow testing the same units.

Also +2, 3, 4, 5 and 6 about the amount of BS out there plus outdated and inaccurate information, especially on the Quadrajet carburetor. There isn't a single day goes by here I don't get a call from someone wanting a "bottom plug kit" because their engine fails to start after the car has set for a week or so. They went on the Internet and every poorly informed soul regurgitating old/outdated/inaccurate information responds to their complaint telling them to glue up the bottom plugs with some useless epoxy that doesn't hold back fuel in the first place. The real problem is either a swollen or failed accl pump cup either not putting fuel into the engine when fuel returns to the carb or it so tight in the bore it woln't let fuel past it to fill the accl pump well or some sort of fuel delivery back to the carb issue. Either way you get a no-start scenario and have to dump some fuel into the engine to get it to fire.

Here are the facts. Rochester fixed the leaking bottom plug issue clear back in 1969 when they made the castings thicker at the plugs and pressed in solid aluminum plugs and swaged over them. From 1965-68 ALL of them will be leaking because they used small brass cup plugs driven and staked in place. Lead was common for the front plugs and we often see those leaking as well.

It's pretty rare to see a later model carb leaking compared to every single early one, but in any and all cases leaking bottom plugs will NOT keep your engine from starting after a long period of not using the vehicle. In most cases they actually cause hot restart problems after 15-30 minutes of sitting after a hot shut-down as they leak fuel into the intake flooding the engine............

[mchell]

Originally Posted by Ram Air IV Jack

You have a good point here and there is some logic in what you say. However, Cliff has been an expert with these Qjets for years and his data is solid and recent. One thing about the net, the BS out there is incredible. I'm a data person and knowing Cliff and having work done by him makes me trust his judgement. Experience is the key, IMHO. I have a 69 273 Qjet that was rebuilt by a renowned carb shop in California back in the 90's. Highly recommended. That shop is now defuncted BTW. I needed service for it 20 yrs later and Cliff took it in. Cliff found a lot of problems with the internals to my surprise and brought it back to factory specs. The RAIV engine has never run better. Now everyone is entitled to their opinion. But Cliff's research and knowledge with Qjets is golden from my experience with him. There is and always will be people claiming to be "experts" with anything. Do your research so you don't get burned!!!

As mentioned, I study many different sources for info…..there are MANY guys out there that are very very knowledgeable, yet may never have chance to pass that knowledge along. I take all into consideration.

At the end of the day, its just a carburetor…….If I screw it up, I’ll build another one without a single worry

[Ram Air IV Jack]

Originally Posted by mchell

As mentioned, I study many different sources for info…..there are MANY guys out there that are very very knowledgeable, yet may never have chance to pass that knowledge along. I take all into consideration.

At the end of the day, its just a carburetor…….If I screw it up, I’ll build another one without a single worry

I have many reference books in my library on Pontiacs written by authors I do trust. I see your point there. However, the internet has opened the door to "supposed" experts that can ruin a job and/or parts you may be working on through half baked and ignorant suggestions. You see that here and especially on Facebook in the Pontiac groups. If your goal is just to get a carb or whatever part to work properly, no issues with your point. However, if you have a rare $6000 original RAIV 273 Qjet and it gets screwed up, there is plenty of worries man!!! You just can't "build" another original as you mentioned. It all depends on what you want done. When you own a rare car and the parts on it are difficult or almost impossible to replace, you normally are a bit selective on whom you want working on them. That is my logic anyway and Cliff R. fit my requirements.

[Ram Air IV Jack]

Originally Posted by Cliff R

Tks for the kind words.

As far as cfm ratings I'll add this. Edelbrock made their 1904, 1905 and 1906 from the late model castings with the large primary bores. They rated them at "795" cfm. Interesting that the secondary air flaps open EXACTLY to 90 degrees same as the testing done in 1973.

The Edelbrock 1910 uses the same castings, but has the airdoors open considerably past 90 degrees with a much shorter stop. Their is a good pic of it in my book in the chapter on Edelbrock Q-jets. Assuming Edelbrock tested their Q-jets prior to selling them their information is exactly what we found here flow testing the same units.

Also +2, 3, 4, 5 and 6 about the amount of BS out there plus outdated and inaccurate information, especially on the Quadrajet carburetor. There isn't a single day goes by here I don't get a call from someone wanting a "bottom plug kit" because their engine fails to start after the car has set for a week or so. They went on the Internet and every poorly informed soul regurgitating old/outdated/inaccurate information responds to their complaint telling them to glue up the bottom plugs with some useless epoxy that doesn't hold back fuel in the first place. The real problem is either a swollen or failed accl pump cup either not putting fuel into the engine when fuel returns to the carb or it so tight in the bore it woln't let fuel past it to fill the accl pump well or some sort of fuel delivery back to the carb issue. Either way you get a no-start scenario and have to dump some fuel into the engine to get it to fire.

Here are the facts. Rochester fixed the leaking bottom plug issue clear back in 1969 when they made the castings thicker at the plugs and pressed in solid aluminum plugs and swaged over them. From 1965-68 ALL of them will be leaking because they used small brass cup plugs driven and staked in place. Lead was common for the front plugs and we often see those leaking as well.

It's pretty rare to see a later model carb leaking compared to every single early one, but in any and all cases leaking bottom plugs will NOT keep your engine from starting after a long period of not using the vehicle. In most cases they actually cause hot restart problems after 15-30 minutes of sitting after a hot shut-down as they leak fuel into the intake flooding the engine............

This is exactly my point. Half backed ignorant suggestions to lost souls that don't know any better. The internet is full of them so beware!!!

[unruhjonny]

...and that was why I was remiss when Cliff left - and I'd hope he wouldn't do that again.

[mchell]

Originally Posted by Ram Air IV Jack

I have many reference books in my library on Pontiacs written by authors I do trust. I see your point there. However, the internet has opened the door to "supposed" experts that can ruin a job and/or parts you may be working on through half baked and ignorant suggestions. You see that here and especially on Facebook in the Pontiac groups. If your goal is just to get a carb or whatever part to work properly, no issues with your point. However, if you have a rare $6000 original RAIV 273 Qjet and it gets screwed up, there is plenty of worries man!!! You just can't "build" another original as you mentioned. It all depends on what you want done. When you own a rare car and the parts on it are difficult or almost impossible to replace, you normally are a bit selective on whom you want working on them. That is my logic anyway and Cliff R. fit my requirements.

Sure…..

We are seeking different things. I am seeking knowledge so I can do my own work. It’s a hobby. Like anything these days you gotta sift…..

You are seeking someone knowledgeable to do the work. Still need to sift.

If I had a 6000.00 carb I would certainly open it up as I couldn’t resist!

Might want to shelve that gold plated piece for the points shows…….have someone build you a “driver” carb if you drive much….

Cheers

[tremo]

I have a '73 Chevy QuadraJet #7042210 and my current secondary metering rods are .0443 DA. Without checking the stamp, I'm not sure which hanger is in use, but it’s probably the original that came with the carb? Anyone know off hand which hanger that would be?

Since I race in Colorado at Bandimere Speedway, which is at 5,800’ elevation, and most often at a density altitude of ~9,500+ ft in the summer, I’m wondering if the carb is running too rich given the higher altitude?

I see Summit offers .0667 CL secondary rods. Would it makes sense to switch to a larger rod to compensate for it potentially running too rich? If so, which hanger should I use in conjunction with the larger rods? Summit has .615 V and .565 K hangers available. Or should the stock hanger be used?

Any advice would be appreciated!

[Cliff R]

DA rods woln't care too much about the hanger used due to the diameter and taper of the upper section plug long tips.

Going to CL rods is a HUGE change. Probably best to try smaller changes to see if it's going in the right direction.

The best way to do that sort of tuning is to use machined rods with the same tip length and included angle leading to the tips. I can machine a few sets if you want to do some track testing to see what it likes at that altitude........

[25stevem]

In answer to the question at the end of post 44 of this string. 1.5”hg that 4 bbl carbs are rated at is the equivalent of a air flow test done at 20.5 inches of water on a flow bench.

The 750 Q-jet used on that 586 hp 482 cid motor was a choke point .
That could have made good use of a 850 or better yet 950 size carb in terms of making higher peak hp numbers.

[tremo]

Originally Posted by Cliff R

DA rods woln't care too much about the hanger used due to the diameter and taper of the upper section plug long tips.

Going to CL rods is a HUGE change. Probably best to try smaller changes to see if it's going in the right direction.

The best way to do that sort of tuning is to use machined rods with the same tip length and included angle leading to the tips. I can machine a few sets if you want to do some track testing to see what it likes at that altitude........

Thanks Cliff! I was reviewing the chart at https://www.carburetion.com/rodshang.asp and saw the leanest was CL (.0667) and was wondering if that may be overkill.

I'd like to take you up on your offer to machine some new rods for testing. According to the chart, I see the two rods in-between are:

CK .0527
AY .0567

Would you machine these diameters or other diameters? You can send me a PM to let me know the associated cost and I can PayPal you. Thank you!

[Cliff R]

Your information is inaccurate. There are quite a few rods in the .052" to .068" range but they vary considerably in upper section diameter and tip length, etc.

To make accurate tuning changes they need to mimic the DA rods or at least have long tips or you might really lean it out or get inaccurate results.

The closest rod similar to DA/s are CV's at .052". Then they are all over the map from there to .068". CH and DP are common but have short tips.

When I tune at the track I use custom machined rods in .003-.005" increments all cut with the same length tips. If you try another path you can actually lean it up at WOT without using different hangers to get the shorter tip rods up and out of the jet holes further.......

[mchell]

[QUOTE=tremo;6349087]Thanks Cliff! I was reviewing the chart at https://www.carburetion.com/rodshang.asp and saw the leanest was CL (.0667) and was wondering if that may be overkill.

I'd like to take you up on your offer to machine some new rods for testing. According to the chart, I see the two rods in-between are:

CK .0527
AY .0567

Would you machine these diameters or other diameters? You can send me a PM to let me know the associated cost and I can PayPal you. Thank you![/QUOTE

I have simply chucked rods in a drill and used a small file/sand paper to get the upper section and tip diameter i wanted…of course you’ll need a decent measuring tool…. Ghetto , yes, but I learned a few things

rarely mess with them any more.

Personally, I’d simply run a .04x long tip and call it good….. you won’t notice much anyways

[Skidmark]

I guess I see why cliff still hasn't returned my carb for 6 months. He's too busy typing. Lol. Hey Cliff you get my emails?