Fine tuning Q-jet

[Cliff Ruggles]

Another thing few if any folks realize with these things is that a carburetor is a metering device an it is NOT, and I will repeat NOT a "fixed" delivery system.  By design, and I absolutely LOVE this part, it will vary fuel delivery based on throttle position, engine load, air flow across the venture/boosters and pressure differential above and below the throttle plates.  Very clever these engineers were with designing them and they are very effective at not only fuel delivery in the correct proportions but also very fine atomization of the fuel molecules BEFORE they hit the intake tract an make it to the combustion spaces.


I'll slow this down a bit but anyone reading this needs to understand the importance of finding the best "recipe" for your particular combination.  This includes a lot more than just setting up the carb and distributor. There are a LOT of other players in the game and things effecting the final outcome.  To mention a few is the compression ratio, combustion chamber shapes, camshaft events, intake chosen, spacers used, scavenging efficiency of the manifolds, headers and the rest of the exhaust system.  I'd also mention engine temperature (thermostat setting and actual temperature of the engine parts) is a player here, plus the air induction system (heated or bringing in cool/fresh outside air).  I'll go on to mention engine load from a "tight" converter, lower numerical gearing, vehicle weight, rolling resistance......etc, etc.

I certainly hope this doesn't "muddy" the water. Nothing at all wrong with using monitoring devices to help you in your efforts for perfection, but folks need to realize that they are simply tools to assist us and most to see the results of our tuning efforts.  Used or not we ALWAYS tune for best results in all areas.  IF you are using a Quadrajet in your efforts I have the knowledge and experience to help you cut very quickly to the chase.  At this point in my learning curve I know all the part numbers, what they came with, how the factory set them up, and what is needed to dial them in for what you are doing.  I can get you so close in most cases it's basically plug and play.

This is why I retired from carb building a few months ago and have moved on to selling parts, rebuild kits and custom rebuild kits with tuning parts.  I've spend the time and funds to make sure you get the best parts out there, the correct parts, and tuning parts needed to dial you in, or basically "plug and play" right off the bench.

So as these discussions go on folks should realize that this is NOT something you can nail down going out and buying an A/F metering and simply monitoring.  You are going to have to spend some time with your set-up to nail down the best overall results.  This Forum is there to help you that effort........Cliff

[bob69]

Hey all, I appreciate the discussions. Never too old to learn something new  .

old cars, the car does not "hesitate" as such. But it just does not accelerate like it should.  Slow uptake in that first second or two. The acceleration after that is ok but again kinda weak for a 350 when compared to my other cars.

I had a 1969 nova with a nicely built 454. It would smoke the tires easily from a standing start with just 1/3 throttle. I got real good at rolling starts before hammering it. Very strong motor, plant you in your seat real good. I had it at a 1/8 track once. Ran 93 mph with a rolling start. It had a 7029207 with mabs at .070. Ran great. Loved that car right up to the day it was stollen.

I had a 350 hp 327 with with a 7028213 q-jet. That motor would easily rev to 6500 quickly. Loved the way it would raise the front end when I jumped on it. The 350's acceleration is not even close to the 327. The same goes for the other chevys I've had.

At this piont I'll probably chuck the carb and try another.

Before I bought Cliffs book I followed the q-jet tuning steps outlined by Lars. His steps are basic the same as Cliffs. No surprise. So I've done things like tip in and removing a vacume line as tests.

As Cliff mentioned,  you have to tune the ENTIRE set up not just the carb and distributor.  For example, I scored a low mile GM remanufactured 700r4 from a 1990 nine passenger Chevy van. Took the gm rebuilt tq too. After the 700r4 / tq was installed the engine would not idle. To get a 650 idle in drive, the park idle had to be around 1300. After a lot of tuning and installing an idle stop solenoid the engine was still not running great at idle.

After much research I found that the 1990 nine passenger chevy van 350/700r4 tq had a stall around 1100-1200 rpm. The tq was dragging down the motor resulting in a too high idle rpm. Changed the tq to a 2200 stall. I dropped the park idle to 900 and drive idle was 600. The idle stop solenoid was removed. After some tuning to the carb the idle is silky smooth and the engine turns off when the ignition is turned off.

You may have heard  the phrase - no replacement for displacement. Well experience is much the same. That's why I  enjoy reading these posts. Never know what you can learn.

[old cars]

It is hard to reflect sometimes what someone means as problematic online without seeing/touching/hearing in person because we all describe things differently.
AFR gauges (readings) can be controversial to say the least. I will agree there not for everyone. Even the installation location and leakage can affect readings. Not sure if I would call carburetors stone age. Maybe my posts have not been helpful. Good dicussions with relative information are always good though , even when it slides off the topic.

[Cliff Ruggles]

I'll add that the WORST calls I get here are from folks who have installed A/F gauges and are trying to tune their carburetors.

They will call up here all upset and say things like "I saw 15.9 clear up to 17 to 1 AFR a few times cruising and changing speeds in traffic". 

The FIRST question I asked was IF you noticed any hesitation, stumble, bog, "flat" or lack of power, surging, or any other negative symptoms.  "No" they reply, "it seemed to run just fine".  Then WHY are you worried about?

Well, I was over on the Chevelle Forum and a "guru" who posts there said that my A/F should be reading 13.61427427 to 1 at cruise or my carb was all messed up. 

See what I mean?  We tune for results, and certainly NOT what any resident "expert" posts on the subject.

The very same "guru" also told him that he can't use ported vacuum to the advance.  I asked him why?  "Because he said the engine will run hot, overheat, plus it will continue to add timing at heavy/WOT throttle, detonate my engine, crack the piston ring lands and knock all the rod bearings out of it".

LOVELY, another well educated "guru" that doesn't even understand how vacuum advance works and should as quickly as they can get back to their "day job"....IMHO.......see what I mean about using the Internet and an A/F meter to tune with.......FWIW.....

[bob69]

Cliff, one last thing to check. Could the carb be draining the fuel from the accelerator pump well while running? What would the possible causes be.

[old cars]

I’ll share some results:      454BBC /approx 500hp        turbo 400/2.73 axle          227@.050 camshaft

First a Quadrajet
Idle AFR 13:1     Cruise 15.5:1  50mph 2100rpm     Light Acceleration: 16.5:1       WOT: 12.5:1


Holley  780cfm (originally)
Idle AFR 12.6:1             Cruise: 13.5:1                     Light Acceleration: 13.5:1       WOT: 12:1
After
Idle AFR 13:1                 Cruise 15.5:1                     

Results will vary but these AFR ratios produced no hesitations or loss of power you could feel on the street.
The Holley carb stopped blubbering at lower speeds. WOT was not finish tuned at the time.
Idle fuel reflects camshaft being used. Idle vacuum was 12” approximately in gear.

[Cliff Ruggles]

You will find the Holley carb (no mention as to part number, model, mods made to it or type of boosters used) more difficult to tune for part throttle A/F than the Q-jet.

You will NOT find your results changing jets and PV's.  A large portion of the fuel at part throttle comes from the transfer slots as the large primary bores and inefficient booster design aren't in the mix like they are on the Q-jet.

I remember my early tuning efforts with Holley carburetors dating clear back to the 1970's.  I remember locking the emergency brake and blocking the tires, putting the car in gear, then climbing up and bracing myself over the carb, looking down in it and watching for fuel to start streaming from the boosters as I applied load to the engine against the converter.

NOTHING....WTF?  ALL the fuel was coming from the idle discharge holes and transfer slots, not the first drop from the boosters.  The Q-jets I was tuning at the time, or attempting to tune as it was all a new game for me had TONS of fuel coming from the boosters in a nice cone shaped well atomized pattern with the slightest movement of the throttle off idle.

Hum?  An early lesson in Holley tuning and by that time I had worn the bowl thread clear out of the main casting changing jet sizes in vain to try to get some decent fuel economy out of them.  Not having any luck sent me into the air bleeds and then into the metering blocks.  I got good at them early on then really good at them after a decade or so of tuning them to the brink of extinction.

By design I was never able to equal the Q-jet or the TQ's in fuel economy and seamless transition right off idle and for normal driving, but any heavy throttle movements without the secondaries was very impressive in comparison.  This simply happens due to the much greater CFM available w/o having to get into the secondaries like I did on the spread bore units.

In later years I played around more with the larger annular booster Holley carbs and found them to be better in that area, but it comes at a cost of CFM lost stuffing a much larger booster into the larger bores. 

I finally decided to pick a direction and run with it, so in 2003 when I went full time into the carburetor rebuilding/restoration business I dropped just about everything from the list and stuck almost 100 percent with Q-jets, but would take in a factory Tri-Power set-up (2-JETS) on occasion.  That didn't last long and very quickly my backlog went out past two years.  So I dropped everything but Q-jets, hired more help and we ran full time till a couple of years ago.   Most of my employees moved on, I did not replace any of them as it became a perfect opportunity to retire and not have to put anyone on the street.

There's the short version of a long story but at this time I'm only selling parts, rebuild kits, and custom rebuild kits with tuning parts.  I have the best accl pump seals currently available and NOTHING will make them swell up like the cheap soft light blue seals showing up from all the other sources.........Cliff

https://cliffshighperformance.com/simplemachinesforum/index.php/topic,4546.0.html

[bob69]

I've been adjusting and testing for the past few months and finally worked out the issues.  So here is a recap.

The original carb was defective. Tearing it down I found some sizable voids in the base plate casting. The voids were located inside the idle needle screw holes. Probably why the carb was showing a lean off idle afr spike. Also, this lean off idle afr did not show up in any of the three other q-jets I tested.

I tested jets from 67 to 71 with various rods. Except for one test, none of the tunes gave the performance I felt the combination is capable of. The one test, with 71 jets, smoke the tires for about 30 feet and then nothing. Like turning off a light switch, performance disappeared. The car ran OK at low engine speeds but had no performance.

It became obvious there was a problem with fuel starvation. The fuel pump was replaced about 5 years ago, so it was probably a restriction in the tank or fuel lines. On these older chevys, the fuel pick up has a strainer or "sock" to filter out debris. I pulled the tank and it looked new inside. Very clean. Pulled the sock and found it was almost completely plugged up.  Don't know why. It was a replacement sock that came with the reproduction fuel pick/gauge line. Perhaps the stock's material was reacting badly with today's fuels.

Replaced the sock, blew out the lines with compressed air and replaced all rubber fuel lines. Did another test and still no performance. Pulled the fuel pump and found that the pumps internal plunger was partially stuck open. Limiting the pumps stroke and limiting fuel flow. Replaced the fuel pump. The fuel starvation problem was solved.

The 71 jets were now way too rich everywhere. Put in 69 jets which cleaned up the afr's and still has performance. May try 68 in the future.

[quadrajam]

Good. So how are you liking those SUMMIT heads?

[Cliff Ruggles]

At least you found the root problem.  IMPOSSIBLE to effectively tune when fuel delivery issues are present.  So basically you are back to square one.

The good news is that you can make predictable changes at this point........

[bob69]

Quadrajam, the jury is still out on the summit heads. I've let the 200r4 control the shifting while testing. This 200r4 shifts around 4200 rpm on wot (a 1986 monte carlo 305 - not the SS). No where near what the engine is capable of. After I finish tuning in the carb I'll be able to see how they run at higher rpms.

[quadrajam]

10-4 thanks.
Nice work finding the fuel pressure issue. That will
drive you nuts.

[von]

bob69, My 2004R also shifts around 4200-4400 rpm even after I've done considerable governor lightening. It's a bone stock 350 engine but I'd like the WOT shift point a little higher. If you figure out a mod that works well, please let me know. Thanks.

[bob69]

Von, the 200r4 is not an easy tran to hop up. Changing the governor weights is the easiest mod, but may not get you to where you want to be.

The vast majority of 200r4s were installed in low performance v6 and v8 grocery getters. The tran, specifically the valve bodies, were designed with early shift points for low performance engines. Modifying the governor may not be enough to get the higher shift points you're looking for.

However, GM did make a 200r4 for high performance buicks, oldsmobiles and chevys. They are found in Buick turbos, olds 442 and monte carlo SS. The valve bodies were designed for high performance. They were designed for higher shift points. From what I've read the low perf vb can not be modified to match the hp vb.

Look up the buick forums (turbobuick, gnttype, v8buick) for in depth information on the 200r4s. The buick GN and T-types have loyal followers and they love their 200r4s.

[von]

Yes I knew all that about the 200 4R. I built mine. I was hoping to find out a valve body mod to raise the WOT shift points but sounds like unicorns. My part throttle shift points are perfect.