Technical SBC and quadrajet issues

My mild 350 is giving me grief. Engine consists of 9 to 1 comp, 150lbs on compression test, comp cams xe262h mild cam, I set total timing at 36 degrees which is 18 initial. Rebuilt quadrajet by me (meticulous), RPM air gap intake.
Problem: I get stumble going into WOT, just doesn't pull like it should. In the garage runs good great throttle response.
At idle there is NO vacuum at distributor vacuum port and idle mixture screws have very little effect and can close them off. Manifold vacuum on gauge is just under 10.
Any ideas?

 

You realize this could cause a lengthy dissertation , right?
'course I'm not gonna do it, two finger typing style.
Do you have the Doug Roe book?

 

Try setting the initial advance at 8º.

 

its a quadrabog , its supposed to stumble when it tips over AT WOT .

for full time advance make sure you have the right port( better to pull from manifold ) as the normal port does not allow vacuum to be applied at idle only during transition ( off idle and more throttle ) some carbs have the "timed port" and a full manifold port but not all .

as for the the idle screws not having any effect means the throttle blades are open to far and its going into the intermediate circuit of the carb or you have a vacuum leak , if the engine will not idle with the blades closed you will have to either have a vaccum leak ( throttle shafts or base plate is worn which is common on older used unit or bad base gasket , or you need to make bleed holes in the throttle plate to help the mixture due to the cam , to check for throttle plate wear see if the shaft wiggles forward and back , when you open the blades up if it does excessively you should rebush the base plate ( they sell a kit for this ) if the shaft is tight then you will have to drill the 1/16th holes in the blades by the transfer slots , if the holes are already there ( some oem truck carbs and high lift cam cars have them OEm ) slowly open them up 1/32 " at a time and see if the mixture screws work ,

I definately say to people if your gonna tinker with a q jet get Doug Roes book .

 

1. stumble going into WOT: secondary air valve is opening too soon, tighten the spring.
2. use full manifold vacuum for distributor and reset initial timing (without vacuum advance hooked up) to around 8 degrees^
3. check for vacuum leak (s)
4. did you seal the main wells with epoxy?

 

Gets way too choppy at 8 for such a mild cam. A lot of info out there says to time total timing as that's where it counts the most....????

I've had other qjets that wailed all day long, this is my first troublemaker. I'm going to try the distributor on manifold vacuum next.....

1-air valve seems to have proper tension, its not flopping....
2-im trying this next
3-im trying this next too
4-no epoxy, wasn't aware of this

 

Is That dizzy fucked up ?
if total is 36 and initial is 18 ???
Setting it at 8 initial will be 26 total ???

Double check your methods and make sure that's right reading. If your readings are accurate the dizzy needs work.


If the mixture screws have no effect, there's obviously a problem someplace in the carb.

 

I've checked multiple times. I've checked base line with two different timing lights, both same. I only have one knob adjust timing light. Distributor feels tight everywhere but I could try another.

 

is the intake a spread bore type? pics I looked at have it as a square bore.

 

It is a square bore and I'm using a adapter.

 

Remove quadrabog. Open trash can. Toss. Close trash can. Locate and open Summit catalog. Order up Holley 600cfm vacuum. Be much happier.

j/k man. I've never had any luck with those unless they are new. Not a fan. I'd think that engine would run weak and stumble worse on 8 degrees initial even with that mild cam. If it hits fine with no load and bogs while driving it and don't seem to pull I'm thinking it's an accelerator pump issue or jacked up timing curve. Lack of adequate fuel when engine demands it for the overall pull. Maintaining proper fuel pressure? Pump shot when you wing it open? Kink or blockage in fuel line? (this would not show on tip in but in pulling power afterwards).

If it's in the dizzy, have you watched what the timing is doing while stomping it in the shop?

 

The Qjet is arguably the most sophisticated 4bbl ever made. People who crap on them typically don't have the patience or skill to adjust them properly.

The epoxy on the main well plugs only applies to the 1966-67 Qjets with the sheet metal plugs. All later QJets have solid aluminum spun-in well plugs that don't leak and don't need epoxy. The air valve spring wrap needs to be set properly. Simply saying "it doesn't flop around" is not an indication that it is set correctly. For starters use the number on the carb to look up the factory spring wrap setting. Once you know it's at the factory setting, then you can experiment with small changes one way of the other, if needed.
And as others have noted, be sure this is really a carb problem and not ignition or fuel delivery. In forty years of working on them, I've found the Qjet to be a very adaptable street carb.

 

No other carburetor has won more races than the quadrajet. It's has its own language.

 

Let me mention the obvious here. This says one of two things, you either got a killer vacuum leak or your throttle blades are open too far and you are not making use of your idle slots.

You don't have much of a cam and you got good compression and at idle you should be pulling in the 14-18" range. Make sure that you are checking your vacuum from a good source and not from ported vacuum. Your idle speed should be in the 600-700 RPM range by the way you didn't state that but just to touch 3rd base here.

The GIMP suggested 8 degrees initial advance and a lot of books say that. I usually start at around 12 degrees myself. I read someplace when I was a kid that the Corvette motor had 12 degrees initial advance, it was probably wrong from what I have read since then but it works for me. 18 degrees is a ton my friend. That's almost half a cylinder. Anyway the GIMP usually gives good advice and that is what the books say.

Sooooo you got a starting place here, now grab a wrench and get after it.

 

They made a ton of Qjets for different applications do you know what this one is for maybe you need one better suited for your needs.

 

also some of them get messed up by folks who don't' really understand how to work on them. If you have a few extras, you might try one and see if it works better.

 

Have you done a piston stop test and verified TDC markings? There's a bunch of ways they get way off and taking the way off marks as true can make you crazy- timing light is lying to you.

What rpm are you setting total at?
And then did you wing it past that to see if there's more advance there? Some factory dizzys won't give all advance till 4000 rpms or just above. You'll want to be all in at 2500 or so and you should have about 24* in the mechanical, absolutely more than 20 and you only have 18* showing here.

 

Yeah, run the numbers.
Internally tuned for tiny tiny cubed engines up to 500 inch monsters and they all are the same basically outside.

 

It gets worse starting in I believe '85 they were electronically controlled.

 

Ok,
the carb is 17056213 (1977 G20-G30 VAN) carb looked virgin, no slop in shaft.
I don't have a tach but I total timed to about 3000 approx. by ear. Idle 600-800.
Confimed TDC and mark, bang on.
Brand new intake gaskets and basically new intake.
The dizzy is unknown stocker, pricing a MSD street fire HEI right now.
I can't throw too much money at it right now, I don't have extra carb.
Its gotta get to the exhaust shop too yet.

 

Check for vacuum leaks with that carb adapter, too!

 

Where do I look up the spring wrap specs?

 

Told you this thread might get involved.
Not mentioned yet is the primary choke pull off, which is actually the one to the rear.
It also acts as a vacuum brake for the secondaries.
I've seen one which was actually a slow one , go bad and become too fast , letting the secondary flapper open too quick.
You need to check that too. Push it in , hold your finger over the port, then release .
You'll see the difference if you compare a few.
Generally, the smaller the engine , the tighter you want the flapper and pull off.
This also applies to the tighter the torque converter, the slower you want the secondaries.

 

Yup the vac pull off also keeps/adjusts how fast the secondaries open... also the flapper spring must be set right as mentioned earlier. The secondary needles and the hanger also have to be close to get it working right.. the needle hangers are tunable.. they have a letter stamped on them that tells you what one you have. If you carb came from a engine thats way different than yours you will have to tune it to work on your motor. There are some great quadrajet tuning books out there that can help you. There are also small wells with very small fill holes that some quads have that feed the brass tubes from the cover that act as a type of accelerator pump by temp enrichening the secondarys just as the flappers open. Make sure the are open and working

 

Distributor is a whole 'nother thesis to be written.
18 deg at what rpm? 36 deg at what rpm?
What kind of springs and counterweights on top ?
Is the stop bushing still in place?
Hard to get from 8 * to 36* total without modifying something on top.
Normally , lowering timing smooths out idle. Unless of course it's loading up.

 

If somebody could throw some new parts over the fence I'll get started

 

Yeah they made a few different rod holders and metering rods for the secondaries.

 

correct, there is only 2 basic castings the superduty /buick-olds 800 cfm unit and the 725 unit ( other than fuel entrance locations). all the changes are internally thru jetting which is what makes them so nice as the carb only pulls what air it needs on the secondaries and keeps the vacuum up so the fuel stays mixed / as for a nice transition on the secondaries besides the airvalve spring they put buffer dashpots which most people remove on them that allow the valve door to open slower and keeps the vacuum signal strong and not slam open and bounce shut .
if you need parts go here http://quadrajetparts.com/

Doug Roe used to sell them but unfortunately he passed away in the early 2000's

 

actually CCC came out in 1980 on the 301 Pontiac , and its actually not that hard to understand them its basically a closed loop system that checks the o2 sensor and pulse modulates the main /power needles , and the same computer system actually was used in GMs first efi (TBI ) setups .

 

Yes ,but hard to retro fit them to anything HAMB friendly.
Good for a few parts...in my opinion.