Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

M4ME 17085580 primary circuit

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theraymondguy:
1985 Pontiac Trans Am, Canadian non ccc on lightly modded LG4 305 (9.5:1, 416 heads, headers, Accel distributor and the wimpiest cam gm ever put in a small block).

My first automobile carb rebuild, Ive rebuilt dozens of motorcycle carbs and thrice more small engine carbs.  I rebuilt this carb and installed primary throttle bushings. I have approximately 14 months of run time on the rebuild.  I believe the carb was all original, however the caps were removed from the primary adjustment screws, likely had been rebuilt at some point.  The accelerator pump has the blue seal, the float assembly is new.

Engine comes off choke perfect, idles at 550rpm in gear (as per the under hood decal) pulls a smooth 19 inHg and has all emissions installed and plumbed as per the hood decal (there *might* be a ‘bb’ in the egr vacumm line…).  The engine pulls clean off the line and all the way to 5000. 8 degrees of initial timing and 36 degrees all in.  1.5” primary headers and factory air cleaner with a wix filter.

However. 

It takes 5 (five) full turns out on the adjustment screws to achieve this, which seems out of line.  I’ve sprayed enough carb cleaner around it the carb to have removed most of the coating, and still can’t find a vacuum leak at the intake or anywhere else.

The primary here are marked “72”, I cannot find any identification on the primary needles, but they clearly have three diameters or the main body and two smaller diameters if you prefer.

In a motorcycle I’d have already swapped in larger primary jets, but I’m in unfamiliar territory - almost need permission to move forward.  Some messing with a spare accelerator arm has proven that there’s more power to be had with more fuel (probably could use some tweaking on the apt valve).

I have questions about tuning for power as well, with plans to install an intake (old school performer) and mild cam (204/214 112 LSA) this winter.

I did chat with Cliff briefly on a Facebook Quadrajet page.  Cliff recommended the “M” style primary needle iirc and a phone call about jets.

I’d Rather work through issues here, where a guy doesn’t have to worry about what I’m taking someone else away from or misinterpreted dialogue.

Thanks for your thoughts and input,
Rob.

Kenth:
Replacing main jetting rods and jets will do nothing for the idle mixture at the mixture needles/screws.
The idle circuit is a stand alone circuit in the carb.
It is very common this circuit is too lean set up for todays fuels as these contain less BTU´s than yesteryears fuels.
In the idle circuit you have idle tubes (jets), upper and lower idle airbleeds, idle downchannel restrictions, off-idel slots and last idle needle holes.
To get control of the idle mixture needles one has to open the idle tubes .002"-.005", idle downchannel restrictions the same amount and the idle mixture needle holes to .085"-.095".
Start small until you are satisfied.

If you don´t already have Cliff´s book, i´ll recommend to buy it.

theraymondguy:
Thank you, that sounds like the right direction.

Ive been through Cliffs online store a number of times and not seen the tools to complete these adjustments, where do you purchase them? Mcmaster Carr?

old cars:
"It takes 5 (five) full turns out on the adjustment screws to achieve this, which seems out of line."

I assume you are talking about the idle mixture screws. Remove screws and drill passage to .090"

With 19" vacuum at idle you could try this first (before moving on to idle tubes ,if necessary)
Also quality of fuel depends where you live. Are you in Canada?

old cars:
Ontario, Canada gasoline is 10% ethanol in regular fuel (approximate) Premium is ethanol free.

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