Quadrajet Hesitation On 400 Pontiac

Arjen

Well-Known Member
hello everyone,
the Firebird we rebuilt has done more than 3000 mls sice we rebuilt it.
the only problem that we still have a small isue with the engine.
between 1600 and 2000 rpm , when hardly touching the accelerator,
the engine hesitates in all gears.
it has a ( rockauto) "rebuilt " rochester quadrajet on it, which otherwide performs flawlessly.
I am now reading some books on the quadrajets. ofcourse any suggestions on what could be the problem
would help us greatly!

thank you in advance,

Arjen
 
The simplest thing first, check the accelerator pump assembly. The later model pumps with garter spring and Flourostomer seal usually addresses this problem. This keeps the seal in constant contact with the pump bore, and the seal will not swell or shrink in contact with modern fuels
Try an HD accelerator pump duration spring, and matching pump return spring.
 
hello Again,

I bought a lambda sensor kit and today we fitteled with carburation and ignition.
first off all we tested the advance and it went up to 49 degrees !!!
we took the distributor out and found that the someone elongated the springs!!
this made the weights hit the rotor.
wedid a testdrive , but there was still some hesitation between 1500 and 2000 rpm.
the lambda read 0.7 !!!
back in the workshop we took the plugs out and some of them where faul.
we cleaned the plugd and then set the carb and ignition to get maximum vacuum.n
we drove the car again and had a reading of 0.9 between 1500-2200 rpm slow
on the trothle.
after about 15 miles the vaue dropped to 0.7 again!!!
back in the workshop, looking at the plugs and 2 where black.
these are plugs that where fitted 2000 miles ago!!
we replaced the plugs with new ones and h-gapped them to 0.45 instead of 0.35.
no more hesitation and back to 0.92 lambda.
we still have issues with the accaleration pump , when you floor the carb suddenly, well
the value of lambda goes to 0.60 and a big hesitation.
we'll get back on the car later .
we are in the final stages of building a Bugatti type 55
that's a 1937 2.2 liter engied car with over 180 bhp .
we rebuilt it back to original spec.
you will be able to see some movies of it on my facebook and instagra page.

best regards,
Arjen
 
Just " pressed " the piston off the acceleration pump and compared it with other carbs..
There's hardly any resistance...
 
changed the acceleration duration spring.
on the pontiac,
for some reason the lanmdba dropped to 0.8????
will fiddle some more (again) with the idling.
found out quite a bit now about Qjets.
like " fast idle" , which comes in handy on cold starts ( laugh)
anyway , next week I am rebuilding a 1930's solex carburator for a Bugatti type 55.
simple little thing , we had to cut the membranes ourselves as they are no longer available.
hope the car runs next week.
 
Auto Restorations (Toronto Canada) sells your membranes. Sorry, I don’t have their information.

Sounds like you don’t have the correct size pump for the QJet, what is the number on Carb), I’ll check what I have for NOS Pumps and HP pumps.
 
It is most likely going lean as the big secondaries open because the choke dash pot is bad or wrong for your application. That little accelerator pump cannot deliver enough fuel for the 2 inch secondaries.
Is there a lnk from the top flapper to the choke pull off?
 
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