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UngerMarkus 08-23-2012 03:39 PM

Qjet stumble 360 Amc

I recently installed a brand new Jet Performance Qjet on a perfectly working 360cui Amc. The car is a Jeep Grand Wagoneer.

The car idles great and the low end throttle responce is very good. It even feels fine under light acceleration. But as when you hit the gas the engine stumbles until you back off the throttle. It does not matter if you drive in D or lock it in a lower range.

I already tried tightening the secondary spring with no success.

I checked the timing and set it to 34 degrees at 3500 rpm.
The fuel pump is new so is the filter.

What could cause the stumbling?



RWENUTS 08-23-2012 07:21 PM

Look inside the carb when you stroke the throttle with the motor shut off. You should see two strong shots of fuel beside the venturis. Maybe a plugged accelerator pump discharge line.

UngerMarkus 08-23-2012 11:33 PM

Thanks for your reply! I just checked the accelerator pump. It looks like it works properly.

The problem is not just a bog or a short power hole. The engine keeps stuttering as long as you keep the pedal pressed. similar to a lack of fuel. When you back off the gas the stuttering goes away and the car accelerates.

RWENUTS 08-24-2012 08:13 AM

Have you checked for vacuum leaks?
What's your initial timing?
Does your carb have access to the APT (adjustable part throttle)
What intake you got?

UngerMarkus 08-24-2012 10:43 AM

Hi! It's an Edelbrock Performer intake. Initial timing is 12 degrees, total is 34.
I searched for vacuum leaks with brake cleaner and could not find any. Vacuum at idle is 20hg. Vacuum while cruising is about 16 hg. The needle provides a steady reading.

Today I checked the float level. I set it to 16/32". Yes, it has an accessible APT screw. The setting is still stock. I never touched it.

Fang16 08-24-2012 11:11 AM

Secondary problems?
1st check the cam on secondary and make sure it's there and raising the secondary needles.

2nd Check the secondary "jetting". Years ago ran across one that hadn't been drilled.

UngerMarkus 08-24-2012 06:56 PM

Thanks, that's a great suggestion! I'm gonna check that ASAP.

RWENUTS 08-24-2012 07:57 PM

Vacuum to distributor hooked to ported or manifold vacuum?
What distributor?

raguza123 08-25-2012 07:11 AM

It sounds like your air valve is opening too soon, is your air valve opening too soon? if it opens too soon you'll get a rush of air and fuel and a bog.
Do you have a manual on tuning a Qjet, I have a factory manual I could send, I do believe your Qjet is out of adjustment? or maybe I'm wrong

UngerMarkus 08-25-2012 03:35 PM

Hi guys! It's a mallory hei distributor. I tried both, ported vacuum and manifold vacuum. Manifold works best, but it didn't cure the problem. The secondary spring tension was the first thing I thought about too. I tried everything from very loose to very hard. The stuttering seemed to be a little less intense with the hardest spring tension.

The theory with the secondary wells not drilled seems more and more plausible.

What do you think about this test?:

In case there is something wrong with the secondaries, I could keep them from operating using some duct tape and tape them shut. Now the qjet should work like a 2bbl carb. If the stuttering at wot is gone, its obvious there is a fuel delivery problem that causes the engine to run lean.

raguza123 08-25-2012 04:53 PM

It's worth a try, that will tell you if it's the primaries or secondaries that's causing the stumbling. One thing what will keep the throttle plates closed, they will still open?

UngerMarkus 08-28-2012 06:39 AM

Hi guys. Today I disabled the secondaries. The engine has the same problems. As soon as you open the throttle more than half way it stutters, regardless of the speed. In neutral it revs up nicely.

So I assume the problem is on the primary side.

Any suggestions? I'm running out of ideas...

UngerMarkus 08-28-2012 06:41 AM

In case the APT is not adjusted right could it cause problems like that?

RWENUTS 08-28-2012 07:58 AM

Apt set too lean could cause a stumble or bog. Weaker power spring could cause that stuttering thing too.
Reach in the air vent in the top centre of the carb with a small screwdriver and see if you can push the primary metering rods down while the motor is shut off. They should move down easily and spring back up. I've had them hangup on the gasket before.
How did you disable the secondaries?
What's the elevation where you live?
Apt could be set too lean!

raguza123 08-28-2012 07:58 AM

Also you could be too rich on the primary side for jetting. as it was suggested check the APT.
Check your pump stroke it may need to be larger stroke or decreased.
Check your choke if so equiped, it may not be opening up fully.
Your Power Piston may need a weaker or stronger spring.
A vacuum leak could be at the Throttle shafts.
Which Carb do you have, a number of the unit

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