Quadrajet Questions

[OldsFan]

My carb's float went bad (filled with gas), so I pulled the carb apart and rebuilt it.  I have a 17085226 Q-Jet (non-computer).  It isn't my first time rebuilding a Q-Jet, so no big deal, I thought. 

Put the freshly rebuilt carb back on the engine and it starts up, runs and idles fine.  I adjusted the idle mix, etc. etc.  I drove it around a little bit and everything seemed fine.  I didn't run it hard and part-throttle acceleration was and is fine.

Today, I took it out to setup the secondary air doors and found a problem, though.  If I just stab the throttle hard, it falls on its face for a second or two and then gets up and goes.  I put a vac gauge on it and it hits and holds zero vac during that stumble.  I figured it was that the spring for the air doors was just too loose. 

But here's where it gets strange:

It has the same stumble in neutral.  If I just punch it in neutral, it actually stumbles (but for a shorter time).  I pulled the air cleaner and watched as I cranked open the throttle with my hand.  I don't even see the secondary doors moving but it still stumbles!!! 

What could cause this?  Could it be the accelerator pump shot being wrong or not enough?  Any other ideas?

[bd]

There are a large number of potential causes for stumble on acceleration.  Since the carb was just rebuilt, initial thoughts are low float level, inadequate accelerator pump shot, sticky power piston, warped fuel bowl cover.  Engine bog can also be caused by inadequate ignition timing, or a sloppy timing chain.

Consider this question carefully:  When you romp on the throttle, is the bog instantaneous with throttle motion... or does the engine begin to accelerate for 1/2 to 3/4 second and then bog?

[Big Katz]

I had a similar problem a couple of months ago. I installed a cheap carb kit in a quadrajet and is ran good for a day and then developed the same stumble on acceleration. It turned out that the rubber seal on the accelerator pump swelled up and came off of the piston. Apparently this is becoming a common problem with todays fuels. I ended up doing a search and found www.cliffshighperformance.com/‎ He claims the materials in his kit are resistant to this issue. I installed the kit and it has ran smooth ever since. It is worth removing the top and taking a look. 

[BADAZ chevy guy]

Just a quick tip! If you remove the roll pin from the accelerator pump arm, just enough to remove the arm, the top comes off a lot easier.
Seeing that you have done your fair share of Q-Jet builds I'm sure you know this, but I thought I'd mention it for readers not so familiar with them.

[bd]

I had a similar problem a couple of months ago. I installed a cheap carb kit in a quadrajet and is ran good for a day and then developed the same stumble on acceleration. It turned out that the rubber seal on the accelerator pump swelled up and came off of the piston. Apparently this is becoming a common problem with todays fuels. I ended up doing a search and found www.cliffshighperformance.com/‎ He claims the materials in his kit are resistant to this issue. I installed the kit and it has ran smooth ever since. It is worth removing the top and taking a look.

Google: GM 17110591 Viton Pump Kit

[rich weyand]

Another couple of possible issues that masquerade as an accelerator pump issue. 

- Talking to an old speed shop mechanic, he said that the connectors on the wires from the distributor shaft sensor to the ignition control module can get flaky.  When you get on it, and the vacuum advance comes off, the sensor rotates around the shaft and you can lose contact to the ICM.  The stumble is similar to an accelerator pump issue.  Not likely in your case, because it would have to be a coincidence to happen at the same time you replaced the carb.

- The other ignition timing issue can be too little advance.  At low rpm (centrifugal advance not in), you open the throttle (dumping the vacuum, so vacuum advance is not in), and the timing goes way retarded.  Again, the stumble is similar to an accelerator pump issue.  Again, probably not your issue, but it could be if the distributor isn't clamped down hard and you moved it replacing the carb.  You probably should be running 14-18 degrees of base timing (with the vacuum disconnected) if it is an SBC.

[OldsFan]

Wow!  I thought for sure nobody would bother with my question about my Qjet.  You guys have given me EXCELLENT responses and ideas!!

So to answer the questions:
bd - The stumble is IMMEDIATE.  There's no beginning of an acceleration, but the engine does not completely stall either (comes close, though).  I figured if the pump shot were not working it would just stall out altogether...  On the float level, I got a brass float which has the rounded edges so setting the level perfectly was hard.  I thought I got it close, but it is worth a second look.  I'll also check the accelerator pump... See my note on the timing in response to bd below.

Big Katz - The kit I got was from NAPA and included the new "blue" style accelerator pump seal...  BUT, it doesn't say anything about ethanol compatibility.  Still, I've used probably 5 Napa kits on other carbs and not had an issue.  I'll check this though!  Are the NAPA kits generally ok?

BADAZ - Yes, good point.  I like to put a flat screwdriver between the roll pin and the housing - leaves a little space making it easier to get it back into place later.

Rich - I have the 5 pin HEI module with the knock-sensor electronic setback.  It did still work last time I checked it, but it shouldn't be hard to check again. 

Now - the timing is another thing.  I set it way back to about 9 degrees BTDC (sticker actually says 6 is stock, :-) ) from about 14 degrees.  I did this right after installing the rebuilt carb just to add a measure of "safety" against knock in there until I have everything worked out.  Do you think that could really be causing it??? 



So one other thing came to mind.  My carb is an M4MED model with the "D" standing for "Dual Capacity Pump".  There's an electrical connection on the top of the carb which pushes down a pin inside.  I think when the engine reaches operating temp that switch reduces the pump capacity. I am wondering if that has something to do with it.  I am going to try and check that switch as well just with a 9 volt battery or something.  For those reading this later - it is the grey connector, not the blue one on Computer Controlled Quadrajets.
There's a great post on M4MED's here: http://forum.73-87chevytrucks.com/smforum/index.php?topic=11916.0

Thanks again guys!  I really appreciate the suggestions and more are welcome.  I'll follow up once I have a few minutes to tear into it again.

[bd]

The blue pump diaphragms are Viton and should function fine with enriched fuels.

"Immediate" hesitation with base timing correctly adjusted is generally insufficient fuel delivery.  But... you stated the problem began after retarding the timing 5° from where it had been (done at the same time as the carb OH).  So....

Things to do:

Return the timing to its pre-hesitation setting to eliminate base timing as a variable - what happens?

Try hovering the carburetor with a gloved hand and a shop towel to enrichen fuel delivery as you rap the throttle - what happens?

The M4MED dual capacity pump functions just as you're thinking, except it energizes before reaching operating temp.  Unplug it - what happens?

Finally, with your posting on timing I have to ask:  Have you verified timing chain wear and the balancer timing marks?

[OldsFan]

Ok, your suggestions led me right to it.

I checked the balancer just recently with a piston stop and there was "almost" no slop that I could detect in the timing chain.  The motor only have about 67K miles on it...

Anyhow, looking down the primary barrels with flashlight (at night), I can see that there is very little pump shot.  On older Q-Jets I get a solid stream coming out, with this one I am getting just some dribbling down the side of the barrels. 

 Even after shutting the engine down and rapping the throttle open I just get a little drizzle - that can't be right.  The dual pump energized or not doesn't make a difference.  It's both sides.

I'll pull the top off the carb this weekend and see what is going on with it.  Maybe the umbrella-type pump seal got unseated or something...  I'll let you know what I find.

I really appreciate the help on this, though.  If nothing else, I finally learned how the dual capacity pump deal functions. 

[OldsFan]

Ok, sorry it took so long but I finally had time to track a few things down.

I am pretty sure I am having an issue with the dual-capacity solenoid.  I pulled the carb apart and replaced the accelerator pump.  It runs much better, but when cold there is still a stumble.  Once it warms up the stumble is gone.

I tested the solenoid itself and it is working (pushes the little plunger down when it gets 12 volts).  Also, I checked the electrical switches and it appears it gets power when cold and not once it warms up.

So here's the question...  How does that dual capacity system work?  Can I take the bottom of it out somehow?  I read through my info on the carb, including Ruggle's book, but there's not much coverage on the actual function of that system inside the bowl.  Could it be plugged or otherwise defective?

Any help on this appreciated!

[bd]

See page 6C1-29 (pg 749) of the 1986 Service Manual for the M4MED dual capacity pump diagnostic steps.  The procedure is pretty straightforward.  Energizing the solenoid should decrease the pump shot...

I checked the electrical switches and it appears it gets power when cold and not once it warms up.

You need to figure out why it's energizing when cold instead of when warm.  Faulty or incorrect thermal switch, perhaps.  Did someone cobble the wiring or modify the circuit?

[OldsFan]

Ok, first thank you for that link!!!  That does that make it a heck of a lot easier to troubleshoot.

Second, I got that mixed up.  The solenoid gets power all the time (key on), but it never grounds.  That is, I hooked up a multimeter and checked and even when fully warmed up (180* thermostat open), it only shows about 5 volts between the solenoid connection and the thermal switch contact.  I think the thermal switch is bad.  When I short it to ground, I can hear the solenoid clicking, so I am going to work on finding a new thermal switch.  Fixing that may help some things...

Another question, though -  I noticed that if I press it and hold it down for a few seconds, it takes 3-5 seconds or so to come back up.  I don't seem to remember that on any other carb I've got.  How long should it take the accelerator pump to come back up from being fully depressed and held until it empties???  Should it spring right back or take some time to refill?

[bd]

Expand on this...

...even when fully warmed up (180* thermostat open), it only shows about 5 volts between the solenoid connection and the thermal switch contact....

Where are you connecting the voltmeter leads to measure 5 volts?  I don't have a reference on the temperature calibration of the sensor.  The 180° thermostat maybe too cold.

But, I want you to think about something for a moment.  Energizing the solenoid decreases the pump shot.  You're experiencing a lean throttle opening, now.  What do you suppose will happen when you get the dual-capacity solenoid working?

...Another question, though -  I noticed that if I press it and hold it down for a few seconds, it takes 3-5 seconds or so to come back up.  ...How long should it take the accelerator pump to come back up from being fully depressed and held until it empties???  Should it spring right back or take some time to refill?

There's a spring under the accelerator pump that should return the pump immediately.  You have a pump cup that is sticking in the bore - not good.  What pump did you use last (brand & P/N)?  Did you replace the pump shaft seal that is staked into the bowl cover (if the M4MED uses one)?

You may need to polish (hone) the accelerator pump bore and use a different cup material.  More on that after you respond to this round of banter.

[OldsFan]

Hey Rich...  Thanks for the responses.  So here goes:

Where are you connecting the voltmeter leads to measure 5 volts?

I hooked up one lead (+) to the battery positive.  I hooked up the other to the pin coming out of the temp switch on the water neck.  I got 5 Volts and a high Ohm reading (don't remember what).  I also tried hooking the positive to the lead that goes to the solenoid and kept the negative lead in place (engine on, full temp) and got a slightly lower rating but near 5 Volts.  Also, it seems to be 5 volts regardless of temp.

What do you suppose will happen when you get the dual-capacity solenoid working?

Yeah, I know...  I was just hoping for some sort of miracle.    Either way, I intend to get this working, so I want to make sure I find the right switch to replace it.

You have a pump cup that is sticking in the bore - not good.

Just what I was thinking.  The odd thing here is that it moves down smoothly and it comes back up smoothly, just slowly.  If I pull upwards on it, it almost feels like I am working against a vacuum - hard to explain, but it pulls back down sorta'...  I've never tried to just yank it, so I am talking about gently pulling on it with fingers, no tools.

Did you replace the pump shaft seal that is staked into the bowl cover (if the M4MED uses one)?

It does and yes, I replaced it.

What pump did you use last (brand & P/N)?

Napa ECHLIN, part 2-4055 (V12268, Made in USA).  I reused the return and duration springs on the new pump because it did not come with one.

You may need to polish (hone) the accelerator pump bore and use a different cup material.  More on that after you respond to this round of banter.

How would I polish it?  I mean, I need something that will clean up the bore without increasing it or mis-shaping it...  Maybe a really fine grit sandpaper?  Polishing compound...?  Thanks for all the help - I am pretty determined to get this working.   

[bd]

Where are you connecting the voltmeter leads to measure 5 volts?

I hooked up one lead (+) to the battery positive.  I hooked up the other to the pin coming out of the temp switch on the water neck.  I got 5 Volts and a high Ohm reading (don't remember what).  I also tried hooking the positive to the lead that goes to the solenoid and kept the negative lead in place (engine on, full temp) and got a slightly lower rating but near 5 Volts.  Also, it seems to be 5 volts regardless of temp.

With the dual-capacity solenoid connected to, or disconnected from the switch?  Ignition on or off?  The way you describe it I'm not convinced that the measurement is valid or meaningful.

You have a pump cup that is sticking in the bore - not good.

Just what I was thinking.  The odd thing here is that it moves down smoothly and it comes back up smoothly, just slowly.  If I pull upwards on it, it almost feels like I am working against a vacuum - hard to explain, but it pulls back down sorta'...  I've never tried to just yank it, so I am talking about gently pulling on it with fingers, no tools.

What pump did you use last (brand & P/N)?

Napa ECHLIN, part 2-4055 (V12268, Made in USA).  I reused the return and duration springs on the new pump because it did not come with one.

Pick up a GM 1711591 pump kit.

Did you replace the pump shaft seal that is staked into the bowl cover (if the M4MED uses one)?

It does and yes, I replaced it.

Lubricate the accelerator pump shaft and seal with a miniscule smear of silicon dielectric grease.  But, use it very sparingly so none of the silicon compound drips into the fuel bowl.  Even small amounts of silicon will damage O₂ sensors, making them ineffective.

You may need to polish (hone) the accelerator pump bore and use a different cup material.  More on that after you respond to this round of banter.

How would I polish it?  I mean, I need something that will clean up the bore without increasing it or mis-shaping it...  Maybe a really fine grit sandpaper?  Polishing compound...?  Thanks for all the help - I am pretty determined to get this working.   

Procure a ~3" long 1/4" bolt.  Cut the head off the bolt.  Use a hack saw to split the threaded end of the bolt lengthwise ~1.25" (or ~0.25" less than the width of a roll of 120-grit emery).  Now you have an arbor.  Clamp the arbor in a 1/4" die grinder.  Wrap about a 3 - 4" length of emery around the slotted end of the arbor. 

Remove the valve and check in the base of the accelerator pump bore.  Then, use the makeshift hone just as you would use any hone, moving in and out of the pump bore to create a crosshatch pattern.  120 emery will remove material very gradually and nicely polish the bore.

Clean the bowl and bore with some carburetor cleaner and compressed air.  Sparingly lube the pump cup with a light smear of ATF and reassemble.  The pump should slide in the bore very smoothly with little resistance.