Pump shot on q-jet

[Justin Fleming]

I just bought this truck and it had carb issues....So I rebuilt it and everything is to spec. So I took the 86 k5 out for a maden voage today and remind you it is just a stock 305, but when I jumped on it it fell right on its face.......In the holy world it is what you would call pump shoot. Is there any tips to fix this on a q-jet?


thanks

 

[dyeager535]

Pump either works or it doesn't. Run the truck, turn it off (or leave it running I guess) pull the air cleaner, and depress the accelerator pump piston with your finger. If the engine is off, you'll see a shot of fuel if there is any there and it's working right.

I have personally NEVER seen a failed accelerator pump on a Q-jet. I'm sure it happens, but typically it's things like timing or choke that affect how they run and accelerate.

 

[Justin Fleming]

Pump either works or it doesn't. Run the truck, turn it off (or leave it running I guess) pull the air cleaner, and depress the accelerator pump piston with your finger. If the engine is off, you'll see a shot of fuel if there is any there and it's working right.

I have personally NEVER seen a failed accelerator pump on a Q-jet. I'm sure it happens, but typically it's things like timing or choke that affect how they run and accelerate.

No the pump and everything is good......Just tryN to figure out how to get the stumble out when I jump on it...

 

[rumbly truck]

Checked for vaccum leaks? you can move the rod that goes between the throttle body to the accelerator pump arm out to the outside hole. This will give you a harder shot out of the pump, but it will be shorter in duration. keep checking the basics qjets rarely havethis problem. Are you sure the nozzles squirt gas?

 

[Justin Fleming]

Checked for vaccum leaks? you can move the rod that goes between the throttle body to the accelerator pump arm out to the outside hole. This will give you a harder shot out of the pump, but it will be shorter in duration. keep checking the basics qjets rarely havethis problem. Are you sure the nozzles squirt gas?

Yea it is deffinetly pumping gas when I push the pump down....... I will continue to look for the obvious

 

[Cricket]

I'll second that. I've had luck moving the power piston actuating rod to the outside hole on two seperate occasions with a stumble on acceleration. It's easy to do and worth a try.

 

[dyeager535]

I've also bent the arm doing that. You have to be careful that you don't bottom the piston out if you try and get more travel out of it.

There are various piston designs, I've had the wrong one come in a rebuild kit before. As I had rebuilt the carbs, it was pretty obvious though, as the spring was almost at coil bind assembled.

 

[diesel4me]

quadrabogg..

Vacuum leaks are a prime cause of off idle stumble--wear in the linkage that moves the accelerator pump causes a delay in the response of the pump to throttle opening too,and cause a stumble.(there is a spec to adjust the pump stroke by bending the pump linkage rod that must be correct also)--the "Adjustable Part Throttle" screw inside the carb on emission years carbs were often set too lean,and caused a stumble off idle,or surging at cruising speed---but every carb I drilled the plug out of and messed with that screw either leaked after,or never ran any better--GM says to "never touch" it--I should have payed heed!

An EGR valve opening too soon,or not closing all the way can cause hesitation....timing being off,or a defective vacuum advance can do it too(might try using manifold vacuum rather than ported on the vacuum advance--some GM trucks are designed to be hooked up that way,and wont idle or accelerate good if hooked to ported vacuum)...A long shot could be the "check ball" for the accelerator pump might be rusty or even missing,but that usually results in a poor or no "squirt" from the pump jets..

Good luck!..I've had a few Q-jets that I could never get rid of the off idle stumble,even after rebuilding and adjusting everything..I got pissed and put an AFB Carter carb on them instead!..no more "dead spot!"

 

[mikras1]

Timing/EGR

Justin,

A few days ago you were asking about Timing – did you get that worked out? If not this might help:

The peephole on the timing tab was for some kind of automated timing light – I never heard of anyone using one though.

The indicator tab is very hard to see – I usually position myself over the fan shroud where I can look down behind the water pump. I hold the timing light a little bit on the driver’s side of the truck and aim at the tab.

The tab has marks for –4, 0, +4,8 & 12 degrees. Starting from the peephole – the first high point is –4, the deep groove is zero, the next 3 high points are +4,8,12.

When the vacuum hose is reconnected, the timing mark moves way off of the indicator on mine. You say you only saw a couple of degrees of change – so maybe there is a vacuum leak or the advance is sticking.

Finally, my 86 pings/knocks(whatever is the right term) very bad at +4 with 87 Octane gas and pings slightly with 89 Octane. I run 89 gas and I think I have the timing around +2-+3 (I set it back a tiny bit to where it would run smooth).

Now for today’s problem – the hesitation – on my truck an off idle hesitation turned out to be a bad EGR. You say the truck is emissions stripped – is the EGR still there and connected? If so, try to disconnect it. If the hesitation goes away but other misfiring type problems appear – then you need a new EGR. Take the old one with you if possible to the parts store, because the number that comes up in their computer doesn’t fit on our trucks. I had to go back and have them cross reference the Delco number.

 

[Justin Fleming]

Justin,

A few days ago you were asking about Timing – did you get that worked out? If not this might help:

The peephole on the timing tab was for some kind of automated timing light – I never heard of anyone using one though.

The indicator tab is very hard to see – I usually position myself over the fan shroud where I can look down behind the water pump. I hold the timing light a little bit on the driver’s side of the truck and aim at the tab.

The tab has marks for –4, 0, +4,8 & 12 degrees. Starting from the peephole – the first high point is –4, the deep groove is zero, the next 3 high points are +4,8,12.

When the vacuum hose is reconnected, the timing mark moves way off of the indicator on mine. You say you only saw a couple of degrees of change – so maybe there is a vacuum leak or the advance is sticking.

Finally, my 86 pings/knocks(whatever is the right term) very bad at +4 with 87 Octane gas and pings slightly with 89 Octane. I run 89 gas and I think I have the timing around +2-+3 (I set it back a tiny bit to where it would run smooth).

Now for today’s problem – the hesitation – on my truck an off idle hesitation turned out to be a bad EGR. You say the truck is emissions stripped – is the EGR still there and connected? If so, try to disconnect it. If the hesitation goes away but other misfiring type problems appear – then you need a new EGR. Take the old one with you if possible to the parts store, because the number that comes up in their computer doesn’t fit on our trucks. I had to go back and have them cross reference the Delco number.

Thanks for the response

I have set the timing and I have figured out the degrees and what not.I think I am about 6 degrees currently and I do not have any ping. The truck idles and purs like a kitten. If I am just accelerating slowly there is no problems. put if I just punch it it stumbles and then recovers and goes....The emissions are stripped the EGR is in place but not connected. The timing only jumps a little bit when I hook up the vaccum. If I bring the R's up and look at the timing it is definetly advancing....

 

[Justin Fleming]

Vacuum leaks are a prime cause of off idle stumble--wear in the linkage that moves the accelerator pump causes a delay in the response of the pump to throttle opening too,and cause a stumble.(there is a spec to adjust the pump stroke by bending the pump linkage rod that must be correct also)--the "Adjustable Part Throttle" screw inside the carb on emission years carbs were often set too lean,and caused a stumble off idle,or surging at cruising speed---but every carb I drilled the plug out of and messed with that screw either leaked after,or never ran any better--GM says to "never touch" it--I should have payed heed!

An EGR valve opening too soon,or not closing all the way can cause hesitation....timing being off,or a defective vacuum advance can do it too(might try using manifold vacuum rather than ported on the vacuum advance--some GM trucks are designed to be hooked up that way,and wont idle or accelerate good if hooked to ported vacuum)...A long shot could be the "check ball" for the accelerator pump might be rusty or even missing,but that usually results in a poor or no "squirt" from the pump jets..

Good luck!..I've had a few Q-jets that I could never get rid of the off idle stumble,even after rebuilding and adjusting everything..I got pissed and put an AFB Carter carb on them instead!..no more "dead spot!"

I have not set the measurement of the pump rod. I will try this tomorrow. I have completely rebuilt the carb, plus new floats and power piston. I know that the check ball is in there and in great shape. I have also cut the plus out so I could adjust the are fuel. Currently I think I am 5 turn out witch this could possible cause somewhat of a lean condition. I have manifold vaccum set up currently. The problem I notice when setting the timing and then plugging in the vaccum line it only moved a few degrees. Is this right?? As far as emissions go I due not have the egr hooked up. It is in plave but disabled. I guess it is possible that it is not closed all the way.

thanks for the reply......let me know what you think

 

[rumbly truck]

If its doing that, you have a accelerate pump problem.

 

[Justin Fleming]

Vacuum leaks are a prime cause of off idle stumble--wear in the linkage that moves the accelerator pump causes a delay in the response of the pump to throttle opening too,and cause a stumble.(there is a spec to adjust the pump stroke by bending the pump linkage rod that must be correct also)--the "Adjustable Part Throttle" screw inside the carb on emission years carbs were often set too lean,and caused a stumble off idle,or surging at cruising speed---but every carb I drilled the plug out of and messed with that screw either leaked after,or never ran any better--GM says to "never touch" it--I should have payed heed!

An EGR valve opening too soon,or not closing all the way can cause hesitation....timing being off,or a defective vacuum advance can do it too(might try using manifold vacuum rather than ported on the vacuum advance--some GM trucks are designed to be hooked up that way,and wont idle or accelerate good if hooked to ported vacuum)...A long shot could be the "check ball" for the accelerator pump might be rusty or even missing,but that usually results in a poor or no "squirt" from the pump jets..

Good luck!..I've had a few Q-jets that I could never get rid of the off idle stumble,even after rebuilding and adjusting everything..I got pissed and put an AFB Carter carb on them instead!..no more "dead spot!"

Well I took your advice and measured the pump lever/rod....It was off some. I actually had to bend the arm up witch would allow the pump rod to come up further. Now would this increase the pumpshot?? I have not had time to drive it yet as it is still all apart. So I dont know if it fixed it. Also you mentioned this "Adjustable Part Throttle" how do you set that? I think your referring to the one thay they mention in the rebuild kit never to remove unless deemed necessary. In my case I removed it as the carb was packed full of carbon from the carbon canister. Now the Vaccum advance deal mine does not seem to advance much when I connect it. Should it advance more than 3 degrees???

thanks

Justin

 

[diesel4me]

well...

You must back off the idle speed screw until the butterflies are completely closed before measuring or attempting to change the setting by bending the lever!..(dont know if your instructions told you that or not!)...

The "APT" screw IS the one they say to never mess with!..I fooled with a few,and they all ran worse with only 1 exception,and that one I could never get the "plug" to stop leaking I even tried smooshing a lead fishing sinker in place of the plug,and it still weeped!..ended up ditching that Q-jet in favor of a Carter AFB... I see no need to mess with it..I doubt if any carbon got in the jet that screw adjusts..others here have adjusted them successfully,maybe one of them can advise you there--if it were me I'd not touch it!..

The amount of degrees a vacuum advance will advance the timig varies among models,but most will advance the timing 8-12 degrees,with full vacuum applied..(manifold vacuum)--if its hooked to "ported" vacuum,it wont advance at all at idle,or at least very little--might be why yours is only 3 degrees..it should advance more as the throttle is opened with ported vacuum...

 

[Justin Fleming]

You must back off the idle speed screw until the butterflies are completely closed before measuring or attempting to change the setting by bending the lever!..(dont know if your instructions told you that or not!)...

The "APT" screw IS the one they say to never mess with!..I fooled with a few,and they all ran worse with only 1 exception,and that one I could never get the "plug" to stop leaking I even tried smooshing a lead fishing sinker in place of the plug,and it still weeped!..ended up ditching that Q-jet in favor of a Carter AFB... I see no need to mess with it..I doubt if any carbon got in the jet that screw adjusts..others here have adjusted them successfully,maybe one of them can advise you there--if it were me I'd not touch it!..

The amount of degrees a vacuum advance will advance the timig varies among models,but most will advance the timing 8-12 degrees,with full vacuum applied..(manifold vacuum)--if its hooked to "ported" vacuum,it wont advance at all at idle,or at least very little--might be why yours is only 3 degrees..it should advance more as the throttle is opened with ported vacuum...

I did back off the idle screw per the directions.....I am lost about what plug you are talking about.....the screw that I am talking about did not have a plug...but someone else has been in this carb prior to me... I have my vaccum hook right to the manifold port behind the carb and I only see a couple of degrees advance