Qjet Question

Robert Simpson · 04-17-2017, 09:25 AM

I am having a problem with my Q-jet on my footbrake street car and I could use some advice on how to fix it. I have a nova street car with a sbc/t350 combo that I drive and bracket race. It has a stock valve body with a shift kit and I use a Q-jet. I leave at 16-1700 and it flashes to around 25-2800 and it shifts on its own if left in drive or second. It runs low 12's to high 11's. The problem that I have just started to experience is if I have caught someone early in the run and try to pedal the car it starts to slow down like you took your foot off the gas! But it is on the floor. On this particular run I pedaled once at about 1000' and lost .30sec,12mph and lost the rd! The engine makes a different noise like it is straining but still running but just not pulling. Any idea what this might be in the carb? Could the secondary flap close and get stuck in the closed position? Any idea what this is and how to fix it? I have not modified the linkages at all, and only have done the usual things to rebuild and set it up to run on the car and leave clean without a bog etc. Any advice would be appreciated. Thank you

R. Thorne · 04-17-2017, 09:46 AM

Choke mechanism allowing choke plate to pop shut and, in turn not allowing secondaries to open when back on the throttle?

Hacksaw · 04-17-2017, 10:31 AM

Check the rear flap. Maybe hanging shut somehow.

Robert Simpson · 04-22-2017, 12:43 AM

Thank you all for the suggestions. The choke is wired open and it does not serve any function. I will look and see if somehow the secondary flap is getting caught on something. Please keep any suggestions coming. Thank You.

tj310 · 04-22-2017, 07:51 AM

Quote:

Originally Posted by Robert Simpson

I am having a problem with my Q-jet on my footbrake street car and I could use some advice on how to fix it. I have a nova street car with a sbc/t350 combo that I drive and bracket race. It has a stock valve body with a shift kit and I use a Q-jet. I leave at 16-1700 and it flashes to around 25-2800 and it shifts on its own if left in drive or second. It runs low 12's to high 11's. The problem that I have just started to experience is if I have caught someone early in the run and try to pedal the car it starts to slow down like you took your foot off the gas! But it is on the floor. On this particular run I pedaled once at about 1000' and lost .30sec,12mph and lost the rd! The engine makes a different noise like it is straining but still running but just not pulling. Any idea what this might be in the carb? Could the secondary flap close and get stuck in the closed position? Any idea what this is and how to fix it? I have not modified the linkages at all, and only have done the usual things to rebuild and set it up to run on the car and leave clean without a bog etc. Any advice would be appreciated. Thank you

The secondary air flap is controlled by the forward located choke pull (air valve dash pot / secondary dampener ) . This will not allow the flap to open after a high vacuum for a second , so it sounds like normal operation . The size of the air bleed restriction inside the pip controls the time it takes to open .
Got this from Doug Roe' s book Rochester carburation H.P. Books
...Trevor

Hacksaw · 04-22-2017, 08:57 AM

Is your linkage reworked to the secondaries or still factory stock. If still stock, could the rear throttle blades somehow stick closed when you pedal?

Mark Yacavone · 04-22-2017, 12:15 PM

Not sure of the exact problem here, but I would say to look at each possibility.
Start with the sec. flapper, and make sure it doesn't come anywhere near the top opening.
You can loosen the screws and shift the plates around a bit, and then re-tighten.
Or, just file the casting a little bit, where ever it's close.
Of course, you wouldn't want to do this on a S/SS carb. I'm sure you know this.

Hacksaw · 04-22-2017, 01:35 PM

Just remembered this. I once had the split pin that secures the cam for the hanger walk out slightly and that interfered with the flap.

Alan Nyhus · 04-22-2017, 02:41 PM

What's the float level measurement? And fuel pressure?

Vic Santos · 04-22-2017, 07:30 PM

Had this problem approx. 30 years ago. What happens is that when you shut the secondary throttle at high vacuum the pressure across the throttle blades holds them in place when you try to open them again especially if the bores are a little oxidized and the actuating lever is NOT PINNED TO THE SHAFT. The fix is to pin the throttle shaft to the actuating lever; do not rely on the torsion spring to open the throttle. A $.10 rollpin is most common fix.

04-17-2017, 09:25 AM	#1
Robert Simpson Senior Member Join Date: Aug 2004 Location: Jacksonville, Florida Posts: 803 Likes: 6 Liked 20 Times in 12 Posts	Qjet Question I am having a problem with my Q-jet on my footbrake street car and I could use some advice on how to fix it. I have a nova street car with a sbc/t350 combo that I drive and bracket race. It has a stock valve body with a shift kit and I use a Q-jet. I leave at 16-1700 and it flashes to around 25-2800 and it shifts on its own if left in drive or second. It runs low 12's to high 11's. The problem that I have just started to experience is if I have caught someone early in the run and try to pedal the car it starts to slow down like you took your foot off the gas! But it is on the floor. On this particular run I pedaled once at about 1000' and lost .30sec,12mph and lost the rd! The engine makes a different noise like it is straining but still running but just not pulling. Any idea what this might be in the carb? Could the secondary flap close and get stuck in the closed position? Any idea what this is and how to fix it? I have not modified the linkages at all, and only have done the usual things to rebuild and set it up to run on the car and leave clean without a bog etc. Any advice would be appreciated. Thank you

04-17-2017, 09:46 AM	#2
R. Thorne Member Join Date: May 2009 Posts: 170 Likes: 35 Liked 43 Times in 24 Posts	Re: Qjet Question Choke mechanism allowing choke plate to pop shut and, in turn not allowing secondaries to open when back on the throttle?

04-17-2017, 10:31 AM	#3
Hacksaw Senior Member Join Date: Sep 2016 Posts: 838 Likes: 148 Liked 276 Times in 151 Posts	Re: Qjet Question Check the rear flap. Maybe hanging shut somehow.

04-22-2017, 12:43 AM	#4
Robert Simpson Senior Member Join Date: Aug 2004 Location: Jacksonville, Florida Posts: 803 Likes: 6 Liked 20 Times in 12 Posts	Re: Qjet Question Thank you all for the suggestions. The choke is wired open and it does not serve any function. I will look and see if somehow the secondary flap is getting caught on something. Please keep any suggestions coming. Thank You.

04-22-2017, 08:57 AM	#6
Hacksaw Senior Member Join Date: Sep 2016 Posts: 838 Likes: 148 Liked 276 Times in 151 Posts	Re: Qjet Question Is your linkage reworked to the secondaries or still factory stock. If still stock, could the rear throttle blades somehow stick closed when you pedal?

04-22-2017, 12:15 PM	#7
Mark Yacavone Veteran Member Join Date: Mar 2002 Location: Miles From Nowhere Posts: 7,414 Likes: 2,579 Liked 4,447 Times in 1,693 Posts	Re: Qjet Question Not sure of the exact problem here, but I would say to look at each possibility. Start with the sec. flapper, and make sure it doesn't come anywhere near the top opening. You can loosen the screws and shift the plates around a bit, and then re-tighten. Or, just file the casting a little bit, where ever it's close. Of course, you wouldn't want to do this on a S/SS carb. I'm sure you know this. __________________ We are lucky we don't get as much Government as we pay for..... Will Rogers

04-22-2017, 01:35 PM	#8
Hacksaw Senior Member Join Date: Sep 2016 Posts: 838 Likes: 148 Liked 276 Times in 151 Posts	Re: Qjet Question Just remembered this. I once had the split pin that secures the cam for the hanger walk out slightly and that interfered with the flap.

04-22-2017, 02:41 PM	#9
Alan Nyhus VIP Member Join Date: Dec 2002 Location: Lower Dakota territory Posts: 1,163 Likes: 1,013 Liked 761 Times in 225 Posts	Re: Qjet Question What's the float level measurement? And fuel pressure? __________________ "That'll never work....."

04-22-2017, 07:30 PM	#10
Vic Santos Junior Member Join Date: Dec 2008 Posts: 26 Likes: 0 Liked 1 Time in 1 Post	Re: Qjet Question Had this problem approx. 30 years ago. What happens is that when you shut the secondary throttle at high vacuum the pressure across the throttle blades holds them in place when you try to open them again especially if the bores are a little oxidized and the actuating lever is NOT PINNED TO THE SHAFT. The fix is to pin the throttle shaft to the actuating lever; do not rely on the torsion spring to open the throttle. A $.10 rollpin is most common fix.