Bog when secondaries open

[JohnM]

I have a 396 out of a 1966 Chevelle in my 1978 K5 mated to a 3 speed automatic. Had an old Edelbrock and it ran poorly when I bought it so I had Bob at Custom Carbs in Reno NV build me a Qjet about 11 months ago. Whenever I would open the secondaries the motor would bog and then the RPM would surge up and down between 300-400. Same surge when I would hit a bump. He said the core matched my motor and all other parts were new and it ran fine on the 396 test motor in his shop. He finally got a different core and built me a new one. I can now drive around all day on the primaries (no more surge or RPM bounce) but when I open up the secondaries it bogs hard around 2500 RPM! For the rest of the day whenever I try to climb a hill it will immediately die, then fire right up and idle perfect, add another hill/mix/repeat. Bob told me to get a fuel pressure reducer (not the clicker kind?) because the Qjets can’t take more than 4.5 PSI??? New parts= Intake/distributor/fuel pump/fuel lines+shielding/fuel tank w/no improvement. I have already put enough $$$ in to it to have installed a TBI... On other blogs I have seen similar issues with a recommended fix of  adjusting secondary air valve tension, although no one ever followed up on whether or not this solved the problem.

[429bbf]

john it sounds like your running out of gas .(low float level, to small of needle and seat,fuel delivery problem,(volume )if you pull the line off the carb and stick it in a bucket is there a steady stream of fuel with the engine running?when i run into problems like this i feed the carb with a secondary source of fuel to help me find the problem.hth.

[Cliff Ruggles]

Your builder is incorrect about fuel pressure.  The later units can take high fuel pressure and function fine.

Any "bog" at the onset of the secondaries is usually a lean condition, which is easily corrected.  If a new secondary cam/spring wasn't installed during the "rebuild" (unlikely that it was) get them and put them in place.  Set the spring tension about 3/4 to 7/8 turns for initial tuning.

Check the secondary hanger to make sure it isn't bent.

Get one of our accl pump assemblies to make sure the pump shot is sufficient when going quickly to full throttle.  Most secondary problems are actually a lack of fuel from the accl pump.

Check the secondary POE system to make sure the passages are open in the main casting and airhorn.

Last, check the choke pull-off to make sure it isn't releasing too quickly, or that the link up the secondary airflaps has play in it when the pull-off has vacuum to it.

I suspect a few simple things missed during the build and slightly out of adjustment is all that's going on with your carburetor.  I see this a lot, folks use crappy parts, incorrect fuel inlet seat, and NEVER change the worn out plastic cam and weak secondary airdoor spring......Cliff

[JohnM]

Thanks for replying. Truck is running perfect until I try to climb any long hills. Will pop a couple of times and then die when the engine loads up. Immediately restarts and idles perfect for the descent. My MPG has increased from 6 to 10 and no more bog when I open the secondaries. I did install a pressure regulator and I am going to try increasing the pressure to see if that helps. Here is a video of it dying on a hill. You can hear the RPM's fall off then recover then fall off again and die. Had steady pressure on the accelerator.
https://www.youtube.com/watch?v=kX5NdOGDS94&feature=youtu.be

[JohnM]

Took it out this morning to the same hill. When it started to pop again I had to floor it to keep it running, but made it to the top. Hit a couple of steeper hills same thing. I don't recall having to open the secondaries every time I climbed a hill when I was a kid. Is this just normal op, or is there something I can change/adjust?

[von]

I had a somewhat similar problem once where it would run normally except on long uphill grades where the engine would bog and eventually die. Turned out to be the nylon sock on the fuel pickup in the tank would get clogged with debris and severely restrict the fuel supply. After sitting a few minutes the debris would fall off and it would start and run OK til the next hill. I had the sock removed and an in-line filter installed and no more problem.

[JohnM]

Thanks for the reply Von. Starts immediately after, no problem or need to press accelerator.
 

[Cliff Ruggles]

It doesn't appear you're running it hard enough to suck the bowl dry.  May be flooding out, I'd try lowering the float to see if it improves.....Cliff