Hey guys. I'm posting because I think this might be a very distinctive problem that someone can immediately diagnose. I've never run into an engine hesitation like this one before! The car is a 1980/4speed stock except for headers and true duals.

The hesitation happens only during one very specific circumstance. Driving normally (cruising or casual acceleration), mash the pedal down and it sounds like the engine shuts off for maybe a quarter second, then it returns to running normally. Sounds like a fairly normal thing right? Nope.

The main odd thing is that if I let off the pedal after this happens, drive normally, and do the same thing again say 30 seconds later there is absolutely no hesitation at all. If I drive the car like a maniac (moderate to hard acceleration and braking without any cruising) the hesitation doesn't present at all.

The other slightly bizarre thing is that while it sounds like the engine was literally shut off for that quarter second, there is never any stumble or hesitation when the power comes back on, like the hesitation never happened. Oh and I get popping in the exhaust during engine braking so I assume there's some rich mixture going on that could be related, but I have the quadrajet power adjustable part throttle kit installed in the carb and between that and the idle mixture screws I've not been able to tune out the popping so far.

I have a few ideas for what could be causing it, but I'm really hoping someone can save me a bunch of hours of troubleshooting by instantly knowing the cause of this behavior. If not, I'll just have to dig in when I can find some time and start going through the usual suspects.

I'm not listing all the details of what I've done to the car already etc. because I'm only asking if this is a very specific known problem rather than just asking for help in general going through all the usual stuff. I'll be happy to provide that info if someone wants it, just post here and let me know.

Thanks!

 

1st issue could be accel pump.
2nd issue sounds lean issue
headers lean out mixtue.

what is but I have the quadrajet power adjustable part throttle kit?

 

Accel pump and spring new, but maybe need a different spring stiffness.

for an explanation of the APT setup

 

Wires on the pick up coil on the verge of breaking??
Only showing up with a sudden vacuum advance change??

 

Thanks I'll check. I actually bought a new distributor and intend to put it in hopefully by the end of the month. Time is difficult to come by.

I'm pretty sure I drove the car with the vacuum advance disconnected while trying to figure this out last year but I'll check that too. Something I hadn't thought of before, I installed an adjustable vacuum advance and following Lars tuning papers and IIRC I wound up maxing it out and got close but still not quite reaching the amount of vacuum advance Lars recommended. This is all from memory from over six months ago so apologies if I say something incorrect about the tuning papers.

Anyway my point is that maybe maxing it out was incorrect, so all the more reason to make sure I verified the vacuum advance isn't triggering the problem.

I've currently got the APT kit (vid above) adjusted very lean right now... to the point where I think I'm just barely detecting a tiny bit of surging at cruise, and still get a bit of popping during heavy engine braking like the off ramp of the interstate but I do think it helped a bit. I need to play with it a bit to make sure I'm not imagining the very slight lean cruise condition.

When I bought the car it was surging badly at cruise, very unpleasant to drive. I rebuilt the carb and set it up with some help from the tuning papers and also installed the APT, replaced the plugs, wires, cap, and rotor, installed the adjustable vacuum advance, stripped off all the emissions stuff, and installed the headers/exhaust... Oh and also when I tried to shoot the timing for the first time I found the balancer had a slipped ring so the timing may have been way off. I replaced the balancer (all of this was done over the winter) and in the spring finished putting it back together and timed it based on Lars advice. I had already moved the distributor before I realized the balancer ring was slipped so no idea if it had been set to something reasonable before. At any rate something I did fixed the surging 100% but I got this hesitation in it's place. The fix might have been the carb work, incorrect timing advance, or may have been the cap and rotor. I remember finding the little spring/button thing had literally fallen apart when I pulled the cap and rotor. I can't really say whether it was doing this hesitation thing before I did all this work, the surging was so bad I didn't even notice anything else that may have been going on. This week I decided to replace the coil and it turned out a whole new high performance HEI distributor with excellent reviews on Amazon was basically cheaper than buying a decent coil so I bought one LOL. Hopefully that will pretty much guarantee there's nothing ignition related causing any of my little problems if nothing at all changes when I install it. The only problem there is finding time to work on the car, I'm insanely busy working extra hours and making critical repairs to my house, but I'm also too broke to pay someone else to solve my problems!

 

Check your secondary pulloff for proper function.......
When you hit the gas WOT......the vacuum for the pulloff disappears and releases the secondary air valve so it can open.......the hesitation before it opens allows the secondary fuel tubes to catch up and release fuel as the air valve is further open, or else it will bog......in fact, this $15 pulloff is the #1 cause of the "Quadrabog" that millions of "gearheads" bitched about in the 70's and 80's.......you can also tighten the rotating tension spring on the secondary air flap as well......
Accelerator pump lever rod should be on the inside hole.

Jebby

 

When's the last time you changed the fuel filter(s)? Quick and easy to do.

 

To diagnose your popping, search for "lean afterfire".

80 4-speed is peak Corvette! And mine does that, too. Tweaking the timing helped a lot (make sure your vac can is connected directly to manifold vacuum, not ported). Further tweaking the idle mixture helped a bit. But read some posts by @lars describing what's happening, and get his timing and Q-Jet papers, if you don't already have them.

The real solution is likely rejetting to compensate for the headers.

 

Read post 6 above. What do you have the secondary air valve spring tension set to and have you checked the vacuum pulloff?

 

I did a full rebuild on the carb last summer with all new parts including the filters.

 

Mel, Bike, and Jebby thanks for the excellent suggestions. I'll dig into them as soon as I get a chance, this looks like great advice but I probably won't get to tinker with it until next week.

 

If the choke pulloff has been changed (or if the carb has been "rebuilt"), chances are damned good that the pulloff is defective: Most replacement pulloffs do not have the proper bleed-off restrictor in the vacuum nipple, and have an "instant" relaxation/extension rate. This will cause the problem you are seeing. To test it, just put a long vacuum hose on the pulloff nipple and suck on the hose. Does the pulloff retract instantly as you suck on the hose? It should retract at a restricted rate. When you release your suction, does it instantly extend? It should not. It should take about 2 seconds to fully extend. If it does not delay, you need to replace the pulloff with one that has the proper bleed rate, or fabricate a .018" restrictor to install into the nipple.

Also, verify your secondary airvalve spring is set to 3/4 turn windup.

Lars

 

I suspect that the issue has been correctly identified above - lots of good info.

I had a similar issue with my '79 (actually several issues) that we thought we had addressed. This included a new fuel pump. It was then that we had different significant bogging issue...
Of course, the new fuel pump was a huge improvement over the old one it replaced. When I went to drive more aggressively, the car would start to respond, then bog as if starved for gas. It was, but not because of the fuel pump.
It turns out that the intake sock in the gas tank was old and dirty and became a huge restriction when the new pump asked for more gas. I replaced the sock and all was great!

I don't think that's what's happening here, but those socks are not expensive and it is an easy thing to check and cross off the list of possibilities.

 

Excellent thank you! I haven't been able to work on the car yet since these messages but I expect to find you have pointed me at the correct problem. How do I purchase a replacement pulloff with confidence that it will be the correct type with the restrictor?

 

It was exactly what I was looking for and I absolutely love it. An added bonus of sorts, I was born in 1980. It's silly, but makes me smile anyway. Besides, it probably makes me a little more willing to tolerate that she's got some rattles and lots of imperfections when I think about my knees and sore back etc. being the same age she is! :P

I'll confess, when I was shopping I wished I could afford to buy a 68-69 roadster like I had twenty years ago but I love the '80's looks almost as much and she's so much more comfortable and polished feeling to sit in and drive. I always coveted these deep bucket late C3 seats too.

Wait, did we already talk about Corvairs last year? I see a Stinger or Stinger clone in your photos... I was a hardcore Corvair Nut for many years, and always will be in my heart. I think maybe I asked you this already and you said you didn't own that 'vair you just like them?

Your 80 looks fantastic too, I love the red/black interior for some contrast. I keep thinking I might put black carpet (including door panel carpet) in my 80 when I eventually replace the carpet.

 

https://cliffshighperformance.com/pr...choke-pull-off

 

Perfect, thanks!

 

I'd estimate that when I release it, it takes approximately one second to retract. When I suck on it, it takes maybe 1/2 to 3/4 of a second. I'll go ahead and order a replacement. I'll check the airvalve spring, but I followed your papers as best I could when I rebuilt the carb last summer so it's theoretically already set to what you recommend.

Thank you Lars

 

you also may want to buy this book

 

Done, thank you.