Ok my 78 is done here is what I did.

Vortec Heads
1.6 roller tip rockers
dual plane intake
Rebuilt Q jet twice
Electric choke
Re-curved dist 36 deg all in at 3000 idle at 12 deg
New cap, rotor and coil.
New spark plugs and wires, rechecked order 7 times.

She starts great, idles great, under load backfires thru exhaust only. Sporadically but annoyingly and predictably, worse when cold.

All suggestions welcome!

 

mechanical advance distributor? check springs .. see if not advancing

 

Seems good all clean, free and new i'm stumped.

 

First I re set the valve lash. No difference.

Then

I had bought a Mr Gasket 929 advance kit. The holes in the weights were to small and the weights were not swinging free. I dremmeled them out a touch reset the timing and wow she runs great!

Thanks Bebezote for making me look at dist again.

 

Great to hear !!...

 

Little longer test drive its still there much better though. Carb?

 

Popping in the exhaust, referred to as "afterfire" ("backfire" is popping up through the carb), is almost always caused by a lean condition. The modifications you've done to the engine will require that the carb be richened up. If you're running the carb in its stock configuration with the mods you've made, you have a significant lean condition, which will cause an occasional lean misfire. When you get the lean misfire, the misfiring cylinder will pump the unburned fuel and air into the exhaust system, where it then will explode. Richen the carb up and it will go away.

Lars

 

Afterfire got it. Thanks Lars 17057253 carb looks stock but I can't find the stock jets and rod combo no marks on the ones in it. I've read your paper 3 or 4 times but I could use a starting point.

I thought I might drill out the Idle discharge ports and the Idle Channel Restriction to fatten everything up.

 

Lars and Cliff Ruggles are both very generous with their time and knowledge on this forum. With the help of this forum, their writings and a wideband O2 gauge I was able to really dial in my Qjet.

I am not an expert at all. I only share some things they shared with me. I have a similar carb to yours. The first thing Cliff told me was I needed to choke up the air bleeds. He sells sets of tap-in restrictors for a couple of dollars. I installed two sets and did not drill anything. I also bought a set of rods from Cliff. Give him a call and let him know your specs and I'm sure he can get you lined up.

The combo that is working in my small block is 44 rods from Cliff and 69 jets. The eureka moment for me was understanding the APT system of these carbs.

 

is the power piston enriching it..?

 

but it still sounds like timing to me..not getting advance...as under load conditions causing it..

 

this is from another site...but it sounds like timing..or power piston problem..it if was running ok..and now seems to want to pop.... sounds lean..

this is a write up by Lars !!!

Incorrect Power Piston Spring
Results in: Poor idle, rich idle, ineffective idle mixture screws, hesitations and stumbles.
Comments: At some time, there must have been a popular article published that told people to cut their power piston springs to hop up their Q-Jet: I see more cut springs that you can imagine. A cut, or soft, spring will keep the power piston seated in the full lean position during part-throttle power, resulting in a sag and poor throttle “feel” during part-throttle acceleration. In contrast, if a stiff spring has been installed, the piston will never seat, and the carb will be running in a full-rich condition even at idle. This causes incredible tuning problems for idle and off-idle performance, and the idle mixture screws will have little effect. New power piston springs are available in packs of 10. If you suspect the spring to be non-original, stick in a new one to eliminate this as a problem.

Jammed Power Piston
Results in: Poor idle, rich idle, ineffective idle mixture screws, hesitations and stumbles.
Comments: The power piston can be jammed in either the full lean or full rich position, producing the same symptoms as an incorrect spring. You can test the power piston with the engine “off” by inserting a thin, long screwdriver down the bowl vent in the air horn. You should be able to depress the power piston and feel it pop back up. A jammed piston is usually caused by dirt entering the bowl, but can also be caused by carbon sooting up through the intake manifold from bad valves or valve timing problems.

 

When I rebuilt the carb for the 5th time I was smart enough to drill out and Tap APT port.

I was at my wits end because I never had a Qjet with APT before. Idled great, WOT great, transition between primary and secondary great. everything else bucking and snorting and "afterfiring" light throttle ran like crap. Turned that silly APT screw 1 1/4 turns CC and voila runs like a raped ape! I'm so excited the car is fun to drive woooohooooo!!!!

No direct answers but Lars and bebzote pointed me in the right direction with the timing and power piston position (which the APT dictates).

My neighbors think I'm crazy cause I'm lighting up the tires from every stop and turn.

 

I'm one of the lucky ones! I live in Colorado Springs (6100 ft above sea level) and bought a '68 Shark from Fayetteville AR. As you can imagine it needed some timing and carberurator adjustments. I emailed Lars and he said sure bring it by, so I drove it the 90 miles or so north to his home garage. He was amazing to watch, like a ballet dancer on this side then on the other, adjusting this and then that. He found a bad vacuum advance and changed it...........then engine suddenly became smooth at all rpms. About the time I thought we were through, he turned it off and off came the carb and over to the bench. Seemed like only a minute and he had the carb apart with all the pieces all laid out. He rejetted it and what amounted to a carb rebuild for altitude right there. Put it back on the car and final adjustment and told me to take it for a spin. Big smile on my face as I got rubber in the first 2 gears (L-79, 327 ci 350 hp) when merging on a hiway.

On the was home south of Denver I ran it quickly up to 90 a couple of times just to stay out of traffic's way.

Amazing difference even at this altitude. I now have added "Tuned by Lars" to my car show storyboard.

 

Cool deal......I'm just so jealous ...touched by the master

 

Lars is the man for sure wish he lived in Alaska. Lars Being a rocket sceintist, id bet that this stuff to him is like mowing the lawn to us.

 

can you even see your lawn today?

 

No I havnt seen my lawn since the end of October. Merry Christmas.

 

Merry christmas. To you too.

 

Ok did my final (LOL) tune today.

Initial timing at 18 all in by about 3,000 (screaming engine in driveway is distracting). Recurved dist with cheapo Mr. Gasket advance kit, set mixture screws at 3 turns out, She rips!

Last question is I'm pulling like 21-22 inches of manifold vacuum at Idle 700 RPMS and had to turn APT out to almost 6 turns (8.5 is max before it falls out) to get rid of lean stumble and afterfire is this due to the performer (2116) intake and Vortec heads? Sure sounds like a lot of negative air pressure/Vacuum.