Author Topic: Chugging, rough running, popping in exhaust after QJet rebuild  (Read 799 times)

Offline steve.dinino

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This project started after I decided to ditch the Edlebrock 1407 that was on the car when I got it and go back to the stock QJet. Beyond the ancient design of the 1407, I was tired of fooling around with hack throttle brackets and downshift cables. It also had no choke and that made cold starting difficult.

I have a 77 Corvette w AT & A/C  (but not the original engine - I believe it's an L65 from a 72 Camaro). The original exhaust and intake have been replaced with a Weiand 8004 and Flowtech 11106FLT headers. Other than that, I believe the engine is stock. Heads, cam, etc. are original.

Before switching carbs the car ran OK. It was dog slow (as you'd expect with 77 gearing and a TH350) and drank a TON of gas, but it didn't miss or chug.

I found a couple usable 17057204 cores and used your rebuild kits to make one good QJet. I used Lars papers and tutorials to go through all the usual trouble points - years and years of dirt and neglect, worn shafts, bent linkages and tangs, weird float heights, clogged tubes, etc. I did my best to return the QJet to its stock setup. It has the stock 77/52 primary jets/rods and the CH secondary rods installed.

The only non-stock mod I made was one I read in your book:  drilling out the idle mix holes in the throttle plate to .090. I felt safe doing this because both you and Lars commented that it was common to run out of idle mix adjustment on these carbs.

I report sadly that it has never come close to even the lackluster performance of the Edelbrock. I've checked and rechecked the timing curve, checked valve lash and valve action (fearing a wiped cam lobe), replaced plugs, cap/rotor and wired - no joy. I went so far as to get a wideband AFR bung welded into the header so I could check AF ratios. At idle I'm sitting at about 14/15:1. It idles great. When I give it gas it will go lean, but not crazy lean: 15-16:1. But it most definitely runs like hot garbage.

What am I missing here? Any suggestions? At this point I'm either going to box this up and send it to Lars and beg his mercy, or I'm giving up and buying a Holley.

Steve
 

Offline mcx

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Re: Chugging, rough running, popping in exhaust after QJet rebuild
« Reply #1 on: June 11, 2023, 04:15:15 AM »
…I would remove the air lid plug and make the APT functional( it’s in Cliffs book!) and start adjusting it by turning it counterclockwise to richen the part throttle response (make small changes and road test each time)..you have the wideband o2 gauge so put it to work.

Offline quadrajam

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Re: Chugging, rough running, popping in exhaust after QJet rebuild
« Reply #2 on: June 11, 2023, 08:24:52 AM »
Popping from exhaust may be ignition related.Do you have another dist you could swap?
Or maybe just the module and coil...???
Got full 14 volts to the Dist. terminal?

Offline steve.dinino

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Re: Chugging, rough running, popping in exhaust after QJet rebuild
« Reply #3 on: June 11, 2023, 12:04:17 PM »
Thanks for the replies.

I failed to mention tapping the APT plug hole when I had the carb apart. I did do that bit of modification as well. The weather hasn't cooperated today (leaky t-tops = no rain driving) but I will get it out and try incrementally adjusting APT as soon as it's dry.

As for the ignition suggestion: good thought. I replaced the distributor a couple years back because the old shaft was worn. The new one was a GM drop in replacement, complete with coil and module. Doesn't mean everything is good though. I'll pull out the meter and test the coil. I can also swap back a different module just to see if it makes a difference.

Oh, one more bit of data: I checked compression. All cylinders are within a couple PSI of 120. Not stupendous, but no cylinder is way low either.

Offline steve.dinino

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Re: Chugging, rough running, popping in exhaust after QJet rebuild
« Reply #4 on: June 12, 2023, 07:47:28 PM »
Well, I found the problem. The #8 plug wire had a crack in the insulation near the plug boot and must have been arcing against the block or header. I couldn’t see it because that plug is hidden behind the AC box. Replaced the wire with a spare, and it runs better than it ever has.

What’s the old saying again… most carb problems are really ignition issues?

Offline Cliff Ruggles

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Re: Chugging, rough running, popping in exhaust after QJet rebuild
« Reply #5 on: June 15, 2023, 12:21:16 AM »
Good news!....