I have a 1969 L46 coupe, 4-spd close ratio gearbox. Car is stock except for a Pertronix Ignitor III breakerless ignition. Carb is Rochester Qjet.

Car is tuned, primary ignition is new. I've owned the car for 18 months and the engine is in great shape. There is no documentation to indicate an engine rebuild since new and it shows 116K miles. It blows no smoke and consumes very little oil. No oil leaks.

Car was tuned a year ago, including carb tuning. 1200 miles added since then.

During acceleration I experience 'flat spots', where depression of the gas pedal does not result in linear response from the engine. It's most noticeable around 3000 rpm. Once the secondaries open it seems OK. This condition has always been present and I've just lived with it but now I want it gone. I'm pretty good with troubleshooting and repairs, but not an expert.

My thinking points to either the distributor or the carb. I am pretty sure the distributor is the original one. Could this be a problem caused by a worn distributor, maybe a problem with the timing advance curve? If it is the carb I'm completely clueless since I've very little experience with them.

Ideas are appreciated before I take it to the shop. By the way, I've used Brad's Custom Auto in Seattle for repairs. He's great. Specializes in Vettes and Mustangs.

 

That sounds like carb to me. A tune is one thing rebuild is another. If the carb has never been rebuilt and 116k on the clock it probably needs it. These guys aren't cheap but they're supposed to be very good.

http://www.allamericancarburetor.com

 

An air valve spring on a Qjet that is too loose will allow the engine to sag or bog as the carb transitions to the secondaries. Check the windup against the spec for your particular carb.

 

Thanks. The carb settings were adjusted when I took the car in a year ago but no disassembly/rebuilding was done. I'm going to get a distributor rebuild kit under assumption that it needs one and will check out the link you sent.

I've always heard that before you check the carb, check everything else, but in this case I'm stumped to think of anything beyond the distib or carb that could cause this symptom.

 

Look up Lars on this forum and find his email. He is the Qjet expert. Ask that he send you his technical papers on the Qjet and distributor. Then, if you can't get things sorted out, you could arrange to send your carb to him. It would be worth it.

DC

 

Already downloaded the Word file. Amazing. Very thorough. I'm not sure I have time to do the investigative work though.

Thanks to everyone for their feedback.

 

If you found a Lars paper on the internet, it's likely to be an old one. He doesn't upload his papers so someone else probably did. You have to email him for the most recent version as he modifies them periodically.

Here's his email address: V8FastCars@msn.com

DC

 

If this happens when you start to 'mash' the accelerator, my first thought would be that the accelerator pump seal has torn off or fallen off the piston rod. Most of these seals are not meant to be used in alcohol. But, modern fuels now have ethanol, which is chemically similar and has the same effect on lots of these seals. Only Viton seals are impervious to ethanol laced fuels.

With engine OFF, remove air cleaner, look down into primary venturi openings and use hand to quickly move throttle linkage to WOT position. If you see a strong squirt of fuel go into each of the primary venturi, then this is NOT your problem. But, if the streams are weak (or nonexistent), you will need to install a replacement accelerator piston with Viton seal to solve this issue. Where to get one? Your guess is as good as mine. I've heard that NAPA can get them, but I really can't confirm that. You might search "Google" for "Q-Jet accelerator pump with Viton seal" and see if you have any luck.

The other strong possibility is that the problem is with the ignition and not with the carb.

 

make sure your advance weights and vacuum advance canister are working correctly. According to what you said, the distributor hasn't been touched since new - they do wear out.