Engine Pop

[Driftin John]

I need some tech help on engine performance. I'm pretty decent with cars and motors, but most of my experience is with Musclecar Iron and I'm not up to speed with the Z motor. Let me describe the situation:

'73 240 Z, all stock (yeah it has the flat top carbs) with 150,000 miles. Compression seems decent at 160 - 170 PSI

Ran decent before I took the car out of service for a bare metal restoration beginning in April of '03. While out of service, I did the tune-up basics to the motor including:

- set valve clearance - thought I got it right, but it's possible the motor cooled off too much due to my inexperience with this procedure. Plan to re-check this weekend

- new plugs, wires, cap, rotor - car had Pertronix ignition in it from previous owner that I left in place

- checked the timing, but I'm at the end of adjustment and I haven't achieved the 5*BTDC that I believe is correct. If I'm reading it right by the Chilton's manual, I can't get it further than about 10*BTDC before I run out of adjustment turning the distributor.

When I get the car warmed up and drive it, at anything more than the slightest amount of throttle, it starts to buck like a Wild Bronco as it misses and pops. The timing issue would be my leading suspect except that it didn't do this prior to when I gave it the tune-up. I've run the timing all of the way to the extremes and in between without much success. I didn't get into the timing chain, so I haven't got it off a tooth or anything since I've owned it. I have not touched any carb settings since I am very unfamiliar with these carbs.....don't look anything like a Holley spreadbore do they? If I put the choke on partially, it seems to help with the Pop, but performance doesn't seem stellar.

Lean pop in carb, valve clearance, timing.....I'm open to suggestions.

[2ManyZs]

Before you go to far, have you looked at your fuel supply yet?

Does the car idle OK and only get the pop under acceration? How about under steady throttle?

I'm wondering if you either have a fuel supply problem due to junk in the tank that might only be clogging the fuel pickup, or a clogged fuel line or filter.

Have you tried to richen the mixture on the carbs or do anything with the carbs yet?

The timing issue might be that someone had the dristributor drive gear out at one time and it is one tooth off on the carnk drive gear. Easy enough to fix if you have to later.

I'd be willing to bet it's a carb related (most likely a lean condition since the choke seems to help) or fuel pickup related problem. Best thing to do is if you aren't going to replce the flat tops with round tops, you ought to do a thorough check on the carbs. Mainly vacuum leaks at the throttle shafts, sticking needle, low fuel level in the float chamber... the list is long....

[beandip]

John , I agree with 2many , sounds like a lean pop. Is there a reason you are keeping the flat top carbs ? If you do change to the round top SUs , keep the same intake manifold that you are running now. I have a '73 also but not the flat top ones and I cant help you at all . They are a completely different breed of cat. There are a few members still running them and understand there ins and outs. If you dont get information on them , try to post a request for help with them here and try the help me section. Good luck .

[Driftin John]

I did have the tank out for the body restoration, so having some sediment stirred up makes some sense. I changed the fuel filter at tune-up, but I may just need to check the entire fuel system to be sure.

Thanks

[Driftin John]

I have the flat tops and would change them out if it were to make the car more reliable, smoother, etc. Performance is not my top priority for right now since this is a Father/Son restoration project for our 16 year old son. Funny, my Dad didn't buy me my first GTO and help me restore it????

[beandip]

the round tops are far and away superior to what you have now. They will make a big difference in the over all performance and the ease in tuneing.:classic: