Backfiring when heated up...Why?

[yellowhammerjam]

Long time reader, first time poster. My 1972 240z cranks easily and when cold starting with the choke I get no backfiring. After the Z warms up I get the backfiring on high revs when i release the pedal. Also, I get a chirping sound that worries me like the valves rattling after it warms up. Is this probably all related to timing and SU carb adjustment? I ordered the flow meter today and will try to balance the carbs better. The car had a slight smoke out the tailpipe but comes and goes. I was going up an incline the other day and it sounded like the valves were about to jump out the motor.

Love the car and tinkering with it, but as far as mechanical reasoning I have been reading older posts to try and figure out the problem

Any advice would help.

[Gary in NJ]

Timing.

Have you checked the timing with a strobe? If so, was the idle set correctly with the vac-adv disconnected and plugged?

[cygnusx1]

I can't answer your question exactly but I can give you some advice. When the exhaust manifold gets hot, the unburned fuel mixture can ignite in the manifold causing a backfire. To exacerbate the situation, an exhaust manifold leak can allow oxygen into the manifold for even more likelyhood of combustion. Chirping, is the sound that a leaking exhaust manifold gasket often makes. Under load, an exhaust manifold leak can sound like a loud ticking that could be confused with valve noise. All in all, I am betting on you needing a new gasket. If you do, check the manifolds for straightness.

[Zed Head]

The sound of "valves about to jump out of the motor" while climbing an incline might actually be "knocking" or detonation, which could be from incorrect timing. It's very bad for the engine.

[Bruce Palmer]

I'm leaning toward timing too far advanced and the carbs being lean, which is why things are okay when the choke is pulled on, richening things up.

Does this car have an electronic distributor in it?

Edit-- When you say "backfire" do you mean spitting back through the carb/s. If so that would be a definite lean thing especially in the off throttle mode

Edited October 11, 2010 by Bruce Palmer

[yellowhammerjam]

Well I have almost finished the front suspension and moving to the rear. I will check the exhaust manifold gasket for leaks and get the timing right. The flow meter has yet to arrive but will get the carbs balanced soon hopefuly. I love tinkering with the car and learning how things work and how to fix when it breaks down. The aricles and advice from you guys are so valuable.

Maybe soon I will have it back on the road with a big smile on my face!!!

Also, running stock points. Have toyed with the idea of electronic ignition. One baby step at a time though.

[yellowhammerjam]

Could my anti backfire valve be bad? I checked into replacing that and ouch the price is around $200.

Are the timing strobe easy to read even for a novice such as myself? I found the advance and retard marks at the bottom of the dizzy. It is set at just past the middle mark slightly. I guess I just need to get a timing strobe and figure it out. The engine had some work before I bought the car. It had been sitting for about 12 years garaged. All stock except for headers installed some time back. The engine ran ok when I bought it except for a little smoke. Then I noticed the ticking sound soon after. When I first crank the car with the choke it sounds pretty smooth. No ticking or backfiring. After it warms up and choke depressed the ticking starts and the backfiring starts upon decelleration from high revs along with puffs of smoke.

[SteveJ]

In addition to what the others have said, do you still have the smog pump attached? If not, have you plugged the port going into the exhaust manifold? Either condition could increase the amount of oxygen in the exhaust. Combined with bad timing, mis-adjusted points, etc., this could lead to backfiring.

[yellowhammerjam]

I will check the smog pump to see if it is still installed. I bought a timing strobe today and will be checking that tonight.

[yellowhammerjam]

All the smog equipment is still hooked up. I am thinking it could be the anti backfire valve causing the backfire out the exhaust. Only does it upon deceleration after the engine is warmed up. Thoughts?

Also in trying to balance out the carbs I hear a whisteling sound coming from the rear carb. I can push up on the pin underneath slightly and it will stop and idle smoother. Thoughts on how to adjust so the whistle stops?

I am close on the carbs. The engine runs smooth on aceleration just can not get the idle smooth at 750 rpm. At 1000 rpm pretty smooth. I am thinking the whistling of the rear carb is to blame.

[SteveJ]

Did you check your dwell and timing, yet? If you have too much unburned fuel in the exhaust, the smog pump will provide enough oxygen to support combustion on decelleration. You can check this by pulling off the smog pump hose and capping the inlet to the exhaust. If the backfire goes away, you know you have too much fuel in the exhaust. A big help to eliminating that in my 73 was the installation of an electronic ignition. Better combustion eliminated the unburned fuel problem.

[yellowhammerjam]

Thanks I may try that.