Attempted to tune SU Carburetors but still backfiring on intake

[jalexquijano]

My mechanic adjusted the TIme to 18BTDC at idle maybe it should be lowered to 12 or 10 BTDC at dile and it will supress the deceleration backfire. I am trusting that the distributor timing marks aligned correctly and thus can draw such measurements! 

 

Any advice?

[Chickenman]

Im having this same precise problem, My car is a 1972 240z with a Schneider 274F Camshaft and remanufactured SU Carbs by Ztherapy!!! Also have pertronix ignitor and have set the Distributor timing to 11 BTDC. Please note i have also removed all the emmissions crap including the antibackfire valve from the balance tube.  Should i lower the timing to 10 BTDC at idle to avoid the sudden gunshot when decelerating?

^ above is linked from a 13 year old thread.

 

http://www.classiczcars.com/topic/1382-backfiring-should-i-be-concerned/#entry482317

 

 

Ok. So you never did get back to me in the other ( this ) thread when I asked for clarification on how loud the back fire from the exhaust was. That's the problem with starting or discussing the same problem in numerous threads and scattering information all over the place. People can't follow what you have done. And it is really bad form to revive 13 year old threads from someone else and expect people to follow what you have done. George, please stick to ONE current thread!!

 

However, from your latest description it sounds ( no pun intended ) like you have a fairly big backfire as you describe it like a " Gunshot". Is that correct?

 

Most cars with a bigger cam and properly tuned will pop or sneeze through the exhaust. That is perfectly normal.  What is NOT normal is a big Gunshot or Shotgun backfire.

 

That type of backfire is usually caused by air leaks into the exhaust system. What happens is that you always have some " over run " with carburetors on deceleration. This means that partially un-burnt fuel gets blown out of the cylinder on the exhaust stroke. Higher duration cams such as your Shneider 274 add to the problem due to the greater valve timing overlap. By itself, this partially burned fuel is not a big problem.... but add some extra Air into the mix via an exhaust leak and and it can create an explosive combination. Most people don't realize that exhasut leaks work both ways. The exhaust gases flow " pulses " blowing exhaust gases out of a bad joint... but also sucking fresh air in on the Negative pulse.

 

So any leaks in the exhaust side needs to be located and fixed.  

 

1: First thing that comes to mind is that you removed some of the emissions control parts. If exhaust manifolds had air injection plugs did you plug the system off properly. Note that the air nozzle threads are NOT a standard pipe thread size. Brass or steel Pipe fittings will fit if you force them in... but they can leak air. That can be enough to cause backfires.

 

2: Look for any broken studs, blown exhaust gaskets or cracks in the exhaust manifold/header. All are sources of air leaks and can cause backfires. Check tightness of all nuts and bolts.

 

3: If you still have the factory manifold, check for broken studs or blown gasket. Check that the fasteners are tight. Any signs of black carbon or white markings around exhaust parts can be signs of an exhaust leak. 

 

4: Check the rest of the exhaust system all the way back to the tailpipe. Best to do this on a hoist. Look for leaks at pipe joints, cracks or holes in mufflers. Big backfires can blow the seams in a muffler. Then the problem gets worse. Black traces from leaking carbon will usually be a clue.

Edited December 4, 2015 by Chickenman

[siteunseen]

Sounds like that "antibackfire valve from the balance tube" isn't "crap" afterall. 

[jalexquijano]

Thanks for your advice Chickenhead. However, i wanted to comment my car presented the following symptoms today:

 

Accelerating fine at high rpms but whenever it got stuck in 2 to 3 minutes traffic jam, the idle started to become erratic to a point the rpm fell to 100 RPM and suddenly the engine stalled. I had to pull the choke up or keep the foot in the accel pedal to avoid the car from stalling. The car stalled 3 times,  Could this be due to the fact that the exhaust valve on cylinder number 4 was opened to .013 with the purpose of running on 6 cylinders as mentioned on my former post number 42!

 

I guess i will need to perform another compression test as i am beginning to suspect there could possibly be a burnt or warped valve somewhere.

 

What directions should i take first before spending money on new valves and seats?

[Chickenman]

I highly doubt it's a valve problem... and your mechanics tale of having to adjust the one valve to .013" never did make any sense at all. That's a wild goose chase IMHO.

 

It's going to be either a fuel or ignition problem that is affected by heat. What is the temperature gauge reading when in Traffic and the problem occurs? More likely fuel. Having to pull the choke lever on when it gets hot indicates engine is running out of fuel. Could very well be vapor lock.

 

Can you take a few clear pictures of your engine. I want to see a few things. Some good shots of the carburation and fuel lines and fuel filter would be very helpful. Take a few general pictures of ignition system and wiring as well.

 

BTW... do you have the factory heat shield in place under the carbs?

 

Edit: You need to find a different mechanic IMHO. Your Porsche " expert " has some strange ideas or you are not understanding him. Either way, these are simple engines and any decent mechanic should have it running like a top by now.

Edited December 12, 2015 by Chickenman

[bartsscooterservice]

What's the condition of the engine itself ? How many miles ? Does it use oil ? Sounds more like leaking valves to me, and bad or worn piston rings. Backfiring through the carb or exhaust can be an ignition timing problem, or leaking valves.

 

The carbs can run to lean or to rich, but if the engine is in good condition it should not backfire..

Edited December 14, 2015 by bartsscooterservice

[Chickenman]

^ Hey Bart. Thanks for trying to help. But unless you have been following all of George's posts, you may have missed some very important information. We've already covered that valves and rings are not the issue in this and other posts. Engine has very good compression. Within 175 to 180 average with a high of 185. Well within specs.

 

Unfortunately, Georges mechanic, has got him chasing his tail around in circles. And George himself has got so many different about the same subjects threads on this and other forums that it is difficult for members to follow exactly what is going on.

Edited December 14, 2015 by Chickenman

[jalexquijano]

The car is back with the mechanic. The backfire was eliminated by setting the Timing to 10BTDC at idle. Now i have to tackle with the shutting down when the car is in heavy traffic and the plugs getting black at idle. He will need to adjust the valve to .010 and see what happens.

[jalexquijano]

^ Hey Bart. Thanks for trying to help. But unless you have been following all of George's posts, you may have missed some very important information. We've already covered that valves and rings are not the issue in this and other posts. Engine has very good compression. Within 175 to 180 average with a high of 185. Well within specs.

 

Unfortunately, Georges mechanic, has got him chasing his tail around in circles. And George himself has got so many different about the same subjects threads on this and other forums that it is difficult for members to follow exactly what is going on.

Mechanic is still checking valves opening on 4th and 5th cylinder! He says the compression on cylinder 4 is the highest of all and that is the one where the exhaust valve had to be opened to .013 for the car to change the idle when pulling the spark plug off cylinder 4. If compression is good and no leaking or warped valve, why is the car idling erratic tending to stall after car warms up and shutting down when stuck in traffic??

[Chickenman]

Mechanic is still checking valves opening on 4th and 5th cylinder! He says the compression on cylinder 4 is the highest of all and that is the one where the exhaust valve had to be opened to .013 for the car to change the idle when pulling the spark plug off cylinder 4. If compression is good and no leaking or warped valve, why is the car idling erratic tending to stall after car warms up and shutting down when stuck in traffic??

George we keep trying to help you out but you never seem to answer our questions properly. You just go off track and ask more questions, often repeating yourself.

 

How about reading post #53 ( to start with ) and answer each question fully and post pictures as requested. Remember... we aren't there and we can't mind read. So you have to be as clear and logical as possible. Otherwise, people are just going to get fed up and stop answering you.

[Chickenman]

Mechanic is still checking valves opening on 4th and 5th cylinder! He says the compression on cylinder 4 is the highest of all and that is the one where the exhaust valve had to be opened to .013 for the car to change the idle when pulling the spark plug off cylinder 4. If compression is good and no leaking or warped valve, why is the car idling erratic tending to stall after car warms up and shutting down when stuck in traffic??

Here are the compression figures that YOU posted way back at the start of this thread. A 5 psi difference in compression is NOTHING and the figures are well up there. Perhaps ypou mechanic cleaned the fouled spark plug when he took it out,. But from these figures there is absolutely NOTHING wrong with your compression and that means nothing wrong with your valves or rings.

 

And even IF you did have a bad valve you would have a constant rough running and a misfire and , it would NOT cause stalling in traffic as the engine heats up.  Understand??

 

 

THe compression test showed the following results:

 

cylinder 6 --------------------- 175 psi

 

cylinder 5 --------------------- 180 psi

 

cylinder 4---------------------- 180 psi

 

cylinder 3 ---------------------- 185 psi

 

cylinder 2 ---------------------- 180 psi

 

cylinder 1 ---------------------- 180 psi

 

 

I'm about to give up on this. Someone else take over please. I tried but this is a waste of time.

Edited December 21, 2015 by Chickenman

[jalexquijano]

George we keep trying to help you out but you never seem to answer our questions properly. You just go off track and ask more questions, often repeating yourself.

 

How about reading post #53 ( to start with ) and answer each question fully and post pictures as requested. Remember... we aren't there and we can't mind read. So you have to be as clear and logical as possible. Otherwise, people are just going to get fed up and stop answering you.

Can you specify which pictures you need and from where exactly? Do you want pictures of the cam with the valve cover off??