Backfireing Through Carbs

[sawdust2005]

I have a problem with my 70Z backfireing though the carbs. I the stationary timing marker is gone and i really don't have any idea where the timing should be or if I get it close. Whenever I think I have the car running correctly it changes after running a while. I replaced the distributor with a Malory electronic one and also replaced the coil.

Help!!

[biddljj]

Did this problem exist before the ditributor was replaced ?

[sawdust2005]

Yes, but there were other problems too.

[beandip]

You need to determine top dead center . Remove the spark plugs and turn the engine over with a socket on the crank pulley. Either you or have someone else put there finger over he plug hole on # 1 cylinder . When the piston comes up on the compression stroke you will feel the air pressure build. Now if you look in through the plug hole with a flash light you will be able to see the piston rise . right when the piston stops , before it starts down . That is top dead center . It may take a few tries to get it. When you accomplish this then confirm by also looking for the timing mark on the pulley . Mark this with a pointer or paint a mark . This is a crude way but it will at least get you in the ball park. Why is the pointer missing ? Make a new one or replace it. Some times backfiring through the carbs is caused by too lean of mixture . That is if the timing is correct. The timing is the first thing to fix . If to far advanced major problems can be the result . If the engine pings under load , retard a few degrees , and do so until there is no ping . This will get you by for a short while , but do fix the missing pointer situation .

[bloxman]

Whenever I am looking for top dead center I remove the spark plug and use a long piece of gas welding rod to determine top dead center. While you are doing this have the distributor cap off and see if the rotor is pointing at the #1 position at the sme time your welding rod seems to make its zenith in travel. Then check that your plug wires seem to be in the right order.

You can generally fab up a pointer figure out where the old one used to bolt up. do the process above with the rod and rotor and mark (cut it , scrape it ,use a file whatever works best for you) it it to the timing mark.

Bloxman

[ezzzzzzz]

Get the basics in place (timing, plug gaps, wires). As stated by Bloxman, pull the distributor cap and watch the rotor while turning the engine over by hand and bring the #1 piston to TDC. #1 on the distributor cap is basically facing the radiator. Check for a timing mark on the damper and make some mark on the front cover or a simple wire pointer to attach the front cover. I'd back up and put the old distributor in first. You may have actually introduced more problems on top of the existing ones. Firing order is 153624. DON'T throw parts at it. Make only ONE change at a time and note any improvements or deterioration. Be sure all vacuum hoses are good and no other vacuum leaks exist. I'm assuming the backfiring is coming when trying to throttle up the engine? You also never stated what the other problems were. You could be starving for fuel too but you need to know the ignition is close before moving on to that.

[sawdust2005]

I've been having a problem keeping the car running without backfireing. That's why I changed the distributor. The carbs were leaking and I could not adjust the jets. I installed rebuilt carbs but I need to get the timeing right before I try to adjust the carbs.

[beandip]

listen to what EZZZZZZZzzz said. you see , backfireing through the carbs can be from timing issues and lean mixture issues. Among other things . How many years have you been working on SUs and or Z engines ? None of us want to argue with you , we just offer knowledge and experience . Thats all . Gary

[Ramses]

Sawdust2000,

I had the exact same problem on my 73. The timing is the 1st thing you should check. This is what I learned form these forums and from talking to 3 mechanic friends of mine.

Remove your spark plugs. Remove your valve cover. You should be able to turn your motor over by hand. Turn it till the heels of the first two valves closest to the front of the car are at 10 oclock and 2 aclock respectively. That means that the 1st cylinder in under compression and neither valve is opened. Replace valve cover and spark plugs. Remove the distributor cap. The rotor is pointing to spark plug #1. The firing order is 1-5-3-2-6-4. Move counter clockwise on the distributor cap and connect the spark plug wires to each corresponding spark plug. Replace cap and start the car.

If you look at the distributor there should be a several markes next to the adjusting plate that has an A on one side and an R on the other with several slash marks inbetween. The marks are towards the front of the car. Line the mark on the distributor exactly dead center to the marks on the adjusting plate. Start the car.

Take a phillups screwdriver and loosen the adjusting screw, located towards the rear of the car on the distributor, and twist the distributor a bit towards the advance side. The car should idle a bit more and no longer backfire through the carbs. You will need to use a timing light on your new timing mark on the crank pully, but this should get you running pretty smoothly.

If the problem still exhists, then it is something to do with you fuel system.

Good luck and use these forums, they have been very helpful to me.

Chuck