Hi all,

I just swapped out my cam in my 70 road runner and I am now getting a backfiring through the carb. I know the usual reason are lean, vacuum leak, timing, or valve train issues.

I changed the cam without pulling the engine and when I put the distributor back in and went to fire it up I somehow ended really really advanced. It was kicking back against the starting but it ran just wouldn't idle.

So I retarded the timing and it starts up and runs fine. But if I rev up the engine it backfires through the carb. I can bring the rpms up slowly and it seems fine, but if I stab at the gas it backfires.


I have not had a chance the mess with it too much, but was looking for any tips on what to check for besides what I listed above. I will throw a vacuum gauge on it tonight also, but what should I look for on that and what will it tell me exactly.

Check, check and double check your firing order and wire location. It is opposite of a small block as far as rotation. After all the years of doing this stuff you tend to get lax. I know because I did just that.

Firing order first. Then pull the wires one at a time to see if you can tell what cylinder it is.

New or used cam and lifters?

It is a used cam, from a previous motor I had. Ran fine when I pulled it. I only ran it about 500 miles or so. New lifters though.

Didn't think of pulling each plug one at a time. I am really hoping that I didn't wipe a cam lobe.


Could it have hurt anything when it was really advanced and kicking back against the starter?

If it's got a Holley carb you might have blown a PV if it backfired and doesn't have protection. But i believe you would have a rich condition if it did.

it does have a holley carb, but I thought they had some type of protection in them now that prevents the pv from being blown....?

Sounds like the chain is off a tooth or two....

MORE

Thanks for replies. I got some garage time ahead of me.

Yeah, I will check everything I possibly can before I pull the front end the motor apart again...

If you can't find a couple of wires in the wrong spot, then I think BigBird nailed it. I did this once when I was in to much of a hurry and the engine acted exactly like you discribe. Ended up that I had lined up the timing mark on the cam sprocket with one of the dimples on the crank sproket, not the timing mark. I was very lucky not to bend any valves and after I fixed the problem the engine worked perfectly.

Jack

Ok, so I finally got back out the garage checked the firing order and everything was as it should be.

I ended up taking the front end apart and made sure the timing marks were lined, they were. But I pulled the timing chain off and re installed it just to make sure.

I got it all back together and fired it up and it runs a lot better now...but there is still a couple issues.

The first is the timing mark is jumping and it seems rythmic. The timing will be there then every second or two is dissappears and the comes back....

I took it for a spin around the block and if I accelerate normal everything seems fine, but if I put my foot into it backfires.

Is this my timing chain is toast? or a vacuum leak? I did throw my vacuum gauge on it and at idle in gear the vacuum gauge showed 15 and it pretty steady..... I threw in the towel late last night and am going to step away from it for a day or two, but any ideas would be great! thanks - Brian

As you may notice, I am not much on posting, and I have not been here in a while, but I am sure some of the seasoned gear heads can answer the question as to the resutls of a cam without proper valve spring pressure?

Rob (First post!) :-)

Where are you setting the base timing at? (degrees)

trying to set it at 15 intitial, but it was hard with it bouncing. It was at the end of a long day, so i just as i started getting frustrated, i decided it was a good time stop for the night.

IF i had a vacuum leak, wouldn't my vacuum gauge been all over the place? it was steady at 15....

Steady 15 sounds good especially if a non stock cam.