bbc new stock cam, misfire, backfire; timing?
 

i just installed a new stock cam in my 402 bbc. i also put in a new timing chain and gears. the truck starts and throttle response is pretty good, but it sounds like its missing on one cylinder and occasionally backfires. it has backfired at the carb and at the muffler. when i installed the sprockets for the timing chain, i had the engine at TDC on compression stroke. the dots on the gears were both at 12 o'clock. i pulled the gears off and installed the new ones the same way. i used a straight edge to make sure they were aligned. i have not let it idle but i have started and stopped it a few times. i have checked and rechecked my plug wires. i have a couple of questions:
first, is it possible that i am a tooth off on the timing chain? would that cause a misfire?
second, should i break the cam in with the engine misfiring, or should i address that issue first?

Did you verify and turn the crank one full revolution as it will put the cam dot at 6:00 and the crank dot back at 12:00 to know fore sure it is in alignment (TIMING GEARS)? If so then you need to just rotate the crank one more full turn placing the cam dot and crank dot both back at 12:00 and you can set in the distributor pointed to the #1 post on the cap.
Should be right there and be in time.

Cole

If you put the cam in correctly then I think your distributor is off a tooth, making your timing way off. Have you checked the timing? Put #1 at TDC. Make sure it's on the compression stroke. See where the rotor is pointing. I wouldn't fire it up to check the timing until you do that.

I'd also recheck the plug wires once more. Maybe one is swapped. The backfire makes me think it has something to do with ignition.

no i did not. i wish i had tho. if i was off by a tooth would that cause my problem?

If you have already started but did not follow the breakin procedure, chances are you now have a bad cam. To properly breakin a cam, you can not shut it off after starting it. You might check all of the valve adjustments and start it again, this time following the breakin procedure. You may get lucky.

You can use a timing light on it at 2000 or 2500 rpm . It doesn't have to be at idle to check the timing.

The instructions that I have read on cam breakin procedure do allow you to start and stop to make adjustments . They just don't allow for any idle time.

yes, i did put it at TDC on compression stroke and i set the distributor body to line up with the rotor. but when i tried to start the truck i had to turn it a bit counter clock (advance?)to get it to run at all. i just rechecked the wires again and they are correct.

it is a timing chain. i didn't rotate the crank to put the cam dot at 6 but i used a straightedge and lined them up. i wish i would have though. when i set the rotor to point at the #1 post, the truck runs really bad. i have to move the distributor counterclock a bit to smooth it out, but it still misses.

i have not let it idle, but when i first started it, it stumbled and wouldn't rev up to 2000 rpm. so i killed it right away. the other times i have started it, i brought it up over 2500 immediately and have shut it off at that rpm as well. i haven't run it over four minutes yet.

I would say your are simply out of time .IMHO Go back and pull the #1 plug and bring it back up and see where it's at from there. see if it's at #1 on the distributor or not. JMO Take a breather and don't get overwhelmed. :sweat:


Cole

As long as you were varying your engine speed from 2000-2500 when you first fired it up, and kept it there until you shut down and didn't do a bunch of idling, your cam will be fine. When you fire it up next time, just run it to 2000-2500 again and you'll be fine. Make sure you have cam break-in supplement added to your oil though.

My buddies and I have been building engines, racing, and swapping cams in cars and motorcycles since the mid 70's. We've had to stop break-in several times for things like an angry psycho neighbor (open headers in town not popular), police (open headers again), overheating (not my car), and an oil leak (not my build).

As long as you shut it down without idling much and you restart with normal cam break-in procedures, you'll be fine. I wouldn't be real comfortable doing it with a solid flat-tappet with .630 lift and springs to match.

I have no idea where all this cam break-in paranoia comes from, especially with a stock profiled cam and matching valve springs. A lobe will not flat because it turned over 300 times between the time you let it idle down and then shut off the engine. Flat tappet cams were used for maybe 100 years before the big three ever even invented roller tappets.

Stop worrying about your timing chain-for now. If you used a straight edge to check it you're probably just psyching yourself out. Check the distributor and if that's good come back to the forum. It'll take 10 minutes.

It's hard to give you advice unless you tell us how you started it, how long you ran it, what rpms, and did it run fine at first and then start running bad, or was it the same the whole time.

I'll PM you my phone number.

i did as you suggested. (took a breather.) lol. then i pulled the number 1 plug. i plugged the hole with my finger and turned the crank until no more air could be heard hissing out between my finger and the block. the timing mark was actually at 10 degrees btdc. (should air be coming out all the way to 0 degrees tdc?) then i pulled the dist. cap and the rotor is pretty close to the number 1.

ok. thanks to Rineholio i got things semi-straightened out. i put the crank to 10 degrees tdc on compression, then i set the rotor to point straight to the number 1 post on the cap. its not backfiring anymore. got kinda hot tho. 225. gonna check coolant after she cools off.

Most likely after setting the timing at 12 deg static with a timing light the overheating problem will be gone. Timing just a little retarded will make an engine run hot .