Thread: new cam-wont fire

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Hey guys, just installed a new cam and now im trying to fire it for the firstc time.It turns over and catches but backfires through the carb and dies.My dist. is right ive checked it.What next? hate to keep turning it.

Originally Posted by blumule

Hey guys, just installed a new cam and now im trying to fire it for the firstc time.It turns over and catches but backfires through the carb and dies.My dist. is right ive checked it.What next? hate to keep turning it.

I know you say the distributor is right, but based on my first fire problems a month or so ago first double check your firing order at the distributor, and then be sure you're not one tooth off on aligning your distributor with the cam gear. It sure sounds like a basic advanced timing issue if it's popping back through the carb, or a couple of plug wires swapped.

Crossed wires, dist. out of time, cam out of time, vacuum leak, just to start the list!

Originally Posted by blumule

Hey guys, just installed a new cam and now im trying to fire it for the firstc time.It turns over and catches but backfires through the carb and dies.My dist. is right ive checked it.What next? hate to keep turning it.

Sounds like you may be 180 degrees off......Check it, recheck it, switch the wires on the distributor and try it again.


Bill S.

Backfire through the carb usually means that you've got spark on one or more cylinders on which the intake valve is not completely closed. That allows the burning mixture to go up the intake tract into the manifold. As well as the usual distributor/ wire/ firing order check, verify valve position on number 1 TDC compression stroke.

Bob

It does sound if im off on firing order.I made sure air came out of #1 piston . At the top of the stroke i installed dist. with rotor pointing at #1 terminal. Isnt this right? How do i verify valve position?

Bring number one cylinder up on the compression stroke, then check that both valves are closed. You didn't say which engine you have, but if it's ohv, there shouldn't be any spring load on the rocker arms at TDC compression stroke. Also, run a compression check on number one cylinder. If the cam timing is off, it will affect cylinder pressure.

Bob

Mule, Yes your procedure is correct. Pull the number one spark plug, turn the motor over until you have compression on the number one cylinder, use what ever terminal on the dist cap that the rotor is pointing to as number one, install plug wires from that point.

Now it sounds like you have done that, and you have a problem. So lets "ASSUME" that you are off a smidgen, move all of your wires back or Counter Clockwise one terminal on the dist cap and try and retart. If you are off like is mentioned just a tooth or so on the distributer this just "May" solve the problem. Now if its worse, then advance all of your terminals or go "Clock Wise" one terminal from the initial setting, in this case that would be two terminals as you had already moved them counter one. Got it??

May or may not solve the problem but it is at least doing something and will probably solve this.

Roland

Valves are adjusted too tight. Assuming this is a small block or big block Chevy, follow this procedure I wrote for the Crankshaft Coalition wiki.....
http://www.crankshaftcoalition.com/w...valves_SBC_BBC

this is a ford 390 with non adjustable rockers.Hydraulic cam. Plug wires checked out. Both valves are closed at #1 cylinder. piston pushed out air on stroke and rotor pointing at #1 terminal inside cap.Im starting to freak a little.When i installed cam i lined up cam and crank gear marks as described. New gears and chain. could it have skipped? maybe i need to advance timing more. I hate to keep spinning motor blindly.

When I did mine (SBF) I brought it to #1 compression turning it with a breaker bar, finger in the #1 plug hole (passenger side, front plug) to verify compression and stopped it with my timing mark at 10 degrees BTDC. Then I checked the rotor alignment and adjusted the distributor to point rotor centerline at my #1 mark on the distributor housing. You can still be off by some amount of rotor phase angle in your distributor, but not more than the width of your rotor electrode. That's how I determined I was a tooth off on my distributor, and it worked for me. Just to be sure, here's a link showing cylinder numbers and distributor rotation for the FE.
http://boxwrench.net/specs/ford_352-428.htm

why stop at 10 degrees? why not TC, and then line up rotor?Either way ive done both and it still pops at my carb. Im very frustrated.Could it for some ungodly reason be an coil or ive got a pertronix igniter in distributor.

10º is a good starting point for initial advance. Line up plug wire terminal and rotor at that setting and you're closer than you would be to the actual firing event than if set at TDC. Not likely to be a coil issue. As for the Pertronix, was it functioning properly before the cam change?

Bob

Pull the valve cover on the number one cylinder side.

Turn the engine over by hand, when the number one intake valve closes watch for the timimg mark to come up on the damper. Stop at 10 degrees advance, now put the number one plug wire where the rotor is pointng, either by rewiring the cap or (preferred) by repositioning the distributor.

It should start right up.

Bear with me im learning . I know theres an intake and exhaust valve for each cylinder. Is the intake the first or second one from front of motor. After intake valve closes doesnt yhe exhaust start to open?