I would then start it with the 'running adjustment' valve covers on and look for a hanging valve.

It could be something as little as a bad cap, wire or plug.

But I would check the valve train ASAP because 1) mechanical valve trains should be checked after (or even during) their initial firing, things settle in and clearances can change alot, and 2) A valve train is a terribal thing to waste.

Whatever it is, sounds like it's specific to one cylinder.

SteveW

I may be wrong but I think it's too lean. And it may have a vac leak on the right side, either a pcv or power brake hose/booster. The warmer the engine gets the leaner the mix can be and run ok. Right? I'm also pretty sure when I set the valves on the new 351w cold, they loosened up after the break-in(when warm). That could have just been the cam/lifter breakin that loosened the clearances.

SteveW

That's good info! I appreciate it, as I haven't set the valves cold since the initial build.

I would set them cold (just to make sure everything has clearance), then run the engine to temp and reset.

I've seen hydraulic valve trains where the intake valve wore to zero lash (propane fueled). They would cause a popping on startup and while cold because of the intake remaining slightly open. Compression was being pumped back into the intake and sometimes the flame would travle past the valve. Either way it was a dead cylinder till till the engine(s) got warm.

Then I have seen exhaust valves cause backfiring for both too much and not enough clearance.

In extreme 'no lash' I have seen exhaust valves held open. During combustion (what combustion there is) the pressure goes straight out the valve.

In less extreme cases the popping is the exhaust valve opening too soon in the power stroke. The cylinder pressure hasn't dropped 'enough' and the valve opens against high pressure.

Too much lash and the valve opens late in the power stroke when cylinder pressures are climbing. The slug nears it's bottom most position and slows but the combustion gasses are still expanding, pressure climbs and the valve opens. Pop!

Then theres the possability the valve event are so mistimed by impropre lash that the cyllinder just isnt pumping air.

How important is lash?

My 920 Virago (V-Twin scooter) was having real problems idling. The problem had been getting worse for miles and years. I figured it was the "junk" hitachi carbs and the fact that I rode her hard. I mean, Real Hard.

One day while adjusting the valves I had the test gauge (0.0005" indicator) home to check the rim run out.

I set the clearance with a feeler gauge then set the indicator up to measure the actual lash travel. Thousanths off. WTF??

Rocker faces and valve stems had worn and I was measuring the high points. Bummer.

I set "lash" to "measured travel" on all cylinders. Idle returned as did low speed running, torque and throttle response.

So, valve lash is a pretty important part of an engines operation. There is a temptation to think a few thousanths of an inch doesn't mean much when the valve travles hundreds of thousanths of an inch.

Those few thousanths mean one helluvalot.