Cylinder Head Swap blues....

[Beatnik]

So I just finished swapping cyl heads (J's for ported X's) along w/ intake, carb, starter, plugs & wires, coil, alternator and it fired right up! Yea! Thanks to the new mini-starter and carb. Idle's fine and nice. However it's popping/snapping intermitantly when you blip the gas or hold it between 1500-3000. Above 3000 it seems to smooth out (no popping). It sounds like popping/snapping inside the muffler (not a backfire out the rear). I'm thinking it's timing? However I never removed my distributor or rotor (just the cap) so technically never messed up timing. I already checked my wire terminal connections at the cap and plugs, and the routing. That looks good.

Any ideas what else would cause these "misses" or "popping"? I'm going to put a timing light on it tonight, but if it's not that I'm at a loss?

 

[rmchrgr]

Popping inside the muffler sounds like it could be unburned fuel. Maybe check that all your connections are in firmly for each cylinder and check your plug gap. Easy stuff first.

Also, maybe your engine is running less efficiently now with the new carb, intake and ported heads. Since there's more air/fuel going in, it's possible your original timing spec isn't lighting off the mixture at the right time.

If you're going to experiment, play with the timing first, that's easiest and dosen't cost anything. Adjust, drive, repeat.

 

[mikesduster]

ditto on the timing.

 

[Beatnik]

Thanks guys - Didnt get a light on it last night, but glad to hear you think it's timing too. Didnt think about unburned fuel that could have something to do with it too. It's a new 650 AVS Eddy carb and for being out of the box, the idle setting is spot on and it doesnt bog/choke/cough/stumble at all when you hit the throttle (just popping). I'll have time this weekend to play with the timing, then adjust the carb...

Question - What do like to gap your plugs at?

 

[rumblefish360]

I'd call it lean before timing since the distrib. didn't move. But it should be looked into anyway. There was alot of change going on.

 

[rmchrgr]

I'd call it lean before timing since the distrib. didn't move. But it should be looked into anyway. There was alot of change going on.

True, it could be a lean conditon. More air = lean conditon and since more air is coming in with the new components, you might need more fuel to compensate. Also, check for leaks like hoses, carb base, etc. to make sure everything is sealed up tight and not lettting extra air in.

To my knowledge though, exhaust 'popping' usually occurs on deceleration. This is because less air flows into the chamber when the throttle plates are suddenly closed. Less air = rich condition, unburned fuel into exhaust stream.

To me, a lean condition would sound like marbles in a can (pinging) otherwise known as detonation/pre-ignition. That would be heard from under the hood instead of the exhaust.

Take out your plugs and see what they look like. Fouled with gas? You're rich. Look burned or destroyed? You're lean and/or detonating.

I'd adjust the timing both ways to see if the problem goes away. Read; advance to see if it gets better then maybe retard to see if it comes back or vice-versa. Tuning just takes time and experimentation. That's the fun of it.

Standard/typcial plug gap is .035".

 

[Beatnik]

Thanks again guys! Yea, time and experimentation. Unfortunately time is in short supply these days. Wish I could just lock myself in the garage until I'm done but life gets in the way!

I'll pull my plugs and look at them. How long should you run new plugs to get a good idea of rich/lean condition? Mine probably only have 20-30min run time at most - most of it at idle.

It's definately a popping/small backfire sounds coming from the exhaust, not pinging from the engine (detonation). I'll keep playing with it...

Thanks again!

 

[C130 Chief]

If you have a slight exhaust leak upstream you may be pulling in enough oxygen to cause unburned fuel to light off in the mufflers. Typically this will give you snap-crackle-pop as you decelerate, but can occur any time.