rebuilt 350 backfire

[smcurro]

Hello,

I have recently started and broken in my rebuilt 350 for the first time. I now have started to drive it and notice that sometimes on medium acceleration I hear a pop on the drivers side and lose all power for a quick second. I let off the gas and again all is well. If I don't let off the gas right away the popping continues and I have zero power. This occurs regardless of whether the engine is up to temp or not but seems to happen more when cold.

Engine is a stock rebuild with 9:1 compression. Cam is comp 42-227-4 "252H Good Low end torque and reliability", which I think is set 4 degrees advanced - it was degreed. Ignition is Pertronx HEI ignition with 8 degrees of initial timing and vac advance not hooked up for the time being. Not sure what max timing is. Plugs are AC delco (whatever # is for '70 350) 0.045 gap. Carb is a new 600cfm #1850 holley. Idle jets are 1 3/4 turns out. Idles at about 700rpm. Edelbrock performer intake. #6 heads. Everything is new (filters, tubing, etc).

The engine runs great in every other way except for the backfire now and then. Vac at idle is between 16 and 17in. Runs smooth and seems to have good power (until it backfires).

I have O2 sensors on both sides (dual exhaust) and two gauges. In medium acceleration the pass side is basically right in the middle (0.45V - 14.7:1) while the drivers side is pinned lean. I can hear a small exhaust leak upstream of the sensor on the drivers side at idle, especially when the engine is cold.

Could the backfire be due to air entering the exhaust pipe through the leak? I'm in the process of fixing the leak. I didn't think this could be the problem because I've had lots of similar leaks on a few different cars and I never had this happen before, but I really have no idea. When the backfire occurs, both O2 sensor gauges go completely lean until I let off the gas.

Any ideas? This is really annoying especially since it runs so well otherwise and really I don't want to break anything due to this occurring! Thanks for the help!

[MDchanic]

What happens if you really nail it, rather than just press the pedal for "medium acceleration?"

- Eric

[smcurro]

Haven't tried, I think it would be worse though because sometimes I give it just low medium power and it pops a little, and on higher medium power, like when going up a hill, it'll pop a bit worse. I've been trying to be really nice to it since it literally has 22 miles right now. and again sometimes it doesn't happen at all. It happens maybe 25% of the time upon acceleration

[coppercutlass]

this may seem dumb but how do the spark plugs look on the side that is lean., And how the they look on the opposite side.

[smcurro]

Oh that's a good idea to check - I'll take a look today. thanks

[MDchanic]

Thus far you've described a whole lot of leanness.

Fuel pump, jetting, float level all come to mind as initial things to check.

You have the advantage of having not one, but two A/F meters to guide you.

Check those things and let us know.

- Eric

[smcurro]

Plugs look the same on both sides. They still look new and are just starting to get a light tan color to them. I have a chart of pictures of different plug problems and they are similar to the picture of a good plug, just newer looking. Fuel pump is new. Carb is brand new but I have no idea what the float height or jets are set to. I'll have to look into it. The engine idles on the rich side but not pinned rich. Its also on the rich side during mild acceleration/cruising/coasting

[nofiveo]

Fix the exhaust leak if it's upstream of the wideband first, the leak will give you a lean reading like you're experiencing. Your reading of 14.7:1 is the scientifically perfect ratio.. real world conditions will put it on the lean side. Look to your carb settings first, if those check out get some carb spray and start looking for a vacuum leak.

[smcurro]

I figured out the problem. The 1850 shipped with a 6.5 in-Hg power valve. As I accelerated after the throttle initially opened, The value closed way too early as vacuum increased and caused the engine to go very lean. I replaced it with a 10.5 in-Hg valve and it runs 100% better with zero stumble. I might change to an 8.5 or 9.5 though because I can sorta feel a surge when the 10.5 valve opens - not sure, I'll have to experiment. Plus with the valve opening more often now I can almost see the dollar bills going out my tailpipe.

I guess Holley didn't expect my engine to pull 17" of vac at idle...

Thanks for the help!