new 454 in my 71, backfiring issues

[Marc33]

I put a new motor in my old truck, mild 454. with a cam, new edelbrock 750 carb, new performer intake, thorley tri y's etc. HEI ignition

I am taking it tomorrow for the exhaust, currently running open headers.

Under load, the motor backfires. At cruzing speed, like 35-40 it is smooth. Idle in neutral smooth, revving at idle smooth. Once I get moving smooth.

The timing is set to 4 degrees btdc and I think I am good on the vacuum side of things. Any advice would be welcome.

[BigCarl]

What color is the smoke coming out? Are you backfiring through the carb too? Have you tuned the carb or is it dropped on out of the box?

I had a hell of a time last week with my truck. I was backfiring through both holes.

It turns out for me it was a lean mixture.

Try putting on a stock air cleaner (reducing the amount of air going into the engine) Or you can throw some painters tape around your air cleaner if its open. Try running your truck then. If it runs better, your air/fuel mixture is too lean.

[Newtz67]

Try advancing a little more. Try 10 degrees advanced. I'm running my 454 at 13 degrees advanced.

[junkman]

I'd give it a bit more timing too, and double check valves. Is the motor new? If not, I've had a couple that acted like that, wore down cam lobe, bent pushrod, things to look for too.

[68 c10 ls2]

This may sound dumb but check your firing order. If you have one in the wrong spot it could to that to. It happened to me when i had the old motor in my truck. It ran awsome at idle and crusing but under load i had a backfire.

[junkyardjohn]

That timming or firing order issue has got me a bunch of times, & I'm no newbee to the world of old chevy small & big blocks. I ussually put the tranny either in reverse or drive (doesn't really matter witch) stand on the brakes, & excelerate. (trying to see how it acts under a load) If it bogs & backfires thru the carb, it's usually a fireing issue. Either a plug wire or maybe I stabbed the dist. one tooth off. First i check the wires (it's easy & I'm basically a lazy sh!t) & then I'll pull # 1 up to tdc & restab the dist. I've put on about 50 new edelbrock carbs & 99 times out of 100 they work just fine right out of the box. Don't start second guessing yourself & trying to adjust things you think are probably o-k. This old man I worked with as a kid always used to say... when you hear hoofbeats.... don't look around expecting to see unicorns. John

[Newtz67]

Originally Posted by junkyardjohn

That timming or firing order issue has got me a bunch of times, & I'm no newbee to the world of old chevy small & big blocks. I ussually put the tranny either in reverse or drive (doesn't really matter witch) stand on the brakes, & excelerate. (trying to see how it acts under a load) If it bogs & backfires thru the carb, it's usually a fireing issue. Either a plug wire or maybe I stabbed the dist. one tooth off. First i check the wires (it's easy & I'm basically a lazy sh!t) & then I'll pull # 1 up to tdc & restab the dist. I've put on about 50 new edelbrock carbs & 99 times out of 100 they work just fine right out of the box. Don't start second guessing yourself & trying to adjust things you think are probably o-k. This old man I worked with as a kid always used to say... when you hear hoofbeats.... don't look around expecting to see unicorns. John

I just want to agree with the carb advice. I use these carbs (if it's the performer series) and love the fact that they're so simple, and good to go outa the box. I also admire your honesty about being a lazy sh!t! I'm also lazy, just afraid to say it out loud. I wanna be just like you when I grow up!

[junkyardjohn]

Originally Posted by newtz67

i just want to agree with the carb advice. I use these carbs (if it's the performer series) and love the fact that they're so simple, and good to go outa the box. I also admire your honesty about being a lazy sh!t! I'm also lazy, just afraid to say it out loud. I wanna be just like you when i grow up!

thanks.. Thats probably the ultiment compliment.... Lazy folks seem to think & come up with the simplelist & fasted solutions to a problem, instead of just jumping in & doing stuff that winds up compounding a problem you already had. Sometimes thats a good thing. john

[Marc33]

well, I was looking for a less obvious reason for this issue, but John reminded me to double check the obvious. I had the dist in and out so much over the last few days that wires 5 and 7 got switched accidentally.

All better I think, I will test drive her tomorrow morning. Getting a little late to test drive the big block with open headers around here.

Thanks John,68 c10 ls2, and everyone else too!

[Newtz67]

Right on!