350 tuning problems. Backfires and pops, runs rough.

When i drive down the road and the motor is at about 2000rpm and up, it backfires out the exhaust quite frequently.. about once every second or so.. and sometimes backfires out of the carb.. and it stumbles on acceleration and generally just runs like ****.When it's in park and i blip the throttle it falls flat on its face. What causes this?? It idles OKAY..ish. not terribly. And it will pull right around 14 to 15 on vacuum at idle. This all started after I changed my distributor springs to a lighter weight, changed 3 burnt plug wires, and put on collector gaskets.. and took off my timing cover to check cam timing. That's it. Nothing insane, didn't even remove the distributor or mess with anything major. I'm pretty baffled here and I really feel like I'm running in circles here. So now the carburetor is back to all the original settings I had, and I put the other springs back in the distributor. Still runs like ****. I think I'll check the plug wires tomorrow. That's about all I can think of right now.

350 9.6:1
Edelbrocker performer RPM air gap intake
2in carb spacer
Edelbrock performer RPM heads
1.6 Comp roller rockers
Comp cams 282s magnum cam (.495/.495 w/ 236° @ .050")
Comp double roller timing set (installed dot to dot)

HEI distributor with MSD weight and spring kit (heaviest springs installed.. which it seemed to like the best)
Initial timing 20° and total timing 36° by 4000rpm

Holley 670 street avenger:
Originally 65/68 jets, 31 squirter, orange pump cam position 1.
Now it's Rejetted to 68/71, 37 squirter, blue pump cam on position 2.

 

I had two plug wires crossed and it ran exactly as you describe. Idling in neutral it sounded ok but under any load it popped out of the exhaust and backfired through the carb.

I would check that first..

 

I'd be looking at the HEI springs and recheck timing. I'd have changed all the plug wires. What did your plugs look like?

 

rockers making noise...?

 

What do you mean crossed? As in literally two wires just crossing over each other?

 

Sounds like WRONG FIREING ORDER!

 

The rockers are tapping a bit, but it's a solid flat tappet cam with a .022" hot lash. The plugs were cocoa brown on the insulators, however they were wet and dark around the ring.

 

I really hope it isn't wrong firing order hahaha. That would make me feel dumb.

 

As an example, 3 & 5 mixed or crossed. Right wire, wrong plug.

 

It happens a lot.

 

Can happen to anyone at some point!!!

 

Hey boys. 8 and 6 were crossed. HahahahahHa

 

That would explain A LOT of things. Wow. Thanks for the input, however I cant start it up and verify until the morning, but for now I'm 95% positive this is my problem, other than just needing a good timing/carb tune.

 

Glad you got it worked out. I've got those little number tags on my wires because I got mine crossed one day. Never again.

 

Thanks again. I'll let y'all know if that plug wire swap fixes the problem tomorrow when I get around to starting it.

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You musta been using that infamous MSD firing order chart...

 

I've done the same thing many times. I will do it again in the future. I feel dumb when I do it, too. Somebody should start a support group.....

 

Sign me up for the support group. 64 years and I'm still doing stupid things. LOL

 

Ever install a new clutch in something and put everything back together and get ready for the test drive - only to realize you left the throwout bearing out? I HAVE.

 

Haha I need to join this support group. This is probably the 4th time I've done this.. I thought I was past this jack***ery lol

 

Alright!! That did the trick! Runs great now. Just needs a different set of distributor springs (probably go medium and light instead of light light) and idle needs to be kicked up about 100rpm.

 

Sat on the trans bell housing and installed the plugs to the distributor. Had the shop manual in front of me. More than one came out wrong. Number tags are your friend.

 

I changed the plug wires,carb and double checked everything else before I realized 7 and 8 were reversed.. talk about dumb***.

 

Trust me. I feel the same way. I tried so many things from ignition timing and curve, to jetting and squirters. Even plug gap. Just goes to show that anybody can over look something so simple lol.

 

If you changed out three plug wires, change out the other five - then you'll know you're starting back at square one with plug wires.
I generally dislike advance kits with their myriad wimpy springs and light weights. To just use the "by-gosh-and-by-golly" technique of designing a proper advance curve for your engine (with its mods) in the ch***is you are running is a shot in the dark at best. A good Ch***is Dyno Technician is your best bet here to obtain the best curve for a wide range of speed (RPM) and loads.

 

How's the engine running now? You said you went to the light/ med spring rate vs light/light, ill get the timing light back to you so you can check total timing and see where your at. What was the final carb tune your running? Blue cam set to 1 pos, and 68/71 jets right? Squirter size? Myrtle is going to be sweet this weekend, Scott's car is almost wired, Carl said he just had to run a few more circuits and he's ready to roll.


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Runs beautifully... just a tiny tiny stumble off idle (which that's just tinkering with pump cams), and I'm running medium/light which it seems to like. The carb is at 68/71 jets, and I went down to a .031" squirter and orange cam on 2 after I put it in the garage to tune. Still haven't tested it like that. Then all I need to do is.verify total timing and I'm good bro. And yeah dude it'll be bad ***! I went to Scott's last night to see the car. It's close. Carl should be done by tonight, if not wednesday.

 

I was gonna go over to Scotts but I was tired as hell after duty and the beer wasnt helping either... hahahah

 

Haha yeah i don't doubt it dude. I really wanted to go to beedubs with y'all and have a beer, but I had to go to work.