87 350 TBI Running Rough

[adamj]

Hello everyone, this is my first post here. I recently bought an 87 burb with TBI'd 350.
I did some needed maintenance yesterday: wires, plugs, dist. cap + rotor, air filter, oil change.
When I fired her up after doing that, I noticed it running much rougher than before. Lot's of visible and tactile vibration at idle. I was hesitant to run it, and just moved it out of the garage and parked it outside. When I put more than a little load on it, it backfired/missed. It also smelled like it was running rich. What could be the problem here? (none of those problems before the swap)

One thing I did notice when doing this work was that my #1 spark plug terminal on the dizzy was one off from what is most common with 350s. Firing order was the same, just shifted back one post. I did try swapping wires to the standard position, but that only resulted in serious backfire and I shut that down immediately. Not sure if this is related or not, but I'd thought I'd mention it.

It ran fine before this tune up work, and I was surprised to see it running poorly after all those new parts.

Maybe I knocked something loose? I did fiddle with routing the wires quite a bit to get them nice and neat. Still learning what the different sensors, plugs, vac lines are, but I checked the MAP sensor vac line and all seems good there. Watched the fuel injectors, them seem to be spraying evenly.

CR43TS Plugs gapped to 0.035
AC Delco Pro Wires
AC Delco cap + rotor

HELP! What did I do wrong here?

 

[smoothandlow84]

Check all of the vacuum lines for loose or cracked connections. Is your distributor loose? Also, it is rare but sometimes your new spark plugs can be bad right out of the box. Are your spark plugs all gapped properly? Just a few things to check. I too replaced all the parts that you mentioned my last tune up. I had a bad brand new ac delco plug that was causing itermittent firing issues.

 

[adamj]

Thanks for the reply.
Will do the carb cleaner vacuum check, maybe I cracked a line.
As for the plugs, I was very diligent in gaping to .035, but I still plan to remove them and check for cleanliness, and also that they're all working properly.

For the distributor, what do you mean by loose? Do you mean the cap?

 

[smoothandlow84]

Thanks for the reply.
Will do the carb cleaner vacuum check, maybe I cracked a line.
As for the plugs, I was very diligent in gaping to .035, but I still plan to remove them and check for cleanliness, and also that they're all working properly.

For the distributor, what do you mean by loose? Do you mean the cap?

Yes, check for a loose cap. It's possible that it may have rotated slightly enough to alter the timing. You may want to check the spark plug cables and boot connections too.

 

[adamj]

Interesting... looking into timing now. I did not think timing was adjustable on this fuel injection/electronic distributor engine.

 

[Georgeb]

I could see it missing from a bad plug but not backfiring and popping. Did you recheck that all the wides are run correctly as well as the check for a vacuume leak. How about checking the timing?

 

[CSFJ]

Ran into the exact same issue not too ling ago, turns out I transposed 2 of the wires. Time to recheck everything you did.

 

[adamj]

Problem solved!
Smoothandlow was right! It was a bad spark plug after all. I took all the plugs out again and found #5 was completely clean and the resistance tested very high. Replaced with a brand new one, and everything is back to normal.

One thing that I'm still not sure of, is why my distributor would be 1 terminal off from the usual layout. This is a rebuilt engine and maybe an aftermarket distributor was put in. But the firing order is still the same, and it runs fine now, slightly better/smother than before the tune up.

 

[Georgeb]

Could have been put in a tooth or two off.

 

[adamj]

Could have been put in a tooth or two off.

Would that affect timing/performance?

 

[Georgeb]

Would that affect timing/performance?

I dont think so.

 

[smoothandlow84]

Gla dto have helped. Sparks plugs are mass produced and unfortunately a bad one will make it through to the end consumer. It sucks to have it happen. Even since I ran across bad new plugs, I always buy 2 extra, when buying a set. Nothing worse than doing a rebuild project, turning the key and having issues. Btw...Don't feel bad if you crosssed two plug wires either. Hell...that's why mine are number labeled now....lol. !! Moroso and Accel make some nice little plastic clips that are numbered 1-8 that clip on each plug wire. I picked up a set when I made my own custom length msd plug wires recently.

 

[smoothandlow84]

I could see it missing from a bad plug but not backfiring and popping. Did you recheck that all the wides are run correctly as well as the check for a vacuume leak. How about checking the timing?

Back firing and or popping was an issue I had from two crossed plug wires. I felt like an idiot since it took me all day to finally figure out what I did wrong

 

[Georgeb]

Back firing and or popping was an issue I had from two crossed plug wires. I felt like an idiot since it took me all day to finally figure out what I did wrong

I can see popping from that. I have done it too.