84 k10 timing issue? Misfire under load

[andyc1121]

Hey guys,

Had my truck running good I thought until a mechanic set my timing back and now it misfires under load, my timing when I got my truck back was 6btdc and would misfire bad on cylinder 6, 7, 8 around 1500-2500rpm. Now my initial is about 16 deg. Another issue I'm having is when trying to set the total timing it retards and I can't get a steady reading with a timing light. New plugs, coil, ignition module, year or so old msd 8mm wires n same with cap n rotor.  Here's a video of it, you can hear the popping sound.



http://s1283.photobucket.com/user/andyc891/media/160_zps9f89cd45.mp4.html

[bake74]

    The video was pausing too much for me to see it, did you try to set your timing back to where it was before the mechanic reset it ?  Are you plugging off the vacuum advance when you are setting it ?

[bd]

Couldn't really hear the popping clearly enough from the vid - is it coming from exhaust or intake?

Four things to check:  1) make sure the dampener hasn't spun on the hub, so you are setting the timing accurately, 2) check rocker and valve motion for a cam going flat, 3) use a different timing light - you described mistriggering of the strobe, and 4) any chance one or more spark plugs were cracked during installation?

[Irish_Alley]

did this "mechanic" test drive the truck after and thought it was ok?

[roundhouse]

I never bother with a light

Just advance it a little. Take it for a full throttle test drive up a hill.  If I doesn't ping then advance it a little more

Do this till it pings then back off just a little

You can also set it with a vacuum gauge  adjust for max vacuum at idle will be close to the best timing

My 74 Bronco with a fuel injected 5.0 from a 90 mustang has 44* advance  according to the light

But it Runs great

[roundhouse]

I never bother with a light

Just advance it a little. Take it for a full throttle test drive up a hill.  If I doesn't ping then advance it a little more

Do this till it pings then back off just a little

You can also set it with a vacuum gauge  adjust for max vacuum at idle will be close to the best timing

My 74 Bronco with a fuel injected 5.0 from a 90 mustang has 44* advance  according to the light

But it Runs great

However misfiring under a load sounds like a bad cap or rotor or plug or wire

[andyc1121]

Hey guys,
Thanks for all the help, when I get back to town gonna try roundhouse's advice n see what happens. Irish, I doubt he took it for a test drive Bcuz when I left the shop it was misfiring a lot so I went back n picked him up for a test drive n he said it was a bad fuel pump. So I changed it and no better so I upped the timing and it ran better. The mechanic said my initial was at 21-22 deg n kinda pinging but it ran fine n accelerated good no misfire or ping.. Umm truck has a new harmonic but it's not the right size but I don't have timing tape that's the right size on it n marked tdc on the balancer.

[bake74]

Hey guys,
Thanks for all the help, when I get back to town gonna try roundhouse's advice n see what happens. Irish, I doubt he took it for a test drive Bcuz when I left the shop it was misfiring a lot so I went back n picked him up for a test drive n he said it was a bad fuel pump. So I changed it and no better so I upped the timing and it ran better. The mechanic said my initial was at 21-22 deg n kinda pinging but it ran fine n accelerated good no misfire or ping.. Umm truck has a new harmonic but it's not the right size but I don't have timing tape that's the right size on it n marked tdc on the balancer.

      Bad fuel pump ? = bad mechanic 

[Irish_Alley]

Hey guys,
Thanks for all the help, when I get back to town gonna try roundhouse's advice n see what happens. Irish, I doubt he took it for a test drive Bcuz when I left the shop it was misfiring a lot so I went back n picked him up for a test drive n he said it was a bad fuel pump. So I changed it and no better so I upped the timing and it ran better. The mechanic said my initial was at 21-22 deg n kinda pinging but it ran fine n accelerated good no misfire or ping.. Umm truck has a new harmonic but it's not the right size but I don't have timing tape that's the right size on it n marked tdc on the balancer.

      Bad fuel pump ? = bad mechanic 

X2
when was your last tune up?

[andyc1121]

Ya bad mechanic is putting it nicely lol I actually had the same misfire when I had a plug wire fall off a spark plug last winter going up a hill, put the wire back on n it was fine.. Even told the mechanic that n he swore it was the fuel pump. Last tune up was about a month ago new plugs, wires, coil, ignition control module( ran better after icm), fuel filters. The truck has a stock 81 Camaro 350 with newer gasket n seals, would a holley 670 street avenger carb with edelbrock performer rpm intake(high rise) n shorty headers cause the engine to need more or a different timing? Last time the timing was set a mechanic friend did it by ear. I should have the weekend off so I'll go thru and check the basics and what you guys suggested and report back, thanks for all the help guys I really appreciate it.

Spelling mistake in my last post, I do* have msd timing tape on the harmonic that's the right size for the harmonic.

[Irish_Alley]

a mild 350 engine wouldnt need more that 600cfm. have you inspected your plugs lately?

[andyc1121]

Hey guys,

Spent the day yesterday tearing into her and turns out my distributor had lots of play, you could see on the cap how the rotor was moving up n down. Plugs looked good but changed the wires n plugs and it's a night n day difference! I set the total timing to 36 deg (no more bouncy timing while trying to set it) and it doesn't miss at all. Ended up going 4x4in last night n we got 2 Chevys stuck until 2am thanks for the help guys!

** installed new distributor to fix the issue.

[bake74]

     Worn distributor bushing on the shaft.  Glad you got it figured out and are now able to enjoy her.