Rough and shaking 350 after changing spark plugs

[Dave15]

Hello,

this is my first post here.

i own a 1986 c20 350 in pretty neat condition, i did a lot of rework on the truck this winter (spark plugs, engine-, axle-, and trans fluid change, all filters, new power steering pump) and took the truck out for a testride last week, first 30 miles went perfect.

this weekend i was driving again after about 20 miles it startet to run rough at P and even worse in D, when accelerating you can feel that something is wrong like not all 8 are firering correctly. Also when driving at constant speed you feel a constant rythmic failure of the engine.

anyone knows a possible solution?

 

[77 K20]

Check to make sure you didn't have a plug wire pop off. I've had them back off on a spark plug or on the distributor end before.

 

[Trucksareforwork]

Make sure you didn't knock a vacuum fitting loose somewhere. If it's a stock '86, you also might have cracked one of the plastic lines which are pretty brittle at this age. Your symptoms sound like a vacuum leak or plug wire issue.

 

[ChevyBlood]

Check all of you spark plugs very close , I recently did a tune on my corvette that just like what you are saying and after it driving me crazy rechecking everything it turned out to be a sparkplug was broken the porcelain inside of the metal housing where the electrode is was broken couldn't tell by looking at it just when it finally discinagrated enough to come apart, it was a ACplug

 

[Dave15]

thank you for the input so far guys, will check them on thursday when i have a day off... thankfully its not the daily driver car...

i just need to figure out where those vacuum things are.... got quite some knowledge on engines but its basically European cars and Mustangs

 

[77 K20]

Ugh. I didn't see you had a 1986. I had one and there are plastic vacuum lines everywhere like what Geoff said. My dad back then had a 1987 and it was way simpler!

I found a vacuum diagram: it is for a 305 but think it is the same? Might be of some help. Before poking around at each one can try starting it and spraying them down with carb cleaner and see if your engine idle changes (a change in the engine idling means the carb cleaner was sucked into the engine there from a vacuum leak)

You must be registered for see images attach

 

[Dave15]

Poped the hood and checked the lines starting from the canister and immediately saw this.... hopefully this is the reason for the rough engine

You must be registered for see images attach

You must be registered for see images attach

 

[Dave15]

by the way, anyone here who can tell my what that thing is for? saw that there is a cable attached on it which was loose, i think wasn't pinned in for a long time because its quite dirty in there

You must be registered for see images attach

 

[Dave15]

UPDATE: Reattached the vacuum line, fired it up and ran great after a few miles when the engine starts to get warm the problem comes back......
i read somewhere that the sparkplug gap for my truck should be 0.045... is it possible that the gap causing that problem because i looked at the plugs in the cataloge and found out that the installed sparkplugs are 0.035

 

[Snoots]

Old points style distributor = .035.
HEI distributor =.045.
Some people set theirs at .060. Don't ask me why. Whatever gives you a very light tan on the plugs is good.

 

[77 K20]

How is your distributer cap looking? Terminals on the inside look good? Rotor look good? A lot of carbon?
Or if you pull the coil pack up inspect the rubber washer and the carbon button. I've heard stories that the high resistance versions of these can get very got and can cause failures. Aftermarket companies make a low resistance version.

 

[Dave15]

will check the gaps tomorrow also the distributer, don't know when this has been changed last time since the truck got out of the USA 2019 and was brought to a barn in Germany for 3 years until i imported it to Austria. thiking of ordering a new cap and new wires from rockauto anyway.
how can i identity if its a HEI or not?

 

[77 K20]

If it is newer than a 1975 HEI became the standard.

Try to find a cap and rotor that uses brass contacts vs the cheaper aluminum ones.

You must be registered for see images attach

 

[rusted nuts]

UPDATE: Reattached the vacuum line, fired it up and ran great after a few miles when the engine starts to get warm the problem comes back......
i read somewhere that the sparkplug gap for my truck should be 0.045... is it possible that the gap causing that problem because i looked at the plugs in the cataloge and found out that the installed sparkplugs are 0.035

Not plugs

 

[newguy11]

You must be registered for see images attach

I don’t think this is your problem but it should be connected to your electric choke if you still have one