Rebuilt Top End - Starts and Idles, then misses/rough - at wits end!

[SuperOldSchool]

Hello

Just finished clutch and rebuilt heads on my 87. Engine put back in and it started and ran well - I ran it for about 1 minute then had to shut it off to leave (went out with wife).

About 5 days later I go back into garage to restart - and it runs like crap - missing. Sounds like its on 3-4 cylinders and idles at like 500. The gas is older (1.5 years) - I did add some octane boost to the 1/4 tank of gas and was planning to fill up with fresh once I got wheels back on. I thought it might just be the old gas, but then why has it run 'great' twice but other times is crappy?

I shut it down and have done the following:

-Recheck ignition wires - and sprayed with water mist to check for arcing
-Clean/check ground strap on body and on manifold
- Tap fuel injectors with wrench (some posts said this could loosen them up?)

Restarted it tonight and it ran great again, for about 30 secs, then rough (same as before)

I am ruling out these:
CHT was new
fuel filter new
O2 sensor new
Ref sensor properly gapped
new fuel lines
new plugs, cap and rotor


Would the older gas cause this? I thought it might, but like I said it has run like a clock twice.....

I was so psyched when it started and was purring, now am really upset and frustrated....

Who's been here and what did you do to get back on the road?

Thanks
SoS

[LJ851]

Are you getting any loud sounds (like very loose valve clearance) when it runs rough, or is it quiet and rough ?

[SuperOldSchool]

There was a knocking sound or ticking sound this last time I ran it ( I think ). I say "I think " because I am running cat bypass with straight pipe right now, just till I get going. I will then rehang muffler. So, the car is really loud and it's tough to hear other sounds....
Not sure it that was lash sound or detonation? I could remove valve covers and recheck valves. Not sure how it would run so smoothly though during first couple runs if valves were loose? It was quiet when running smoothly.

[LJ851]

Old gas turns very sticky and can stick the valves open in the valve guides. Usually, after running an engine on old gas once or twice the valves will stick. Not sure if that is your problem, a compression test when it happens should prove the theory.

[SuperOldSchool]

I hope I didn't bend or break anything if what your saying is true, ugh.

So what is my best bet?

Drain fuel, add fresh, remove valve covers and inspect?
Should I be worried at this point?

Thanks

[RedCoupe]

You may have one or more new valve guides that are a little tight. Couple that with the old gas, and you have a valve hanging open a little bit. If you heard a ticking, it's probably just temporarily large rocker arm clearance rather than piston-valve contact. I once had a similar problem and couldn't get a valve to stop hanging open until I pulled the valve cover and shocked the problem valve by tapping on the valve spring with a piece of hardwood and a hammer, while I had my air compressor connected to the spark plug hole. I added some Marvel Mystery Oil to the fuel and the problem didn't reoccur. But, first step would be to pump the old fuel out of the tank and I think I would again change the fuel filter. A fuel additive that could help dissolve the varnish from the old gas wouldn't hurt either. Best of luck!

[SuperOldSchool]

Thanks! What about the fuel in the lines already? Just get out what I can from the tank and filter and try to run it through with the new gas? I think I am just scared I really messed things up . Thanks again

[E Sully]

1 1/2 year old gas, especialy if you did not add Stabil is not good. I would drain the tank and put in fresh gas.
I really doubt you have a sticking valve, but if you are worried about it, do a compression test. You can also see what the spark plugs look like.
I would think more in the line of a sticking injector.

[brighton911]

Seconds on the old gas. Without stabilizer of some kind, the volatiles go away leaving something that either doesn't burn at all or very poorly. Good time to check the screen on the tank too (via the drain plug).

[Lapkritis]

I hate draining old gas. My method is to top the tank up with fresh gas and techron as long as nothing is varnished. Dilution is the solution.

[cgarr]

You didn't happen to have a guy "the mad German" aka the elbow Stefan helping you??

[irobertson]

Stuck injectors.
First, start with some fresh fuel.
Mine did the same thing upon startup after one winter. Pull plug wires until you find one that makes no change, then rap on the injector with a wrench. Hopefully that will free it up. Worked for me.

[SuperOldSchool]

Hi Cgarr, yes Stefan was over.... .

Thanks to all for the responses! I'll get out most of the gas from the sender hole and fill up with fresh. Let's see where that goes.....it did have Stabil, but not sure how well that works after over a year.