258 Engine Vibrations

[cdcutsha]

86 CJ Fresh AMC 258 i6 / 4.2l 6k miles on it. HEI/nuttered

Noticed that upon neutral/park high rev. ( 3500 to 4500 ) rpm. there is a noticeable vibration coming up through the gas pedal. I've looked for a long while trying to find an answer with no luck.

Any ides?

Reaching the limits of the cam/crank and things getting loose?
Spark Jump?
Missfire?
Torque Converter/Flexplate?
Harmonic Balancer?

I know on mine 4500 is the redline on the tach, and the engine wouldn't ever see these kinds of numbers in the daily driver or trail.. But i'm curious to what's causing them..

Thanks!

 

[Lumpy Grits]

WELCOME

Your '86 has a manual-No flex plate or torque converter.

Your other 3 choices are what you want to check along with the correct ign timing to start with.
LG

 

[cdcutsha]

WELCOME

Your '86 has a manual-No flex plate or torque converter.

Your other 3 choices are what you want to check along with the correct ign timing to start with.
LG

I threw in the Torque converter, because my 88 auto does the same thing.. that's why I was curious. Ign on the cj is 9* and yj 10* I was curious to see if there could be something wrong, or perfectly natural with the type of engine and valves beginning to float or something.

 

[Lumpy Grits]

Common for an engine to do this if it isn't 'loaded' some.
How does it run on the street with a 4300 rpm pull in 2nd or 3rd gear?
LG

 

[cdcutsha]

Street runs fine, though i've never had the b@lls to wind it up like that in gear going down the road. but normal driving it does fine.

 

[Hedgehog]

Why not? You don't have a problem spinning the thing up to high RPM with no load to properly soak up the power you are generating. No big deal, just don't shift it as quickly as normal. The engine has proven that it can take the RPMs without flying apart.

 

[Lumpy Grits]

What HH sez-
Your do'n more harm to that engine spin'n up like that without a load than you know.
Don't 'fix' what ain't close to 'broke'.
LG

 

[Torkrench]

What HH sez-
Your do'n more harm to that engine spin'n up like that without a load than you know.
Don't 'fix' what ain't close to 'broke'.
LG

Aint doing any harm to a AMC 258 i6 / 4.2l winding up to redline...jeez you ack like the cranks were whittled out of scraps & bolted together. Theres 7 mins for chrissake! They arent any more out of balance then any other street engine. Oil pressure? Check...no nocks or rattles? check...wind it up & let er eat, it can take it

 

[Heavychevy]

If you feel no vibrations out on the road. Don't worry. The first warning will be your oil pressure dropping from what you consider normal.

 

[mtnwhlr]

Mine developed a vibration and it turned out to be the water pump.