Engine Miss.. rounded lobes?

Status
Not open for further replies.

MrSony

Geezer
Nov 15, 2014
6,791
6,675
113
Des Moines, Iowa
I'm 99% sure my engine is misfiring on a cylinder or two. I doubt it's the plugs or the wires. I took all 8 plug wires 1 by one and took them off, held the end of the boot about a cm away from the spark plug, and there was a nice blue spark arcing from the boot to the plug. I think I may have a rounded cam lobe or two. The motor does have a rowdy cam @ .469/.469 lift , 268 duration on intake and exhaust, but you can see it shaking left to right when it misses. My engine's doing the shimmy and I don't really like it. I wouldn't be surprised if it was a rounded lobe or a few, I forgot the oil galley plugs when I first built the motor and broke the cam in with barely any oil getting to the top end. I replaced them almost immediately, but the damage was probably already done. It seemed to run as well as it ever has a few days ago, but recently it's become more noticeable especially at idle and part throttle cruising. I don't have a video to describe it, but imagine buh-buh-buh-bbuh-buh-buh-bbuh-bbuh, and that's what it sounds like. I've never had any shiny bits or shavings in the oil that I've seen or in the filters I've cut open. But I rev this thing past 5500 more often than Buick ever intended so it's not a shot in the dark. The RPMS also fluctuate 100rpm or so up and down and it never used to do that. What say you guys?
 

Texas82GP

Just-a-worm
Apr 3, 2015
7,981
18,676
113
Spring, Texas
Depending on how much overlap the cam has, some misfire at idle is normal. What's the LSA of the cam and what is the duration at 0.050" lift?

Worn cam lobes would not be an intermittent problem. You could measure valve lift at each rocker with a dial indicator to check the cam.
 

pontiacgp

blank
Mar 31, 2006
29,270
20,391
113
Kitchener, Ontario
If you have headers and an infrared thermometer it easy to find a cylinder that is misfiring....and I agree with Jared mechanical problems don't come and go
 

Clone TIE Pilot

Comic Book Super Hero
Aug 14, 2011
3,827
2,565
113
Galaxy far far away
Could be a vacuum leak or restricted fuel flow causing a lean misfire. Could even be bad gas. Does the exhaust smell?
 
  • Like
Reactions: 1 user

MrSony

Geezer
Nov 15, 2014
6,791
6,675
113
Des Moines, Iowa

MrSony

Geezer
Nov 15, 2014
6,791
6,675
113
Des Moines, Iowa
If you have headers and an infrared thermometer it easy to find a cylinder that is misfiring....and I agree with Jared mechanical problems don't come and go
NO headers. Just manifolds and downpipes.
 

MrSony

Geezer
Nov 15, 2014
6,791
6,675
113
Des Moines, Iowa
have you adjusted the valve lash since you did the cam break in?

350 Buicks don't have an adjustable valvetrain. Just shaft rockers held in place with nylon buttons. I may take the valve covers off to see if any of the buttons has broken (common on Buicks) but a rocker arm doing whatever it wants in there would make more of a stir than just a weird miss-sounding burble, or at least one would assume so.
 

axisg

Comic Book Super Hero
Jul 17, 2007
2,685
2,354
113
YYZ
It would be really easy to pull off the oil filter and de-can it to see if you have any metal in there
 
Status
Not open for further replies.

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot UMI Performance

Contact [email protected] for info on becoming a sponsor