85 Vette Cam

Reply

  #1  
Old 11-20-08, 10:45 AM
palmcoast's Avatar
Member
Thread Starter
Join Date: Dec 2006
Location: New Jersey
Posts: 105
85 Vette Cam

What is the easiest way to tell if a cam is shot in a car that won't stay running?
I have an older fuel inj. Vette that my friend had and he said the cam is shot and after I replaced that it should run fine. I was going to do this as a winter project but don't want to fix the wrong thing.
Thanks
 
Sponsored Links
  #2  
Old 11-20-08, 11:27 AM
Member
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,751
You can pull the lifters and check the base of them and the lobe that drives them. A worn out cam will show wear on both components. The cam lobe/lobes may be scored or flared and the lifter base severely cupped.

Also if a cam is responsible for poor performance it'll show in the lift of the lobe against the lifter. So, assuming you have the lifters up (either solid lifter or hydraulic with no noise like tapping) you can tell if one or more of the lobes isn't moving all the tappets.
 
  #3  
Old 11-20-08, 12:06 PM
Member
Join Date: Mar 2006
Location: USA
Posts: 269
The usual failure of a flat tappet camshaft is wear on the cam lobe. I would start by removing the valve covers and observing the movement of the rocker arms to see if there may be a wear issue that is bad enough to be observed in the reduced movement of the rocker arm(s). If you can't tell by just watching the movement, you can measure the movement. I would use a dial indicator on the pushrod end of the rocker arm and measure the lift of the cam lobe. It would be handy to know what the lift should be, however, just comparing the lift of the various lobes should give you a good idea if you have a wear issue. It shouldn't be necessary to disassemble the engine to see if a camshaft lobe or lifter is worn excessively.
 
  #4  
Old 11-20-08, 02:17 PM
Forum Topic Moderator
Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 4,292
Your friend says replace the cam. What does he base it on. There are many reasons for not continuing to run. I would start with fuel pressure, injectors, coil packs, and timing before starting a major tear down.
 
  #5  
Old 11-20-08, 03:03 PM
Speedwrench's Avatar
Member
Join Date: Apr 2008
Location: Oklahoma
Posts: 1,698
will it run long enough to listen to it? if so when running most of the time bad cams will give you loud lifter sounds, and backfireing through the intake, or exhaust. usually with a very bad missfire., you should also be able to check with a compression test. to fully check out the engine.

Murphy was an optimist.
 
  #6  
Old 11-20-08, 06:59 PM
Member
Join Date: Jun 2008
Location: Eastern Georgia
Posts: 486
It is a small block chevy from the 80s, if it has more than 100 miles there is a 75% chance the cam is worn out. Chevrolets factory camshafts sucked in the 80s (bad metal??). Check rocker arm motion if it is bad you will be able to see it, the rocker on the affected lobe will be nearly motionless. When you replace it use an aftermarket cam kit from Crane, Comp Cams, etc they seem to be more durable. On initial start-up with new cam and lifters do not allow the engine to run slower than 2500 rpms for the first 20 to 30 minutes. If you need to make adjustments shut the engine off, make the adjustment, then re-start it.
 
  #7  
Old 11-21-08, 10:49 AM
mikeTN's Avatar
Member
Join Date: Dec 2007
Location: Nashville
Posts: 635
Cool cam problems

You didn't say how many miles are on the car. I am guessing a lot. When you say "won't stay running", that could bring on all kinds of problems and solutions. I would advise that you make a vacuum check on the engine. It should have 15 to 18 inches of vacuum with everything off. That is presuming it is a stock cam and not a high lift. If the cam is worn(especially on the intake lobes) there will be a lack of vacuum. Changing a chevy cam is not that big a deal. Would be a good time to upgrade if you are planning on keeping the car. You also didn't say what engine you have. Some of the smaller engines had a run of bad camshafts where the intake lobes did wear prematurely. If all else checkes out and you are SURE that the cam is at fault, change the lifters and cam as a matched assy.. If there is a lot of miles, winter is a good time to do a rebuild. Just a thought. Pay me noway me later!! Good luck.
 
  #8  
Old 11-21-08, 11:20 AM
palmcoast's Avatar
Member
Thread Starter
Join Date: Dec 2006
Location: New Jersey
Posts: 105
The car is 1 1985 small block with 125,000 miles. It was bought on an online auction. The last owner told my friend that it was running fine until he replaced the ignition key assembly.
It will run rough at an idle but when I try to give it any gas it pops back through the air intake. I was going to start by pulling the injector rails and try cleaning the injectors first and then just work my way down from there
 
  #9  
Old 11-23-08, 02:21 PM
mikeTN's Avatar
Member
Join Date: Dec 2007
Location: Nashville
Posts: 635
Smile

Originally Posted by palmcoast View Post
The car is 1 1985 small block with 125,000 miles. It was bought on an online auction. The last owner told my friend that it was running fine until he replaced the ignition key assembly.
It will run rough at an idle but when I try to give it any gas it pops back through the air intake. I was going to start by pulling the injector rails and try cleaning the injectors first and then just work my way down from there

Backfire in the intake suggests intake valve stuck open, cracked, chipped, carboned up(holding it open). Check timing first. #1 cyl. top dead center (on compression stroke), rotor button pointing to #1 plug wire, timing marks on harmonic balancer and timing chain cover aligned. If all is well, check firing order of plugs(18436572)clockwise.

Changing the ignition switch should not have caused this problelm.
 
  #10  
Old 11-23-08, 06:56 PM
Join Date: Jan 2007
Location: North Central Indiana
Posts: 912
Bad cam?

Another way to get a rough idea of cam lift with limited tools is to hold the edge of a putty knife against the push rods while cranking it over. There will usually be enough varnish on the rods that the knife will leave a bright scrape mark on them. If that 85 engine wasn't out of a vette it was equipped with a carb. ?????? RW
 
  #11  
Old 11-23-08, 08:15 PM
Speedwrench's Avatar
Member
Join Date: Apr 2008
Location: Oklahoma
Posts: 1,698
backfire in intake is also indicative of exhaust valves not opening fully forcing exhaust back out the intake valve when it opens, gm had a lot of bad cams in the early 80's i heard it was from bad heat treating at the cam factory, but it doesn't mater what caused it they had many bad ones. but with the description he is giving i would go with bad timing chain and gears, probably jumped one tooth and is now out of time. however with that high of mileage i would think seriously about doing the cam and lifters if i did the timing chain and gears any way just to pep that tired old vet up.

are you going to cowboy up or just lay there and bleed?
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes