As some of you may remember from my last post, I was having trouble adjusting the valves on my 72 small block. It's a stock motor with a cam installed. A couple of rockers came loose on the drivers side which I had trouble adjusting to re-tighten them. Now that I have them adjusted correctly (I think), I now have a miss. The shop that installed the cam thinks it's a plug wire or cap. I've spoken to another mechanic and he thinks it may be a flattened cam lobe. How can you tell if you have a bad cam lobe? The rocker that I was having trouble adjusting does not appear the have the same amount of travel as the other rockers, but I was told by the shop that installed the cam that if a cam lobe was flat, it would be back firing through the carb. Can anyone shed some light on this.

Thanks all!

 

If you have excessive cam wear there will be metal in the crank case oil. Drain the oil through a fine filter like a coffee filter or something similar. Watch for metal shavings. Also pull your oil filter and cut it open looking for more material.

Other than that, pulling out the lifter may provide enough of a view of the cam to see if it's wiped out.

 

You can also try holding/placing a strong magnet in the oil stream as you drain it to see if anything sticks to it.

Good Luck!

 

I will assume you adjusted the valves using the engine running method, if not please say how you did.....

well i would start with easy first, see if the plugs are firing using either spark tester, or an inductive timing light in a pinch. if the plugs are firing, find out if its the intake or exhaust, then check that plug for fouling, or damage and check the wires eliminate the ignition first, then if you still suspect somthing else, it could possibly be a problem in the valve train ( but if it was fine before chances are not)

how does the engine turn over? does it struggle a little?


the easiest test is the vaccuum test. hook up a vaccum guage to the intake port and see what it does. if you get a jumping needle but in a constant range at idle it is related to a single Problem (cylender), it the vacuum drops and stays jumpy when you increase rpms, it is most likely that a valve may be suspect.

once you determine the bad cylender in question, the best way to verify lift is to use a dial guage indicator and measure the lift at the push rod from the suspect cylender to a good one. Short of that, a quick way is to measure the height of a good spring both unloaded and compressed, then compare this hight between a good cylender and the suspect one.

if the measurement shows short of a good cylender, then check the rocker arm, and stud in question ( could have walked out), the push rod (damaged or bent), then the spring. If the rocker, stud, spring and push rod are good to go, then you have to pull the intake and check the the lifter, and all that is left would be the cam. work the problem, and eliminate from easy to hard across the valve train

if you have a bad intake lobe than you would not be getting sufficient fuel/air for combustion in the intake stroke, and have a resultant miss, but the plug in question would most likely be fuel fouled. if you had a bad exhaust lobe, the yeah you could see some back firing up through the carb
how old are the lifters and cam?

 

Asianlimodriver, methinks like durangoboy, except don't waste your time draining the oil. If you've wiped your cam (and I suspect you have), the filter will be FULL of chips. Pull your $4.00 filter off, cut it open, and look at it. If it's full of metal, my bet is you have CrispyCam....

 

You're right. I adjusted the valves with the car running. Changed all of the plugs and none of them looked fouled. I bought a new set of wires and will be installing them hopefully tonight. I did notice a short down by the starter and I'm hoping this short is affecting the Pertronix ignition that was installed. Any thoughts on that?

 

My experience with the pertronix unit it is if its bad you get nothing that is either the module works or it doesnt. I would still run the check on the vaccum, its easy, and will tell you a grreat deal on how your engine is running

 

You're already onto it. If you can obviously see a difference in rocker/pushrod travel, that's a bad sign. Measuring the travel is a positive way to tell. You can do it by turning the engine over by hand. A dial indicator makes for an accurate job, but a friend turning the engine while you measure with a decent scale will work too. backfiring through the carb isn't necessarily a sign at all, especially if it's an intake valve.

If you have that rocker properly adjusted and there are significantly more threads exposed on that stud than the neighboring studs, that's a sign that the lobe base circle is worn.

Have you checked to make sure the valve springs are ok?

I'm personally not a fan of adjusting the valves with the engine running. I've done it both ways and I prefer to do it by feel. Tighten the rocker just until the lash is gone from the pushrod. Not until you can't turn it or when it gets hard to turn. Back the nut on & off just a few degrees until you're certain you've found the point where ther lash finally disappears. Then another 3/4 turn and you're in business.

 

if the springs, rod and rocker check out, and the stud hasnt walked out, you're gonna have to pull the intake and inspect that lifter chief, if you're lucky its the lifter if not you talking about yanking the cam

 

IMO it's the LEAST accurate method....ever notice none of the cam manufactures recommend it?

 

What cam did you install?

Should the shop that installed the cam not figure out what is wrong with it? How long ago was the cam installed?

 

Thanks for all of the input all. I went out last night to try and change the plug wires. Thought I'd start with the passenger side since it's the hardest side to change. I have the ignition shielding on the car. What a pain. Do I need to drop the starter to remove the ignition shielding on the bottom of the engine? This miss is really driving me crazy. The cam and lifters are a year old with I'd say around 5k miles at most. I thought I'd start by trying to trouble shoot the problem myself. I don't have big funds to be taking my car in for every little repair. I've got a wedding to pay for next year. Any suggestions are greatly appreciated.

 

no the starter stays in, but if you are gonna route through the stock shielding, do yourself a favor and put the car on ramps, get underneath to route the wires in through the shielding against the block

did you verify the missing cylender plug was firing or not?

you said that the bad cyl was not making full travel yeah? did you do it by eye, or did you measure it ( dont mean to insult you), if its not getting full lift, for sure, just changing the wires out may not help the situation, though it cant make it worse

Im happy to help ya out, shoot me a PM if you want me to talk ya through this


cheers


tim

 

Good show Tim.

 

Well, heere's the latest. I took my car to the shop that installed the last cam and they confirmed that my number 5 cylinder cam lobe is in fact going flat. I guess I was right about the rocker arm not having the same amount of travel as the other rockers. I guess there's a little good news in this. He's offered to split the labor cost on installing a new cam and lifters, so the total bill should be around $600 including parts. At least I should have my car back by Wednesday. I've been dying to drive it given the nice weather here in California. I'll let you know how this all turns out.

 

Careful on just replacing the cam.

If you have wiped a lobe, then all that metal is everywhere in your engine. Oil passages, bearings, etc. IMHO, the right way to fix this is to pull the engine and clean the block and replace all the bearings. Sorry, but that will avoid problems down the road. A new cam will be quick fix but not clean all the junk out of the engine.

 

I just stripped down (completely)a 350 that had at least one bad cam lobe. I did not find any bad damage or large traces of foriegn material on/in any bearing surfaces anywhere in the block ,that would do short term harm . That being said, I do not know if this is the usual case in bad lobes. My dial calipers indicated at least a 1/4 inch difference on one lobe. The 1/4 inch was by eye, since I saw the problem and didnt care to procede futher in any measurements. It doesnt take much to cause a miss. I would think that all the bearing,EXCEPT cam bearing,can be replaced with engine in car.Not sure about cam bearing,as I have always had engine out of car and had a machine shop do that chore for me.

 

Spoke to the shop owner yesterday and mentioned my concern about the metal shavings in the oil. He is going to flush out the motor before he installs the new cam. Not quite sure how this is done, but I think he knows what he's talking about. He builds a lot of race engines and has a couple of dragsters with his sons.