Hi Gents,

I just bought a 1971 Corvette with numbers matching stock engine. As I continue to evaluate the car, I am realizing the idle is too rough/lumpy. I was told the engine was completely rebuilt (2 owners ago) with a mild cam.

Question:
I am trying to figure out if the rough idle is being caused by the mild cam, or do I have other problems? Off idle it seem smoother. Carburetor mixture adjustments don't seem to help. Vacuum bouncing between 14-16psi (I think?, need to check again). I never owned a car with a performance cam. How smooth should a mild cam idle? Is there a way to determine if my rough idle is being caused the cam?

I know many people love the lumpy idle, but i hate it :-(

Thanks!

 

Without a video or at least audio clip it would be impossible to say however I would take a guess that your cam is likely the reason.

'Mild' is a relative term but even what I consider mild has a bit of a lump to the idle.

Try to get up a video clip...

 

Trying to determine a cam type by idle lump is very dubious. There are many very mild cams pretty much equivalent to the stock cam with different duration and LSA from stock that sound sporty/high performance, just like there are many high performance cams that idle as well as my OEM stock L-82 cam did (in fact better) with .525/.525 lift and LSA 110. You would never know that my Howards cam is MUCH more aggressive than the L-82 cam (.450/.460, LSA114)...

 

When I bought my 69 L46 last spring, I thought I had a lumpy idle due to a cam also. I fixed a couple of vacuum leaks and had the carb rebuilt by Lars.....and now I've got a nice smooth idle (and much more performance).

 

Pull a valve cover and measure valve lift. You can get close with a small steel ruler having somebody bump key to turn over. Now, you can get a degree wheel or tape and get all the cam specs, but the amount of lift will give you a rough idea.

 

Tuning has a lot to do with how a performance cam will behave.

 

Thanks for all the feedback!

Interesting idea on measuring the valve lift at the rocker arms. The concept makes sense, but I would love to hear more about this. Proper way to do this, OE specs, comparison to mild cam specs? I really want to get a better idea of what cam I have. I would hate to go all crazy rebuilding everything only to find out my cam was the cause all along. In parallel, ill take a closer look for any vacuum leaks as well as check the points, timing, wires and plugs. As soon as the snow melts, Ill roll the car out and take a video clip of the engine idling.

Any addtion feedback is certaily welcomed.

Cheers!

 

Lift is easy to measure. Pull a valve cover. Put a dial indicator on the top of a valve ( you will want to do at least one intake and one exhaust as they will be different if you have a dual pattern camshaft.) Rotate the crankshaft manually through 720 degrees and read the highest and lowest points on the valve. The difference is your lift. If you measure at the pushrod, you will have to multiply by the rocker ratio (usually 1.5) to get the lift at the valve. Measuring duration can be done as well, but it will be difficult, if not impossible, in the chassis. Go to Youtube and check out videos on degreeing a cam for more information.

 

Assuming that the engine hasn't been retrofitted with solid lifters I don't see what this procedure would achieve?
When the engine is turned manually the hydraulic lifter will collapse when it is on the lobe, so the measurement will be considerably lower than the true lift of the cam.

 

If you have engine vacuum at 14 to 16" Hg (measured as "inches of Mercury"), you do not have a cam big enough to cause a "rough" idle. I build a lot of mild performance engines using the CompCams XE262, which has specs very similar to the L82 and L46 factory performance hydraulic cams. This cam idles at about 16 inches, and will idle completely smooth with no roughness at all when the engine is correctly tuned. I also use the XE268 in engines with headers and good exhaust. This cam is regarded as a modest performance cam, and is usually not used in engines with stock exhaust manifolds. This cam will idle with about 14 inches of vacuum, and only has a very slight "coarseness" at idle once the engine is correctly tuned - you could take a tall socket and stand it up on end on the running engine, and it will not fall over. In my personal car, I run a 285 (adv) solid cam which pulls about 12 inches of vacuum. This cam has that "Harley-Davidson" sound to it, but still has a steady, slightly lumpy, idle that is uniform and steady with no "rough" feel - the engine does not "shake" at idle.

If you have a "rough" idle with 16 inches of vacuum, you have other tuning issues you need to look at. Verify you are running enough timing - you should be in the low 'teens for initial. Verify your vacuum advance is working, and verify it is hooked up to manifold vacuum. With the vacuum advance hooked up, you should have timing at idle of about 30 degrees. Make sure you do not have any vacuum leaks (very common on Corvettes). Unhook every vacuum hose (including the power brake booster line and PCV) from your carb and plug every vacuum nipple and hole and see if anything changes. If the idle mixture screws do not affect idle (you should be able to kill the engine by screwing them all the way in), you have a carb problem, a vacuum leak problem, ignition problem, or an internal engine problem. Not a cam size issue - a cam needs to be awfully darned big in order to cause a "rough" idle.

Lars

Edit add-on: Measuring the lift of the cam at the pushrod or valve will not tell you anything about the cam's specs that would affect idle quality. Idle quality of a cam is determined by duration, overlap and lobe centers - not lift alone. You can't measure the parameters that determine idle quality with common hand tools with the engine and cam in the car. A valid check, however, would be to verify that all 16 valves are opening (approximately) the same amount: If you have a lobe or two going flat, it will caused the rough idle problem your are describing, and this is not an unusual problem on a new engine. This can be visually observed by removing the valve covers, disconnecting the ignition, and cranking the engine over while visually observing the movement of the rocker arms. Flattened lobes are very obvious with rockers arms that will have noticeably less movement than the other rocker arms.

The more likely cause is a vacuum leak: A vacuum leak will affect low-rpm idle quality, but will smooth out and be less noticeable at higher rpm and under load. A vacuum leak can also make the mixture screws appear to be ineffective.

 

nice write up and good info.


sorry thread starter i don't get it.
there is a missing engine and one that is smooth with a cadence.

 

Lars- Thank you for the outstanding write up! Excellent information.

I did not realize that a "lower than normal" vacuum at idle (all things being equal) is a possible indicator of a performance cam. Based on all the feedback, I am now thinking my engine is perhaps out of tune. I will start with the basics, confirm correct initial trimming, check for vacuum leaks by plugging all inlets and checking the gaskets. If that does not help, Ill proceed to disassemble the carburetor and give it a good cleaning. Ill provide an update to thread once completed.

Thanks!

 

The lift will tell you a little. Nobody is gonna make a longer-duration cam on bone-stock lift. By the same token, nobody is gonna use ultra conservative stock duration on a cam that is .100 or more higher than stock lift. So it can tell you if it is a factory cam. Harbor freight has a dial indicator with a vise-grip locking base. 25 or so bucks. That and a timing tape that fits your balancer and some google-YouTube homework, you can get all your timing events. Assuming TDC mark is in the right place.

 

Update:

To recap the following are some base line specs. Still trying to solve the shaky rough idle.

-Previous owner claims the engine was rebuilt ~2000 miles ago
-Previous owner claims engine has a mild cam which is why engine does not idle smooth (suspecting this was a lie to explain the shaky idle?)
-Carburetor from a 1973 Oldsmobile. Jetting is unknown
-Compression 155 at all cylinder (+/- 5 PSI)
-Plugs look OK
-vacuum now at 12 in
-idle set to 675 (but can also run as low as 550)

I went on to check the idle timing and found it to be set WAAAY too advance (something like 40 BTC). With the vacuum advance disconnected, I set it 10 BTC. This helped improve the rough idle a little, but still not very sooth. Total timing seem to be spot on at about 36 BTC. Now that the timing has been brought down to 10 the vacuum is a bouncy 12 in.hg. To insure I dont have a vacuum leak in the lines, I removed all vacuum lines from the manifold and ports and plugged them, but that did not help. I then proceeded to spray all areas with carb cleaner and found no leak. I tried playing with the mixture but they have little effect unless I screw them in 1 turn before the stop. engine runs the same weather 2 turns out or 7 turns out. One last point. past 1000 the engine does smooth out but still not silky smooth.

I am stumped! Do I perhaps just have a performance cam? should I tackle the carb next? maybe still a vacuum leak somewhere? I welcome another thoughts.


Thanks
Agaon

 

I had a similar problem a few years back. Engine suddenly started running rough and drove me nuts for months. Finally pulled the intake manifold and found the gasket was leaking on the valley side. New intake gasket solved the problem. I now retorque the intake bolts regularly.

 

One thing I do for a poorly idling engine is set initial timing with a vacuum gauge. I adjust timing to get the highest vacuum reading. This is helpful with timing marks that are whack. A good ignition curve requires more than finding happy idle, but it's a start.

 

Gents Thanks for all the feedback and Ideas. Its comforting to hear that most feel it not the cam. I think I exhausted all possible timing settings. it diffidently runs a little better but still far from perfect. The only thing that remains is perhaps a bad intake gasket or clogged carb. Ill move on to the carb next by soaking it over night and blowing out every passage (otherwise visually it looks rebuilt). If that does not help ill proceed to replacing the intake gaskets. I am located in NJ 08831. If someone is near by and has an hour or two to spare, I can supply bear, soda, Lunch, and good laugh.

Thanks

 

 

I just installed the zz4 cam in my L31 Vortec a few weeks ago and there is a big sound difference in my truck. I’m using comp springs that are 360+ pound seat pressure, retainers and locks, 1.5 roller rockers, and still have stock push in studs. It sounds good. Don’t have the specs on hand but I will post if interested. But there is a total difference in the sound and performance in my L31

 

Have you double checked your firing order? For some reason I think there is one set of cylinders you can get swapped and it will actually run pretty well but not correctly.