What causes cam timing error

rexav8r · 02-01-2006, 04:39 PM

First, I just got the car back from my mech and truely, it's never run better, since I've owned it !!

That said, I was a bit surprised at the sequence of events. My car (the '70E) has a rather fresh engine rebuild of approx 22K miles or so. I've done quite a bit of Auto-cross but I'm finally taking the plunge into DE this spring at Mid-Ohio, and I wanted the car to be mechanically fit.

I'm not that sure about the age of my MFI timing belt so I thought that along with an overall tune-up, since I had a new belt laying around I'd have it installed and the MFI timing reset along with the mixture which had gotten a bit rich.

I got a call from the shop the day after they started to let me know that my cams were pretty far out of time. In addition to setting the MFI timing they first would have to set the cam timing. Also a later (& larger) SC pulley had been installed along the way to facilitate cooling the engine. The car had been in Colorado for the previous 15 years with thin air. Unfortunately, the SC pulley is not indexed for a '70 motor.

Fortunately my shop had a "donor" 1970 S engine in back that they canned the correct pulley off of to reset the cam timing.

**Here's my question: What causes cams to come out of time? They can't really slip on the chains or can they. Is it simply a function of the valves losing their set position? Did the mech who did the previous cam setup screw it up ?

oneblueyedog · oneblueyedog's Garage

Chain stretch? Left a locating pin out? I think that the cams can be out of time for how you want to drive the car, but that's more of a setting. I'm not as experienced as others here but I set my cams on my rebuild and you can accurately set them with the dial indicator between the 2 banks. You probably have a high lift cam engine and I hear they are harder to time. That might be a control issue with the tools to set it. I'd like to hear how this engine could go out of time due to running. I say nearly impossible by engine runs, except for a few decades of use. I'm a street guy though, not a racer.

syncroid · 02-01-2006, 05:22 PM

The likelyhood of your cam timing going out of spec is really slim! The mechanical timing belt going out of time is not un-common. You do not need to reset your cam timing to set your mechanical injection pump timing. You do need the correct pulley though. The mechanical pump timing is set by going to T.D.C. #4 cylinder and then moving the pulley to "FE". (on the correct pulley of course) Then you line up the appropriate mark on the mechanical fuel pump. I'm not saying your cam timing was right in the first place. Its possible it was out of whack before your mechanic worked on the car.

rexav8r · 02-01-2006, 05:24 PM

Here's a bit more input...

My car has been running fairly rich as I said. For the past 2 to 3 years though, it's been a pretty fast little car performing very well at Auto-X events. Many guys have commented saying there's no way it's a stock 'E', which for the most part it is.

The first comment my mech said is that the cams were set for a relative smooth low end but I was losing a lot of top end speed ! This from an already fairly fast car. Also that it was pretty rough above idle.

He also said that he could set it to maximize top end but I'd end up eating clutch after clutch & it'd not be fun around town anymore. So we opted to smooth out both low and top ends, resulting in a better low end than when I took it in, and supposedly a fair gain in top end performance.

Now I've been talking speed, because I don't know what the torque curve looks like on this 36 year old girl anymore, though it feels great !

rexav8r · 02-01-2006, 05:38 PM

Noah,
By partially tearing it down, he DID have the valve covers off, plus the timing chain covers off, and the oil fed tensioners pulled out. He had a guage mounted to a cylinder on each side measuring (I assume) valve clearances.

Is there more that is normally done?

I've never worked on the insides of an engine, though I hope to as soon as funds allow. That has been a goal of mine for some time.

Syncroid,
I think that's what happened. He wasn't planning on even checking the cam timing. Went to look for the FE on the pulley and obviously there wasn't one. When he tried to line up #4 TDC it didn't line up right which led him to check the cam timing which turned out to be off. After setting everything he DId go back and index my pulley for the FE setting. (Though I didn't look to see how he marked it yet.)

javadog · 02-02-2006, 04:42 AM

Richard,

That's the normal amount of teardown to set the cam timing. The dial indicator gauge would have been set up on each side to measure the valve lift at a certain point, which is how Porsche times their cams. The cam timing is adjustable, within a certain range. Rotate them in one direction from the stock setting and you'll shift the torque curve down a little and rotate them the other way and you'lll move it up a bit. If the timing was off by much, it was probably set that way previously by another mechanic. This is of course assuming that the cams are E cams and haven't been changed to something else in the past.

I'd be careful about deviating too far from the stock setting for two reasons. The MFI space cam was designed with a stock engine in mind. It may run better overall at the stock cam timing. Secondly, you need to be sure that you have adequate piston to valve clearance. I don't know how far you can adjust the timing on that specific engine before that becomes an issue. Not trying to alarm you, just want you to be aware of the issues.

JR