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| Joined: | 09-17-2007 |
| Location: | San Francisco, California USA |
| Posts: | 221 |
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Got the cam degree'd in and spent lots of time with it to get a very good feel of what was happening. With the help of Tim I was able to find MOP using the excellent Dave Bean guide, and confirmed a 272 duration with opening and closing points at around 31 and 61 degrees. I marked up the cam pulleys with lots of info- opening and closing points, MOP point, and the arbitrary TDC marks to line everything up for service in the future.
Wow was it difficult to get a setup to measure the tappet movement. There is practically no access to the back of the tappet because the cam is hogging all the real estate. I had to use a very thin extension for my dial gauge, and then it was very difficult to get a solid reading because it was always at a slight angle. I kept trying and finally got something close to a perfect parallel angle to the valve travel. We're talking about 2mm of space if everything is lined up correctly and I needed to grind a relief into the dial gauge extension to get it to fit.
One thing I have noticed is that if you can't get the full 360 degrees of rotation and actually measure the travel on the tappet fully across its travel, it's really a waste of time. You need to see the full travel to accurately measure MOP, period. Taking a partial measurement and finding the opening or closing points or trying to extrapolate the MOP is silly.
The Dave Bean directions were very good once I wrapped my head around the concept. At first I didn't really understand, but the method of finding the MOP and going from there is very helpful, as well as taking measurements at higher lifts. At zero lift, your reading error is highly magnified. Also, there is a dwell at MOP and you have to measure it and average out where exactly it is- sometimes a few degrees of rotation yields the same reading.
In general it helps to have a deal gauge that is digital and reads in .0005" instead of the usual .001". When using an analog gauge it gets confusing as to what rev you are on (even if it has the little rev counter built in). With the digital, it simply reads the position in absolute terms with no fuss.
It's difficult to get a setup to hold the gauge well. Most kits come with a magnetic base and there really isn't anything magnetic to actually clamp to anywhere on the Lotus 907. I ended up taking an iron plate from my hydralic press and setting it on top of the flats of the intake manifold and kind of wedging it against the cam tower hoops. This provided a very nice, stable, and heavy base to clamp the arm to. I initially tried with a disc brake rotor measuring kit which has a flexible metal arm- it was useless and way too wobbly.
Another thing that helps immensely is a quality degree wheel. I first used a simple Jegs kit that was made of basically an aluminum that wasn't much thicker than a soda can, and these things can be bent just looking at them. I returned it to Amazon because it insulted my intelligence. A friend lent me a monsterous 1/4" thick aluminum degree wheel that was easily 20" in diameter. It was so stable and beefy you could turn over the crank by just grabbing the wheel- it felt like I was in the Navy on a boat opening a door. Because the actual degree wheel being such large diameter it makes the resolution that much more accurate in terms of the degree measurements.
My setup process is what Tim recommended and it went flawlessly:
1) Make sure the crank is at 90 degrees (all piston at about midway travel)
2) Install the cams at the 10 o'clock (intake) and 3 o'clock (exhaust) position (guesstimate in my case because NO pulley marks, on the green dot you just line it up). Bolt it down and check shimming if needed.
3) Gently raise the crank back up to TDC. If you feel anything, back off and reposition the cams. Make sure nothing is touching.
4) Install the belt and rotate the crank a few times to visually confirm all marks line up (since I had no marks I made some temporary ones)
5) Degree the cam, starting with the MOP point. Measure everything and mark it on the cam pulley.
I ended up using my adjustable vernier pulleys to help eliminate slack in the belt and get everything to a very high tolerance. Usually the vernier is set at a 0 degree arbitrary point so that, in the future, you can slightly retard or advance the cam for tuning purposes. I didn't see too much of a reason to do that and instead simply found the most accurate position to achieve the correct timing.
Thanks to everyone that helped! She started up right away and I can already tell that my years of deteriorating performance was most likely due to really bad cam timing.