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| Joined: | 04-16-2018 |
| Location: | Santa Barbara, California USA |
| Posts: | 22 |
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While waiting for the distributor and tach conversion, as I said I am going through with relooking at some of the basics. Rebalanced the carbs and tried it from a different direction. Adjusted all of the idle mixture screws to the same turns (ended up at 4 turns) after very small adjustments to the air bleed screws on 1 and 4 to give equal CarbTune readings. Getting to 4 turns was what it took to get an 02/fuel ratio average of just under 14:1. The reading is bouncing around much more than I remember in the past though. Not sure if that means something wrong with the meter. At 3000rpm average is about 12.7 also with a lot of variation, somewhat less. For those with O2 meters are your readings reasonably steady?
Did a compression test, with engine warm.
1- 200
2- 208
3- 184
4- 200
That's a little disturbing. This engine still has very few miles and I did a test when new and they were closer then.
The plug condition seems to indicate that cylinders 1 and 2 have been running rich and 3 and 4 lean. I'll check again to see if these latest adjustments even that out.
Before freaking out too much about the relatively low compression on cylinder #3, how many miles are on the engine, and have the valves been re-adjusted, and if so, how long ago?
I would do a leakdown test to make sure the cylinder is sealing properly. If so, I would then check the valve clearances (which I know is no small task on the 907 engine). I tend to set the valve clearances on the loose side on an engine with a fresh valve job, because they will tighten up once everything seats in during the break-in process. If the clearances get too tight, the valves are open for a longer duration than normal, and this affects the dynamic compression.
I am currently assembling the engine on our (somewhat) recently acquired '74 Jensen Healey (#19250 for those keeping score), and while this is my first 907 engine, I have a good amount of experience building/breaking in engines as a professional Porsche/BMW technician.
The usual routine at my shop is to install and run the engine with break-in oil, do the initial break-in run (say ~100 miles, and make sure the engine sees some load to seat the piston rings, no babying it!), change oil, re-adjust valves. Then re-adjust the valves and change the oil again at the 1,000 mile mark, and go to normal service intervals after that. Whether Porsche 356, VW Type IV, Porsche 911, or BMW M10/M20/M30, this routine has always worked well.
I did the same on our '81 Alfa GTV6 after a fresh valve job, and the shim-and-bucket intake valves (same concept as a Lotus 907) certainly tightened up after the valves seated in!