Do you have the M117 shop manual? It is available on-line at MBUSA Startek under the 107 chassis.
Also here M117 4.5L
Some of the numbers in this thread seem a bit inaccurate for a 72 4.5L M117 (i have one!) Check the manual (
timing in section 07.5)
A 'few' points:
- Timing marks are closer than I could ever get. Don't worry about them!
- The 72/73 have metal backed chain guides. Unlike later cars with plastic guides, these do not break and being unobtainable are better left in place.
- Fuel pressre spec is 2bar +0.1 or 29-30 psi. I run my car at 32-34, but have adjusted the MPS using an AFR gauge to compensate for the higher pressure.
- The knob on the ECU adjusts idle mixture only. Mixture under load conditions is adjusted on the manifold pressure sensor (MPS). Only do that if you have a good exhaust analyzer that can measure %CO or AFR. Specs are available. (I installed a full time AEM wide band AFR with dash display)
- Getting the trigger points back to under 120deg was a good move. To do that, a tool/gauge is usually needed. We call it Norbert's tool. It ensures all points are adjusted equally. I acquired a brand new set of triggers and measured them. They were just over 100deg, I believe. The exact angle is not so important, but lower is better. I would say 100-130 range should be OK.
- Trigger points fire injectors in pairs and tell ECU when to inject. The MPS measures manifold vacuum and in part determines how much to inject. However, the other main input to ECU, is engine rpm. This also comes from the trigger points. When closing angle is too high, points may bounce, it seems and the EU may think the rpm is higher and therefore inject more fuel?
- Re vacuum - you can set timing quite accurately using a vacuum gauge either off tee in MPS hose or off a connection at the back of the engine that goes to locking system. I found that I could rotate distr until I got to max vacuum - about 16-17 "Hg, then back off a little. Then check timing. With vacuum retard line connected, about 5deg BTDC at 700rpm seems to work best. Then check at 3000rpm. The centrifugal advance should get you to about 27degBTDC. If not, the centrifugal advance is sticking.
- A bad TS2 (coolant temperature sensor) can contribute to rich mixtures. This is an important input to the ECU. Check all sensors vs specs in Section 07.4 of FSM.
- One vacuum leak that is not obvious, is right through the AAV. AAVs are prone to sticking. It should be removed, taken apart and cleaned if you suspect it is sticking. It should be totally closed at 150F, otherwise you have a vacuum leak!
- New Bosch injectors hard to find. I see some by Standard Motor Products. Still 8x$56!. The originals in my car are 48 yrs old and all I did was clean and flow test them.
- One of best upgrades I made, was rebuilding my injection harness. cushjbc on Benzworld R/C107 forum is the expert and helped me with parts/advice.
- Many of us have installed a Pertronix 1885 to replace points. Eliminates one wear component. For under $100, a good upgrade.
Info here and on BW