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Old 06-14-2002, 08:17 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I have seen more early 4.5s mistimed than any other car I know. The main problem is that their idle timing spec is designed for vacuum on the vacuum retard. The vacuum retard, when functioning properly, will pull the timing about 10-13 degrees. This is the state which is to exist when the timing is set to 5deg ATDC (the spec of most early 4.5s). If the A/C is on the vacuum is shut off and the timing if set to 5 deg ATDC will be 10-13 degrees retarded. Here in Florida I have seen this maybe a hundred times and it really makes a dog out of it.

As these cars get older the retard system gets poor for a number of reasons. Many freeze totally, many get bad diaphrams and a number get reduced movement. These problems should be fixed, but timing in the mean time needs to be deslt with.

I creat a new setting. I view the reduction of timing when the vacuum is applied. Should be 10-13deg (no more). I add that to the 5 dgrees ATDC and get 5-8deg BTDC with no vacuum. THIS is basically factory setting and assures no loss due to malfunctioning timing retard unit.

To this I would suggest adding 5-8 degrees timing for a new spec of 10-16 deg. The more timing you take the more need you will have for high test gas and pulling boats becomes a No-No.

If you can raise the idle vacuum to 17in you can make 20mph with the help of some lean tactics on the MAP sensor. MAP sensor being the current generic description for the manifold pressure sensor (manifold air pressure). A device the size of a grenade on these cars (in front/below of the brake booster on 107s)

Setting the sensor up requires the use of an exhaust gas analyzer. One disconnects the throttle switch to remove the idle compensation circuit. (The engine is then running on sensor mapping only). The MAP is then adjusted by the 4mm allen at the end under the black plastic cap (the center pops out with a very small screw driver - most are already mising). I set mine to 1% CO and then replug the throttle switch and adjust the idle compensation to 3% CO. This will get 15+ mpg. To get 20 one must take the timing to the max and then set the CO (w/o throt switch) to .5% or less and set the idle comp to just below best running probably 1-1.5% CO.

I do not know which way to turn because I do not need to know!! I use the analyzer to find the precise settings. To do it really properly I now do it on the Dyno under various loads. The settings can not be aproximated (well maybe I could {bg}). The device is tooo sensitive.

An after thought makes me also suggest that to get 20mpg one will also probably have to dial indicate and modify with offset keys the CAM timing. A new chain on old gears probably could use 2 deg on the left cam and 6-8 on the right cam. I am NOT suggesting a modified setting here!!! I am suggesting that to get, exactly, the spec,MB designed, will take such corrections with old gears maybe even new ones.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Last edited by stevebfl; 06-14-2002 at 08:28 AM.
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