1972 280SE 4.5 timing-pertronixII-disrtibutor - Page 2 - PeachParts Mercedes-Benz Forum

Palolo · #16 12-25-2012, 05:51 PM

Quote:

Originally Posted by Adv_rider

Question: to install the distributor, which is more important to consider cam at mark or crank at OT?

Crank position. The crank position is a direct indicator of the piston's position, and when the spark plug fires (distributor rotor's angle) is more critically timed to the piston's position at various RPMs (before or after TDC, advance/retard etc)

Not saying the cam's position is not relevant, but when considering the distributor, you are essentially discussing spark timing and piston position.

The cam's orientation impacts the valves' positioning during the cycle and in terms of the distributor timing, would affect whether a valve was possibly still open at the time of a spark, or perhaps prematurely actuated leading to insufficient air/fuel. Also, depending on where the piston is in its cycle, both piston and valve/s can try to occupy the same physical space at the same time, which is definitely a bad thing on an interference motor. (On some engines like the old volvo 4 banger, the valves would never meet the piston)

Quote:

Originally Posted by Adv_rider

I guess next I'll need to check timing chain wear.

Note: i didn't change the base, it's adjustable CW or CCW by loosening a couple of screws.

Could this be the timing chain is off by a tooth??

You should be able to determine chain condition by checking to see that the crank is at 0deg TDC for cyl 1, and observing the cam's position. Pull the motor through a few times in the proper direction and observer the cam rotating and see where its timing mark ends up relative to the crank's position.

A tooth is a lot to be off by on the crank sprocket (smaller gear, fewer teeth, one tooth represents a larger % of 360 degrees) but less noticeable on the cam side (larger gear, so one tooth is a smaller % of the rotation) due to the way a crank spins at 2x the rate of a cam.

Great news that it is running, but like above, I too believe in checking the settings and starting from even-stevens and not trying to set timing based on cam position.

Good luck!

Adv_rider · #17 12-25-2012, 10:01 PM

Quote:

Originally Posted by Palolo

Crank position. The crank position is a direct indicator of the piston's position, and when the spark plug fires (distributor rotor's angle) is more critically timed to the piston's position at various RPMs (before or after TDC, advance/retard etc)

Not saying the cam's position is not relevant, but when considering the distributor, you are essentially discussing spark timing and piston position.

The cam's orientation impacts the valves' positioning during the cycle and in terms of the distributor timing, would affect whether a valve was possibly still open at the time of a spark, or perhaps prematurely actuated leading to insufficient air/fuel. Also, depending on where the piston is in its cycle, both piston and valve/s can try to occupy the same physical space at the same time, which is definitely a bad thing on an interference motor. (On some engines like the old volvo 4 banger, the valves would never meet the piston)

You should be able to determine chain condition by checking to see that the crank is at 0deg TDC for cyl 1, and observing the cam's position. Pull the motor through a few times in the proper direction and observer the cam rotating and see where its timing mark ends up relative to the crank's position.

A tooth is a lot to be off by on the crank sprocket (smaller gear, fewer teeth, one tooth represents a larger % of 360 degrees) but less noticeable on the cam side (larger gear, so one tooth is a smaller % of the rotation) due to the way a crank spins at 2x the rate of a cam.

Great news that it is running, but like above, I too believe in checking the settings and starting from even-stevens and not trying to set timing based on cam position.

Good luck!

Thank you. The way you explain things is really understandable.

Graham · #18 12-25-2012, 10:29 PM

Quote:

Originally Posted by Adv_rider

Thank you. The way you explain things is really understandable.

Yes that was a great explanation. This link relates to a similar subject Distributor installation advice for dummies needed - Benzworld.org - Mercedes-Benz Discussion Forum
Maybe it will help a little.

Tomguy · #19 12-25-2012, 11:15 PM

I think 1 crank tooth would be 40° whereas 1 cam would be 20° but I don't have any of the gears in front of me to double-check. Either way I doubt you'd be off a tooth and run reasonable at all since your timing chain, if original, probably has 10°+ of wear / stretch in it. Mine had about 12° and I replaced the chain, leaving the original aluminum-backed rails, and that got it within 2-3°.

Graham · #20 12-26-2012, 11:25 AM

Quote:

Originally Posted by Tomguy

I think 1 crank tooth would be 40° whereas 1 cam would be 20° but I don't have any of the gears in front of me to double-check. Either way I doubt you'd be off a tooth and run reasonable at all since your timing chain, if original, probably has 10°+ of wear / stretch in it. Mine had about 12° and I replaced the chain, leaving the original aluminum-backed rails, and that got it within 2-3°.

We don't have confirmation from Adv_rider yet, but it does sound like the chain is likely stretched. It the marks on the passenger side camshaft were aligned, and there was say 10deg of chain stretch, it is easy to see that the distributor would not be in design position.

Best to set crankshaft at TDC, then insert distributor. Because of the short piece of chain between the crankshaft sprocket (8) and distributor drive sprocket (3), the distributor should then be positioned approximately correctly even if chain has stretched.

If at this point, passenger cam marks are more than say 5 or 6 deg off, then time for new chain and perhaps sprockets, if they are worn.

Bear in mind, I am no mechanic. Just an owner who has done this once!

This pic shows the chain routing and might help understand why chain stretch would throw the distributor position out.

Adv_rider · #21 12-26-2012, 12:34 PM

Thank you guys for all the information. I will get into the car later today.

Something interesting I found on the manual! It says that my 1972 117.984 should have distributor part # 0231 401 003 or 0231 402 002

It seems the previous owner replaced the distributor with the 1970/71 engine 116, distributor! which is 0231 401 002.

I dont think there should be too much variation, but could this be a reason?

Tomguy · #22 12-26-2012, 12:58 PM

Spark curve is likely different. My hunch is the 3.5 doesnt advance as much due to higher compression?

Graham · #23 12-26-2012, 02:13 PM

Quote:

Originally Posted by Adv_rider

Thank you guys for all the information. I will get into the car later today.

Something interesting I found on the manual! It says that my 1972 117.984 should have distributor part # 0231 401 003 or 0231 402 002

It seems the previous owner replaced the distributor with the 1970/71 engine 116, distributor! which is 0231 401 002.

I dont think there should be too much variation, but could this be a reason?

If you have the 1972manual, the USA 117 and 116 do have quite different specs for advance at 1500/3000 rpm (Greater advance for the 116 engine). Other specs, for example at idle, are not much different. This would likely mean that the centrifugal advance cams are different.

I may be wrong, but I don't think this would be the cause of the distributor install alignment.

Palolo · #24 12-26-2012, 02:19 PM

Quote:

If at this point, passenger cam marks are more than say 5 or 6 deg off, then time for new chain and perhaps sprockets, if they are worn.

ADV: This is well worth considering, as well. If the sprocket teeth have a lot of wear, that can throw off the cam position by a lot, especially when added to the old chain's extra length. Just threading a new chain in to the timing system might not fully address your situation, given that the crank's position won't be true due to the worn teeth on the drive side of the crank sprocket. The cams' "driven" sprockets will exhibit less wear on the opposite side of their teeth, and if you see noticeable wear or asymmetry on your cam sprockets, there's a very good chance that the crank sprocket needs to be replaced as well.

Which leads to a new front seal... Which leads to...

Oh yeah, I have ended up redoing way too much after opening something up and saying "I'll just change/clean this one little thing..."

Adv_rider · #25 12-26-2012, 10:42 PM

Guys, I was kind of expecting a lot of wear on the chain, but to my surprise when i set Crankshaft at TDC, the camshaft mark is aligned!!!... Really!

..

Too good to be true, but it seems the PO did a great job with this car... The top (tensioning?) rail is still the metal one!
I even have a picture, but don't know how to posta pic in here.

Grab the timing light and, at idle with vacuum connected, it is around 8* BTDC.
The engine feels great, I even drove around the block and it seems perfect.

My strobe is very simple and I don't have a tach so I will need to visit a mechanic to help me with correct timing.... But which numbers should I use?

I think the distributor not being the correct one is really messing with the position and im afraid the numbers I have for timing may not be the correct option.

Manual says 5*ATDC at iddle with vacuum.

Adv_rider · #26 12-26-2012, 10:48 PM

Quote:

Originally Posted by Graham

If you have the 1972manual, the USA 117 and 116 do have quite different specs for advance at 1500/3000 rpm (Greater advance for the 116 engine). Other specs, for example at idle, are not much different. This would likely mean that the centrifugal advance cams are different.

I may be wrong, but I don't think this would be the cause of the distributor install alignment.

In deed they are very different.

Quote:

mean that the centrifugal advance cams are different.

I have no idea on how to eat that! .... Is there anything I should consider doing to compensate that?

meltedpanda · #27 12-27-2012, 10:18 AM

since this seems a bit unusual to all of us here, and the fact that it is now running "perfect" in your words, I'd leave well enough alone. Set the timing by ear and feel. Go out and punch it up a hill if you have knock ,retard it a bit
You can do it this way on the old engines

Adv_rider · #28 12-27-2012, 10:47 AM

Quote:

Originally Posted by meltedpanda

since this seems a bit unusual to all of us here, and the fact that it is now running "perfect" in your words, I'd leave well enough alone. Set the timing by ear and feel. Go out and punch it up a hill if you have knock ,retard it a bit
You can do it this way on the old engines

Can you tell more about the air-mix screw? Hw many turns from close is the aprox. Standar? I have it on 1-1/2

dirtyharry · #29 12-27-2012, 11:01 AM

I'm running my M117 at something like 14Btdc, it really likes it, with a modified curve to bring full timing in by 2000rpm, takes off like a rocket, doesnt knock... with the compression so low and no vac advance, its all I can do to make a halfway decent driving W108.

Graham · #30 12-27-2012, 11:07 AM

Quote:

Originally Posted by meltedpanda

since this seems a bit unusual to all of us here, and the fact that it is now running "perfect" in your words, I'd leave well enough alone. Set the timing by ear and feel. Go out and punch it up a hill if you have knock ,retard it a bit
You can do it this way on the old engines

Trial and error can work.

8deg BTDC with vacuum indicates a lot of advance. What happens if you disconnect the vac line and plug it? In other words, is teh vacuum retard working?

6deg atdc spec is for emission reasons. Mycar runs much better with more advance. Best setting on my car is about 27-30 btdc at 3000rpm. I set it at idle and then just check at 1500 and 3000 because I hate to have engine revving so high for long!

Maybe start set at 0deg at idle (without vac) and drive car. I use a standing start from bottom of hill near us and check if car labours or not. I have also checked 0-100km runs vs spec on a flat deserted road near us.

The different cams will affect the amount of advance you get as rpm increase. It seems your car will advance more quickly and further than standard. So maybe starting at 0deg no vac might work.

If you over advance, you could get pinging or maybe burn valves?

Let us know how you make out. Good luck.