Setting dwell and timing.

[travellerdogs]

I am going to set the dwell as I just found a dwell meter with tachometer abilities. I've read through all the forum threads and need some clarity  on the settings. The car is a 69 280SL with the 051 cast iron distributor. From what I can discern Dr Dan says 38 degrees for dwell. When I set the timing, do I disconnect the vacuum line and block the line or do I leave the vacuum connected. And is the timing 8 degrees BTC like most other cars.  When this is done i will then start on the fine tuning of the fuel system. BTW, the car runs fine but I have to get the RPM up hopefully after the dwell and timing is set.  Thanks, Carey

[garymand]

Rpms will go up when you pull the vacuum line then block the leak.  check your dwell and timing. 

[travellerdogs]

Hi Garymand, I know that the RPMS goes up when you unhook the vacuum line.  But do you disconnect the vacuum line to do the timing  and of course block the vacuum line or do you set the timing with the vacuum line connected. And I read that the Timing is at 8 degrees BTC not ATC. I am used of doing timing on most cars with the line unhooked and blocked to stop vcuum leak. My experience is timing is set at BTC with vacuum line disconnected. Thanks, Carey. Need some clarity on this issue.

[Benz Dr.]

Rpms will go up when you pull the vacuum line then block the leak.  check your dwell and timing. 

Blocking the leak won't do a thing for you. Once you pull the vacuum line off of a VR system like the 051, the advance plate will spring ahead. I leave everything hooked up because that's the way it's going to be working once you drive the car.

I set the total ignition advance to 38 degrees BTDC at or above 3,000 RPM - it won't advance any more after that. At idle you should then have about 8 degrees BTDC. If your distributor is working properly you will have 8 degrees. If it's less, the advance cell isn't adjusted properly.
Total distributor advance should be 30 degrees - the extra 8 degrees comes from your basic ignition timing set at 8 degrees BTDC at idle. Not to be smart or anything, but you would be surprised how many people really don't seem to understand this simple concept.   

[travellerdogs]

Thanks Dan, that's what I wanted  clarified. I understand how it works now and I know about adjusting the vacuum rod to set the total advance. Wasn't sure about the vacuum line. Carey

[stickandrudderman]

It's interesting that Dan advocates and has success with 38° whereas I, with my factory indoctrination, will only use the factory setting. Since it's nearly 30 years since I became independent I think it might be time to try Dan's settings out!

[garymand]

Interesting, I didn't expect the depth, and left off a lot of detail.  Good point Dan, Its a little faster and more to the point if all is well.  Its a long step between following the book and getting to the level of knowledge and experience where you understand what is going on and can comprehensively explain it to someone else. 

But to clarify for a person new to adjusting dwell,I didn't realize what you really were asking.  To your specific question, yes, you adjust the mechanical advance to the ( degrees with the vacuum not working on the distributor.  Its not much of a vacuum leak so it might not change the rpms, but I plug it just becasue the book says to.  I've not stopped to think about what difference the vacuum leek makes.  I'm more concerned about where the advance is and how the weights are working.

 it That is when I want to check or adjust the mechanically advance of the spark separately from the vacuum advance.   I adjust the mechanical timing of the spark to #1 to  8 or 10 degrees ahead of TDC, first.  Then I take notice if it smoothly adjusts for rpm changes (up to what ever the springs limit is) The stiffer the springs the higher the rpm at max mechanical adjust.  BTW, I've seen a few distributors with stuck weights or weight shaft, where the timing jumps back and forth or doesn't move at all with rpm change. 

I move my timing more than most.  I drive over a range of altitudes and like to run the lowest cost gas I can for the heat or type of driving I'm asking the SL motor to do.  The 250 motor had the neat graduated aluminum scale and cam'd adjustment lever so I realized the Germans knew they ccould adjust the timing for gas and altitude.  I would set the max to 10 or 12 degrees for sea level and hi octane  and back it down for low octane and advance it again up around 2000 feet when going skiing.  In the heat, if I heard pinging, I would just lower the timing.   ( just pulled the head to replace the gasket (first time on this motor) and the valves looked new, very little exhaust deposits, much to my surprise.  I've only put 10k on this motor but it had 90k on it when I put it in.  so much for Techcron or high priced gas.)

With a 280 distributor I couldn't swap the timing adjustment device from the 250 and just blindly back the timing off if I need to.  But I know the timing is 10 degrees advanced at hard CCW stop.

Given that hook up the vacuum to the vacuum pot and the advance should move appropriately down.  Then raise the rpms while watching the timing mark move up to ~38, smoothly, both up and down.  Sophamoric, I suppose, but I've just gotten used to looking for how it should work and I get a kick out of seein this machine do the right thing so well.  Can't think of anything else to add.

[Benz Dr.]

I guess I`ve sort of become used to pulling the distrbutor and setting it up with my tester. The 051 should run 20 degrees of mechanical and 10 degrees of vacuum advance. If it only makes 28 degrees of advance I usually turn the pull rod about 1 full turn and then I test it again unil I see the full 30 degrees needed.

Dwell angle affects timing. If you are droping dwell angle you will also loose ignition timing. The factory allows for a 3 degree max drop over the whole range. I usually spin the distributor up until the points start to float while watching the dwell angle. A good unit will hold dwell to within 1 -2 degrees right up to point float which can be as high as 8,000 RPM ( high as my machine will go ) on really good units. 6,500 - 7,000 is more common. These were very good distributors when they were new.

The dual point unit that I use in my own car came out of a 300SE. Rather than the normal 38 degrees of dwell this one will make 52 degrees. Two sets of points placed on opposite sides of the point plate along with a three lobe cam makes this happen. This unit will easily spin to 8,000 RPM because each point set only has to open half as often as a single set. This was a very high tech unit at the time - the  advance plate is set on ball bearings for smooth operation. I was using a distrbutor cap from a 280 twin cam but found one to fit that is used on a BMW.

[travellerdogs]

Hi Dan, I've got 40 degrees of dwell. Now the problem is my RPM is only at 500. I have marked 30 and 35 degrees on the balancer. Do I have to set the idle timing. I checked the timing with vacuum connected at idle at 500 rpm and it is sitting at TDC plus or minus 2 degrees. Can I just run it up to 3000 RPM and set it at 30 The engiine runs fine, starts instantly and idles good but at low rpm's . I am going to warm it up and then pull a plug to check the  color. I think it is running too lean. When I unhook the Fip rod and push it down a bit the idle  goes up. The throttle valve and pump rod all move at the same time. The pump rod is set at 233mm. I have checked all my linkages and they seem right. \i have to check to make sure the throttle valve is resting on the stop and also at the pump to make sure it is on it's stop. I have to take the battery out to get at the distributor bolt so I will run it with the battery on the floor with heavy jumper cables to do the timing. Thanks, Carey