another timing post

[jeffc280sl]

 My 1970 280SL 4 speed manual was runing very strong.  Cold idle was fine.  No leaks in the CSV.  Timing was set at 3 degrees atdc. But lately when the engine is fully warm I have a very low almost stall idle.   

So I went through the linkage setup and made some minor corrections. FIP is on its stop and the butterfly valve is closed properly in the throttle body.  Making good vacuum  I disconnected linkage to the butterfly and when I opened the valve I got an 500 to 600 RPM increase. .  I adjusted the fuel knob on the FIP and the air valve screw without much of a change.  I thought the mixture was too rich.  So I started to lean the mixure knob on the FIP.

Here is where I think I got off track.  I read that we should setup the timing at 10 degrees BTDC without vacuum.  The idea being the vacuum retard when connected would change the timing to 8 degrees ATDC.  So I did that.  My idle speed picked up so I set up the idle speed with the air screw and FIP knob with timing set at 10 degrees BTDC without vacuum.  MISTAKE  Now the car will not run with vacuum on the distributor.  I’ve fooled around with the mixture screw both ways so much I don’t know where I started.  My sense is the mixture is leaner because that’s what I was originally trying to do. 

Without vacuum idle is nice and smooth but the exhaust smells.  On the road the car misses in the lower rpm range and runs okay above 3000 rpms or so.  Well now I'm a little confused so I thought to write and ask for common sense from this group. My thinking is that it was wrong to setup idle rpms at 10 degrees BTDC without vacuum.


So now I want to go back to where I was before I started making changes.   Is it correct to think my mixture is too lean even though the exhaust smells ? Here is why I ask.  In theory idle is supposed to be lean.  A lean mixture takes longer to burn then a rich mixture so it needs to be lit earlier in the compression cycle (spark timing advanced).  So my car will not run with the vacuum retard and later spark timing.  Does this make sense?  I think the mixture needs to be richer at idle.  Comments please.

[Allenh]

DOes this make sense?  for the idle speed to have dropped off it would generally make sense that something had changed.  If all of the engine components seem to be OK, could you have gotten fuel when this started that affected your idle?  Like regular grade or a cut rate fuel? Even in a modern car where the engine controls take over, I know I see a marked drop in MPG's when I get some poorer quality fuel, even when "named" brand.

I would set it back the way it was and run the gas out of it and try some fresh fuel.  Sometimes are simple and not a malfunction. 

Allen

[Cees Klumper]

Here's what I, as shade-tree mechanic, would do first.

• double-check to make sure all of the linkages and their supporting brackets are tight and operate smoothly; check the measurements as far as rod length and so on (see linkage tour)

• install a fresh set of sparkplugs. It's amazing how different an engine can run once one or more spark plugs foul up and no matter how you clean them, they sometimes just turn bad
• hook up my basic CO meter (Gunson) and work out an acceptable warm idle mixture - 3-4% off the top of my head should work. I would do it with the vacuum attached, and if getting to a smooth idle means I have to set the timing different than spec, so be it

• try the car out on the road with these idle settings
• assuming after this it runs well (if not, there are other issues I would suspect) at speed, I would start playing with the timing settings, seeing what happens if I move them closer to 'spec'.

I also wonder what brought about this change in running characteristics. Normally settings don't start wandering and something else than ignition timing or FI pump settings is the culprit.

[jeffc280sl]

Thanks for the advice Allen and Cees.  I need to find my exhaust analyzer.  I should have thought of that.  At this point I'm looking for advice to return my setup to where it was before.  At this point I don't know if I'm too rich or too lean with the mixture. If it only runs on the advanced side does that indicate a lean condition. A side note is that I think the distributor advance is intermittant.  Sometimes the advance at 3000 rpms stops at 20 degrees BTDC and other times it continues to beyond 45 degrees BTDC.  Thinking about a 123 distributor.

[Naj ✝︎]

Hi, Jeff,

You can open up the distributor and have a look.

A spring may be broken or weight sticking?

Normally, the max centrifugal advance is 20* to 22*, so if you set the timing without vac at 30* BTDC at 3000 rpm, you will get idle timing at 8* to 10* btdc. When to replace the vac, it will retard 8* to 14* to ATDC. These figures are for an 051 distributor. Other dizzy specs are in the wiki.

good luck

naj

[jeffc280sl]

I've been able to get back to 0 degrees timing and the car runs almost as good as it did before I started fooling with it.  I had leaned the FIP so much that it would not idle at the correct timing setting.  I'll continue to adjust and improve things.  I did discover a problem with the advance on my distributor.  Found a reasonably priced 123Ignition and will order one of those tomorrow.  Also replaced the brass bushings on the cross over linkage rod.  Nice improvement in linkage movement.  Will replace a few balls and sockets then I'll be done with that part.

Thanks for the help

[Cees Klumper]

Jeff- while you are improwing the linkage joints and bearings, don't forget about the one that is below the exhaust manifolds, on the engine block. It's hard to see and get to so gets neglected sometimes. There are little nylon bushings in it and replacing those with new can be a good thing.
If putting in the 123 distributor makes as much of a difference on your car as it did on mine (and my original 051 distributor was in pretty good shape) you won't be disappointed.

[jeffc280sl]

Thanks Cees.

[glenn]

Re: rich vs. lean at idle.  MB says tune to 800 rpm with minimum fuel, then close off idle air to 750 rpm, i.e. 50 rpm rich.  Engine burns about 8 ounces(1 cup) per minute at 60/70 mph and 1 ounce per minute at idle.  Engine will run about 6 hours on a tank of gas on the highway and idle for 48 hours or so. 

This has worked for me.  1. Warm-up engine.  2. Unhook linkage rod from butterfly and FIP.  3. Adjust idle air to get max rpm.  4. If rpm is above 800, close FIP knob(CCW).  If rpm is below 800, open FIP knob(CW).  5. Repeat 3 and 4 until rpm is 800.  i. e. least amount of fuel necessary to get 800 rpm with right air amount.  6. Rotate distributor to see effects.  Should be close to max rpm as you slowly turn each way.  7.  Turn idle air screw in to reduce rpm from 800 to 750--50 rpm rich.   8. Reattach linkage.     

[jeffc280sl]

Thanks Glenn.  You mention turning the distributor to see the effects.  If I advance timing in my current setup I'll get a 200 or so pickup in RPMs. If I retard the timing even a little the engine stalls.  In order to get the timing set at factory specs I need to enrich the mixture to get there.  I've got some issues with my distributor which complicate setting the idle.  Once I get the 123Ignition system I hope to be able to fine tune the idle.

[glenn]

Are the cam shaft and crank shaft in sync?

[Tonys113]

Jeff,
Glenn makes a great point regarding the CAM and Crank being in SYNC. That turned out to be the major problem with why my car would not idle, nor run very well at all. Easily checked by removing the valve cover, rotate the engine over either by hand or remote starter switch until the crank shaft timing mark in at '0'. Look at the front of the cam, there is a cast mark in the front cam tower that should be lined up with a mark on the CAM Gear. Be sure the engine is on the compression stroke, (Distributor Rotor should be pointing at the #1 Cylinder). If these marks are not lined up you may never really get to where you are going. Once I corrected mine, fine tuning became relatively an easy process.

Tony

[jeffc280sl]

Thanks guys.  I'll check into it.