Flat spot when driving at light throttle, OK at idle and accelerating

[CoreyJ]

We have an issue where we believe the engine is running lean, it starts and idles fine but at 1,800 to 2,000 rpm it will backfire through the intake acting like it is running lean. Step on the gas and at half throttle and beyond it takes right off with good acceleration. The distributor has been replaced, spark plugs, wire set and the fuel injection pump was just sent out and rebuilt. Even with the richness control on the fuel injection pump at it's maximum adjustment it is still running lean. We have performed a compression test which all cylinders were close (138 psi). We have adjusted the valves, double checked the timing - but still have this flat spot backfire at mid-range. We would greatly appreciate input on the next step to correct this problem!

[Nicolas Aristodemou]

Which model/ year do you have? Manual / Auto?

[jeffc280sl]

Where did you send your FIP for rebuild?  Do you have any calibration test data?  Attached you will find some test data from my on road dyno runs.  There is a brief moment during start of full load acceleration when the AFR leans out.  Its very brief while the FIP reads a new position on the space cam. Its possible with some equipment to measure AFR during on road acceleration.  You can see a pic of my lap top on the passengers seat.  This is a very challenging problem to discuss and remedy on the road.  My thinking is the FIP calibration company should have tuned the pump to make perfect runs.  I don't think any richness adjustment should be needed on your part to get perfect performance from a recently tuned FIP.  Scales on the charts are as follows.  Time is on the bottom.  Manifold Air Pressure, Air Fuel Ratio and RPMs are on the left side.

[CoreyJ]

This is a 1971 280SL Automatic. The fuel injection pump was just rebuilt at Bud's Benz - they didn't provide any calibration data. We appreciate your input!

[Jonny B]

Have you checked the emissions reduction gear? These are the black boxes on the left side of the engine bay by the battery. There are two black boxes and a round device (two-way valve) in the vacuum line. These are related to idle and advance.

We have been doing some trouble shooting, well I should say Tyler has been doing the checking!. He found that one of the boxes, while having the correct wiring diagram on the cover, has the wrong internals! Still working through this, but it may be causing some issues.

A lot of times these are disconnected.

[ja17]

Check the fuel pressure and volume. You should also check the fuel pressure when the problem is occurring. Try clamping shut the return line to the fuel tank to see if the problem improves.

[jeffc280sl]

I'm in favor of following the axiom here that all other systems and operations should be checked, adjusted and proven before moving on to the FIP.  Jonny and Joe are good to follow this practise.  I have a 1970 4 speed with transistor ignition and emissions control.  I know the emissions control system looks to cut off fuel to the pump to reduce emissions under deceleration.  It may be part of the emissions control during idle and acceleration.   I just don't remember right now.  In my car I have disconnected the emissions system and replaced the ignition and distributor with a 123 distributor.  I would like to think your FIP is not the issue because of your recent tuning.  With that said I would not think any enrichment adjustments should be needed.  Best of luck.  These things take time and patience to resolve.

[Nicolas Aristodemou]

I'm in favor of following the axiom here that all other systems and operations should be checked, adjusted and proven before moving on to the FIP.  Jonny and Joe are good to follow this practise.  I have a 1970 4 speed with transistor ignition and emissions control.  I know the emissions control system looks to cut off fuel to the pump to reduce emissions under deceleration.  It may be part of the emissions control during idle and acceleration.   I just don't remember right now.  In my car I have disconnected the emissions system and replaced the ignition and distributor with a 123 distributor.  I would like to think your FIP is not the issue because of your recent tuning.  With that said I would not think any enrichment adjustments should be needed.  Best of luck.  These things take time and patience to resolve.

I am also of the opinion that the problem would be with the function of the emissions control system and/or the replaced distributor and its retard function related to the emissions control system (vacuum solenoid valve that operates removing the vacuum from the distributor at around the  2000rpm mark. The fact that a distributor has been replaced does not mean that it would be operating and/or set up correctly.

To clear the distributor issue I would strongly suggest getting an 123 electronic unit, although setting it up to work with an emissions control system is not the easiest task. There are, however, several posts in this forum about this issue to guide you.

Again best of luck.

[Cees Klumper]

I know it's an obvious point, but have you done the linkage tour, and confirmed that the air and fuel are 'synced' properly and all the linkages / brackets are tight? I once had a poor running issue just due to the fact I had not properly torqued down one of the brackets holding the linkage rod that goes across the valve cover. Also you might want to check for a vacuum leak and check the injectors/lines.

[specracer]

If you and the car are in Saco ME, you have a fantastic resource in your back yard

http://www.sltechw113.com/

[CoreyJ]

Thank you for all the input! - we will go through the linkage tour to make sure there is nothing we have missed. It sounds like most members are in favor of converting to the 123 electronic distributor to eliminate that as a possibility, right now the timing is letting it start and idle fine - and once it gets past half throttle it takes right off, it's the slower speeds  it has this miss fire through the intake. We will check the emission boxes as well, we haven't seen this be a problem before but everything is getting older and possibly breaking down.

[jeffc280sl]

the emissions system is described in the BBB.  try reading the tech manual for more details.  i think i put stuff in there when i was into this issue years ago. 

[jeffc280sl]

my review indicates the relay boxes are involved in two operations.  they are ignition changeover and fuel cut off control.  the ignition changeover is pretty straight forward and is described in the attached photos. 

i believe the ignition changeover subsystem reduces emissions when the engine is cold, less than 17C or when very warm, over 100C by adjusting timing.    the second subsystem is basically an on/off switch for the fuel shutoff solenoid.  it gets inputs from engine sensors to include rpm and transmission pressure to indicate a deceleration mode and charges the solenoid.  fuel cutoff reduces the release of unspent fuel during this mode. a failure mode in this subsystem will be evident in an intermittent, on or off state of the fuel cutoff.  the operation of the subsystem can be eliminated by disconnecting the solenoid.

the ignition changeover subsystem can be checked using the procedure below.  you can compare the wiring diagram below and the one in my last post to see these two operations

[ctaylor738]

When you say "Even with the richness control on the fuel injection pump at its maximum adjustment it is still running lean," do you mean the thumbscrew on the back of the pump?  That's the idle mixture adjustment, which is not going to help your flat spot.  I recommend going a couple of clicks rich on the main rack aka "full load" and see if that helps.  This adjustment is the opposite of idle (left rich, right lean).  You will probably have to dial back the idle adjustment.

[bracurrie]

my review indicates the relay boxes are involved in two operations.  they are ignition changeover and fuel cut off control.  the ignition changeover is pretty straight forward and is described in the attached photos. 

i believe the ignition changeover subsystem reduces emissions when the engine is cold, less than 17C or when very warm, over 100C by adjusting timing.    the second subsystem is basically an on/off switch for the fuel shutoff solenoid.  it gets inputs from engine sensors to include rpm and transmission pressure to indicate a deceleration mode and charges the solenoid.  fuel cutoff reduces the release of unspent fuel during this mode. a failure mode in this subsystem will be evident in an intermittent, on or off state of the fuel cutoff.  the operation of the subsystem can be eliminated by disconnecting the solenoid.

the ignition changeover subsystem can be checked using the procedure below.  you can compare the wiring diagram below and the one in my last post to see these two operations

The ignition changeover subsystem is designed to keep timing retarded until over 2500 RPM or if the engine is cold or hot. On mine the speed switch has gone wonky and the system kept turning off then on. Turns out you dont have to have the retard system at all but then you need to make other changes to get full performance potential.

[mrfatboy]

On mine the speed switch has gone wonky and the system kept turning off then on.

This sentence caught my eye because several of us have been working to get the bugs out of a friend's 1970 280sl. Besides finding out that the outboard black box relay had the wrong circuit board in it (turns out the relay was tampered with) the throttle body switch was installed upside-down so the on/off signal was reversed to off/on.

The moral of the story is you really have to check everything 🤣

[Tyler S]

There are a lot of things that influence the timing vacuum changeover valve besides the speed relay. There is also the potential of a faulty low or high temp switch, transmission switch, or the changeover relay itself. Any of which can cause the advance to kick in intermittently at the wrong time/temp/rpm. The Changeover relay (pictured below) uses a latching setup on some systems depending on the variation. Another thing to check on the back side of the relay boards is for cracked solder joints where the relays mount to the board. Intermittent connections here cause all sorts of strange behavior.

The second photo below are the differences in the 2 relays that MrFatBoy referenced. They are not interchangeable between specific systems/ variations.