How much vacuum should there be at the Venturi?

[dwilli3038]

I am trying to get my 230SL to run more like a Mercedes  ..... The idle will not go below 1400 RPM. When i decoule the linkage, it seems that the mixture is close to what it should be.  After a couple hundred miles my plugs show some signs of carbon buildup. The vacuum connection for the distributor at the venturi has extremely low vacuum......you can see the needle move on my vacuum gauge but so little you cannot get a reading. I seem to have strong vacuum at the brake booster and at the idle speed adjustment screw. I did clean the port on the venturi with carburetor cleaner but still seem to have only negligible vacuum at this connection point.

[ja17]

Hello Daryl,

It depends on if your distributor is a vacuum advance or vacuum retard version. An early vacuum advance version will have low vacuum at idle and more as rpms build up. Just the opposite for the vacuum retard versions.

In any case the ignition timing should be around 30 degrees before TDC at 3,000 rpms.

As far as your fast idle, make sure the venturi is closing up all the way. Close down the main air screw on the intake so that the idle is around 700 rpms. Split the linkage and check the mixture. Adjust the IP thumbscrew (engine off) if fuel mixture is incorrect. Maintain 700 to 650 rpms warm idle.

[dwilli3038]

When I turn the screw all the way it still idles around 1400 RPM. The screw will only change the idle from about 1600 RPM to 1400 RPM.

[ja17]

Hello Daryl,

It sounds like you have a vacuum leak or the venturi is not closed down all the way. Also, the WRD may not be closing all the way.

First, disconnect the linkage rod to the venturi and set the small slotted screw so that the venturi closes all the way, You will feel is stick closed when it is all the way shut. Set the screw so that it just keeps if from sticking shut. (See linkage tour).  Nezt check the WRD to make sure that the aux air intake at the filter stops after the engine is warm. You could possibly have another vacuum leak but these two items are the most common cause of a high idle.

Keep us up to date,
Joe

[glenn]

You've got to get the fuel(gasoline) flow from the FI pump down to 750-800 rpm rate with the accompanying proper air flow(butterfly shut and idle air screw adjusted to max rpm).  If the FI pump push in knob(controlling the low speed-CCW) will not do it(reduce fuel input), then the WRD, CSV or atmospheric correction might be it.  Also, PO may have adjusted the rod clamp position on the shaft coming out of the side of the  FI pump.  A NO-NO per JA and others. .. i.e. PPP  Pagoda Perdition/Puerility

Once you get to 800 rpm, screw in the idle air to 750 rpm (or 50 rpm rich) per the BBB.

[dwilli3038]

I took off the hose at the air filter and plugged it the idle slowed down. I removed the idles screw assembly and cleaned it and found I gained at full turn on the screw, the result was the idle slowed to 1200 rpm. I took the hose off again and plugged it and the idle went down to 750RPM. .... and I could hear a sucking sound and found that I need a new O-Ring at the cold start valve. As soon as I obtain the O-ring I will let you know how things go

[ja17]

Hello Daryl,

Sounds like your on the right track !

[glenn]

You want to adjust the idle air screw to get max rpm.  If the rpm is over 800 rpm, reduce fuel flow at the knob on the back of the FI pump (CCW).  (This assumes the butterfly is on the stop and closed and the FI rod is on its stop at minimum.) Then adjust the idle air screw again to max rpm. repeat until you get 800 rpm.  Then screw in the idle air screw to get 750 rpm.   The BBBs have this procedure.

Basically you got to fuel flow thru the injectors to the minimum that gets 800 rpm with 800 rpm worth of air thru the idle air screw.   This means no fuel thru the CSV or anywhere else.

[Benz Dr.]

Here's one you don't see often but it will cause high idle speeds.

Sometimes the idle speed is high and sometimes it's just about right. Distributor is working and not hanging up and yet the problem won't go away. No vacuum leaks and the throttle linkage is set up perfectly. IP and WRD are perfect.

  So what is causing this problem?

[glenn]

PO adjusted the linkage rod clamp on the short shaft that comes out of the FI pump. ?maybe???

[J. Huber]

How about something to do with the gas pedal?

[stickandrudderman]

Looks like a thread hijack!

Short to ground on idle speed solenoid?

[glenn]

Vacuum on the downside of a closed butterfly in the venturi should be 17-19 in Hg.(out of 30 Hg.)  Any auto mech book covers interpreting 'vacuum gauge' reading on engines. 
A lot of cars were tuned with just vacuum gages.

[dwilli3038]

The O ring at the cold start valve helped a lot but I still have some problems. It idles smooth between 800 and 1000 RPMs. It I opent the air screw to 1000 RPMs I have 15 inches of vacuum and if I close it down to get 800 RPMs the vacuum is 800. I did gop through the linkage proceedure and that seems good. I have a hard time trying to adjust the screw on the injection pump. When I try to turn it counter clockwise it seems to want to stay pused in when I stop and it does not seem to want to go further counter clockwise. Also when the engine is running the screw is turing (is that supposed to do that?) If I seperate the linkage and give some gas it seems to be running very rich because if I give more air that gas with the seperated linkage I get a lot of additional speed.

any thoughts?

[ja17]

Hello Daryl,

The screw should definitely not be turning when the engine is running. Screw it back in enough clicks so it dis-engages when you let off the screw. Sounds like you may need to do a rack adjustment or a baro adjustment to get the IP to lean down. Do a split linkage test after you adjust the idle thumbscrew on the IP.

[dwilli3038]

I did turn back the idle screw on the injector pump 2 clicks and it no longer spins with the car running. If I adjust the air screw for highest vacuum and smoothest running it is 1100 RPM. When I did the split linkage test at higher RPMs giving it more air made the rpms go considerably higher.

I guess the whole thing is too rich. How do I adjust the injector pump to lean it out over the entire range?

[glenn]

In the big picture- the engine burns(IP injects) about a cup(8 oz) per minute(4 gal/hr) at 60-75mph(3500 +/- rpm??).  At idle(750 rpm) it is about a tenth of that.

Besides the idle knob on the back of the FI pump, the barometer and engine cooling water temp affect the engine speed.  Higher barometric pressure and colder water give more fuel.  Also, the start solenoid maybe energized -unlikely.  Cold start valve gives more fuel.

A PO may have adjusted stop on the throttle shaft, - a NO NO.

[dwilli3038]

I have already checked the cold start valve and it is fine. I have checked the warm up devise and it is working correctly. Since it is rich at all speeds I doubt it is the stop on the injector pump. I think I need to adjust the rack but I am not positive which adjustment is used to affect all ranges.