Idle adjustment at FIP

[280sl1968]

I have gone through all of the linkage adjustments and everything appears to be in order as shown in the linkage tour. I adjusted the throttle/venturi so that it was fully closed/slightly binding when against its stop.

Afterwards, the car was barely able to idle and very close to stalling. This suggested to me that someone had been adjusting the throttle stop in order to tweak idle. Having the throttle correctly closed at idle now meant the engine wasn't getting enough air.

Adjusting the idle air adjustment screw on the manifold increased idle RPMs to around 1,600, so I have been trying to use the idle air/FIP idle fuel adjustment procedure as set out on this site.

The problem I'm finding is that CCW clicks on the FIP screw don't seem to be having a noticeable impact on reducing idle RPM.

Closing the manifold air idle screw clockwise reduces RPMs, as would be expected. Opening it again, increases RPMs, but opening it further doesn't seem to make the RPMs fade away again.

Overall it all seems to be running too rich, but I can't seem to reduce the idle fuel flow.

The FIP is on its idle stop.

Any ideas as to other things I should investigate?


Thanks

David

[Larry & Norma]

Is the cold start valve leaking, could introduce too much fuel.
Where are you in the UK?

[280sl1968]

I was just looking at the cold start valve as you type.

My car has the larger type CSV and with the small side screw removed and the fuel pump running, there are definitely drips of fuel coming out. The tech manual suggests this is a symptom of a leaking internal valve, so it is likely adding to the rich running problem.

I'll get that removed and look at it now as it obviously isn't helping.

My question is, would a leaky CSV be enough to cause it to run so rich that I see no reduction in idle RPMs at idle from 1,600 as I back off the FIP idle fuel screw?

(BTW, the car is currently in the North West - sadly haven't yet found any mechanics nearby that I trust with it)


Thanks for the input.


David

[Benz Dr.]

You need to also look at the auxilary air valve on the top side of the IP. Remove the small air filter and listen for air rushing in as the engine warms. It should shut off and no air should enter once it does. You can place your thumb over the hole and no vaccum should be felt. If you feel any vaccum, the slide valve isn't fully closing and you will need to add a shim under the push pin from the thermostat. If a large amount of vacuum is felt, you may need a new thermostat or the slide valve could be stuck.

Idle speed is a combination of fuel, air, and timming adjustments. Change any one of those and you may need to adjust the other two.

[ja17]

Good advice from Dan, I agree.

[stickandrudderman]

I posted a link to my YouTube video which demonstrates cold start air filter function on here a while back but I can't find it at the moment. It will help you see what others are talking about above.

[dante53]

I think that this is the right link http://m.youtube.com/watch?v=VXZwHvdlNTE

[garymand]

I think Larry found the first thing to fix: the CSV.  You can't get the idle correct with the extra gas enriching the mixture.  I've only fixed this once.  I tried renewing the surfaces with no success, but a new valve fixes the problem impressively. 

Nice video. It would be good to continue it as the WRD closes off the air.  Put your finger over the hole to block the air as it warms up, repeatedly block the hole as the motor warms up to hear the valve closing and finally that it is fully closed at about 80 degrees.  That is a great demonstration of how the WRD should work.

[stickandrudderman]

Occasionally there's some leakage passed a closed auxiliary air valve even when hot but as long as the leak is small it can be accommodated by adjustment of the idle air screw. Application of heavy grease to the slide will also help to seal it.
So, in this case at least, a little less than perfect is OK. The main objective is that the air slide actually moves with temperature but there's plenty of detailed info on this site that has all that covered.

[280sl1968]

Thanks for the replies everyone.

So, I pulled the CSV apart and cleaned it. Looks like that has resolved the small fuel leak, so I can hopefully eliminate that now.

The aux air valve could be heard drawing a lot of air on startup and as it warmed, the reducing air flow could be heard. After about 5 minutes running, there was still a little bit of vacuum to be felt when I put my thumb over it, but nothing huge.

All of this has left the car with a warm idle around 1,350 rpm according to the dash tach.

I attached a new tach/dwell meter today, and for some reason the 6 cylinder tach setting gives me a reading of around 380, which x10 to get the final rpm suggests 3,800 rpm at idle! Unless I'm completely missing something, there is no way it is idling at almost 4,000 rpm. The dash reading seems to make more sense.

The dwell angle setting reads 25.5 degrees.

The knurled FIP screw doesn't seem to be reducing the idle level to any significant amount with CCW turns.

Also, at the moment, the engine stalls when I put it in gear.

Something isn't right.....


EDIT: Rather than start a new thread, I'll also add this which I only noticed today. The distributor vacuum connector has no pipe attached, and the vacuum connector at the throttle manifold has been capped off with a rubber plug. Any reason why someone would have done this?

The car was in the shop for a long while about a year ago for emission test compliance 'adjustments' and the FIP was sent away for rebuilding at the same time. Is this vacuum pipe removal something they might have done at the time as a temporary/permanent mod?

I don't really understand the implications of this well enough to decide whether I should be getting this vac pipe reinstalled.

I adjusted the points gap and the dwell meter now reads 36 degrees.



David

[jameshoward]

David,

Why would a 1968 car require emission test adjustments? It's exempt emissions under the MOT, isn't it?

Assuming you have a stock distributor and are on points, you would need to have the correct installation - ie the small vacuum line - in order to set the timing properly because, depending on the distributor you have, it either retards (051) or advances the timing. Not sure what the implications are if you've moved away from points to something else, like a 123 dissy, but someone here will.

As for the tach meter, either it's screwed or you've got it set up wrong. On some they recommend that you attach the cable to the spark lead to a particular cylinder. Could that be it, perhaps?

You obviously don't turn the FIP idle adjustment screw with the engine running, but perhaps someone else has and has goosed it?

JH

[280sl1968]

Hi James

The car was with me in Canada until earlier this year and some of the provinces there include these old cars in the "air care" program. Ridiculous, I know.

I hope the FIP isn't screwed - as I say, it was all redone and the car has done very few miles on it since, so I'd be pretty disappointed if they gave me a duff unit.

I thought the tach meter setup was pretty straightforward - red to -ve on the coil and black to earth. Maybe I'm missing something.

The whole absence of the vac line is puzzling me. Just thought I'd ask here in case it might have been done deliberately to get it through the emissions.


David

[garymand]

I suspect you have a vacuum retard distributor and with no vacuum connected to it the idle will be high.  Without the vacuum, it is advanced too far at idle.  I also guess you had a plastic vacuum connection tube at the vacuum pot and an inexperienced mechanic broke it off.  It true, you will see a black plastic stub left on the pot where the tube used to be.  It that is the case, and you are industrious and mechanically inclined you can work a repair.  At the least you can run a vacuum hose from the throttle valve and hold it temporarily on the pot stem and see if you can get the vacuum pot to suck in and retard the disty, bringing the idle down.  If the pot works, I have drilled and tapped the stem.  It looks a little like a plastic pop rivet.  Use a thread tap and thread the hole.  Put corresponding threads on a 3/4 inch piece of 1/8 od copper tubing.  You probably get the idea.  If not pull the distributor and take it to a dealer and but a new pot.  

[Benz Dr.]

Not sure which Province the car came from in Canada but here in Ontario anything older than 1988 is exempt.  The 051 distributor has a metal vacuum cell and nothing can break off. The plastic vacuum hose will break off and maybe that's what you are looking at. You can get a replacement hose and and seal for it.

[280sl1968]

It was British Columbia - the AirCare fees go to fund the TransLink public transit system and those non-polluting diesel buses they have 

Anyway, here are some pictures of the vacuum port on the distributor and on the throttle manifold, along with the rubber cap found on the throttle port. It doesn't look as though anything has broken off on either side; they both look like short straight metal fittings that could take push-on rubber or plastic pipe.

The fact that there is a cap on the throttle manifold port makes me think this is no accident (although not restoring the vacuum connection later might have been??).








Appreciate everyone's help with this

David

[garymand]

You're fine.  A temp fix is run a vacuum hose between the two.  The correct setup uses a white plastic tube for the majority of the route and rubber hose for the last few inches at each end.  With out the vacuum connected, the motor will idle up around 1300rpm.  with vacuum, it will retard the advance and bring the idle down below 1000.  And start your tuning from there.

[280sl1968]

Thanks everyone - I was beginning to worry that I had bitten off more than I could chew with this one, particularly with the problems caused by previous 'repairs'. Things seemed to be getting worse instead of better.

I have fitted a temporary vacuum pipe between the manifold and distributor, and after that, the fine tuning of the fuel/air mix was much easier. I actually found from the split linkage test, with the vac line in place, it seemed to be running a little lean, probably as a result of my backing out the FIP screw to try and get RPMs down.

Anyhow, the car is running better than ever now and the gear changes are so much smoother.

I'm getting to grips with the whole timing, vacuum and retard principle, but I don't quite understand the relationship with idle RPMs. Can someone explain in simple terms why the missing vacuum hose was making my idle RPMs too high?

Also found as a result of this work, that putting the car in gear causes a significant drop in idle RPMs. I had someone watch the constant speed solenoid as I put it in gear, and there was apparently no movement at all. It's not so bad that it stalls but it needs attention.

I'm going to call the repair shop that did the emissions work on it last year to find out about that missing vacuum line. If they didn't remove it, they surely must have noticed it was gone. They purported to be MB oldtimer specialists too with rates to match, so I hope the explanation is good.....


David

[jameshoward]

It would help to know which model distributor you have (sorry if I've missed that). Vacuum does different things to different distributors. It should be printed on the plate on the body of the dissy.

You could read this: http://en.wikipedia.org/wiki/Ignition_timing

[280sl1968]

It's a 009 vacuum retard distributor.

I've read quite a bit of material on timing and the effect of the vac retard system etc, it I can't quite make the connection between losing that vacuum and stubbornly high idle RPMs I had.


David

[garymand]

The idle changes because the vacuum rotates the plat the points are sitting on.  Its like rotating the wghole distributor relative to the rotor.  you can use your hand and rotate the distributor a few degrees each way and see what it does to your idle./  That is the same as what the vacuum is doing.  Its adjusting your timing when your m,otor doesn't need the advancement.  High vacuum is at idle or down hill and the vacuum is used to move the spark closer to top dead center.  The mechanical advance moves the spark in advance of TDC for more power.  Briefly

[ja17]

Hello David,

Go back and double check to make sure that the venturi flap is closing all the way at idle. Disconnect the linkage to it and back off the 10mm lock nut til it starts to bind, then adjust just a bit so it does not bind. High idle is usually caused by an air leak when the venturi flap is open a bit.

Over-advancing timing at idle can cause the idle rpm to increase also, to a certain point. A "vacuum retard" distributor holds the timing "retarded" at idle, which results in  lower idle rpms. This only happens if the vacuum line is connected!  If the vacuum line is left off, no timing retard happens at idle and the rpms increase as a result. It sound like a tech has modified the timing from specs to compensate for some idle or timing problem.