W115 rough idle - PeachParts Mercedes-Benz Forum

nachi11744 · #1 11-22-2003, 09:00 PM

Hello,
The car is a 1976 W115 200 gasoline with the Stromberg 175CDT carb. I have a new carb on it and I have checked the ignition system which is working fine.
The symptoms are:
1. Rough idle after running at 40-60 mph, VERY rough idle after running at 70-90 mph. If it is left to idle for 4-5 minutes, it eventually smooths out.
2. Normal idle if it runs around in commuter traffic around 20-30 mph.
It pulls away from a stoplite without any problem, but there is some snatching in the driveline at constant throttle.

I have checked for air leaks, there are none.

I am suspecting the fuel line,fuel pump or the fuel pump drive cam that is on the oil pump. I know that the drive cam wear was an issue on carb engine BMW of the 1980s, but has anyone experienced this on any Mercedes engine?
This has been driving me nuts, anybody here have any ideas or has encountered this before?

300SDog · #2 11-22-2003, 11:11 PM

Based on what you're saying, typical MB culprit is sticking distributor advance that gets held wide open on high speed runs. Search 'distributor advance' here...... this is common problem. How many miles on engine??

nachi11744 · #3 11-23-2003, 07:23 PM

Hello,
The engine is about 40,000 miles since rebuild, but the distributor is only about 2 years old, the old unit was shot and a new Bosch replacement disributor was fitted. It is the corrrect part# and all that. Nonetheless, I will check the timing when it is running rough, this unit could have seized as the car was driven very little in those two years before I acquired it.
Has anyone encountered a blocked fuel line on W114/115 cars?
Thanks again.

Tomguy · #4 11-24-2003, 12:20 AM

The mechanical advance absolutely NEEDS oil to keep it working properly (I found this out the hard way). If it hasn't been oiled in over 3 months, it's overdue! Pull the rotor and put a drop or 2 (or more, but don't drench) on the pad below the rotor (in the shaft). Work it back and forth - it should be able to be turned clockwise (advanced) a bit without too much difficulty, and when let go, should automatically spring back all the way (or at least near all the way) counterclockwise. If you can't turn it CW, try to turn it counterclockwise, that means it was stuck wide open.

psfred · #5 11-24-2003, 07:04 PM

Think plugged fuel filters or tank screen.

Peter

nachi11744 · #6 12-04-2003, 02:30 AM

Hello,
Update on the M115/W115 rough idle saga.

1. Checked distributor, no sticking centrifugal or vacuum advance.

2. Checked fuel tank screen, fuel lines and inline filter, all are clear.

3. Checked fuel pump delivery AT IDLE, OK up to specs.

4. Finally, I took the carb apart and reset as well as realigned the floats(someone had mangled the alignment of the two semi floats to cure flooding, a new float valve cured it easily), with the float level at 15-16mm, the result is that:

a)at normal speeds from around 40-70mph there is no more driveline snatch and stopping at stoplites after running at 60mph does NOT result in rough idle BUT if stopping after runnng at 80-90mph result in the same rough, misfiring idle.

b)The same thing happens after running at 60mph or so in slightly cooler, wet weather(if you can call 78F cooler than the daily 86-90F !), there is distinct miss firing and rough idling for a few minutes that sort of smooths out after 5-8 minutes, but the car will pull away from a stop without any snatching even if the idle is poor.

Is it possible that the vacuum controlled fuel return valve is returning too much fuel and reducing the volume of fuel in the float bowl? I have never come across such persistent incurable poor running in a carb fed engine.
Thanks and have a good week.

neileg · #7 12-04-2003, 05:30 AM

Could it be fuel vapourisation? A high speed run in already high ambient temperatures could see some real heat under the hood.

bobboyer5 · #8 12-04-2003, 10:26 AM

Unless done already, I would replace and properly clamp new fuel line hoses throughout system, and go for the new fuel filter.

Can't hurt.

psfred · #9 12-04-2003, 06:17 PM

Sounds like you are going lean at hot condtions. You may have to up the idle mixture a bit, to keep it from going too lean.

The Stromberg is a rather complicated carb, it's a variable venturi type, like the Holley 2700 series, but earlier and not as reliable.

Getting it to work properly under all operating conditions probably isn't actually possible. I'd let it run a bit rich cold rather than have it lean out at idle hot.

Peter

nachi11744 · #10 12-04-2003, 07:51 PM

Hello,
Thank you for the replies.

I have already replaced all the 26 year old rubber fuel lines and blown out the metal lines to make sure there is no blockage anywhere.

I am suspecting two areas now:

1. The fuel pump drive cam may be worn out and causing the fuel volume in the carb float bowl to drop far enough to cause a lean out at idle after running at high speed.

2. The vacuum fuel return valve is overenthusiastic and returning too much fuel at speed, though the car easily pulls 90-95mph with the a/c engaged in 90-105F, with engine temp holding around 90C. I am thinking of rigging up a vapour separator canister(Solex carbs in the 1990s had these) and disable the vac return valve, this has a simple restriction in the fuel return line, just to see what happens.

As Peter suggested, I have it running slightly rich and even when the idle is rough, the car will pull away from a stoplite without any problem, but thats because the Stromberg uses only one jet for ALL operating conditions. I have always been able to sort out carbs, even the Solex 4A1 is no real mystery to me, but this one has me stumped.
Under 85-90% of my driving conditions, the car more than impresses passengers with it's glass smooth idle, good performance and of course the ability to smooth out the worst of badly worn out and potholed urban roads, but this poor idle bit has really got me.

psfred · #11 12-04-2003, 09:25 PM

Are you using motor oil or ATF in the hydraulic damper? If you are using ATF, it may be too thin.

Just a thought, I've never had the "privilege" of fighting one of these. Another though is a stuck heat riser if you have one (I don't remember if there is on on this application).

I will check with my friend Hans, though, just in case he knows some trick.

When the work, they work very well. Ditto for the 2700 variable venturi Holleys. They didn't work well enough to stand up to electronic fuel injection, though!

Peter

nachi11744 · #12 12-05-2003, 12:34 AM

Hello,
I would appreciate any input, Peter, thank you. Please do ask your friend Hans if there are any tricks(!).

At all other times, it runs almost like a FI car with a smooth idle, no sags under acceleration and it can pull 4th gear WOT from 25mph on a flat road without any snatching.

I am using motor oil, 20w in the damper. With ATF, the acceleration is poor, what with the 100% humidity and almost 100F weather here in the tropics.

This application does have a heat riser, it is *non operational* as in it is stuck somewhere, I cannot ascertain if it is on full heat, or fully closed or somewhere in between!

Have a good weekend

autozen · #13 12-05-2003, 10:52 AM

Nachi,

I have never seen the problem you describe and I am very familiar with this carb. the 175 Constant Depression Stromberg is about as close to fuel injection as a carb will ever get because of its tapered fuel needle and ability to constantly adjust the air flow. I can't see any fuel delivery problem, because of the high speed performance you describe. I would bet on vapor lock. You feel that the return is returning too much fuel, but if it did that, you wouldn't get the performance you are getting. I'm wondering just the opposite. What if the vacuum operated return isn't returning enough fuel causing the fuel under the hood area to get too hot? It has been years since I worked on those carbs although I still have some NOS parts for them. Also there is a special small diameter hose on the carb. I can't remember what it is for. You have me curious now. I'll dig out my old factory M115 book and see if I get any new thoughts.

Good luck,
Peter

autozen · #14 12-05-2003, 10:59 AM

Nachi,

One more thought. I believe that engine has a vacuum booster pump for the brakes. It is possible to suck oil into the engine at high speeds because of a torn diaphragm whoch could cause rough idle until the oil burns out of the cylinders and you won't always see smoke. This might be a stretch, but hey, we are grabbing at straws here now.

Peter

nachi11744 · #15 12-05-2003, 08:10 PM

Hello,

There is no vacuum pump on this model, it is a W115.015, a 200 RHD.

The small diameter hose would be the vacuum hose for the fuel retun valve's suction tube from carb mounting flange ?

I have done simple tuning and cleaning stuff on the Stromberg 175 before because I had a Volvo 240GL (here the 240s did not get FI until 1988-89) so I do know that it is not a *troublesome* carb. In fact, it is much more user friendly than ANY German Solex carb , and I ought to know, having had to work on 32/32TDID, 35PDSI and the infamous 4A1.

I guess I am clutching at straws.

After raising the float level from the specified 16.5mm to 15mm, it only does the unstable idle bit after running at 70-80mph for a few minutes. If it is revved hard in the lower gears, eg. 3rd. gear up to 60mph to climb a hill, there is no rough idle, where is used to be very unstable before.

I am thinking along the lines of the fuel in the float bowl boiling, but the carb has an insulator, a rubber flange and a nasty little heat deflector shield! Perhaps I need one of those nifty fuel coolers that are installed on the W124/140s, that ought to stop any vaporisation.

Psfred mentioned the intake hot spot, I had a look at it, it is jammed in a position where the slot in the back(spiral spring end) of the shaft is horizontal and from what I can make out from the W114/115 CD and parts diagrams, that would place the flap in the *closed*(no heat) position.

Thanks again for the useful insights.
Have a good weekend.