Carburettor experts please help
Discussion
I need answers to a few problems with my car which I think all relate to the Stromberg 175CDT carb on my Mercedes w123.
The first is as follow:
There is a idle shut off valve attached to the bottom of the carb. Turning this also adjust the air/fuel mixture. There is a very minor leak from where the mixture adjustment attaches to the carb.
Looking at parts diagrams show 2 o rings on the valve so I assume it is those that are failing.
First question... When I unscrew the idle shut off valve assembly and remove it will petrol pee out the bottom of the carb?
The first is as follow:
There is a idle shut off valve attached to the bottom of the carb. Turning this also adjust the air/fuel mixture. There is a very minor leak from where the mixture adjustment attaches to the carb.
Looking at parts diagrams show 2 o rings on the valve so I assume it is those that are failing.
First question... When I unscrew the idle shut off valve assembly and remove it will petrol pee out the bottom of the carb?
OK, so I fixed the leak by replacing the o rings.
The next issue is whether the following is a carb problem or something else.
Even before the leak, the car has a very mild on/off hesitation at about 1800-2000 rpm.
This doesn't happen when accelerating.
It's only present when on a trailing throttle and only in that rev range.
If richen the carb fuel mixture, I can get it to stop. Doing this has the side effect of a lumpy idle and fuel smell though. The car sometimes runs on as well and I think this happens when it's running too rich.
So I can lean out the mixture and get an absolutely perfect idle but end up with the hesitation as above.
Is this likely to be a carb issue or ignition timing. If the latter, which way does the timing need to go?
All the ignition components are around a year old.
Any ideas as it's doing my head in.
The next issue is whether the following is a carb problem or something else.
Even before the leak, the car has a very mild on/off hesitation at about 1800-2000 rpm.
This doesn't happen when accelerating.
It's only present when on a trailing throttle and only in that rev range.
If richen the carb fuel mixture, I can get it to stop. Doing this has the side effect of a lumpy idle and fuel smell though. The car sometimes runs on as well and I think this happens when it's running too rich.
So I can lean out the mixture and get an absolutely perfect idle but end up with the hesitation as above.
Is this likely to be a carb issue or ignition timing. If the latter, which way does the timing need to go?
All the ignition components are around a year old.
Any ideas as it's doing my head in.
If the carb is old enough to perished O rings in one bit, then the rest of it is probably worn out too. Get a full rebuild kit (they are not expensive), remove it, strip it, clean it, rebuild it. Things like float height, leaky gaskets, or even completely missing jets (vibrated and fallen out, or even not replaced by previous carb fiddlers...) are normal on old carbs..........
There’s only one jet on a Stromberg like that, it could well be worn as could the needle but sourcing a good replacement might be tricky. The size of the jet and profile of the taper of the needle will vary to engine spec and has to be dead right. And even then it may not sort your symptom.
You could try tweaking the timing in either direction to see how it reacts, advance is probably the most likely to help, but you may well find that much like the mixture adjustments you can gain in one area while losing in another.
The issue might be that we have got sufficiently used to cars that run pretty perfectly we have forgotten that they didn’t always go that well, what you are describing sounds to me like the kind of thing you live with rather than the kind of thing you try to fix................
You could try tweaking the timing in either direction to see how it reacts, advance is probably the most likely to help, but you may well find that much like the mixture adjustments you can gain in one area while losing in another.
The issue might be that we have got sufficiently used to cars that run pretty perfectly we have forgotten that they didn’t always go that well, what you are describing sounds to me like the kind of thing you live with rather than the kind of thing you try to fix................
I changed the flange and gaskets some months ago.
The vacuum pipes are plastic which terminate with rubber pipe. I've changed all the joints apart from some that have multiple outlets. I ordered replacements for those last week. I maybe look at replacing the actual plastic pipes too as I'm sure they'll be brittle and past their best.
One other symptom. When I first got the car, starting it required no throttle at all. It would just fire up immediately with the choke engaged and rev to 1200rpm as it should. It ran very smooth and the choke would disengage after a few minutes and settle to a steady 800rpm idle.
Now it needs some throttle input when starting and will run a bit rough until warmed up. I can't remember what I messed with to cause this.
The vacuum pipes are plastic which terminate with rubber pipe. I've changed all the joints apart from some that have multiple outlets. I ordered replacements for those last week. I maybe look at replacing the actual plastic pipes too as I'm sure they'll be brittle and past their best.
One other symptom. When I first got the car, starting it required no throttle at all. It would just fire up immediately with the choke engaged and rev to 1200rpm as it should. It ran very smooth and the choke would disengage after a few minutes and settle to a steady 800rpm idle.
Now it needs some throttle input when starting and will run a bit rough until warmed up. I can't remember what I messed with to cause this.
buggalugs said:
Have you got a strobe you could check the timing with? Check if it’s steady or jumping around etc
Yep I've checked timing. I have one of those lights that you set the advance on and then adjusted timing to TDC mark. It should be 13 (+ or - 3). I was told that it should be retarded slightly due to lower octane fuel so it's been set to 11.I've just been out gradually altering the timing and at 14.5, the symptoms have just about gone.
The timing mark does not appear completely static and moves about very, very slightly. Is that not normal?
Engine only has 106000 miles.
GreenV8S said:
I assume it's got a mechanical distributor with an external vacuum advance canister - have you removed that at any point? If so, check the pull rod is still mechanically connected to the advance mechanism; if it's come unhooked, you'll have random timing.
Yup, it has vacuum advance, but I haven't touched that. Rollin said:
buggalugs said:
Have you got a strobe you could check the timing with? Check if it’s steady or jumping around etc
Yep I've checked timing. I have one of those lights that you set the advance on and then adjusted timing to TDC mark. It should be 13 (+ or - 3). I was told that it should be retarded slightly due to lower octane fuel so it's been set to 11.I've just been out gradually altering the timing and at 14.5, the symptoms have just about gone.
The timing mark does not appear completely static and moves about very, very slightly. Is that not normal?
Engine only has 106000 miles.
Rollin said:
Just capped off pipe to vacuum advance and now idles like a sewing machine. Bogs down slightly when pulling away and hesitation as described above is worse.
Vac advance should come in just as you go on the throttle, excessive advance will make the idle lumpy. The pickup pipe for the advance should be just on the airfilter side of the butterfly when it’s at idle so it gets manifold vac as soon as you go on the throttle, if the throttle stop is a bit too far open it will get vac advance at idle as well.
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