weber conversion results-long post

[MunichTaxi]

You might also confirm the vacuum advance is plumbed correctly

How does the timing advance look with a timing light?

[nhdoc]

Originally Posted by MunichTaxi

You might also confirm the vacuum advance is plumbed correctly

How does the timing advance look with a timing light?

There are two types; vacuum and centrifical. I check the vacuum advance by using a mityvac and drawing a vacuum on the line while watching the timing mark at idle with the timing light. If the timing mark advances under vacuum and returns when the vacuum is released it is working. You'll also be able to hear the engine's speed change when it advances. It should also hold vacuum pretty well...doesn't have to be perfect but should not leak down fast or require constant pumping on the mityvac to hold it. If it leaks too fast it usually means the diaphragm has a small hole and should be replaced.

Then, with the vacuum advance disconnected I rev the engine and watch the timing mark. If it advances then the centrifical advance is working...if not then you know you have to look at it and see why it is not working. The shop manual will tell you at what engine speeds the advance should be in degrees and there is usually a curve they follow which you can use as a reference. It does not have to be perfect but should be working.

[bluebenz]

Thank you Marty!

I removed the air cleaner and moved the throttle, could smell gas but didn't hear any squirting, so it might very well be an accelerator pump problem, like my mechanic was suspecting. Waiting for weber redline reply to my email.

I checked the vacuum ports. On the new weber,there are 2 ports: vacuum advance port and another one called manifold ported vacuum (vacuum retard). I have one connection going from the distributor to the vacuum advance port, but nothing plugs in the manifold ported vacuum.

[bluebenz]

Just thought I would post a few pictures to show you what it looks like:

[MunichTaxi]

has 1 port, then it is the vacuum advance port.
If you don't have the 2nd port on the distributor, simply plug the Carburetor retard port.

That is what I did on my Weber install.

[bluebenz]

Originally Posted by MunichTaxi

has 1 port, then it is the vacuum advance port.
If you don't have the 2nd port on the distributor, simply plug the Carburetor retard port.

That is what I did on my Weber install.

That's the way it is set up now. Hopefully with a little fine tuning I'll be back on the road!

[Kimo]

It's been a few years and I don't remember the details but a few years ago I had a 1969 250S with the Weber carb conversion. I also had a "flat-spot" problem like what you describe. I discovered through a lot of trial and error that if I adjusted the idle mixture a little rich, the flat spot would smooth out. I also discovered that I would not pass emission tests with the carbs running rich so on inspection day I would lean out the idle screw.

[t walgamuth]

Originally Posted by nhdoc

There are two types; vacuum and centrifical. I check the vacuum advance by using a mityvac and drawing a vacuum on the line while watching the timing mark at idle with the timing light. If the timing mark advances under vacuum and returns when the vacuum is released it is working. You'll also be able to hear the engine's speed change when it advances. It should also hold vacuum pretty well...doesn't have to be perfect but should not leak down fast or require constant pumping on the mityvac to hold it. If it leaks too fast it usually means the diaphragm has a small hole and should be replaced.

Then, with the vacuum advance disconnected I rev the engine and watch the timing mark. If it advances then the centrifical advance is working...if not then you know you have to look at it and see why it is not working. The shop manual will tell you at what engine speeds the advance should be in degrees and there is usually a curve they follow which you can use as a reference. It does not have to be perfect but should be working.

I agree with everything nhdoc says but will add two subtle different ideas. First, most carbs in my experience have three positions on the accle pump. There is usually a linkage and a lever and three holes to choose from. I would expect it to come from the factory in the leanest or the middle hole. You want to move the linkage so that it makes more movement on the pump side with your throttle movement, so you would want to shorten the lever on the pump side if I am picturing it correctly.

Secondly, I always remove the ac and just look down in the throat to actually observe the fuel stream from the accel pump. It is thicker than a hair but not as big as a pencil lead. You should be able to see it clearly.

The accel pump could be worn out prematurely if the carb sat on the shelf too long or if it happened to have a fold in the seal (depending on the design, some are diaphram as nhdoc said and some are a plunger design.) I have a weber book so if you have specific questions I can look up your carb family and offer specific advice if needed.

Oh, yes, another way to check the vac advance is to take off the dist cap and suck on the vac line going to the dist. You should be able to observe the top plate moving back and forth (rotating).

It is funny to me to think that all these little tests are completely unknown to a person born 20 years later when carbs were virtually non existant. When I started fooling around with cars, only Indy cars had fuel injection! (And some of them still ran carbs, I think).

Carburators are so very simple in concept but in actual working practice they are fiendishly complex devices and will give problems sometimes if there is just a little spec of dirt in the wrong place.

Tom W

[Rick76]

If I recall correctly, the early North American M110 (73 to 76) had only vacuum retard. Look around for a European or 77 and later US distributor.

[t walgamuth]

Some cars actually had a vac advance and vac retard, my 84 bmw did. I think the mbs were set up differently though, as noted above.

The bmw would never run properly with correct power til I modified the distributor to give pre emission controls performance.

Tom W

[bluebenz]

Spring has finally arrived here in Canada, time to put the car back on the road.

I installed a fuel pressure regulator in january (fuel pressure is now as required by weber redline) and after speaking with a weber redline representative I had the timing set to 10 degrees BTDC and now it finally runs well!!!

I'm not crazy about the gas pedal feel (not progressive, almost like an on-off switch) but I think I'll be able to fix that.

Thank you all for your input!

[t walgamuth]

Sometimes there is a very fine adjustment of the throttle plate that covers or uncovers ports which direct air to different places. So the fine adjustment might need another look. There should be complete instructions on it. Off throttle response is also related to the accelerator pump.

Tom W