weber conversion results-long post - PeachParts Mercedes-Benz Forum

bluebenz · #1 11-17-2007, 09:05 PM

Sorry for the long post!!! This great forum has helped me a lot in the past, and hopefully some of you can give me pointers so I can enjoy my car again! I spent a lot of money on a weber conversion and so far the results are not as good as expected, although it is an improvement!

I fell in love with the MB 114 when I was a kid, and I bought one off ebay last fall. A 1973 280C with 90000 miles. Great looking car and I enjoy it a lot although the past months have been frustrating at times!

When I got the car last year, I had a tune-up done: new NGK spark plugs, new plug wires, distributor cap, rotor and points (no condenser, couldn't find one), new air and fuel filters, oil and filter change, transmission oil and filter changed. I also replaced the battery, new battery cables. The previous owner had recently changed most belts and hoses, fuel pump and rebuilt the solex carb.

The car ran extremely well for 8 months, then some problems started: Very hard to start when cold, rough idle, stalling, sometimes hard to start when warm. First step was to remove a leaky fuel return valve (could not find a new one so just removed it) and that seemed to help a lot, at least temporarily. I then installed a crane XR-700 pointless ignition system with a new crane coil, and thought the problems were solved for good.

Unfortunately in september the cold start problem got worse and the stalling at intersections made me nervous. The solex carb was leaking, float was stuck, and after reading a lot on this forum I decided to get a new redline weber 38 dges conversion kit. Got the new carb installed by my trusted mechanic. Removed the egr and some other emission stuff at the same time. Timing was set after carb installation.

Now the car starts well when cold or warm, the idle is smooth, and it doesn't stall anymore. Car is powerful and smooth. The only problem is an annoying flat spot or hesitation on take off (the car does not move for 1-2 seconds when you press on the gas and then it goes) and a shorter hesitation when accelerating hard from a cruising speed. The only way to avoid this initial hesitation is to accelerate very slowly from 0 to 10 mph and then you're OK.

The weber web site says the fuel pressure should be 1.5 to 3.5, mine is 4.5 but I dont think it is causing my problem. I suppose I could install a pressure regulator but dont want to spend more money if it isn't necessary.

Hopefully we will be able to fine tune the carb (accelerator pump adjustment maybe?-could not find out how to adjust it on the weber web site) and enjoy this car again! I was wondering if anyone who had done this conversion had similar problems?

Thanks for your input!!!

cth350 · #2 11-18-2007, 06:59 PM

Did you keep the original fuel return setup with a vacuum controlled return valve?

Also you should have the fuel pressure match the specs. Aside from preserving the carb gaskets long term (not that only 1psi would cause a problem there), the various metered bores that the fuel flow through are calibrated based on assumptions madea bout operating conditions, including fuel pressure. -CTH

nhdoc · #3 11-18-2007, 07:11 PM

Has it always had the flat spot since the weber was installed? Typically flat spots like that are a sign that the accelerator pump is not functioning. You should be able to hear liquid gas squirt with the engine off and the throttle opened if you remove the air cleaner. It is possible you got a defective carb or the pump is clogged. It should not be too hard to determine if the accelerator pump is working and if not to replace it.

I suppose it could also be a vacuum leak if the pump turns out to be working. Could also be timing or the timing advance not working right but assuming all was well before the weber was installed I'd start with that being the culpret.

It's a very nice looking car...love the color too!

bluebenz · #4 11-18-2007, 07:59 PM

First of all thank you both for your replies!

To cth 350: I didn't keep the original fuel return setup, figured that since the main function of the return valve was to prevent vapor lock on hot days and were I live in Canada we unfortunately don't have many hot days, I would be OK, so just plugged that line. And I will get a weber or holley pressure regulator, only 20-30$, and at least everything will be as specified.

To nhdoc: Yes, ever since the weber was installed I have had the hesitation, a problem I didn't have before! The weber did solve the hard starts, rough idle and stalling issues though, but the hesitation on take off makes driving it in traffic very dangerous!!! The hesitation when you accelerate hard from a cruising speed is very subtle though.

I suspected a vaccum leak too, but everything looks good under the hood, although didn't measure the vaccum. Timing was just reset after weber installation. Mixture has been adjusted too, was a bit too lean, which got rid of the rare backfires. Idle speed has also been adjusted and seems fine. Other than that it seems to run fine, no smoke, oil pressure very good, running temperature very good, no strange noises, good power, etc..

I will check if I can hear fuel squirting when I press the throtlle (engine off but warm I imagine?)

I did email weber redline again, maybe there is a way to adjust the accelerator pump! Will keep you posted!

Again I am very grateful to have found this forum!

Patrick

nhdoc · #5 11-18-2007, 10:00 PM

Yes, listen for the fuel squirt with the engine off...doesn't matter if it is hot or cold but you probably should remove the air cleaner to listen in the throat. I don't think the pump is adjustable...most consist of a diaphragm that squirts fuel and if they are stuck or torn they just don't work...you simply replace the diaphragm as a unit. My bet would be that it is the problem.

You might also confirm the vacuum advance is plumbed correctly and there's vacuum from it and that the advance itself is working. On my '64 there were two vacuum ports on the carbs...the one which produced vacuum with the throttle open was the one the vacuum advance was supposed to attach to not the one which has vacuum on it at idle so you might want to check that too because at some point they changed the design. I believe they call it "ported" vs "manifold" vacuum and mine used the ported source but later ones used the manifold source. Your tech may have plugged it into the ported when it should be manifold vacuum.

MunichTaxi · #6 11-19-2007, 12:31 AM

You might also confirm the vacuum advance is plumbed correctly

How does the timing advance look with a timing light?

nhdoc · #7 11-19-2007, 06:50 AM

Quote:

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 · #8 11-19-2007, 11:37 AM

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 · #9 11-19-2007, 12:05 PM

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

MunichTaxi · #10 11-19-2007, 09:03 PM

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 · #11 11-20-2007, 07:33 PM

Quote:

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 · #12 11-22-2007, 09:45 PM

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.

dquance · #13 12-04-2007, 02:41 AM

Although I know nothing about weber carbs, might I suggest you install the Pertronix ignition and flame thrower coil to insure you have the best spark possible. Especially if you plan to keep the car.

Doug Q

t walgamuth · #14 12-04-2007, 08:03 AM

Quote:

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

bluebenz · #15 12-04-2007, 08:51 AM

Quote:

Originally Posted by dquance

Although I know nothing about weber carbs, might I suggest you install the Pertronix ignition and flame thrower coil to insure you have the best spark possible. Especially if you plan to keep the car.

Doug Q

I did install a crane XR700 and a crane PS40 coil in september, and that made a huge improvement in driveability, although stalling and hard starts reappeared in october. However I think a crane (or petronix) ignition system is a very good investment!

Thank you Tom for your suggestions, I will look in to it next spring, when the weather is nice enough to put the car back on the road! And I did order a pressure regulator, so everything will be as specified by weber!