1972 250/8 W114.001 130 Engine Zenith to Weber JAMENG C501 Kit Conversion - PeachParts Mercedes-Benz Forum

tom&jen · #1 01-13-2008, 01:13 AM

We've taken the plunge and decided to convert from Zenith carbs to Weber carbs and have gone with the JAMENG Kit C501. For those familiar with the kit's installation instructions, we have reached Step 21 and been successful in starting the engine and running my car for a short while. We are now at the tuning stage and assistance is being sought for this aspect.

Apologies for the long post, but some background may assist. This has been a long term project, with some personal health issues getting in the way. My Merc has been off the road for some time, with the original issue being a cracked cyclinder head. The head was replaced and it was static timed to TDC. The car has not been on the road since then.

The Zeniths have always been a problem on the Merc even when it was on the road and ultimately required rebuilding. It was unfortunate that we hadn't found this website prior to letting a mechanic loose on the Zeniths (for tuning) and the problems started from there. Even a carb kit and rebuilding didn't do the job. We decided that the best way forward was to go with a Weber conversion and the JAMENG C501 kit was decided upon. (We were originally thinking down this conversion path in 2003!!)

We have searched this website looking at various posts and threads and also sought additional material where available - 'The Driveway Mechanic: Zenith to Weber Carburetor Conversion' article and the 'Carburetor Set Up and Lean best Idle Adjustment' article off the Redline web-site, prior to starting the conversion.

In some way, we're looking for confirmation of a couple of things and advice from anyone who has gone down a similar conversion path. Step 11 deals with installing the connecting link between the two bellcranks - this part is reused off the Zenith carbs, with the length of this link being adjusted so that it is identical to the center to center distance between the bellcrank pivots on the Webers, to ensure identical throttle action in both carburetors. Our distance is 7 3/8 inches and this is spot on as per instructions.

Step 12 installed the two short curburetor pushrods ( 1 3/4 inches) such that the lower arm of the bellcranks points between the 4 and 5 o'clock position. The lower arm points in that position.

My first confirmation is at Step 13 - it is here that the linkage rod supplied with the kit is adjusted to 5 3/4 inches center to center and this replaces the 'stock front pushrod in the throttle linkage'. The first check is made to see that 'both carburetors open fully when the accelerator pedal is fully depressed and that it is possible to depress the accelerator pedal sufficiently to engage the electric kickdown switch located behind it.' We believe that we are engaging with the kickdown switch and hence that further adjustment of the throttle linkage / accelerator linkage (intermediate rod & transmission control pressure pull rod) isn't necessary as per suggested remediation at Step 13 and 15 if that is the case. However, what is meant by 'fully open'? We get travel of the butterfly valves when the accelerator is pressed to the floor (photo 7 below), but certainly not the same as when the carbs are operated by manually pushing the rods (photo 6). Are our carbs behaving as they should at this stage? Any comments would be welcomed. The Driveway Mechanic article emphasises checking to make sure that the throttle travel is OK by pressing down on the accelerator and watching to see that the carburettor throttles opened fully. In their instance, this happened, and none of the suggested adjustments (identified above) were necessary.

Photo 8 below shows the carbs when there is no accelerator being applied and the engine is off.

Photo 9 shows the carbs and butterfly valves in their 'stopped position', we got the car started only through priming them with gasoline and run it for a short while.

We've assumed that everything is OK at this point and the next part is tuning. Would it be appropriate to use the ‘Redline Weber Carburetor Set Up and Lean Best Idle Adjustment’ tuning instructions, given we have a JAMENG kit? What approaches have others taken?

An album with a couple of additional photos and accompanying text for the photos below has been created at webshots.com

t walgamuth · #2 01-13-2008, 08:03 AM

When the foot is off the gas pedal the butterflies should be fully closed. When the foot is to the floor they should be fully open. If the carb plates do not shut at rest the idle circuits on the carburator will not work.

Don't fiddle with any mixture adjustments until you have the linkages sorted out so that they will do as stated above.

The mixture adjustments are probably set at the factory and are probably very very close to what is needed, so fiddling with them before all other items will result in getting it good and fouled up.

(a carburator in principal is a pretty simple device, flow air through it and it will mix in fuel with it at about 14 to one ratio, so if you take a carb and (somehow) manage to adapt it to a cadillac (I have done this, actually) it will run fine until the little carburator runs short of total volumn to provide what the bigger engine needs. The car will feel like you have leveled off on the gas pedal but it will run just fine up to about half the normal operating range)

So please get the linkages sorted before adjusting any mixture screws.

Also, once you get it running more or less normally be sure the vacuum has no leaks and the ignition is tip top before fiddling with any mixture screws too.

BTW....nice looking setup!

Tom W

Chas H · #3 01-13-2008, 11:49 AM

The balance of the carburettors must be set with a synchronizing device such as a "uni-syn". This is done at idle with the linkage disconnected, since this is the speed most affected by balance. Until you have done this any other adjustments are wasted.
I believe you have confused the choke plates with the throttle. The plates at the top of the carb are the choke plates. The throttle plates are close to the bottom of the carb. Don't worry about full throttle until you have a good idle and drivability.

MunichTaxi · #4 01-14-2008, 12:20 AM

is the adapter for the CarbSync device to the carb throats.
I haven't used the empty Cool Whip plastic containers (yet), but I understand they fit the carb throat tightly for a good seal.

The idea is to get both carbs pulling equal vacuum thru the gauge at idle ( with the linkage dis-connected between the carbs).

Then set the linkage for both carbs to open together as rpm's increase.

JimFreeh · #5 01-15-2008, 06:25 AM

Quote:

Originally Posted by MunichTaxi

is the adapter for the CarbSync device to the carb throats.
I haven't used the empty Cool Whip plastic containers (yet), but I understand they fit the carb throat tightly for a good seal.

I have.

Works great on my 71 250C with the webers and JAM kit.

Jim

230/8 · #6 01-15-2008, 12:57 PM

As Tom said, sort the linkage first to ensure this basic system functions. You should be able to lengthen the front link, this is the one that reverse pivots the foot pedal motion back to the carbs. Lengthening this link will, or should raise the foot pedal in the car so that you have greater travel. Get the carbs to seat fully at rest and open fully at full throttle. When you remove the linkage at the carb, there should be no more movement available in the throttle, either at rest (idle) or at full throttle.

You should be able to use the Redline lean drop method...same carbs. This is a crude early smog control adjustment technique that might work well for you, but it might not, depends on your engine. If you get a stumble off-idle, you may need to richen the idle mixture slightly if all other adjustments are on specification.

The project you are engaged in opens up all the potentials for small niggling bits to be a problem...vacuum leaks, timing off slightly, heat flaps stuck, valve adjustment not optimum, linkages less than optimum...you get the idea. Never assume anything is right without personally checking it against specifications, and ensure you have a good understanding of auto mechanic fundamentals. Each of those small problem areas can lead you down a rabbit hole because of the similarities they share in symptoms...a vacuum leak can mimic bad timing, which can mimic a poorly adjusted fuel mixture, which can mimic out of balance, and so on. In this respect a vacuum gauge can be a good trouble-shooting friend, but its usefulness depends on the aforementioned mechanic-ing skills.

Good luck, I'm sure you will be happy when done. Final thought, you mentioned troubles having begun when your mechanic was given the task to deal with your Zeniths. This tends to support my thesis that most problems are related to poor mechanics than to bad Zeniths. Mine perform without problems, as do those of many others who have a well-developed understanding of their basic function.

Again, good luck,

230/8

p.s.: Whatcha going to do with those old Zeniths?

todds · #7 01-16-2008, 12:18 PM

What did you do about the internal transmission switch on the front zenith that is missing from the webers? Did your car have one?

tom&jen · #8 01-23-2008, 05:45 AM

Thanks todds for your comment on the internal transmission switch. We don't believe that our car had one on the front zenith but your comment is sending Dad back to the Zenith to have a look and also to the Chiltern's book and the Jamiekop carb manual (downloaded off the web) that we have.

tom&jen · #9 01-23-2008, 05:57 AM

Thanks Tom, for all the advice especially about getting the linkages right before touching anything. The JAMENG Step 13 check is 'to see that both carburetors open fully when the accelerator pedal is fully depressed'. We have assumed that this related to the choke plates, which is what I called 'butterfly valves' in my original post. When the foot is off the gas pedal the choke plates (butterflies) are fully closed. As per photograph 7 in my original post, when the foot is to the floor the choke plates are not fully open (if 'fully open' is defined as being in the vertical position). Instead they open approximately 1/4 inch when looking at the plate's movement in relation to the rear carby housing. Hence, my original concern and checking as to what 'fully open' meant. Are we 'fully open' in this position? The linkages have been set with the measurements as per the instructions from JAMENG. I tried to illustrate the carb operations we're seeing through the various photographs. We have rechecked these measurements and they are exact in accordance with the instructions.

tom&jen · #10 01-23-2008, 06:06 AM

Thanks Chas H, wrong terminology on my part (the jen part of the tom&jen team) when I called them butterflies in my original post. I was however referring to the choke plates and I've corrected this in subsequent posts. Your advice on the balancing technique is certainly appreciated.

Fortunately, from other advice on this site, we purchased from the USA a Synchrometer Carburetor Flow Meter for use with the Zeniths - so we are certainly looking forward to making use of it.

tom&jen · #11 01-23-2008, 06:14 AM

Quote:

Originally Posted by MunichTaxi

is the adapter for the CarbSync device to the carb throats.
I haven't used the empty Cool Whip plastic containers (yet), but I understand they fit the carb throat tightly for a good seal.

The idea is to get both carbs pulling equal vacuum thru the gauge at idle ( with the linkage dis-connected between the carbs).

Then set the linkage for both carbs to open together as rpm's increase.

Thanks for the advice on using the CarbSync device. As mentioned, we purchased the 'Synchrometer' Carburetor Flow Meter for the USA and seeing posts on the site about the Cool Whip plastic containers (we don't have this brand in Australia) but we found a substitute and made two perfect fits that fit both the Zenith and the Webers. Dad did a fantastic job (if I can say so myself). I'll be posting a photograph of what he made - unfortunately the one he sent me (showing it in situ on the Webers) doesn't do justice to it - so I'm waiting on getting another one in a couple of days.

By the way, we actually live in two different states in Australia, separated by about 450 or so miles.

cth350 · #12 01-23-2008, 07:49 AM

Not sure how I missed that chassis number when I first read the thread. A W114.001 is a stretched sedan (more precisely a "special body"). That long car should make the 450 mile span feel a little shorter. I have the W123 stretch but would dearly love one of the W114 stretch cars too.

-CTH

todds · #13 01-23-2008, 10:33 AM

Since yours is a '72 it might not have the 4spd fluid coupling type transmission, in which case you wouldn't have the switch either. If you did have it, there would be two black wires running to the (original) front carb and some screw terminal attachment points on a built-in throttle switch which is supposed to close only at idle and open again at around 1200rpms. It's not a kickdown switch, which is located on the accel pedal inside the car. Instead it does something to lessen the creep at idle and smooth the upshifts when decellerating, particularly into first. I think maybe it affects the fluid coupling somehow. I had a caddy with a similar switch that would alter the angle of the torque converter blades, a so-called "switch-pitch". At any rate, if you have that type of transmission it'd be imperative to have a switch of some kind, just check your old carbs.

230/8 · #14 01-23-2008, 12:45 PM

I believe the carb switch was used on the K4A series transmissions, your car uses a K4C...

230/8