Zenith carbs follow-up - making progress?? - Page 2 - PeachParts Mercedes-Benz Forum

ctaylor738 · #16 04-10-2003, 01:03 PM

NOT!

It should hold vacuum. So that's a small leak. But what it means is that you are not getting advance under load, which could cause a flat spot.

We need to get to the bottom of this vacuum leak issue. What sort of vacuum are you pulling at hot idle, and is it steady? Measure it at the back of the manifold where the hose to the cabin connects.

gmask · #17 04-10-2003, 03:17 PM

Here's how I set my fast idle..

It's nearly impossible to adjust it withthe engine running.. I have tired with a mirror but still it is nearly impossible to turn the screw without burning yourself etc. So what I did was from a cold start I place the Tach meter where I can see it from the driver's seat. Make a quarter turn on the fast idle screw and then start engine and look at RPM. Turn off engine and repeat until the RPM is where it should be with the transmission engaged.

It does sound to me like your carbs are still out of sync and that is why you are getting the drips. You really need to flowmeter which you said is on the way. You will also need a hood adpater which I think can be gotten from baumtools.com or you can find a round piece of tupperware or something similair that fits perfectly over the carb and cut a hole in the bottom to fit the flowmeter.

I recently changed the fuel pump in my rear carb and after tuning it I was still getting a rough idle and I obviously still have a vac leak somewhere. I discovered that if I closed the choke flap on the rear carb that the idle smoothed out.. the carb is getting so much air that it doesn't need the choke flap to be open so I just disconnected the lead to the bimetal spring and then drove 600 miles with it that way. I plan to investigate the problem this week to see if I can get it to work properly but this worked in the mean time.

So I guess I'm saying is that if you close one of the other choke flaps and then engines smooths then you still have a vac leak some where.

The other thing I would like to tweak more is getting better acceleration. I fell like the engine is plenty strong but it takes a while for it to wind up.. I'm going to check the linkage to see if it is stopping short and is not allowing me to give the engine more gas or not.

On the subject of ignition.. I doubt that it will solve your idle problem. I'd like to get mine rebuilt once Randy is back to health. For my car this wol dprobablyhelp the acceleration and the upper limit of how fast it wil go as currently 95 is about tops.. I wouldn't want to go faster but to go 80 I pretty much have to floor it.

rgnprof · #18 04-10-2003, 10:36 PM

OK, here's where I'm at tonight:

1) Checked the idle and it was low 600-650 rpm. Upped it to 800-850 rpm, rebalancing the carbs. Mixture screws set at 1 full turn out - using the hose method carbs sound the same and idle smoothly.

2) Pulled the front carb accelerator pump nozzle, cleaned and blew it out, replaced and redirected the flow. Both pumps producing a good stream when car is off - flow is a little more diffuse from both pumps when the car is running.

3) Replaced the tube that runs from the exhaust manifold to the air cleaned snout - I forgot about it after the my last "adjustments".

4) Checked vacuum at intake manifold (18 psi), front carb (19 psi) and rear carb (19 psi). All pressures are steady and following the vacuum pressure guide from the Durrance web page, all readings are within normal ranges. I can't find a vacuum leak - except that the distributor vacuum advance slowly leaks.

Car currently runs better than it has since I have owned it! Took it out for a spin - accelerates smoothly and just "feels" right. It is warm here tonight and engine had been idling for a while, so it was very warm - we'll see about things in the AM.

I'm not exactly sure what helped, but...

I think I am going to try and check the vacuum advance by teeing off a hose into the cab, so I can check it when the car is under load. Thanks for the suggestions.

Ryan

rgnprof · #19 04-29-2003, 08:21 PM

A quick update and a couple of questions: Got the JCWhitney carb balancer and I adapted a 1 quart Rubbermaid Seal 'n Saver container for the hood - it fits almost perfectly. Carbs were pretty close, I made a few adjustments and car runs better. I have noticed that my engine seems to be running a little 'louder', impossible to diagnose over the internet, but it just sounds like it's running a little rougher - sounds a little like a card in the spokes of a bicycle wheel??? I am thinking I need to adjust the valves??? I really don't know - gas mileage is the same.

Per some earlier suggestions, I have disconnected the retard vacuum line from the distributor and I have bypassed the 2 and 3 way control valves - vacuum is running directly from the rear carb to the rear carb dashpot and from the front carb to the advance side of the distributor vacuum unit. I set the timing - with the vacuum advance line connected - to 8 degrees BTDC. I did have to back off the idle from around 1200-1300 rpm back down to 800 rpm and then balance the carbs. Everyone has assured me that the car would run better, better acceleration etc., but it is running a little worse - hesitates on acceleration and seems to be less responsive and a little sluggish. Once I'm above 20-30 mph, I can't tell any difference. Should I fiddle with the timing and leave it this way - or switch it back?

One other question: I have white smoke out the exhaust when I start the car - when I hit the gas the first time after starting, light cloud of smoke, and then I don't notice it anymore. Any ideas?

Thanks, Ryan

gmask · #20 04-29-2003, 08:37 PM

>>>I have noticed that my engine seems to be running a little 'louder', impossible to diagnose over the internet, but it just sounds like it's running a little rougher - sounds a little like a card in the spokes of a bicycle wheel??? I am thinking I need to adjust the valves???

Did you allready check the valve clearance?.. I don't think that's the issue. The last time I was working on mine at some point I had this sound ..could be the same or similar.. it was sort of like rapid backfires but not really.

>>>One other question: I have white smoke out the exhaust when I start the car - when I hit the gas the first time after starting, light cloud of smoke, and then I don't notice it anymore. Any ideas?

This is probably just water in the gastank.. you should run the car until the tank is almost empty and or add some water remover to the gas tank. It allways freaks me out when I see this because it reminds me of my blown head gasket which give sme cold sweats to this day. I still have nightmares occasionally about the car over heating and blowing the gasket.

However if you start to see very tiny black specs in the radiator water or start getting white smoke after the car has gotten to full temperature it's a blown gasket. :-(

ctaylor738 · #21 04-29-2003, 10:34 PM

You could back the timing off to 6 BTDC and see how that feels. There is nothing terribly scientfic about it. Never hurts to adjust the valves, either. If you have any that have loosened, then replace the adjuster.

psfred · #22 04-30-2003, 11:24 PM

Actually, your timing may be late with the vac connected amd then set to 8 BTDC -- I don't know how that particular system works. Will look in the manual I have and see, they usually list a cranking (static) timing spec along with the running ones.

If the vac is either on or off, it may not be appropriate to set the timing with it on (gives advance above a certain rpm and when the engine is hot, etc) -- in which case, try setting the timing with the vac disconnected and see what happens to performance.

Can't help with the noise.

Peter

rgnprof · #23 05-01-2003, 08:59 AM

I have reconnected all of the vacuum lines and set the timing to 2 degrees ATDC - WITH VACUUM CONNECTED to both sides of the distributor advance/retard unit. Runs much better, accelerates more smoothly from a stop and has better pick up. I am getting some vacuum advance, but not what the Mercedes factory manual says - 42+ degrees at 1500 rpm! I am getting closer to what my Haynes manual says - 25-30+ degrees. A friend of mine told me that the vacuum advance advances the timing under load, and the car just sitting in the driveway revving the engine, is not under load - and that's why I'm not getting the advance indicated in the factory manual. Any thoughts? Thanks.

Ryan

300SDog · #24 12-24-2004, 10:51 AM

High and low speed idle jets, atomizer, accelerator pumps, electric two phase chokes and carb synchronizing. Not to mention linkage adjustments. And all those itty-bitty brass jets and valves freeze in place as properties of metalurgy take over.

Sounds easy?? LOL - and these are microscopic measurements on 30 yr old auto parts prone to gasoline varnish deposits, warpage and air leaks, minuscule blocked passages and just plain wearing out of components which have never been available off the shelf.

Sounds like DIY Rolex watch repair to me!!

DANTRCAV · #25 01-06-2005, 01:22 PM

Ryan:

Just happened on your post this morning. I think I might know why your car did not run as well as expected with you piped the vacuum from the front carb direct to the distributor advance. However, this caveat: I sold my 250C and all my manuals last spring so I am relying on memory here.

As I remember, the vacuum pipes on the Zeinth carbs are BELOW the throttle plates which means you are piping manifold vacuum to the dizzy advance. This would apply full vacuum advance at idle. Since manifold vacuum varies inversely with engine speed, as you increase speed the vacuum drops and the timing RETARDS up to the point where the centrifigal weights take effect. Normally, dizzys that are conneted directly to carbs (or intake) come from ported (s-port) vacuum which is tapped ABOVE the throttle plates. Ported vacuum varies directly with engine speed so, at idle you have zero (or very close to zero) vacuum and as the engine speed increases so does the vacuum and so does the vacuum component of the advance curve.

When I first got my 250C the PO had totally screwed the existing vacuum system and nothing worked. I disconnected everything, set initial timing at 4 degrees advanced and ran without any vacuum advance while waiting for my Weber conversion. When I put the Weber carbs on the engine they had a true ported vacuum pipe so I hooked up the vacuum advance and all worked well.

Not sure this is any help at this point in time but I thought I would add another nickle to this thread.

Good Luck.

Dan

230/8 · #26 01-06-2005, 06:35 PM

Just an additional thought or two to share regarding dual diaphragm distributors and other arcane stuff for Zenith equipped cars...The two distinct values for vacuum, low for venturi vacuum and high for manifold vacuum at idle, and vice versa at higher speeds form the basis for the dual diaphragm operation. At idle, the manifold vacuum signal is stronger than the ported venturi signal, so the spark is retarded. As engine speed rises the venturi vacuum signal becomes stronger and over-rides the manifold vacuum signal. By simply eliminating the manifold vacuum line to the back side of the distributor vacuum cannister the venturi vacuum line attached to the advance side of the cannister will allow for no advance at idle speeds and normal or perhaps a bit faster advance above idle...this is similar to european models. Set your basic timing to the setting that gives you the correct total advance at the specified running RPM in your manual. Or simply set it 0 to 5 degrees BTDC, depending on your fuel grade. By the way, if your vacuum cannister is leaking then why not just replace it with a single line cannister from an earlier model?

The centrifugal advance weights are speed-governed and operate independently of either vacuum signal. When cruising both combine to produce a total advance greater than either system working alone.

With respect to the idle quality problem, I think the balance of the carbs is the primary issue and should be verified with a proper bonnet adaptor in place, one that forms a very tight seal so the syncro-tool functions reliably. Under no circumstances should you ever try to "ear balance" a dual carb set-up. It's a recipe for failure. And, using a bonnet adaptor that does not fully seal the carb top is unreliable and a waste of time. Even a slight amount of leakage will produce bad results.

Just some random thoughts from one who has worked on the Zentihs on his 230/8 for more than 30 years. They run perfectly and never give me any of the problems that seem to afflict others.

For what it's worth...

230/8

MikesRadioShop · #27 01-14-2005, 07:55 AM

Good morning Ryan,

I'm a new member, by chance, and responded with a lengthy reply yesterday PM. I don't think this system delivered the message but I thought I would try to determine if it did. No response from the webmaster either. Please reply to this, thanks. Maybe I can help solve at least some portions of your dilemma that others have missed.

Mike

Tomguy · #28 01-14-2005, 08:59 AM

Mike: If you see, the first post is almost 2 years old - I would sure hope he has it figured out by now!

rgnprof · #29 01-14-2005, 03:26 PM

I'm not exactly sure how this thread got re-opened. I have solved many of these problems already following similar approaches mentioned in the last couple of posts. Balancing the carbs the right way definitley helped, but I haven't been able to bypass the emissions systems and the dizzy advance box and get the car running well. I did find a new distributor vacuum advance, so I have no leak there. I have run into new problems and if any of you are interested please check out the link below to my most recent post. I haven't been able to work on the car much since before the holidays, but I'm getting ready to try and attack some of the problems.

Update on my 250

Ryan

MikesRadioShop · #30 01-17-2005, 02:54 PM

Good afternoon Ryan,

I responded because I saw a response of Jan '05. I figured that person had talked with you as of late, maybe by phone - etc., and that the "carb" problems were still an issue & topic of conversation.

I have a 1965 220Sb, 582,800 miles, gas-auto, with two Z's and I love 'em.

It took a while but I got all the settings and external kinks in linkages etc. settled. The biggest problem was that there is an engineering quirk on these at the throttle valve cam lever. To get these Z's to run optimized you must get the zero throttle plate opening set right-on. It can't be done with the cam plate installed. They figured it would be done as an off car work. I reworked the cam slot and the rest is history.

I don't know what kind of car you have with the Z's but the same premise of correct setting of the zeroe's must be true. If the zero reference is off all other adjustments are off. Your carb symptoms are all reminiscent of what I had.

I do agree with most all the positive information that I see posted, except for one. I set and tuned the carbs by ear and a tach and it runs beatiful, about 15mpg. Just as shown in our Benz club magazine, idles so smooth with a styrofoam cup of coffee on top of the valve cover with little bulls-eye ripples and no spills.

If you would like more info on what I did & why/how I'd be glad to share.

PS. The reason I was visiting was to determine the availability of parts/info relating to my 75 450SL. Sunvisor snap in mounting clip in particular. Anybody have any thoughts?

Mike