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

rgnprof · #1 04-07-2003, 05:03 PM

OK - can someone explain what this means??

Zenith carbs - 1972 - I've had trouble getting them to idle right without having fuel come from the preatomizer (main driving circuit) in the center of the primary barrel.

Tried to synch today - thinking this was the problem. I have been able to get it to idle, at 760-789 rpm w/o fuel dripping - so maybe some progress - but they don't sound synchronized to me - rear carb is louder than the front (using the hose method).

Rear carb - 1) remove mixture screw and car stalls, 2) cover the carb and car stalls, 3) unplug the idle control solenoid and car stalls, and 4) cover idle air hole and idle slows just a little

Front carb - 1) remove mixture screw and idle slows to 400 rpm, but doe not stall, 2) cover the carbs and car slows to 200 rpm, chugs but doesn't stall, 3) unplug idle control solenoid and idle slows (400-600) but car does not stall, and 4) cover idle air hole and car slows a little - similar to rear carb.

Which one - if either - is right? Anyone can any ideas? I'm thinking the front carb is not performing correctly. Also, I have checked and checked for vacuum leaks by spraying carb cleaner around both carbs - earlier I thought it was leaking around the base, but now no changes in idle that I can find.

Thanks, Ryan

rgnprof · #2 04-07-2003, 08:44 PM

I think I have found the source of my problem. I had tried to adjust the fast idle screw on the front carb earlier and I think that I must have screwed it in too far, which may have been keeping the throttle valve open. I backed off the screw and the carbs synched much better and no fuel dripping!

I have actually tried several times to adjust the fast idle and I just can't do it right. I have a salvage carb that I have tried and I can't even get it right. Can't get the screw to set against the 'fat' part of the cam. The fat part of the cam won't rotate far enough foe the screw to set against it.

Ryan

ctaylor738 · #3 04-08-2003, 07:44 AM

Sounds to me that one or both of the following are happening -

- the choke linkage is binding and not allowing the cam to move to the fast position

- the linkage is in the wrong position, and the little stops that actually move the cam are not contacting each other

Sounds like you have the vacuum leaks under control - that's most of the battle.

rgnprof · #4 04-08-2003, 09:47 AM

Hello Chuck, meant to email you. I drove the car this AM - cold here, around 34 degrees. I am almost positve I got the idle problem solved, but now the car is back to running poorly - immediately upon acceleration from a dead stop - car lurches and when it's really cold it backfires a little. I feel like I am back to square one! The idle is a little slow this AM and I may try and increase it some, but the fast idle is not working at all. I have checked the linkage, and I just can't figure out where it may be binding. The car ran better with fuel dripping from the preatomizer, which I think was happening because the primary barrel throttle valve was partly open at idle.

Could this be an ignition problem, or something to do with the distributor advance mechanism not working properly?

Ryan

psfred · #5 04-08-2003, 06:59 PM

Ryan:

You must synchronize the carbs with an air flow meter, you cannot do it by "ear" -- any more than you can set the timing "by ear".

The easiest way to get close without a flow meter is to set the idle speed screws and mixture screws so that you get equal rpm drop when you cover the air hole in the carbs.

You will probably have to set the idle speed screws and mixture screws to the same setting to start with (that is, the same number of turns from all the way closed -- BE CAREFUL with the idle screw, it is very easy to break it when it is screwed all the way down!).

From there, twiddle the idle and mixture screws to get the same idle speed drop, then set the idle and fast idle. Make sure you have the cam and choke links installed right side up and right way round -- I've never taken one of these apart, but it is possible on a Rochester to get some of the links on the wrong side of the parts, and they bind.

Backfire cold is a result of running to lean, probably on the front carb.

Peter

ctaylor738 · #6 04-08-2003, 07:35 PM

Peter is right, but I think you can still get the sync close by ear and the idle-drop test. It sounds to me like you are back to a choke problem. Suggest you run through the test to see if your choke gap is correct and the same on both carbs.

One way to do it might be to start the engine cold, and measure the gaps right after the engine starts

#7 04-08-2003, 08:16 PM

Don't know if this is any help but my '69 230, the M180 without ANY pollution contols ran much better when every 10,000 miles I adjust the valves, put in a new set of plugs & points, & set the timing to 0 degrees ATDC. It idles around 650 RPM so it stalls sometimes when cold; think I need to raise the idle a little. Everything has to be set just right with these guys. And they really like high test gas. Good luck.

Cal

rgnprof · #8 04-08-2003, 08:40 PM

Thank you for your replies. I have driven the car all day and regardless of how long I drive it, I still have a "dead spot" early on while accelerating. I know I have a fast idle problem and that I'm not doing something right in adjusting it - I just don't know what! But separate from that - I'm back to having the same consistent hesitation problem - I give it gas and it doesn't go anywhere (after this early hesitation problem, car runs great - no problems accelerating from 30 to 70). If I ease off the accel pedal then back on, it will get past this 'dead spot'. What's odd, is that it had quit doing this when I had fuel dripping into the primary barrel.

I have ordered the JC Whitney synchronizer - should be here next week. I "think" I have the carbs close - they sound much more alike than they did before I backed out the fast idle screw on the front carb- when the rear carb air intake 'sounded' a lot louder than the front.

I don't know even anymore if this is a fuel problem. I'm back to thinking it's an ignition problem - I know I'm not getting proper advance on my timing. For instance, the Mercedes shop manual indicates that when I disconnect the temp switch, in the thermostat housing, and ground it, the timing should advance. Mine advances momentarily, but then drops back, and I'm not getting 40-50 degrees at 1500 rpm either. Could all of this explain my symptoms, or am I totally lost???

Also, if I wanted to replace the ignition - any suggestions? I know some have said the Crane 700 (??) or Pertronix (??). Can I install that?

Thanks, Ryan

psfred · #9 04-09-2003, 12:55 AM

Ryan:

Dead spot on acceleration means either too lean on the idle circuit or the transition orifices are plugged in the carb, or that you are way out of synch, so that one carb is providing no air/fuel mix at light throttle, probably because the throttle linkage isn't correct.

You have been doing all these adjustments with the link between the carbs disconnected, haven't you? Fit the link to go on exactly, with no change in throttle opening on either carb, when they are syncronized.

Peter

rgnprof · #10 04-09-2003, 09:14 AM

Yes, I have been synching the carbs with them disconnected from each other and from the throttle. I have followed the procedure 'to the letter'. I believe I have them close, at least. The car idles fine - even in Drive.

Thr heistation occurs the first time I step on the accelerator pedal from a stop - if I ease into the pedal and accelerate very slowly, I don't notice it as much. If I punch it to accelerate quickly- it may even lurch so badly that it backfires. Once I'm past that, no problems through acceleration. It also does it if I'm coasting (Foot off the accelerator and at any speed) and then I hit the accelerator pedal. Again, this is why I started tearing into the carbs, and actually had it fixed when the car was idling wrong! I'm confused - but what else is new! I hate the idea of changing the carbs to Webers (I'm something of a purist) but I'm a little frustrated.

Thank all of you for your patience!

Ryan

ctaylor738 · #11 04-09-2003, 12:14 PM

If this is happening when the engine is warmed up, then look at the accelerator pumps. Lack of vacuum advance could also cause this.

psfred · #12 04-09-2003, 08:48 PM

Check both carbs to make sure the accelerator pumps work properly -- Chuck is correct in that the accelerator pump is there to add fuel when you make the transition from idle circuit (high vac, fuel delivered out the idle port) to running (low vac, fuel from atomizer in venturi). This is a classic carb problem.

The fun part is that a vac leak will seriously complicate things, as will operational condition of the engine, cam timing, and distributor mechanical advance!

If the throttle plate is either open too far or too much, you won't get the correct amount of fuel out the accel. pump, and you get stumble. Late valve timing will result in low air flow, so that you will have the throttle valves too far open at idle, and the transition ports (or the Zenith equivalent) won't work properly, causing a loss of air flow and fuel deliver lag when you open the throttle. Late of lagging ignition timing/advance will make all this worse. Thios is why guys with "big" cams sometimes drill small holes in the throttle plates -- it is absolutely necessary for the throttle plate to cross the transition bores at the correct time with the accel. pump.

To fix: check output on accel. pumps -- amount should be 0.7 to 1.0 mL for one full stroke of the pump. You must remove the top cover and pre-atomizer to check (I'm sure, somehow, you are famliar with how to do this.....).

Injection of fuel must start simultaneous with throttle plate movement. If it's late, you will get stagger.

Make sure the spray hits the side of the carb, 10-15 mm down from the top of the plate block.

And also, make sure you don't have a vac leak! Classic vac leak problems are erratic idle, hard starts cold, throttle lag, stumble on opening throttle, and difficulty in setting idle speed and mixture. You have all the symptoms, and no amount of adjusting will make the carbs work properly if you have a vac leak -- all will be much worse if it is only on one carb, too! The brake booster and associalted plumbing, the vac emissions controls, and vac locks/AC controls are all possible leak spots.

Hope I'm not preaching to the converted.....

Peter

Peter

rgnprof · #13 04-09-2003, 11:21 PM

Preach away! I am coming to the conclusion that at least part of the problem is the accelerator pump. The spray from the front carb is more diffuse than it should be - at least according to the factory manual. The spray is so wide that it looks like part of it is hitting the preatomizer. I don't know how to measure .7 to 1.0 mL of fuel - I know there is a special tool to use, but I don't have it. I need to track down something that will fit and collect the fuel - any ideas?

I know the symptoms I describe fit a vacuum leak - but I have checked and re-checked the carbs and things appear tight! How do I track down leaks elsewhere? I seem to have good pressure from the intake manifold - 15+ psi.

Thanks - I appreciate the suggesions!

Ryan

psfred · #14 04-10-2003, 12:36 AM

Ryan:

The outlet pipe for the front carb is partially plugged, preventing proper fuel deliver AND proper mixing -- the fuel must spray down the side so that it is atomized by the air fow around the throttle plate.

Take the delivery tube off and blow backwards through it and get the plug out, and I'll bet all your troubles will disappear. A piece of fine wire will probably help, too.

MB makes (made, probably) an nifty little cup on a stick that fit down into the carb. No chance of finding one now at a reasonable price -- I will see what I can come up with that is similar volume (I've got some pipets and can actually measure that small). Hopefully something around the house.

You will have to check for a vac leak by pulling the hoses and applying vac -- I'll bet you find one that doesn't hold.

Peter

rgnprof · #15 04-10-2003, 10:28 AM

The only place that I am pretty sure I have a vacuum leak is in the distributor advance unit - I THINK! When I pull the advance hose (facing the firewall on the rounded side of the vacuum can) and apply a vacuum pump, it moves the arm and holds vacuuum momentarily, but slowly loses it. I can't figure out - for sure - if this is what the advance unit is supposed to do, or not???

Ryan