Zenith Carb backfire after Crane installation

[DANTRCAV]

Ryan:

FWIW:

I had a 4 row oval aluminum radiator in my 250C and with the exception of those days when the temps got above 90 and I had the AC on I never got much above 175 either. I think if the engine temp was a problem you would see your condition when the engine was cold after start-up and you don't appear to have the problem.

Dan

[mzsmbs]

Originally Posted by rgnprof

I have also been told by an old-time mechanic I ran into in Portland last weekend that you have to set the timing by ear on this car (chain stretch, cam wear, etc) - which doesn't really help me but it does indicate that I may need to fool with the 4 degrees ATDC setting which is the factory setting.

Ryan

good advice.. just set it where it accelerates and decelerates well.. no coughing, pinging backfiring, missing... smooth... i do all my cars this way although i haven't messed with the merc in a while but crane is going in probably yet this winter...

[rgnprof]

OK, here's where I'm at now - and I think I making progress.

Checked the rotor at high rpm (approx 30 degrees advanced) and rotor remains still and fixed on the number 1 terminal - just where it should be. But, I am not getting enough timing advance (max 30-35 degrees, and according to manual it should be 42-54 degrees). I am also NOT getting any timing change on the vacuum advance at high rpms (4500) - supposed to get 8-12 degrees, so I'm pretty sure I am having timing advance issues. Vacuum retard works fine but the control valves (part of the emission stuff) is not switching to vacuum advance at high rpm.

So,

I set the timing to 4 degrees BTDC, replaced the Bosch plugs with NGK BP5ES gapped to .040 and I have no more backfire. Car seems a little sluggish in low gear (plug gap issue maybe??), but greatly reduced stumble and no backfire from carbs or exhaust, yet.

Questions:

1) Is the lack of timing advance causing some of my problems?

2) When I advance the timing the idle speeds up - how far BTDC can I advance it (I adjusted the idle after setting the timing and got it back down to 900 rpm, or so, but I don't know how much more I can adjust it if I change the timing again), and:

3) Since my vacuum advance is not working, how should I best bypass the emission valves - Dan, I guess this question is primarily for you since I know you have done this before. Since the retard is working fine should I disconnect them?

Ryan

[milterino]

Hi: After reading your correction I went out and confirmed.. Mines a 76, rotation clockwise and ATDC is to left as you say. Thanks for the correction. MIlt.

[DANTRCAV]

Ryan:

A thought and you might have checked this already. Have you applied vacuum to each side of the dizzy to see if it will hold vacuum? It "might" be possible that your advance/discharge diaphrams have a leak?? Just a thought.

I was running 10 degrees BTDC on mine with all of the "emissions" stuff disconnected and the car ran very well. However, as I remember since you are running the stock Zeinth carbs (I had Webers) you don't have a source of ported (s-port) vacuum for the dizzy vacuum advance. The vacuum ports on the Zeinths were, I believe, below the throttle plates thereby they would produce manifold vacuum. You can verify by checking where the vacuum pipes on the Zeinths are located. I "think" I remember then being on the base plate below the throttle plates. You could also just attach a vacuum hose to the front pipe and see if you have vacuum at idle and none (or less) as the engine speed increases. If so, then you will not have correct vacuum advance on the dizzy. Ported vacuum increases with engine speed, manifold decreases.

On the other hand, if I am having a "senior moment" and you do have the correct vacuum signal at the carb then just run the advance line from the front carb to the advance side of the dizzy and cap the retard side.

One more thing, I never ran anything except premium (91 to 93) octane fuel in my 250C and an initial 10 degree advance was not a problem. I have no idea how it would have reacted with regular - maybe no difference.

As far as the emission system, you can just plug the advance and retard lines that go to the dizzy and not worry about it.

My guess is that your being sluggish in low gear is more a factor of timing and advance. I ran my plugs (NGK) at .045 and all was well. As far as how much idle adjustment you might have I can be of no help there since the system on the Webers differed from the Zeinth's. I assume you are adjusting via the star wheel adjusters and if that runs out you should be able to back off the throttle stops (carb top plate) a tad to compensate - then rebalance.

Keep us updated. Sounds like you are making good progress. Once I got my 250C sorted out it was a total blast to drive.

Dan

[rgnprof]

Dan,

This is a new distributor vacuum advance/retard box, but the advance side does not hold a steady vacuum (retard side does), loses it slowly but I wonder if that is the proper operation? Regardless, at all speeds I am not measuring any vacuum at the hose with a vacuum gauge. The Carb vacuum ports show 16-17 inches at idle and increases under load, so I'm not sure what this means (I am still confused about port vs manifold vacuum).

I came across this on the Mercedes ponton site regarding a Crane installation:

"Now we can set the timing to Mercedes-Benz specifications with a timing light. From your engine manual, find the amount of spark advance specified for high RPM operation. For example, if your ignition is specified to advance to say, 30 degrees at 3200 rpm, then adjust the timing to show 30 degrees advance with the engine at 3200 RPM. In this manner, your engine timing is optimum for operating engine RPMs. Never mind what the idle advance is - you don't drive the car at idle RPMs."

Since my specs call for timing advance of 42-50 degrees at 3000 rpm, should I set my timing to 42 degrees with the engine running at 3000 rpm? And, should I reset the spark plug gap back to .032"? I still don't have any carb backfire, but engine is definitely a little 'sluggish'.

Ryan

[DANTRCAV]

Ryan:

It might be possible that the advance side has a slow bleed-off or will not hold a stady vacuum. It might also be possible that by not holding a vacuum it will not allow full advance. I know, not much help there. If I still had my car I could check but I don't.

I assume by your saying you have no vacuum at the hose you are referring to the hose at the dizzy with the emissions stuff connected. If so, then you obviously have problems with something in that circuit.

Pulling that much vacuum at the carb port will put the dizzy pretty much into full advance at idle and you don't want that. Not sure why or how it increases with load if it is piped to the manifold other than it might be picking up more vacuum due to the velocity of air after the venturi.

Ported vacuum is taken above the throttle plates somewhere in the venturi. If you look into the primaries of your carb you see two very small holes. These are ported vacuum holes that apply vacuum to your secondaries via the secondary vacuum "motor". When the engine speed gets high enough, enough ported (or venturi) vacuum is created across the holes to cause the diaphram to move and the secondaries to open. This is pretty much how all multi-barrel carbs work that use vacuum (not mechanical) to open the secondaries.

I am at a loss as to explain how your vacuum at the carb is working - I do not remember mine behaving that way but it has been almost 8 years since I had Zeinth carbs and I did not spend a lot of time trying to make them "nice".

Again, when I had the Zeinth carbs I plugged the advance/discharge lines and set the timing to 10 degress before and just drove the car until I did the Weber conversion. It ran ok and I don't remember it being too sluggish in low.

Wish I could be more help. Sounds like you are making good progress so hang in there.

Dan

[ctaylor738]

AIUI, ported and manifold vacuum work in an opposite manner when the throttle is opened. Manifold vacuum decreases, where ported vacuum shows a sharp increase. So which is used depends on how the advance is wired.

If manifold vacuum is used, I think that the vacuum holds the timing in a retarded position and allows it to advance when you hit the gas and vacuum drops. Ported vacuum increases as the throttle opens and pulls the dist plate into a more advanced position. You can tell what you are hooked up to by putting a guage on the line and cracking the throttle.

I am not making this up.

[250 Coupe]

Ryan,
CTAYLOR783 has the correct description of the vacuum ports and their action.

Think of the engine as a vacuum pump and the throttle as a valve. With the valve closed (at idle) you will see a high vacuum below the valve but none above. As the valve opens you will see a decrease in the vacuum below the valve due to air flowing in and a increase in vacuum above the valve due to restrictions in the carb body. By changing where the port is in relationship to the valve you can use the vacuum to change timing, operate EGR valves and work door locks.

Our cars use manifold vacuum or the one ported below the valve. At idle the vacuum is routed through a three-way valve to the retard side of the dizzy. As engine RPM increases, the vacuum is routed to the advance side. Other cars used the "ported" vacuum port, which is IIRC, just above the valve. At idle you would have no advance and as the throttle is opened and vacuum is increased, you would get progressively more advance. This is how the Webers work.

As for the leaking diaphragm, I think mine leaks also and that makes me wonder if that is to let the retard side work if the advance side is not opened to atmosphere at the three-way valve.

On my 250C, I disconnected the three-way valve completely and routed the vacuum line straight to the advance side of the dizzy. There is only 4 degrees of advance in these dizzys but I figured some is better than none. I set the timing at 40-42 degrees at whatever RPM the book called for.

The PO did the carbs just before I bought it but I don’t think he did anything beyond a gasket kit and setting the fiddley bits according to the manual.

I also only use premium fuel as it will rattle other wise.

The end result is a car that is a blast to drive. It will bark the tires on the 1-2 shift and sounds cool with the dual exhaust that could use a new muffler.

Michael

[rgnprof]

OK Gentlemen, I think I am starting to get this and I really appreciate the help. I am now pretty sure that my advance/retard box and the valves/relays that control them are not operating properly.

I just re-checked the vacuum at the box on the dizzy and from both carb ports. At idle, both carb ports read 15", hit the throttle and needle jumps to zero and then back up to 17", or so and with a steady 3000 +/- rpm, vacuum stays at a steady 17".

The hose coming from the control valves and connected to the retard side of the vacuum box holds a steady 15", regardless of engine speed (and retards the timing about 12 degrees) and the hose going to the advance side shows 0", again regardless of engine speed. I'm pretty sure that is not right so the speed relays are not changing the vacuum from retard to advance under the proper engine speed, so I have constant vacuum retard and no vacuum advance. I am sure this is part of my driveability problems, but I don't know how to fix it??

I even have spare speed relays, but they don't work either. I would like to just bypass the emissions stuff, but with vacuum constant from the carb ports, does it make any sense to do that?

I hope this makes sense, but this has been an ongoing problem. Thanks for the help.

Ryan

[250 Coupe]

Ya know what? Even though I understand how the vacuum works, it never occured to me that I was using the manifold port and therefore my advance is probably not doing much of anything.

A thought about your speed relays. With an aftermarket ignition, you may no longer have the signal to the speed relays. I bet mine don't work either.

Michael

[ctaylor738]

I don't have a 250C anymore, so some of this is from my fading memory. But the way I recall this working is that the advance worked off manifold vacuum so the vacuum actually held it it a retarded position at idle or normal load. Then you hit the gas, vacuum drops, and the timing advances until the vacuum builds and the timing returns to an un-advanced position. The retard allows vacuum to the retard side when the RPM's are over 2500 and temp is in the normal range.

The advance works by rotating the plate that the trigger sits on CCW so the trigger fires earlier relative to engine position.

So what I suggest is that you verify that this works with the Mityvac. Put the Mityvac on the dist and note that as you apply vacuum, the plate moves CW - retard. Attach only a single vacuum hose to that side of the dist. Then play with the timing starting with maybe 8 advanced. with vacuum. When you blip the throttle, you should see a quick advance, followed by a return to the starting point. Rev it up slowly to 3000 and see if you have the right total advance which I think is 38-42.

[dkveuro]

Originally Posted by rgnprof

I need some help!

I it occurs when the car is warmed up and when I accelerate from a stop or when I hit the accelerator through a turn or after slowing down. It doesn't do it everytime.

carbs are balanced...............car doesn't have the lurch it use to and runs great after the initial backfire problem - more power and better acceleration in the upper range.

What am I overlooking? I've checked the Crane ignition, per the manual, and everything checks out so i don't think I have a problem there. Thanks.

Ryan

You first off need to check the basics...

#1.
Engine off...Check to see if at #1 TDC, piston is at TDC and timing mark is at 0I0.
by inserting a long thin screwdriver down #1 plug hole and slowly rotate engine towards TDC.
With the valve cover off, also check cam is at TDC with both #1 cylinder lobes away from the rockers...valves closed.
Cast raised mark on cam housing and thin notch in cam sprocket boss.
....The crank dampers do move out of alignment .

#2.
With engine running...Vacuum disconnected...check ignition timing with timing light....should be 10 to 12 BTDC.
Raise rpm to 1300 rpm and timing should not advance...if it does, the advance weight springs are
loose/weak and must be re-tensioned to prevent mechanical timing advance below 1300 rpm.
An early ignition timing creep will upset idle speed .

#3.
Now check for a signal vacuum signal...should be about 10 to 18 inches.
This vac' is applied to distributor retard pot, and will move timing back (ATDC) about 5 to 10 degrees.
Apply this same vac' signal to the advance pot on distributor and timing should advance 12 degrees or more.
When vac' removed from distributor pot, timing should rapidly return to original setting.

#4.
With engine running...check the advance pot gets no vacuum at idle but vac' as throttle opened slightly.
This vac' will come from the throttle body port just ahead of the carb' throttle plate.

#5.
With engine running.....check vac' to retard pot...
...this signal will be at retard pot always....... unless full throttle applied...should be same as intake manifold vac', about 15 to 18 inches.
This signal should be unmodified and from the intake manifold after the carb' (Downstream.).

#6.
If everything checks out or after correcting any problems...with engine running...no vac' to distributor...
and vac' gauge attached to intake manifold port for full time vac'..
chock the rpm's to 3500. Now turn the distributor towards advance until highest vac'
reading is seen on gauge, which should be around 20 to 23 inches.
At this point, back timing down until you loose 2 to 3 inches of total vac' reading.
This should be ideal max advance for you motor.

#7.
Refit vac' hoses to correct ports on distributor and road test.
There should be no ping or rattle at full throttle in high gear from 1500 rpm to 4000 rpm.
If rattle or ping is heard, retard timing 2 degrees at a time until no rattle or ping heard.
Record set timing spec's and keep for future reference.

You should now be set.
If you still experience back fire, the carb' acceleration squirters are not injecting enough fuel or are late on injection.
Correct this condition with carb' adjustments to the accelerator injection cam and linkage.
Acceleration jets may need cleaning or enlarging if back fire continues.
Depending on you local emission laws, this should allow a pass at inspection time.

The retard function does two jobs...idle correction as warming up and reduced co% emissions at idle.
the distributor retard/advance capsule is designed to allow advance vac' to overcome
retard vac' at all times..ie: advance vac' is given priority when applied.
The only time retard should be applied to timing is at idle or throttle closed conditions.

Mercedes used various temperature sensors to switch vac' signals off and on at different engine temperatures and throttle positions.
Unless you have a diagrammatic layout of this system and know the vac' switches are working correctly, bypass these for drivability.

[230/8]

All this vacuum talk, port/manifold, is very interesting, but doesn't answer the question, "how to fix it?" Simple really...

Whether manifold or venturi (aka "port) your engine is producing a vacuum in order to run. The distinction between manifold vacuum sources and venturi vacuum sources is only important when needed to identify the one that is dominant at certain engine speeds. Hence the routing of vacuum lines from both sources to each side of distributor. At idle the manifold vacuum is higher than venturi vacuum, so it is hooked to the back side of the advance can and retards spark. Most cars during the early smog years used this method to regulate timing advance. As engine speed rises the venturi vacuum rises and overcomes the manifold vacuum source, but only slightly. Distributor diaphragms are often made from lighter materials to respond to these smaller signals, usually only a couple of inches of difference between them.

Here's how you fix the problem:

Without vacuum, set the timing to gain the normal advance at 3,000-3,500 rpm. Check the manual for an older version of your engine, i.e. before smog crap was added. If you want the vacuum advance to work at idle speeds, i.e. adding in that extra 4-5 degrees of advance, then hook up the manifold source to the advance side of the can. (Your idle speed will increase a bit and sometimes that can be problematic. On my car this rise causes the centrifugal advance to engage and run up speeds even higher, and I don't like that.) That's it, forget about the second hose, plug it off. (This type of hook-up will often reduce a slight timing related off-idle stumble, if you encounter that little problem.)

However, if you like a steady idle with vacuum advance assist coming on only at engine speeds above idle, then hook up the venturi source to the advance side of the can. It will provide a signal only when engine speed is raised, i.e. running along down the road, and no vacuum advance at idle. This is sometimes helpful in passing smog checks. Again, forget about the second hook-up. Just cap off that manifold source.

You can pitch those relays and three-way solenoid valves. Not needed. (Look at a picture in any old chilton manual and you can see how non-smog engines were set up a'la european version.)

Here's another thought...if you have a three-way valve then the valve may be diverting a single vacuum signal to either side of the advance can depending on engine speed or temp or a combo of the two. Check the source of vacuum for the three-way valve. It may be the front carb, and only that source. If this is the case, then simply time the engine as I suggested previously, and run the vacuum line to the vacuum can directly from the carb. You can attach a vacuum gauge to the line to check the vacuum. If zero at idle then raise the speed gradually. You should see a vacuum signal when speed rises. This is a venturi source. It may be a good idea to also change out the vacuum can to a single inlet style from and older model.

One final point, as you tinker with this, remember, your engine speed and timing related to it will affect the vacuum at that speed. This can have an affect on the idle speed dashpot at the rear carb, if your car is equiped with one. This dashpot is used to set idle speeds and may need to be readjusted as you work your way through this problem with the distributor advance/vacuum.

For what it's worth, this has worked for me and I've been running my 230/8 this way for 32 years.

230/8

[dkveuro]

Originally Posted by 230/8

All this vacuum talk, port/manifold, is very interesting, but doesn't answer the question, "how to fix it?" Simple really...


230/8

Did I just WASTE my time in my last post or what ?
Hell's bells.....If it was being made so bloody simple...how's come we are at post #30 now !