Zenith Carb backfire after Crane installation

[230/8]

Cool your jets podna, I started to draft my post and then had to leave for a meeting before finishing it. I finished it when I returned and posted it without knowing yours was up. No ruffled feathers intended. Your advice was excellent. Mine merely offers an alternate way to look at a possible solution, one that has worked well for me. My old 230/8 only began to drive well after I worked around and through all those pesky smog mods. MBs of our vintages were originally designed for much different conditions than we have here in the USA. They were modified to work with the evolving smog laws of the early 70s. Most of them can be made to run far better when we can figure a way return them to a condition close to their original design...or as near to that as we can get, given the pieces we have to work with.

As to simple, everything is simple when one follows the methodical step by step approach to problem solving you outlined so well.

Cheers,

230/8

p.s.: re-read your closing motto, it seems most appropriate in this case, would you not agree? Cheers, again...

[250 Coupe]

Hey guys,
I'm printing both of your "Patented Methods For Timing An Old Benz" and tucking 'em into my MB folder as they both have valuable information.

Oh, by the way, if some fool (not me ) has put the wrong head on the car, it will NEVER run quite right. The cam will end up a half tooth off no matter how many times you try to set to rights.

Michael
Former owner of a very poor running 250/8 sedan.

[dkveuro]

[/QUOTE] p.s.: re-read your closing motto, it seems most appropriate in this case, would you not agree? Cheers, again...[/QUOTE]



'scuse me? Is this a 5 minute argument, are you here for the 30 minute session?

I'll have you know I'm a lot simpler than you might think ?? wait up...that's not right !

This is kinda like..... " I'm not as think as you drunk I am."

[dobrodan]

I am real late to this thread so here's a question that may have been answered already. Have you done a compression check? A bad intake valve will cause a carb backfire under load, like acceleration. Also, is your valve timing OK?

[rgnprof]

Just back in town - spent the last part of the week in Colorado Springs - much prettier part of the country than OK!

Appreciate the responses - I have checked the compression and it is fine - have not checked the valve timing.

I have made another discovery though. I have a MB supplemental manual that includes a lot of carburetor info, crankcase ventilation, '72 modification info and the emissions diagrams. I have had the emissions hoses connected for a '71 and not a '72 model - (slight changes made in the system from '71 to '72). After rerouting the hoses I have isolated my vacuum advance problem. There is a speed relay that is supposed to change the ignition from retard to advance at rpm's > 2400. When the engine is idling at oil temp > 17 C and coolant temp < 100 C I have 12 volts to the 3 way valve and it is open and energized and vacuum (controlled by the 3 way valve is run to the retard side of the dist. vacuum box. At rpm > 2400 the relay is suposed to shut-off the valve (voltage should drop to 0 at the control valve connector)and vacuum is routed to the advance side of the dist. vacuum box - THIS IS NOT HAPPENING.

My 3 questions -

1) The schematic shows this speed relay connected to terminal 7 - where and what does that mean?

2) How does this speed relay know when the engine is running greater than 2400, and would connecting a Crane ignition affect that? (BTW, this relay was not switching the vacuum advance before the Crane installation - I don't know if the relay is bad or if it has something to do with the signal TO the relay. I have another relay that I got from DANTRCAV that does the same thing as mine - 2 bad relays??).

3) What problems does this cause - not getting proper advance?

I know that I can adjust it per some of the ideas posted earlier, but I am exhausting my options, I guess.

Thanks, Ryan

[250 Coupe]

The speed signal came from the old switch gear.
I've forgotten what model we're talking about. I'll try to look around but I don't think it will work with the new ignition. I have my solenoids bypassed for now.

Michael

[rgnprof]

It's a '72 250. By the 'old switchgear', your talking about the ignition box under the battery that the Crane bypasses, right? Is there another way to send the speed signal to the relays? I've read and re-read the suggestions from yourself and from dkveuro and 230/8 earlier in this thread and I'm still not sure exactly how to bypass the relays and valves effectively.

You said to 'set the timing at 40-42 degress at whatever RPM the book called for', and I'm not sure what this means or how to do it?

It just doesn't make sense to me to run vacuum to the advance side of the dizzy vacuum box all the time and to just cap the retard side - not considering the way the vacuum works in my car. I'm sure I'm making all of this much more difficult than it needs to be... besides, my car runs pretty good right now, although I am concerned about the consequences of not getting proper advance (mileage, etc.??).

Ryan

[250 Coupe]

Originally Posted by rgnprof

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

Ryan,
This how I have mine timed.

Yes. By "old switch gear", I did meant the module under the battery tray. Mercedes called it switch gear since it isn't really an ignition. It acts as a high power switch for the coil and uses the distributor points for a low current signal. It's supposed to extend the life of the points.

You are correct about the vacuum lines. How mine is hooked up doesn't make sense since I have full vacuum advance at idle, lose it when first opening the throttle only to get most of it back when cruising. I guess it really doesn't matter since it's only 4 degrees.

It (the advance can) should connected to ported vacuum but there isn't a port on the Zeniths.

Glad to here that your car is running well.

Michael

Oh ya, I get 13MPG for my 9 mile commute and closer to 15 on the highway. I also have a very heavy right foot.

[DANTRCAV]

Ryan:

Just for grins why don't you try what I did before I got my Webers?

Plug the advance/retard and set the initial advance to 10 degrees before and let the mechanical advance give whatever it gives. Drive the car and see if that is acceptable. Worked for me (not perfect) until I got the Webers and a ported vacuum pipe.

Dan

[rgnprof]

OK, I'm about ready to wrap this up - just some experimenting left.

Dan, do you mean to set the initial timing at 10 degrees BTDC w/o any hoses connected - plug everything off at the carbs and the control valves and leave it at that? Then I clearly have no vacuum advance, right?

Also, is there a way to get get a ported vacuum signal to the advance side of the distributor vacuum box, or is there no ported vacuum source on this car ('72 250)? I can hook up a good vacuum source to the advance (so that side holds a vacuum) side and timing advances and idle speed increases, but if I just run vacuum from the front carb (manifold vacuum) then I am always advancing the timing, even at idle, so I don't know if that will help.

Thanks, Ryan

[Arthur Dalton]

What some do is to grab vac from the manifold [ highest vac at idle when carbs are closed] and run that line to the vac retard. Then they set the timing at 3000 rpm first. What happens is when at idle , the vac retard brings the timing down and as soon as you start to open the throttle, the timing instantly advances [ collapsing vac.]
This is just a copy of the older vac retard FI engines that only used vac retard and mechnical advance. Not a Bad timing curve..The -051 distributors used this system..
That was pre emmissions mandates and they worked just fine..

[speedy72t]

Sell it and buy a Honda?

[DANTRCAV]

Ryan:

Yes, set the initial timing at 10 degrees BTDC. Disconnect both advance and retard lines and cap them. You might have to adjust the idle downward.

Correct, you are running with no vacuum advance/retard. The only advance you will get is mechanical.

As I said, I used this until for about 4 months until I got the Webers. I didn't have a problem and my car ran a lot better. Doubt it would make your situation any worse. If it doesn't improve the situation or the improvement isn't enough then it is easy to reconnect and retime.

Good luck.

Dan

[Arthur Dalton]

By hooking the retard up to manifold vac , you will not have the idle problem and you will also have a wider curve.

[rgnprof]

Thanks Dan, I will try it again. I think last time I tried it this way I did not have the carbs as well balanced as I do now.

Arthur, I'm still not sure how to set the timing according to your suggestion. You state ' Then they set the timing at 3000 rpm first'. So if I hook up the vac retard and disconnect and plug all the vac advance lines, how and to what degree do I set the timing?

Secondly, since I have replaced the stock coil and igniton with a Crane ignition and coil, how do I now measure the rpms?

Ryan