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1969 Swiss 404 - 1977 Porsche 911 S
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148 Posts
Discussion Starter #1
My 404 was running just great.

Suddenly while driving, it backfired through the carb. Since then, once the rpm reach a certain level it backfires throught the carb.

This change occurred very suddenly so something must have changed.

Its been running great until just now. I've recently installed the civilian ignition kit which included a new cap, wires, plugs, rotor and ignitor and I've recently checked the fuel filter which was very clean. The timing was checked when I installed the kit and then once a week later. I've got at least 20 hours on it since this was done, during which it has run perfectly.

Backfiring throught the carb suggests a timing problem to me.

Any suggestions as to what I should check first would be appreciated. Like I said, something changed suddenly to cause this, one second it was business as usual, the next second it was running poorly.

It starts normally and idles fine. It just won't run well above a moderate rpm level.

I've got it here in my shop right now so if you think of something let me know before I dive in.

Thanks.
 

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1969 Swiss 404 - 1977 Porsche 911 S
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148 Posts
Discussion Starter #2 (Edited)
I guess it was more of a miss than a backfire.

I took off the dist. cap, everything appearred to be in order there.

I hooked up the timing light and noticed that the strobe was a little erratic. When I revved it up to the point where the engine would miss, it corresponded to the timing light. This seemed indicate an ignition problem.

I took the cap back off and the only thing I could think of to change was to disconnect the condenser wire which we had grounded back to the distributor body. After I installed the pertronix some time ago I noticed that some others had left it loose or cut it off. The instructions didn't even mention it so we screwed back on to the body of the dist.

Anyway, this seems to have fixed the problem but I'm still wondering why it went so long running just fine and then suddenly started acting up. Does anyone have a theory on what happened here? Have I partially fried the ignitor or the coil by leaving the condenser attached? I'm a bit puzzled. I really don't like it when I fix a problem but don't understand how. The problem often comes back to bite me somehow.

Anyone?
 

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1967 404 Unimog (Belgian), 2010 Ford Fusion Hybrid
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253 Posts
Last spring I had a problem that sounds similar to yours. My truck would idle fine, but after driving for about 15 minutes it would begin to misfire at high rpm. As you drove it longer the misfire would get progressively worse until it was misfiring as low as 2000 rpm's.

I took me a while to find the culprit. I had installed the Petronix ignitor more than a year before and never had any trouble with it. I didn't want to believe the ignitor was the source, but after putting the points back in and the truck running well I had to believe it.

I spoke to Pertronix about it and they helped me solve the trouble. The most common way that the UM-161 ignitor is installed is to put the red wire between the coil and ballast resistor so that it is at +12V. In reality the ignitor should be run at +24V. It will usually work at lower voltages, but sometimes variations in coil or ballast resistance can cause the voltage between them to drop below 12V which is what was happening to me.

After wiring the ignitor up to +24V (or closer to +28 really with the engine running) no more trouble. Most people wire their ignitor at +12V and have no trouble, but it should be run at +24 according to Pertronix. I spent quite a while talking to them on the phone, they were very helpful.

Anyway, not sure if that's your problem, but it sounds similar to the issue I had last year.
 

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78 406
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251 Posts
Another great reason to leave in the points. What a great, reliable and simple settup. I love the points system on my M-127 motor. You always can get parts, I carry the old points, condensor and rotor in my trunk just in case. I have never needed them though. I always hear of trouble with a missing engine at higher RPM's with these retrofitted electronic ignitions. Just looking at the military cap, wires and plugs on the M-180 engine impresses me as a great heavy duty system. I guess I am just old school and grew up on engines with points. You just need a dwell meter to set them up, although I have done many with just a matchbook cover for a feeler gauge. The Petronix can cover up the symtoms of a wore out distributor though. I would guess the Petronix is close in price to a rebuilt dizzy, just a guess though. Has anybody ever rebuilt there distributor? Dan Caron still has a Bosch disributor analyzer and can rebuild and settup your dist. to like new condition. He is also the ultimate guru on these engines. He has installed the dual point distributors out of other Mercedes engines into the m-127, he calls the car The Red Rocket. He sells parts and does work on the M-127, M-129 and M-130 engines, many parts also fit the M-180. Check his site out http://slbarn.********/about/
 

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I am no expert, but what I found was that the springs deep in the distributor were rusted so that would no longer extend and the advance would not work properly. Once I found this, I cleaned them and was able to get the power back, otherwise it would hardly run above idle. Yes, I have civilian ignition setup.
 

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1969 Swiss 404 - 1977 Porsche 911 S
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148 Posts
Discussion Starter #6
The distributor weights are advancing the timing as rpm's increase.

I've tried connecting the + wire (red) from the ignitor to both the input for the noise suppressor and the coil itself.

I had a look at the trouble shooting tips on Expedition Imports site. I tried moving the ignitor closer to the magnetic ring.

The problem has not gone away. My thoughts that the condensor wire had something to do with it were wrong, although it did work fine for a short time after disconnecting it. The condensor shouldn't be doing anything at all now anyway.

So it would seem that I've either had the new coil or the new ignitor suddenly go bad on me. I'm leaning toward the ignitor.

Is there a way to test it? I guess I'll have to call Pertronix and see what's up.

This conversion kit was supposed to make things more reliable...

Scott at Expedition Imports - any suggestions?
 

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63 UNIMOG S404.115
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116 Posts
I've tried connecting the + wire (red) from the ignitor to both the input for the noise suppressor and the coil itself.
Whilst I haven't had any problems with my conversion I did have a to and fro with Scott over the take-off point for the ignitor. A previous post stated a view that 24v was required and this is supported by some, but not Scott. I've stuck with his recommendations. What you have done by taking the ignitor from the coil is further reduce the voltage.

You would need to take the voltage from BEFORE the ballast resistor to get 24v or greater when charging voltage is taken into account. You can try that but make sure it is a switched supply.
 
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