Timing/ignition! problem?

[Paolobuchberger]

Hi Guys,

Hoping someone can help. My new engine is missing at low revs (under 1500) and high revs (over 4,000). To try and improve things, my friend just overhauled the distributor with its electonic ignition insert because cylenders 1 and 4, especially 1, was getting very little spark. Leads are new.

The real mystery for him in diagnosing the propblem is that if you remove the lead from no 1 while the engine is running there is no discerninble difference, the engine carries on. Same with no.4. If you remove the lead from no. 2 the engine starts running quite rough, which is what he expected it to do when removing no.1 or 4. In fact the engine started with both plugs 1 and 3 out!!

I knew our little boxers were bulletproof but this is rediculous!

Does anyone have an idea or know if it is a characteristic of our engines that they can just keep idling and reving seemingly undaunted with one of the two front plug leads removed??

[evilgidget]

Have you tried putting plugs on the leads out of the engine & checking for spark? I've bought new leads before that were faulty out of the box, so I wouldn't be surprised if you've got the same problem.

[Greek]

Take a check on the spark plugs. I had almost the same behavior a couple of weeks with brand new sparkplugs (1 month old) and for some reason one of them went bad and didnt work. i indication was that it had different color on the top thatn the other and it might be caused for gas overflow that shortcircuited it. Once we changed it the engine is working fine...

[Paolobuchberger]

Hi Guys,

We tried swapping leads and changing plugs as well (three different brands) same result. Plus after the dizi was cleaned up, the spark on plug no.1 Was actually much, much better. Advancing and retarding the timing made no difference either!

After I posted the thread, my friend tried adjusting the fuel but strangelly enough, the right hand carby front throat (over no.1 cylinder) does not respond at all to the idle adjustment screw. You can screw it all the way closed (or all the way open) and it makes no difference to the idle. If you try that with the left hand throat (cylinder 3) the engine immediately revs lower and starts to stall, which is what we expected the other one to do.

Could this be aa fuel feed problem rather than electrical?

Thanks for the thoughts though, I really appreciate it. Any more welcome.

regards

paolo

[BigSud]

Sounds like you've got blocked jets on the cyliners that aren't running properly. Certainly that will cause it to idle badly, and depending on what else is blocked, it may also have something to do with the high speed miss.

Pull the idle jets out and blow through them - DON'T use a pin or file or any other metal pick to clean them. If you must, use a toothpick - but don't damage the brass. Next, do the main jets.

It might also be helpful to pull the carb covers off and check for dirt etc in the float bowls - that can be a sobering experience if you haven't done it for a while.

Chris

[jol]

If first cylinder is dead (caused by no spark), of course you wont get any answer when adjusting idle on it.

[Paolobuchberger]

Yeah we are going to check the jets tomorrow. We are definatelly getting spark on cyl. 1 now, though maybe it's not firing at the right time, I don't know if that's possible when the others do seem to be. Certainly the plug in cylinder 1 ends up dirty from what seems unburned caked on fuel rather than anything else and after swaping the plugs in 1 and 3 as an experiment, we eended up with the one in 3 cleaning itself up and the clean one in 1 caked in muck again.

I,ve found someone who will get me a second hand Bosch electronic ign. system that came with the twin carbs (dizi and coil etc). I was going to get it anyway, but now if we can't sort the miss we can always swap them over as a last resort I suppose. Mind you the existing system was working on the single carb engine and seemed fine on this one for a couple of hundred k's.

It actually doesn't run all that bad considering! Amazing what a Sud can do on three cylinders!Just trying to look on the bright side

Thanks

Paolo

[jol]

"if you remove the lead from no 1 while the engine is running there is no discerninble difference, the engine carries on."
This means #1 is dead.
I'm having this same problem myself, this thread gave me inspiration to check the ignition part again (swap back to the old leads and another plug)

I tried cleaning the plugs from the sht making a short-circiut but no change

[damonb]

Paolo - as Bigsud says, check for blocked jets on the No 1 cyl's carb throat. If you've got good spark then it's most likely fuelling. If a jet is badly blocked, fuel may be sort of dribbling through and not atomising properly, causing the plug to foul quickly.

You can fit the Bosch ign system from twin carb engine - this will be at least correct in terms of advance curve etc if indeed there is a difference from single to twin carb in that regard. Don't think it'll solve your current issue though.

[mrbram77]

I put new leads on yesterday.No spart at all on none of the cylinders. SO after messing with it i think somehow i broke the coil i'll get a new one and see what it does. any other sugestions (was running fine before)

[Paolobuchberger]

Hi guys,

Appreciate the advice. Did check the jets and the idle jet was blocked on no.1. Cleaned it with compressed air and hopefully we are eliminating potential causes of the miss. Mind you now it isn't running well at all. I think these carbs are going to need some carefull attention.

I'm hoping the different dizi. will improve things but not optimistic.

regards

Paolo

[jol]

Add a fuel filter just after the fuel pump to longen the service intervals of the carbs.

[Greek]

What kind of spark plugs are you using?They dont seem to be golden lodge for certain.

I have founδ this site that has different kinds of spark plugs and the cause of that. iIt mught be usefull....

SparkPlugs

[Paolobuchberger]

The plugs are NGK now but untill last week I had Golden Lodge. I do have a fuel filter attached after the pump.

[Paolobuchberger]

Hey guys,

Sorry in advance about this novel.

We've been doing a lot of investigating on my no.1 cylinder problem today and I need some info.

My colleague has diagnosed what could be described as blowback through the carby throut on no.1. With the air filter off and engine running, you could hear a hiss coming fro no.1 carby throut. placing some paper over that throut, you could see that air was being blown back up the carby and fuel spitting onto the paper from below.

We tested compression and got 150 for all cylinders except no.1 which was about 143. When you choke off the carby throats (just by covering them over) one by one the engine starts to run down as you would expect, but no effect on no. 1. We cant find an air leak, though I realise it must be getting air from somewhere even if its not running well.

After all that, this is the real question. My friend knows lots about engines but not these engines and there design. After removing the carby, he manually turned over the engine slowly with paper over the inlet manifolds, watching for suction and the exact moment where there is blow back.

What we found was that there is blow back up the inlet manifold for about 10 per cent of the compression stroke. So there seems to be overlap between closure of the exhaust and opening of the inlet valves. This happens on cylinder 3 too, not just the potentially dodgy one. We were shocked to see that the person who set the tappets after re doing the heads had set them to less than minimum tappet clearance! We re-set one tonight as a test, but there was still some blowback (though less), so that does not seem to be the whole problem. All the tappets were set below tollerance. but only no.1 was acting up.

Does anyone know wether the "overlap" or extent of it is normal on our boxers?

We cant really get any further without knowing, so if anyone does know, I'd really appreciate hearing it.
regards

Paolo