Monday, 14 January 2013

Misfire Woes:

Went on a Maccies run with some work colleagues on New Year's Eve and I may have been pushing the old girl a bit hard, when a horrible clanging noise began from the engine and got worse quickly. Some of the guys at work suggested the first stop should be the rotor-arm and dizzy-cap so we managed to get some delivered through the parts dept. that afternoon, but it was still chugging on a mixture of cylinders and it limped home for me then refused to start again. When it eventually did the old M10 barely managed to fire 3 cylinders.

My list of diagnostic checks was:

Replaced distributor-cap and rotor-arm.

Tested all 4 plug-leads.

Replaced all 4 spark-plugs.

Removed, cleaned and rebuilt Weber carb.

Checked valve-clearances and confirmed all valves appear to be moving correctly, turning the engine by hand.

Finally diagnosed cylinder 4 as no firing.

Plug and lead tested again, this time sparking against the suspension turret.

Compression test revealed Cyl. 1 and 2 @ 150psi, 3 @ 45psi and 4 @ 25psi.


This all seems to point to a bust head-gasket between the 3 and 4 cylinders. It is unlikely the valves for 2 cylinders would have bent or suddenly need adjusting at the same time, so off with its head and lets look at that gasket.

*UPDATE: The head came off pretty quickly and revealed one seriously blown head-gasket right between the 3rd and 4th cylinders [detailed pics in this post]. Diagnosis spot on, but the culprit may be a slight warping at that end of the alloy-head - not very much at all, but still a worry considering it looks to have been skimmed before. Either way, the new head-gasket is on [I went for a full head set off eBay for £27.99, which includes the manifold gaskets etc.] and the car is running on all four cylinders again, though not as happily as it could be. I was forced to remove the distributor and seem to be having some issues with ignition timing since putting it back [in this post], and the carb. still hasn't been tuned properly since the rebuild, so a range of fettling awaits before I can drive it for the first time in 2013.


Sunday, 13 January 2013

Head Gasket - well and truly blown!

Here are some pics of the blown head-gasket that turned out to be the misfire culprit [from this post: ] and it turned out my forum-eductaed guess was right and it had failed right between the 3rd and 4th cylinders. Kaboom!

No damage to the water/oil galleries thankfully, but still a catastrophic way for the gasket to fail so quickly under a bit of load. You can see best in the bottom pic of the head where the gasket has begun to fail at the left side of cylinder 4, taking that down and then blowing through to cyl. 3. Not good. An examination of the cylinder-head when removed showed that it is warped a tiny bit, so it looks like the engine has been overheated at some point before I got the car and this is probably going to happen again. Oh well, priest-like driving will be the order of the day unless I just go seat-of-the-pants until it goes again, but in the long-term, an M44 engine-swap from a more modern 318is is looking like a must!





Saturday, 5 January 2013

Weber Carb. Rebuild [Twin DMTL]


As mentioned in this post [], rebuilding my Weber DMTL carburettor was one of the first things on my list of curing my misfire problems, which turned out to be a more serious blown head-gasket. Still, restoring the carb. is one of the first things the previous owner told me I should do and gave me a brand new Weber rebuild-kit with the car, so I guess it can't hurt the old girl.

This job isn't for the faint hearted though, so if you have a disposition towards botching fiddly things as I generally do, then it's probably best left to a pro - I messed up my throttle-cable and had to replace it, and fettling the throttle-spring arrangement was all but impossible to get back to how it was. It's still not right, throttle springs etc. a bit loose, but at least its running ok again.

Here's a great guide for tuning the carb. once it's rebuilt: http://www.carburetion.com/Weber/adjust.htm

**CAUTION: If you're running a Weber on an E21 then your 'Bowden' throttle-cable will likely have been cut to fit through the cable-eye on the throttle linkage. Cutting the metal end off the cable causes it to lose its tension and the strands begin to unravel immediately so be very careful when removing and fitting the cable to the carb. [especially if you need to do it more than once which broke mine], as once the cable has begun to expand inside the sheath it binds very easily and you will need a new one. These aren't in production anymore either and I got the last one from BMW Classic in Park Lane, though I'm sure plenty are still out there, but still save yourself the worry!