316, M10B18 rough running after new head-gasket

[e21_james]

Hi,

The head-gasket blew between cyl. 3 and 4 on my 316 earlier in the month and I quickly swapped a new one in and rebuilt the carb. while I was at it. Trouble is she's running a Weber 32/34 DMTL carb. instead of the stock and I also had to remove the distributor entirely to access the head-bolts. With everything back together it fired up no problem and is back to all cylinders, but is running very loudly, as if the exhaust is blowing badly and there are slight misfires or backfires when the car is rev'd, as in the vid. below.



I'd replaced the dizzy-cap and rotor-arm while fault-finding, as well as fitting 4 new spark-plugs, but I have now gone back to the originals, which are all OEM BMW replaced parts, and this has it running slightly better. I have tested compression and all 4 cylinders are spot on. This weekend I bought a timing strobe-light and checked it against both the steel-ball mark on the flywheel and the groove a previous owner has cut into the front crank-pulley.

With the vacuum-tube removed from the carb. we could get the engine to run much better and react better to adjustments to the distributor, but the sweet-spot is still more or less the same place and it's not perfect. Replacing the vacuum-tube though caused the engine to die first time round, and now it will run I'm back to the noise and misfires, regardless of distributor-timing or the idle mixture of the carb and, having spent over £150 now, I'm at a loose end and am on the verge of calling someone in to tune the car, provided something else isn't deftly wrong.

Sorry to throw a bit of an essay in as my first post, but any help and advice would be greatly appreciated guys, so thanks in advance!

[Jeroen]

OK sounds like timing is way off indeed. First step is to make sure the static timing is spot on, meaning with the vacuum hose(s) disconnected from the ign dizzy. I don't have the ign details on hand here (required pre-ignition in degrees and at what rev) but to check you need to have a look at the Z mark on the flywheel, I wouldn't base myself on some mark on the vibration damper.

After that you can reconnect the vacuum hoses and she should be fine. Should, but reading your experiences you may have the vacuum hose layout wrong which may lead to the wrong vacuum values causing the vacuum advance system to shoot to an extreme causing the rough running. But it's worth another try after you're absolutely sure static timing is perfect. I guess a decent workshop manual will be able to supply you the correct values, but with a bit of luck someone else here can help you out as well.

[Jeroen]

Ignition timing values: 25 degrees pre ignition at 2200 rpm should give you a stable steel ball and the Z mark right next to it through the bell housing inspection hole.

Added info to FAQ section as well

[e21-Mark]

Hi James, where are you based? It may be that one of us is nearby and could take a look?

On the Weber there are 2 screws. One is the mixture and the other is the idle screw. Carefully turn the mixture screw fully in. (gently till you just feel it bottom out) Next screw it out again by one and a half full turns. (between one and a half & one and three quarters is usually fine)

Once the engine is running at idle, let it warm through. You can then undo the pinch-bolt that keeps the dizzy in place. You could now try advancing or retarding the ignition just slightly. Easy does it though, as you won't need to move it far to have an effect.

I would also whip out the plugs, turn the engine by hand till number 1 piston is at top dead centre, then see to make sure the rotor arm is facing number 1 lead in the dizzy. It's possible to accidentally time the cam out by 180 degrees and end up with these same issues.

[Reck]

25 degrees at 2200 sounds right. Hard to keep steady at 2200 if it's running like a pig though as me and Nas found!

If anyone has a vaccuum diagram for connecting the weber I'd be grateful too.

[e21_james]

Well, thanks for all the replies guys, it's been an interesting weekend. Prior to me posting we had been timing the ignition to about 25 degrees before TDC to 2200rpm - where the silver ball appears. I rechecked the static timing this weekend and we again tried timing it in, but if anything the noise and spluttering had got worse, so we decided something else must be at fault. This weekend, despite the pouring rain, I got the E21 on ramps and had a better look at the exhaust and it turns out one of the manifold gaskets slipped off its stud during fitting and was miles out of line. Ironically, it was cylinder 4, the only one I couldn't see properly with the car on its wheels. Whoops...



As you can see from the pic the gasket had been blown apart by the exhaust. I rooted for a spare in my gasket set but only have ones with the oblong hole I think for E28, so I tentatively replaced the blown one with a liberal helping of Holt's FireGum. Once that the manifold was in place timing the ignition took about 10 minutes, now I'm playing with carb. tuning. There is still a ticking/clapping noise coming from the engine somewhere, but i'm pretty sure it's the broken exhaust gasket - I've ordered 4 more from ECP. Apart from that the old M10 is running like a dream again after a fraught month.

I'm based on the Wirral by the way in the NW, so pretty far from the South, though I did buy my E21 from Sutton, Surrey - still has the Coombs of Guildford rear-window sticker. It's a 1983 Y in Henna Red.

[Jeroen]

Glad you found the cause... most importantly the head doesn't have to come off again!

[Reck]

A swine to get at the manifold studs with the engine in the car I found. Glad you had your issue resolved. Looks like you may have had the same cheapo gasket set I did off ebay - I also found that many of the manifold gaskets were the wrong shape.

Plenty of us in the North. I'm in Manchester - also with a 1983 Henna 316!

[alschux]

Cool, another 83er from Coombs of Guildford - those stickers must have been made of good stuff!

[nas80]

Always is something simple on an m10