Boxerworks.com

Hi, new to forum, not BMWs. Bought a '74 R90S in '87. Had it worked on by someone who I thought knew more about machines than me - Big Error. Anyway, after this "service" The valve train developed a strange behavior: after ~5 miles it starts to sputter and backfire through the intakes. In checking the valves, all are so tight as to not close. Resetting them just allows it to go another ~5 miles then misbehave again. Eventually, it broke the rockers by being So out of proper angle.

The "mechanic" had no idea what was wrong and suggested the valves has "tulipped" from too much unleaded gas on leaded valves. This didn't sound even close to right, but not having the $ to get it fixed right, I parked the bike.

Zoom ~20 years, now with the $ to reassemble and try again, I bought & measured a set of used heads - same dimensions on both. It's a very simple system and I now have enough background to do my own mechanicals. But I still have no idea what's wrong. I don't think they're hydraulic lifters, so: could they be put in backwards? Has anyone seen / heard of this? Thanks in advance, <<<)))

I'm trying to picture what would cause the valves to simultaneously close the clearance to zero. Usually it's the exhaust valve more than the intake. Here's a head from my R75 a few years ago:



The exhaust valve wasn't tulipped, but the seat was eroding and you can see that the face of the valve is almost below the face of the seat. The intake valve still has some margin above the seat and is closer to right but it was also closing up. The final few thousand miles on those heads the clearances closed from .004" (I) and .008" (E) to zeroish in a few hundred miles.

Post some pix of your heads when you get a chance. Here's how my heads looked after the rebuild:

IMO valves ca 1974 don't tulip...that is a term used for the 1981-1984 valves when BMW had the wrong metallurgy for the seats. These seats didn't conduct heat back to the head, so the heat remained in the valve face and warped them. Stock valves from the 1970s would recede because of the lack of lead in the gas...they basically wore down.

If you're going to go forward, you should use valves and seats from 1985-on...BMW finally got that right. I would send the heads out to someone who knows how to deal with Airhead top ends. Ted Porter in California is at the top of the class on that.

Kurt in S.A.

Hi, Much thanks to you both. The eroding seat makes total sense. That said erosion accelerates rapidly would explain my problem.
Engine gets to temp, valves recede & not close, readjust simply lets it repeat. I'm thinking, pull & send out the heads. Then reassemble with a fresh gasket set. Funny: in Three HOURS I get an answer to something gnawing at me for 20+ years. THANKS Again <<<)))

Ted Porter is highly recommended. About ten years ago I had him rebuild my stock '74 R90s heads. (130k and all original).

There wasn't any valve recession but I had him install new, modern seats, new valves, new guides and springs. The bike has 141k on it now and it's probably running the best it ever has.

Don't want to be a party pooper but...

VSR doesn't normally affect inlet valves and even with the exhaust valves, it normally takes several thousand miles (not 5) to close up the gap. Also, it doesn't stop being a problem when the engine is cold. I think there must be something else going on here... but I can't think what...

What work did the mechanic (if you can call him that) do?

Oh, and no, theyre most certainly not hydraulic lifters...

Rob

Hi, and THANKS for the reply. I'd been riding this several (`3) thousand miles, can't know about previous owner. Neither of us aware of valve seat consequence. So it might be a matter of deterioration of the seats.

Stupid truth, get the heads upgraded to unleaded tolerant fuel and see what happens.
I suspect the prior seats got to a "tipping" point where prior seat recession accelerated.
Can't / don't know and Don't care -- Just need to FIX IT.

Could you give us the mileage on the bike, may help us in diagnosing issue. Also what was original issue that prompted you to use the "mechanic" .

Hi, it's 368xx miles, but unknown how many were on unleaded fuel. After 20+, I don't remember the initial issue, but suspect it was the valve recession first causing trouble. Heads get sent out 7/30, not going to look any further until they return and I put some miles on it. The seat recession just makes too much sense. Valves needed the upgrade anyway. Thanks to All <<<)))

I guess we won't see pictures? General consensus seems to say you have more going on than VSR. Good luck.