A little help needed for a new guy

[chipysbmw]

Hi all,

I’m a new member but have lurked on the fringes for many years and wonder if you all could help me out, as I have an interesting mechanical problem.

I’ve got a ’78 r100, which was ridden by my father and passed on to me, and I have ridden her for years but put it out of service last year as work became heavy. At the beginning of this summer (uk) I decide it was time to get her back on the road and when I tried to fire her up the engine slowly turned. Ahhh… flat battery I thought. It wasn’t. Anyway to cut a very long story short I’ve got to the point where the gear box is off as well as both cylinders but the engine is still binding. The symptoms are these: when I turn the engine, with an Alan key in the alternator, it is free from binding on one revolution but extremely hard to turn over on the next. The pattern continues. Any ideas?

The beemer is a lifelong bike and wont be leaving my hands, I am of moderate mechanical skill but in no way an expert so I’m also looking for a dummies guide to removing/replacing the block internals, timing chain, main bearing etc but I cant seem to find one.

Anyway that’s my problem, thank you for reading and I hope you help.

Regards,

chipsybmw

 

[Rob Farmer]

Starter motor.

You have the earlier Bosch (as opposed to the later Valeo). I've recently had exactly the same issue in my 78. Stripped the starter, measured the coils and the rotor windings all ok. replaced the bearings and it was exactly the same. Fitted a valeo and it's spining freely and starting easily. The Bosch is a great starter motor but it draws a huge amount of power compared to the valeos, it's also a bit of a beast in terms of weight. The Bosch draws 320 Amps when in good condition so the slightest increase in electrical resistance internally or in the starting circuit will render it useless.

Take the plugs out before turning the engine and see if it makes a difference I suspect the binding you are talking about is just the extra resistance of the compression stroke. Airheads are so simple inside the chances of needing a bottom end rebuild after it's stood for a while are pretty remote, especially if it was running well before being laid up.

Count the number of teeth on your starter before buying a replacement. Yours should be 9 tooth. They changed over from 8 teeth in 77 so its always worth checking with bikes from the era. Motobins have a new starter for your bike for £118 inc VAT (Part No.71020A) a Valeo is £294 and a Bosch is NLA but you can buy the parts for it.

 

[Kenny]

Certainly try what Rob says re starter motor as I doubt very much anything else has seized in the block.

 

[chipysbmw]

Hi Rob and Kenny,

Thank you for trying to help me with problem. The starter motor was one of my first thoughts. I removed the it and changed brushes and bush but no difference. So i brought a new one. Still problem was there so continued on and got to the point where both cylinders are off and the pistons can move freely in open air, pushrods out, the gear box is off so the engine cant bind through sticking clutch plates but still the engine binds on one cycle and is free on another, when i turn it over via the Alan key in the alternator. Surly the whole engine should turn easy? I cant for the life of me think what it is.

Regards

chipysbmw

 

[Rob Farmer]

If it was me I'd go the whole hog and put the front timing chain cover and separate the timing chain. that will break it down to the crank or camshaft/oilpump.

This is a 78 engine with the timing chain cover removed.

Before you do that though drop the sump and have a look for a small chrome pin. 2 or 3mm in diameter and maybe 5mm long.

 

[chipysbmw]

Hi Rob,

I've had the day off work and spent the day researching, I to have come to that conclusion. Could you tell me what finding the chrome pin would mean? Also do you know any good books for main bearing and seal replace meant? I think ill strip the engine completely

 

[Rob Farmer]

The pin holds the front main bearing in place. If it comes loose the bearing can move in the housing. The bearings have ports in them to provide a pulse of oil to the top end. If it moves too far it can block the oil flow completely. I'm not saying it has done but if the bearing has moved it could possibly be causing your problems. If you split the cam chain you will have a better idea if it's the cam or crank.

John Roberts posted some pictures a while back but I cannot find them.

You can get some very strange things happening. I had this a few years back. Suddenly lost oil to the front big end and main. Pulled the bearing housing to have this pop out

it was a square oil seal from a oil cooler cover. The strange thing was the engine didn't have an oil cooler so it must have been in the back of the block for years before working its way up the front oil way.