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Engine won't turn over

5K views 15 replies 4 participants last post by  AWJDThumper 
#1 ·
Having fixed what I thought was a sprag clutch problem, I still cannot get the bike to start. The engine tries to turn but stalls at what seems to be max compression. I have fitted a new battery and fully charged it, this made no difference. I have removed one of the twin spark plugs and this allows the engine to continually turn over with no compression. It's as if there isn't enough grunt to turn over the engine, but I believe I have eliminated the battery. Does anyone have any ideas what the problem could be please.
 
#2 · (Edited)
There is a compression release on these engines. Maybe that's not working?

Google decompression lever - there seems to have been problems with them over the years on this model. I did read a thread about somebody who replaced/upgraded theirs but I haven't found it yet.

This is more info about the decomp lever https://advrider.com/f/threads/the-g650-gs-thread.760254/page-285

(Assume it's ok to link to useful info on another forum? If not, apologies to mods and please delete)

Buzz.
 
#3 ·
Thanks for the reply, very interesting and it does seem as it could be a contender for what the problem is. The link gives a good description of how to change the lever and it doesn't look like a difficult job. I couldn't really find much more regarding it being a common issue. It doesn't appear to be a common item to buy either. I can only find the spring available, so it may need a trip to BMW.
I have had the starter motor apart to clean and paint up the cover, so I was thinking it may be a problem with my rebuild. However, it spins and turns the engine with the plug removed so I don't now think this is a concern.
The decompression lever looks like an investigation project for next week.
Many thanks
 
#5 ·
Thanks, I noticed on the few other references to decompression levers I did find, it did seem to be later bikes or the G650.
I really am at a loss for other ideas though, I was hoping someone on this forum might have experienced something similar.
 
#6 ·
I guess the other option is to try a new starter motor in case it's weak but it's a bit of an expensive gamble.

You could try jumping direct from a good battery to the starter motor terminal bypassing all the bike electrics to rule them out (e.g. starter solenoid, battery cables, connections etc)

Buzz.
 
#7 ·
I did try with jump leads direct from a fully charged remote battery to the starter motor and it was exactly the same. IE.turned over a part revolution before stalling against what appears to be compression. Decision time!!! I'm currently thinking that I will progress as follows:
Strip the starter looking for anything obvious.
Remove the cam cover to check the decompression lever/spring for anything obvious
Bite the bullet and replace one or the other depending on what I find
Anyone disagree or have any other ideas?
 
#8 ·
I've just had a look at your previous posts and can see that after replacing the Sprag clutch and the battery, you found that you got a click from the solenoid but the engine would still not turn over. It might help to understand what you did get from there to where you are now. Also, it might also help if we can understand the history of the bike. First off, what bike is it, how many miles has it done and was the bike running ok before the problem developed or is this a bike you acquired in this condition?

I can't say for certain but it would not surprise me if the engine would fail to turn over against compression if the decompressor stopped working. I have a bike with an air cooled Rotax which does not have a decompressor and the starter motor only just manages to turn it over with a compression ratio of 10.5:1 which is lower than the 11.3:1 on the F650GS. That said, it's possible the starter motor is not developing full power but I believe you have wired it straight to a new battery with the same results. However, I would check the starter motor rotates freely by hand but otherwise it sounds as though that side of the engine is probably ok.

If the engine has done a high mileage, it's possible that the cam that operates the decompressor has worn sufficiently to stop it operating. On the later G650GS bikes, the problem was that when the engine got hot, expansion of the parts caused the same thing to happen due to a design/manufacturing fault which required the decompressor to be replaced. To put it into perspective, the decompressor only opens the exhaust valve a small fraction of an inch and so it wouldn't take much to stop it working effectively. Unfortunately, I don't think there is any way of checking the condition of the decompressor and so if it is suspected of being at fault then the only option would be to replace it with a new one.
 
#9 ·
Brief history:
BMW F650GS Twin spark 2005
Got bike April, full working order with no starting issues and had a couple of nice sunny day run outs.
Only 8.5k miles but I would describe it as untidy. No other background info, so took as a project.
Removed starter and both engine covers to re-paint. Exhaust been off, but left hand side only now back on.
Done a deep clean to all items around front of bike, removing parts where necessary. Air box, battery, hoses etc. All back on now with fresh oil, filter, plugs and coolant. Water pump checked and in good condition, just polishing to the shaft with no grooves and cogs fine. Probably confirms low mileage.
On replacing covers and re-assembly it would not start and came with a graunching sound from right hand side of engine. Engine not turning, hard to tell what else was turning from starter. Removed cover, checked position of gears, spacer and thrust washer. Retried and still the same. Eventually concluded it was the sprag, so replaced.
Now back to current position. Engine tries to turn, but stalls. Replaced battery as this seemed obvious issue. When fitting the sprag I spun up the starter with all gears on and the sprag/dynamo removed, it spun up fine. I have to say that the splines on the starter shaft end are not in the best of condition that may be a sign of past starting issues. I'm not now convinced either the sprag or battery had failed.
Enjoying the project, but getting frustrated at the moment.
 
#10 ·
Thanks for the background information. With that low mileage, it's very unlikely that the engine will have suffered any appreciable wear unless it had been run with very poor quality oil (ie oil that had not been changed for a considerable period). Therefore, it's highly unlikely that you've got a problem with the decompressor.

From what I've understood, the bike was running fine before you started work on it which tends to suggest something might not have been reassembled correctly. It is definitely unlikely that the Sprag clutch would have failed just after the overhaul if it was ok before. If it was me, I would take it all apart again and carefully check the way it's all supposed to go back together with reference to the BMW or Haynes manuals. You have to make sure in particular that the arrow mark on the freewheel is properly aligned. It sounds that under no load conditions (spark plugs out), everything turns ok but under load something is binding enough to prevent the starter motor being able to spin the engine past compression.
 
#11 ·
Now then. I might be getting somewhere but I still need to work out how to fix it. Convinced that everything behind the engine cover was fine (checked it so many times whilst off) I have just removed the starter and noted that location marker was nearly a quarter turn out of position. Pulled it apart and re-assembled it in the right position and put back on bike. The 'graunching' sound has returned with no turnover. Pulled the starter and spun it on a remote battery. It is spinning the 'wrong way' that means the 'graunching'sound is the unloaded gears spinning in the freewheel direction with the sound magnified through the cover. There is obviously something wrong with my starter rebuild, but not quite worked out what.
 
#12 ·
Out of curiosity, what do you mean by the location marker on the starter? As far as I can remember, the starter motor simply pushes into engagement with the starter gear wheel? Also, do you really means that the starter motor spun the wrong way when powered by a remote battery? I believe it needs to rotate anti-clockwise when viewed from the right-hand end so that the generator flywheel then rotates clockwise with two gear wheels in between?
 
#13 ·
Ditto!
I didn't want to say which direction the starter goes in case I made a mistake, but I agree with you - should be turning anti-clockwise viewed from the RHS of the bike.
It's very hard to see how you could get the starter going the wrong way.

Buzz.
 
#14 ·
Right, obviously it's my fault, but here's what went wrong.
When I removed the starter and broke it down to paint the corroded barrel I re-assembled the unit barrel to the non drive end cover with its location indentation matching up to a small cast lug on the cover. It should have aligned to the +ve connection on tdc. Being approx 45deg off, it caused the starter to run in reverse and make the horrible graunching sound from the unloaded free wheeling gears. (It really does sound bad).
Concluding it was a failed sprag clutch I ordered a new assembly (non BMW). When this was assembled it matched with the starter rotating in the wrong direction, obviously trying to turn the engine in the wrong direction. Only last night after a few pots of ale and head scratching did I piece things together when I concluded that the new sprag clutch must be working opposite hand to what it should. Having had them both on the bench this morning assembled identically, the new one rotates one way and the existing the other. Mitigating circumstances to my stupity me thinks and reason to get my money back. I have Corrected the starter build and re-fitted the original sprag.....Bingo.
Its been an education and kept me busy doing lockdown. There's a lesson to be learned in trying to save money and buy non original parts. And a bigger lesson in that the devil is in the detail.
Thanks for all your help, it's difficult trying to bounce ideas off my wife and the dogs.
 
#15 ·
I'm still trying to catch up .... and learn.

So the starter barrel was assembled wrong way round and 45 degrees out which makes them motor go backwards?
I'd have to think about that a bit - the barrel contains the permanent magnets which were back to front causing the reverse direction?

Then on top of that, the aftermarket replacement sprag clutch was also back to front?

Blimey you did well to unravel that lot! Glad it's working now, sorry couldn't actually help!

Buzz.
 
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