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Discussion Starter · #1 ·
About once a week, the engine makes a 'CHUNK' noise and stops dead, usually when sitting at traffic lights.
Ive not had one of these engines apart so am unfamiliar with what may be causing this...

Once it has stopped, it will restart but is a little reluctant (Ive heard this can be normal for this engine to be reluctant to restart when hot).

It has also (very) recently started 'surging' slightly - i.e. its not an even idle and the revs rise and fall a bit and you can also feel this while riding. Ive ordered new plugs and caps as a start as these have been in there a while.

Any suggestions on things to check/look for please?
 

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Surging sounds like you may have an air leak on the inlet side.
Check the inlet manifolds for cracks carefully.
Also if you have not replaced the carburettor O rings, diaphrams and given them a good clean then it may be time.
Hard starting could be put down to fuel problems or air leaks on the inlet side.
If you do the carburettors, then replace the O rings in the inlet manifold where it bolts to the engine.
The clunk could just be the engine stopping suddenly, unless the cam chain is very very loose.
Pete
 

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As GODOH posts, sounds like fuel issue, worth checking carbs / manifold 100% on bike this age
Suggest check that battery connections are tight and firm, these big singles do vibrate, affects voltage etc, lots of other stuff too FAQs useful. Voltage check stationary and running Worth doing on these pre 2000 bikes, should max out at 14.6v (ish) VR known fault again all sorts various issues can occur.
 

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Discussion Starter · #6 ·
Thanks guys.

I replaced the carb diaphragms about a year ago as one had split. I didnt do a complete strip and rebuild but replaced gaskets/seals as required. Inlet manifold was good then.

I did fit a Scottoiler recently and Im wondering if that is allowing some air to be drawn into the inlet, as it works using a vacuum takeoff from the carb. Next thing I will check.

Hard starting is only when warm. It starts first turn from cold. Ive read of other people having this and not finding anything 'wrong' so maybe its just the way the carbed engines work..?

If the cam chain was very very loose. would I not hear that rattling? Ive previously checked the tensioner but it appears OK. Does the cam chain have a link? i.e. can it be replaced without an engine strip down..?

The battery is relatively new and I have a voltage gauge fitted which is showing 14.6 (ish) when running.
 

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I thought about the clunk the engine does, and mine does that if I stall it in tight spots.
With the cam chain yes you would most likely hear the rattle if it were really loose. The tensioner is pretty easy to check and clean.
I am fairly sure the cam chain is one piece so replacing it is a fairly big job,. The main things that cause problems are the tensioner and the cam chain guides.

Just wondering how often you clean the air filter?
I had a tenere once it was an absolute mongrel ( kickstarter) to get running when really hot. I fitted a valve and a tube to the inlet manifold so that I could allow more air in when it was hot and it started much easier then.
If the engine is running too rich then that will make it harder to start when hot

Other things that can affect starting when hot is valve clearance, if a valve is a bit tight then compression will be low when hot.
If the engine is idling too low it will stop too. Idle speed should be around 1500 rpm, being a big single they need to keep turning over.
Voltage looks fine.
When you did the carbie diaphrams did you check the needles for wear. Sometimes the needles get wear grooves on them especially if they do a lot of idling. This will make the bike run rich at low speed and could make it stop and harder to start.
Hope this helps
Pete
 

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Discussion Starter · #8 ·
Thanks Pete,

The chunk noise is not as a result of me stalling, the engine can be idling happily at about 1500 and suddenly it stops dead with the chunk noise without me doing anything. Rarely it has done it while Im riding along on a lower gear.
Being unfamiliar with the innards of the engine, I dont know what could cause it.
I was wondering if the starter motor could be engaging or something...

No chain rattle that I can hear. I'll pull the tensioner out again and check it.

Ive a new air filter waiting to be fitted so I'll do that as one of my next moves. Good thought.

I think Im running a bit rich as my fuel consumption seems higher than I would expect but its hard to tell as I live on an Island and most of my riding is in 1st-3rd.

From (failing) memory the needles appeared to be OK.

I think as a standby Im going to order another kit in case I need to remove and rebuild the carbs ( 2 Sets Motorcycle Carburetor Repair Rebuild Tool For BMW F650 93-00 Mikuni BST33 | eBay )

Ive a few things to check so hopefully will get that done later.

Thanks!
 

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There is a sprag clutch on the starter motor, they are a one way device that allows the starter motor to tun the engine over but not the engine to turn the starter motor. Some of the clunk could be it.
See if you can get the fault to occur at home, maybe start the engine and wiggle the wiring loom, this will help rule out a break in a wire causing the problem.
Also try temporarily disconnecting the scott oiler and block the hole in the manifold off, to see if that is causing the problem.
The air filter ( at least on my 96 funduro) could be washed and re oiled. Mine was a foam filter.
You could take it out temporarily and check the air box is clean and free of oil. There is usually some oil in there but if the engine has been overfilled with oil there can be quite a bit of oil in there.
Running it without the filter will show you whether the filter is totally blocked up.
Pete
 

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One easy thing to do is to drain the carburettor bowls.
Some of the early bikes also had a fuel filter that sits between the carbie bowls. I know a stupid place to put it.
From memory it is in the T piece that the fuel line connects to at the carburettors.
If you can get to it it may be worth drilling out the filter section and adding an inline fuel filter to the bike.
Water in the fuel can cause lots of problems, bikes are hard to start, run rough and stop lots.
Dumping the contents of the fuel bowls will show you how much dirt or water is in the fuel. If you dump the contents onto concrete or in a bowl you can see if there is water because the fuel and water don't mix.
If there is water in the bowls, then put a siphon hose into the fuel tank to the bottom and see if there is water in the tank too.
Or take the tank off and clean it, there is an inline strainer in the tank, basically a plastic gauze connected to the fuel tap. It can get clogged too depending on how clean your fuel is.
Pete
 

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Discussion Starter · #11 ·
Thanks for all the suggestions.

I had water in the tank a while ago (2020?) as a result of leaving it uncovered and the drain hose around the filler cap was blocked so rain found its way in past the cap/seals.

This was when I drained and partially stripped the carbs and found I had a split diaphragm so I got a kit and replaced a few bits. I removed the tank and drained and rinsed it then, and I fitted an inline filter between the tank and carbs so I could see if fuel is flowing and check for any debris etc.

I wasnt in a position to remove the carbs back then and do a complete strip and rebuild, but it worked 100% fine after (until now).
It lives on my driveway but I do keep it covered now and have unblocked the drain hose!
No sign of water or debris etc...

Could the starter motor sprag be 'catching' and causing the engine to stall..?

It is a foam filter, but from memory I think the one in there is one I cut from a larger piece so Im going to fit a 'proper' one and see if it helps.

First thing to check will be Scottoiler, then filter.

I was hoping to look at it yesterday but got held up working on something else.

Cheers for the replies!
 

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The starter sprag could do that, it would have to be fairly worn though. They often fail because the spring around the sprags that holds it together fails, then they slip and the starter won't engage.
The repair is to get a large oil seal with a spring the same size and replace the spring.

The fact that it starts again is the hard part,
When you have changed the oil in the past have there been metal flakes in the oil?
Pete
 

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Discussion Starter · #13 ·
Starter seems to work fine otherwise. No problems starting etc.

When it happens it definitely sounds like something mechanical/metal catching and stalling the engine. I wish I could catch it happening on video but its very random.

Oil is nice, so sparklies and remains fairly clean...

Thanks
 

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One other thing to check just came to me.
Check that the fuel tank breather is not blocked.
When the bike stalls just open the fuel cap and listen to see if there is a sucking noise.
It could be that the breather in the fuel cap is blocked and the engine is being starved of fuel.
Pete
 

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While you are checking things, try these two.
Check the sidestand switch if you have one.
Check the Kill switch to make sure it is not faulty.
Both of those will stop the engine dead if they have a problem.
With the sidestand switch, just put the bike on the centrestand, start it and move the sidestand up and down to see if it the switch is working. See how much movement is needed to get it to work.
The kill switch likewise, just rock it a tiny bit, and see if it stops the motor.
Or just undo the screws and check that a wire has not come loose.
Pete
 

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Discussion Starter · #16 ·
Thanks for the suggestions.

Ive not had the chance to look at anything yet as Im struggling to finish some other work on the car which has delayed me this weekend :/

I dont have a sidestand switch and Ive had the kill switch apart. Fuel tank cap has been taken apart and checked but I will try opening the cap next time to check if theres a noise.

It definitely sounds like something mechanical stopping the engine. I dont get the noise if I stop it normally with the key or kill switch.

Thanks
 

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Is the gearbox smooth? and easy to find the gear you want an neutral?
Another thing the noise could be is the decompressor.
If there is a problem with the decompressor then it could cause hard starting, as it holds a valve open a bit to allow the engine to turn over easily.
Pete
 

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Discussion Starter · #18 ·
I ended up spending three days sorting out the turbo on the defender so only just had the chance today to check a few things on the bike.

Found an answer to the surging issue (I hope), the Scottoiler elbow ('vacuum damper?') on the card had split. Its in such a wonderfully accessible place that I hadnt spotted it before(!).

Ive ordered one from ebay and since I have no expectation of receiving it within a week, Ive tried to bodge it with silicone aquarium sealant... Fingers crossed!

Ive not experience the 'chunk' for over a hundred miles now... :/
Im not naiive enough to hope that its cured itself, but you never know...

Is the gearbox smooth? and easy to find the gear you want an neutral?
Another thing the noise could be is the decompressor.
If there is a problem with the decompressor then it could cause hard starting, as it holds a valve open a bit to allow the engine to turn over easily.
Pete
Gearbox is clunky, but Im lead to believe they are all like that?. No baulking or slipping out of gear though, just a good positive 'CLUNK' when changing gear...

Starting is fine. How would one check if the decompressor was causing an intermittent issue..?
 

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I ended up spending three days sorting out the turbo on the defender so only just had the chance today to check a few things on the bike.

Found an answer to the surging issue (I hope), the Scottoiler elbow ('vacuum damper?') on the card had split. Its in such a wonderfully accessible place that I hadnt spotted it before(!).

Ive ordered one from ebay and since I have no expectation of receiving it within a week, Ive tried to bodge it with silicone aquarium sealant... Fingers crossed!

Ive not experience the 'chunk' for over a hundred miles now... :/
Im not naiive enough to hope that its cured itself, but you never know...



Gearbox is clunky, but Im lead to believe they are all like that?. No baulking or slipping out of gear though, just a good positive 'CLUNK' when changing gear...

Starting is fine. How would one check if the decompressor was causing an intermittent issue..?
Check drive chain tension also Cush drive rubbers , either could be the cause . Gearboxes are agricultural at best , they all do,that sir
 
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It sounds like you have the problem sorted. If the t piece was letting in enough air the carburettor diaphrams would not get enough vacuum to lift them. Also the bike would be running lean at idle.
I would think that as the bike stopped the decompressor would be what was causing the clunk.
I hope it is as simple as that.
Pete
 
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