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Old 11-19-2012, 03:08 AM   #1
sam_2555 OP
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640 LC4 Supermono issues

Ok, I've been lurking here and gathering information for a while but still can't sort out the issue with my LC4.

The story so far: I bought an LC4 640 engined supermono off a friend at work. He had got the engine mounted in the Aprilia rs 125 frame, installed all required running gear and had been trying to get the thing running for about 6 months with no success. During this time he had replaced the starter clutch but that was it as far as engine work. It's an all black 640 from an Enduro (apparently) but due to the sketchy history I don't know for sure which model it's from.

I bought it thinking I'd have it running in a couple of weeks. 6 months later and it's still a non-runner.

The list of things I've checked so far is:

Mechanical:
Cam timing - checked, rechecked, rechecked, flipped it 180 degrees to check that the engine was in fact sparking at every TDC. Then rechecked the timing, then flipped it back 180 degrees to the original timing.
Autodecompressor - checked it each time I've had the cam out, spring is still good and the peg still has nice sharp edges with no signs of rounding. Makes a good 'CLICK' sound when turning the engine over by hand.
Flywheel - each time I've had the cam out at TDCC I've checked the position of the timing mark on the flywheel and it's been at the correct point in the sight glass. I checked this to make sure the flywheel wasn't slipping and knocking the timing out.
Compression test - run a compression test and got around 160psi (can't remember exact figure, it was a while ago and I remember thinking it was easily high enough).

Fuelling:
Carb - it's got a BST40 on it, I've stripped it 3 times, cleaned it in a parts cleaner at work, rebuilt it, checked float heights, checked the float vavle is actuating at the correct float height, checked free movement of everything. The carb had a massive jet in it when I stripped it the first time so I put a 157.5 (I think, it's the jet size recommended on here for the BST 40 with a straight through exhaust). There was a TPS on there as well but that was removed by the previous owner, leaving a peg sticking out of the side of the carb. I don't think this leaks air, but I could be wrong.
Choke - replaced the crappy plastic nut where the choke cable goes into the carb, also bought a new choke cable and plunger/spring assy.
The 'make shift' fuel tank I'm using is a funnel hung from the garage roof that I fill with fresh fuel for each start attempt - definitely clean, good fuel.


Electrical:
Flywheel pickup - resistance about 100 Ohm (from memory, it was in the middle of the allowed range of resistances, I think it's 80 to 120 Ohm.)
Ignition coil - resistance wasn't correct so I bought a second hand one from a running 660 LC4 off ebay.
CDI unit - the only thing I couldn't check with a multimeter so I bought a new one. It's a DC-CDI from Ignitech (Czech company). The DC bit means the CDI doesn't need the AC input from the stator like the standard unit so this eliminated two possible problems that might have been stopping the engine from running.
I've put a couple of new spark plugs in (incase the first one had some sort of defect).
Checked continuity between all electrical components.


All of this and the bloody thing still won't go!

I've been really thorough when checking/testing everything so I havn't missed anything (I don't think). I've had covers off both sides of the engine and given everything a good visual inspection and couldn't see anything that might stop it from running (no big chunks of metal/piles of swarf etc..)

So currently I have a bike with fuel (plug is wet after long starting attempts). Spark - nice fat blue spark when testing with the plug out and also testing with a plug in so it's got enough power to spark and turn over under compression. Correct valve timing. And compression. Why then, will it not run?

I am on the edge of pulling the head, splitting the cases and fully rebuilding the thing from the ground up. Or the other option is buying a grey engine and get a bit more power. But I don't want to give up and buy a new engine, I want to make this one work.

So, any help, advice, opinions or general banter would be appreciated. As I say, I'm ready to take drastic action.

Cheers,

Sam.
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Old 11-19-2012, 06:56 AM   #2
McBoab
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Its a looooong time since I had an LC4 640 but I do have etched into my brain the starting issues it had from new...
Mine was a Dellorto carbed bike but the principle issues/fixes may help.
Mine had the same problem from new with the plug wetting out and not starting, technique and ambient temps/humidity requirements seemed to be on a knifeedge for successful starting... I ended up reducing the idle fuel jet size by from about 40 odd to low mid 30s (it was a long time ago so sizes more a guide) and starting got a whole lot easier , just seemed far too rich at idle and any choke made it worse. I later installed a SommerKTM larger needle jet and that made it worse which led me to belive the big lung was also able to pull off the needle jet at idle ! not something you get in the carb tips tuning manual so I had to lower the needle a touch to reduce the fuel available. The BST may also suffer a touch from rich idle/weak air jet and will be suceptable to needle/needle jet wear which could also be affecting starting in a similar way to my old dellorto did. HTH
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Old 11-19-2012, 06:59 AM   #3
bmwktmbill
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Do you have the workshop/LC4 Repair manual?

Please post a picture of the mc.

bill
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Old 11-19-2012, 07:27 AM   #4
gunnerbuck
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Did you try starting with the stock 640 CDI unit? The CDIs have built in map to control spark...


If you have a good spark at the right time, compression and fuel it should start unless too much fuel is getting into the combustion chamber... You could try a shot of starting fluid through the airbox to see if it will fire...
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Old 11-19-2012, 07:34 AM   #5
sam_2555 OP
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Images

As requested, some images (this is my first go at this so bare with me if it doesn't work).

The mono when I picked it up:


Right side of engine + rusty chain:


Full right side as it stands now with Street Triple R front end:


It's now got a black front wheel and some clipons.

It looks ok when it's all dressed up. The guy I bought it off spent time making it look pretty and not long trying to start it up.

Any ideas on what might be preventing it from going?

Sam.
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Old 11-19-2012, 07:40 AM   #6
sam_2555 OP
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P.s.

I have the workshop manual for the engine.

I've tried easy start and still nothing. All I got after a while was easy start coming out of the exhaust.

I did try it with a standard LC4 cdi unit to no avail. The engine will spin over and cough but won't fire. The reason I bought a new cdi was that it was the only component I couldn't test using a multimeter so I assumed that this was the thing preventing it from starting.

Could too much fuel prevent it from starting all together? I thought it would just make it run rough when it started. I forgot to mention originally that I've replaced the rubber manifold piece between the carb and the metal manifold that bolts to the head so there's no air leaks there.
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Old 11-19-2012, 08:37 AM   #7
kyns
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Have you replaced ignition wire and spark plug cap ? I know you have spark but......


Side stand switch ? ( not sure if 640 E has one )


Have you tried another carb ?


If you have compression and timing is correct, there's no need to split the engine imo.
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Old 11-19-2012, 09:53 AM   #8
sam_2555 OP
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Kyns, the coil I bought off ebay had a HT lead and cap attached to it. I checked the resistances of them and they were within the parameters detailed in the workshop manual. I've also tried stripping back the insulation on another HT lead and wrapping the wires around the tip of the plug to make sure the plug was definitely sparking when it was in the cylinder.

The side stand switch would cut power to the CDI I think, so I would get no spark at all if that was the causing the fault.

I havn't tried another carb because they cost so much to buy on ebay and I though that it looked ok. I am familiar with the black magic that goes on inside carbs and just because everything is as clean as new and everything is moving right does not mean your carb will work as its supposed to.

If anyone has an old BST carb they aren't using anymore (perhaps after an FCR upgrade) I would be up for buying it for a reasonable price.

There are so many unknowns with this whole project, I don't know any history of the carb or engine or electrics - where they've been or what they've been used for. I have taken this back to first principles to try and find the fault and have got nowhere, I shouldn't have to split the cases and rebuild everything but if I can't figure out why it won't start I'm going to have to dig deaper OR buy a new engine.

Thanks for the input guys. I really appreciate it.
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Old 11-19-2012, 08:16 PM   #9
bmwktmbill
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Sam, that is a beautiful machine. It just has to run.

I don't think the carb is the issue, it should at least run badly or idle and not rev with what you have and with using the starter fluid...it should do something ugly at least.

How is the compression when you put your thumb over the spark plug hole?

You must have set the valves many times if you flipped the cam timing so you set then at TDC using a steel rod to find TDC. You should be able to lock the engine at TDC and set the valves there.

Is the intake tract open as in no rag jammed in it blocking the intake valves?

Do you have slack in the decompression lever system?

Is the plug wet?

bill
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'02 KTM 640 Adventure-lowered
"On the road there are no special cases."
Cormack McCarthy-The Crossing

The faster it goes the faster it breaks.
And high performance=high maintenance.
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Old 11-19-2012, 08:56 PM   #10
rz35027
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Pretty bike!

Adjusting the valves on the exhaust stroke would be an easy mistake to make... the old "rag in the intake" has defeated me before...

I can't tell from the photos but I don't imagine access to the carb is very generous... If you are thinking of replacing the carb, have you considered an FCR?
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Old 11-19-2012, 09:18 PM   #11
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Bill said, "Do you have slack in the decompression lever system?"

I just got done with a valvetrain refresh and my bike wouldn't fire. Turns out I forgot to thread the cable adjuster in. There should be a tiny bit of slack in the cable.

I hope you get it figured out. I'd drain the carb and leave it out of the equation for now. It's prolly what the problem is.

If you have the carb out, drain it and work the throttle to get the accelerator pump to squirt all the fuel out.

Pull the plug and crank it for 10 seconds to clear it. Then, can use starting fluid, but I prefer fuel squirted with a spray bottle or a small water bottle with a little hole in the cap.

Open the throttle and spray some fuel straight thru the carb into the head. Close the throttle to 1/4 open and see if it fires.
If it fires, it's the carb that needs work. Or upgrading.

What a bike! Hope you get it sorted out.
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Old 11-19-2012, 09:51 PM   #12
beek-02
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compression+fuel+fire=running

It sounds like you have a real mystery. I have had a few of these and starting can be a bear if you haven't found the right technique. It is rather hard to find the right technique on your own. I have to ask if the choke is still hooked up on the carb? Carb setting can be one of the settings that has to be right to even get a puff out of these beasts. Another thing is voltage, you need a good battery or a capacitor in the system that will hold voltage, I'm told a good solid 5 volts, in order to run. You can try using jumper cables to a higher power source, say a car battery, and try cranking it that way. Those are the 2 places I would check first. I have one I just put together and it will run fine until I disconnect the battery then it will do nothing. Also it would crank over many times and do nothing until I figured out the carb settings and where they needed to be, now it fires almost immedietly.

Don't give up, you'll get there. Besides the dreams of riding that should be motivation to keep going. It looks like it would be stupid fun!

Good luck, Beek
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Old 11-20-2012, 01:22 AM   #13
sam_2555 OP
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Thanks for all the responses.

Bill, compression is good, I have been using the crankshaft locking bolt when setting the cam timing.

Looking down the inlet manifold you can clearly see the inlet valves, so no rag/blockages.

The decompression lever system isn't set up at the moment. I thought a while ago it might be the manual decomp sticking on somehow but the compression test would have come out really low if it was.

The plug is wet after long periods of trying to start it.

I have considered an FCR but I need to get the thing running before I start throwing money at performance improving parts. I don't know for sure that the fault lies in the carb, if I did then I wouldn't bother replacing it with another BST - FCR is definitely the way. Just too much at the moment to buy one without knowing for sure that I need one.

Idle, there's no accelerator pump on the BST. I'll try your easy start method when I'm next in the garage. The bike's stored in my dads garage at the moment because I don't currently have anywhere else indoors to work on it.

Beek, you forgot a term in your equation: correct timing. I'm sure it should run, the maths says so! As far as power, I'm using the battery from my Street Triple to start it. I was using another battery for a while that was pretty weak but using the Street's battery it spins over fast and the starter sounds strong.

I need to ride this thing, I can't wait to get it running. Beek you're right, it does look like stupid fun, combining my 2 favourite bikes - tiny 2 stroke well sorted frame and a big dirty single.

Thanks for the input guys.
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Old 11-20-2012, 05:04 PM   #14
beek-02
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Quote:
Originally Posted by sam_2555 View Post
Thanks for all the responses.


Beek, you forgot a term in your equation: correct timing. I'm sure it should run, the maths says so! As far as power, I'm using the battery from my Street Triple to start it. I was using another battery for a while that was pretty weak but using the Street's battery it spins over fast and the starter sounds strong.

I need to ride this thing, I can't wait to get it running. Beek you're right, it does look like stupid fun, combining my 2 favourite bikes - tiny 2 stroke well sorted frame and a big dirty single.

Thanks for the input guys.

I thought I saw it had been checked multiple times. Plus I assumed all the valve train was correct because you were getting approx. 160psi on the compression tester. But now that you mention timing there is a couple things you could check there. One being the cam and cam gear timing marks lining up, but you have to pull the valve cover and water pump. The other item to check is the autodecompress mechanism on the cam. It might have already been mentioned though. You should also check to see which cam is installed and make sure the the follower bearings on the rocker arms are tight as well. There are a few instances where they came apart and caused some pricey damage.

I know you are just trying to get it running but there is some good info on this forum and others about cam options and carb setups. I would think with the purpose of this bike you would be looking for a high flowing high rpm motor combo. The TM42 carb from mikuni is supposed to be a good pumper carb setup. It's a varient of the fcr for a bit less cash. You can do some port work on your own head or look for a 03+high flow head. There isn't a lot of difference. The exhaust valves are a bit larger 32mm vs the 30mm and the intake plenum is all one piece not bolted on and it seems that it frees up some room for extra flow. I guess the intake side has also had some porting done at the factory on the intake side. It's not that pretty but I guess they must have found it beneficial enough to pay a guy to hand port them at the factory. I'm not sure what you are going to run for exhaust but there seem to be a few options out there. Just some food for thought.

pics are worth a thousand words.
cam marks.


Can kind of see the rough porting done on the intake side.

cam with autodecompress
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Old 11-20-2012, 08:16 PM   #15
bmwktmbill
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Sam,
What does it do when you try to start it with the carb, with starter fluid?

Does it ever fire?
bill
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'02 KTM 640 Adventure-lowered
"On the road there are no special cases."
Cormack McCarthy-The Crossing

The faster it goes the faster it breaks.
And high performance=high maintenance.
Bill Shockley
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