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525exc won't start after rebuild

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Hi guys!

I have replaced the piston and cam chain on my 525exc -03. After this it will not start, it fires occassionally when kicking it over.

The thing is that I know that one thin washer, that sits in the ajduster screw (please see below, red arrow), is missing after assembling the carb. Could this washer really cause the bike not to start?

I also did one mistake and used old gas (the bike has been sitting since six months back) at my first attempt. Realised that and drained the carb+tank and tried with fresh gas, but no difference.

When mounting the cam I locked the crank with the screw that sits in the crankcase. Put on the cam with the marks aligned. Put on the cam chain and tightened the cam chain tensioner lightly. So i guess I did it right there. Valve clearance checked carefully.

Will try a new spark plug tomorrow and try to dry kick excessive gas from the cylinder. Any other tips before I head to the garage? Thanks!

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Think more information is needed.

What is your experience in rebuilding 4 strokes?

Have you rechecked valve adjustment?

the washer/oring you are referring to is to keep crud out of the fuel screw circuit, plus keep it from vibrating out.

Could be a multitude of reasons as to why it isn't starting. Would be best if you could write a detailed report of tear down and rebuild, noting tear down and reassembly of all parts (i.e. the carb).

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The item you have marked with the red arrow should be the O-ring. The thin washer is used to prevent the spring from damaging the O-ring. I don't have a numbered schematic drawing to check but I think that is the correct order — fuel screw, spring, washer, Oring.

As for the bike not starting I guess a couple of things could be wrong. Make sure that the cam and flywheel both agree that the engine is at TDC. Easy mistake to get it 180 degrees off. It sounds like you did the job right but can't hurt to check.

Did the carb have old gas in it during the 6 months the bike sat without being used? If so it could be that the pilot jet is plugged with gas residue. It might be easiest to just replace it as they are difficult to get perfectly clean if that is the case.

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Do a compression test. Should be around 115 PSI.

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Today I put in a new spark plug and put in the oem adjuster screw with o-ring,washer,spring. Tried to spray brake cleaner through the air intake so it would fire up easier, but it only makes some single firing. Didn't had the time to check timing and valve clearance this time.

Did the carb have old gas in it during the 6 months the bike sat without being used?
No, the carb was drained.

I will try to remember what I have done with the bike. And I should say that this is my first time doing a top-end (on a four-stroke).

Changed complete piston, top-end gasket kit, new cam chain, cleaned carburetor thoroughly.

When assembling I put the crank in the locked position with the screw going in underneath the engine case. Mounted cylinder, feed the cam chain. Put on the head with the cam. Made sure I didn't put the cam 180 degree wrong, aligned the cam chain wheel according to the two marks/dots. Fastened the cam chain. Put on the valve cover and adjusted valve clearance, sealed with loctite "glue". Tightened the cam chain tensioner. Mounted carb. and so on.

Next time, which unfortunately won't be in the next month I will check valves and timing.

Thanks for your advice!

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Another possible problem: Did you have the slide out of the carb? If so make sure that you got the vacuum release plate on the front of the slide re-installed the right way. It can fit two different ways and if it is in wrong you will sometimes get the occasional pop or backfire but it probably won't start. The hole in the release plate should be located to the bottom. BTW, don't ask how I know this.

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Yes I had the slide out. Will check that aswell.

But it will take some time before I might be able to get to the bike again. Will keep you updated how it goes.

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