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Timing Question


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#1 TitanNeil

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Posted 20 May 2019 - 05:25 pm

Good day all,

 I replaced the timing chain in my Mk1 850. I THOUGHT I had it at left cylinder TDC, timing mark on mag lined up on mark. Could I have been a full rotation off? Wouldn't the left piston be bottomed out rather than TDC if that were the case? Trying to solve my left jug backfiring through the carb...



#2 Bjørge

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Posted 21 May 2019 - 05:57 am

Well, on a 4 stroke it wouldn't be possible to be one full rotation off and still have timing marks on cam shafts aligned...your crank shaft makes 2 full rotations on 1 camshaft rotation, so you would have been 1/2 rotation off - and bike wouldn't run...also because valves would be cnackered......


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#3 Studley Ramrod

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Posted 21 May 2019 - 08:44 am

Yes you could have been one rotation off.  No1 piston is at TDC when the timing marks on the camshaft sprocket line up.  If the crank is lined up at TDC and the camshaft sprocket marks don't line up, you have to rotate the crank 360 degrees.

 

No the piston wouldn't be bottomed out, but you probably know that already from what I've written.  The engine uses the wasted spark principle.


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#4 TitanNeil

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Posted 21 May 2019 - 09:51 pm

Which is what I was wondering - because the reason I had it all apart was because the cam chain was loose and when I 'thought' I had it at TDC I removed the tensioner. Well, of course it jumped a tooth and when I started it up I bent 2 valves. So, when disassembled I did not have any reference point other than cam markings and mag T mark. Its making me feel like I don't have a clue. I popped the cam cover up enough to see that when the mag mark is at 'T' then the E and I stampings on the cam sprockets are level and correctly oriented. The punch mark on the cam itself was upwards at reassembly too. It all looks correct.

I bought a super-cheap compression tester and it left me more confused - it won't hold pressure. When cranking the gauge goes to 120 but flicks back to zero instantly, on either cylinder. I either have a bad gauge or the valves are being held open, which is a possibility too. I will test with another gauge to confirm either way.

Is it possible to have backfiring with correct valve timing and sealing?



#5 Studley Ramrod

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Posted 22 May 2019 - 09:29 am

Yes. The carbs could be causing it to backfire.  Have a look here  http://www.carpe-tdm...p?showtopic=556

 

You need a good seal of the threads on the compression tester, sounds like you might not have got one when testing.


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#6 TitanNeil

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Posted 24 May 2019 - 04:57 am

Thanks, I had forgotten about the wasted spark ignition. The compression tester is undoubtedly a piece of total junk, but the readings threw me off so much I

started to question what was going on. I will update once I thoroughly go through the carbs.



#7 TitanNeil

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Posted 30 May 2019 - 09:06 pm

So. I think my timing is a full rotation off. I had the head off the bike when I rotated the crank to put the left cylinder at TDC. Everything went together as I thought it should, with both cams back in place I was able to install anew cam chain and have all the markings line up level with the top of the head. Stamp marks on cams facing upwards. It turned over fine, was hard to start and backfired immediately.

Let say, 'theoretically', I reassembled the engine with the 1st piston at TDC on the POWER stroke, rather than the INTAKE stroke. Is there any way to tell other than running it?



#8 Studley Ramrod

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Posted 31 May 2019 - 09:54 am

You can't reassemble on the wrong stroke.  Assuming no1 piston was at TDC and the timing marks on the camshafts lined up, along with the cam lobes facing outward/away from each other, then you've set it up correctly.  Also assuming the camchain didn't slip when you put the tensioner back in.

 

I would lift the rocker cover and check everything, prolly be quicker in the long run.


Mk2a 2000 in Silver. Top end Refurb @ 41100 miles, Scottoiler, Renthal Road High Bars, Up & Back Bar Risers, Bellypan, DL650 Handguards, Capt. Picard Bar Ends, House of Henty SS Wheel spacers, New Seat Cover Fitted. 58 MPG !!  Now owned by chrisbee !   Studley's mk2a Bloggerydoodaah !  Photos of my first MK1  Photos of my MK2a  TPLQHCSRSFC No. 1 Fan


#9 TitanNeil

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Posted 31 May 2019 - 09:46 pm

I removed the cam cover to check - everything lines up as it should. Cams facing outwards, markings on sprockets parallel, stamping on top, TDC on mag correct and left piston at top. Valve clearances rechecked and all good. Carb pilot screws were 2 turns out, I did an external float level check and they are both the same and not overflowing. Stabbed myself pretty good with a frayed throttle cable - and guess what? They are no longer available from Yamaha in North America, so I will have to  find a cable end kit and make new ones. So far no problems discovered that would explain the backfire.

Will be back at it tomorrow afternoon after work....




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