Solved: Desperate for help with Cam Timing: piston hitting valve

[zaphoid18]

Hi Guys,

I am getting REALLY frustrated with this one. I recently pulled the head and bores off my bike to do some freshening up and new head gasket. Now when buttoning her up as I turn the crank bolt the piston contacts the exhaust valve. I've probably redone these steps 7 or 8 times now.

I've followed the Haynes and service manual to the letter and cannot wrap my head around what could possibly be wrong, especially as my bike was running before this.

I set the left cylinder to LT
aligned the sprocket pip to 12 o'clock
aligned the camshaft pip to 12 o'clock
lines on sprocket angle slightly retarded but within reason (pic)
insert bolt and rotate install second sprocket bolt
rotate back to LT confirm position of markings and lobes
front cam chain blade in
cam chain tensioner in
rocker cover installed, bolts tightened gradually in star pattern, rocker arms are all centered on valve stems
rotate engine slowly and feel soul crushing boop.



-The pistons were never removed from the con rods so they aren't mixed up
-the rockers and valves were never removed or disassembled so they're the same
-pistons went back into bores nice and simple engine rotates smoothly without head
-cam chain is not bound up and engine rotates smoothly without rocker cover.

Help please totally at a loss here. Whats really frustrating is its all so simple and straightforward, Its a SOHC!

 

[16VGTIDave]

I know little about SOHC engines, so take the following with that in mind!

You can't be using the correct timing marks. I was under the impression that the "LT" mark was for setting the ignition timing. Which would be advanced relative to Top Dead Center. There should be a TDC (Top Dead Center) mark. This can also be verified by gently inserting something like a wooden pencil or dowel into the spark plug hole and resting on the piston. Gently turn the engine over while making sure the dowel doesn't bind, and you will be able to accurately determine when the piston is at the top.

Good luck!

 

[BBS360]

You're on the right mark. LF for ignition timing, LT for left top dead center.

I've managed to get the lines on my timing chain sprockets perfectly horizontal during the few top end reassemblies I've done.

It's only a guess but I'd suspect maybe you're one tooth off.

 

[Jeff0133]

I know nothing about timing but read somewhere that when rebuilding and engine there are spec measurements for the conrods ...could it stretch with mileage/abuse....just a thought...

 

[xschris]

Confirm your at tdc on the felt piston like dave said. You may be 180* out.

 

[zaphoid18]

Thanks for the replies guys,

I'm sure i've got the cam and sprocket positioned correctly ( pips up, lobes down) so that makes me question the crank.

When i set the left cylinder to LT which is supposed to be TDC the piston has gone well past the top of its stroke and is travelling down again, pictured here

Should i not trust the rotor markings and go by the pencil method as dave suggests? Or is this normal for our bikes?

 

[BBS360]

It's possible that the rotor with the timing marks may have been removed previously and reinstalled without the woodruff key to align it properly.
Wouldn't have affected anything before if the timing was already correct before the rotor were reinstalled.

You could get TDC close enough for reassembly by the pencil method but it'd be a good idea to figure out why the marks aren't correct.
I'd say you could mark the rotor when you're at TDC and measure back 10° to get the LF mark, but if the woodruff key is missing the rotor might not stay in its current alignment.

 

[Greasey Fingers]

Did you set the crank position before installing the cam/cam chain etc? (its a step that could easily be missed if you put it together in a hurry) You will need to open it up and set the crank position correctly if you didnt.

 

[zaphoid18]

HelloGentlemen,

As it turns out the problem was the woodruff key. Somehow it came loose in the rotor housing and lodged itself in the starter clutch. This allowed the rotor to spin a bit and throw off all my efforts to time the crank.

Thanks for your input. The best thing to learn is if what youre doing doesnt feel right, stop and revaluate. Sometimes the solution lies deeper than you thought. Theres a reason for everything

Best of all shes a runner!
Fired up like a champ. Went for a little rip and she runs great. No smoke or leaks.

 

[HoughMade]

Good find!

 

[Winnipeg_79_XS400]

HelloGentlemen,

As it turns out the problem was the woodruff key. Somehow it came loose in the rotor housing and lodged itself in the starter clutch. This allowed the rotor to spin a bit and throw off all my efforts to time the crank.

Thanks for your input. The best thing to learn is if what youre doing doesnt feel right, stop and revaluate. Sometimes the solution lies deeper than you thought. Theres a reason for everything

Best of all shes a runner!
Fired up like a champ. Went for a little rip and she runs great. No smoke or leaks.View attachment 27361

I'm about to dive down this same rabbit hole. Where is the woodruff key?

Cheers

 

[BBS360]

The Woodruff key he's referring to is #3 on this diagram:
http://www.boats.net/parts/search/Yamaha/Motorcycle/1981/XS400 - XS400H/CRANKSHAFT PISTON/parts.html
It keeps the generator rotor with the timing marks in alignment.

I'd caution against unnecessary disassembly.

 

[WelderDave]

Quick question regarding your timing mark picture. I’m doing mine right now and I lined up with that line and it appears I’m slightly off maybe a half a tooth or less...
I did have the head resurfaced slightly to make sure it’s flat. Any input?

 

[Scott J Hendry]

Hey WelderDave. My engine rebuild is the same, I can't get the lines horizontal and lobe at 12 although it is only marginally out. It's out in the direction that would make the valve timings retarded however so I'm not taking any chances, I've just ordered a new chain and rear chain guide. I'm suspecting I've got either excessive wear or stretched chain. I read on a XS650 forum that its possible for the chain to stretch if the bike hasn't been looked after properly which mine wasn't previously. If you want I can give you my WhatsApp and let you know how this plays out for me.

 

[Durney]

This post maybe too late for these issues in this tread. With my 1981 XS400 I was getting only 39 MPG and the power was that of a 250 so I checked the compression it was low - 119 on both sides. I must have not had the end gap correct on the left side when in stalling new rings back at 21k because the top ring was broken and the ring groove had a chunk missing. Murphy's law would have the broken ring piece fall into the crankcase when I pull of the barrels. Luckily I fished the chunk out with a magnet. I learned to pay attention to details and double check measurements. I bought new pistons and barrels on eBay from a bike with less than 12k on the engine. After assembly it wouldn't start unless I turned my ignition plate CC very advanced to get it to run. I moved the timing chain over one tooth by holding the sprocket with the 2 bolts out and moving the chain over one tooth and it still wouldn't run unless advanced. Had the rocker box off and adjusted the timing 6 times and the pick up rotor still off by one tooth. Turns out moving the timing chain with sprocket loose over one tooth also moved one tooth on the crank so it made no change. Also I found the hard way that the LT mark is for valve adjustment at TDC and the LF mark needs to line up with the nipple on the top cam sprocket bolt so the ignition pick up coils ( no points ) are in time. When put the new rings at 21k I had much easier back then.
I use my bike as a daily driver to commute 70 miles per day. I now have over 53k on the bike and will get over 100k before it dies or I do.

Always double check measurements and don't be in a hurry when working on these old gems