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1977 Yamaha XS750
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Discussion Starter · #1 ·
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Hey guys, my compression was really low so after finding the shims were leaving too little clearance and causing two of my valves to stay open I ordered the tool needed to remove my shims. However, not the tool. The original doesn't look to be found anywhere outside of the UK and these new versions don't quite reach the lifter body (anyone else had this problem?). Anyways, since I can't get that tool, I figured the next COA would be to simply remove the cams. Anyone see any issues with my thinking here? Split cam chain, remove chain (replace with new after), remove cams, remove shims, reference shim numbers to measurements I already have, order new shims, insert new shims, replace cam, replace chain, and close her up?
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1977 Yamaha XS750
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Discussion Starter · #3 ·
I don't see mention of either in the manual. Just that they also split the chain. No mention of the replacement either.
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cool,
sometimes the top sprocket is bolted onto the camshaft and that facilitates the removal, chain replacement is way better but you need to change the tensioner guides even more likely before the chain. They take most of the wear.

Cam will be fine if it was never starved for oil, the buckets take the wear more.
 

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If the clearance is that tight the head may need new valves as the seating surface has badly receded or the stems have stretched badly at the valve and will stretch again.

What I would do or suggest to a customer:
Get a new chain and remove the old one. Perform a leak down test with the cams out.
If it passes reshim the valves (now easy since you can pull the cams) and replace the chain guides if necessary, install new cam chain.
If it doesn't pass time to pull the head and regrind/replace the valves or head itself.
 

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Valves are hard, seats are relatively softer, in a battle between the valve and the valve seat the seat loses.
In a battle between a piston and a valve the entire engine loses.

In a battle between the cam chain and the chain guides and chain tensioner that is made from nylon, the nylon is the sacrificial part. The chain is done once it elongates by 3% and to measure that you just lay the old chain along side the new chain and compare their lengths.

imo Buckets and shims are a pain to adjust and potentially less accurate compared to a screw adjuster with a lock nut on it. Give me a SOHC over a DOHC any day for that reason. He's going to need to calculate and record the shims and clearances for all the cylinders and install all at once or remove and re-torque the cam shafts multiple times, unless he can hold those springs down and swap shims. In the battle between the cam shaft and the shim bucket, the contact area on the cam lobe is very large in comparison to the contact patch on the shim bucket unless it is fitted with roller bearings, the bucket is going to lose that one. In a SOHC engine the rocker arms will wear faster then the cams unless they are riding on roller rockers.
In a SOHC engine that needs to withstand low oil pressure, the design upgrade is to put roller bearings on the cam shaft.



... leak down test failure can be just as easy a piston and rings problem as valves and if it was bad that should have been revealed when the engine was compression tested prior to disassembly. If you are trying to address low compression the valve clearance adjustment is the least intrusive and involved service to attempt, everything else gets complicated fast.

Some engines feature an automatic decompressor built right onto the camshaft (the one above does), if valve clearance is off even a little bit with these engines, the decompressor will fail and the engine becomes difficult to start.
 

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How are you going to stretch the valve :unsure:
Valves don't stretch :geek: they expand with heat to grow in length, the valve springs are the only thing 'stretching' the valve length and the springs are not strong enough to deform the valve stems length. Ever.
 

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How are you going to stretch the valve :unsure:
Valves don't stretch :geek: they expand with heat to grow in length, the valve springs are the only thing 'stretching' the valve length and the springs are not strong enough to deform the valve stems length. Ever.
You're right, the stem doesn't stretch but have seen them break at the neck on car engines running huge spring pressures and solid lifters. What the valve will do is tulip, can post a pic later. Seats can recess.
 

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Seats absolutely do recess. You would be posting pictures of a bent valve.

The purpose of setting the valve clearance is to accommodate the expansion of the valve stems length due to heat.
That is why the clearance is greater on the exhaust side then the intake side, there is far more heat.
Once the engine is at operating temperature the clearance approaches zero on all the valves.
 

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If this were my bike I would just skip the testing and reshimming and pull the head, have it and valve cover cleaned and blasted, valve job, bench reshim, and reinstall with new chain and head gasket. The peace of mind is worth it plus it will look super nice.
 
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