How critical is lobe position when checking valve clearances?

[sacruickshank]

Resurrecting an '81 GS550T with 8k miles. Don't know if the valves have been checked so I opened it up. Most of the valve clearances are within the spec ranging from .04mm to .07mm, but a few only hit the spec as the cam shaft turns thru it's rotation. Some of them are a little tight in other positions during the rotation, albeit not when the lobe would be engaging the shim.

So ... how critical is it that the lobes are the the official position per http://gsarchive.bwringer.com/bikecliff/images/gs850valve_adjust.pdf and only that position when checking clearance? I've always assumed that the lobes were a constant radius during the portions that do not contact the shim.The clearances are so close that swapping in a different shim may throw them out of spec. unfortunately they are currently close on the tight side, rather than the safer loose side.

 

[Redman]

I too was wondering about the off side of the cam.

but what do need to concider about cam position when checking clearance is to have the 2 cams that are next to each other , both off the bucket at the same time. If one is depressing it’s spring, it is also pushing the cam shaft around and is effecting the clearance reading of the other cam.

 

[Redman]

So, yes, cam lobe positions is important.

so the answer to if should be pointed up, or pointed forward or pointed back is : one forward and the one next to it backward, so neither on that side (say, exhaust 1 & 2) is pushing againnst spring.

 

[Nessism]

When using the factory cam positioning the valves are checked in PAIRS, with the adjacent valve to the one you are checking on the base circle. This is important because if you check the clearance with the lobe pointed away from the bucket the adjacent valve may be depressed, and when that's happening it skews the cam in the journal clearance which increases the measured clearance.

I've done comparisons both ways and when the adjacent valve is depressed it results in a measured clearance about .002" (.05mm) more than with the factory positioning. That's not much, but considering how tight the factory spec is, it's significant.

Bottom line, if you want to adjust the valves with the lobe pointing away from the bucket you should target a higher clearance range: at .07mm at a minimum. And speaking of clearance, you should always target the top end of the clearance range and consider increasing the clearance up to .12mm. Kawasaki KZ bikes use clearances up to .15mm and the GS 8V engine is a KZ clone so what's good for the goose is good for the gander. There is no reason to target the tighter Suzuki clearance unless you are willing to keep on top of your measurements checks per the manual. If you are prone to skipping checks, target the higher range.

 

[sacruickshank]

Thanks @Redman and @Nessism for the info. That makes sense.

I'll check them again once I get the engine back to life. I suspect some residue from sitting may be affecting the clearances. Plus I'll need to change the petrified gasket and I'm not yet ready to tackle that bit of torture. The valve cover came off with minimal damage to the gasket, so it's re-usable for short test starts but not long-term riding.

 

[ukjules]

Just checked mine on a 1980550e
This gets it spot on ! from proper workshop manual.
and yes you when in right position do 2 at once.

p.s let's have a pic of the bike .... I like to see
these old bikes.

 

[ukjules]

this may help to get started .....

 

[sacruickshank]

Just checked mine on a 1980550e
p.s let's have a pic of the bike .... I like to see
these old bikes.

I have problems uploading photos directly, so here's an imgur link. Not a beauty queen but relatively low miles, looks to never have been dropped, and no one has chopped anything off. Some PO did go a little crazy with spray paint.

And the "need it gone" price was right.

https://imgur.com/a/CGy8eOT

 

[Big T]

http://members.dslextreme.com/users/bikecliff/images/how_to_post_pictures.pdf

http://members.dslextreme.com/users/bikecliff/

 

[Rich82GS750TZ]

sacruickshank
Since you already an Imgur account, it?s just a few extra steps to post pics so they show up in the thread instead of just the link. Follow the link in my signature about using Imgur, post #6 in that thread.

 

[sacruickshank]

Thanks Rich, i'll use that next time.

For the record, the bike does have the fenders and throttle, they just weren't mounted at the time of the photo. It does not have a headlight (will buy), functional seat or side panels, so those will be make or buy decisions.

 

[Steve]

Bottom line, if you want to adjust the valves with the lobe pointing away from the bucket you should target a higher clearance range: at .07mm at a minimum.

I have a slight problem with that suggestion, Ed. :-k

You had mentioned that the cam could be skewed in the journal by the adjacent lobe pushing on a valve. That is very correct, but it only happens for one of the two valves in that pair. If you set the clearance loose for both of them, one might end up just right, but the other one will be too loose.

Loose isn't all that bad, within reason, but it also helps of the clearances are somewhat consistent.

.

 

[Nessism]

Steve,
When adjusting the valves with the lobe pointing away you can't adjust the valves in pairs. The crank/cams must be repositioned for each valve you are checking. I haven't checked every position but pretty sure that for each valve, then the lobe is pointed away from the bucket, the adjacent valve will be depressed, thus it's important to target a higher clearance range.

 

[Steve]

Very true, you can NOT adjust in pairs if you do it the wrong way,which is what many of the new guys think is correct. And that is because that's the way that most cars get adjusted.

It's definitely best to follow the factory procedure to position two adjacent lobes so they are pointing about 45? away from their respective valves and adjust them as a pair.

All three service manuals (Factory, Clymer, Haynes) show that same goofy picture that says "position the cams like this", but only the factory manual puts into text that you position the cams for a PAIR of valves and check them BOTH without moving the cam.

Once it is understood exactly how to position the cams and WHY, it makes the procedure go so much easier and quicker. :encouragement:

.

 

[Grimly]

Once it is understood exactly how to position the cams and WHY, it makes the procedure go so much easier and quicker. :encouragement:

.

This ^^^^^^^^^^. For years I adjusted them by appearance, one at a time (heel of cam at bucket) and by adopting the proper method it made things a bit easier and quicker.

 

[sacruickshank]

UPDATE - Thanks all for the guidance. I've now re-checked all the clearances using the pair technique and lobe positions described. Five are within spec, but three (1E, 3I, 4I) are <0.02mm. The buckets still spin freely, so at least they are not touching which should be good enough for a few test short starts. I'll re-check & adjust once Spring and test rides gets nearer.

 

[salty_monk]

You'd want to get those shims swapped out fairly quickly though, loss of compression, burnt valves etc

 

[red1100]

A degree wheel and dial indicator will get you spot on at every lobe. Not that hard to use.

 

[Nessism]

A degree wheel and dial indicator will get you spot on at every lobe. Not that hard to use.

And what would spot on be?

 

[red1100]

Spot on is finding exact tdc of cyl 1 with a degree wheel and dial indicator. It's the only way to know the piston is truly at the top. It is a little involved but dead nuts accurate.Make a wire pointer that points to the edge of your degree wheel. Screw in a positive stop bolt in the spark plug hole,bring the crank around by hand till it touches the stop,Mark down the number the pointer hits on the degree wheel. Now turn the crank the other direction slowly all the way around till the piston stops again,read the number.In the simplest terms if the degree wheel sez 20 going one way and 10 going the other TDC is at 15. Whatever the wheel reads it will be halfway between the two as noted on the degree wheel.