78 GS550 Blowing fuel vapour back out carbs.

[GS550_MATT]

So going off whats been said I will check the valve clearances and adjust if necessary, Is it the valve lapping in that has caused the probem or this and the head skim, as i think it could only be the lapping in that could possibly make a difference to valve clearance from the work ive done, im not sure if laping valves would remove enough material to make a difference.

What do you guys think, has anyone experienced similar problems

[Brendan W]

removing material from the valve seat or the valve head will allow the valve to rise higher in the guide and decrease clearance.
Skimming will not do that. What it can do is mess with the synching of the timing marks, but .006", I don't think that would have a big effect.
I've only ever seen timing blow-back when the chain skips a tooth or two

[Steve]

I thought if the valves were tighter the compression would be better and sealed , however your saying the valves are tighter making them close later, if this is the case do i fit thinner shims or thicker , I imagine thinner shims would be the way to go as they would return the vqlve fractionally sooner (well thinner as long as the clearances end up being in parameters of the specification in the service manual)

Yes, good sealing on the valves will definitely help with compression numbers, but it's not the tight seal that we are concerned about, it's the tighter (smaller) clearances between the valve and the cam. (Actually the shim and the cam, but who's counting?)

I don't recall seeing how long since the bike had been run regularly before doing all this work. If it had been sitting for a while, some crud might have accumulated between the valve and the seat, which would hold the valve open ever so slightly off the seat. If you had measured the clearance, it might have been OK. But now that you have cleaned off all that crud, the valve is sitting a bit deeper into the guide, which pushes the bucket and shim a bit closer to the cam. That is where you measure the clearance.

When you measure the clearance, be sure to follow the procedure in the FACTORY manual. It is the most accurate, but is still a bit confusing. I like to start on the left side of the bike and do the exhaust first, so rotate the crank so that the lobe on EX1 is pointing FORWARD, EX2 will be pointing UP. When the lobes are in those positions, without moving anythnig, measure the clearance on BOTH EX1 and EX2. Record them. Turn the crank 1/2 turn (180 degreres), you will see that IN1 is pointing UP, IN2 is pointing REARWARD. Measure BOTH, IN1 and IN2. Give the crank another 1/2 turn, EX3 and EX4 will be in position. A final half-turn on the crank will set up IN3 and IN4.

Now that you have all your clearances, it's time to inventory the shims. This is helpful, even if all your valves are in-spec. Next time you need to check clearances, you will know what shims you might need to have on-hand before you start the job.

Remove the shims ONE AT A TIME and PUT THEM BACK BEFORE MOVING THE CRANK. Do I need to put that in bigger letters? Yes, it's VERY important that you do not move the crank unless every bucket has a shim installed. Inspect/measure the shim's size, record it (somebody mentioned my spreadsheet, look in my sig), move to the next shim. When you have them all recorded, you will see whether you can move some shims around to minimize the number of shims you need to purchase.

Yes, it's tedious the first time around, plan on at least two hours. After you have done this a few times (book says every 3,000 miles), you can get the whole process down to less than 45 minutes.

.

[GS550_MATT]

I remember when I first took the valves out the did have tiny black dots on them which i imagine was carbon which like you said once removed brought the valve clearances down too much.

None the less I measured all my valve clearences and got the results :

EX 1 less than 0.03mm
EX 2 ABOUT 0.045mm
EX 3 ABOUT 0.045mm
EX 4 LESS THAN 0.03mm

In 1 less than 0.03mm
In 2 less than 0.03mm
In 3 less than 0.03mm
in 4 less than 0.03mm

Thats the best accuracy i can get wth my feeler guage , however Given I calculate everything spot on I know I can get the valve clearances exactly where I want them, I will try get the clearence as close to 0.08mm as I can, allowing sme room for error either way.

Im going to remove my shims and measure them with a digital vernier calliper and write them all down and see if I can switch a few around possibly.

What is the correct way to use the C shape tool that compresses the valve springs allowing the shims to be withdrawn/replaced, do you lever it against the bucket edge?

[Brendan W]

I've gone to the dark side and use the zip-tie method.
I hear a lot from people who use the c tool a lot and they have the knack but inferior tools will make it problematic.

[GS550_MATT]

Exhaust
1 2.65mm shim
2 2.60
3 2.60
4 2.65

Inlet
1 2.70x
2 2.75
3 2.75
4 2.75


Because the cams are touching the shims and there is no gap I assume that fitting thinner shims will bring me to 0.05mm clearance, or the next size will be 0.010mm, before I buy them should I set them at .05 OR .010, Specification is 0.03 - 0.08.

Im not sure if its best to fit them at 0.05 or 0.010 , im tempted to go 0.010 because as the engine gets arm the clearances actually get smaller.

[steveb922]

Fitting a .05 thinner shim will not necessarily get you .05 clearance, because you have to allow for the valve spring tension. You could fit a .05 thinner shim but only end up with .02 clearance. You could take your thinnest shim and move that around the buckets to try to get definitive clearances, and then calculate from there.

In general terms, the higher end of the factory approved range (0.03 - 0.08) is better, because, as you say, the clearances will decrease. In practice, 0.10mm (not 0.010!) will be fine.