78 GS550 Blowing fuel vapour back out carbs.

[GS550_MATT]

Recently on my bike the timing chain snapped , so I took the head off stripped it down checked if valves bent; they were okay, had the head skimmed 6 thou at the local engineer shop I use. I also decoked the chambers valves and ports myself with a wire brush attatchment on a drill. Then I lapped the valves back in and fit new stem oil seals.

With the head off I was able to pull out the timing chain and removed all broken pieces , next I fit a new headgasket and put the head back on, I fit a chain that could be broken and riveted with a masterlink. Then I put the 1 and 4 at TDC and put the cams in at the correct timing marks with arrows 123 Lined up and 20 pins inbetween 1 and 2. I have built the engine back up and fired her up and she wont run correctly.

It will idle but not the same as it used to, its smooth and firing on all four cylinders but stalls a lot. and when i rev it up it will very slowly come back to idle, It revs very well and is not misfiring but will stall whn the revs come back down, aswell as this there is a strong smell of petrol near the cone filters, and when i rev it with them off you can see fuel blowing out the carbs. something is not right, im going to do a compression test when i get one, not sure if its blowing compression back past the inlets.

Does anyone have any ideas , i think that she is running lean , i cant see any obvious air leaks in intake. Would the carbs normally need setting up after the work ive done? Really want to get her running just like she was.


I got lots of pics to put up , will do it when I can;

Regards Matt

 

[tom203]

".. arrows 123 Lined up and 20 pins inbetween 1 and 2."

To clarify this, do you mean 20 pins between 2 and 3 with the first pin counted right over 2 mark?

 

[tkent02]

CAn't remember, is twenty pins the right number for the 550?

 

[GS550_MATT]

Yes 20 pins exactly from number 2 arrow on exhaust camshaft to number 3 arrow on inlet camshaft, I have the engine timing set up exactly how it should and says in the book, It is critical the timing is exactly acurrate and I would always make sure that everything is correct TDC etc..... It revs up alright and is very smooth indicating to me that the timing is definately correct as if you were perhaps even a tooth out it would run crap. Im pretty sure the ignition is working well and doesnt misfire, experience tells me its a fuelling and air problem the way its not returning the revs down, as if it was sucking air in or something, the key thing i noticed ithat doesnt seem normal is fuel blowing back out the carbs as if some compression is going past the inet valve when its closed on compression stroke, so maybe the valves arent sealing, when i do a compression test this will indicate what might be wrong. but ive just rebuilt the head and all seemed fine? couldnt possibly be the head skim that has messed things up somehow?

any insights and help much appreciated

 

[Nessism]

I'm confused as to how you could have broken the cam chain and not bent any valves. How is this possible?

 

[Steve]

Did you check the valve clearance? Depending on what all you did to the valves and/or the seats, you might have reduced the clearance a bit. Tight valves will close later, allowing spitting back through the carbs.

.

 

[GS550_MATT]

Cam chain snapped because I removed the tensioner to clean and check operation, however when I refitted it I did not turn the knurled adjuster spring 1 turn anti clockwise which is what creates the tension on the pushrod keeping the chain tensioned. So anyway I fit a new chain headgasket etc... and now I am here. All i did to the valves was wire wheel the carbon off of them, then lapped them all in by hand using coarse then fine paste, and fit new seals, all valves buckets collets and springs were returned to the exact same place.

What valve clearances do you mean ? I made no measuremnts of anything to do with the valves, what should I have checked, valve guide clearance , valve seat (I didnt know that was a thing) You may be onto something here, If the valves are too tight and closing later how would I rectify this, by changing the cam timing possibly?

 

[tkent02]

Adjust the valves as described in the Suzuki Service Manual, not Haynes or Chiton's, they have it wrong. When you lapped the valves you made the clearances even tighter, but they tend to get tighter with mileage anyway. Running them too tight will burn them.

 

[GS550_MATT]

How do I post pics on here from my iphone guys?

I hope it is the clearances because I am happy to do that than take the head off again.

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)

 

[Brendan W]

Click on the invite in Steve's sig for a valve clearance worksheet.
Inlet valves closing too late will allow fresh mixture to be blown back where it came from.
Thinner shims increase the clearance.
Posting pics can be done through photobucket. See the article about it here http://members.dslextreme.com/users/bikecliff/
It's on the top left just below the lists of wiring diagrams and manuals.

 

[GS550_MATT]

 

[GS550_MATT]

 

[GS550_MATT]

 

[GS550_MATT]

I did a compression test and got 85 psi on cylinders 123 and 100 on 4 , is this normal readings or quite low ? The pic where the chain is slack is because I hadn't fit the tensioner yet

 

[GS550_MATT]

 

[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.