78 GS550 Blowing fuel vapour back out carbs.

[Guest]

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]