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It's not a vacuum leak! Valve clearance?

  • Thread starter Thread starter scuttle
  • Start date Start date
S

scuttle

Guest
Hey,
I'm new to this forum, but I've been reading it for a while.
I'm hoping that posting my problem might give me a second opinion, as well as giving others an alternative to just "vacuum leak" for this problem.
I'm rebuilding a 1982 GS450 and I've had a hell of a time. Something has been causing the engine RPM's to race really high once it warms up and I've been chasing it for days. Everything I've read on this forum indicates it's likely a vacuum leak, but I've isolated everything I could to trace it down, but it just isn't there.
Yesterday I tried synching the carbs, hoping that might help, but they seem to be well balanced from the bench. Then I took the synch off and put it away, I thought I'd fire it up and see if I could find anything. She ran like a top!
I couldn't think what was different between the time I was running with the sync on and now, but I looked at the boots and the synch ports were still open! I had forgotten to put the plugs back in, in my frustration, and it seemed to run really pretty nicely.
I put my finger in front of each port, one at a time, and noticed that one side was sucking and the other was blowing. BINGO! My intake valve was out of spec on my left cylinder. (if I had a compression tester....)
Took off the valve cover and sure enough, couldn't get a shim under the intake OR the exhaust.

Ok, here's my questions:

1: Would this be what is causing the racing; blowback from one cylinder?

2: I have no way of determining what size shim to get to replace them! Gonna try taking the existing shim sizes and subtracting the outside gap tolerance, which should get me close, but still inside tolerances, but I'm open to suggestions that might save me extra trips across town and buying 4 shims when I only need 2.
 
Well, the first thing you need to do is make sure you are doing your clearance checks when the engine is stone cold. Next, check your clearances. If they are all tight, check to see if you can at least spin the shim bucket when the cam lobe is pointing away from the valve. If you can, you can likely just go one shim size smaller. If it is too tight to turn, you can keep guessing or just substitute a thinner shim that is several sizes too small so you can measure the extra clearance and make a better guess what you need. Either way, you will need to remove each shim to see what size it is. You might be able to move them around and not have to buy more than a couple. Look on the bright side ... at worst, you will only have to buy FOUR shims. Most of the rest of us have EIGHT valves to adjust. :o

To help you determine what shim you need and to help you keep track of what's in there from one adjustment to the next, send me an e-mail request (address is in my profile) and I will send you a copy of my Excel spreadsheet that helps you with the math.

.
 
scuttle said:
Hey,
Yesterday I tried synching the carbs, hoping that might help, but they seem to be well balanced from the bench. Then I took the synch off and put it away, I thought I'd fire it up and see if I could find anything. She ran like a top!
I couldn't think what was different between the time I was running with the sync on and now, but I looked at the boots and the synch ports were still open!
Click to expand...

If you sync'd your carbs and then found that they were noticeably on different sides of ambient pressure, I would be very suspicious about the accuracy of my sync tool :-k
 
Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. scuttle,

Don't forget, once you get your valves in spec you'll have to re-sync your carbs. There is lots of valve adjustment information collected on my website. There is even a link to a video documenting the procedure for the 2-valve/twin cylinder GS motors. From my website:

*****************************
Valve Adjustment Video:
Kerry Burton of GStwin.com has put together a very informative video chronicling the process of valve adjustment on his GS500. The procedure will be the same for all GS twins. The principle is the same for all 2-valve GS motors.

CLICK HERE to go to the Google video.
******************************

Here is your official "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike! :D

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff
 
Depending on where you are on this planet, there might be a GS-er in your neighborhood with a shim collection.

Post your approximate location, and help might be just around the corner.

I would also add to the above that it's sometimes helpful to have a very thin shim around for these zero-clearance situations. For example, I have a 2.15 and a 2.40 I can slip in, measure, and then at least get in the ballpark for the shim I actually need.

Some details about what you did to address your vacuum leaks might be helpful too. Did you install new intake boots and new o-rings? Many other tactics have been tried in the past with little success. You can't fix old intake boots.

You'll also want to make sure the petcock vacuum port is plugged whenever the tank is off.
 
Wow, hey guys. Thanks for all the feedback and the warm welcome!

For the tappets, the buckets would rotate pretty easily and by hand, but I couldn't even get a .0015" shim under the cam (it was 36 hours cold).
I ended up buying 2 shims smaller than the original by a little more than the outside tolerance of the gap. That did the trick perfectly for the intake, right into spec, but for the exhaust I had to make a second run and go even smaller. I went from 2.6mm intake, 2.55 exhaust to 2.45 intake, 2.35 exhaust. *WHEW!*.

I'm watching the video on twin valve adjustment as we speak, but I've already got the valve cover back on and she finally runs without acting like there's an exhaust leak.
I have new boots and rings, but just in case I did the ol' starter fluid around the seals trick.

Like I said, I've been reading this forum for some time and found it invaluable many times. Based on what I've found here on the GS models, I've already replaced the stator and reg/rec with aftermarket ones recommended here and the ignition with a new Boyer Bransden and I've loved the improvements! I figured it was high time I join up.

I'll update my profile so it has my email and indicates that I'm in Portland OR. I've seen a couple GS's on the road, but for older bikes around here, it seems to be Honda 350 country. I have a soft spot for the GS450 because the first bike I ever had about 17 years ago was an identical model.

I'll definitely keep you guys posted as I go and eventually post pics, but for now, I still need to adjust the timing and sync the carbs (starting with new plugs!). I'm getting some missing and backfired right now, so I don't want to run it much yet.

Wish me luck!
 
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