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Discussion Starter #1 (Edited)
I recently bought an '81 GS450L with 15,000 miles that stalls under throttle and coughs back through the carbs. Compression was 150+ psi on both cylinders, and the oil filter was clean, save for a few tiny metal specks. I had the carbs removed and decided to check compression again - I looked in the intake ports, and saw some very small bubbles / foaming coming through the bit of starting fluid left around the valve seats.

My question is whether these valves are leaking which causes the engine to cough, and does this warrant re-lapping the valves? I figure I should check valve clearances before I remove the head, just to do the easy stuff first. My knowledge of cars tells me that the valves should seal 100% perfectly, especially when pressure is forcing them onto their seats, but I don't know if a small amount of leakage is acceptable. Does anyone else have experience with a bike that does this?

Any help is appreciated. Thanks.
 

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Any engine that has sat for awhile will be left with at least one valve open. Corrosion and crud can easily build up. It may take some running to clear it. When you have a problem, especially with a bike with carbs, always look at those first instead of tearing the engine apart. Then x100 on looking at carbs on a bike that's been sitting.
 

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Discussion Starter #3
That's a good point about the bike sitting for a while - I can guarantee this one sat for some time before I bought it. I retarded the timing as far as I could, and also cut down the exhaust to just where the mufflers start to flare (someone took out the baffles and welded perforated plates on the ends of the mufflers - I'll be putting stock exhaust back on it soon so I don't feel bad about cutting down an already ruined set of pipes), and this made it run a little better. It still sometimes coughs though. Like you said it might just be a matter of letting the valve seats clean themselves by running.

I should also probably add that I cleaned the carbs after I brought it home to see if that would fix the issue. They were pretty clean, and the diaphragms were intact and flexible.

Here's a video I took looking down one of the intake runners. Sorry about the harsh starter sound - I think I had my phone held against the frame.

Thanks
 

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Seems to me you wouldn't get 150 psi on a compression test with any serious leaking, but your video definitely looks like the intake valve is blowing bubbles. That was a clever way to inspect!

Intake backfires are reported to be caused by too lean, too rich, leaking valves, and ignition timing issues, so that really narrows it down. Did you check the rubber boots that connect the carbs to the intakes? Small cracks in these are pretty common on bikes of that vintage and will cause a lean mixture.

You might try doing the Seafoam suction top end cleanout, where you spray liquid Seafoam into the intake while the engine is running. I've convinced myself that I had noticeable improvement in my valve sealing with that treatment.

And cattzap, it is possible to store your bike with both valves firmly closed. It's easy with a thumper!, a single cylinder bike.
 

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Discussion Starter #5
I will pull the intake boots and check the o-rings under them, and try Seafoam. I have a strong suspicion that the bike is running rich, so I might adjust the floats and see if there's any improvement. I actually didn't have a spec for float level when I reassembled the carbs, but they looked fine to me. I have a shop manual in the mail that should be here by the end of the week, so I'll take a look at that and adjust the floats.

Thank you for the input.
 

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Discussion Starter #6
So, it actually turns out that most of the issue was caused by...bad spark plugs. They had some soot on them and the spark was very weak, which I didn't realize since I was checking spark with a different pair of plugs. Putting a set of stock exhaust pipes on the bike also helped a lot, and it's now running well and road worthy. I also replaced the o-rings on the boots between the carbs and the head.

So lesson learned - check spark with the plugs you have in the machine, not a screwdriver.

Thank you all for the help.
 

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Thanks for the update and glad you got it sorted out. :thumbsup: :thumbsup: :thumbsup:
 
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