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79 gs850 popping

  • Thread starter Thread starter Chuck Fryer
  • Start date Start date
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Chuck Fryer

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I have a 79 gs850 that I've had since February, managed to get about 3k miles on it this year. I'm having a popping when I let off the throttle, especially when upshifting 1-2, and 2-3. The plugs say lean, well 3 of them fo. When I first got it I replaced the orings at the head, ajusted the valves, sealed the air box. So, where should I look? Re check valves? Carb jets? BTW, I recently cleaned the carbs. The issue was present then, that is why I cleaned them.
 

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Yes, replaced all exhaust gaskets, and 6 broken studs.


OTE=jsandidge;2417993]Have you checked for exhaust leaks?[/QUOTE]
 
I assume the plug photo is 1-4. left to right? #3 is rich?

If so, you've got a leaky petcock. You're running lean on all 4, just the flood of gas is making #3 rich

When you had your carbs apart, did you check the jet sizes? You have the stock airbox, stock exhaust?
 
Yes, that is 1-4 L-R. The tank doesn't leak when sat on ground and petcock pointing down. I guess I could block off the vacuum line to carb and run petcock on prime to see if it helps?
 
Just test the petcock and know....

Try this... remove both lines from petcock: get some clear vinyl tubing that fits snugly on petcock vacuum fitting. In "on" and "res" fuel,show flow when you suck on this tubing- otherwise it should not. Any signs of fuel in the tubing means the petcock diaphragm is likely leaking.maybe just when bike is running but maybe at alltimes.​
 
Ok, tested with mighty vac and no gas was passing thru. I now blocked the vac hose with a bolt. I plan to turn it to prime & test ride to see if it makes a difference.
 
Test ride with blocked off vacuum was the same. Any other ideas? Sure could use the help. Thanks
 
A little random popping is somewhat normal but a lot suggests an air leak somewhere.
 
I should add that it is down on power with this miss/popping. Also the comp test was a bit low at 110 +/- for all cylinders and the leak down was at 5% and same for all cylinders. The leaking was into the crankcase, meaning it's ring time. Has just under 40k miles.
 
Before I went ringing, I'd get new points and condensers and hope that my problem is misfiring in ignition circuit.
 
Do you think a float that is off do you think a float that is off could cause this?
 
You definitely need to fix the carb #3 rich problem. Could be a float related problem. Check the fuel level in each bowl to verify whether it's leaking or not.
 
Is that as simple as pulling the drain plug and seeing if one bowl has more than the others? I agree the number three problem needs to be fixed but I am not sure whether it is an ignition problem causing the fuel not to burn or the carburetor putting too much fuel in
 
Well, pull all 4 plugs, lay on the cylinder head and hit the starter button.
They should all spark blue/white. It helps to do this in the dark

To check the static fuel height, you need an adapter and some hose to see the fuel height on the outside of the carb. You can buy one from Z1 Enterprises

You didn't reply to the jet size question. With those Harley mufflers, you're probably lean
 
Last edited:
I had popping on one cylinder:


which turned out to be bad carb synch:


You may also have flooding in no.3 due to poor needle valve seat O ring:


You are getting incomplete combustion OR over fuelling on that cylinder. For incomplete combustion problems, make sure the valves are closing and seating (compression testing or leak down test), check plug and leads (try switching plug with another cylinder and then, as a separate test, switching plug leads 2 and 3 - un-plug leads from spark plug on cylinders 2 and 3 and reattach the other way round).

For overfuelling, look at float height, float damage/leaking float, wrong jets or wrong needle clip setting, badly synched carbs (carbs are synched to no. 3 so if it was a bad synch then carbs 1, 2 and 4 are not opening at the same time as no.3), check the working of the petcock (looks like you've done that) and the vacuum hose.

Greetings
 
Thanks. Jets were factory size, as far as the info I found. I bench synched the carbs with a caliper on top of the slides, so they are close, +/- .002 or so. I need to purchase a set of gauges to vac sync. Just finished adjusting valves, all good, #3 exhaust was on the loose side.
 
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