Greetings from Newfoundland,

I own a silver 1984 GS with 79000km's. Pretty much stock with the hot start assist, sub-zero start and shutter valve(troubling shooting failure) removed.

I just left the driveway one day and while changing gear from 1st to 2nd I heard a slight backfire and then a high idle started, revving around 1500rpm.
Idle and lumpy, black smoke from exhaust(didn't do this before), sightly backfiring(spitting).
When I quickly touch the gas pedal it bogs down a little then revs up in it's own as I let go of the gas. I assume a vacuum leak but I cannot find it. internal leak?

Shutter valve was sucking air. Removed coasting valve/shutter and butterfly. Kept shaft in place and siliconed. Capped all vacuum lines. No effect. Still high idle. Could a leak somewhere cause the coasting valve to operator at idle? (butterfly was closing shut at idle).
Checked for leaks with carb. cleaner, no effect. Checked the hoses close to the intake, looks good and tested by blowing through it with my hand on the other end.

I've been slowly working the bugs out of this car, but this problem is making be frustrated.
Please help me get this car back on the road, Thank!

 

I discovered the problem. It turns out my Secondaries were sticking slightly opened. I sprayed them with a lube and work it opened and closed.
I now have an oil leak on the front of the engine gasket covering the old pump linkage.