Johnson 1990 GT200 Idles high

I recently replaced the reed box gaskets and the gaskets on both sides of the intake manifold to fix an issue with carbs spitting back. The engine starts and runs much better at the high end. Unfortunately the engine idles at 1200 to 1500 and won't come down even after warming up.

-I've double checked the seals and reassembled with a thin bead of RTV.
-Checked my idle timing (6 degrees).
-Checked the throttle bodies to insure they are fully closed at neutral.
-Capped off all intake vacuum ports that need to be capped.

The engine continues to run at speeds of 1200 - 1500 rpm. It will many times proceeds to sneeze and sputter when put into gear. With minimal throttle advance it stalls. With a jump to 1/2 or 3/4 throttle it will accelerate great.

I assume it is getting access air causing it to idle high and possibly lean.

Any suggestions?

Re: Johnson 1990 GT200 Idles high

Chances are excellent that your QuikStart ignition is not disabled when you reach 96 degrees or 1100 RPM and that your timing is still advanced. Check the black and white wire running from the temperature switch on the cylinder head to the powerpack.

You can also tell by listening to the engine when making a cold start. There should be a noticeable slowing down when the Quikstart senses 1100 RPM or 96 degrees, whichever comes first.

Re: Johnson 1990 GT200 Idles high

Thanks for the quick response Ezeke. I ran the engine again today and double checked the timing and RPM. With a cold engine the timing is several degrees retarded and the rpm is roughly 1500 in neutral with the engine in the water. After a minute or so the rpm drops to about 1200 and the timing is advanced to 6 degrees.

Please let me know if there's a way to rule out these two scenarios.
1) Is it possible that my RPM gauge is incorrectly high and the engine is running properly? (The engine sounds fairly high to be at idle)
2) It the throttle bodies are closed completely, the Quikstart appears to function. Is there any other way for this engine to idle higher than normal once warmed up?

As a side note: I think much of my engines problem with sneezing and dieing on slow acceleration could be due to a lack of spark in all the cylinders. The engine started just fine and ran rough on 3 cylinders. Upon testing the other side of the engine I found that I could not get a spark in any of the cylinders. I pulled off one of the wires and it arced. Right after the arc accorded the rest of the cylinders started firing. Any chance this is common or should i look into a new power pack?

Once again thanks for all your help.

Re: Johnson 1990 GT200 Idles high

First, you have to disable QuikStart when you set the timing to be sure that it is not advanced; Ground the black and white wire on the powerpack side.

The engine has a shift assist switch behind the shift lever on the engine. When you shift out of forward into neutral, it is supposed to cut off three of the cylinders which makes it much easier to shift. If the switch is faulty or is misaligned, the three cylinders can get cut off when they should not. You can temporarily disable the shift assist switch by disconnecting the black ground wire.


S.L.O.W. is activated by the temperature switches via the tan wires, which are also wired to the horn for the entire alarm circuit. Normally, blocking diodes control which functions can reach which circuits.

S.L.O.W. is also a powerpack function that disables several cylinders.