IO-360 idle problem

[Monticello Flying Club]

We have a maintenance issue on our '67 M20F that has us stumped.  If we bring the throttle back slowly she idles up at 1,000RPM.  If we bring her back quickly, she goes all the way back to the normal approx. 600RPM.  Mechanic thought it was a failure of the throttle cable sleeve pulling out of its set and installed a new cable.  Unfortunately problem did not go away.  We checked it with the cowl off and strangely the throttle body is at it's idle stop in both cases, whether its at 1,000RPM or 600RPM.  Anyone got any thoughts? 

[jetdriven]

Wear on the throttle shaft? That could let air in. Also, when was the fuel injector servo last overhauled?

The factory ball link connector gets slop in it and that could easily be a symptom. Slamming the throttle shut will close the throttle fully but bringing it back slowly, the slop won't allow the cable to pull the throttle fully shut.

Sucks you had to spend 500$ on a cable that didn't fix it. Misdiagnosed.

[N201MKTurbo]

It sounds like an intermittent air leak. Check the "Sniffle Valve" on the bottom of the sump.

 

When it is idling fast put your finger over the drain line and see if it slows down.

[KSMooniac]

Sniffle valve is a very good guess and easy to check.

Sent from my VS985 4G using Tapatalk

[DrBill]

Also check the standby vacuum tap if you have one.   My A&P put the output of a shop vac into the air intake packed in with rags and used a water/detergent mix in a spray bottle to find the leaks you could not hear.

BILL

[philiplane]

a completely blown sniffle valve will only add less than 100 rpms to the idle speed. 

[Monticello Flying Club]

Thanks everyone.  Turned out to be a loose bolt where the throttle arm connected to the throttle body that allowed for play.  Nothing like paying $500 on a new throttle cable just to find a loose bolt.  My mechanic claimed the cable needed to be replaced anyway, but I think they all say that in situations like this just to make you not feel so bad.