77 930 Turbo Carrera problem

I just bought a 77 930 with 57k orig miles and when I hit about 4000 rpm and lift to shift the engine will surge another 1500 or so rpm as if I am still on the gas.It does this every time and has since I purchased the car.

I have checked the throttle linkage and replaced the return spring and it's fine with no binding, I'll even tap the pedal while its surging and it has no effect so it must be a CIS issue.

Any ideas appreciated.

Hi Jeff,

Do the 77's have an intercooler? I know 76's don't, been sooooo long since I've seen anything earlier than a 79 I can't remember.

Check the spring & diaphram in the Manifold Overboost Release Valve. If they are sticky or bad the intake can't release the incoming pressure that's built up when you shut the door (throttle) on it, causing an anomoly.

HTH's

JRV

Thanks JRV

No intercooler

Where would the valve be located?

Hi Jeff,

on 79's it's on the left side manifold back corner, engine lid open facing the engine .

on 79's-up it's the square piece that the overboost shut-off switch is plumbed into. 76-77's have to have this boost release diaphram also, to divert the boost when the throttle is closed. So look around it's there somewhere.

Thanks

None of the shops here could see me till next week, I'd like to get this fixed since I can't take it past 4500 rpm so I can rap this thing out and see what it will do and try and scare myself as it has a few mods SSIs, dual out exhaust, 1 bar spring etc.

Regards,

Jeff

Jeff,

also check that the Distrib isn't sticking.

JRV

First thing i would do if it is modded for additional boost, is add an intercooler, before it goes BOOM.

Yes,I want to add an intercooler, it would need to be a custom made unit or 2 smaller custom units to fit under the smaller 77 Turbo tail.