Fast Idle

I have a 1978 small block chevy 305 with a Rochester 4 barrel, the boat was laid up for a long time. When I got it I had a guy look at the engine, I had to get the crankshaft reground with new bearings, new oil pump. The engine now runs well, starts fine, but I have to adjust the idle high to stop it cutting out when I shift. I have try'd a new shift interrupt switch also a carb clean and new gaskets new water filter . The carb had signs of water inside. The boat is hard to control as it moves to fast in gear at low revs. I have to idle at around 1100 rpm, I think it should be 700. If I set the idle at 700 it cuts out when shifting. I have tuned the carb also checked timing. Any suggestions ?

Re: Fast Idle

Instead of trying a new shift interrupt switch, see if the switch is activating when you put it into gear - it shouldn't. Sounds like a symptom of a bad shift cable.


EDIT - let us know a little more about what drive you have, ignition system, etc.

Re: Fast Idle

Sounds like a vacuum leak to me.
Temporarily unplug one of the leads to the shift switch. When the problem continues, start looking for a vacuum leak around the carb's base and throttle shafts, secondarily along the intake gaskets.
Sticking secondaries (not closing completely) will also promote you condition.

Oh.......explain your "new water filter" statement.

Re: Fast Idle

Hey guys thanks for the quick reply..
I have put a new shift cable in, the switch is not activated when in gear. When I got the boat there was no shift interrupt switch. I put the switch in thinking it would solve the problem, it helped ok but there is still the problem ..
I have a pre alpha drive, ignition no idea..

I think Eddie might be on the right track..
How do you sugest checking for the vacuum leaks ? Will I take the carb off ? I will have a look at the secondaries, but I think they are ok. I have the Engine out of the boat at the moment, I have replaced the plywood on the transom.

I should have said fuel filter...

Re: Fast Idle

Have a look at this thread, HOW TO: How the Shift Interrupt system works - Page 1 - 288089
It really sounds like you have shift cable adjustment problems if the cable is installed and working correctly.

Does the shift plate activate the shift interrupt switch WHILE you are shifting. It should NEVER activate shifting into gear.

Re: Fast Idle

"How do you sugest checking for the vacuum leaks "

If your problem continues with the shift switch disconnected, probe suspected vacuum leak sources with an unlit propane torch. If the propane is drawn in via a leak, the engine speed will increase. In auto use, starting fluid was often used but not too bright an idea in the boat.