High Idle on 470 Merc w/Rochester 2 jet

[TIJA99_470_merc]

170hp ThunderCraft

Can you give me tips as to why my carburator keeps going to high idle after it warms up. And of course when this happens Shifting into FWD or REV happens with a banging sound.

I can adjust the or choke settng and the Idle RPM will mellow down and I can drive the boat for a while and shifting is smooth w/o any banging ( 1-2 hours ) and the Idle will creep up again. I try to leave the IDLE & Misture screw at 2 turns out from home position

The Service manual just sasy check the the choke setting.

Am I missing something, I had someone replace the Carburator KIT gaskets and internals last season.

Thanks in Advance

 

[Silvertip]

Re: High Idle on 470 Merc w/Rochester 2 jet

Yes you are missing something. First -- this post belongs over in the Mercruiser I/O forum. Second -- the reason the idle keeps going up is that you have not adjusted the choke properly, the choke pull-off is not functioning, and/or the choke heater is not working. NEVER adjust the choke unless the engine is stone cold. Next, disconnect the throttle linkage at the engine. Blip the throttle to "set" the choke. Did it fully close? It should just close, not snap shut which means the choke heater is adjusted way too tight. Loosen the three choke housing screws and rotate the disk so the choke plate opens, then rotate is the opposite direction until the choke plate just closes. Even though you have the throttle disconnected, start the engine and see if the choke plate opens about 1/4 inch. If it doesn't the choke pull off is not working. That's a vacuum diagram that opens the choke right after engine start to eliminate running to rich. Replace it. The fast idle cam should be in play at this time and idle speed should be 800 - 900 rpm or so. Continue to watch the choke plate. The choke heater should continue to to pull the choke open until the plate is perfectly vertical. Blip the throttle and the cam should allow idle rpm to drop down to normal idle. Adjust idle speed with the idle screw. You may have two idle screws - one on that rides on the fast idle cam and other that controls normal idle. Make sure you know which one is which. Having fixed all the bad parts and made proper adjustments, adjust the throttle cable so it does not affect idle speed when being connected. Idle mixture is not a a simple set it two turns out. That's a starting point. Rotate each one in 1/8 turn clockwise until the engine begins to gag or bog. Not the position. Rotate it outward until it does it again. Happiness is somewhere in between the two points. You may need to lower idle rpm a little when this is properly set. If the idle mixture adjustments seem have no effect you have a serious vacuum leak somewhere.