Changed water pump, now won't idle properly

[alhersch]

Long story short: Took boat out, no tell-tale water stream took boat home. Changed water pump and ran on muffs, no water stream. Found plugged tell-tale hole, used airhose to unplug, ran on on muffs, ran fine and water stream okay. Took boat to lake, won't stay running at low idle (800rpm), but will stay running at 1K-1.5K and higher. If I let run at low rpm's motor will die out, if I pump the fuel bulb motor will jump back to correct idle rpm's. Jammed boat in gear and choked the tar outta here to get it going then she ran fine once above 1.5K and planned out fine. Found hole in vacume line and replaced also. Motor ran fine all summer so far. Compression on 5 of 6 cylinder is 65 -75psi I believe, i'm not sure the exact reading but know it was about that low and all cylinders were within 20psi of each other. Fuel pumps replaced last spring.

Is there anything that could have affected the motor by changing the water pump at all, in any way shape or form?

This is a 68' Merc 1000, I6

Thanks,

Alan

 

[freddyray21]

Re: Changed water pump, now won't idle properly

seems rather low for compression to me. Should be about 100 at least and all within 10-15% of each other. If you have one cylinder that is 20 lbs from the other five you have something going on. When you took it out before changing the impellar did it get hot? Bad compression could be indicative of idle problems. Might have got it hot enough to damage the head or blown a head gasket. Although a head gasket will usually show two adjacent cylinders low. Check the compression again on all six cylinders and post them. You might also try checking the compression with a little oil in the cylinders.

 

[alhersch]

Re: Changed water pump, now won't idle properly

Thanks for the reply, and yes I was thinking (from reading on here) the the compression should be closer to 120... However, I will recheck the compression. I don't know if I can check the bottom cylinder though, I don't have a 90 degree elbow and my hose won't bend that sharp I don't believe, but i'll see what I can do.

I don't believe the motor got hot, not to say it didn't. I did try to run full throttle to see if I could get the tell-tale water stream then about half throttle back to the ramp. But when I did replace the pump it still didnt' pee out. Which was found to be a plugged pee hole. So I 100% believe it didn't overheat.

Also, why will it run perfectly fine at home on the muffs but not in the water?

Anybody else have a thought about this?

Thanks,

Al

 

[diaric]

Re: Changed water pump, now won't idle properly

on muffs, no backpressure of exahaust under water. in lake rpm will drop 100 or more from backpressure. thats why when you set up carbs, you can get it running smooth on muffs but for fine tuning and settings, it should be in water