120 mercruiser bogging down

[plts333]

I have a 82 starcraft with a 120hp mercruiser (serial 6124460) motor with a rochester sg2 (I think) carb. It starts easy and idles just fine. However, it is bogging down under load. It doesn't happen right away, some times it take a 15 - 20 seconds of good power for it to bog down and then after it idles sometimes it will go for a while, other times you have to baby it just to get it to move again. We have rebuilt the carb, replaced all the filters, check the check valve in the tank, replaced the plugs, coil, and pionts. We've tried new gas and gas treatments. Nothing we have done seems to affect the bogging. We checked the fuel pump, it holds 5psi of pressure (not sure if that is correct). Any ideas?

 

[Kainon]

Re: 120 mercruiser bogging down

have you done a compression test ? and a leak down test ?

I'm wondering if you missed a clogged hole under the power valve, or that the power valves bad. The power valve is at the bottom of the carb has a spring and pin that moves with vacuum changes in the engine.

 

[libellav15]

Re: 120 mercruiser bogging down

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[libellav15]

Re: 120 mercruiser bogging down

yep - check the powervalve hole - and general operation.

have you tried running from a remote fuel tank direct to pump - this will eliminate any fuel and filter issues if it still bogs.

does the temperature stay normal

and in your service item list you don't mention the condensor - worth changing for the price of them - and it could be dodgy under load - even straight out of the box they can be cream crackered

cheers

adam

 

[plts333]

Re: 120 mercruiser bogging down

I tried to clean the carb out as well as I could. There was no rebuild for that valve, so I did'nt mess with it (other than soaking and cleaning). It said that it was not replacable. Is there any way to test it?

 

[plts333]

Re: 120 mercruiser bogging down

Yes we did try running directly from a gas can, with no change. Temp is consistanly good. I was wondering if the float in the carb could be bent too low and we're just starving the carb under load? That was going to be my next move, bend it up a little to allow more gas in. I'll check the power valve as well.