Low RPM at WOT on Mercruiser 260

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

So I now have major problems. I decided to change the carter carb to the Qjet from the engine that came out of this boat. So I took off carter and scraped gasket. went to stick QJET on and fuel inlet hits the thermostat housing so now I have to get a Banjo fitting from quadrajet.com. Went to day to buy another gasket for carter and put it back on. I do have a rebuild kit coming for QJET. Then I noticed that throttle lever on Volvo Penta setup is different than my Mercruiser cable wise:facepalm: Anyway went out to run it with carter and couldn't even make 2500RPM. Serious knocking from engine compartment. Turned around and went back to dock. Now while idling there is a knock and the RPM gauge is bouncing all over the place like from 500-1200. Also there is an odd "loping" vibration like something out of balance. Almost like half the cylinders are not firing. Distributor appears to be tight. When I turn the engine off and turn it back on it acts like batteries(new with Pro Mariner 3 bank smart charger) are dead and barely cranks over then it catches. I am wonder if I have a rod knocking. So it looks like I am going to have to pull engine again. I have the 350 that came out of this boat. I put a wrench to it and was able to rotate the crank shaft so it is not seized even though it was an insurance write off because boat had taken some water that got to starter and alternator. I dont know why this change the boat has been running OK just not able to WOT RPM. Went out with the kids last weekend an fished a little. Now all I did was take one carb off Try to put another one on. Ended up putting original one back on just to get boat going. Now????

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

I should be around there today. I'm putting he bat back in the water this afternoon. I'll look around for a guy with the engine cover up

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

I should be around there today. I'm putting he bat back in the water this afternoon. I'll look around for a guy with the engine cover up

Thanks Dogdave I am heading down there now to start evaluating. My slip is South H number 38

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Many carbs and motors benefit using a 1 inch carb spacer.

Is it possible that when assembling this motor that you have the cam/crank alignment off by even one tooth, this would radically change the power capability of the motor. If the cam is very close to stock this could be checked by pulling one valve cover and a degree wheel on the crank.

First get a carb spacer and try the Q-jet.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Many carbs and motors benefit using a 1 inch carb spacer.

Is it possible that when assembling this motor that you have the cam/crank alignment off by even one tooth, this would radically change the power capability of the motor. If the cam is very close to stock this could be checked by pulling one valve cover and a degree wheel on the crank.

First get a carb spacer and try the Q-jet.

So i bought the 1" spacer but the straight fuel inlet still hits the thermostat housing. So I will order a banjo fitting and try that.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

What was the issue with the distributor? I sure hope that is it.

 

[cantaris]

Re: Low RPM at WOT on Mercruiser 260

Ok so I may have found the problem with the help of Dogdave. I was explaining all the ins and outs of my engine and we were baffled. then I mentioned that I was seeing carbon dust in distributor. So I took dist cap off to show him. He asked if advance weights were working I said yes that I just oiled it the other day. And then I proceeded to show him by grabbing rotor to advance it but it kept turning freely. Now we're thinking the lower drive gear sheared the pin, which would be a definite engine pull. I pulled the distro and the the gear is fine and there is no sign of metal in the oil. Whew! Now when I hold gear I can turn rotor. So I got the distro home and disassembled it and found that the plate that has the slots for the advance weights to ride in completely freely on the shaft that holds rotor cam. Looks like it is supposed to be interference fit or swagged on. So I pulled the distro from the engine in my garage and will try it tomorrow. This may have been my problem entirely and it finally let go. On a side note I spent the entire day laying across the engine and upside down in the direct sun changing out the power steering pump which I had unknowingly bent the pulley when I installed engine the first time when I set the weight of the engine on the stringer on that pulley. Had to grind and reglass stringer Thanks Mercruiser for that design. So that was a job I thought I would have to pull engine for. So now everything is back together except the distro. Hopefully....

 

[CharlieB]

Re: Low RPM at WOT on Mercruiser 260

Post #24

Did you put your hand on the rotor and turn it?

I glad you finally found the problem. Now you can check out which prop gets you into the correct WOT range.

How old is this unit?

Have you had the Distributor cap off?

With the cap off, remove the rotor and look at the advance mechanism, springs, weights, the pivot points look good? Lightly lube, just one drop of oil or a very light weight grease. Look at the slots and limit pins, turn the upper shaft by hand, will it turn freely pulling against the springs to the end of the slot against to stop pins? If not then this could be part of the problem, the ignition is not advancing fully and limiting power.

Q-jet carbs work very well, the secondary air gate adjusts air flow to the demand of the engine. The same 800 cfm will work on a number of small blocks as well as a few big blocks with minor adjustments. As long as fuel flow is good and the throttle cable pulls wide open, and of course the carb is clean, shouldn't be a problem.

 

[Dogdave]

Re: Low RPM at WOT on Mercruiser 260

How did the run go today? I would really focus on valve lash it seams like it may be tight and you are dumping compression. If the valves are good then that leaves the bore, pistons and rings or a bad valve seal a leak down test would help or a wet compression test.