92' Mercruiser 7.4 Bluewaters Won't Idle

[gage_keeble]

My dad just bought 1992 Wellcraft Corsair 35'. It has two 7.4 Mercruisers that were in horrible condition after nearly 3 years of no matientence and 6 months of sitting. They were supposedly rebuilt a while ago, but it doesn't look like it. I'm on the job of getting the engines up to par, but I'm having a lot of trouble. We put new carbs with electric chokes on them, most of the ignition stuff has been replaced on both, but no no change. Both need a fair bit of throttle to get started, and will not run without a bit of throttle to keep them above 1k RPM. I cannot get them to idle to tune them. I have the idle mixture screws set to 3 turns, and they still don't idle. The choke is set correctly. If I bring the throttle back to normal, they stumble and die. Fuel filters are replaced, two new fuel pumps, I'm stuck on where to go next. It has thunderbolt IV ignition and the engine serial numbers are 0D00685. It has a Hurth V-Drive. Any help is appreciated, I don't know where to go next!

 

[Bt Doctur]

manifold vac leak, carb base gasket leaking

 

[gage_keeble]

manifold vac leak, carb base gasket leaking

Yeah I need to get a Vaccum gauge on there to see, but both engines not wanting to idle doesn't make sense. I need to get a torque wrench and make sure that they are to spec. It's in inch pounds and I don't have a wrench that goes that little. Good idea though I'll check on that.

 

[gage_keeble]

A new problem has occurred, only one wants to run at a time. Last weekend the port side wanted to run and not idle but would run with throttle, but the starboard side would not start. Now the problems have flipped sides and the port won't start while the starboard will idle with throttle. I'm really confused now and will try to start them again after sitting for a while.

 

[alldodge]

With things moving look at electrical. Remove the wires on the starter and clean them up shinny metal. Measure the voltage at the coil and see what it does during cranking and while running. Use a spark gap gauge and connect to coil

 

[gage_keeble]

With things moving look at electrical. Remove the wires on the starter and clean them up shinny metal. Measure the voltage at the coil and see what it does during cranking and while running. Use a spark gap gauge and connect to coil

That's a good idea, I'll be sure to try that when I go down to the boat today. For the coil, I should be looking for jumps, or low voltage? The coil on the port side engine is new so it might not be that but I'll check on both, you never know.

 

[alldodge]

Thinking the voltage is either to low or not there at all when cranking.

 

[gage_keeble]

Thinking the voltage is either to low or not there at all when cranking.

Now that you mention that, that's a good test. Enough voltage to crank but not enough to spark, although they ran at one point. But I'm willing to try anything at this point.

 

[Rick Stephens]

Sitting three years, what is the condition of the fuel in the tank(s).

What was the source for the new carburetors? How do you know they are calibrated to your motors?

 

[gage_keeble]

Sitting three years, what is the condition of the fuel in the tank(s).

What was the source for the new carburetors? How do you know they are calibrated to your motors?

The fuel was a big problem before, new gas was put in after the old stuff was drained out. One tank is clogged and turned off right now, so both engines are running off one tank. That could also be a problem. Both tanks are full of fuel. The carburetors are from National Carburetors, in Jacksonville, Florida. I was skeptical of their tuning, but I talked to the guy just an hour ago making sure both were tuned on a 7.4L and he said they were. He gave me a few troubleshooting tips, so I'm heading down to the boat in a bit to make sure I check all of his suggestions off the list. I honestly think when they do run, it's a Vaccum leak somewhere. I bought a Vaccum tester and a 1/4 torque wrench and I'm going to bring a 3/8 down there and make sure the intake and carb itself is actually to spec. I'll post the result and the Vaccum readings when I do so.

 

[gage_keeble]

Ok, so I torqued down all the carb bolts, made sure the gasket was on properly, reset idle mix screws again, and they started. The port had a lot of gas in the intake, which made me think it was leaking. The starboard still needs throttle to run, but will run well. The port started up and actually idled! A slow and lopey 400RPM idle but it idled. I tried doing a bit of adjustment to the screws and now it won't start, most likely flooded I bet. The port also needs full throttle to start and then brought down suddenly, so that definitely needs work. Although it's progress so I can't complain.

 

[gage_keeble]

Just checked starboard, Vaccum is at 15-25 at 1200 rpm

 

[alldodge]

Can you get the idle closer to 700?

 

[gage_keeble]

Can you get the idle closer to 700?

Yeah, I'm cracking down on 750 right now with the screws. Almost there, I'll get another reading at true idle

 

[Rick Stephens]

I am always leery of rebuilt or even new carbs that are 'tuned' to a given motor. Seen all too many with issues over the years. Some thoughts for you on what to check out:

Fuel system: you might consider purchasing a vacuum/fuel pressure gauge, if the one already got isn't made for both. They are relatively cheap and you can do some fuel system checks by teeing the fuel line before the pump and seeing what the suction side vacuum is. Then hooking up after the pump and seeing what the carb side pressure is.

A carburetor that is dumping raw fuel and flooding probably has the float level wrong or crud in the needle valve seats...

Did you pull the anti-syphon check valves at the tops of the tank draw pipes? Those are cheap to replace and easy to corrode and plug up.

I would consider pulling the carb apart to see if it is full of crud from your tanks.

Rick