Mercruiser 3.0 wants to die only at 1800 RPM

[frostystorm]

I have a new 2019 long block with 150 compression across all the cylinders, and a brand new fuel filter in the pump.

It was built up from a blown up 2008 3.0 TKS, so long block replacement, toss all the old parts on the new engine, so the carb is definitely original, and it shows signs of being rebuilt at some point, and whoever did it stripped the screws holding the carb together out to a condition that is unrecognizable, so drill baby drill...

The Issue is if you hit like high 1700s to 1900 rpm the engine will either want to die, or it will drop back down to idling. I'll idle at 800-1100 all day fishing, or tow tubers all day at 3500-4500 without an issue. But if you put it right in the slot under load at 1750-1850 RPM, it'll just start being bitchy.

I am guessing this is carb related, although I have not put a timing light on it, but given it idles fine and runs fine all across the rev range, and is an HEI distributor, I can't imagine it is timing related.

My TKS module blew the 20amp fuse once this year, but given voltage it extends in and out to the range. Otherwise it hasn't given me any issues in that regard.

My conundrum, I cannot rebuild this carb, remember how I said it came from a 2008, well whomever rebuilt it prior stripped every screw on the carb beyond recognition, the only way it's coming apart is by drilling the screws out, and I know i'll **** that up.

I don't want to throw a new carb at it but probably will, so what should I check first? I am checking timing this weekend but I doubt it's off at idle, if its doing something it's behaving incorrectly at 1700-1800 rpm.

My guess is fuel related, and carb, because it idles and runs at full throttle fine. probably around when the idle circuit hands over to the main mix i would guess?

 

[alldodge]

Sounds like the power valve. The power valve transitions the idle circuit to the main. Only way to fix it is get into the carb.

Could replace the TKS with a standard carb

 

[nola mike]

An impact screwdriver can work wonders, otherwise screw extractors/left handed bits might work. The screws aren't in very tight. Well, shouldn't be very tight.

 

[frostystorm]

Sounds like the power valve. The power valve transitions the idle circuit to the main. Only way to fix it is get into the carb.

Could replace the TKS with a standard carb

Yeah I came from the world of smaller outboards and dirt bikes and stuff where the carbs aren't quite as complicated, but I figured there was some type of internal valve or passage or something that handles the different circuits.

My accelerator pump definitely shoots strong and that's probably what gives it enough fuel to actually run well at 1800 RPM for the first 30 seconds or so before it idles down again.

The fact that a standard carb costs less than a TKS solenoid alone, i'd definitely grab a standard carb.

 

[frostystorm]

An impact screwdriver can work wonders, otherwise screw extractors/left handed bits might work. The screws aren't in very tight. Well, shouldn't be very tight.

I can assure you these ones are in pretty tight. Since they're the "you should use a phillips but we cut a flat blade extension in" screw heads, someone took a way too big flat head and greased them as tight as humanly possible, and turned the heads into what looks like security screws.

It's impressive someone would do that to a carb that costs like $900 brand new. I don't know why you would do that. I might try to fix her up, but i'll probably grab a second standard carb anyways.

 

[cyclops222]

Stupid idiots work on engines........ I have met a LOCKTITED carburetor. Had to drain the carb of gas & fumes. Then used a tiny torch unit on the screws and casting.

 

[frostystorm]

No one ever posts the resolution to these kinds of posts, so when you search them nothing worth reading comes up

I took it out today and pulled the flame arrestor, I let it warm up until the hissing stopped, venturis were still bone dry.

I throttled up to 1800-1900 RPM, it ran fine for a few minutes, then all the sudden it bogs down and dies, the top of the throttle plates are soaked in fuel but the venturis are still dry, which makes sense at 1800 rpm the plates are only open a crack. The accelerator pump is dribbling and the area near the TKS solenoid is soaked in fuel.

In the garage later when I go from idle to full throttle my accelerator pump doesn't sound like a quick nice shot of fuel anymore, it sounds like it's taking easily 2x as long as it used to to spray fuel. My guess is it's sticking and then dumping fuel eventually right around that throttle position.

So yeah the whole carb needs to come apart for a cleaning, or in my case, im just getting an electric choke model, because I have been messing with the TKS module every year consistently and it's $400.