5.0 Mercruiser Problems

[dsull]

I just recently purchased an 09 Glastron with the 5.0L Mercruiser carbureted motor and an Alpha One lower unit. Engine serial # is 1A323012. I am having issues with it not performing as I'd expect. The boat cranked up and ran fine on rabbit ears. Idled smooth, revved in neutral, sounded good. When I got it on the water and tried to get it up to speed, however, it vibrated violently and did not accelerate. The RPMs also stayed under 2K. I could eventually get the boat on plane but it was clearly lacking power.

I came home, checked compression. It was good - 125 to 130 in each cylinder. Pulled the plugs and all 8 were fouled and black. I assumed the carb was running rich. I replaced the plugs, distributor cap and rotor, wires, and changed the lower unit oil. When I took the boat back out, the vibration was gone and it sounded much smoother but it still did not accelerate like it should. RPMs climbed slightly to 2200 but the manual says 4400-4800 is WOT RPMs. Next, I looked to the prop - previous owner had a 15.25 x 21 prop so I thought that was too large and too much pitch so I stepped it back to a 15x17 prop. Ran the boat yesterday and it ran better but I'm still not reaching RPM range specified.

I can't imagine I need to step back to an even less pitched prop. I feel like the motor is not generating the power needed. I plan to adjust the carb (Mercarb two barrel) when my carb adjustment tool shows up. I BELIEVE it's running too rich. I also plan to check the timing because the motor tries to continue running a little bit when I turn the key off.

Does anyone have any other suggestions? I'm at a loss.

Thanks.

 

[NHGuy]

You are on the right track. Absolutely check all the tuning setup stuff. On the Mercruisers there is a process to set timing which depends on which ignition you have. See if it's Thunderbolt V. I'm not up on later years like an 09.
I'd not buy another prop yet.
I'd suggest externally clean the carb. Run some cleaner into it while the engine is running. I'd start it up, disconnect the throttle linkage, and spray some carb clean through it while giving it some rpms.
Change the fuel water separator, change the oil (after warming the engine a bit, cold oil is hard to extract) Verify your ignition wires are on the right plugs and that they are good.
It's usually something that make you facepalm!

 

[Scott06]

Start with basics- remove fuel filter and see if there is anything but fresh clean gasoline in there. Is the choke opening fully or do you have a TKS carb? If tks is the enrichment module kicking on and off like it should ?What do the plugs look like now that you changed them? Set ignition timing - that will be a TBolt 5 so you will need to put it in base mode and verify timing is at 10 deg BTDC at idle, then total advance should be close to 30. if the ignition module is bad it may not fully advance the ignition.

 

[achris]

Sounds like TKS is being TKS.... The enrichment system... Another one of Merc's not so great ideas... Read up on how it works (or is supposed to work) and check that it is actually working...

Chris....

 

[dsull]

It DOES have the TKS module on it and I have read up on it a little. From what I read, it sounded like it kicked the idle speed up to around 950 for 6-10 minutes after cold start before lowering it back to 650 under normal operation. Sounds like it may not be shutting off. It did read like it was similar to an electronic choke so I wonder if the TKS module not shutting off is choking the motor at WOT. I need to read up on if I can disable it.

I plan to pull plugs and check timing this weekend. Maybe it'll be something simple like timing. It looks like the marks for timing are on the BOTTOM of the engine so I bet it's a pain to get to. I've read up on base timing and it sounds like I need to ground the purple and white wire coming from the ECU on the right side of the motor.

Thanks for the tips!

 

[Scott06]

It DOES have the TKS module on it and I have read up on it a little. From what I read, it sounded like it kicked the idle speed up to around 950 for 6-10 minutes after cold start before lowering it back to 650 under normal operation. Sounds like it may not be shutting off. It did read like it was similar to an electronic choke so I wonder if the TKS module not shutting off is choking the motor at WOT. I need to read up on if I can disable it.

I plan to pull plugs and check timing this weekend. Maybe it'll be something simple like timing. It looks like the marks for timing are on the BOTTOM of the engine so I bet it's a pain to get to. I've read up on base timing and it sounds like I need to ground the purple and white wire coming from the ECU on the right side of the motor.

Thanks for the tips!

The tks is an enrichment module that lets extra fuel in for warm up. It should hiss until it warms up at like 135 f or so. It was meant to replace an automatic choke so folks who don't drive carbureted cars and dont know how to set a choke can start it easily... IMHO a dumb system when two pumps of the throttle and advancing it a little never failed...

Yep ground the purple and white it will be hanging around the left side of the carb and have a black rubber plug in it. Unless there is a timing tape on the dampner you will need and advance light to see the total advance

 

[Scott Danforth]

If your compression gauge is correct, your motor may have issues. Hopefully it's just that it has been running rich

Get the carb TKS issue fixed, then after running, check again

 

[dsull]

If your compression gauge is correct, your motor may have issues. Hopefully it's just that it has been running rich

Get the carb TKS issue fixed, then after running, check again

I've since learned I was supposed to check the engine compression with the throttle wide open. I'm not sure how that would make a difference without the engine running but that's just what I was told. 125-130 was with it in neutral. Seemed low but I'm not sure.

I'll check timing tomorrow. Any ideas how to disable the TKS?

 

[Scott Danforth]

I've since learned I was supposed to check the engine compression with the throttle wide open. I'm not sure how that would make a difference without the engine running but that's just what I was told. 125-130 was with it in neutral. Seemed low but I'm not sure.

I'll check timing tomorrow. Any ideas how to disable the TKS?

Pull the carb and go thru it

 

[dsull]

Pull the carb and go thru it

I have pulled the carb and checked it out. It was surprisingly clean. It looks like that TKS module just plugs in but I don't know if there are any other adverse affects if you simply unplug it.

 

[nola mike]

I have pulled the carb and checked it out. It was surprisingly clean. It looks like that TKS module just plugs in but I don't know if there are any other adverse affects if you simply unplug it.

It needs 12v to keep it closed/off. Unplugging it will make it rich I believe. Whether it can be removed completely I'm not sure. Should be able to just swap in a standard carb though.

 

[dsull]

Well I tried adjusting the timing yesterday but was unsuccessful. When I grounded the purple/white wire and put my timing light on the #1 plug wire, I was nowhere near hitting the timing marks on the pulley. It's almost like the wire isn't grounding and putting it in base timing mode. Seems like I'm missing something. Before I adjusted the distributor, I wanted to verify I wasn't overlooking a step. See the attached photo. I'm assuming that notch on the block is my mark for timing but the timing light isn't hitting the paper with the timing marks at all.

 

[dubs283]

Even without the pur/wht wire grounded you should be able to be within the range on the timing marks when looked at with a timing light. Are you for sure on #1 cylinder wire? Furthest fwd cylinder port side of engine

Kinda confused as to what you have done so far. You say you've checked stuff out like the carb which "looked okay/clean" but did you actually disassemble and clean it/replace anything? How about tune up stuff, plugs, wires, cap/rotor, filters, etc....

 

[dsull]

I was on the correct plug wire based on your description and the corresponding numbered wire from the distributor. Sounds like the timing is bad far off. I would have thought the motor would sound like it wouldn't idle properly with it that far off but maybe not.

Thus far in this process I have replaced plugs, plug wires, cap, rotor, and disassembled and reassembled the carb. The carb was very clean inside with no gunk on the jets or in the bowl. I checked the float level and it was correct.

Sounds like I'll try turning the distributor slightly and seeing if I can get the marks to line up to 10 degrees BTDC. Thanks.

 

[dsull]

Well I loosened the distributor and rotated it clockwise until the "10" in the above picture was lined up with the notch on the block. The motor did not sound like it was running very well. I think the mixture is too rich but I won't have the tool to adjust it until tomorrow. It doesn't seem like this should be guess work. The motor says timing is 10 degrees BTDC and I lined that mark up but it just doesn't seem right. Also, in order to get the idle speed to 650, I had to run the idle screw almost all the way in and that also doesn't seem right. I'm clearly missing something but I'm not sure what.

 

[dubs283]

Could be a fuel supply issue, pump/carb or other component. Is the firing order correct?

 

[nola mike]

I was on the correct plug wire based on your description

Verify this. Description doesn't cut it. The distributor markings might not correspond to the actual engine position. Rotate the engine until you're at TDC in the #1 cylinder. Make sure that the rotor is pointed at the #1 plug. Then do your plug order from there

 

[achris]

Verify this. Description doesn't cut it. The distributor markings might not correspond to the actual engine position. Rotate the engine until you're at TDC in the #1 cylinder. Make sure that the rotor is pointed at the #1 plug. Then do your plug order from there

Yep, and make sure it's on #1 firing stroke, not #1 overlap!

 

[dsull]

I'm not overly versed in engine timing. I know I can pull the #1 spark plug and rotate the engine to the cylinder being TDC and pull the cap and see where the rotor is but I'm not sure what the overlap stroke vs firing stroke is.

 

[achris]

In a 4 stroke engine the crankshaft completes 2 full revolutions per cycle (suck, squeeze, bang, blow)... The pistons will be at TDC TWICE per cycle. When setting up the ignition timing you need to be sure you are on the correct TDC (the one that goes 'bang'), or the engine will be trying to fire one crankshaft revolution out of sync.