2012 3.0L TKS runs great with muffs, but slow on lake...

[nola mike]

I wouldn't mess around with a sticking cable that wont allow it to return to idle. This is especially true for an alpha drive that requires a shift interrupt to get out of gear, and may not work at higher rpm.

Hard to say if the cable needs adjustment or replacement

Normally trying to lube a cable is a losing bet. The throttle cable needs some preload on it to close the throttle completely though. Remove the cable and see if it's smooth. Certainly not related to the low rpm problem though.

 

[Bondo]

Normally trying to lube a cable is a losing bet. The throttle cable needs some preload on it to close the throttle completely though. Remove the cable and see if it's smooth. Certainly not related to the low rpm problem though.

Ayuh,..... The throttle cable should be pushin' the throttle linkage, firmly against the idle adjustment screw,....

 

[GeorgioP07]

Ayuh,..... The throttle cable should be pushin' the throttle linkage, firmly against the idle adjustment screw,....

It is. It's very tight indeed.

 

[GeorgioP07]

Hello I just measured the resistance of the ignition wires / spark plug wires using a digital voltmeter sent to 40K Ohms.
Compared to the wires I replaced these are reading 2.03, and the older wires are 4.07, I believe they are well within spec, showing less resistance.

 

[cyclops222]

Are there shift cable CLAMPS on the ends of the throttle cable end brackets ? Are they adjusted correctly ?

 

[ESGWheel]

@Lou C would you mind reading thru this thread to see what we may be missing? George (the OP) has been great at sticking with the forum and following thru on a ton of items but still stuck. My thoughts are a restricted exhaust that he still needs to investigate but would appreciate yet another set of eyes on this one. Thanks.

 

[Lou C]

you have enough fuel pressure, but your low WOT RPM might be a lack of fuel volume....that could be a sticking anti-siphon valve on the tank, or a pick up tube that has crud getting pulled against the screen at the bottom of it. Any inboard I think is going to have an anti siphon valve unless someone removed it, I would think about checking that, because it could be your problem.
think about it like this, to idle and run at low rpm takes relatively little fuel in terms of gallons per hr., but to plane the boat will take between 2-3x as much in terms of gallons per hr.
My boat had this problem when I first bought it back in 2002, a good mechanic found it and after that was fixed it ran well for years. I do think that these parts should be inspected on a regular basis, because it doesn't take much of a restriction and you're having this problem even if everything else is perfect.

 

[ESGWheel]

George, I do not recall if you got on the water with the fuel pressure gauge installed and was able to monitor it under load / when it was bogging down. Recall for safety this will take 2 folks, one to drive and one to look at gauge. My concern is that this was only done on the muffs. Thanks.

 

[nola mike]

you have enough fuel pressure, but your low WOT RPM might be a lack of fuel volume....that could be a sticking anti-siphon valve on the tank, or a pick up tube that has crud getting pulled against the screen at the bottom of it. Any inboard I think is going to have an anti siphon valve unless someone removed it, I would think about checking that, because it could be your problem.
think about it like this, to idle and run at low rpm takes relatively little fuel in terms of gallons per hr., but to plane the boat will take between 2-3x as much in terms of gallons per hr.
My boat had this problem when I first bought it back in 2002, a good mechanic found it and after that was fixed it ran well for years. I do think that these parts should be inspected on a regular basis, because it doesn't take much of a restriction and you're having this problem even if everything else is perfect.

Pressure was measured at speed I believe. If still present that should rule out the valve, no?

 

[Lou C]

I'm wondering if you can have enough pressure but not enough volume. Pressure only needs to be adequate enough to keep the float bowl full, but think about how much GPH increases from idle/low speed to planing/WOT. 2-3x as much.

 

[Scott06]

I'm wondering if you can have enough pressure but not enough volume. Pressure only needs to be adequate enough to keep the float bowl full, but think about how much GPH increases from idle/low speed to planing/WOT. 2-3x as much.

If the volume wasn’t there the pressure would drop as the carb end of the fuel line is open into the bowl

 

[Lou C]

I'm going to see if there's a fuel pump volume test in my FSM. Curious now.

 

[Lou C]

for the 4 cyls is 3.5-6 psi
for the 4.3, 5.0 and 5.7 is 5 3/4 to 7 psi
for the volume test, 1 pt in 45 sec or less
interesting they give a vacuum at cranking speed test, 9-10" of mercury

 

[Lou C]

they (OMC) also gave specs for the anti siphon valve and the pick up tube in the tank.

 

[ESGWheel]

Geroge, my focus is now on post #69 along with a couple of suggestions about fuel flow, most recently by Lou.

In re-reading post #69 you asked, “Would the fuel pressure inline from the pump to the carburetor change when the motor is under a real load on the water compared to running on muffs?” and it did not get fully answered. In fact, when I initially read it, I thought you had put it under load but it seems as if was only a question and did not actually do it.

The answer is yes, it consumes more fuel under load. This is no different when you are towing your boat vs. not towing it, your truck gas milage goes way down when towing.

Lou is suggesting that a partial blockage in the fuel delivery, like the anti-siphon valve, could be responsible. So even though you are seeing good pressure on the muffs at all RPMs does not mean will have good pressure under load.

If you still have an opportunity to run the boat on the water my suggestion is now to do so as per post #88. If all is good, then I am back to the restricted exhaust.

 

[Lou C]

And getting back to possible exhaust restrictions, it’s easy enough to slide down the rubber exhaust hose and check to see if the exhaust flapper is still in place & moves freely. If it’s missing then it could be blocking the exhaust. All it takes is one overheat in the past. You could also remove the cover over the fuel sender to inspect the anti siphon valve & fuel pick up tube. These are basics that have to be right for the engine to run right under all conditions….

 

[GeorgioP07]

at low rpm takes relatively little fuel in terms of gallons per hr., but to plane the boat will take between 2-3x as much in terms of gallons per hr.

The fuel pressure definitely did not double at 2500RPM, in fact, it went lower than it was at 850RPM idle.

 

[GeorgioP07]

And getting back to possible exhaust restrictions, it’s easy enough to slide down the rubber exhaust hose and check to see if the exhaust flapper is still in place & moves freely. If it’s missing then it could be blocking the exhaust. All it takes is one overheat in the past. You could also remove the cover over the fuel sender to inspect the anti siphon valve & fuel pick up tube. These are basics that have to be right for the engine to run right under all conditions….

Someone here mentioned that I would have to take the exhaust manifold off then (replace the gasket) to check to see if the flapper was in place. When you say, "missing and blocking the exhaust", do you mean that it might have overheated, crumbled, fallen down the exhaust and it remains are now blocking out drive? The videos that I've seen, this flapper looks like a rubber butterfly that barely touches the exhaust. either round or oval shape. I've attached a picture of my exhaust where the flapper might be. The large U-shape piece is cast iron, then a rubber boot, then back to cast iron. Would I be taking off the rubber boot only, or the entire pipe to check? I do have a video borescope with a 15' cable if I can find a place to insert it. Thanks...

 

[ESGWheel]

Geroge,

My apologies, I should have been clearer about what to look for re: fuel pressure under load.

If you can run the boat on the water, with another person on board, record the fuel pressure at all speeds up to and including when it starts to bog down. So run at idle (in gear) for two minutes, record fuel pressure. Run at 1000 RPM for a minute or two, record fuel pressure. Now go to 2000, record pressure after a minute or two, and continue to do so all the way up to it bogging down and when it does, record pressure. Then please post a table like this example (I made up the numbers)
Idle – 4.7 psi
1000 – 4.7 psi
2000 – 4.5 psi
3000 – 3.2 psi
3500 – 1.5 psi (bogging down and cannot get higher RPM)

More info: there is a mathematical relationship between PSI and Flow (Gallons per minute) and since can only measure PSI with your set up, having low PSI at higher RPMs under load will be the indicator of low flow and thus the issue. You fuel pump should put out about the same PSI regardless of the engine RPM.

 

[GeorgioP07]

If the volume wasn’t there the pressure would drop as the carb end of the fuel line is open into the bowl

The pressure showing on the inline gauge did drop as the throttle was increased. It was idling at like 4.5 PSI, then at double the throttle, the PSI went down to 4.2 PSI closer to 3000 RPM.
Anti-Siphon Valve - What am I checking for here, the valve with the green marker line on it? Checking it for any blockage, and if the check ball is missing? Thanks.