70 2 stroke horrible bog

[mitch184]

Working (and getting stumped) on a buddies '02 70hp 2 stroke. It's on a 16' Arima with a 17" pitch prop. Motor idles great, and eventually will run very smooth on plane. WOT is a little low at 4700 so we're getting a different prop.

However, it bogs terribly bad and it awful to get on plane and most of the time won't. As in smoothly hit the throttle and it just bogs and surges and cuts out and eventually most of the time you can wait it out and it will get on plane. Sometime you just pin it WOT and just let it work through the bog/surging and it gets on step.

What's been done:
-Removed all 3 carbs and prime start. Thoroughly cleaned, blow out. Re-assembled and performed linc and sync.
-Drained 6 month old gas
-Verified timing is correct per the panel at idle and WOT
-Cleaned the grounds for the coils and every other ground we found on the motor
-new spark plugs
-new fuel filter
-new fuel pump
-Spark plugs look rich

New plan is to check compression in the motor. Then go through fuel lines one more time to look for leaks at fitting, clamps, etc. Other than that.. I'm kinda stumped at this point. Any ideas? Is it even possible that the little arima is just too heavy and that combined with the prop is just too much for the motor? It would seem like it would still power through it.

Thanks

[rejesterd]

Hi Mitch. Did the plugs look wet and dark? And did they all look the same?

Also, when you say it cuts out, do you mean it stalls completely? Surging I assume means the revs go up and down.

Finally, what is the idle speed after it warms up?

[mitch184]

Plugs all look very similar. Probably 10 hrs on the plugs and they are black. Little wet.

It won't stall and die, it surges and struggles and bogs. Almost like electrically something is missing.

Idle speed is rock solid 800. Thing fires up every time cold or hot.

[rejesterd]

If they're all getting black and wet after 10 hours, yeah.. check compression. If you find compression to be good, then check the crank position sensor (peak voltage test). Find the service manual for it and you'll see the specs listed. Assuming the carbs are actually clean and adjusted properly, link/sync is correct, and compression is good, I don't know what else it would be. Everything else I can think of is ruled out by the fact that it starts up and idles perfectly.

[rejesterd]

Oh.. maybe a spun hub? Unlikely but easy to check for.

[99yam40]

spun prop would make the motor rev up not bog.

Op did you make sure the prime start system is functioning as it should?
you can check the heater unit to make sure it heats up and extends the needle.
if it does not close off then there will be too rich once motor warms up.
is the red lever in the auto/normal position?
if in the open position it will run rich.

Is the rubber cap #22 in place over the pilot jets in each carb?
if they come out high speed tends to run rich, but idle is not affected
https://www.boats.net/catalog/yamaha...lra/carburetor

[mitch184]

Compression is 120, 120, 100. But I messed up and the timing at WOT was only set to about 16 BTDC. I believe the factory setting is 22 BTDC so I'm to set it there. Can someone confirm it is 22? Everything I see says it is.

Idle is 7 and wot is 22. I'm going to set it with a timing light

[rejesterd]

I have a 70hp 2-stroke manual from 1999 that says it's 20 for the 70 hp (and 22 for 50/60/90 hp). You can try that, but that 3rd cylinder is 20% off. Might be a head gasket leak or something with that cylinder. If it was just slightly worn rings, they'd all be around the same. Correcting the timing will probably fix the engine cutting out suddenly and surging, but I wouldn't ignore the compression readings you got.

[mitch184]

Well i made a pull at 22 and the surge is totally gone. It lugs a hair but I think that has more to do with the prop.

Everything was running smooth...... and then now it wont shift into nuetral.... linkage works.. something on the motor is bound up. On to the next problem

This motor is a 70TLRT so I think it's a 95 instead of 02. Mist have misread it originally.

[99yam40]

did this motor have a problem with the fuel pump at on time leaking into the crank case?
if so it may have washed off oil from cylinder walls on #3 and damaged the piston and/or rings.

but it could be carbon sticking rings

[Rossy299]

Hey mate i have the same engine and can send you manual if needed. Chris

[rejesterd]

Yeah, 17" seems pretty high for a boat that size. So a lower pitch will help the WOT speed, but I would at least do a leak test on that cylinder after you drop the lower unit. Seems like you'll need to drop it anyway if it's not shifting into neutral.

[mitch184]

No fuel pump issues that I know of. Going to look at the piston tops with the bendalite shortly.

Yes it needs a 15" but he bought a 17"

[rejesterd]

Well, I don't think you would know if the fuel pump was leaking into the crankcase unless you took it apart and inspected the diaphragm. Worth a look before you run it again.