787 siphoning gas out of tank after shutdown

[pincusa]

So here's what I've found. This has happened to me three times, each time I've thought it was an isolated incident.

I go to start the engine, and it's hydrolocked. Won't spin at all. Remove the plugs and crank, and lots of raw fuel shoots out of the spark plug holes. Dry the plugs off, put them back, and after lots of cranking, I can usually get it running on one cyl but the other one doesn't catch.
Yesterday, I pulled the head off and with a narrow tube, sucked a lot of raw fuel out of the case. Put her back together, and it started right up. I go to start her today, and hydrolocked again!

787 engine is stock. Only thing changed is I replaced the single port fuel pump with the higher flow dual port pump that feeds each carb by it's own supply. Return lines are still linked together.


I'm sure there's some valve in either the fuel pump or carbs that isn't working and hoping someone else has seen this issue before. I'm tempted to look for a fuel shut off valve and just put that in the supply line.

 

[Dr Honda]

Well... the needle and seat should seal off the pressure when the engine is shut down. But, even if it was bad... the fuel shouldn't keep flowing.

Check the needle and seat... but check to make sure that the return line isn't plugged up. If your return is open... any extra pressure will go out that hose.


One thought. is the pump leaking down the pulse hose?

 

[pincusa]

Tony -
I'll check the return lines and seats. I too couldn't see a stuck float or needle filling the case. It just didn't make sense.

Great thought about the fuel pump! It makes sense that if the diaphragm were ripped, that would be an easy way to fill the case with fuel. I'll disconnect the pulse line and see if fuel is coming out of there.

Do you think a ripped diaphragm would still supply the carbs with fuel to keep allow the motor to run?


One last problem now, after having the engine hydrolocked three times now, there's a clunking noise when I turn the flywheel by hand. The noise is the worst at TDC of each piston. I'm now concerned that a wrist pin or rod is damaged....

 

[Dr Honda]

Tony -


Do you think a ripped diaphragm would still supply the carbs with fuel to keep allow the motor to run?

Depends if it's a rip, or a small pin hole. If it's small, it could still pump.

One last problem now, after having the engine hydrolocked three times now, there's a clunking noise when I turn the flywheel by hand. The noise is the worst at TDC of each piston. I'm now concerned that a wrist pin or rod is damaged....

Since its been filled with fuel... it washed all the oil away.

If it's clear now... I would give it a good shot of fogging oil in each plug hole, and into the carbs. AND... if possible, get a helper to manually turn the engine while fogging into the carbs so you get some oil into the crank.

Also... I would mix up some fuel pre-mixed 40:1 just to make sure you are getting oil into that engine.


Last thing... I'm not sure how good the seals are between the engine and the counter balance, but I would pull the plug, and make sure that the oil in the gears aren't polluted with fuel. (open the plug and give it a sniff)

 

[pincusa]

Tony -
Good thoughts. I premix my gas since I didn't trust the oil pumps. But when I had the head off, I gave the cyl a wipe down with an oiled rag just in case. I'll fog her up just to be safe. Gas is very good as removing any oil residue!

I'll check the balance shaft oil. That's still a mystory on how well isolated that is from the rest of the case. My SBT engine doesn't have an oil fill, while the stock engine still takes oil.

Guess I won't be using her over the weekend

 

[Chester]

Replace the needles and seats in the carbs. If they stick open, the engine will fill up with fuel.

Chester

 

[pincusa]

Well, I spent the day with my butt in the air and my head in the engine compartment. The fuel pump was holding vacuum and showed no signs of a leak.
And as it turns out, Chester was right. Though the needles and seats are only 1 year old, one of them was stuck. Over the winter the fuel looks like it gummed up one of them and was causing issues. Put it back together, and now she runs again.
Hope to get her in the water tomorrow!

Thanks Chester and Tony for the help!


Now to figure out what the clunking noise is when I spin the engine by hand is... Hope I'm not looking at a bottom end!