5.7 Mercruiser Thunderbolt surge at high rpm

[muc]

manual #17 p/n 823225 1
page 5C-6 covers how MerCruiser wants it adjusted
I have seen other methods to adjust this that don't require the gram scale. Sorry I can't help with that because I have the gram scale and that's what I've always used.

But my guess is that the spring is broken and needs to be replaced.

Maybe somebody else can help with how to adjust it without the special tool. Seems to me it involved turning the screw until the valve just started to open and then turning back 1/4 turn?

 

[alldodge]

Have a look here
Air Spring Adjustment

 

[xltier]

this explains it some https://www.youtube.com/watch?v=xXRl6rcT1no

 

[Iamjtm]

Alright thanks, I’m going to remove the carb and see if I can figure out what’s going on with that. All the stufff I’ve read about it seems to be a full throttle stumble, when mine isn’t that simple. The boat will run great at full throttle every time, it happens when your at a steady rpm, there’s nothing I can do to make it start; full throttle, let off, full throttle again etc...

 

[Scott Danforth]

You have to resize your pics to about 25%. Tutorial in the rules and guidelines forum

 

[Iamjtm]

Sorry for the delay, been pretty busy. Rebuilt the carb, same problem. The boat starts up perfect, gets up to plane quick and has plenty of power, cruising anywhere above 3k for more then 30-90 secs the boat starts to stumble and the rpms drop. Any other ideas?

 

[Scott Danforth]

what is fuel pressure?
if you crack your fuel fill cap, do you hear a sucking sound?
when you rebuilt the carb, did you chase out every passage and orifice with a wire?

 

[Iamjtm]

I’ve opened the cap before, I’ve also taken the vent line off and blown thru it and didn’t feel any restriction. I never got the pieces to make a fuel pressure rig but I can do that if that’s what you recommend. I had it rebuilt by a local speed shop that specializes in older carbed cars, and they mentioned that the carb wasent very dirty at all I’d assume they chased out all the holes..

 

[Iamjtm]

Today I pulled the pickup tube out, I was going to replace the fuel lines and fuel pump, the pickup screen was very very clogged. I can’t find the pickup kit with the 90 degree elbow and tube anywhere. Does anyone have a part number for those? I went to west marine and a local boat shop, neither one of the young kids working there could find anything close to it.

 

[muc]

https://www.iboats.com/shop/moeller-fuel-pick-up-with-aluminum-pl3-elbow-legacy-igf-729899.html

this it?

 

[alldodge]

the pickup screen was very very clogged.

For it to be clogged, you should be able to use a soft wire brush and air to clean. Rinse with gas, naphtha or other

Also with it being clogged your tank is probably has crud build up in it and needs cleaned.

 

[Iamjtm]

Yes that looks like the right one, but it doesn’t have the anti syphon valve on it that I broke, the screen was also sucked up to the top of the tube and I’ve tead it’s suppose to be at the bottom. Will I be ok to replace it without the screen and change my fuel filter every week or 2 for a while to filter the stuff out or do I need to cut the floor and remove the tank to be cleaned...

 

[alldodge]

Were all working off what you say, and mentioned, screen and clogged. Many of the tubes don't have a screen. If yours was like the one MUC posted, then it wouldn't have one. If the clog was at the antisiphon valve then replacing the tube and valve should be enough

To find out if your tank is dirty inside, remove the fuel tank sender. Have plenty of ventilation and use a flash light to have a look inside.

 

[Scott Danforth]

There should be a cover over the tank sender. Use that to access the tank for cleaning