350 feels like I am letting off the throttle and back on again

[Karl.]

I have a 1986/7 350 5.7 V8 mercruiser engine.


It has a refurbished quadrajet,
2 heads refurbished,
Mallory distributor with new cap and converted to electronic ignition,
New plugs,
new alternator,
New fuel pump,
Tank cleaned out (the lines were dirty)
Fuel lines replaced.

All these parts are roughly 2/3 hours old on the engine.

I still need to get a replacement fuel filter but that was replaced roughly 5/6 hours before the tank and lines were replaced.

Once get to about 33mph/3300 rpm it feels like I am letting off the throttle and putting it back on again.

Previously it got to about 3300rpm then it struggled to get to 2500rpm and just keep getting slower and slower untill I couldn't even put it in gear.

It has got better and I have no problems cruising at 3200rpm after cleaning out the tank and replacing the fuel lines.

I would like to just have the boat running smooth.

 

[Karl.]

Also the ignition timing has been set to 10 btdc per the spec on the rocker cover. We have 96ron fuel so I think I could run slightly more timing for more power if needed.

 

[Scott Danforth]

Surging is fuel system

Did you check the contents of the fuel filter?
Dud you verify float setting?
What is your fuel pressure?
have you checked the anti siphon valve?

 

[Karl.]

The fuel filter was dirty, it have cleaned it out with fresh petrol 3/4 times to the replacement arrives.

I haven't checked checked the float level, I assumed it was ok since it was only recently refurbished.
​​​​​​
how can I check the fuel pressure between the pump and the carb? What sort of fitting do I need to buy? It's a hard-line between the pump and the carb.

What is the anti siphon valve? I don't think my boat has one.

I drove without the fuel cap on to make sure the tank wasn't vacuum locking and it made no difference.

 

[Bondo]

I haven't checked checked the float level, I assumed it was ok since it was only recently refurbished.
What is the anti siphon valve? I don't think my boat has one.

Ayuh,...... Welcome Aboard,..... Even a brand new carb needs to be checked, 'n tuned to the motor,......

The anti-siphon valve is the fuel tank's outlet fittin',....

Yer motor should be revin' to 4800 rpms, 3300 is luggin' it, Badly,....

 

[Scott Danforth]

You cant clean a filter. Good chance you contaminated the clean side

 

[alldodge]

how can I check the fuel pressure between the pump and the carb? What sort of fitting do I need to buy? It's a hard-line between the pump and the carb.

Take the hard fuel line to the local auto parts store. They should have some fittings to match those fittings so you can use rubber line. Also get a Tee so a vacuum fuel pump gauge can be connected. The pump tester is normally under $20

https://www.homedepot.com/p/GearWre...C_UQQAvD_BwE&dclid=CK-1jcOtodwCFYIkgQodkeMP7w

 

[Karl.]

Thanks for all the help so far guys!

I will get a rubber fuel line made up so that I can check the pressure.

I have 2 new fuel filters ordered so I will replace that as soon as it arrives.

We took out the pick up pipe and blew all the lines out with a airline and replaced whatever other fuel lines we could.

I will report back once I am able to check the fuel pressure and new fuel filters.

Thanks

 

[alldodge]

The antisiphon valve screws into the fuel tank fitting and the rubber line is attached to it. Inside the valve is a spring a ball which can get clogged

https://www.iboats.com/shop/moeller-anti-siphon-valve-3-8-barb-3-8-npt.html

 

[Scott Danforth]

my guess from the accent vernacular in your text your probably in the UK.

 

[Karl.]

I am from the uk yeah. Hopefully that is ok with your forum. I can't fix any good forum's in the UK that deals with American v8s.

I will have a look for a anti siphon valve tomorrow.

 

[alldodge]

No issues with anyone around this big blue marble, there are several here from your side of the pond, and a whole lot of other places

 

[Karl.]

Hi, I found the anti siphon valve, there was no dirt but I blew it out with the airline and sucked on it and the marble is moving fine

 

[Karl.]

I forgot to mention, while changing the fuel pump this was behine the diaphragm. It seems like a clear plastic washer. Does anyone know where this could come from? The diaphragm was in perfect condition and I couldn't see anything wrong with the pump but changed it out anyways.

​​​​​​

 

[alldodge]

Sure don't, maybe some adhesive

 

[Karl.]

Ok, I decided to run a pipe of out a jar of clean fresh fuel and I can gladly say I have no issues.

​​​​​​After cleaning out the tank and replacing the fuel filter I was able to run it using the OEM tank setup without any problems.

I have some mayo coming out of the two breathers when I'm wot about 44mph but I'm hoping this is just condensation from not being running as the last 2 and a half years and not been running properly for the last 2 months.

Thanks for all the help and advice

 

[Scott Danforth]

a little condensation is normal. it should go away on a long run. if it gets worse, tell us.

 

[Karl.]

Hi, I have another question.

I am trying to set my ignition timing.

I have a Mallory distributor with mechanical advance. Do I need to lock the mechanical advance to set the base ignition timing to 10° BTDC?

The boat has a lot more power if I set my base timing to more than 10 BTDC but I obviously don't want to risk any detention.

Thanks
​​​

 

[Karl.]

Or would I be better pulling a skier to put the boat under load and set the timing to 36° at wot?

 

[alldodge]

Suggest setting it to 10 and then adjust timing curve to match total

http://www.loco4fomoco.com/MalloryAdvanceCurveKit.pdf