Chev 350 Inboard

Hi All
Its been awhile since I have posted on here
Just to recap ive got a 21foot fibreglass tralier boat with a smallblock chevy 350
Rockcheshter Quaddrajet carburetor
Mechanical fuel pump about 3 years old
Yesterday I went out marlin trolling for the day and had engine problems
I ran at about 20knts out to the marlin grounds for 2hrs with no engine problems
Then trolled at 7knts for about 8hrs with no engine problems
Once I had finished trolling I had a 2hr run back to the boat ramp and this is when the engine dramas started
I have done this trip heaps of times and never had any engine problems until yesterday
On trying to get the engine up to speed anything over about 8knts the engine would start to surge and would die so it took me hrs to get back to the boat ramp
When the engine started to surge if I quickly pulled the throttle back to about 5knts to engine would recover and run fine
At one point I tried to hold the revs up to see if the engine would come good but it just surged really badly and would rev
I'm thinking it may be a fuel flow problem but am not really sure
The boat has 2 fuel tanks and has the problem on both tanks so if it is ful lines its between the ful tap and engine
I'm not wanting to start pulling things apart until I have some idea were to start
Any thoughts would be greatly appreciated
Many Thanks Mike

 

Sorry should have read
At one point I tried to hold the revs up to see if the engine would come good but it just surged really badly and WOULDN'T rev

 

Check the water separator and fuel filter?

 

Thank you jehardim
I will make that my first job
Mike

 

A few items

Not sure about down under but in North America each fuel tank requires an anti siphon valve at the top of the fuel tank to stop fuel siphoning into an engine bilge if a fire should burn through a fuel line.
They can be an extremely small fitting at the top of the tank, at the draw tube, a spring loaded check valve, about the diameter of most tank fittings and about an inch long.
Sediment can impact the check valve from opening fully under vacuum from the fuel pump. With two tanks, this should not happen but it could.

If you have to check these check valves, you have to be extremely careful to hold the fitting between the check valve and the fuel tank with another wrench to not let any of the torque be carried by the draw tube. Sometimes with the old fittings
it takes a lot of torque to break the fitting lose and you don't want the top of the draw tube to carry this load

I would rent a fuel pump pressure gauge or hire a mechanic to check the fuel pressure down stream from the mechanical fuel pump in case the diaphragm has perforated and is letting fuel bypass. If the fuel pump is a marine mechanical fuel pump, this perforated diaphragm will not present raw fuel into the bilge as there will be a return line to the carb for that purpose.

A fuel filter almost filled with water can create the same conditions as mentioned above.

 

Hi Barry
Thanks for your reply
My tanks don't have the anti siphon valve but I'm now going to put them in
I had to replace the pump awhile back because the diaphragm had perforated
The new pump is aprox only 3 years old but I will pull it if the water trap and filter dont fix the problem

 

The anti siphon valves can be small as seen above

 

sounds to me like blocked jets in the quadrajet. probably needs a carby strip and clean. and replace fuel filters. if you have used e10 at any time it may have water in the fuel system. also normal unleaded goes bad if the boats been parked up for a while.

 

Hi Brendan,
Thanks for your replay
Ive not used E10 in it
Fuel doesn't get a chance to sit in the tanks long as I'm a keen fisho and get out a lot LOL
Im going to check water filter to see if its full

Cheers
Mike

 

no worries. i think it will most likely need a carby clean and new fuel filter. good place to start anyway.

 

Hi again
Ive decided to replace the mechanical fuel pump on my chev 350 while fixing the above problem
I've relised that the existing pump is not marine as it doesn't have a return line
My question is once I install a new marine pump does the line go to the small tube on the back of the Chockchesor Quaddrajet
This tube currently has a small black rubber cap over the end of it
Am I correct in saying the carburetor needs 6-7psi, what psi range mechanical fuel pump should I get
Many Thanks Mike

 

That vacuum port on the back of your Quadrajet (just be sure it's pulling full manifold vacuum) can take the line from your mechanical fuel pump. It's not exactly a return line. In the event that the diaphragm on your fuel pump fails, the leaking fuel will be contained inside the pump body. Engine vacuum will pull this fuel directly into the engine causing the engine to stall due to flooding. This keeps the fuel out of the bilge. You probably already know that but I just thought I'd clarify for anyone else reading. Good idea to get that automotive style pump out of there whether it's bad or not. 6-7 psi sounds about right for pressure any marine pump for a small block 5.7 liter Chevrolet engine will be fine. I'd change the filters while I was in there, they're cheap and it can't hurt. Good luck, MIA

Holley 712-327-11 110 GPH Mechanical Fuel Pump -Marine https://www.holley.com/products/fuel_systems/fuel_pumps_regulators_and_filters/fuel_pumps/marine_pumps/parts/712-327-11

 

You might want to upgrade your fuel filtration and water sep filters to increase capacity, perhaps even a dual system so that if one clogs up, you can switch to a clean one.
Not likely that your tank has a bottom drain, but the bottom of the tank is where the problem usually lies, and a cleanup is probably in order.
Once that’s sorted, then look at cleaning up carb, etc.

 

Thank you Missinginaction and kapnD great advice from you both
Greatly appreciated
Cheers
Mike

 

Just a quick question
The small hose size of the carburetor vacuum today fuel pump is very small
Are there hose clamps this small