Technical Mercury 8CM Fuel Requirements

Hi All,

Having a problem with my Mercury. At full throttle it's spluttering and cutting out. I've got a 6v electric fuel pump fitted.

Engine is a completely standard 8CM.

I've found the pressure in the manual should be 3.5 to 4.5 PSI, but what volume of fuel should be supplied, especially at full throttle?

My current pump supplies a maximum of 60 L/H is this enough?

Thanks

 

I can only figure in US units, so lets see : There are about 4 liters in a gallon. Let's assume 15 miles per gallon for your car (my '51 Ford with a Merc engine is similar). At 60 mph yout car should use 4 gallons (or about 16 liters) of gas per hour, so that shouldn't be a problem.

At this point, I would remind you that 90% of fuel problems are ignition related. When was your last tune-up? I would suspect the coil, condenser, or other deteriorated parts somewhere.

 

Thanks @tubman so the reason I think it might be fuel is that it only happens at near to full throttle.

In reaching the conclusion it might be fuel related we have:
- checked all the plugs
- checked the timing (2° at idle)
- checked compression
- replaced diaphragm in the vacuum advance
- checked points
- coil and condenser are new (within the last few months).

The pump supplies enough fuel at cruise, but if I floor it I get issues, so trying to find out what volume is needed at full throttle.

 

I've found in the manual that at idle it should consume 1 pint in less than 45 seconds.

This equates to 38L/H or about 10G/H.

So at full throttle would it need more than 60L/H 16G/H?

 

Inspect plug wires? Could be jumping when the combustion chamber is at full pressure? Inspect inside of cap?


-rick

 

@69fury we've checked the plug wires, no crossing and all look to be in good condition.

Missed the distributor cap off the list above, but that's also new alongside the rotor arm.

 

When it comes to ignition components, "new" does NOT equal "good". Folks have had to go through 2 or 3 condensers until they finally found a good one.

 

No, it should SUPPLY a pint in 45 sec at idle with the fuel line from the pump delivering into a container. Big difference. You have enough pump IF it's getting to the carburetor.

 

Thanks @chicken won't it need to supply more at full throttle though?

 

Could anyone tell me the volume of fuel the engine requires at full throttle?

 

If you have fuel pressure at wide open throttle you are getting enough fuel. A friend had a Pontiac 421 he put 2 4's on. At WOT it would just start to slow down. We put a fuel pressure gauge on it and the fuel pressure would go from 5lbs to zero when it started to slow down at WOT. This was with the stock fuel pump. Put an electric on with the stock pump and the problem was fixed. It would hold 5lbs at WOT.

 

I'll go at this my way again. At WOT, you will get maybe half the gas mileage. Just like my previous calculations, that yields 32 gallons/hour, slightly more than half what your pump is supposed to deliver.

 

@tubman Google suggests there are 3.785 litres to a US gallon. So 32 US GPH would be about 121 LPH, which is double the max of my pump (60 LPH).

So potentially my pump is not up to the task.

 

GPH = HP/ 10.

 

Matt, does your engine start to sputter immediately when you go full throttle or does it take several seconds? Reason I ask is because the float bowl of the carb will be full as you cruise along at part throttle, then when you floor it it'll take a few seconds (maybe five) before a less-than-adequate pump will fail to keep the bowl full.
The suggestion to plumb in a gauge right ahead of the carb is a good one. If you have good pressure at full throttle the pump isn't the problem.

 

@chicken it's fairly instantaneous, maybe a couple of seconds at most.

 

I'd sure keep an open mind and consider something other than the pump, but I'd check pressure at the carb if possible with a long hose and gauge you can see while driving.
This could also be a problem with a partially restricted main jet in the carb-rare but not unheard of. When it's messing up does pumping the throttle help? The accelerator pump will give it an extra shot of fuel for a pump or two even if the bowl is getting low. Pulling the choke partway will do the same.
Lastly, condensers are highly suspect as others have said. A try with another one isn't a bad move.
I wish I were there to help diagnose!

 

First thing, what size are your fuel lines? If you are worried about the volume of fuel your pump is supplying, you're not going to get any more fuel at a specific pressure than what will pass through the line you're using. If everyone else with a flathead uses 3 1/2 to 4 PSI and don't have your problem, there has to be a restriction in the line somewhere. My fuel line is 5/16 inch and my stock 239 runs just fine with 3 1/2 PSI.

 

If there is a rubber hose connection in any part of the fuel line it may be collapsed from the inside preventing the flow of needed fuel at high speed.

 

I have found brass fittings, especially 90 degree elbows with tiny holes in them. I have drilled out lots of them, both air and fuel lines.

 

Wide band O2 sensor.

 

Thanks @alanp561 I would need to check, but I suspect they're whatever came on the car.

I understand what you're saying, but if you have a pump that does 4psi and 25gph and a pump that does 4psi and 50gph surely there is a difference?

@19Fordy that's certainly another possibility, there are definitely sections of rubber hose.

 

Matt, Just as a point of information, I also have an 239 cu. inch 8BA with stock internals, a 390 Holley carb with #51 jets and am using a Facet 40106 electric fuel pump. Gas line is 1/4 in. Runs fine. Pressure is 4 to 7 psi.
12 Volt DC Facet 40106 Electronic Fuel Pump | Fuel Pumps | Engine Accessories | Engines | www.surpluscenter.com
You might find lower pump price but, be sure it's a Facet pump made in the USA and NOT a China reproduction.