Loosing Prime
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Parts Nerd Extraordinaire
Joined: Sep 2010
Posts: 4,381
Likes: 1,791
From: Western NY
Loosing Prime
I feel as though my F6 this year is unable to maintain prime on the fuel pump (stock mechanical). My F5 on the other hand, will start in an instant (same stock mechanical fuel pump). What can I do to keep it in prime (no, I don't want an electric pump!).
Tuned
Joined: May 2013
Posts: 408
Likes: 1
From: Yucca, AZ
If your pump looks like this or at least close you'll note that it may contain a set of check valves so that the fuel won't back wash to the tank. One or both could be compromised as well as the diaphragm itself. Most all mechanical fuel pumps have a diaphragm air vent. If the pumps diaphragm is split or has a pin hole in it you can usually see fuel seeping out the vent. Post a pic of your pump if you can. There are so many types out there that could have been mounted on that truck over the years it's hard to give you advise on what to check first. 
Capt'n
Capt'n
Fleet Owner
Joined: Jul 2004
Posts: 27,299
Likes: 1,059
From: NM
I think two things cause that. One is a leaky hose that goes from the firewall (end of steel line) to the pump. The other is a steel fuel line full of crud. The crud gets under the check valves (see above) and lets them leak enough to drain back, but not so much they affect normal operation.
When BunnyBlue52 was over at my place, we replaced that hose on his truck, and also rebuilt the carb, and added a fuel filter between the pump and carb, so it was completely empty. I figured we'd have to crank forever to get it running. It cranked less than 5 seconds before firing off. His steel fuel line had been replaced earlier and everything was clean.
When BunnyBlue52 was over at my place, we replaced that hose on his truck, and also rebuilt the carb, and added a fuel filter between the pump and carb, so it was completely empty. I figured we'd have to crank forever to get it running. It cranked less than 5 seconds before firing off. His steel fuel line had been replaced earlier and everything was clean.
Postmaster
Joined: Mar 2013
Posts: 3,066
Likes: 7
From: Over There
So the question then becomes what kind of carb do you have and where is the fuel going? IDK if your stock, but for example if you have a 2100/4100 series carb on it they are notorious for problems like this. It's a design flaw in the power valve.
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