Mallory 4309 return regulator

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66Valiant528

66Valiant528

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My fuel pressure has been dropping after 20 minutes of run time. The vac/boost nipple on the top doesn't hold vac. I assume this is an indication of a leak in the diagram. Does that port hold vacuum?
8AN front to back 100/10 micron filters both repla ed and Aeromotive 11203 pump. Voltage and relay is good. Pump flows a ton.

20230427_113532.jpg
 
Yea that diaphragm should hold vacuum and not leak. But the 20 minutes of run time is throwing me off. If the diaphragm is bad you won’t have good fuel pressure adjustment to begin with. Are you using a fluid filled gauge in the engine compartment? They can change FP reading based on heat.
 
2 fluid filled but they do it when I'd consider them cool. Pressure drops. Turn it on cold 8 to 9 psi.
 
Got it. It’s probably time for a rebuild kit.
 
I don't have the second gasket on my set up. Never had this apart in 14 years.

Screenshot_20230427-111934_Chrome.jpg
 
Maybe I am missing something, but if the diaphragm leaked you would have fuel being pumped into the engine...& you would sure know it.
I would suspect the fuel supply to the reg: pump, pick up, filter.
 
Do you have a vacuum pump? You could put a vacuum on it and watch the vacuum gauge to see it losing vacuum. That would at least confirm it.
 
Yes human vacuum pump. My mityvac died. Put a hose on it. Won't hold vacuum. Pulled the line off the forward filter. Fuel pumpin out like a garden hose. Fills a 5 gallon pail quickly. Maybe I'll try it when the gage reads 3 lbs. Doesn't take long.im beginning to suspect the feel line is collapsing inside.. This is about 15 year old aeromotive 8an hose. They told me yesterday that the hose really only has a service life of 8 years. The ethanol could have softened up the rubber and the pump is sucking it closed. I am going to change the feed line to the pump just in case.
I agree if the diaphragm is blown how come no fuel leaking out? But again it doesn't hold vac/boost. So I believe it's bad. 25 bucks shipped from Holley on its way.
 
Bewy said:
Maybe I am missing something, but if the diaphragm leaked you would have fuel being pumped into the engine...& you would sure know it.
I would suspect the fuel supply to the reg: pump, pick up, filter.
Click to expand...
I do not have a hose attached to the nipple. No need but I agree it should squirt out fuel.
 
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