Author Topic: 1973 Blazer fuel prime issue and vapor question  (Read 2245 times)

Offline KyleK

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1973 Blazer fuel prime issue and vapor question
« on: December 21, 2018, 09:47:19 AM »
Hello all. I’m hoping the group can help me with a fuel issue I’m having. I am a newbie (but teaching myself quickly), and need a little help. I can’t rely on anything the previous owner has done due to some other issues we’ve found/fixed (but that’s another story). Sorry for long post, I just want to provide all of the details.
 
Vehicle: 1973 Chevy Blazer, 350 V8, 5.7, carb, mechanical fuel pump, single fuel tank (31 gallons I believe)
 
Background: Prior to changes, the motor ran fine (after an intake gasket fix), and pulled fuel into carb with no issues. The two issues to fix were the fuel gauge pegged to below empty, and the fuel filler hose was dry rotted, and spilled a lot of fuel when filling it.
 
Steps Taken:
-          Disconnected hoses, lowered tank
-          Removed fuel sending unit, all looks good and functioning (put back in with new seal)
-          Corrected wiring on fuel sending unit and ground (that fixed the fuel gauge issue)
-          Put back together with new fuel line hose (3/8”), return line hose (5/16”), fuel filler hose, fuel filler vent hose (1/2”)
-          Reconnected all hoses, refastened tank with new straps and anti-squeal
-          Note: The fuel line outlet (shown by red arrow in image) actually swivels a little. Should it do that?
 
Result of Steps:
-          I can fill the tank now with no spills/issues (put in about two gallons I had in a container)
-          Fuel gauge works with no issues
 
New Problem:
-          Once back together, I tried restarting. It would turn over but not fire. I assume it had lost it’s prime since I had disconnected all lines, so with some advice, poured a little gas into the carb directly. After doing that twice, it stayed running with no issues. Drove it over to buddies a mile away and worked on it all afternoon on other items.
-          When starting back up that afternoon, it did the same thing (turn over, but no fire). Buddy swore I was out of gas, so put five gallons in it.
-          We carb started it again, assuming running out of gas killed the prime again. Drive it the mile home no issues.
-          Next morning tried to fire it up (knowing I have five gallons in there), and same issue (turn over, but no fire)
-          The in-line fuel filter appears halfway full
 
Plans Next:
-          Currently tank/hoses dissembled as in the picture
-          Plan to disconnect fuel and return lines from fuel pump and blow air through them just to make sure there isn’t a blockage (since lines completely disconnected now)
-          Also plan to put gas in carb directly to see if it leaks out of the bowl anywhere (but haven’t seen that happen before at all)
-          Could replace mechanical fuel pump, but would seem like that isn’t the issue as it worked fine before
-          I don’t see any holes/leaks from any part of the fuel/return lines (near pump, metal lines, or at fuel cell)
 
Questions:
-          What other steps do you recommend, or any other thoughts?
-          I’m confused about the talk of a “vapor line”. I don’t appear to have one of these, and wouldn’t know where to hook it into. Is that needed? I’m not talking about the vent line, but rather a vapor line I see in some documents running from front to back of the vehicle)
-          Are my hose sizes correct, or does it matter? I see in posts that the return line is listed as ¼” (mine is 5/16” and fits super tight anyway on the fuel sending unit)
-          Should the outlet for the fuel line swivel or move? Mine can move left/right when I was replacing that hose. Doubt an issue, but not sure if it was rusted and sealed before, and then when I changed the hose, I did something
-          I can’t understand why it works once carb started and primed (and can’t see any leaks), but then won’t start the next day  (note that if I turn it off after primed and try 1-2 minutes later, it works again). Just not the next day.
 
 
Thanks so much!
 
Kyle


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Offline JohnnyPopper

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Re: 1973 Blazer fuel prime issue and vapor question
« Reply #1 on: December 21, 2018, 01:31:01 PM »
Hi Kyle, welcome!

So if your first efforts were to work on the intake manifold gasket, I would look to see if when/if you removed the carb, you affected the linkage to the accelerator pump.

Next time its cold, test the pump by looking down the throat of the carb while pumping the throttle, looking for jet(s) of gas being pumped down the barrel. You are duplicating that function when you 'carb start' it.

The swivel just means the tube broke free of the plate it's passing through. It may or may not leak. If you want to fix it, clean with brake cleaner, and apply JB weld.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: 1973 Blazer fuel prime issue and vapor question
« Reply #2 on: December 21, 2018, 06:16:05 PM »
Do you have a vented gas cap? If you loosen the gas cap does it still happen? Make sure you don't have any kinked hoses. No your hoses shouldn't swivel
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Offline Henry

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Re: 1973 Blazer fuel prime issue and vapor question
« Reply #3 on: December 22, 2018, 11:27:38 AM »
Hi Kyle:
As VZ has mentioned, your 3/8 fuel supply hose should not swivel...if it does, you could introduce a air leak which would break the fuel pump "pull" from the tank. If the leak is big it will prevent the pump from working at all...if it is a small leak, it can allow the gas to siphon back down to the tank and prevent the fuel pump from self-priming after a long shut down or at slow speeds. Get some hose clamps on that.

Vent line: yes, my 1973 service manual shows your vent line running from the sending unit to the filler tube. So, in order to vent properly, you should have the right filler cap as VZ has mentioned...it must be a pressure-vacuum cap that allows venting...you can do a quick and dirty test by blowing and sucking on it...it should have resistance to hard pressure and vacuum, but should flow with light pressure and vacuum.

"Vapor line": your Blazer is what I believe to be a "light duty emissions" truck which means it originally came with the ECS (Evaporation Control System). This was a cannister in your engine bay that took vapors from the carb and tank and held them to be recycled back to the carb when engine was running. I would assume your cannister is long gone and the carb is not the original one so all these connections are gone. What you probably had was a tee at the vent line that then had a hose that ran up to the ECS cannister in the engine bay. I dont know where there is a GM diagram of this, but possibly there is a schematic of how the original hoses were on the Autozone website where they show all the emissions diagrams for Chevy vehicles.

Your fuel pump is probably OK since you seem to have a good amount of fuel in the fuel filter.

You said your vent line wants to be 5/16 because 1/4 is too tight...there are so many different tanks, senders, changes in configurations during the 1970s that I would not be surprised if the vent line sizes were different...just make sure you are using the right port on the sender...If you have a 3 port sender, I would expect the vent port to be the shortest of the 3 in the tank. Fuel pump return line would be second longest and the supply line would obviously be the long one with the sock on end at bottom of tank.

Regards,
Henry

Offline KyleK

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Re: 1973 Blazer fuel prime issue and vapor question
« Reply #4 on: December 23, 2018, 06:34:27 AM »
Thanks guys for the response. I’m going to check out these options/checks and let you know the results.


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Offline 75gmck25

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Re: 1973 Blazer fuel prime issue and vapor question
« Reply #5 on: December 24, 2018, 03:31:24 PM »
My '75 has these three ports on the sending unit:
3/8" - fuel feed with filter/sock on the end in the gas tank
5/16" - fuel vent.  It terminates about 1-1.5" below the level of the sending unit, and it has a special fitting on the end - fitting probably stops fuel from entering the line
1/4" - fuel return line. It terminates about 3" below the sending unit, and its just an open line.

Each one of the lines has a rubber hose that connects to a metal line running up to the engine compartment.  The metal fuel feed and return lines end at the frame right next to the fuel pump.  The vent line runs up near the radiator support, and then a rubber  hose goes over to the vapor canister.

Bruce