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1972 Blazer Carburator losing prime


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Steve A
New User

Sep 16, 2020, 3:56 PM

Post #1 of 4 (248 views)
1972 Blazer Carburator losing prime Sign In

When my 1972 Chevy Blazer 4 wheel drive with the 5.7L 350 engine sits for more than 48 hours it will not start. If I push the accelerator down once and release it always starts if it has been less that 24 hrs. If it is more than 48 hrs. then pushing the accelerator, pumping the accelerator, cranking, nothing works. I then have to remove the air cleaner and pour a half cup of gas directly into the carburetor, then it will start every time and run fine. This problem started about 5 years ago and progressively got worse. Since then I have replaced the carburetor (rebuilt), fuel pump, fuel line from fuel pump to carb, fuel filter (replaced with check valve filter), fuel tank including all rubber hoses connecting to the tank. There is no visible gas leaking anywhere or an odor of gas. There is no gas in the crankcase. It does not seem that cold, hot, or any weather makes any difference. Can anyone help!


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Sep 16, 2020, 4:38 PM

Post #2 of 4 (242 views)
Re: 1972 Blazer Carburator losing prime Sign In

Steve A: It's normal for those! Carbs, this a Rochester (or whatever is on it now) 2V or 4V doesn't matter they get hot and vented right outside a bit more than would be allow a model year later. It EVAPORATED OUT of the bowl, that's all.


Just get used to cranking it a few turns of engine either before you set choke or let it crank should race until touching gas pedal at about 1,800 too high so one touch drops to tolerable 1,000 +/- as primary choke plate turns open the idle is riding down a "step cam" you can see that.


There isn't anything come hell or high water you'll do to fix that short of add some rig to squirt raw fuel into some vacuum hose (never done by me they all did that) is part of how they work. Why later than new?


Since I asked I'll answer it: The carb was better insulated by lowest gasket when dry and new so you could just improve it with a new one alone if still sold alone.


Make sure it has a tiny flap on the little fuel filter not wide open that holds fuel inside carb still evaps out just takes more time.


Sorry - it takes the novel but SAID IT'S 100% NORMAL! The hotter the engine when you shut it down the more fuel over a day or two is GONE.
I'll prove it to you: Do this: Start it cold without allowing it to warm up and sit for same couple days it WILL NOT DO THAT.
Hope you didn't turn in the "core" of the carb there was probably less wrong with it than what you got these carbs - either one were excellent the #1 problem was setting choke pull off - the primary one and vacuum diaphragm that pulls it off. Measure that opening if not perfect right away if you know it's normal in all other ways.


Nothing wrong I've not only fixed these brand new owned a zillion of them are the best there was just took YOU or driver of the day to understand it. No two were exactly the same nor day by day just how many pumps or none at all if already warm was acquired second nature best if you drove it last yourself and knew or paid any attention where it was when run last,


Tom



Steve A
New User

Oct 20, 2020, 6:03 AM

Post #3 of 4 (93 views)
Re: 1972 Blazer Carburator losing prime Sign In

Hi Tom,

Thanks for your reply. Unfortunately I got some bad advice and replaced the original carb that probably had nothing wrong with it and I turned in the core.
I did try the starting cold experiment. I started the truck cold and only let it run for 20 seconds or so until it ran smooth and shut it down. Unfortunately two days later it did not start until I poured gas into the carb.
I have heard about gas leaking out of the bottom of the bowl. Apparently some people put epoxy on the outside bottom of the bowl. Could this be part of the problem? And a good fix.
Thanks,
Steve


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Oct 20, 2020, 6:21 AM

Post #4 of 4 (89 views)
Re: 1972 Blazer Carburator losing prime Sign In

OK - I don't own or deal with any by chance right now. Will add you can re-route fuel line away from heat helped? It must be metal not rubber that could leak/rub = total fire - duh.


Now with high/fast evaporating fuels the issues or normal just worse + ethanol is not compatible with the OE gaskets or rubber, plastics and more of OE just can toss those.


Other idea once was put in an electric prime pump with a switch fill up carb before starting but must allow mechanical one to pass fuel thru it? Too involved just crank it, wait then touch gas pedal sets choke will then start right away, 2nd time. You got used to these were perfect if you knew how long it had been or how warm engine was when used last.


Owned zillions most needed adjustments then perfect get used to dry carbs with that one trick.
Too bad you turned in core the no doubt make clones now if an OE worth tons to both of us (not many understand these things) that knew Rochester Carbs and their quirks. BEST and most even mixing of fuel of anything made if you only understood them - nobody did so lies then claims of the aftermarket many switched to junk as it turned out all but poured fuel all over engines so BYOFE = Bring Your Own Fire Extinguisher crap.
Shoot - 1972 - last of the best GMs real + best before the idiot attack on 1973 models! Those years known to me as the "Years of Malaise" for engines and almost no power out of anything,


Tom







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