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Old 10-26-2009, 08:23 AM   #1
Green Monster
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Help... Starting Problems, Rough Idle, and Backfire through Carburetor

I have a 72 Blazer with a 350. The truck was originally from California and is now in Pennsylvania. I never adjusted anything on it since I have owned it because it always ran great. The motor was replaced with a crate motor around 2000 (year). The truck ran great and sounded great since I have owned it in 2003. However, two weeks ago the engine would stumble and the carb started to backfire only at high RPMís under heavy acceleration. I though it just might be the colder weather causing this. I tried 92 octane gas and this did not make any difference. I usually run 87 octane gas. I donít really know much about carbureted vehicles. This is my first. This seemed like minor issue.

Yesterday, I filled up with 87 octane as I usually do. Drove three miles. Parked it. Came back out five minutes later and the engine would not start. It would crank but not start. I checked for spark and it had spark. I pulled the distributer and check to see if it had points that needed to be adjusted. It does not have points. I check the dwell while cranking l and it was at 30. I finally got it to start by giving it pushing the gas pedal as I cranked. But when it started it idled very low. I drove it home one mile and it stalled in the driveway. I started it again and the carb backfired when I stabbed the gas. It will start but misfires and stumbles. I need depress the accelerator to keep it running. The RPMs run up to 2,500 then drop like itís going to stall, I give it a little gas and the RPMs race up to 2,500 then drop like itís going to stall.

What could be the cause and how do I fix it? Any suggestions?
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Old 10-26-2009, 08:33 AM   #2
1968SWBBigBlock
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

Look & listen for a vaccuum leak
Also check the timing - it really sounds like a timing issue - to far advanced
Also this would be a good time to replace the fuel filter
New set of plugs and wires, cap & rotor
Lastly if I read your post correctly you have HEI - if that is the case I would also replace the module that is located on the bottom side of the distributor - this seems to be the one minor weak link in the HEI system
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Old 10-26-2009, 08:38 AM   #3
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

point plugs timing vaccume leaks fuel air filters

old vehicles need maintenance
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Old 10-26-2009, 08:59 AM   #4
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

It sounds like its flooding. Needle & seat worn out or sticking or has trash in it, bad float, if its a quadrajet proper choke operation in critical.
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Old 10-26-2009, 09:23 AM   #5
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

Three things come to mind: Try the cheaper, easiest first.

1. As mentioned above, change the filters. Inlet to the carb and any others that may have been added later.

2. Check or replace fuel pump ($15 or so). Mine did the same thing when it had a ruptured diaphragm and wasn't pumping enough gas.

3. Carb for severe flooding or choke adjustment. If it's this bad, you should have dark grayish or black smoke coming out of the exhaust. Have someone observe if you can get it started.

After this, ignition problems may be causing it as mentioned by posts above.
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Old 10-26-2009, 12:00 PM   #6
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

I recall not smelling much gasoline odor when I removed the air filter and thought it was odd. Could that be an indication of a bad fuel pump? I will try to get to it tomorrow afternoon. I will try the cap, rotor, plugs, and filters and see what that gets me. I'll keep checking for vacuum leaks. Thanks for the replies.
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Old 10-26-2009, 02:04 PM   #7
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

Every time gasoline gets reformulated for "better emissions" the result seems worse and worse for carbureted vehicles.

One of the most annoying experiences I had was when California introduced laws that resulted in alcohol and MTBE heavy formulas at the pump. alcohol evaporates after all, which means if you didn't use what you bought within a day or so your tank suddenly had 10% less fuel than you paid for and the residue left behind took the form of crystals that would clog the needle and seat. Since lobbyists drown out anyone who actually understands this, all I could do was rebuild my carb every six months when the formulas changed again.

That was years ago and to me, things are getting worse. While California discovered the flaws in their love of MTBE, many states are still encouraging it because they want to be "just like California"...uhh, California has problems! Adding more and more alcohol doesn't help either, especially if it's the kind that likes to grab loose bits of rubber on its way to the carb or absorb water before it evaporates.

I know I rambled a bit, and I may be wrong about this but it sounds like you have the same problems I faced. The solution for the moment is simply to clean and retune the same system even if the new tune isn't the same as the old (compensating for leaner fuel). The fuels billed as compensating for poor-running engines are bad for good-running engines!

Also, when you know you are going to run through a tank of gas in a reasonable amount of time (planning a trip to a far-off cruise-in?) go ahead and replace the alcohol that evaporated. "Fuel treatments" are mostly isopropyl alcohol, intended to absorb any water in your tank and carry it to the combustion chamber before it causes rust in your tank--more garbage!!!

If you add fuel filters to your system to catch any rust or garbage that may be clogging your needle and seat--I recommend NAPA part# 23002 for 5/16" lines, they are dirt cheap and clear so you can see when they are dirty--add one between the tank and fuel pump somewhere, like the hose between the tank and line, which has probably started cracking from the wonderful fuel mixtures by now anyway.

Hmmm...it must be time for my "happy pill"
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Last edited by CSGAS; 10-26-2009 at 03:17 PM.
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Old 10-26-2009, 02:20 PM   #8
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

Quote:
Originally Posted by CSGAS View Post
Every time gasoline gets reformulated for "better emissions" the result seems worse and worse for carbureted vehicles.

............ Since lobbyists drown out anyone who actually understands this, all I could do was rebuild my carb every six months when the formulas changed again........

....... "Fuel treatments" are mostly isopropyl alcohol, intended to absorb any water in your tank and carry it to the combustion chamber before it causes rust in your tank--more garbage!!!.....................

Hmmm...it must be time for my "happy pill"
Your 100 percent correct... We don't have the problem here in Tx (yet) but I'm sure it's coming with all this green regulations being pushed down our throats.

Every state has their own laws regarding gasoline. Newer vehicles tend to compensate with the oxygen and other sensors along with the computer, but our older trucks can't, so be aware of this when driving out of state.
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Old 10-29-2009, 09:58 AM   #9
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

Problem solved. Threre were three issues causing the problem.

The carburetor gasket was blown. The backfire at the carburetor caused the carburetor gasket at the intake manifold to blow out. This was causing the rough idle. A new gasket fixed the the idle problem.

The fuel had water in it. When my dad and I pumped fuel into a cup there was a layer of water on top of the fuel. We had a lot of rain the night before I fueled up. I wonder if any got into the tank at the gas station? We put two bottles of HEET water remover into the fuel tank.

We took it for a spin and the engine stutered and stumbled under hard acceleration.

We put new spark plugs in and it runs like a champ now.

Thanks for your help, guys.
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Old 10-29-2009, 03:07 PM   #10
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

water wont float on top of gas is what i was tought in science class
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Last edited by cdowns; 10-29-2009 at 06:44 PM.
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Old 10-30-2009, 12:48 PM   #11
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

"water wont float on top of gas is what i was tought in science class"


lol, exactly what i was thinkin. not quite sure what kind of fluid will separate and float on TOP of gas. but i don't think it was water.
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Old 10-30-2009, 01:05 PM   #12
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Re: Help... Starting Problems, Rough Idle, and Backfire through Carburetor

My mistake. The gas was floating on the water.

From BP's website:
WHAT DOES WATER IN PETROL LOOK LIKE?
Water does not mix with petrol and readily settles to the bottom to form a distinct layer which may be clear but usually looks rusty or dirty. Its presence can be confirmed by using water finding paste. If the layer at the bottom of a petrol sample does not give a reaction with water finding paste then it is probably not water and a sample should be sent to a laboratory for analysis.

http://www.bp.com/liveassets/bp_inte...er_in_fuel.pdf
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