The Stovebolt Site Forums The Shop Area Engine Shop 1972 350 - backfires at shutdown

This truck will backfire when I shutoff unless I idle the ending a couple minutes .

First, check and adjust your timing. Fouled plugs?

What carburetor is on your engine? Is the engine stock?

Is the backfire out the tailpipe or through the carburetor?

Your carburetor may be leaking fuel into the intake or the mixture may be set too rich.

My 72 although it was a six had the solinoid that when you shut the engine off it shut the throttle plate ,maybe if your carb has the same type of solinoid it may be stuck.

Here's a pretty good example of the idle stop solenoid that allows the throttle plates to drop completely closed as the ignition is turned off. Engines have a tendency to run-on, Diesel, or backfire on shutdown without it.

www.ebay.com/itm/184573184071?

Jerry

It could be as simple as the idle speed set too high. A little more information would help, what transmission? air conditioning? idle speed in neutral? Does the engine run too hot prior to shut down?

Just to help folks know more about what Svcattle has, below is a screengrab from another of his posts in the Big Bolts forum showing the specs of his truck.

It's a 1972 GMC 5500 with the standard GM corporate (Chevy) 350 V8, 5 speed manual transmission and 2 speed rear axle. IIRC, the typical carb for this 350 would have been a Quadrajet 4 barrel.

Svcattle - how about some pictures of your current engine and carb setup so folks can be sure what we are talking about?

The 1970's medium duty GM trucks that I am familiar with had 366/427 gas engines and factory Holley carburetors, the carbs had the integrated vacuum/centrifugal governors. Did the 350 come with a Q-Jet only?

Buckshot - All the research I've done so far shows the 350 V8 could have been equipped with either a Rochester 2G (2 barrel) or possibly an optional Rochester 4MV (Q-jet)*. The carbs for the Chevy/GMC corporate Big Blocks were the Holley 4150 models (model info taken from a GMC Shop Manual).

* The Rochester 4MV Q-jet is listed in the 1972 Shop and Overhaul Manuals for 350 V8's, but doesn't specify which truck models they were used on.

Below are screen grabs from GM Partswiki from a 1976 Chevy/GMC Medium Truck Parts Book. In it, the only carb listed for the 1972 350 in the 50/55 Series is the Rochester 2G. The second pic shows the Holley 4 barrel listings for the 366/427 big blocks.

I read in another of Svcattle's threads that he found and was ordering a 1972 GMC Master Parts Book specific to his truck. Perhaps he can check in there and provide more info on what carbs are listed for his 350, as well as confirm what he has currently installed on his truck?

PS - The other pics are from a 1972 GMC Truck Dealer handout showing the engine options. No mention of the carbs that go with the engines.

From the Rochester Master Sales and Service Manual

1972 Chevrolet truck:

2G - 7042123 (transmission not specified)
2G - 7042124 (transmission not specified - WITH GOVERNOR)
4MV - 7042910 - California A/T
4MV - 7042911 - California S/T
4MV - 7042210 - A/T
4MV - 7042211 - S/T
4MV - 7042208 (transmission not specified)

Jon

Well, it looks promising then for parts and/or replacement carburetor if needed.

Ok i found this thread again thanks for posting the link . The engine is a stock 350 with Rochester 2bl carb no solenoid . I will try to get engine and carb pictures tomorrow .

If the carburetor is a Rochester 2G with manual choke and a centrifugal/vacuum governor, I would temporarily disconnect the steel vacuum line between the distributor and the carburetor and plug the line at both ends. This will allow you to set the idle speed and mixture without any unknown vacuum leaks affecting the settings. Test drive to heat the engine to full operating temperature and see if the backfiring changes. You may also have to disconnect and plug the main vacuum hose that powers the HydoVac and 2 speed in order to diagnose a vacuum leak.

A quick test, pull the distributor cap off enough to get your hand on top of the distributor. You should be able to twist the rotor and have it snap back. If it doesn't, the advance circuit is stuck. This could be the issue, too much timing.

A colder heat range of spark plug may also help, a hot range will act like a glow plug till they cool off.

Joe's point about the spark plugs is valid. I think the stock plug is/was an AC Delco R45. I had a '72 Monte Carlo and that seems to be stuck in my brain.

Lot's of people hear the word "hotter plug" and think it means better performance when in fact all it does is dissipate heat more slowly. They're generally used to prevent fouling.

I put a dwell meter on and it moves if that helps

I’m running hotter plugs to prevent fouling

Interesting. I'd install the factory spec'd plugs in and see what happens. You'll at least be able to rule out the plugs as the culprit if you still get the backfire.

If the dwell changes with rpm, it indicates that the distributer shaft bushings are worn, a small amount of change( 1 degree) is normal. Timing should change with rpm but dwell should remain steady. My '57 5700 with 283, stock carb and distributor has a spec of 4 degrees BTDC static timing and has no provision for vacuum advance.

I have a Pontiac distributor I working on set up on my distributor machine right now. With new points ( heavy duty NAPA brand ) and a new condenser, the dwell is set at 32 degrees. At 6000rpm it will back up to about 29 degrees. Its a slow back off and doesn't start till around 4500 rpm. Below 4500 rpm, it is rock solid at 32 degrees. With the standard set of points, it won't make it to 5000 rpm without cutting out, the spring tension is too weak and the points start bouncing, yet the dwell is still with in range.

You have distributor issues if the dwell is moving more than a degree or two at higher rpms. Does the timing advance with rpm? Does it advance with the vacuum applied?

Also, when it back fires, is it tail pipe or intake manifold?

Yes the timing is advancing . The dwell doesn’t move much at all but it did change a little on my meter . Backfire is out the exhaust . Truck runs really well just has a backfire if I don’t idle a minute . I do need to slow idle down a bit I turned it up a bit but I went a hair much . I had anti foulers on all 8 cylinders and no back fire then . I removed them and now is when I get the backfire . I was using a gallon of 15 40 a day on an 8 hour day tried Lucas junk didn’t help anything so I changed it to Schaffer’s 20 50 for a while then switched to mystic 15 50 and i don’t have near the oil consumption I did

Backfiring through the exhaust sounds like the fuel is too rich and/or the extreme oil consumption is burning in the hot exhaust after the ignition is terminated. The 2 stroke Detroit Diesels that I drove only used 2 qts in 350 miles, 8 - 10 hour trip, 80,000 lbs gross on the trip home.

It could be you have carbon build-up in your cylinders from the oil consumption. When you shut it off the carbon still has glow and ignites any residual fuel. Back in the day when an engine had problems like you are describing we would run the engine to operating temperature. Take the air cleaner off and at fast idle, pour (trickle) water down into the carburetor (don't kill it). The theory behind it was to steam clean the carbon out of the cylinders.Whether it worked or not, in my opinion it helped. Then we would give them a thorough tune up.

We did the same water trick, about 1500 to 2000 rpm, a cup water in hand and start dumping it in. You need enough to choke it down a little and make steam, not enough to kill it, too little didn't seem to do anything. A few good hard pulls up a long hill cleans it out pretty good also. A little hotter thermostat wouldn't hurt.

I’ve heard of people doing that. I’ve always wanted to try it. After running the Schaeffer and then switching over to 1550 it doesn’t use the 10th of the oil it used to.

Update on the backfiring . It just one day up and stopped backfiring .