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Topic: Flaming Exhaust/ Carburetor Question (Read 5 times) previous topic - next topic
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Flaming Exhaust/ Carburetor Question
Yahoo Message Number: 87498
Wondering if anyone else has had this problem and what did you do to resolve it?
 My 1987 MP, Chevy 350 engine with carburetor and headers/dual exhaust has been developing a problem recently.  It has been backfiring loudly and with flames through the exhaust pipe on the left side.  Gas mileage has also been dropping.  On a recent trip where I had previously been getting over 8 mpg over this same route a number of times I am now getting about 5mpg.
 It backfires when I quickly take my foot off the gas after being at a constant speed say 55mph for approx 3 minutes or more.  It does not backfire all the time.  On that recent trip(approx 450 miles round trip over 4 days) it did it maybe eight times.  At night can see flames coming out the left exhaust pipe when it backfires.

The backfiring started about 3,000 miles ago.  At first it sounded like something was moving around in an exterior storage compartment and did not occur very often.  It was not till I was driving at night and could see both the flame and the noise at the same time that put two and two together that it was backfiring and now the decreased gas mileage plus increased frequency.
 When I bought it this past spring (about 7,000 miles ago) I had a lot of work done to it at a Dyno shop in Santee, CA (Southern California) which included rebuilding the carburetor/rotor/installing headers plus a lot of other stuff.  Recently talked with some folks there at the shop, via phone as I am in Washington state, and they thought I might have a bad spark plug wire, although did not directly speak with the mechanics as my message was relayed via person answering phone(shop owner was out for the day).  The spark plug wires had been replaced by the previous owner and are maybe three years old.
 Any other suggestions on what could be causing this problem?  I am new to the area in which I am living (this past summer), so do not yet have a mechanic that I trust.

John 1987 MP Chevy 350 engine Wenatchee, WA
Re: Flaming Exhaust/ Carburetor Question
Reply #1 –
Yahoo Message Number: 87511
Quote
constant speed say 55mph for approx 3 minutes or more.  It does not backfire all the time.  On that recent trip(approx 450 miles round trip over 4 days) it did it maybe eight times.  At night can see flames coming out the left exhaust pipe when it backfires.
Not really an expert with this symptom, since I haven't experienced it first hand before. However, at a guess, it would seem unburned fuel is igniting in the exhaust system. Cruising under load will heat up the headers, but fuel in the exhaust is not lean enough to burn until the throttle is closed. Then, the AIR pump adds the needed oxygen. I would start by checking plugs, wires, and distributor secondary.

Steve
2004 FL
2013 Honda Fit
Re: Flaming Exhaust/ Carburetor Question
Reply #3 –
Yahoo Message Number: 87522
Quote
Wondering if anyone else has had this problem and what did you do to resolve it?

My 1987 MP, Chevy 350 engine with carburetor and headers/dual exhaust has been developing a problem recently.  It has been backfiring loudly and with flames through the exhaust pipe on the left side.  Gas mileage has also been dropping.  On a recent trip where I had previously been getting over 8 mpg over this same route a number of times I am now getting about 5mpg.

It backfires when I quickly take my foot off the gas after being at a constant speed say 55mph for approx 3 minutes or more.  It does not backfire all the time.  On that recent trip(approx 450 miles round trip over 4 days) it did it maybe eight times.  At night can see flames coming out the left exhaust pipe when it backfires.

The backfiring started about 3,000 miles ago.  At first it sounded like something was moving around in an exterior storage compartment and did not occur very often.  It was not till I was driving at night and could see both the flame and the noise at the same time that put two and two together that it was backfiring and now the decreased gas mileage plus increased frequency.

When I bought it this past spring (about 7,000 miles ago) I had a lot of work done to it at a Dyno shop in Santee, CA (Southern
California)
 
Quote
which included rebuilding the carburetor/rotor/installing headers plus a lot of other stuff.  Recently talked with some folks there at the shop, via phone as I am in Washington state, and they thought I might have a bad spark plug wire, although did not directly speak with the mechanics as my message was relayed via person answering phone(shop owner was out for the day).  The spark plug wires had been replaced by the previous owner and are maybe three years old.

Any other suggestions on what could be causing this problem?  I am new to the area in which I am living (this past summer), so do not yet have a mechanic that I trust.

John 1987 MP Chevy 350 engine Wenatchee, WA
John: I would have the timing checked.  It seems that an exhaust valve is staying open when the plug fires.  Good luck John 23.5 FL
Re: Flaming Exhaust/ Carburetor Question
Reply #4 –
Yahoo Message Number: 87525
I would replace the plugs, the plug wires, and check your timing.  A basic tuneup.  Usually backfiring is a timing issue.  But it could be other issues as well.  If you go through the above and that doesn't fix it, the next place I would look would be the carb.  After that, it could be an internal engine issue.  Sticking valve, or bent valve, or a worn out timing chain.
Re: Flaming Exhaust/ Carburetor Question
Reply #5 –
Yahoo Message Number: 87537
John,
 Sounds like you may have a problem with your vacuum advance operation in the distributor.   Most likely with the mechanical advance arms.  In older Chevys, it is not unusual for these parts to need lubrication to ensure that they not only advance the spark timing on accelleration, but also that they stop advancing the spark on decelleration.  Since your symptom seems to be following a period of constant speed, the mechanical advance arms may not be returning to the "unadvanced" position when you take your foot off the gas.  You could have a vacuum advance issue, but usually that problem shows up as a "delay" in power when accellerating, and as a generally rough running engine (though I would check all the vacuum lines anyway, as they do deteriorate, especially wherever they have been stretched over a connection, such as the carb, the distributor advance unit, etc.)

Best of luck on finding/fixing your problem.

George Culp
Saucier, MS

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john208665 wrote:
Re: Flaming Exhaust/ Carburetor Question
Reply #6 –
Yahoo Message Number: 87538
"Sticking valve, or bent valve, or a worn out timing chain."

A message back in 2006 from Larry W. said:

The 350 is known for worn timing chains. The wise change them at about 90-100,000 miles.

I did the chain on our 83 LD at 95,000 and it was very loose. I dropped the oil pan at the same time to change the oil pump and to check the bearings. The mains bearings were great but the rod bearing were starting to show 'copper', meaning the babbit bearing surface was just about gone. It was caught at last moment. A few thousand more miles and the crank would have been damaged.
 I suspect the previous owner had neglected regular oil changes. I have disassembled many 350s with 150,000+ miles and have not seen such wear except on poorly maintained vehicles.

Andy Baird http://www.andybaird.com/travels/
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Flaming Exhaust/ Carburetor Question
Reply #7 –
Yahoo Message Number: 87544
I used to have a 1976 Toyota Corolla that did this. Ih had a vacuum *and* a centrifugal advance, and the vacuum advance was not working because a temperature sensor failed. The spark failed to advance at speed when the engine was warm. I don't know that this is your problem, but I expect it *is* timing related somehow.
Re: Flaming Exhaust/ Carburetor Question
Reply #8 –
Yahoo Message Number: 87857
I would like to thank everyone that responded with advice, explanation, and give an update.  Yesterday I removed a majority of the spark plugs and they seemed very black and smelled like gasoline.
Spark plug cables looked good.  Made an appointment at a local garage for the end of next week for them to take it from here.  The mechanic & shop owner repeated most of what was said here so feel confident about that shop at least at this point.  Hopefully problem will be simple and inexpensive to fix.

John 1987 MP Wenatchee, WA
Re: Flaming Exhaust/ Carburetor Question
Reply #9 –
Yahoo Message Number: 90988
Update on the Flaming Exhaust/Carburetor Question:
 After some delay in seeing a mechanic the initial problem turned out to be that the choke was staying slightly open plus the throttle linkage was corroded/rusted causing it be run very rich.  After this my 1987 MP chevy 350 engine drove much slower then normal but no backfiring around town.  Decided to take a trip which involved going over two mountain passes into the Seattle area.  On the first mountain pass she started backfiring again.  At the point where we were driving out of the mountains into warmer Seattle she went downhill quickly becoming very sluggish, backfiring any time the gas pedal was pressed, and sounded like she now had an exhaust leak.  Didn't even think we would make it into Seattle but did and limped back home.  A look under the motor home revealed one of the mufflers had a hole.
 Back to see the mechanic again.  This time he ran a compression check and all cylinders were very low.  He also changed out a few spark plugs that looked a little marginal(after being run previously with it very rich).  Mechanic did not have a recommendation as to how to proceed.  More of a try it and if you are comfortable with the power then good otherwise recommended a crate engine along with a new carburetor.

Recently went on a trip which was mostly flat and she drove ok with only an occasional backfire.  Gas mileage was around 7mpg.  So for now am going to keep driving it.  Hopefully the engine will last thru the summer.  Also need to get a new muffler.

John 1987 MP Wenatchee, WA

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john208665 wrote: My 1987 MP, Chevy 350 engine with carburetor and headers/dual exhaust has been developing a problem recently.

It backfires when I quickly take my foot off the gas after being
at a
Re: Flaming Exhaust/ Carburetor Question
Reply #10 –
Yahoo Message Number: 90990
Quote
Back to see the mechanic again.  This time he ran a compression check and all cylinders were very low.  He also changed out a few spark plugs that looked a little marginal(after being run previously with it very rich).  Mechanic did not have a recommendation as to how to proceed.  More of a try it and if you are comfortable with the power
Wow, may be a host of problems, but that low compression in all cylinders sounds like timing chain has jumped a tooth or two. If that is true, then that must be addressed first.

Steve
2004 FL
2013 Honda Fit
Re: Flaming Exhaust/ Carburetor Question
Reply #11 –
Yahoo Message Number: 90997
Yes that backfire is telling you something isn't working right in the motor.  I think I would get it replaced or fixed before trusting it for the summers outings.  Usually backfiring is related to timing issues.  The low compression is also telling.  How many miles does this motor have on it?
Re: Flaming Exhaust/ Carburetor Question
Reply #12 –
Yahoo Message Number: 91045
Quote
Back to see the mechanic again.  This time he ran a compression check and all cylinders were very low.  He also changed out a few spark plugs that looked a little marginal(after being run previously with it very rich).  Mechanic did not have a recommendation as to how to proceed.  More of a try it and if you are comfortable with the power

Wow, may be a host of problems, but that low compression in all cylinders sounds like timing chain has jumped a tooth or two. If that is true, then that must be addressed first.

Steve
Hmm, maybe I ought to get a second opinion from a different mechanic.

John Wenatchee, WA
Re: Flaming Exhaust/ Carburetor Question
Reply #13 –
Yahoo Message Number: 91046
The engine currently has about 50,000 miles.  When I purchased it a year ago it had 41,000 on it.  At the time a compression check was run on one cylinder and it was ok.  I do not remember the figure or even if they told me it, just that the compression was ok/good.  They said they could check all cylinders if I wanted to but felt it was not needed.  The outside of the engine looked very clean and they commented quite a bit about it.

John Wenatchee, WA 1987 MP
Re: Flaming Exhaust/ Carburetor Question
Reply #14 –
Yahoo Message Number: 91049
Quote
The engine currently has about 50,000 miles.  When I purchased it a year ago it had 41,000 on it.  At the time a compression check was run on one cylinder and it was ok.  I do not remember the figure or even if they told me it, just that the compression was ok/good.  They said they could check all cylinders if I wanted to but felt it was not needed.  The outside of the engine looked very clean and they commented quite a bit about it.
That's pretty low mileage to have a timing chain problem, but hard too to believe the alternative, which would be burned valves and or worn-out rings in all cylinders. Pulling the timing chain cover should tell the story.

Steve
2004 FL
2013 Honda Fit