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Sheener

Backfire

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86 f 250, 4wd, 460, manual transmission

Gary Lewis

Re: Backfire

Administrator

Sometimes a backfire on deceleration is due to a blown power valve. You didn't say what the CO levels were, but if it is blown they should be high.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Sheener

Re: Backfire

High CO and HC. I will change the power valve and see.

86 f 250, 4wd, 460, manual transmission

85lebaront2

Re: Backfire

Administrator

Backfire on deceleration can also be a bad or not connected correctly air bypass or diverter valve. Its function is to shut off the air into the exhaust to prevent backfires.

Before I converted Darth to EFI, on a cold engine, pulling our 10K 5th wheel, going up onto the interstate near our house, when I topped the ramp and slight incline on the interstate and started down towards the Monitor-Merrimac memorial tunnel section, letting off the gas got a huge "boom" and my wife, following in the car said there was a large flame out of the tailpipe.

Bill AKA "LOBO" Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 2009 Flex Limited with factory tow package Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

Sheener

Re: Backfire

What fuel injection did you install? 35year old parts that are expensive to replace have me thinking of that. Did not have the backfire before hooking up the diverter valve and the replacement is over $300. Anyone ever rebuild a diverter valve.

86 f 250, 4wd, 460, manual transmission

85lebaront2

Re: Backfire

Administrator

Darth's EFI is a factory setup from a 1990 F250, but upgraded to MAF and sequential running a 1996 EEC-V computer.

I would start by checking all the vacuum lines, particularly where they go over the exhaust crossover in the center of the right head side of the intake. Is your vacuum diagram on the radiator cover plate still legible? If not I can post my old diagram. There are two vacuum lines to each air control valve, a red and a black one. The red line should have vacuum whenever the engine is running, black I believe has vacuum with a warm engine, but it could be cold and no vacuum warm.

Bill AKA "LOBO" Profile

Sheener

Re: Backfire

My tag is still legible, and I have access to the one on this forum. I need to test the diverter valve as well. It is a pretty cobbled up mess under the hood. Thermactor system was missing when I bought the truck. Trying to use a harness I got out of a junkyard, but it isn’t very good. I may have to fabricate my own to get it operating, but if the valves are bad and I need to replace, that is close to the cost of converting to efi.

86 f 250, 4wd, 460, manual transmission

85lebaront2

Re: Backfire

Administrator

Here is what the rat's nest is supposed to look like removed from the engine:

Bill AKA "LOBO" Profile

Sheener

Re: Backfire

Yep, got all that with cut off ends, pinched tubes, broken connectors and despair all over it. May have to fab my own that won’t be as compact as that.

86 f 250, 4wd, 460, manual transmission

85lebaront2

Re: Backfire

Administrator

Unfortunately for you, mine is long gone along with the air valves and pumps. If you can find one in a junk yard and get most of it you would be in great shape.

Bill AKA "LOBO" Profile

Sheener

Re: Backfire

Do you not have emission requirements where you live?

86 f 250, 4wd, 460, manual transmission

Sheener

Re: Backfire

In reply to this post by Sheener

Got the thermactor system sorted out, changed the power valve and still had the backfire. Pulled the new plugs and they were black and had buildup. Hecked the compression while the plugs were out and they went from a low of 90 to a high of 115. Going to do a wet test on the compression tomorrow. Any other ideas?

86 f 250, 4wd, 460, manual transmission

ArdWrknTrk

Re: Backfire

Administrator

Let's be clear, by "backfire" you mean an explosion in the exhaust not in the air cleaner, correct?

Because combustion coming back up the intake will definitely blow out the powervalve of a 4180 carb.
And that will cause fuel to leak constantly resulting in a very rich idle and popping sounds on trailing throttle.

Are your new plugs black and wet? Or black and fluffy?

Black and wet with oil is a definite possibility given your low compression numbers. No amount of tuning is going to fix intake valve seals, broken rings or worn out bores.

Black and fluffy indicates way too much fuel. The 4180 isn't that different from a regular 4160 Holley carb. It can be fixed but you need a very specific kit to rebuild it.
And I'd definitely suggest the addition of a backfire check valve so you don't pop the powervalve when the ignition module fails.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

ArdWrknTrk

Re: Backfire

Administrator

In reply to this post by Sheener

Do you have a good idea of how many miles are on this engine? (emissions records should show how many times the odometer rolled over)

Are you in California where you must use factory original parts?

Do you have any (even tiny) exhaust leaks at the manifolds?

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Sheener

Re: Backfire

In reply to this post by ArdWrknTrk

I am in Colorado, and we have adopted California emission requirements. Had a small exhaust leak on passenger side manifold, but took care of that. Backfire is coming from exhaust system. Carburetor is a 650 holley. I have reduced jet sizes for altitude. Initially went 3, but dropped another when I had it apart to change the power valve. Backfire wasn’t as bad, but power is way down, and it did not want to run when cold. I will do the wet compression test today and see if that improves the compression. 6 of the plugs were black and sooty and two not bad, but they still had some dark spots on them. Truck came out of a state without emissions. No notes on the title that it had rolled over, but if not, it averages out to about 2000 miles a year. Would think that it ran more than that.

86 f 250, 4wd, 460, manual transmission

85lebaront2

Re: Backfire

Administrator

In reply to this post by Sheener

Very limited, and we have an update/backdate rule on the books.

Specifically, if I install a newer engine in an older vehicle, it only has to meet the requirements for the year of the vehicle. Specific example, Darth is a 1986 F350 and is converted to MAF/SEFI as per the 1996/7 CA spec trucks, but, since a 1986 over 8500 lb GVW truck was not required to have a catalytic converter, I do not need one and the items affiliated with it are also not needed.

Parts of the state do have stricter rules, the area around Washington DC at present and Richmond and Tidewater (SE VA) keep getting threatened with actual emission testing. I live in a mostly rural area where the major industries are Chicken processing plants (Perdue and Tyson). We do have one of the 4 US Spaceports at Wallops Island.

Bill AKA "LOBO" Profile

ArdWrknTrk

Re: Backfire

Administrator

In reply to this post by Sheener

My '87 is pushing 470,000

Ok, so you don't have the record keeping that a lifetime Cali vehicle would.
But in Colorado you don't have California requirements that everything emissions must be factory???

'sooty' is a positive, because that can be addressed.

Tell me a little about your carb if it isn't the stock 4180. Because I've run the stock Motorcraft by Holley 4180, a 0-80451-S and now this 750.

Bill is probably one of the best to help diagnose a carburetor. He ran a tune-up shop for years back in the day.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Sheener

Re: Backfire

Holley 650 double pumper. Primary jets were 67’s, i reduced to 64’s when it failed emissions due to hi HC and CO. Plugs appear to be wet and sooty from fuel. Had an issue with starts when hot, so I installed a 1” phenolic spacer and that helped the starting. Cranks over without hesitation when hot. Carburetoris only 2 years old, but that may be the issue. I think as long as it appears to have the emission equipment and passes the idle and 2500 rpm test, it is good to go.

86 f 250, 4wd, 460, manual transmission

ArdWrknTrk

Re: Backfire

Administrator

Well, the 4777's mechanical secondaries certainly aren't doing you any favors in a truck, but we need to address why it's SO rich.

Altitude in Colorado can definitely be contributing but I doubt it is the only factor.
How high are you?
What number powervalve did you use?
(and any Holley built after '92 has blowout protection)

It seems that Colorado might have adopted California's limits but definitely not their standards if the test station will even look at your truck with that carb on it.

If your truck is burning oil it may be impossible to get below the HC threshold.
Fortunately the Federal heavy duty vehicle emissions standards in place when your truck was built are pretty liberal.
And like Bill said, they can't hold you to stricter regulations than were in place at the date of manufacture.

Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.

Sheener

Re: Backfire

We are at 5000 feet and the tech’s at the test station just glance at the engine bay. Looked at it this morning and found I had left the vacuum advance line off. I am sure that did not help.

86 f 250, 4wd, 460, manual transmission

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