Backfiring and wont rev up

zanderman111

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Mar 15, 2009
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1987 Merc 3.0 starts and idles like a dream. Shifts into gear perfect. Anything much over an idle causes backfiring and the engine just wont rev up. Timing is correct. Compression is good. Fresh gas. New plugs. Revs correctly in neutral, but when in gear above an idle it backfires and sputters around. Any ideas? Thanks!
 

Haut Medoc

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Jun 29, 2004
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10,645
Re: Backfiring and wont rev up

Are you sure the ignition wires are right?.....
1-3-4-2....;)
Worth a look...:)
 

zanderman111

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Re: Backfiring and wont rev up

Yup ignition wires are correct. I should edit one of my previous statements: The gas is currently a mix of 10 gallons of roughly 4 year old gas with 10 gallons of fresh premium. Does bad/old gas cause backfiring like this from predetonation or something? But then why would it only be under load? And why does it idle so perfectly?
 

dubs283

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Jul 27, 2005
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Re: Backfiring and wont rev up

the fuel could be an issue, best to remove all old fuel before adding new.

is this a points ignition? if so did you replace the points/condenser, then set the dwell, then set the timing?

did you replace ALL fuel filters?
 

zanderman111

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Re: Backfiring and wont rev up

Yes it is a points ignition. I have not replaced the points or set the dwell. How is that done? What is the dwell anyway? Would this cause my backfiring though?

I inspected the fuel filters but did not replace them. They looked okay to me. I know it is always best to replace, but like I said, it didnt look like it was necessary. I know I probably should replace them, but regardless, a bad fuel filter wouldnt cause backfiring would it? Isnt backfiring too much gas or else gas hitting a hot intake/exhaust and igniting?

Also, I should have noted that the backfiring is coming out through the flame arrestor, not the exhaust. Dont know if this makes the diagnosis any different.
 

dubs283

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Re: Backfiring and wont rev up

if it is coming from the flame arrestor that is an indication of the timing being off, probably being too retarded.

dwell is the amount of time, in degrees of distibutor shaft rotation, that the points are open/closed.

you WILL need a dwell meter to set the dwell once the new points are installed.

download the proper manual for your engine from the adults only sticky at the top of the threads page and follow the instructions explained in there.

set dwell first, then set the timing.

just because the filters look okay does not mean they are good.
 

Sparkscw

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Jul 8, 2009
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Re: Backfiring and wont rev up

I had the exact same symptom's and after adjusting the dwell and then timing she is all better, ( I will add that I also found a major vacumm leak at the carb and fixed that at the same time).
David
 

cr2k

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Mar 19, 2009
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3,730
Re: Backfiring and wont rev up

Pull the flame arrester and have someone pump the throttle and see if there is a squirt of gas going from the center of the carb into both throats. If no squirts it could be your accelerator pump which adds a shot of gas as you accelerate to prevent backfire and allow you to accelerate.

This is all too common in rochester and mercarb carbs on GM based engines.



Weather you rebuild the carb (if this is the problem) or have someone do it, tell them to use which ever spring is stiffer on the accelerator pump, the new one or the old one.

My bet is this is your problem or it could be a bad condenser.
 

zanderman111

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Re: Backfiring and wont rev up

Thanks for all the replies. I will work on getting a dwell meter or at least see if someone I know has one. Looks like that could be my problem.

cr2k, it does squirt gas. Does this mean my carb is ok?
 

Bondo

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Re: Backfiring and wont rev up

Does this mean my carb is ok?

Nope,... I suspect it going Lean, causing the backfiring...
 

jerryjerry05

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May 7, 2008
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Re: Backfiring and wont rev up

WATER!!!! in the gas??? Remove the line to the carb and turn it over into a plastic container and see what you have.If there is water then install a water/fuel seperating filter(do that anyway).J
 

newcdnboater

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May 14, 2008
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Re: Backfiring and wont rev up

I also have an '87 3.0 and after all tune-up to spec was still backfiring through carb like you. Rebuilt the card and performance was much better but still backfiring and dieseling. Had to change from spring loaded needle to solid needle in the carb. Works like a charm now. Good luck.
 

zanderman111

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Mar 15, 2009
Messages
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Re: Backfiring and wont rev up

newcd, its good to see someone else with the same problem. Does the 87 3.0 carb come with a spring loaded needle? And you just changed it out? Where / what kit did you get that had the correct needle?

jerryjerry, I am taking it out today and am going to try and run the engine off an aux gas tank to see if my problem is with gas or not.

Bond-o, two questions: how do I tell for sure if its running lean? and why would running lean cause backfiring?

Thanks!!
 

Bondo

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Re: Backfiring and wont rev up

Bond-o, two questions: how do I tell for sure if its running lean? and why would running lean cause backfiring?

Ayuh,... 1),.. The sparkplugs should tell you a story about it by their Color,+ burn patterns...
2),.. Because a Lean condition Causes backfiring....
 

Bondo

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70,906
Re: Backfiring and wont rev up

Checked plugs and they are sooty black. Certainly not running lean!

Ayuh,... It's a Carbed motor,....
Nothing says it's not Dumping fuel into it at Idle,...hence the Black sooty plugs...
But,...
Still going Lean when throttled up....

Rebuild the Carb....
 
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