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Back fire while decelerating and power loss in fifth gear


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#1 Githes

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Posted 20 March 2017 - 11:50 AM

I have an 86 720 that I have recently replaced the head gasket in. Yesterday, it started to back fire only while down shifting to slow down. I just replaced the O2 sensor this morning and that didn't seem to help. Could this be fixed by replacing the muffler, which I intend on doing tomorrow, or is it something else?

Another problem that has risen is a loss of power when in fifth gear. Are these two problems related or are they two separate issues?

#2 MaxChlan

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Posted 20 March 2017 - 03:22 PM

Have you checked to see if your cat is clogged or getting too hot? I had similar problems when I got my 720 last summer, but I did not really notice it until I yanked my cat last week. I had a lack of power in fifth gear and a weird hiccup when going from a stop. Might be worth checking out!



#3 datzenmike

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Posted 20 March 2017 - 04:17 PM

One wing nut holding the air filter on or do you have the electro - injection with two?? If two then EFI and I haven't a clue. You can still try q/ and b/ below.

 

 

Is this one long rumble of pops? or one sudden loud bang?

 

 

a/ When the engine is cold from over night, re torque all the head bolts to 60 ft lbs. Just snap them loose and immediately torque them. Do not loosen more than one at a time. See if that helps.

 

b/ When the engine is hot reset the valve lash. My '84 FSM says 0.014" hot.


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#4 Githes

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Posted 20 March 2017 - 05:24 PM

I just have the one wing nut. It is a long rumble of pops.

#5 datzenmike

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Posted 20 March 2017 - 07:25 PM

Over on the pass side inner fender are two vacuum switches. Both look the same and would fit in your hand. Both have two wires to them and a vacuum hose. (sorry, don't remember which is witch but diagrammatically it's the front one) One vacuum line comes around to the valve cover side of the carburetor and also to a second (thingie) on the fender. Check them both for cracks, leaks or maybe hose pulled off.

 

High vacuum causes the switch to open and turn the idle cut solenoid off during deceleration. This idle fuel is saved from burning and popping in the exhaust pipe. The other (thingie) beside it is the Boost Control Valve. It also senses excessive intake vacuum during deceleration and opens a valve to let air in and lower it. When the engine slows and vacuum is reduced the switch turns the idle cut back on and the air valve closes. It's all I can think of right now.


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