K75 Backfiring and losing power on acceleration

[adrian_a]

first time posting, hope someone out there can advise.   I've recently replaced the engine, an alledgedly low mileage K75s engine into a 1991 K75RT.   
It ran fine for  200 miles or so but it's now developed an interesting back fire and a random loss of power + missing under acceleration.    I've changed the HT leads and the coils to no effect.    Plugs look a bit too clean i.e no brown at all . Sound familiar to anyone ?   TPS switch clicks OK .  I've cleaned the tank out and new fuel has gone in.   Fuel pump runs and sounds fine.
 Any suggestions ....maybe Hall sensor ?

[The Mighty Gryphon]

When you replaced the engine did it have the airbox and throttle bodies attached, or did you swap them over from the old engine. 

I ask because it sounds like you have air leaks around the throttle bodies.

[Laitch]

Fuel pump runs and sounds fine.

The condition of an operating fuel pump is determined by its fuel delivery pressure rate measured at pounds per square inch (psi) by attaching a fuel pressure gauge somewhere into the output line from the tank. That measurement can also help indicate whether the fuel return line is clogged or the fuel pressure regulator is faulty.

[rbm]

Backfires can be caused by air leaks in the exhaust.  Leaks occur at the point where the headers attach to the block, or at the joint between the headers and the muffler. Check these areas for leaks.  Did you replace the one-time-use copper exhaust gaskets?  Did you replace the mica gasket between the headers and muffler?  Are there any after market exhaust parts on the bike?

[adrian_a]

When you replaced the engine did it have the airbox and throttle bodies attached, or did you swap them over from the old engine. 


Swapped them over from the old engine.... first timer i'm afraid, so very possibly the issue - thanks

[adrian_a]

Did you replace the one-time-use copper exhaust gaskets?  Did you replace the mica gasket between the headers and muffler?  Are there any after market exhaust parts on the bike?

No aftermarket stuff.    New copper gaskets were used. Though on the headers and muffler, mica gasket ...not sure will check.    It was done some months back.   Thanks for all the useful suggestions. 

[Martin]

Back firing on overrun can be caused by incorrect TPS adjustment, throttle body and intake air leaks, incorrect injector spray pattern on one or more injector, not fully closing the throttle completely on overrun, leaking or missing Pixie Dust gasket (mica gasket) ,  or incorrect mixture. The mixture can be checked by pressing the green starter button while idling at or above 1000 RPM the revs should stay the same or rise slightly they should not rise significantly or fall. If it's not right it can be adjusted using either an exhaust gas analyser or the lean drop method.
Regards Martin.

[Steviescumbag]

Had an issue similar to this. Maybe you have a cracked crank vent tube . But what did the trick for me was a new fuel filter. I installed a Napa Gold

[adrian_a]

thanks for the suggsetions , after all that a brand new fuel pump fixed it.   Must have been the fuel pressure

[Laitch]

Thanks for the update, adrian_a.