Custom/Hybrid BOB backfire

Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
Swap the white wires inside your electrical box. Also check the start stop pigtail that goes into the electrical box. Wiggle the wires
Right at the grommet. If one is looser than the others you got a broken wire. Which may be giving you intermittent connectivity in your start circuit.
 
rubbertoe

rubbertoe

X-H20 certified
Location
San Diego
i did see in your build thread all that gas on the carpet,leads me to believe somehow you had gas in your pipe or crankcase when you started it .weather it sheared the woodruff key is another story you would have to remove your flywheel to check that(if the key shears off and the flywheel rotates out of its position this will throw off your timing causing backfires).but also if you have a leaky needle /seat this could cause gas to enter the crankcase or your pipe which could also cause your motor to backfire.usually if your needle and seat is bad you will see signs of unburned fuel/oil coming out of the exhaust when you start your engine
 
stg5685

stg5685

Location
masshole
rubbertoe said:
i did see in your build thread all that gas on the carpet,leads me to believe somehow you had gas in your pipe or crankcase when you started it .weather it sheared the woodruff key is another story you would have to remove your flywheel to check that(if the key shears off and the flywheel rotates out of its position this will throw off your timing causing backfires).but also if you have a leaky needle /seat this could cause gas to enter the crankcase or your pipe which could also cause your motor to backfire.usually if your needle and seat is bad you will see signs of unburned fuel/oil coming out of the exhaust when you start your engine
Click to expand...

how do i test for leaking needle/seat?
 
rubbertoe

rubbertoe

X-H20 certified
Location
San Diego
visual is my best answer or just change the part .when the ski was running last did you notice it spraying black gunk out the exhaust (bong water looking residue)not just water from your water box, but usually it will spray black sh$% on anything behind your exhaust ,thats a pretty good indication of a leaky needle and seat.also, what did you change from when your motor ran god last?did you add a flywheel ?change it back to stock and see how it runs if you did.and just start one by one process of elimination.
 
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KTM434

KTM434

Jamie FN Hickey
Location
Palm Coast FL
DK-Freestyle said:
Are you sure your plug boots aren't switched? I've seen that happen a few times
Click to expand...

Matt_E already said it doesnt make a difference but to elaborate, the stock charging ignition setup ignites both spark plugs at the same time at every TDC and doesnt know the difference between which cylinder is at TDC so both plugs fire at the same time
I'll buy the BOB if you are too frustrated with it :)
 
Proformance1

Proformance1

Liquid Insanity
Site Supporter
Vendor Account
Location
New York Crew
These are hard to diagnose, but keyway, leaky carbs, and or both now would be where i would start. What motor and carbs are you running? jetting specs? You could also have a weak starter, put a voltmeter on you battery and turn it over. Whats it read while cranking?
 
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E

egbrig

The stock ignition is a "wasted spark" system. It fires both cylinders at the same time. Firing spark on the cylinder on the down stroke doesn't effect anything, unless there is excess fuel puddled in the cases and the fumes from this ignite. Now you have a backfire. As this happens the crank is rotating in one direction until a sudden change of direction (back fire) causes the crank attached to a heavy flywheel to change instataniously. The sacrificial link (woodrow wilson) lol, tries to save the crank snout and flywheel by ripping in half. Now that the woodruff key has done its job the heavy flywheel has rotated to somewhere where its not supposed to be and the ignition timing is set to who know's where. Now once again you hit the starter and boom, back fire again but for a different reason. Check the woodrow wilson key first, then move on.
 
stg5685

stg5685

Location
masshole
pulled my flywheel cover and there were no evidence of a woodruff key being broken or do you have to pull the flywheel to figure that out?
photo (2).jpg
 
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element3488

element3488

Location
Minnesota
If your woodruff key is fine. it might be a stretch to say but try swapping the msd with a good working stock cdi i had a very similar issue when i just built my ski and turned out to be a bad cdi.
 
stg5685

stg5685

Location
masshole
i already bought new msd hopefully should be here thursday trying to get my hands on a stock cdi to test before i open the new one. Anyone know where i can find a wiring diagram for the msd so i know everything is correct and also whats the torque specs on the flywheel nut?
 
stg5685

stg5685

Location
masshole
pulled my ebox apart no wires seem to be broken and as you see in the first picture there is a pink wire from the msd connected to nothing is that right? also the red wire from the start switch is spliced? is it just me or does this msd look mad old?
photo-1.jpgphoto.jpg
 
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