PTO Cylinder Not Firing

brethren

brethren

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Apr 6, 2012
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164
Age
58
Location
Markham Ontario
This is my first post so excuse me if I make mistakes. I have recently rebuilt a 2003 Viper 700 triple. The reason for the rebuild was a spun crank (PTO cylinder). Ever since the rebuild I can't get the PTO side cylinder to fire. It's running on 2 cylinders and about every 5 seconds the PTO cylinder wakes up with one fire. Rebuild includes the following:


  1. Specked Crank
  2. Rebuilt PTO cylinder
  3. 3 New OEM pistons and rings
  4. New piston pin bearings
  5. New head gasket
  6. Cleaned carbs (including low speed jets with thin strands of wire)
  7. New main jet nylon cups
  8. New NGK sparkplug caps

Compression test looks to be normal at 1: 90psi, 2: 100psi, 3: 100psi (Compression tester not the greatest so values may be off)
Spark seems to be present when the plug is laid on the head
Plugs on cylinders 2 and 3 are nice cream colour, but the PTO cylinder is black.
I have tried to spray fuel into the carb while its running but it doesn't bring it to life, so I don't think its fuel related.
I've checked the reed valves which look good
Ive checked the primary coil resistance tested okay .5 ohms
Ive checked the secondary coil resistance tested okay at 5k ohms and the cap at another 5k ohms for a total of about 10 k ohms

In all my years of working on motors, I've never been stumped like this before. Any ideas? Might be parting it out soon, but I love this sled, it's mint.
 
if you remove the top half of the airbox, and then start up the sled, slightly covering the pto carb with your hand does it start running?

if yes, then you need to look into the carbs or the reeds in that cylinder, if no, then its not a fuel problem.

since the crank was out i will assume the flywheel is not rusty or corroded on the magnets on outside right?...

nest thing is have you ohmed out the spark plug cap? it should read 5000 ohms, those go bad and cause problems like that.

(one last thing you can do....with the sled idling take the pto spark plug cap off with gloves on your hand, and just barely set it back on the plug tip, can you hear the spark jump to the plug and did this make the cylinder pick up and run??
 
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Covering the pto carb doesn't bring the cylinder back to life.
I'll check the outside of the flywheel but I don't remember it being rusty
Spark plug caps are new at 5 k ohms and coil tests at 5 K ohms as well for a total of 10 k ohms from cap to earth.
Removing the pto sparkplug cap does not change the idol, it's definitely running on 2 cylinders, but once and a while the pto cylinder fires one stroke, also I can hear the spark jumping when I pull the plug cap off a bit (I hate that sound, Ive gotten hit before).
I don't want to even say what I'm thinking but the only thing left is the CDI, ouch!
 
with it running and you pulling the spark plug cap up off the plug end leaving a slight gap inside is it a constant zap,zap zap sound if you hold the plug cap just a wee distance from the plug end?

if it is, then its sparking and its doubtfull its the cdi.

somthing else i see when reading your post, you say the plug is black, is it wet/oily and black or dry/fluffy black?

pics of the plugs after its run may help to see whats going on.
 
ok, what you need to check next is this, get a perm. marker and then make a referance line on the engine case on the pto side where the primary clutch is( I make mine in the center like a clock).

Then remove all 3 spark plugs.

Take a long handle phillips head screw driver and turn whatever cylinder you want to start with to TDC, you can guage this by inserting the screwdriver thru the sp. plug hole, turn the engine via the clutch and watch your screwdriver till its at its highest point, mark the screw driver shaft and also the primary clutch down at your mark on the case,I always put a letter next to my line, (p for pto,m for mag,c center). Repeat this for all 3 cylinders. Your mark on screwdriver handle should line up on other cylinders for TDC. Youll also mark the primary clutch with a line in referance to your mark on the case.
When your done you should have 3 lines on your primary clutch and if they are not exactly evenly spaced(measure with a cloth tape) the crank is out of phase!

I listed this way because I dont know if you own a dial indicator or degree wheel but this way works the same and i think thats where your at with what your telling me. I know you just had the crank done and it may be very well it wasnt done right.
 
Oh my God!

As I'm writing this, I'm still in disbelief. The moron that rebuilt the crank lined up the pto and mag lobes together. This would have never crossed my mind in a million years, you're a genius.
 
brethren said:
Oh my God!

As I'm writing this, I'm still in disbelief. The moron that rebuilt the crank lined up the pto and mag lobes together. This would have never crossed my mind in a million years, you're a genius.
Click to expand...

The pto and mag are at TDC at the same time?? Never seen this holy shit man!
 
brethren said:
Oh my God!

As I'm writing this, I'm still in disbelief. The moron that rebuilt the crank lined up the pto and mag lobes together. This would have never crossed my mind in a million years, you're a genius.
Click to expand...


no, I am not a genius, I just have a path to follow to diagnose problems. I asked if it was backfiring and you replied yes, that was the clue for me to ask you to check the phase. I am sorry to have given you the bad news but at least you know whats wrong with it, now comes alot of work again to tear out the crank.

stick with it, it wasnt the sleds fault they really are a good machine. If you need any help just ask.
 
I just verified it by putting the spark plug cable from the mag side to the pto side and for the 1st time since the rebuild pto cylinder fired right up. Wow, this might be a first
 
I got my crank back from the rebuild shop on Tuesday, and everything back together by Thursday. My son and I went for a blurp on Saturday for a 60 km run. The sled is running strong and smooth, and I am glad to have met mrviper 700 who helped me find the problem. I must agree they are nice sleds and definitely one of the smoothest running motors out there. I even cleaned out the bulkhead and with all new gaskets, and exhaust springs, the thing is shiny and dry like new. Nice work mrviper, your my hero!!
 
Thanx for the update and the pm, your welcome for the info, just pay it forward if you see someone you can help and know what the problem is.
Good luck and ride safe!
 
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