VX750 -94 Crank Phasing / Index

M

MaxMaus

New member
Hi,

Needs some help and clarification

Got a -94 750 that runs really bad! It has Bender Quads and a Bender 90 degree box. The number three cylinder continuous misfires with exhaust bangs and even some times on the carb side. Good spark, new plugs, cleaned carbs, reeds work as they should, no air leaks, good equal compression on all four cylinders. I have tried to switch plug caps between nr 3 and 4 cylinder and the problem resist on the 3 cylinder. I´ll guess the only thing left is a faulty crank!?

On the pic number 1 from the clutch side is at TDC and nr 2 at BDC, number 3 is almost up at TDC and nr 4 a bit lower. As far as I get this, in my case the nr 3 and 4 should have been level somewhre between TDC and BDC?

IMG_3119.jpg
 

Sorry for asking similar question regarding crank phasing. Would be very thankful if somebody would clarify?

/M
 
Yes. If 1 is Tdc and 2 is bdc then 3 and 4 should be dead even. Dial indicator can measure this accurately.
 
mine is the same as the pics above....backfires after idle...Must be out of phase.
It ran fine until I had a bad back fire due to low fuel. Now its out of phase!
 
See the big nut sticking out the side in back of the primary clutch? (not clutch retaining bolt) Put #1 at the point the straw stops going up, make a mark on the nut and on the housing next to it, so the marks are next to each other. Put #2 at the point the straw stops going up, make another mark on housing, should be 90 degree from first mark. Then #3 and then#4 making a mark for each. If correct it will be equal between all marks, if its off you'll see right away.

Edit to remove TDC verbage
 
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rustman said:
Yes. If 1 is Tdc and 2 is bdc then 3 and 4 should be dead even. Dial indicator can measure this accurately.
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You cannot see if the crank is at TDC with a dial indicator do to the fact that the piston dwells at TDC for almost 7 degrees of crankshaft rotation.You have to use a piston stop and degree wheel.
 
That is correct, you cant determine TDC without a degree wheel. However.... we dont need to know TDC. Simply the moment the straws stop going up, or start going down. Something that is supposed to be equal between all 4 cylinders. A person could remove the exhaust y-pipes or flanges and make a mark on the stator cover and crank bolt as I described and mark each cylinder when the piston just reached the bottom of the exhaust port. Or have the carbs and reeds out and do the same on the intake side. We are just looking for something that is supposed to be common between all 4. You dont have to split the difference to find TDC just to figure if its out of phase. To know how much its out, sure. I am trying to provide simple methods for people with out special tools to figure out if their crank has slipped.

A dial indicator will tell you when it stops going up, but yes, not tdc in crank degrees.
 
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dansvan said:
Except I have two 92 engines that are out of phase. And find them all over CL. Only reason I've shown an interest in it.
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Well here's one for you.If you have a good crank that is only moved at the gears.I took one of my out of phase engines apart,moved the crank halves to where the looked to be correcct,checked the phasing with a degree wheel,did this a couple times to get it right,and in the end a couple hours later,ended up being out around a half a degree.Then I put my own new index marks on them.I don't mind doing it this way as opposed to pulling off gear and reinstalling it as everything you do that it gets a tad looser on the shaft.All you need is a good degree wheel ,piston stop and dial gauge.Only needing the dial gauge to make sure degree wheel in concentric on crank.Sorry for the long story.I am bored tonight
 
I'm really hoping that's the case with mine as well. I was even wondering if between the two engines I could mix and match crank halves to get it that close. I dont own a TIG but have a few buddies with them, would be nice to stitch it together and not have to send it out for press work.
 
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