Help! back fire and kick back, my XS650 doesn't start!

[YUE]

Hi legends, I have a 78 XS650, I was riding the XS for a while but it started white smoking from exhaust, and it is gradually getting hard to start the engine. I had backfire from carb and kick back badly.

So I decided to install new VM34 carb and rebore the cylinder with brand new 2nd over sized pistons, resize valves professionally, but the xs still does kickback and backfire and it doesn't start now.

My XS650 got Boyer ignition kit, brand new PMA system with capacitor, VM34 carb, and kick only.

I tried to use stock carb, installed battery instead of capacitor, but it still does same thing. I can see good sparks from spark plugs, they are both new spark plugs.

I tried advance and late timing, readjust the tappet clearance for valves as well.

I am very sad that she doesn't start, and I have been investing a lot of time and good money on it.

Please advice me.

 

[Signal]

Others will chime in but here is my initial thought.
Boyers are sensitive to low voltage that in turn upsets the timing. Check that the battery you used has a full charge and is holding its voltage under load.
As an aside I suspect a Boyer will never run properly with a capacitor in stead of a battery.

 

[5twins]

Kick back usually indicates the timing is off. That could be because it's set wrong or might be because of what Signal said. Mmmmm, that very expensive PMA Kool-Aid may have claimed another victim, lol.

 

[fredintoon]

Hi legends, I have a 78 XS650, I was riding the XS for a while but it started white smoking from exhaust, and it is gradually getting hard to start the engine. I had backfire from carb and kick back badly.
So I decided to install new VM34 carb and rebore the cylinder with brand new 2nd over sized pistons, resize valves professionally, but the xs still does kickback and backfire and it doesn't start now.
My XS650 got Boyer ignition kit, brand new PMA system with capacitor, VM34 carb, and kick only.
I tried to use stock carb, installed battery instead of capacitor, but it still does same thing. I can see good sparks from spark plugs, they are both new spark plugs.
I tried advance and late timing, readjust the tappet clearance for valves as well.
I am very sad that she doesn't start, and I have been investing a lot of time and good money on it.
Please advice me.

Hi YUE and welcome,
a rebore & valve job may or may not have been necessary but the only way having those things done could prevent the engine from starting
is if the camshaft was re-installed wrongly. And if it ran with the stock carbs before the rebore it should run afterwards, too.
A thing to check. Does the Boyer ignition use two coils? Is it possible that they are firing at just before the WRONG Top Dead Centre?
And while a Boyer ignition could work just fine installing a PMA is, like 5T sez "Drinking the expensive Koolade."
Is it possible to re-install your bike's stock charging system and sell the PMA to someone who's build needs could justify it's use?

 

[retiredgentleman]

Wow.......................so much money spent on PMAs and capacitors, and all they get is grief, and bikes that don't run.
The wild voltage fluctuations of PMAs and capacitors has killed more than a few ignition systems.
If only there was a better system.........................oh there is one, the one that the bike had when it left the factory.

 

[Hyde]

The cool-aid is tasty damnit! No one said it was gonna decide to come out on the side of the road

 

[YUE]

Others will chime in but here is my initial thought.
Boyers are sensitive to low voltage that in turn upsets the timing. Check that the battery you used has a full charge and is holding its voltage under load.
As an aside I suspect a Boyer will never run properly with a capacitor in stead of a battery.

Hi Signal, thank you for the replay.
I had tested using a stock full charged battery instead of using the capacitor before, but it still does the same thing.....

 

[YUE]

Kick back usually indicates the timing is off. That could be because it's set wrong or might be because of what Signal said. Mmmmm, that very expensive PMA Kool-Aid may have claimed another victim, lol.

Hi 5twins,
I bought a timing light to make sure the ignition timing is right, I haven't done since top end rebuild, cause it doesn't start. I will try to start and set the timing with timing light again.... Thank you for remind me

 

[YUE]

Hi YUE and welcome,
a rebore & valve job may or may not have been necessary but the only way having those things done could prevent the engine from starting
is if the camshaft was re-installed wrongly. And if it ran with the stock carbs before the rebore it should run afterwards, too.
A thing to check. Does the Boyer ignition use two coils? Is it possible that they are firing at just before the WRONG Top Dead Centre?
And while a Boyer ignition could work just fine installing a PMA is, like 5T sez "Drinking the expensive Koolade."
Is it possible to re-install your bike's stock charging system and sell the PMA to someone who's build needs could justify it's use?

Hi fredintoon,

I did rebore and valve job because there were white smoking from the engine, it wasn't from fuel, it was from oil. I measure the cylinder inner diameter with a motor engineer, and he said that the engine is needed rebore. I did not really think that the oil barn is the issue of the kickback and backfire, but I couldn't think anything else and I had to get it done anyway.....

"the camshaft was re-installed wrongly"
I believe that I installed camshaft correctly, I followed service manual and check the alignment marks and TDC mark properly.

"if it ran with the stock carbs before the rebore it should run afterwards"
I could start the engine after many kickbacks and backfire from carbs before the engine work is done, I might be able to start as before, but the main problem is not solved, Basically this XS was running perfectly with PMA, Boyer ignition with capacitor before started doing kickback....

"Does the Boyer ignition use two coils?"
There is only one coil.

"Is it possible that they are firing at just before the WRONG Top Dead Centre?"
I checked with stock alternator flywheel, because the PMA's magnet wheel doesn't have any mark of TDC. I marked as TDC when I reassembled the engine. I checked TDC without cylinder head with my eyes as well.

" Boyer ignition could work just fine installing a PMA is, like 5T sez "Drinking the expensive Koolade."
Is it possible to re-install your bike's stock charging system and sell the PMA to someone who's build needs could justify it's use?"

I can, but my understand is that it still should start normally with a stock full charged battery even the PMA is not working. Sorry I could be wrong, but I had many old bikes, and it started fine with a battery when recharging system was not working, It was just not recharging to the battery, and slowly dying.

Thank you very much for many tips fredintoon, I will check what you said again.

 

[hooser]

How are you vm34 set up? Needle , needle jet, slide (2 or 2.5) main and Pilot size.

 

[YUE]

Wow.......................so much money spent on PMAs and capacitors, and all they get is grief, and bikes that don't run.
The wild voltage fluctuations of PMAs and capacitors has killed more than a few ignition systems.
If only there was a better system.........................oh there is one, the one that the bike had when it left the factory.

Hi Retiredgentleman!

Ummm.... My knowledge could be the issue and hopefully it is, I don't really want to spend another money to get it back to stock....
I really like the XS as well...

 

[U16]

How are you vm34 set up? Needle , needle jet, slide (2 or 2.5) main and Pilot size.

Hi Hooser,

I bought the VM carbs from mikunioz as xs650 conversion kit,

Needle - 6DH4

Needlejet - 159 0-0

Slide 2.5

Main 220

Pilot 25

These are my current set up, I basically followed the instruction which came with the kit.

Thanks

 

[YUE]

Sorry Hooser,

Somehow I logged in to my old account which I created years ago, but u16 is YUE, haha

 

[hooser]

They seem way of to me, here in the states this is what works for most stock apps. but Australia ?
159-P5
6dp1
25-30 pilot
165-185 main.

 

[YUE]

They seem way of to me, here in the states this is what works for most stock apps. but Australia ?
159-P5
6dp1
25-30 pilot
165-185 main.

Thank Hooser,

Are you running with pods filter? I have no idea of needle and needle jets set up, is your set up is more lean?
I can see obviously the main size is way too big against your set up....
I will try to get 185 main.

 

[BobbedSled]

Not that carb size would cause it but
Single carb? Though most used VM36, I do with pamco and it starts right up kick start.

My bike kicked back lile a mule, dangersousy so. I retared timing with the advance plate on the pamco problem solved.

Try retarding timing way back and see if that helps at all and may point directionally where to go next.

Also the advance spring on the kickstart side can stretch and cause premature advance. I replaced mine.

 

[hooser]

Thank Hooser,

Are you running with pods filter? I have no idea of needle and needle jets set up, is your set up is more lean?
I can see obviously the main size is way too big against your set up....
I will try to get 185 main.

The 0-0 is 8 leaner then the P5 that use and the needle is just one size larger the my 6dp1, the one real main difference is on the bottom end when the needle starts to lift. What is your elevation (sea level down there)? A real lean condition will cause popping back... that is what you have with the needle jet . at least if it was here in the states.