1988 Vmax not firing on all cylinders

[scicluna]

Hi guys, hoping someone can help me with my 88 vmax not firing on all cyclinders? A week ago it was hesitating (breaking down) under load and if you held the trottle open it would clear and accelerate normally? Now it's running on roughly 2 1/2 cyclinders, the front 2 are normal, the rear Lh is doing nothing even though you can hear a spark jump when you pull the plug cap and the same with the rear rh although it's slightly better than the Lh.
Compression on all 4 cylinders is 150 psi, replaced the plug which was wet as though the spark wasn't strong enough to ignite the mixture, the leads and caps are new, tried a coil from the front cylinders which was working fine and that made no difference to the Lh rear cylinder.
Had re-kitted the carbies about a year ago, checked the lh rear carbie fuel bowl and found it clean, blew out all the jets on all the carbies, drained tank, replaced fuel and filter.
Now all of a sudden the lh rear is firing normally again and the rh front is now misfiring how the Lh rear was and also backfiring through the carbie, badly enough to see flame and nearly pop the carbie out of it's rubber mount boot? The Rh rear is still the same?
Had a similar problem a year ago, where it would only run on three cylinders, but the miss would change to different cylinders when it felt like it? It seemed to clear up when I used a lower octane fuel? Which was probably a coincidence.
Also the charge rate is about 14volts under acceleration.
Could the ignition unit or pressure sensor be faulty and causing the misfire and the rotation of the miss from cylinder to cylinder? Hope to get some answers, this is doing my head in? Thanks

 

[Miles Long]

Re: 1988 Vmax not firing on all cyclinders

Hi guys, hoping someone can help me with my 88 vmax not firing on all cyclinders? A week ago it was hesitating (breaking down) under load and if you held the trottle open it would clear and accelerate normally? Now it's running on roughly 2 1/2 cyclinders, the front 2 are normal, the rear Lh is doing nothing even though you can hear a spark jump when you pull the plug cap and the same with the rear rh although it's slightly better than the Lh.
Compression on all 4 cylinders is 150 psi, replaced the plug which was wet as though the spark wasn't strong enough to ignite the mixture, the leads and caps are new, tried a coil from the front cylinders which was working fine and that made no difference to the Lh rear cylinder.
Had re-kitted the carbies about a year ago, checked the lh rear carbie fuel bowl and found it clean, blew out all the jets on all the carbies, drained tank, replaced fuel and filter.
Now all of a sudden the lh rear is firing normally again and the rh front is now misfiring how the Lh rear was and also backfiring through the carbie, badly enough to see flame and nearly pop the carbie out of it's rubber mount boot? The Rh rear is still the same?
Had a similar problem a year ago, where it would only run on three cylinders, but the miss would change to different cylinders when it felt like it? It seemed to clear up when I used a lower octane fuel? Which was probably a coincidence.
Also the charge rate is about 14volts under acceleration.
Could the ignition unit or pressure sensor be faulty and causing the misfire and the rotation of the miss from cylinder to cylinder? Hope to get some answers, this is doing my head in? Thanks

Could certainly be the ignition unit, but start troubleshooting by checking the simple things.
With the key on, check for battery voltage upstream and downstream of the ignition fuse, clean the fuse terminals, replace the fuse with a known good one.
Check the condition and operation of the engine stop switch. With the switch closed, should be full battery voltage on the downstream side.
Check the resistance of the pickup coils (you have two on your bike, later model years have only one)
If you don't have a service manual, details can be provided.
By the way - when your bike is "misbehavin" - do you notice any other problems , such as gauges not working, lights dimming, etc.?
Cheers!

 

[scicluna]

Re: 1988 Vmax not firing on all cyclinders

Thanks for the reply miles long, everyting works well besides the miss, all the gauges and lights work, no dimming.
Replaced the pickup coils last time I had the problem, they were secondhand so will do the resistance check again.
Will check for any voltage drop, thanks again will post again once i find something or need to source something.

 

[Fire-medic]

Re: 1988 Vmax not firing on all cyclinders

My experience w/the pickup coils is that they (or it) will begin to work intermittently and replacement solves the problem. By 'intermittently,' the bike would start and run fine for a few minutes, and then the engine would shut off like you hit the kill button-no 3-or-2 cyl cutting in & out, just stop. The engine would turn over fine, but it was like the plug caps were off the spark plugs. No popping, no backfiring, plenty of cranking, but not a hint of running. Let it cool-down for a 1/2 hour, and it would start perfectly. Evidently there was some sort of short which would occur once operating temp was reached, and once it went 'open,' no signal to the plugs from the ignition box. That was where the pickup coil replacement solved things.

From the service manual: 110 ohms +-10% @ 68*F w/the pickup coil lead disconnected from the wire harness to the rest of the bike.

 

[frank5079]

Re: 1988 Vmax not firing on all cyclinders

I had a similar problem with my bike a few weeks back, up until I did the coil on plug swap....now, no issues.

 

[wildweasel_pt]

Re: 1988 Vmax not firing on all cyclinders

Also check plug gap and plug condition (may be isolated due extra crud deposit) as you say its firing but the spark may not be enough to ignite the compressed mixture. You can trouble shoot the pickup and the TCI circuit by grounding all plugs to frame and seeing them spark (be careful with the fingers not to be zapped). Make sure you have the proper leads going to the proper cylinders (sometimes an inadvertent swap may happen when messing with those components). Good luck.

 

[scicluna]

Re: 1988 Vmax not firing on all cyclinders

Have done the coil to plug swap, no different, leads are a year old and have checked for corrosion and found none?
No corrosion on either end of the leads and they are fully home into the coils and the end caps are screwed tight also?
When the plugs are earthed while the bike is running, they are sparking?
Can the TCI affect the strength of the spark and timing (due to the backfiring through the carbie) if it is on the way out?

 

[blaxmax]

Re: 1988 Vmax not firing on all cyclinders

Any solution?

 

[scicluna]

Re: 1988 Vmax not firing on all cyclinders

Everything checked out fine in regards to plugs, coils, pressure sensor, pickup coils. What I did find was a voltage drop of 1.3v from the battery to the igniter unit, seems excessive to me? I started disconnecting connectors checking for corrosion, found some dodgey corroded wiring between the shunt to the regulator and the shunt to the main fuse? Have cut that out and will be replacing it, also found a lot of corrosion on the side stand relay and plug. This could also could be a cause as it is linked to the ignitor unit, will be checking for continuity and corrosion throughout the loom. will contact CO cleaner remove corrosion from wiring? Sorry it took so long to get back to you guys, could find my original post lol!

 

[scicluna]

Replaced some crappy wiring, cleaned up a bit of corrosion! Still the same, getting pretty frustrated! I have pretty much checked everything twice except replacing the CDI, may have to bite the bullet and take a chance? :bang head:

 

[alorio1]

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