V30 Magna Missfire and exhaust mod, advice sought.

[EnglishMagna]

For some reason my 1984 V30 is only running on 3 cylinders and I can't find out why. The offending cylinder has a good spark at the plug and it is getting fuel into the cylinder but for some reason it won't fire. The rocker on that cylinder is a tad worn so I get a little bit of tappet rattle from it but I can't see as it's that worn it's not opening the valves. So I was wondering if any other users have had a similar problem. I thought perhaps the diaphragm in the carb was holed but I guess even then it should still tick over on that cylinder. Also I think I have asked this before but not got a reply so I'll try for advice once more. If I was to carry out the exhaust mod do I need to re jet the carb's and if so to what jet size.

I look forward to any replies but in the mean time I'm going to close up my valve clearances and see if that gets it firing again.

[TLRam1]

Have you checked compression on your cylinders?

Are you sure your getting fuel And a hot enough spark?

I am not familiar with your ignition but could the timing be off?

[dgc67]

How do you know it is only running on 3 cylinders?

[EnglishMagna]

How do you know it is only running on 3 cylinders?

Because when I take the plug lead off that cylinder it makes no difference to the sound or performance of the engine. When I remove any of the other 3 and the engine is ticking over there is a noticeable change in engine sound and behaviour as the cylinder stops firing. I get non of this when I take the lead of the faulty cylinder, couple this with the fact that overall performance is poor and also the plugs on the 3 good cylinders are a nice biscuit brown colour as they should be and the plug from the offending pot still looks new with no sign of soot carbon etc and when I take it out it's always wet with petrol.

Have you checked compression on your cylinders?

Are you sure your getting fuel And a hot enough spark?

I am not familiar with your ignition but could the timing be off?

I've not been able to get my hands on a compression tester yet. It's getting fuel as when I take the plug out it is wet with petrol. There is also a good spark when I rest the plug on the engine and crank it over. However I'm wondering if the coil or something is breaking down when in use so I'm just in the process of swapping the coils for each other to see if this moves the problem to another cylinder. However when doing so I have just noticed that of the primary wires that run to the coils there is black and white one that runs to each coil (an earth I presume) but they run to different terminals on the coils. I would have thought they should run to the same terminal on each coil but that isn't the case on my bike. I wonder if anyone could tell me if this is correct or not.

[TLRam1]

Do you have the service manual? If not, under the "FAQ Section" of this forum their is a link to a page of various manuals. Look on the schematic.

If you can post the schematic we might be able to help. If the plug is wet, sounds like spark, timing, coil, compression, don't know if timing is individually set or not, but if it has not been touched you wouldn't think that would be the case. Back to compression, maybe a stuck ring or valve. 

[dgc67]

Just a shade tree test here, but take all the spark plug wires off and pull that one plug.  Put your thumb (if you can) over the hole tightly and have someone bump the engine over or better is to have it in gear and have some slow spin the back wheel to turn the engine over slowly.  It should force your thumb off.  you can try it on each one and compare the "feel".  I know this is far from accurate, but it is a start to see if compression might be the culprit.

Not sure how the coils are laid out on this model but if there is anyway at all to switch the coils around and see if that makes any changes might help also.  Just because you see a spark does not mean it is hot enough to combust.

Just ideas.  Hope it helps and good luck!

[EnglishMagna]

Thanks to all who have come up with help so far it really is apriciated..........

I have a clymer manual complete with the schematic, but its not that detailed it just shows the wires going to a box marked coil no sign of what terminals it they go to. The coils them selves do not have any +/- markings in fact there is no markings of any kind. I've swapped the wires around just to see what happens and it appears to have a lot more torque however it is still misfiring. I'll get it sorted my main reason for posting was just to ask if anyone else has had a similar problem. But as that's not the case I'll just keep working through all possible solutions. First thing tomorrow I'm going to stick some new plug leads and caps on as the current ones are the originals and at 24 years old they aren't in the best condition.

Now if anyone could give me advice as to if I need to rejet my carbs if I do the exhaust mod it really would be a great help.

[Greg Cothern]

If the bike has sat for a period of time not being ridden it is very likely that the offending cylinder is not running due to a clogged slow speed idle jet in the carb.
Fire it up, and rev it over 3500 and see if it doesnt come to life....  If so you can either use some sort of fuel system cleaner or pull the offending idle jet and clean.