R75/6 not picking up

[Major Softie]

So I tried to run the bike this eve, it would only run on one cylinder again. Both carbs attempted to pick up, both together and tried seperately, the slides attempted to rise but bounced as the bike started to stall. I tried to raise the slide manually and turn the butterfly as suggested but this didn't work.

I feel that the carbs are not the problem. I need to set the idle and mixture but it is impossible to do anything at the moment.

So I suppose I'm moving on to the spark/leads/coils. I don't have spares so before I commit to buying these am I barking up the wrong tree? I fitted an electronic ignition, the wiring is pretty basic so I couldn't have messed that up, the timing needs some adjustment though. I've tried it in all sorts of positions whilst it is idling. I did double check at the wiring.

I also put in a reg/rec, again pretty straight forward.

Not really sure what else I can add! All electrics work fine.

Feeling quite disilusioned with the bike if I'm honest.

Your "shotgun approach" is both expensive and far too dependent on luck. Before "fixing" anything else, you need to do some diagnosis so that you can address the problem directly.

The first thing to determine is if it is a spark or fuel problem. The fact that it is running on only one cylinder makes it extremely easy to test for this. Switch the plugs between cylinders. If the problem switches cylinders, it's the plug. If not, switch the plug wires as well. If the problem switches cylinders, it's the ignition. If not, you have eliminated the ignition and can stop worrying about that system.

If you eliminate the electrical, try switching the carbs. If the problem switches sides, it's the carb. If not, it's mechanical (valves, compression, etc.).

[Airbear]

Whitey, what sort of electronic ignition did you fit?
Is it driven by the crank or cam?
Does it use the original advance mechanism or its own electronic advance?
Any chance that you can re-fit the points and get things running properly as God intended?

Sympathy attached. It's frustrating, but you'll be grinning soon.

[Whitey]

So I swapped the plug leads over so the coils were attached to the opposite plug, the same side still wouldn't fire. Used different plugs as well. Is this telling me that it is nothing to do with my coils/leads/ignition system?

What next then? Valve clearances again? Compression test? Bloody Bings again?!

[Major Softie]

So I swapped the plug leads over so the coils were attached to the opposite plug, the same side still wouldn't fire. Used different plugs as well. Is this telling me that it is nothing to do with my coils/leads/ignition system?

If I understand you correctly, no. Did you still have the same plug wire going to each side, and just switched it at the coil? If so, it could still be plug wire or cap. That still would need to be checked. You want to do all the easiest checks first before you get into the carbs.

When that has been done, then if it's still on the same side, as I said in the previous post, I'd switch the carbs to opposite sides and see if the problem switches.

[Ken in Oklahoma]

So I swapped the plug leads over so the coils were attached to the opposite plug, the same side still wouldn't fire. Used different plugs as well. Is this telling me that it is nothing to do with my coils/leads/ignition system?

What next then? Valve clearances again? Compression test? Bloody Bings again?!

If the other cylinder fires well then by swapping sides, you have demonstrated that the ignition is OK, or at least food enough to run the bike. I have a simplistic mantra that I think at times like this. It goes:

If the engine has spark, fuel, and compression it HAS to run. Obviously there are 'holes' in the mantra, but it serves to keep me focused.

At this point I would pull the plug on the dead cylinder, drop about half a teaspoon of gas into the cylinder, replace the plug and plug wire and see if the engine will catch. If the engine catches and then runs the primer out you know that you have a fuel delivery problem to the cylinder. And you now have narrowed the problem to gas flow to the carb and whether the carb is working properly.

I haven't been watching this thread closely, so my apologies if this has already been discussed in some form.

Ken

[Whitey]

Right. Updates.

I swapped the leads, still attached to coils, just on opposite side. No change in cylinder firing.

Next I swapped carbs, after putting leads back over, still no difference in cylinder firing.

Finally, I changed plugs around, three different combinations, no change in cylinder firing.

In my mind I have eliminated carbs, coils, ht leads. I am left with one cylinder failing to fire and neither sides picking up under throttle under any of the above conditions.

Does this now leave compression? A problem with internals?

[Major Softie]

Right. Updates.

I swapped the leads, still attached to coils, just on opposite side. No change in cylinder firing.

Next I swapped carbs, after putting leads back over, still no difference in cylinder firing.

Finally, I changed plugs around, three different combinations, no change in cylinder firing.

In my mind I have eliminated carbs, coils, ht leads. I am left with one cylinder failing to fire and neither sides picking up under throttle under any of the above conditions.

Does this now leave compression? A problem with internals?

That would apparently be the case.


So, a tight valve would be the first possibility, so check valve adjustment. While checking valve adjustment, turn engine over and make sure valves are opening correctly (no flat cam lobe or broken follower/pushrod, etc.). If no problem there, then after making sure valves are correctly adjusted, do compression check - or better: leakdown. If valve adjustment is good and compression is not, pull the jug.

With one cylinder not firing, if switching spark makes no difference, switching carbs makes no difference, valves are properly adjusted and working, and compression is good, I personally would be at a total loss....

[Ken in Oklahoma]

. . . Does this now leave compression? A problem with internals?

A decent indicator of compression would simply be to remove the plug and press your thumb against the plug hole. Then run the starter. If the compression blows your thumb off the plug hole with a bit of a pop you know there is some compression there, enough that that cylinder should be making some noises indicating that the fuel air mixture is burning.

If your thumb is not blown off the hole that means you have problems such as a broken piston or valves that aren't closing properly on time, or are being held off the seat, or might have stems that are bent.

Ken.