Dropping Cylinder At Idle/low Load - Not The Common Culprits

Hi All

Having an issue withy '97 Monster 750. At idle/low load, the bottom cylinder isn't firing. When you open the throttle, it seems to come back to life. At first I thought it was a fuel issue, so I rebuilt and balanced the carbs but no change. I then measured the various terminals for the coils, ignition modules and crank triggers and got the expected voltages/resistances. I put in new plugs but no change. I thought maybe it was still the ignition modules but swapping them makes no difference. I thought maybe a coil but I've tried two others and no difference. I've also changed the loom that provides power to the ignition modules/coils but that didn't work either. The only thing that makes a difference is removing the HT lead from the plug and placing it back on so that it almost but doesn't quite physically connect. Then the lower cylinder starts kicking back in but not completely smoothly.

The only thing I haven't tried swapping are the crank triggers but if they were dead, why would holding the lead near the plug make a difference?

Any help appreciated as I've been chasing this for a while now and getting fed up with it.

Cheers

 

Thanks. The two other coils had the leads built in, so I guess I'll have to go with crank triggers. I'll order a pair.

Ta

 

Just tried it and didn't see any. It's so weird though, why would pulling the lead and holding it near bring the bottom cylinder to life? I wouldn't expect that to fix a crank triggers issue. I'd have thought the signal would either be strong enough to cause the ignition module to fire the coil or not.

Is there a way to test it other than measuring resistance? When I measured them before, they were basically the same and at the correct resistance (95 ohms from memory?)

 

Hmm, maybe if it's just in the limit of the signal being string enough to trigger the ignition module? Is the trigger adjustable? Does the gap between the sensor and the crank have to be set?

They are iridium plugs that were listed as being for the bike. The top cylinder has no issue. Only changed them once but have pulled and cleaned many times.

I also compression tested the engine and the bottom cylinder was at 150.

 

Righto thanks. That's good to know.

I think the coils have already been changed. I bought some second hand ones to test and they are physically smaller. I also noticed the ones on there don't have part numbers.

Okay cool, I'll read up on how they are set and give it a go. I'm hoping the oil doesn't have to be drained but I doubt I'll be that lucky.

 

Cool, thanks might just order those pickups. Also ordered new ignition modules about 6 weeks ago but royal mail have lost them it seems.

Also just discovered that both my rear indicators were flashing and my brake light isn't working. I can see a resistor has become detached at one end but it's a sealed unit, so I can't get to it without breaking the lens.

Not having much luck with this thing. It's only got 7k miles on it but maybe that's the problem. Machines like to be used

 

I didn't swap them but I did have the upper cylinder coil disconnected and tried it's trigger on the lower cylinder but got nothing. It's setup so the trigger with black and yellow wires is going to the upper cylinder.

When you open the throttle, it runs well. It's only at idle and steady cruising at low throttle openings that it struggles.

 

I originally thought fuel as well, or perhaps air leak but I wasn't able to find one. If you run the bike and pull the plug, it comes out damp and smelling of fuel. Also putting carb cleaner down it when running doesn't seem to change anything.

I forgot but I've previously put a timing light on the lead and observed it dropping. I've ordered a pickup set and will get it installed. I've seen online that the magnets can come loose and move.

Thinking about how these sensors work, they generate current when the crank trigger moves past the magnet, creating a spike. Modern sensors have a power supply and create a square wave so long as the sensor is in range of the trigger. I wonder if it's misadjusted or has an issue where at low RPM, it's not producing enough signal but does once it speeds up.

 

Righto, fair enough. Hopefully when I open it up, I'll see something visible on that sensor then.

 

It's low milage but definitely not untampered. The loom is a bit of a mess, though I've been through it and couldn't find anything that would cause this. I tried swapping out the loom that provides power to the coils but it didn't help. I don't see how it could be any of the other wiring as the coil harness just splits 12v, so an issue up the chain would cause both plugs to miss. The only thing I haven't tested (other that measuring resistance) is the pickups, so really hoping it's that, even if it means splitting the case.

The bike came with no history, my dad surprised me with it because he threw a low ball bid in and won. I'm beginning to suspect it's low milage because this issue has been present for a while.

 

I believe it's adjustable. I've got some on order as well.

 

The bike came with this issue but unfortunately the cold was enough to finish off the fork seals, so it will be going in for that.

 

Pickups changed over last night but didn't test it as waiting for the RTV to dry. Did they use grey RTV from the factory or a gasket?

Only visible issue on the old ones was the wiring at the plug end, one came apart when I was de-oinning it. They were around .61 and .63mm from the step on the crank. I put the new ones at about .68 but can be difficult to determine as the pickups flex a bit.

Will hopefully test tonight but the old sump plug washer wasn't re-usable and the biggest aluminium one I have is 22mm ID. Will try massage that to size as it's booked in to the garage on Sat.

 

Urgh, not sure where to go from here. The bike is barely running now. I put a timing light on it and it might be a few degrees out but I wouldn't think that would be enough to cause this? It's idling super low and opening the throttle doesn't pick it up at all. It just causes it to start missing on both cylinders. I think the initial problem might have been one dodgy plug and then one cylinder didn't like the iridium, which made me think nothing had changed. I put the stock plug back in the bottom cylinder and it started firing. Putting the other plug back into the top cylinder results in an intermittent miss.

When I installed the new pickups, I left the bracket in place and just changed them on the bike. I was pretty careful to align them where they were. I guess I'm going to have to take it apart and try again? Is there a marking anywhere on the engine that lets me know when I'm at TDC?

 

Okay, had it apart again and redone it. This time I got a dial gauge down the cylinder and I didn't have a degree wheel to measure the dwell, so used and angle torque gauge. If I got it where I think it is, it should be pretty smack on or maybe a degree advanced.

It needed quite a bit of adjustment, not sure how since I lined up the dots on the old ones and then replaced the pickups one at a time. I wouldn't recommend trying this without some way to measure the dwell at TDC, it's something like 3-4 degrees. Makes a massive difference when trying to line it up.

I also tightened up the clearance to the pickups. Should be roughly where it was before or slightly tighter.

 

It's around .61, so in spec. It was around .63 when I took it apart originally.