Hi All I was lucky enough to be given a Monster 750 by my dad, the downside is that it was running really badly and now has no spark. I've ridden the bike once and noticed that at cruise it seemed to miss and wouldn't idle without choke. I put it away and it then leaked fuel all over the floor from the overflow. I decided that the carbs were the issue so pulled, cleaned and mostly rebuilt them. They got new idle mixture needles as well as float needles. I set the idle mixture to 3 turns out and it fired right up though still wasn't running particularly well. This evening, I warmed it up and set the balance (was only slightly out) as well as upping the idle. At random, the RPM kept going up, holding there and then dropping back down. Blipping the throttle would also take a while to settle to idle (It's not sticking). I shut the bike off and put the filter/airbox lid on and then it wouldn't start. Pulled the plugs and neither of them are firing (I ground one to the engine and the other to the battery negative lead at the frame end.) Also, the horizontal cylinder plug was quite wet but not the vertical. Could it be that my RPM fluctuations etc was the bottom cylinder cutting in and out? What should I check? I'm not new to mechanics but I've never worked a bike before. Any help appreciated.
The standard Kokusan ignition system never gave a decent spark compared to the average car of the same year, so worth testing in pitch black conditions because it could be feeble rather than non-existent, particularly as you've had it running recently albeit badly. I'm sure I don't have to tell you, that all connectors involved with ignition circuit are worth checking thoroughly, and then (assuming plugs are ok (I use NGK D8EA)) you could work your way through the various suspects and if you get any visible spark on either cylinder's system, then you could do trial-and-error swaps of coil/coil lead(only if later black, slender type - earlier dumpy grey version has integral lead, but going by year, yours must be later type as std.) and "exciter" unit (below) to try and find the culprit(s). If no joy with these then you might have to delve into the crankcase* to check the pick-up units (below) for either breaking down, or connector wires for same either loose or shorting out as all 3 are possible. *you can test these externally before removing crankcase cap, I haven't got figures to hand, but can add tomorrow if nobody else does. Electrex world do an aftermarket unit which is reliable and (at last look) a fair price.:- https://www.electrexworld.co.uk/acatalog/P8.html and I forgot to add:- what a fine, underrated vehicle you have Sir!
Thanks for your reply, much appreciated. I believe it to be no spark as I also shot some brake cleaner down the carbs and it didn't even attempt to fire, though I'll take your suggestion and try it in the dark. Is there a single component that's required for both plugs somewhere? I'll have to do some reading on how the ignition works as my experience is with ECUs. I have the service manual but it didn't have much in there for troubleshooting. I had a look at all the electrical connections I could find but didn't spot anything. The killswitch is in run and when it's not, the bike doesn't crank. It also starts with the side stand down but I've tried it up and still nothing. Thanks, I do like it from the one ride I've had. Feels light.
you are definitely right to get on the Killswitch systems, because if you definitely have no spark, that’s a real possibility isn’t it? It’s too much of a coincidence for both systems to give up the ghost, even with respect to the pick ups in the crankcase.
How does the kill switch work? Does it go to a master relay or multiple components? With the switch off the bike won't crank but it does when on, so I believe the switch itself is okay. Ta
Not at my house and just spent half an hour searching for a diagram for you, something which popped up on the net easily 10 years ago when people weren't so greedy! - might even be a "sticky" on here already. My memory is no help but it's a simple circuit, I expect that you have a meter - will search a bit more - UKMOC must have one..
Blimey, can't believe it, a generous source, better visual and written detail than I have :- https://drive.google.com/file/d/1I5S0fPKwV3EsV_0q6flDwoBzo-bBx62U/view Page 200
Thanks, I appreciate your efforts. I do have a multimeter. I'm going to spend some time learning how it works but all help is appreciated. It's possible but not likely that both circuits failed simultaneously. I would imagine the place to start is a single component responsible for both. I remembered that this happened the first time I rode it. Cranked for ages but no start. That time though, I went and got some brake cleaner, a little squirt in the carbs and it fired right up, so I put it down to fuel. I'm thinking I have an intermittent problem with spark now though. Ta
Page 200 in above link Cammmy, needs rotating 90 deg though! - English translation of items further along manual.
Okay, so as far as I can see from the diagram and a parts manual, the ignition is completely separate on each side from the crank sensor to the plug? It also seems all the ignition components share a common earth, so if I test for voltage at the CDI units, that should rule that out. Also, as an aside, there's no input to the CDI units for manifold pressure? So the ignition is basically tuned for WOT at all times? Seems like that's leaving a bunch of economy and part throttle performance on the table.
yes, as per diagram, two separate circuits, the exciter unit, BB1105, at a guess, is pretty crude, yes, by today’s standards. there was a sports version with a different curve, but not worth pursuing.
Posting as I go along, I don't believe it's the killswitch as it provides 12v to both the ignition and starter relay. From the diagram, it doesn't seem possible for it to break in a way that would allow the starter motor to work without also providing current to the ignition.
Okay, so, mixed results! Still no spark on both cylinders when I got home, I took everything apart and found battery voltage everywhere I expected it. Checked resistance on the coils, both 4.5 ohms, checked continuity between ground and the other two pins on the CDI unit, apparently if there's any, it's dead. One was fine and the other is flickering. I also checked resistance between coil terminals and plug end of the lead (apparently you should see 10k or more), mine both had no reading. Cleaned all the terminals and crushed them a little for a tighter fit, put it all back together and tested anyway, voilà. Two big fat sparks. My guess is that it was simply dirty/loose connections on the coils/CDI units, though the continuity reading on the one has me concerned it's on the way out. It does have a big old air bubble in the potting that would have likely let moisture in over the years. May get a pair of those to be safe. Anyhow, not going to fire it up tonight as I've run it the last two and it's extremely loud thanks to the exhaust it came with. Will run it up to temp tomorrow and see if I can dial the fuelling with the Gunson Co meter I just bought. They have mixed reviews but the positive ones say that if you follow the results exactly, you can get decent results. Will report back. Also, pic attached. It's got less than 7k miles on the clock but I think it's either suffered for sitting or not had the most loving owners in the past. Thanks for your help, hopefully someone else in the same situation will find it in the future.
Andy did a nice video on a bad Crank pickup here: Might be useful. What a pretty bike btw... Really like that
Ah nice, thanks. I'll check that out. It fired up again, I put a timing light on it and could see some misses that coincided with backfires a few times. It's also still hanging when blipping the throttle. Could be carbs but I want to be confident about ignition first. Have ordered a pair of new CDI units as well as plugs. Will see how it goes.
If it's hanging I'd suspect an air leak. Do those engines have somewhere to put vacuum gauges? Could be leaking there. One if my engines just has some rubber bungs over the vacuum ports as standard. Rubber perishes and voila, throttle is hanging.
The vacuum ports on the manifold are screw in. I do think an air leak is a possibility but I'm looking at ignition currently as when I pulled the plugs, one was wet and the other wasn't. I've just rebuilt the carbs with new seals and idle mixture/float needles. So either one carb is dumping fuel in or my thought is that there's an issue in one of the ignition systems, so a cylinder is perhaps dieing off at idle. If It's not ignition, I may have to take it somewhere as my experience is with EFI. Edit: Just had another tinker with it and I'm fairly certain it's ignition. It randomly idles really high and then drops down and stalls. When It was idling low, pulling the top plug lead caused it do die, pulling the lower plug lead (with the wet plug) made no change. I think one of the CDI modules is kaput. Which would make sense as I got funky measurements from one of them. I guess it could also be the coil but I think that's less likely. I also measured the pickups and they both came back around 95 ohms. I would try swapping the CDI units around but as I already have new ones on the way, I don't want to piss of the neighbours constantly starting the bike and having It rev up and down. They're actually pretty cool about all the cars. There could well be an issue with the carbs or also an air leak, but one problem at a time X-D
