Ignition timing, woodruff keys

[xwhatsit]

Hi,

My engine has recently toasted itself to about as much performance as a GN125, and is massively overheating. I did a compression check, it made 170psi, which is quite good. So that's ruled a burnt/bent exhaust valve out.

I'm working on borrowing an ignition light to check timing. All evidence points to this being the problem, so I have some questions already.

My engine is a CB250RS engine -- same as pre-RFVC XL250/XR250 lumps. It has CDI ignition, but uses a centrifugal advance unit.

My question is -- how would the ignition timing go out? Ixion suggested a broken woodruff key. I was happy with that explanation, but after examining parts fiches and my Haynes, I'm left slightly confused. The woodruff key fits into the crankshaft on the flywheel/alternator side (left-hand case). The ignition pickup and spark advancer is on the clutch side (right-hand case). If the woodruff key is indeed toast (easy fix, less than $10, right?), how is it affecting ignition timing? As they are on opposite sides of the crankshaft.

If it's not that, then what else could be causing the ignition timing to be out on an electronic ignition motorcycle? There's no provision for adjustment.

I'm not sure whether it's over-advanced, or over-retarded. It would seem that once you get excessively out of time, the symptoms are fairly similar for either. I have a suspicion the spark is massively retarded, but that's kind of irrelevant.

Thanks for answering my questions.

[Ocean1]

Can't help with the timing thing dude, did have XL's but they were pre-CDI.

Not sure if retarded timing would cause the symptoms you've given. Down on power sure, but I wouldn't expect it to run that hot as a result.

Sounds more like it might be running way lean to me, that would cause both symptoms. If that' possible or looks likely strip the carb and give everything a damn good clean. Check the float valve isn't sticking, set the float level and try again.

[xwhatsit]

Cheers. I did think retarded ignition wouldn't be a problem at first, but after looking around it seems that really retarded ignition can cause overheating too -- something to do with valve overlap or lack of it or something. No idea, really.

Advanced ignition is still a possibility regardless.

I'll check out the carb. It was left out in the rain overnight, and I think the morning after I first really noticed these problems, but the night before it was already overheating something chronic and running crap. Water in the carb causing lean running? I'll take a look anyway. I've got a spare carb, but it will be jetted/needled very rich. Worth a try.

[paturoa]

What "colour" is the spark plug? (You have checked the oil level?)

[jonbuoy]

Is the centrifugal advance sticking? Can you wobble the advance unit on the crank? I think Ixion means the key that holds the advance unit onto the crank, CB750 is fully advanced by about 2.5K rpm not sure at what stage yours changes? Your not running weak on mixture or something?

[xwhatsit]

What "colour" is the spark plug? (You have checked the oil level?)

Spark plug is... grey-ish, and `rough' in appearance from overheating. The outer ring is black.

I check the oil level every time I ride it, small sump, easy to get into trouble.

Is the centrifugal advance sticking? Can you wobble the advance unit on the crank? I think Ixion means the key that holds the advance unit onto the crank, CB750 is fully advanced by about 2.5K rpm not sure at what stage yours changes? Your not running weak on mixture or something?

A couple of people have suggested a sticky advancer (either open or closed) now. Apparently those type of things are notorious for causing problems. I can't see any sort of key holding the advancer in place on the parts fiches. I've attached some images -- the first one is what's under the right side cover. #16 is the advancer. The second image is the crank, of course, and you can see #5 is the woodruff key for the flywheel. There is no similar sort of part for the other side of the crank.

I think it hits full advance at 4500rpm. Or 4350 or something like that. I've wondered about lean carburettion in the past, but I'm at a loss of how to check it properly. Don't have the money for gas analysers and such things. You can check the plug, of course, but I have to rule out ignition timing for those symptoms first.

[jonbuoy]

Not so much detail in those drawings - mine has a locating hole on the crank and a matching pin on the advance unit. You should be able to pull off the advance unit and check the springs are able to open and close the wings. Mark everything if you don't have a timing light. Alternatively blast the hell out of it with some CRC or see if you can get a blob of two in one oil in there without touching it. If you've not done any filter or exhaust/mixture mods the only reason you might be running lean is a partly blocked jet.

[xwhatsit]

Well the results are back, spent the afternoon fiddling about. The `F' mark at idle is almost dead centre, maybe slightly retarded. No worries. It disappears when you open up the engine, so that's good too.

Ignition is now out of the question !!

Plug is definitely looking quite lean.

So now the question is doing a gas analysis test, just to confirm it. Does anybody know where I could get this done cheaply?

Thanks for the help, guys.

[jonbuoy]

With all the other work you've managed to do by yourself you won't find carbs any harder. Overhauling one is easy - adjusting fueling after engine mods is the hard part - you won't have to worry about that. Odds on there is a lump of snot in your jets or float valve . Even if you don't want to disturb anything just flip it over drain all the crap that will be in there out, and use a can of carb spray down the jets. A gas check will only tell you what you know now - your running dangerously weak.

[paturoa]

Also have a look to see that the air cleaner is intact (no holes) and seated correctly. Also check that there are no holes or places air can get in before and after the carb. That could make it run lean / hot.

I do this by finding somewhere where there are no drafts (closed garage) and get some "old" string. It embers really well and produces a good clear smoke that can easily be seen with a torch. If there are any holes it is quite surprising how the smoke will be sucked or blown just at idle. Cig smoke is too much.

[paturoa]

like this
10 chr

[xwhatsit]

Hey cheers. That's an interesting method of finding air leaks.

I've replaced the intake manifold, but it's a used manifold as well, so I can't be sure.

Air cleaner looks good, but I can't be sure. Might have to ring Econohonda.

[scumdog]

Hey cheers. That's an interesting method of finding air leaks.

I've replaced the intake manifold, but it's a used manifold as well, so I can't be sure.

Air cleaner looks good, but I can't be sure. Might have to ring Econohonda.

Another way to check for intake vacuum leaks is to use CRC (or similar) with the thin plastic-straw nozzle bit on it and squirt it around the mounting flanges or other potential air-leak areas.

If there IS a leak your engine will normally change its revs (normally drop a bit) and you may notice smoke (in your case MORE smoke??) coming out the exhaust.

If you're really game and your bike doesn't make random sparks AND you're outside this method is even better with Ez-Start, no smoke but your engine will change in note and revs for sure.

[Ocean1]

If you're really game and your bike doesn't make random sparks AND you're outside this method is even better with Ez-Start, no smoke but your engine will change in note and revs for sure.

+1

Works well for finding sucky leaks.

Suggest you refrain from this tactic on 455 Buicks with MSD6 ignitions though. Could lose your eyebrows. For example. Just speculating unnerstand.

[vifferman]

I'm at a loss of how to check it properly. Don't have the money for gas analysers and such things.

There are simple signs to indicate leanness, the first of which is overheating. The bike will also have a lean surge: riding at constant throttle, it will tend surge, rather than run smoothly.
Try goooooogling "motorcycle carburetor tuning" - there are lots of useful sites, especially the Factory Pro one.