Bike dropping cylinders?

[STEPHASAUR]

Hey all.
So I'm just going to put this out there. No i am not selling it. Its not worth shit not running so I might as well just fix the damn thing.
+ despite the exponentially growing levels of frustration and self doubt, I am learning a tonne.

Help me debug my bike - 1988 MC17 CBR250R

At random intervals mid ride, the bike feels like it's drops a cyl or two and eventually stalls. It will turn over but only for 1-3 seconds before stalling again. It makes a really low rumbling noise, and when throttle is applied it cuts out and dies. Should also point out that when this happens, power is almost not there and it will not continue to run unless revs stay over 6k.
The initial thought was spark plugs. So those were replaced, along with a faulty coilpack - which stopped the misfire under idle - another past issue

Stuff that I have done to the vehicle so far:

- New sparkplugs
- New coilpack to replace faulty one
- Carbs have been cleaned
- Fuel filter / tank/ tap cleaned
- New fuel hose (old one was pinching)
- New air filter
- New regulator rectifier
- New battery (which is still showing the correct voltage)
- Fuses/ ECU connections all been checked - all OK

I haven't done a compression test, and I have yet to check the plugs are sparking. But I assume they are for it to be able to fire and run the initial 10-30 mins.
The other weird thing is that if i leave the bike to sit for a little while, it will fire up again. It acts like it's not getting fuel, but I've checked and there is fuel getting into the carbs...

I must be missing something. Someone please offer assistance!

[bogan]

Do a compression test. Also couldn't hurt to check tank breather is not blocked up (open cap next time it cuts out and listen for 'whoosh' of air), though I wouldn't have expected it to run at high revs if this were the only problem...

[Akzle]

at 1988. i'm picking your looking at head/rings. but those carbs are going to want some sex too.

[kiwi-on-wheels]

Check the head breather line, mine had an issue with it getting hot and all the blow by collecting In the carbs. Pulled the hose off the airbox that comes from the head, let it vent to air, problem solved. Free to check

[STEPHASAUR]

UPDATE:
So towed the bike via van to my mates workshop for a full breakdown

Compression tested - all fine
Tested spark - very good
Double checked carbs - adjusters were set high, bike was over fueling but otherwise ok (these were adjusted to fix)
Breather was clean

fired up and still same issue

at which point his fathers mate walked in, who's been working on/ racing bikes for a good 30 years. after explaining how it was behaving he told me, no question that my CDI is faulty.

anyone got a running mc17 i can test this theory with?
or would any cbr running the mc14e engine be compatible with the same cdi? in which case mc17-mc22 i could use?

thanks!

[kiwi-on-wheels]

If all else fails I have seen cdi's on flebay for a reasonable price.

Iirc the mc19 cdi will work, some repinning may be required

[OddDuck]

I've been having CDI trouble with the 900ss lately so maybe this is useful... Difficulty starting and running, especially in cold / humid conditions. It got to the point where it very nearly wouldn't start at all. Ran OK after it had been out for a good spin though.

I tried cleaning the spade connectors and making sure they were tight (12V doesn't take much to interrupt; dirt, oxide and oil can do that), some improvement but nothing major. I did the same to the main relay and found the push in connectors were oxidised quite badly, some CRC2.26 and several insertion / removal cycles (to clean the connectors) did wonders for the bike.

Problem resumed earlier this month, following parking up in the shade at work.

I took the CDIs off, wiped the dirt off and had a look at the potting. Most potting is epoxy resin or silicone rubber (flexible). In my bike they used silicone rubber. It's good for the electronics because it cushions as well as seals. Close examination showed what looked like hairline cracks on the surface of the potting. Not too surprising on a 1995 bike.

Now for the good bit... I fired up the fan bake oven and cooked them for 6 hours at about 65 - 70 C.

The reason for doing that was water ingression. CDI's have high internal voltages and if water is getting in then that voltage will start dissipating via insulation leakage. We use this technique at work a lot to dry out electronics that test low for insulation values; it's surprising how quickly it comes back up. You don't have to take it to boiling, just hot enough for long enough in a dry environment to get the water out.

After cooling and reinstallation of CDI's, the bike fired up immediately and ran better than it's run in years.

I want to stress that this is diagnostic or last resort stuff. Do it only when you've got cash and a source for spares lined up. The fix is temporary at best - I've tried coating the outside of the potting in silicone grease, but the bike's showing signs of having trouble again (it's only been three weeks). Replacing the potting would be ideal but that's a saga in itself, and anyway there'll be electrolytic capacitors in there which should be replaced as well. New CDI's for me when I can afford them.

Apologies for the novel, good luck and let us know how it goes. Now for the jokes to begin...

[STEPHASAUR]

I've been having CDI trouble with the 900ss lately so maybe this is useful... Difficulty starting and running, especially in cold / humid conditions. It got to the point where it very nearly wouldn't start at all. Ran OK after it had been out for a good spin though.

I tried cleaning the spade connectors and making sure they were tight (12V doesn't take much to interrupt; dirt, oxide and oil can do that), some improvement but nothing major. I did the same to the main relay and found the push in connectors were oxidised quite badly, some CRC2.26 and several insertion / removal cycles (to clean the connectors) did wonders for the bike.

Problem resumed earlier this month, following parking up in the shade at work.

I took the CDIs off, wiped the dirt off and had a look at the potting. Most potting is epoxy resin or silicone rubber (flexible). In my bike they used silicone rubber. It's good for the electronics because it cushions as well as seals. Close examination showed what looked like hairline cracks on the surface of the potting. Not too surprising on a 1995 bike.

Now for the good bit... I fired up the fan bake oven and cooked them for 6 hours at about 65 - 70 C.

The reason for doing that was water ingression. CDI's have high internal voltages and if water is getting in then that voltage will start dissipating via insulation leakage. We use this technique at work a lot to dry out electronics that test low for insulation values; it's surprising how quickly it comes back up. You don't have to take it to boiling, just hot enough for long enough in a dry environment to get the water out.

After cooling and reinstallation of CDI's, the bike fired up immediately and ran better than it's run in years.

I want to stress that this is diagnostic or last resort stuff. Do it only when you've got cash and a source for spares lined up. The fix is temporary at best - I've tried coating the outside of the potting in silicone grease, but the bike's showing signs of having trouble again (it's only been three weeks). Replacing the potting would be ideal but that's a saga in itself, and anyway there'll be electrolytic capacitors in there which should be replaced as well. New CDI's for me when I can afford them.

Apologies for the novel, good luck and let us know how it goes. Now for the jokes to begin...

Hey thank you for the message!!

admittedly mine reacts exactly the same in cold/ humid conditions
I'll pull mine off and take a look when i know for sure it is the CDI bugging out.

Thank you so much for the "novel" absolutely useful!!
will update!

[OddDuck]

A quick update on the CDI's in the 900ss - just back from a 470-mile trip around the lower North Island, quite a bit of it in light rain. Baked-and-greased CDI's worked just fine.

[STEPHASAUR]

UPDATE:

So since the last post I have sourced a replacement CDI which has been tested against the old with very little success
Tested the voltages coming out of the rectifier (incase it had failed again) to find everything was more or less on point.
Tested voltages being produced by the stator - all of which were also fine (couldnt get a reliable result when the bike was running as it barely ran)
Have also tested the ohms on the pulse generator, as one of the suggestions was a sensor was faulty and going OC at random intervals leading to a loss in cylinders. Yet to pull the cover off and physically inspect the sensors however the readings for the both of them were as followed:
yellow / yellow white: 441 ohm
blue / blue white: 426 ohm
Apparently the OEM readings are meant to be anywhere between 315-380ohm (don't quote me on that as i may be a few off) but that shouldn't make too much difference??

Did pull the carbs off again to find that on the underside of the fuel line from filter-> carb did have a massive split in it which could suggest its hesitation when throttle is applied, and perhaps the issue of it dying full stop.
Will be pulling off each individual carb tonight for a full clean up/ re-adjustment as they were set to 3.5 rotations open as opposed to OEM settings which is apparently 2.5 (fuel adjustment) along with checking the valve clearances and re-balancing the carbs (provided there is time)

I suspect battery #3 has also shit itself (not going for motobatt again) only producing 8v under load after a full charge (and discharging rather quickly).
The choke doesn't seem to engage till at least 3/4 into it's turn, which may also be an issue, but that's probably less to do with the running of the machine and more the initial startup.

On the topic of amazing luck i'm having with this metallic horse, the trusty flatmate kicked my front fairing/headlight housing and broke the brackets attaching the headlight... yay

but yea, any recommendations for stuff to look out for while the bike is more or less in pieces?

Thanks in advance!!

[OddDuck]

Kudos for work done so far!! A bit shit that the CDIs didn't sort it.

Right, stuff to look for:

Rust from the tank clogging the fuel filter.

Rust from the tank clogging the carb jets especially slow jets.

Crappy sealing on the air filter allowing air to bypass, this'll super lean the mixture and make the bike run like a dog. Alternately a clogged air filter will rich the mixture up, fouling plugs and leading to weak spark etc.

Vacuum leaks between carb bodies and intake valves - old rubber is suspect #1, then cracked insulator plates etc. Look for dirt traces. If you can see flats set on any O-ring, chuck it.

Pickup coils going open circuit / intermittent after they get hot - difficult to check with the motor in bits, try warming up after reassembly and see if this resistance increases or changes.

Check coil windings resistances.

Test compression in each cylinder if you haven't already.

Go around and (it can't hurt) unscrew and clean each grounding point. Oxide and dirt can really screw a ground up, you need a decent metal to metal contact. 800 grit wet and dry helps, old crimp joints in the wiring are suss too.

Last thing, possibly the most important: if it's eating batteries then there's a chance that you've got a short somewhere. The wiring loom may have frayed and one of the 12V wires could be shorting to the frame, possibly under vibration while running. PITA to diagnose and find... it might need lifting the entire loom off the bike and visually checking for damage.

Apologies if you've done these before, I've got a bit on and haven't time to go through thread again. Good luck.

[Akzle]

UPDATE:

So since the last post I have sourced a replacement CDI which has been tested against the old with very little success
Tested the voltages coming out of the rectifier (incase it had failed again) to find everything was more or less on point.
Tested voltages being produced by the stator - all of which were also fine (couldnt get a reliable result when the bike was running as it barely ran)
Have also tested the ohms on the pulse generator, as one of the suggestions was a sensor was faulty and going OC at random intervals leading to a loss in cylinders. Yet to pull the cover off and physically inspect the sensors however the readings for the both of them were as followed:
yellow / yellow white: 441 ohm
blue / blue white: 426 ohm
Apparently the OEM readings are meant to be anywhere between 315-380ohm (don't quote me on that as i may be a few off) but that shouldn't make too much difference??

Did pull the carbs off again to find that on the underside of the fuel line from filter-> carb did have a massive split in it which could suggest its hesitation when throttle is applied, and perhaps the issue of it dying full stop.
Will be pulling off each individual carb tonight for a full clean up/ re-adjustment as they were set to 3.5 rotations open as opposed to OEM settings which is apparently 2.5 (fuel adjustment) along with checking the valve clearances and re-balancing the carbs (provided there is time)

I suspect battery #3 has also shit itself (not going for motobatt again) only producing 8v under load after a full charge (and discharging rather quickly).
The choke doesn't seem to engage till at least 3/4 into it's turn, which may also be an issue, but that's probably less to do with the running of the machine and more the initial startup.

On the topic of amazing luck i'm having with this metallic horse, the trusty flatmate kicked my front fairing/headlight housing and broke the brackets attaching the headlight... yay

but yea, any recommendations for stuff to look out for while the bike is more or less in pieces?

Thanks in advance!!

is it wrong that reading this, posted by a chick, gives me a semi?

I would say, young luke, that youllre well on the path of righteousness.

[Akzle]

Kudos for work done so far!! A bit shit that the CDIs didn't sort it.

Right, stuff to look for:

Rust from the tank clogging the fuel filter.

Rust from the tank clogging the carb jets especially slow jets.

Crappy sealing on the air filter allowing air to bypass, this'll super lean the mixture and make the bike run like a dog. Alternately a clogged air filter will rich the mixture up, fouling plugs and leading to weak spark etc.

Vacuum leaks between carb bodies and intake valves - old rubber is suspect #1, then cracked insulator plates etc. Look for dirt traces. If you can see flats set on any O-ring, chuck it.

Pickup coils going open circuit / intermittent after they get hot - difficult to check with the motor in bits, try warming up after reassembly and see if this resistance increases or changes.

Check coil windings resistances.

Test compression in each cylinder if you haven't already.

Go around and (it can't hurt) unscrew and clean each grounding point. Oxide and dirt can really screw a ground up, you need a decent metal to metal contact. 800 grit wet and dry helps, old crimp joints in the wiring are suss too.

Last thing, possibly the most important: if it's eating batteries then there's a chance that you've got a short somewhere. The wiring loom may have frayed and one of the 12V wires could be shorting to the frame, possibly under vibration while running. PITA to diagnose and find... it might need lifting the entire loom off the bike and visually checking for damage.

Apologies if you've done these before, I've got a bit on and haven't time to go through thread again. Good luck.

https://cdn.meme.am/instances/60465496.jpg

[OddDuck]

https://cdn.meme.am/instances/60465496.jpg

It's just how I am.

Bike's sick.

Must help.

[Madness]

So... not going for motobatt again

There was once this epic battery thread. I'd post a link but some of the technology discussed in this thread was of such a commercially sensitive nature that it had to be deleted. Before it's removal, the thread clearly established that Yuasa motorcycle batteries are in fact the best motorcycle battery that money can buy.

You're welcome