Loss of power and stalling
 

Hello,

Was hoping to do a nice few hour Saturday trip. Filled the tank and got to about 40 km away from home. Was cruising on the highway at about 6-7k RPM when I felt the bike loosing power. It did that for a few times, then I pulled in the first parking. I looked and smelled around and nothing seemed off. Pulled back on the highway and it ran fine for a few minutes.

Then it stared loosing more and more power, until I had to go WOT and yet it didn't pass 5000 RPM. Tried to give it more or less acceleration, change gears, but after one more minute it stalled. Tried to restart it while rolling with no effect, so I stopped on the emergency lane. After 2-3 tries it restarted. Then I rode it at around 4k RPM and it worked, but it seemed to lack some power. Fearing it might require a tow home I didn't dare to push it past very little throttle and 4k RPM.

When I got close to home I rode it as hard as possible and it seemed normal.

Some facts that may or may not be relevant:
-before this trip I did this: https://www.superhawkforum.com/forum...31/#post412448
-it's done something similar one time when it almost ran out of fuel and when going uphill
-yesterday I did notice some random short drops in RPM on idle (from 1200-1300 to about 800RPM)
-the last few rides, at random times. it felt as if it lost 10-20% power for half a minute or so

Most likely it's something electrical.

I am so frustrated... Got the bike in 2018 in seemingly good shape and have since spent almost 2 times the value of the bike and a lot of hours hoping it would get as safe and reliable as possible. Now it's suddenly completely unsafe for anything other than city riding...

Measured the coils- both have the same values.

Cleaned the ECU and CDI pins- there was a thin, barely visible layer of oxidation present on all pins.

Then I ran the bike for about 130kms, including the exact same route I took yesterday and guess what- the bike ran perfectly.
The differences were that I was alone, and the tank was only 60-70% full.

After some reading and thinking I think it could be:
a) intermittent issues with the ECU or CDI
b) intermittent stopping of the petcock vacuum
c) intermittent blocking of the tank breather hose

a) for this best case scenario is that the oxidation made something malfunction and now it's fixed. Next best is getting a "new" CDI. Worst case is a "new" ECU and set of locks
b) as I cleaned the petcock, it went from one jolt of vacuum needed for every 30 seconds or so to working properly, needing continuous vacuum (as it should). So there could be some vacuum issues that have been masked by the malfunctioning of the petcock
c) I'm estimating that at high speed it burns around 2.5 liters of fuel every 10 minutes. Air needs to replace that same volume inside the tank. If the breather hose somehow can not provide that flow of air, this would also restrict the flow of fuel.

First thing to do is to try again to reproduce the issue. As soon as possible I'll go on that same highway, and fill the bike up before entering- same as the first time.

I think I'm going to start calling this the spring time riding malady. Most of the bikes I get in my shop that act like this have a clogged fuel vent. Simple check, next time it happens, crack open the fuel cap and see if it runs ok.

If you are correct, I will be very glad.

Just to be 100% sure, I will spray some parts cleaner through the tank vent.

I have thoroughly cleaned the tank vent and vent tube using a part cleaner spray and some nozzles of proper size from a vacuum pump. Now it feels like maybe 10-20% more flow- don't know if that's any difference.

100% sure would be to crack open the fuel cap.

I did a 600km trip.

Got on the highway, 25-30kms later... same issue. Started loosing power, went WOT and stalled.

Opened the gas cap, no hissing sound and a few km later, same issue.

I said fuck it- it'll just ride it till it no longer runs.

I've found some patterns:
-before it really looses power first it has some gradual decrease in power, as if going on a slight incline
-what really triggers it is throttle position- it's not 100% linked to RPM- sitting in full tuck and at a slight downhill allowed "cruising" with almost 1000RPM more
-paying attention and rolling off to very little throttle before it really started loosing power made it work and not stall. Since finding this out I have no more stalls
-when it lost power it sometimes seemed like it was running in 1 cyl- with the clutch in it was slower to rev than normal and it sounded like a KTM 640

One friend said that back in the 90's he had a piece of junk car that did the EXACT same things and it turned out to be carb float issues. Dirty float valve and the float set too low only allowed low fuel flow, lower that what was getting burned at high speed.

Got no other idea than to start tearing the carbs. Are there any parts such as gaskets that I should buy beforehand?

Have you taken a look at the spark plugs?

Plugs look fine.

Took out the carbs and cleaned them, however after opening them it was obvious they were not dirty.

Put everything back on, rode for an hour and guess what- it still does the same crap.

What I meant with this question is, have you pulled the plugs immediately after the loss of power/stall? Is one/both wet with fuel or dry? Color?
Thinking back, I don't remember common issues reported on the forum with the ignition control module or CDI converter unit.
There is a known issue, IIRC, with the float vent hoses. I wouldn't think it would cause the problems that you're experiencing, but maybe??
The symptoms seem more like a fuel delivery problem than electrical.

Petcock diaphragm worn / split?

@xeris there have been enough accidents where idiots ran into people stopped on the emergency lane that I wouldn't risk taking off plugs on the side of the highway.
Are the float vents the tubes that sit between the two carbs? Carb cleaner flows very easily through them.

@Wicky It's not in perfect shape but it still seems to function properly. Tried to rebuild it last month but the new parts were junk, ended up cleaning and reusing the old rubber.

Bike running out of fuel would be the most plausible explanation. But WHY? The circuit is so simple, yet the answer is elusive.
The gas follows this circuit:
1- sits inside the tank - tank breather is ok and passing air and parts cleaner spray into the tank
2- goes through the strainer - it looks like new
3- goes to the petcock input and through the 2 outputs - this tests ok (on a cold bike at least) and both carb fuel lines are clear
4- goes to the carb bowls - they are clean and the floats and valves are good
5- goes through the jets and into the intake - they are clean and I highly doubt that they are sometimes clogged while up to a point they're running fine

Thinking back - I should have tested the fuel flow with the bowl drain screws off and the carbs off the bike...

Next things on the list are rebuilding the petcock (hopefully I manage to get proper parts) and fully removing and cleaning the tank strainer. It's a slim chance it'll make a difference but don't know what else to do...

Best to use a Honda replacement diaphragm as aftermarket are hit & miss.

I had issues with mine too... If you have a hand vacuum pump, attach it to the petcock and apply vacuum... Go for a ride... If everything works fine, look for small leaks in the vacuum hoses.... I had a tiny leak that would not allow the diaphragm to open fully. Fuel was flowing, just not enough to keep up with those thirsty carbs.... Seemed fine buzzing around town, but fell flat on the highway after a few minutes....

@Wicky With shipping and tax the cheapest I could have one home is +120 USD. It sucks buying new OEM parts while in the EU...

@Inland Viking
I do have a hand vacuum pump, however it's quite bulky. Is there any other vacuum hose apart from the one going from the rear cylinder to the petcock?

I have suspected the vacuum hose but visually it seems fine. However, just to rule it out I'd rather just replace it.

Can anyone tell me the ID of the vacuum hose?

Should be 3.5mm id.

For cheaper options wth the diaphram I've never heard of any one repairing an old one - you could try patching it like a bicycle inner tube. Or you could strike lucky with a pattern part. Or take a gamble on a secondhand petrol tap off eBay. Price for new Honda one looks around €50 +P&P from David Silvers.

https://www.davidsilverspares.co.uk/...t_16953ML0034/

@Wicky Mine is still in one piece and does not need patching- it's just that it has that "old and frail and just about to tear rubber" look. Thanks for the link, I'll check the shipping charges later but it's the cheapest I've seen so far.

Good news is that I might have found the culprit: https://redscr.net/L7jtGaPatHymxxcSjNtG.png
Keep in mind that the outside looked perfect. I cut ~2 cm off both ends and now they look like new both inside and outside, and I will use this line until I get a new one.
ID measured ~4mm.

Hopefully that was it with this issue. Will report back as soon as I test ride the bike.

Is that the vacuum line to the petcock?

Yup!