Intermittent Stalling - I'm Stumped
 

Riddle me this. Bike has a habit of intermittent stalling. Doesn't happen often, maybe every 50-100 kms. Driving me crazy. Here's what I have:

-87 Hurricane. Basically stock afaik
-Bike starts and runs fine. I start under choke, and less than 30 seconds later I can turn off the choke, and it runs just fine from there. I've set high idle (1200 rpms) and it's pretty steady. No hesitation, so I think that rules out pilot jet
-Acceleration is normal, no weird behavior or flat spots, no hesitation throughout, and I've had the revs right up to 10k, for testing purposes only of course. I think that rules out problems with the other jets
-Recently cleaned carbs. 70% confidence. I'll probably go back and redo them to rule it out anyway
-Rebuilt motor. Healthy compression. Valve clearance double-checked. Spark plugs brown. Seems like a happy engine
-Stick coils mod. Spark confirmed by taking a temp of each header pipe. They seem hot (like 230 degrees C) but I don't really know what's normal, and they're all at similar temps
-New sealed battery. Fuses good
-Healthy temps, fresh coolant
-Fresh fuel

So what happens is I'll be riding along, and inexplicably I'll get a sudden loss of power. I have a couple seconds to pull the clutch before the engine dies. It's the damnedest thing. Sometimes it happens while the bike is moving, sometimes when I'm at a red light. Doesn't seem to be related to engine or vehicle speed. It'll refuse to start back up until I wait for a couple minutes, and even then it won't start unless I put the choke back on. Once it starts I can drop the choke and it's fine again. Like it never happened. I think that points me towards a loss of fuel, and the bike continues to fire for a couple seconds after it starts to bog down, which leads me to believe that it's stalling lean. I guess it's possible I've lost spark on a couple cylinders simultaneously, but if so, I would expect it to be tough to start and to only run on a couple cylinders

How the hell do I diagnose this thing? I think it's gotta be electrical because the bike is so well behaved normally

-Fuel pump? Maybe an intermittent electrical problem? Will fuel continue to pass through it if it loses voltage, or will it cause a loss of fuel if it goes down momentarily?
-R/R? The battery seems to have plenty of juice, and no problems starting, but I know a lot of people complain about these. Could this cause the problem?
-Spark unit? Do these things ever go bad?
-Bad wire? Any tricks for narrowing it down?

This damn thing... Great when it works, but seemingly just as happy when I'm pushing it out of traffic

OK, in the past, members have had the same problem. In most cases, it turned out to be the tank breather line being plugged.
To check if the line is plugged and your getting a vacuum lock, ride with the filler cap open and see if the problem goes away. If so clear the line.

Of course after posting I do a search and it looks like I'm not the first one to have this problem https://cbrforum.com/forum/cbr-1000f...riding-153616/

Gonna have to rule out a blocked tank breather
Will confirm battery voltage, and check out the R/R. Apparently they absolutely can cause this type of problem. Hopefully it didn't destroy my new battery. To be safe I might wire in a temporary voltage gauge and stick it to my non-existent dash. If I notice it overcharging, that'll be a clue
Because we're already definitely in ratbike territory, I'll strap a small microphone onto the fuel pump to make sure it isn't kicking off during riding. Because I really needed more street cred

If I find the fuel pump is kicking off, I'll check out the relay and wires. If it's R/R then I'll have to bite the bullet and pick up a replacement. If clearing the breather tube fixes it, that'll be just fine w/ me

...But yeah, you're probably right. From what it looks like, breather tube should be my main suspect. I guess the lesson is to search before posting. Sorry for spam ;)

That comment made my day! Haven't put my fairing back on after it blew off, ordered new clips and bolts and need to get the glue before repairing. Can anyone tell me why a fuel pump is necessary for that year bike - Wouldn't you just bypass before replacing. Also, my kill switch seems faulty, maybe something to look at? I couldn't get my bike started once, and it fired back up after toggling the kill switch a bunch of times. Going to take it apart this winter and see if it can be cleaned because I'm cheap.

I'd pop the kill switch off and scuff up the internal contacts with some sandpaper. Pretty common problem with older bikes

My understanding is that the fuel pump on 87-88's is needed to slake the big honkin' engine near the top of the powerband. It's also got huge 7/16th fuel hose on it. Later years used a vacuum-petcock instead, but those were different carbs, presumably tuned to handle the changes. These things make a lot of vacuum, so it's no wonder they decided to go that route. Mine will idle without the fuel pump, but there was no reason to remove it, and I really wanted to make sure it retained that top-end power. Because reasons

I can say that with breakin complete, on a certain straight and quiet country road this Friday I finally got a chance to see what she could do. This was a return to the inline 4 for Honda superbikes, and they really nailed it with this one. I don't want to mess with that kind of magic

That said, some people run these without fuel pumps, and they all say it's fine. Personal preference I guess

I would think that an electrical problem would cause sudden failures whereas fuel delivery would cause lose of power before dying.
As far as a fuel pump, unless you have fuel injection where high pressure is needed, no need for a pump.
Remove the pump and that removes the worry of it carping out miles from home.
The float level remains the same with or without a pump.
Riding around with an unneeded fuel pump is like walking around with your appendix. It doesn't do anything, its extra weight and when it dies you are really screwed.

If dyno time wasn't so expensive I'd love to do a comparison with and without the fuel pump

In the meantime, I'm gonna beg to differ. I don't have any evidence to back up my argument, but I've spent a good portion of my working life alongside engineers and I gotta think they would have been under considerable pressure to keep costs down (particularly on this model) so if they could have removed it, they would have. As I say, total absence of evidence, so purely my uneducated opinion, take with a grain of salt etc etc

You're right on about the electrical problem. It felt like stalling lean, not a loss of spark. Disconnected the tank breather and gonna see if it makes a difference. Battery voltage normal so R/R is off the list for now

It's not for performance, it's for convenience. Before auto fuel shutoff, if you forgot to turn the petcock off the night before you would go out in the morning to start your bike and find an empty fuel tank and a puddle of gas under the bike. On your bike, when you turn the key to the on position, you put power to the pump. After riding, when you kill power the pump stops so that fuel stops flowing. Later on Honda came up with the vacuum petcock where you kill the engine, you kill vacuum and fuel flow. It may also be a liability issue because free running gas on hot pipes could cause the bike to go up in flames.

The job of the needle and seat is to stop fuel flow when the fuel reaches a certain level in the carburetor bowl. This is done by adjusting the tab on the float.
Gravity fed fuel has little pressure and the needle is able to stop the flow. Fuel pumps (on engines with carburetors) provide very little pressure as not to force fuel past the needle and seat, flooding the engine.

A fuel injected engine uses between 35-50 psi where as direct injection uses 500-2,800psi. Carbureted engines such as yours usually have a pump that supplies 4-7psi.
This 4-7psi is terminated by the needle and seat. The needle and seat only allow so much fuel in to the bowl. The bowl pressure is ambient as there is a small hole to the outside somewhere on the carb body to stop vacuum lock. So, if bowl pressure is the same with or without a pump, HP would be the same in either case.

So why do we need fuel shutoff? When a bike is on the side-stand, the bowls are at an angel and the float level is off and the needles drop allowing fuel to run freely into the bowls draining the tank.

Huh. Some really good points there. I'm still a bit sketchy on how carbs work, hence I usually just leave them alone unless forced. This is super interesting to me though, and it's good of you to take the time to share that info. It's great stuff

If I understand, you're saying that the fuel pump is an elaborate workaround to a problem with flooding on gravity-fed bikes. The competing system would have been a vacuum-fed petcock, same as the one on my '79 (yes, after reading your comment, I looked up which of my bikes have fuel pumps) where if I understand they plumb a vacuum line back to the petcock which shuts off fuel flow if the bike's not running. Dang clever. Anyway, for whatever reason, Honda decided not to use a tried-and-true, common, and cheap system, opting for a fuel pump instead. I know they did it on my Goldwing, and also the Magna. Probably others too. It had to have been a deliberate decision - that took work. The engineering and design groups had to find a place to put it, they had to build in a relay, wiring, and overcurrent protection, they had resources allocated, meetings held, spreadsheets prepared, all to justify the decision to their bosses. they had to go to Mikuni and spec it out, ensuring that it would all work, and work well. Why? What benefit did they see? Did one of their designers have a really bad childhood experience with a vacuum-petcock or something? Did they have labour disputes at the plant that made 1/8" vacuum hose?

Either way, they must have thought that their method was better. In hindsight it evidently wasn't, as they went to vacuum-petcocks in later years. I wouldn't disagree that this could well be the superior system, but versus pure gravity-fed alone, I think I'm better off running with a fuel pump. For one thing, I'm definitely going to forget to turn off the petcock and drain the gas into the crankcase and then I'll be late for work. That's just me. Also, I'm not convinced that gravity alone will push enough fuel into the floats at low fuel levels (and especially at higher revs). Sure, you get a lot of gravity when the tank is full, but if it's almost empty, I could imagine that you could run the floats dry. This bike is an absolute pig on gas, so I think maintaining constant pressures couldn't hurt. When needle's shooting up the tach, more flow is better than less, even if the opposite is true when the bike is parked ;)

Most importantly, the fuel pump works just fine. It kicks on when it should, and generally does it's job. I'm concerned that there might be an electrical problem in the vicinity, insofar as it could relate to the intermittent stalling, but I'm definitely not at the point of taking drastic measures. I know if worst comes to worst I can swap the fuel lines around to bypass it by the side of the road, but with how the bike's been behaving, starting it up after a stall hasn't really been a problem. I think the fuel pump might be a red herring

That said, with scarcity of parts for the '87 carbs, I can see a situation where I'd swap to the later carbs, along with a vacuum petcock, while deleting the fuel pump. They advertised more horsepower, and it would simplify things somewhat. Possibly a fight for another day

The more I think about it, the more I'd like to see a couple runs on the dyno though. Do you think they'd give me a discount if I told them it was to settle an argument on the internet?

I think that a dyno shop would tell you right up front that it makes no difference with pump or no pump.
Back in the day the top drag cars ran a couple of pumps because they would burn 5 gallons in a 1/4 mile run. They needed volume.
The older rails mounted what looked like a beer cage between the front wheels so that during massive acceleration, fuel would be forced rearward helping fuel flow.

Back to the original problem,
The fuel pump should have points in it. In just about all pump problems I have had, it's due to point failure. If you can ride with the pump exposed, and it dies, pull over and tap the pump with something like a screwdriver. If the pump suddenly starts pumping, it's pretty much a given that the points are sticking, dirty or in the case of a 30 year old bike, worn out.