Topic: 400F supposed fuel problem

[jakec]

My 400 was running fine for months (riding almost daily) until recently it got a coldish #3 cylinder. I some digging around in there and all cylinders are running the same temp now. However I'm still having a problem where the bike will run OK for a while and then suddenly act like it's out of gas. I can ride 10 miles perfectly fine and then this problem will spring up. I have no power above 3-4k rpm. If I cruise along holding the throttle at 4k rpm, the bike is sputtering along with sudden small surges of power here and there.

I only looked inside #2 and #3 carbs, and both were clean with clear jets. Everything is bone stock, with a healthy battery and new plugs. The bike starts up first kick and idles perfectly. I also had installed an inline filter, but I removed it because it was restricting the flow (sitting in a horizontal position due to the routing). I also blew out the air vent hoses on the top, and the drain hoses on the bottom, and both are flowing fine.

I think I am having a flow problem because if I switch to reserve with a full tank, fuel will start leaking out where the line connects to the petcock. This indicates to me there is something restricting flow, so the fuel would rather force its way out at the petcock than flow down to the carbs.

[willbird]

My 400 was running fine for months (riding almost daily) until recently it got a coldish #3 cylinder. I some digging around in there and all cylinders are running the same temp now. However I'm still having a problem where the bike will run OK for a while and then suddenly act like it's out of gas. I can ride 10 miles perfectly fine and then this problem will spring up. I have no power above 3-4k rpm. If I cruise along holding the throttle at 4k rpm, the bike is sputtering along with sudden small surges of power here and there.

I only looked inside #2 and #3 carbs, and both were clean with clear jets. Everything is bone stock, with a healthy battery and new plugs. The bike starts up first kick and idles perfectly. I also had installed an inline filter, but I removed it because it was restricting the flow (sitting in a horizontal position due to the routing). I also blew out the air vent hoses on the top, and the drain hoses on the bottom, and both are flowing fine.

I think I am having a flow problem because if I switch to reserve with a full tank, fuel will start leaking out where the line connects to the petcock. This indicates to me there is something restricting flow, so the fuel would rather force its way out at the petcock than flow down to the carbs.

When it does this have you tried opening the gas cap then closing it ?? I have a gas cap from the infamous 4:1 on my K2 and the venting sucks. I took it apart and they had a wad of what looks like cotton over a small vent hole, I reduced the qty of cotton and it helped SOME but I still need to burp the cap now and then.




Bill

[jakec]

Cap venting crossed my mind, but there isn't a problem with flow from the tank since the gas is forcing itself out the petcock to fuel hose connection. Also when disconnected, the flow is good. I will check the cap anyways though.

[jakec]

I just looked into the venting hole, and it is crusty. the cap internals however aren't screwed together and it's not clear how it comes apart.

I did find this video but it doesn't show it coming apart the rest of the way. I will give it a shot though.

[Bodi]

Leaking from the petcock is a petcock problem. If you have the cone type, a little silicon grease on the cone may stop the leak. The rubber disc type can often be fixed by rotating or flipping that disk. Rivets holding the plate on can be drilled out, holes tapped, and small screws used. The spring plate can be bent to increase spring pressure on both types.
As for fuel flow, the filter "sock" around the petcock inlet tube could be clogged if you had a lot of dirt or rust flakes in the tank. It can be tough to pull out.

[jakec]

The leak on the petcock is coming out of the joint between the hose and the brass fitting. The brass fitting is pretty small on the 40 petcock. Using 1 length of oem honda hose, the fit is extremely tight on the carbs and somewhat loose on the petcock. I use a fuel hose clip on the petcock. Also the petcock is recently rebuilt with a new sock, which I cleaned recently when I started this investigation.


As for the cap I disassembled it and soaked it in chem dip then stuck a pipe cleaner down there and blew it out with brake cleaner. So the vent shouldn't be a problem anymore.

[HondaMan]

Does the bike have the conventional points-and-condensors type ignition, and when were they last changed?
(Just in case this isn't a fuel problem, thought I'd probe in another direction...).

[jakec]

No, it has a Dyna, which I put in about 3,000 miles ago.

I can ride around town all day below 4,000 rpm, but if I start riding it any harder than that, it will be ok for some seconds, then start acting like it's gona die. Then if I baby it again it gets better. Seems like fuel right? I can't think of anything else.

Just rebuilt cap vent (it was a little crusty but still vented)

Newer cap gasket (5-6 years)

Rebuilt oem petcock (clean filter sock, good flow)

New fuel line (no filter)

95% clean tank (no rust, a little varnish in spots)

Stock air filter (almost new)

Air vent hoses at top of carbs flowing. Maybe a little stiff, but flowing. Flow improved when I removed vent hoses from bottom. Blew thru all hoses clear. The most recent test ride was with the bottom vent hoses removed, so they much not be the problem.

All carbs getting gas and all pipes equally hot (to the touch).

[Mark1976]

   If yer chugging around under 4k and ya have a dyna ignition, make sure you have good voltage going to and from the battery, if that battery runs low on voltage (less than 12v's) you're going to get just what you're experiencing.

[jakec]

I will check that today. All my batteries are pretty new so I didn't consider this. THanks

[jakec]

I just checked the voltage. 3 of my bikes all have the same scorpion sealed battery.

400f: 12.52v (sitting 2 weeks)
Cl350: 13.09v (ridden recently)
Cb750: 13.25v (ridden today)

[jakec]

I do have the original chargin system still, maybe I need to upgrade

[Deltarider]

I do have the original chargin system still, maybe I need to upgrade

Maybe, you don't. Have a look at the charging characteristics of the CB400F in p.62 of the Honda Shop Manual: https://www.honda4fun.com/dwnload/Shop-Manual/CB350-400%20Shop%20Manual.pdf
At 2000rpm a stock system already sees 14,2V at the battery terminals. From 3000rpm it's 15V which is a bit high, so abstain from gel batteries. Most gel batteries can't cope with voltages above 14,35V. Not many shops warn for this. Louis in Europe does. None of the aftermarket electronic devices that I have seen equals the performance of the stock equipment. Both the stock rec and reg live forever, unless an error is made. 

[jakec]

Ok, I'll keep them then. I haven't had a problem with charging before. Do you think the voltage is low enough to matter with the dyna?

I don't want to put gas in the tank again and all that unless I feel confident I've found something wrong (have to go to gas station with gas can, can spills everywhere, etc)

[Deltarider]

Hard to say anything meaningful from a distance. Personally I'd want to be absolutely sure fuel reaches the bowls unhampered. It takes very little to impede fuel flow. If the issue comes and goes 'gradually' I would suspect a fuel problem, if the issue comes and goes abruptly it would indicate electrical.
What you could do, is have a voltmeter in a tankbag and monitor what the voltage does in a ride.
The moment it starts acting like it's gonna die, I'd kill the engine that very moment by the kill switch and with a simultaneously pulled clutch, pull over and examine the plugs noses. If they're whitish, it's definitely fuel related. This usually coincides with a hotter than normal engine and a possibly harder shifting gears.
Maybe I've missed it: what type of battery do you run? A 'Scorpion sealed battery' is that a gel type battery or AGM?

[jakec]

It is an SLA. Well, it looks like I need to put fuel in either way, unless I want to take the carbs back out. I could do a plug chop. However I'm not sure if I will get a lean reading, since the bad symptom is like the bike is sluggish and wont move, in a way that feels different than running lean. Not sure how to explain it. Maybe more like rich. Which could be a symptom of weak spark?

When I'm tuning for a custom application I can tell the difference between rich and lean, lean feels like high loud revving with no power and popping, rich feels like sluggish bike doesn't want to move. I think I'm feeling a rich condition. I should say it also just feels like when you ride away with the petcock closed and it starts to bog down after a half mile.

[Tim2005]

If you aren't too sure if it is weak or lean, take the airfilter element out and go for a ride, if the problem is worse you'll know it is lean, and vice versa

[jakec]

I finally got around to putting gas in the bike and taking it for a ride. I guess the gas cap vent must have been the thing. Because that, and charging the battery were all I did to it, and it's fine now. I waited like a month just to put gas in and find that out... lol oh well. Back on the road.