Consulting a carb wiz plse

Bike starts fine. After 30 seconds, maybe a minute, sudden stop.
Have to wait a minute, starts fine again (won't start if I don't wait), runs a minute or two. Quits suddenly. After third or fourth try, it will usually keep on running fine, unless I do it wrong.

I was thinking it was the TCI faulting (CDI, but on a XS400L). Inspected it with a microscope without finding anything to be worried about - but I still thought that it was the problem. Maybe a worn out condenser.

But this week I found a fail safe way to keep it running without it quitting. I start the bike and then keep nudging the revs up, oh no more than 50 rpms or so. I had twitched the throttle once by chance and noticed it had prevented the engine from stalling. So I do this for a few minutes while the engine warms up and it doesn't stall on me anymore this way. After 5 minutes, it will idle normally without any worry.

In fact, by twitching the throttle this way, I probably catch it when it wants to stall. Or maybe I just prevent whatever was stalling it to happen.

Now I no longer think my TCI is at fault. I think it has to do with the fuel not feeding properly, maybe, to the carb? Would this be a problem with the vacuum? The problem never comes back for the rest of the day, even when my bike cools down for a few hours (I ride it at work). I does this only on the first start of the day, at any time of the day. Makes it weird.

Any clue? Or ways I could go about to test things and figure what I should be looking at to fix this?

 

what carbs are on that bike?
it is not getting a rich enough mixture when cold, turn the mixture screws out 1/4 turn.

 

Right it sounds like the choke is not working properly.

 

The carbs are Mikuni BS34

 

Yep, that's probably it. I left it to a mechanic last autumn and felt the choke wasn't setup like it was before when he gave it back to me. Didn't like the new setting. But figured the mechanic knew better than me what it should be like.

I'm taking my manual out and will adjust it and see.

Mixture out 1/4 turn. Starting with that.

 

Those are just like the Mikunis on the 550s but smaller. Choke isn't a real choke, it's an enrichening circuit. It HAS to be adjusted so that it's full OFF when off, and whatever position it pulls to "on" is nowhere near as important. Watch the linkage as you operate it, it should be lifting little plungers sticking out of the carbs. That's all it does.

It sounds to me like a classic case of "lazy petcock syndrome." Try this: Before you start the bike, put the petcock in "PRI" for about 20-30 seconds. Start the bike, let it warm up; once underway, go ahead and switch back to "on." If at ANY point, the bike starts to act like it's running out of gas, put it back on "PRI" just don't leave it there once the motor's off. If using "PRI" like I described seems to help, you most likely need to clean/rebuild your petcock.

Check the choke adjustment and do the diagnostic on the petcock before you start twiddling carb screws.

 

Fitz,
It could be the petcock too 'cause I used to put it on PRI for a few seconds to make sure the gas was getting to the carb and it did seem to help at times. But it would often quit on me even when on PRI. So I never became convinced that was it.

Unfortunatly I did enrich the mixture already with a quarter turn of the pilot screw 8O . The engine does sound better, more like it was before I took it to a mechanic. The engine sounds better either with the choke on and with it off. So that was probably at least part of the problem. Doing that let me lower the idle from 1,400 rpms to 1,250 or so without it tending to stall. A good sign I think. Hopefully I didn't create more problems :roll:

I read somewhere that there was a filter in the petcock. Maybe I have to clean that filter. If my bike doesn't start on choke without me nudging the throttle all the time tomorrow morning when it will be very cold here, I'll schedule taking it apart on my maintenance list.

As for the choke not being a real choke, maybe that's why I can't find a reference to it in my bike's manual. I'll do like you say and observe what it does when activated.

I hope I don't have to do a carb rebuild. Never did that before but I would prefer to do it myself anyway, preferably in wintertime when I can experiment and learn without loosing a lot of riding time. But maybe it's easier than I think though. :? How often do you have to rebuild carbs on a bike?

Tks for all the input, you guys. This is sure a great forum.

 

Hey dmx_xs400L
How about spending a dollar and replace the vac line to the petclock??

This tip helped another similar problem/.

 

I bet what I did to the mixture solves the greater part of my problems.

Thanks for the tip. That was my first question in fact: could it be a problem with the vacuum. The line seems in very good shape. In any case I'll keep that in mind and see if the local stores carry some that I can use on my bike.

 

Be careful--there is a difference between fuel line and vacuum hose; if the auto parts store sells you the wrong stuff it can make matters worse. Regular hose can collapse under vacuum.

 

If you can suck on the vacume line and fuel flows in the on position, you now your line is good.

 

Yah, I was wary about that. I'll read the labels before using just any hose. Tks.

 

:lol: Good testing technique. I guess the fuel doesn't come back up the vacuum hose when you do that, unless you suck to hard.

I think the hose is good, but next time I have to lift the gaz tank I'll check it at the same time your way just to make sure.

It's fairly cold here this morning. Around 45F. The bike started up with no problem with just a tiny twitch of the throttle (after putting the petcock on PRI for 20-30 sec just to make sure the bowls were full). Had full choke on. Reved up to 3,500 by itself. Released the choke to bring it down under 3,000. Then after a minute, took it down to 2000. Another minute, to idle at 1250.

Never had even a hiccup.

So most probably a too lean mixture was the problem.

I consider it solved. Very happy!

Tks again for all the responses guys. You are my best resource to keep my bike in shape.

 

which position should you leave the petcock in when the bike is off?

 

on (down). Pri will allow fuel to flow without vacuum and REs will allow you to run out of gas when you forget to turn it back and not have a reserve. Unlike many other bikes these don't have an "OFF" position. They auto shut off (if working properly) when the vacuum leaves.

 

and for the few of us that have aftermarket petcocks, Make sure u turn it to the off position! lol

 

UPDATE
Well after the problem with the bike stopping cold after 30-60 seconds of warming up creeped back in, I enriched the mixture again - and fouled the plugs.

Cleaned the plugs, leaned the mixture back and started looking at the electrical system.

Drats, there is di-electric grease *inside* most of the plugs! Turns out the previous owner thought this grease was neutral to electicity. But it is not. It's an insulator. So I did the best I could to clean the interior of the plugs with a Q-tip and a spay can of electric head cleaner. Not perfect. Is there a way to clean the plugs or do I have to replace them all?

The bike still gave me the same symptoms a few times. That is, starts perfectly the first time, runs for a minute, stops cold - even if I'm pumping the gas now - and I HAVE to let it stand still a minute before starting it again - it will not spark if I press the starter switch right away. Then is starts immediatly on a press of the switch after that minute of wait.

It will do this 3 or 4 times, then it will run ok for the whole day.

Lately it seems not to do this any more. But I think the problem will come back eventually. Happened before. So my guess is the grease increases the resistence in the electrical circuit and when it's hot or the battery very fully charged, the problem goes away for a while. When it's cold or the battery has self-discharged a bit, the problem reappears.

By the way, I tried different position of the petcock and that doesn't seem to be the problem. Only thing I haven't done yet is to clean the air filter. Although last time I checked it seemed ok.

What do you guys say?

 

The grease is an insulator however when the plug is connected the grease is wiped off and you get a perfectly functional connection assuming your plug cap makes a good connection to start with. Using the grease on the plugs can help prevent arcing and corrosion. I use it on my bike on all connections.

When under about 2500 RPM your bike is running off the battery. Take it down to a parts store and have it load tested. It may be bad or going bad.

 

Are you saying that the engine turns over and there is no spark or are you saying that it doesn't turnover at all, until you let it sit?

 

Hound:
The engine turns over ok - but there is no spark: it turns like the plugs were not in there or not connected. But it turns strong. Like no current getting to the plug for 30 seconds to 1 minute. After I let it stand, bang, it starts right away like a charm. Really baffles me. Like something turns off when it starts to warm up, sheds some humidity, or melts some grease on a contact. When the 3 or 4 cycles are done - start, run 1 or 2 minutes, stops, wait 1 minute, do it again - the bike is ok for the day until the next morning - I ride it to work and I never had a problem from work to home.

Bill, I bought a new battery a month or so ago. It's fine and I watch it closely to keep it that way. I have a 2 AMP charger, but not a trickle charger that I could leave it on. Maybe I should get one. But the engine quits even when at 3,500 rpms - not only at 2,000.

The grease on the plugs? I'm still worried about that. Sure the plugs are tight but I suspect even then a thin layer of grease will be present between the contacts. I know I wouldn't do that on electronic circuits because the resistance would increase and I could measure the difference, but I can't say if it's ok with automotive circuits. I hope it is!

My theory is that the increased resistance from the dielectric grease makes the TCI turn off from to low a voltage until the connectors warm up enough to melt some grease. After which the TCI is no longer in pain from not getting the right amount of voltage. I run the bike 40 minutes to work and the battery gets a charge so it has an easier time when I ride back. Also, it's warmer during the day and the grease doesn't thicken as much during the day as it does at night. The contacts might be better in warmer weather.

Can I test this theory without changing all my greasy plugs that I can't clean?