Fuel in vacuum line coming from petcock?

[Recreateme]

I have been what i thought to be a vacuum leak. - It will rev all through the range in neutral but when riding it wont get passed 3k in any gear - I have check the vacuum lines and intake boots and carbs and filters and got nothing.

When i removed the vacuum line from the petcock it was wet... and when i looked down under my left exhaust (it has some holes from a piece that was riveted on by the PO to patch a hole (i think), these rivets arent airtight) i saw a bit of a puddle forming, when i smelled it, it seems like its unburnt gas..

So What i am thinking here is that when i rev my petcock is leaking fuel into the vacuum line causing a leak AND forcing fuel into my left side between the engine and the carb and this excess fuel isn't being burnt but just exhausted..

Does this make sense?

Next - I have been looking on xs650direct.com and have seen the rebuilt kits for petcocks at 15$ and new petcocks at like 40$ - should i just replace it with a non-vacuum kind and be done with it ?

Thanks!

 

[Yamaguy55]

The diaphragm on the petcocks rot or tear with time. The concept was for those that didn't remember to turn off their gas, it would do it for them. Works ok, but you can live without it.
Depends on what you want to end up with. If you like the auto on-off fuel tap, rebuild yours. If you don't like or care about the original function, then replace it with a manual one.
Neither solution will be forever, ethanol kills off all this stuff.

 

[Recreateme]

Yeah i dont care to much for the auto-on-off thing.. I just want something that is an easy clean replace..

xs650direct.com has 2 LH petcock options... Part #20-0019 and Part #20-0050 can anyone comment on which would work in a 1979 xs400 2-F ?

 

[Recreateme]

OK I looked further into this and actually think its a choke lever problem... I think there is a leak coming though the choke. Is that possible ?

I think its the choke because it doesn't increase my idle anymore and I may have felt some fuel leaking out of it. It will rev just fine but as soon as it goes into gear it bogs down and won't rev over 2-3k...I was playing with the choke, just wiggling around and thgastsd when it took of like a normal bike lol but then died back down

 

[XSLeo]

This carb guide was written for the XS650 carbs both the BS38 and BS34. The XS 650 BS34 carbs are very similar to your XS400 BS34 carbs. Follow the guide to tear down clean and inspect your carbs. You may find it was the carbs but I think your petcock is bad.
On the petcocks the stock non vacuum petcocks have a different bolt pattern where they mount to the tank.
The #20-0050 is one of these. The mount bolts are closer together.
The 320-0019 is the regular petcock with a larger base plate so it can have mount bolts the right distance apart to fit where a vacuum petcock goes.
Just be sure the XS650 petcock fits an XS400 tank. My friend XJ750 the XS650 petcock won't fit. That bike uses a taller petcock. The distance between the body and the mount flange is about 1/4 more. But then again making a spacer that will adjust the petcocks is easy.
Leo

 

[Recreateme]

Thanks Leo, I have a hard time thinking its the carbs (other than maybe the choke leaking air) because in neutral it will rev fine and gives no sign or issue until a gear is selected and then it bogs down and won't have enough power or rev over 3k. But if I pull the clutch in it will go back to acting normally.

That's gotta mean air leak somewhere right?

When I buy the new petcock I'll get the carb intake boots as well. From mikes, these would be their part 20-3068, it the only 34mm boots they sell, they would fit a 79 400-2f right?

 

[XSLeo]

I forgot to put in the link, stupid me. www.amckayltd.com/carbguide.pdf
Those are the symptoms that lead me to believe it's carb related.
I can't recommend Mike's carb boots, or much of their rubber parts, won't hold up to ethanol fuels. Get Yamaha or Tour Max.
Read the carb guide. It has a tuning section.
Checking for air leaks around or in the carbs is carb related.
If the choke, It's not a choke but an enrichener. Has no effect then the enrichener circuits in the carb or plugged. This sounds like a carb problem too.
Read the carb guide.
Leo

 

[Recreateme]

Good stuff. Thanks again!

I think I'll most likely take some time today to check and removed the carbs and boots (again lol)

Ill also probably remove the tank, drain it and remove the petcock for inspection and cleaning. There seems like a good chance at one time old fuel may have sat and could haves gummed it up and the PO only cleaned the carbs.

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[Yamaguy55]

Good stuff. Thanks again!

I think I'll most likely take some time today to check and removed the carbs and boots (again lol)

Ill also probably remove the tank, drain it and remove the petcock for inspection and cleaning. There seems like a good chance at one time old fuel may have sat and could haves gummed it up and the PO only cleaned the carbs.

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The part in the petcock that seals against the lever part that has four holes in it and is made of rubber: you can flip it over and get some more life out of it. Not a long term solution, but does allow a month or so to gather up your parts.

Item number 9 on the Yamaha parts list for the 1980 model in the fuel tank illustration. They call it a valve. I call it a four hole rubber thing.

One thing about parts: as some have found out with aftermarket carb mounts, they often don't last long. The factory parts always fit and last as long as the one you're removing. The danger with factory parts with old bikes is that some of the rubber parts have half died sitting for years on some shelf. The stuff you buy should be reasonably soft and pliable. Check for cracks and "dry rot" for lack of a better term. Avoid aftermarket cheap fixes. You'll just be doing it again in no time, this time with what you should have used the first time. I learned this the hard way years ago. I'm not saying factory is the best, I am saying that aftermarket can be crap.

Many things are just fine from other suppliers. Sunstar made the factory sprockets, and will cost a lot less from them than Yamaha. Good chains are good chains. Motion Pro cables are excellent. Some of the engine parts like valves and such should either be Yamaha or top drawer like Kibblewhite unless you want to go back in there. Same for pistons, although I am not aware of any others. ART makes the factory pistons. They do for the japanese motorcycle makers like TRW did for US auto manufacturers in supplying almost all of the OEM pistons. That may no longer be true, but was when the 400s were built.

 

[Recreateme]

Yesterday i removed the petcock and tank and drain them and dismantled the petcock, and to be honest it looked almost brand new inside.. at least the rubber 4 holed washer thing looked brand new lol I flipped that anyway. I did notice that the tube at the top with the filters did have a bunch of gunk collected at the bottom, so i washed everything out and put it back together.

It seemed to flow freely in the PRI setting and remained closed in both ON and RES. I ordered a new one anyway (it was half an excuse to buy Euro Bars in the same order anyway lol)

Before i took it off i sprayed WD-40 around the carbs and boots and spark plugs looking for air leaks and got nothing.. I did notice that it would die (or lower its RPM) whenever the choke was fully out, and wouldn't rev past 3k with the clutch in.

It was weird it would idle, and rev in Neutral but when i pull in the clutch and rev it wouldn't pass 3k, but as soon as i released the clutch it was right back to seeming normal...

I now think it is either that the petcock was starving the carbs or the Choke has an air leak or gunk in its path perhaps.


BUT i was riding normal for about 10 minutes yesterday lol I got on it took and it stayted below 3K and i kinda giggled the petcock and played the with the selector and it took off and would rev great for a couple trips around the block.. That is what made me think it was the petcock rather then choke this time lol

 

[Recreateme]

UPDATE: I installed the new petcock for mikesxs and it fits great but I had to put it in facing inwards or else the RES setting wouldn't work due to the selector hitting the tank lol
Works great otherwise.

Now that that I replaced I am fairly certain this is a choke issue since it didn't solve the problem at all.. Putting the choke full out causes the bike to drop in RPM and stall out. That says air leak to me. I'm going to pull it next week for repairs, it kills me to wait that long but I've got my safety course this weekend and my anniversary Friday.

Ill update next week!

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[Recreateme]

ugh.. this is getting very frustrating..

I put on a new petcock. Its working well.

I removed and took apart my carbs again, and cleaned everything very well, but still can't get it to rev past 3K under load. It will rev fine until i put it in gear and engage, then it just gets up around 3K and regardless of throttle position never passes it..


I thought it was the choke because it dies whenever the choke is fully opened, essentially it seems like its doing the opposite of what its supposed to, at 1 click out the RPMs drop, and full out the bike dies.
I took it apart and cleaned all the passages, and looked it all over and found nothing that would suggest a problem

I need some ideas here...

 

[16VGTIDave]

Is the engine surging or just not accelerating? Smoke from the exhaust? What do the plugs look like? Have you carefully examined your carb diaphragms? What about the ignition advance?

 

[Recreateme]

I'll run through it here, this is typical 90% scenario..

-I have learned the choke doesn't work so i leave it all the way in, and push the starter/kick it over a couple times it eventually starts.

-If i give it throttle it will rev as if everything is fine, all the way up to redline.

-If i pull the choke 1 click out the RPM drop a bit, but not much, if i pull the choke all the way out it just sputters once or twice and dies. No smoke from the exhaust.

-At this point i usually restart it and it fires right up, it revs normally, no smoke, no pops nothing.

-I get on it to ride it, pull in clutch, click into first and throttle up around 2K to engage the clutch, and then it enages and I'm off.

-At this point im going like 8kph and give it throttle and it revs up smoothly to about 3k and at that point it just holds it, regardless of if the throttle is wide open or not. No smoke, engine sounds healthy as to where it should sound going 10KPH in 1st at 3K lol

-If i let off the throttle it will drop back to idle speed and i can pull in the clutch and it will rev fine, when i let the clutch out it goes back to maxing at 3K...

-So i can cruise around at WOT going 10kph lol If i open the choke at any point the RPM will drop down, and full choke it dies, as if its got no gas..

-Occasionally it will rev over 3K and running seemingly normal but that only lasts like 30seconds and then goes back to staying low...


I pulled the plugs yesterday and the right plug has some carbon on it, looks rich, and the left looks lean..

BUT the left is lean because of my broken idle mixture screw tip... This shouldn't stop it from pulling past 3K though..

However these plugs are from the PO and i haven't cleaned them or anything, so they may not be accurate (i know, i will buy new ones this weekend)

I have not checked my timing advance at all.. it scares me a bit lol I was hoping i didnt have too since it seems to idle well..