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Message started by DragBikeMike on 04/15/18 at 15:10:54

Title: Vacuum Petcock
Post by DragBikeMike on 04/15/18 at 15:10:54

In my prior post regarding the TEV I noted that the manifold vacuum drops as low as 0.5"Hg under WOT conditions. That 0.5" value was noted at WOT going up a grade. That concerned me because I know the petcock depends on vacuum to hold it open. I decided to test the petcock to see how much vacuum is required to keep it open.

This was a hard test to pull off because the required vacuum is so low. As best as I could determine, that valve opens about 1.25"Hg. That would imply that it will close under WOT conditions. I can't believe the designer would deliver something like that....but I guess they didn't expect anyone trying for a land speed record on an S40.

I plan on getting rid of that little gem ASAP, and until I have something else to replace it with, I'm stayin away from the WOT stuff. Also, all should keep in mind that long winded cams with lot's of overlap send manifold vacuum into the toilet. If you have a big cam, or intend to install one, plan on ditching that petcock.

I see lot's of posts on here about the Raptor petcock and lot's of comments about bogus parts floating around. Can any of you attest to the bonafide Yamaha part number, year, make & model of the sourced vehicle? I appreciate your consideration.

Title: Re: Vacuum Petcock
Post by Serowbot on 04/15/18 at 16:38:11

This is the Raptor part no. 5LP-24500-01-00
2003 Yamaha Raptor 660 YFM660R

NOTE... ( all Raptor 660 petcocks fit,... I included a specific model/year for internet ordering, and anal retentive counter salespeople...) (Raptor 350 petcocks won't fit)...

Title: Re: Vacuum Petcock
Post by DragBikeMike on 04/15/18 at 20:00:00

Thanks Serowbot. That's the info I was looking for.

Title: Re: Vacuum Petcock
Post by norm92de on 04/15/18 at 20:52:36

FWIW,
I had a brand new bike. The first week I had it I removed the stock petcock, warranty be darned. That thing is an engine failure waiting to happen. Of course I fitted a YAMAHA raptor petcock to replace it.

P.S. anybody want to buy an unused stock petcock. :)

Title: Re: Vacuum Petcock
Post by justin_o_guy2 on 04/16/18 at 16:17:15

For sale
Headache
Cheap

Title: Re: Vacuum Petcock
Post by norm92de on 04/16/18 at 17:00:25

You've got it Justin.

Title: Re: Vacuum Petcock
Post by HAPPYDAN on 04/16/18 at 19:05:52

I've got one, too. Low miles, hardly used. Doesn't work. Go figure.

Title: Re: Vacuum Petcock
Post by Blueraven on 04/17/18 at 08:56:35

Is there any reason specifically to replace the petcock if the OEM one is working fine?
I'm always faired well going off the premise "that if it ain't broke don't fix it".

BTW- I do have the Raptor petcock sitting in my toolbox just in case the OEM one does fail.

Title: Re: Vacuum Petcock
Post by batman on 04/17/18 at 09:14:44

Blueraven, It's not if it fails , it's when it fails. the carb pulls the least amount of vacuum at WOT , and can fail at the worst time . as in a passing ,and the diaphragm closes and the motor dies ( and so may you).

Title: Re: Vacuum Petcock
Post by Blueraven on 04/17/18 at 13:38:24

2526332A2629737F470 wrote:

I see you're point.
I guess I'll go ahead and swap it out since I already have the Raptor.

Thanks,

Title: Re: Vacuum Petcock
Post by norm92de on 04/17/18 at 13:41:56

Batman is right. Get the petcock changed pretty soon. As he said "not if but when" Ethanol really does a job on the diaphragm over time.

Title: Re: Vacuum Petcock
Post by Aaronjkjkk on 03/26/21 at 22:49:46

Hi I did a vaccum test and the needle jumping all over so my wife showed me book that says that could mean a head gasket problem so my question is do u think the head gasket would be bad

Title: Re: Vacuum Petcock
Post by DragBikeMike on 03/27/21 at 01:57:31

The jumping needle is normal. It's a big single cylinder engine so the vacuum pulses like crazy. You need a snubber in the line. A small 1/4" needle valve will do the trick. Adjust the snubber valve until the needle steadies out. It (the needle valve) will be almost completely closed. I used a rig like this.

I doubt that you have a problem with your head gasket.

Title: Re: Vacuum Petcock
Post by Dave on 03/27/21 at 03:41:57

5E7E6D707175747574741F0 wrote:

Hi I did a vaccum test and the needle jumping all over so my wife showed me book that says that could mean a head gasket problem so my question is do u think the head gasket would be bad

Yep, I tried putting a vacuum gauge on my little Ninja. The needle was swinging so fast it was just a blur! The intake manifold is subject to wild pressure pulses....and it is normal.

The stock vacuum petcock works for a while - then it doesn't. Unfortunately the failure mode is not always the same and can cause owners to look for problems that don't exist.

One mode of failure is the diaphragm gets stiff, and it begins to limit fuel flow. As noted in DragBikeMikes original comment the engine vacuum drops as the throttle is open wider....and very often the engine will run fine until the rider gets out on the interstate and rides fast. This failure is a real disaster waiting to happen - 75mph in traffic on a 4 lane highway and your engine quits! Riders often claim that the bike starts back up after sitting for a while - which often has folks looking at ignition or carb problems that don't exist.

Another mode of failure is the diaphragm that gets porous. This causes fuel to leak down the vacuum line to the carb. This can result in the engine running rich if the leak only occurs while the engine is running - or it can leak gas continually and fill the air box and engine crankcase with fuel.

For a reliable running bike - it is important that you replace the stock vacuum petcock with a manual one. And then you need to remember to turn the petcock on when you start your ride each day, and then turn it off at the end of each day you ride. I am a bit more focused about the issue and at the STOP sign down the street from my house I reach down and turn the petcock OFF.....and by the time I reach my driveway the engine is running out of gas (This allows my bike to be parked with a nearly empty float bowl and reduces the chances of my jets being clogged up when the fuel evaporates or gets stale).