'05 YZ125 Acting Like Outta Gas w/ Float Set by FSM

[cbh148]

I've got a 2005 YZ125 that I've performed a top and bottom end rebuild on 2 years ago.  The bike has about 52 hours on the meter (which I put on during the rebuild), and I've been diligent about replacing the piston/ring every 20 hours.  Engine-wise, the bike is all stock except for a set of VForce 3 reeds that I put on about 20 hours ago.  The pipe and silencer are stock, and I've repacked the silencer on 2 different occasions.  I run Pro-X pistons, Cometic gaskets, Klotz premix (30:1), 93 octane non-ethanol fuel, and NGK BR9EG sparkplugs.  I sealed the cases with Yamabond 4.  I've rebuilt the carb about 4 times using Outlaw Racing kits.  It's never been jetted, and I've put all of the carb settings back to the original positions listed in the Yamaha FSM.

I've been dealing with a frequent yet somewhat intermittent issue for more than half the time that I've owned this bike.  When it decides to misbehave, it will begin idling high during hard riding, much like it's about to run out of fuel, and if it's still ridden hard, it will cut out -- often on the face of a jump, which is very scary.  The bike will foul plugs from time to time if it's not being ridden at the track, and the plugs are always black, wet around the outside, and dry in the middle, so I assumed that the stock jetting was just a rich setting for safety (at the expense of the occasional plug), but figured that whenever I'm riding the bike hard, then perhaps a low float height setting in the Mikuni carb might be causing that intermittent lean condition any time the engine load depletes the low amount of fuel in the bowl.  

So I followed the FSM procedure of connecting a clear tube to the bowl to see the fuel level that the float valve stops it at.  I inscribed 2 lines into the side of the carb for the 9.5mm and 10.5mm boundaries that the level is supposed to be in when the carb is vertical.  Through a lot of trial and error, I was able to bend the float tab to a position where the fuel level satisfies that FSM spec.

I thought the bike was fixed after a few hard test rides with no sudden high-idle nonsense, but unfortunately the problem returned once I actually had the bike off at a track.  I just tested the float level again to see if the setting was repeatable after all the recent rides, and it looks like it's still dead-on to me -- or at least not low enough that I'd expect it to cause a fuel-starvation type of issue.  See the pic below, and notice the 2 long lines I scratched into the carb body indicating the 9.5~10.5mm range.  The small scratch up above is for showing how high the overflow tube reaches up.

I thought I might have a vacuum leak, but I made a pressure tester and it was able to hold rock-solid for days.  I also tested vacuum on the engine, and although it wasn't as capable, it still never dropped (raised?) back to atmospheric.  I even tried heating the crap out of the engine with a hair dryer to see if thermal expansion would open up the leak and thus cause a change in either the pressure or vacuum tests, and didn't see a significant change.  My VForce 3 reed setup has a gasket on top and on bottom, and I coated both sides of both gaskets with Permatex Ultra Black RTV (like I have seen recommended here on TT for this particularly troublesome model of VForce 3 reeds).

Any idea what it could be?  For reference, I've been riding the bike at 600~1100 ft. above sea level at different tracks around North Alabama.

[YZDOC]

Welcome to the TT YZ forum. 

Looks like you have a great seal on her. How do the reeds look after the 20 hrs you put on them? Check for chips and cracks, carcks you might not see unless removed and inspected. 

[diesel_dean1]

you could have a bad gasket, letting air thtough. But my 250 will rev up when riding hard so its not too too bad, just lower your idle when its really hot. another thing you can try is move your clip one position lower

[cbh148]

9 minutes ago, P84ONE said:

Welcome to the TT YZ forum. 

Looks like you have a great seal on her. How do the reeds look after the 20 hrs you put on them? Check for chips and cracks, carcks you might not see unless removed and inspected. 

Thanks!  Reeds don't seem to be in bad shape.  I bought the VForce setup used, so they've got more hours than the ~20 that I've racked up on em.  They appear to be fine though and don't seem to be letting light pass through.  Furthermore, the super-high-idle-after-riding-hard issue was present prior to installing the VForce reeds anyway.  I recall halfway hoping that the reed swap would remedy it way back at the time because my stock reeds had a little bit of delamination and chipping at the tips.

2 minutes ago, diesel_dean1 said:

you could have a bad gasket, letting air thtough. But my 250 will rev up when riding hard so its not too too bad, just lower your idle when its really hot. another thing you can try is move your clip one position lower

I was expecting the stock carb settings (including the needle clip position) to be able to maintain a pretty consistent idle, even if ridden hard, albeit without making the most power or without possibly fouling some plugs.  Seems like a dealership selling a bike that would behave like what mine's doing would result in some seriously pissed off customers, and/or some injured ones because once the high idle works up, it'll cut out during a pull out of a corner or up a jump face.

I was thinking that a bad gasket letting air through would've shown up in the pressure & vacuum tests.

[speedtoad]

Since its intermediate would look for an electrical problem.
Spark plug wire?
Grounds?
Stator?
I'm thinking stator. Sorry.

Sent from my SAMSUNG-SM-J320AZ using ThumperTalk mobile app

[cbh148]

1 minute ago, speedtoad said:

Since its intermediate would look for an electrical problem.
Spark plug wire?
Grounds?
Stator?
I'm thinking stator. Sorry.

I put a brand new OEM stator on during the engine rebuild, so I'm inclined to believe it's fine -- especially since the high idle craziness seems to be so consistent with the bike being rode hard.

[mlatour]

You should re-seal the reed cage assembly with Yamabond,

even if not in direct contact with liquid gasoline, 

over time the vapors alone will turn regular automotive RTV to mush and create air leaks.

 

I once smeared a small dab around my YZ125's carburetor's top cover (cover where the throttle cable connects)

to keep the cover's o-ring in place in it's groove while changing out needle settings.

In no time the usually high strength Permatex Ultra Copper turned to a mushy mess simply from the vapors.

[mlatour]

Your 0-60psi gauge isn't accurate enough to read any loss for this type of test,

you can easily order a 0-15psi gauge thru auto parts stores, a 0-10psi even better.

 

Pressurized at 6psi, absolutely zero drop of pressure for 20 minutes is the norm,

tested last summer my YZ125 went 30 minutes before seeing a 0.2 psi drop. 

Edited August 2, 2017 by mlatour

[cbh148]

22 minutes ago, mlatour said:

Your 0-60psi gauge isn't accurate enough to read any loss for this type of test

I think it serves just as well since I made up for the low gauge resolution with really long amounts of time (like "2 days later" as seen in that pic).  Because, as we can see, it didn't really leak that pressure down.

[speedtoad]

Ignition timing may be cause high idle.

Sent from my SAMSUNG-SM-J320AZ using ThumperTalk mobile app

[cbh148]

15 hours ago, mlatour said:

You should re-seal the reed cage assembly with Yamabond,

even if not in direct contact with liquid gasoline, 

over time the vapors alone will turn regular automotive RTV to mush and create air leaks.

 

I once smeared a small dab around my YZ125's carburetor's top cover (cover where the throttle cable connects)

to keep the cover's o-ring in place in it's groove while changing out needle settings.

In no time the usually high strength Permatex Ultra Copper turned to a mushy mess simply from the vapors.

I've just ordered some Yamabond 4 (I borrowed some of my friend's 2 years ago when rebuilding the bottom end).

I'm about to remove the carb boot and reed cage to see if perhaps that black RTV has broken down to mush and caused a vacuum leak there, which would be ironic because that was the first place I suspected a leak back months ago when I started investigating everything, and that's when I got a second VForce gasket and applied the black RTV.

It was weeks later that I suspected the float level to be low (which it was) and began sorting it out, so if it all comes full circle to currently being a vacuum leak from the reed block gasket that is causing pretty much identical symptoms to what the previously low float height caused, then holy crap -- what a diagnostics nightmare.  Lol

[cbh148]

So I ended up deciding to not remove my reed cage right away because that could cause me to miss the opportunity to truly diagnose the problem.  Ended up going to Home Depot to get a low pressure gauge to put on my pressure test rig.  I figured, if the RTV on the reed block gaskets was the issue, then it ought to fail a pressure test (and presumably a vacuum test too).  However, so far it's looking like it's holding pretty steady (see pic).

I got a float height gauge in the mail today.  Figured I'd see where my current float height setting is when going by the method Honda uses in the '06 CR125R FSM since they used a 38mm Mikuni as well.  For those that don't know, Yamaha's procedure is to attach a clear tube to the drain plug of the carb bowl, fill the carb with fuel, then compare the height that the fuel stops filling at -- like what I showed in the original post.  Kawasaki uses the same method on the '01/'02 KX125 as Yamaha, but with a different spec (2~4mm above the bottom of carb body, as opposed to Yamaha's 9.5~10.5mm), which is strange given that their both Mikuni's.

Both Honda and Suzuki use a different method -- their FSM's say to check the float height by the more common approach of tilting the carb over at the angle where the float tab just contacts the spring pin on the valve in the closed position, then measuring the distance from the body of the carb to the bottom-most tip of the float plastic.  They have slightly different specs (7.5mm for Honda, 7.7~9.7mm for Suzuki).

My Mikuni's current float level setting, which I believe meets the Yamaha spec, measures out to 9.75mm by the Honda/Suzuki method.  If that other manufucturers' style of measuring float height has any credence to my YZ125's high-idle-leaning-out-only-when-ridden-hard issue, then perhaps I should try bringing the float level up to Honda's 7.5mm spec?  Because, by comparison, my current 9.75mm setting is lean.  But on the same hand, compared to Kawasaki's spec, mine has too much fuel in the carb bowl, so perhaps there's other differences in the 38mm Mikuni's between these manufacturers?  I know some have a throttle position sensor, but I wouldn't be surprised if the float functions are no different.

[Kye Lippmann]

Hi cbh148 You have put a solid effort into the troubleshooting of your carbie fuel issue. Have you solved the problem?

I have the same bike, and the same problem! only this bike is fitted with a Athena 144 Big Bore kit.

So i'm glad to hear that a stock bike is facing the same issues.

I have found that basically its a case of the bowl running dry when on WOT for some time. and it doesn't want to refill quickly until the engine slows down, then it'll fill up happily and keep on going just fine until the next lot of WOT.

This problem seems to be worse with less fuel in the tank, obviously because of less fuel pressure in the fuel supply pipe.

I have also done a heap of troubleshooting and have eliminated everything except the carbie.

I also have a very old KX250 which had exactly the same issues which is now in storage, i would love to get to the bottom of this issue.

[Kinger317]

 Sorry if it’s been covered already but are the float needle and seat being replaced with OEM?  Vent lines all good? 

[Kye Lippmann]

Yeh, Sure thing! Vent lines all blown out with compressed air, no blockages found.

I did fit an aftermarket needle and seat to combat the TMX overflowing issue, this made the problem worse. So ive switched the new oring back to the original needle and seat, reset the float level etc......

in the process of all the googling, ive found the TMX needle block gasket issue.

So i pulled the needle block out and found that it had been previously sealed with blue RTV which should never be used in fuel applications and didnt even have a gasket.

is it at all possible (without the gasket or deformed gasket) for the pressures in the carbie to now prevent the bowl from filling up?

I have ordered the JD improved gasket kit https://jdjetting.com.au/product/tmx-jet-block-gasket-kit-mikuni/ and will see if this fixes the issue!

 

 

Edited January 4, 2021 by Kye Lippmann
spelling

[mlatour]

Make sure that RTV hasn't dissolved and blocked any air/fuel passages.

[Kye Lippmann]

1 minute ago, mlatour said:

Make sure that RTV hasn't dissolved and blocked any air/fuel passages.

 

1 minute ago, mlatour said:

Make sure that RTV hasn't dissolved and blocked any air/fuel passages.

Its the first thing I did. No blockages found. Also noted that the jet block just fell out like the RTV wasn't holding onto anything so definitely wasn't making a good seal. Now that I know about the gasket that is supposed to be in there, obviously there was a significant amount of bypass between the idle circuit holes and the main needle, probably causing all sorts of havoc! Cant wait to see what difference the gasket makes!

[mlatour]

The one in my 2010 YZ125 never failed but was quite deformed upon preventive inspection/replacement.

Likely not common in Australia but another source for that seal is thru BRP / Skidoo snowmobile dealerships

as some older 1990's high performance snowmobiles used a very similar carb, BRP p/n: 404161865

It's not an exact replacement as the BRP version has 3 small holes but can be carefully trimmed into a 'slot' like the original.

 

Is your gas cap / vent tube original or, do you have an aftermarket cap / stubby hose? (known to be problematic)

 

Edited January 4, 2021 by mlatour

[mlatour]

Forgot to mention, a tip to keep that seal in place during re-assembly is to coat it with tacky foam air filter oil.

 

[Kinger317]

40 minutes ago, Kye Lippmann said:

Yeh, Sure thing! Vent lines all blown out with compressed air, no blockages found.

I did fit an aftermarket needle and seat to combat the TMX overflowing issue, this made the problem worse. So ive switched the new oring back to the original needle and seat, reset the float level etc......

in the process of all the googling, ive found the TMX needle block gasket issue.

So i pulled the needle block out and found that it had been previously sealed with blue RTV which should never be used in fuel applications and didnt even have a gasket.

is it at all possible (without the gasket or deformed gasket) for the pressures in the carbie to now prevent the bowl from filling up?

I have ordered the JD improved gasket kit https://jdjetting.com.au/product/tmx-jet-block-gasket-kit-mikuni/ and will see if this fixes the issue!

 

 

 In my experience when the jet block gasket fails it leads to a rich condition. I don’t see how it could prevent Fuel from entering the bowl. Of course get that taken care of but I would still be suspicious of the needle and seat. I know old worn needles can stick when the Viton tip gets worn.  Is the needle hooked on the float correctly? Is anything bent?