Yz 85 Bog Safety Concern Help !!!!!!!!!

johnwilhite

Member
Nov 21, 2002
79
0
Need some help really bad !!!!

My son is just coming back from a broken leg. Bike has been sitting almost 3 months. Been riding it for the past 3-4 weeks.

First race back is this coming weekend.

While the bike was sitting I went completly through it. He locked the tranny up right before he broke his leg. I bought a complete bottom end (BRAND NEW) out of a parted out new 08 yz 85.

I had to install our complete clutch and side case plus our ignition and flywheel. Put our top end, intake and carb also on it.

Before, on the old engine, never have we encountered a bog the 2 years we have owned the bike.

Since he has been back riding the bike (with rebuild) we have encountered a mysterious bog !!! It only happens part of the time.

The first week back on the bike about the 3rd day riding it hard he was coming out of a corner in third and it just fell flat on its face for about 2 seconds and took right back off.

I pulled carb and looked for debris in bowl and found nothing, checked jets and they were clean. Put back on the bike and turned him loose again. Ran great for the next week and bam, it happened again. This time bogged on a 3-4th gear straight.

Pulled carb again thinking had some kind of blockage or debris.
Found nothing !!!

Placed back on bike and has about 2 more hrs of ride time of no bog and last night bogged on face of a double !!!! Scared the shi@ out of both of us.

I am @ a loss here and need some advise on where to begin. I am thinking of messing with the float to put more fuel in bowl but I have never changed it since we have owned bike.

If it was fuel level shouldn't it bog all of the time instead of periodically like its doing.

I really want to get this fixed before I turn him loose racing this weekend.

I am going to have him ride it again tonight and I am going to video it and hopefully it will do it and you all can hear it !!!

Any advice is greatly appreciated !!!!
 

rmc_olderthandirt

~SPONSOR~
Apr 18, 2006
1,533
8
Check the tank, petcock and fuel lines.

If something is restricting the flow of gas to the carburetor you will experience this kind of random bog. You will always start off with the carburetor bowl full of gas so the bike runs fine at first. You could ride easy all day and not have a problem. When he starts riding hard and fast he is using the fuel faster than it flows into the carburetor and the level drops. Once the level drops too far it will bog, and if it drops even farther it will simply cut out.

Rod
 

Patman

Pantless Wonder
LIFETIME SPONSOR
Dec 26, 1999
19,774
0
Also CLEAN the carb just for the heck of it, a visual inspection isn't always going to show the problem.

Maybe the vented cap has crapped out, had that happen on a bike and it cause a mysterious bog or flame out. Once I tried riding with the cap loose but duct taped on and had no issues that pointed to the cap. Replaced it an no problems.
 

johnwilhite

Member
Nov 21, 2002
79
0
Thanks all for the advice !!

Here is a list of what I did last night.

1. Pulled fuel line from carb and turned pet **** on, fuel ran out STRONG !!

2. Pulled carb yet again. Tore carb completely down. Inspected everything
in there, all is good !!! Blowed compressed air throughout entire carb.
Was real careful reassembling.

3. I pulled seat and tank and checked all wiring. I did find one problem.When
I reassembled bike 3 months ago I noticed I had one of the kill switch wires
pinched between the radiator and frame at the upper bolt that hold
radiator to frame. I got somewhat of a SEMI and was thinking I was on to
something !! The wire was smashed pretty good and flat but there was no
signs of exposed wire. I went ahead and disconnected the kill switch
altogether.

4. I pulled gas cap and blew through the vent hose down through the cap.
Nothing seem blocked, blew through good. I then tried the vent hose, I
couldn't blow, but I could suck air. One way valve I guess. Seemed fine !!

5. I put bike back together and turned the kid loose. He rode the hell out of
that thing for a good 30 min. About 20 min. into the ride I was thinking
I had her dicked, no bog yet. THEN, it bogged ever so slightly on a
straight away. It was just a little bog, but, it still bogged. He continued
riding strong for another 10 min. and all seemed fine.


We will ride it again tonight and see what happens.

Keep the advice coming !!!
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Since it never seemed to be a lean bog, the revs would be real erratic before the bog, replace the float valve and seat. Not a visual inspection item, not even if it seemingly is shutting off the fuel by manual inspection, it should be replaced yearly, or when you are having odd jetting problems. If that does not fix it, get a leak down test done, could be an oval bore, or a bad crankcase seal.
 

Chili

Lifetime Sponsor - Photog Moderator
Apr 9, 2002
8,062
15
whenfoxforks-ruled said:
Since it never seemed to be a lean bog, the revs would be real erratic before the bog, replace the float valve and seat. Not a visual inspection item, not even if it seemingly is shutting off the fuel by manual inspection, it should be replaced yearly, or when you are having odd jetting problems. If that does not fix it, get a leak down test done, could be an oval bore, or a bad crankcase seal.

For the limited cost of the brass I'd likely replace all the jets with new just for the piece of mind as well.
 

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