Vintage Dirt Bike Q & A

I have finally got My YZ 465 up and running and somethings just not right?
New bore and hone, new piston and rings, new pipe and silencer, new filter, clean carb and jetted as follows:

Pilot Jet - 50
Main Jet - 380
Slide - 3.0
Needle - 6DH3
Needle Jet - Q8

It is hard to start but once its warm it will re-fire with a couple of kicks.

It just feels weak? Maybe more like a 125? It doesn't spit and spur and sounds fine? I tried flipping the choke on while riding and it didn't make much difference? Plus it will back fire once and a while when trying to start it and the plug appears ok?

Timing maybe?

Help

Air-leak maybe?
Have you done a comp test on it?
Looks great by the way, well done!!!

your pipe and silencer may also be coked up.

this will certainly kill the power for sure.

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Brand new pipe and silencer so I don't think thats it? It also has new intake boot, fresh bore, new piston and rings, new air filter and new ignition coil. It is at the Dealership now and hopefully they can figure it out? I have My first vintage race coming up in about week I haven't even got to ride this thing yet

Let us know what they find. Always good for future reference. I bought a 82 yz125 and it had a fresh rebuild from the previous owner, but it lacked power. After some checking, I found the power valve was installed backwards. I turned it around and and it felt like it gained 15 hp. Good looking bike btw.

Ok, It's almost ready to race The lack of power and back firing was from the head not torquing down due to the head bolts running out of threads. Replaced the head studs and head nuts. For some reason the jetting was way off and the bike acted like it had an air leak. My mechanic suggested a new carb and we started from scratch. It took about a day but We now have it pretty close to perfect Now the bad news My sub frame is bent about an inch and My swing arm is tweaked
Luckily the swing arm I picked up on ebay is straight and the bearing kit will be here Wednesday. I am going to try and heat the frame up and bend it back into place It is getting close

Sweet!

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

any updates Frank?

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Updates?

Well in my first race back in over 7 years I finished first in the 500 expert class and second overall in the +50 expert class. But, in my last moto of the day, I was all excited to pull a holeshot and win, but when we took off the 465 started to foul the plug .... I popped, spitted and sputtered around for about a half of lap before luckily it came back to life. Then I went all crazy and through all the dust I fought my way back up to second place. The problem found out later was the seal on the air box was leaking and sucking in all that dust The day was over and I didn't find out about this until the following week when I found that I could kick the big 465 over with my hand Instead of a piston ring clearance of 13 thousands, I had around 1/4" I don't know how it didn't break the piston! Luckily I had a spare cylinder that was ready and all I needed was a new piston and rings Luckily everything else appears to be undamaged? So for now it is back together, Air box re-sealed and ready for this weekend ..... Temps are forecast to be in the 50's so I may richen it up one on the main to be safe? I still have to find time this week to braek it in though before then

thanks for the update.i hope it works out for you!

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Ok I may have a problem

Fired her up last night to break it in. Started on 3rd kick, let it warm up for a minute or so, took off down the road and it would barely take any throttle. Continued slowly increasing speed going thru the gears for a few minutes. Still felt like it was not cleaned out so I tried to open it up and it started the plug fouling popping and spiting again I was able to limp back to the car. Keep in mind it already had a new plug in it to start but I went ahead and changed it again? Rode it again and this time it ran better but still felt like it wouldn't clean out so I didn't push it?
Tonight I fired it up on the 2nd kick, warmed it up and took off again. This time I was more aggressive and it came to life finally but I'm not sure what is going on with the plug fouling Dealer was afraid it had a leaking crank seal but it doesn't smoke bad and tonight it ran fine. I just hope this doesn't happen again this weekend at the races!

Brand New Carb
380 Main
Middle clip on the needle
50 Pilot
NGK B8ES plug
93 Octane Gas Silkolene 40:1
Brand New Coil

Any Ideas

might get a TTO thermometer like this one.
http://www.ebay.com/itm/Trail-Tech-TTO- ... 01&vxp=mtr

i use one on a 1978 DT175 that has been ported and it has been a big help jetting the carb on it.

it cycles between actual real time temperature and max temp.

this picture shows the sensor lead on the spark plug.


for a long time it would not rev out at WOT like it should and it turned out the main jet was too large.

it the sensor goes where the spark plug washer goes,you just un thread the spark plug washer and put the cooper washer/sensor in place of it then button up the bike and ride it and get the engine fully warmed up and note the temperature at various throttle settings.

this link is to a Mikuni jetting guide online.
http://www.mikuni.com/pdf/vmmanual.pdf

it is a PDF so you will need Adobe installed on your computer.
here is another one in HTML so your browser should read it fine as it is a standard web page.
http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm

looks like you have some tuning to do pretty much. also make sure the air filter is clean,ignition timing is properly set,no restrictions in the exhaust and so on.
if set up right,you have a very potent bike there!

_________________
(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!