My YZ 250 from 78 has absolutely no bottom power nor much torque. I have to get it screaming to move forward at decent speed. When I roll the throttle on, it moves but the motor has some difficulty to get going. Once it reaches a certain RPM, the power comes on suddenly and then it's ok. It really is annoying and it makes the bike difficult to ride. What should I look at? The piston is new, the reeds and the exhaust system is ok. I suspect a jetting problem but don't really know in which direction to go.
Problems with 78 YZ250
- Thread starter yzf426
- Start date
Does it idle?
What color is the plug after a ride?
Can you get a compression test?
Any blow by at the base gasket?
How do you know the timing is correct? (could be retarded)
Does it "ping" (pre-ignition) under load?
Does it double stroke under load at low RPM's?(Indicating a rich condition)
Does the expansion chamber have a big "dent" in it?
As compared to a modern 426-450 class 4 stroke it is going to be pipy as hell! Remember a 250 of that era MIGHT run as well as a current 125...and actually the current crop of 125's will have 4-5 more HP on top and at least as much bottom end if they are setup well. That era 250 was lucky to have mid 30's HP at the rear wheel. They didn't have much bottom end either. Pretty pipy things. Thats why I focus on the Big bores bikes of that era. They are like a current 250 power wise that vibrates like an out of ballance washing machine! You build forearms to ride these things. Have to eat your spinach to ride those things for any period of time.
If all the mechanicals are good and the timing is right (HUGE assumption), being boggy on the bottom is usually aggrevated by lean jetting. Pilot & needle & slide. If you can assume they were correct check for corrosion in those jets.
The other reasons for it to be soft on the bottom include retarded timing and low compression (both top end and crank case compression have to be right). Top end compression from rings/cylinder/piston & bottom end from base gasket/piston port timing/reed valves.
Kinda hard to debug online without hearing that motor.
If that plug is light colored I would start by going one groove richer on the needle (clip down one slot pulling the needle out further) to see if anything changes. That would give you a clue if its a jetting issue. If you were good at feeling how your motor is running I would also consider advancing the timing a bit...just don't advance it to where it will ping (pre-ignition usually under load)& pay close attention to the spark plug plug to make certain you don't got too far advanced & melt that thing! If you NOT good at sensing your motor, let someone who is make those changes. Old Yamaha parts are not easy to find.
What color is the plug after a ride?
Can you get a compression test?
Any blow by at the base gasket?
How do you know the timing is correct? (could be retarded)
Does it "ping" (pre-ignition) under load?
Does it double stroke under load at low RPM's?(Indicating a rich condition)
Does the expansion chamber have a big "dent" in it?
As compared to a modern 426-450 class 4 stroke it is going to be pipy as hell! Remember a 250 of that era MIGHT run as well as a current 125...and actually the current crop of 125's will have 4-5 more HP on top and at least as much bottom end if they are setup well. That era 250 was lucky to have mid 30's HP at the rear wheel. They didn't have much bottom end either. Pretty pipy things. Thats why I focus on the Big bores bikes of that era. They are like a current 250 power wise that vibrates like an out of ballance washing machine! You build forearms to ride these things. Have to eat your spinach to ride those things for any period of time.
If all the mechanicals are good and the timing is right (HUGE assumption), being boggy on the bottom is usually aggrevated by lean jetting. Pilot & needle & slide. If you can assume they were correct check for corrosion in those jets.
The other reasons for it to be soft on the bottom include retarded timing and low compression (both top end and crank case compression have to be right). Top end compression from rings/cylinder/piston & bottom end from base gasket/piston port timing/reed valves.
Kinda hard to debug online without hearing that motor.
If that plug is light colored I would start by going one groove richer on the needle (clip down one slot pulling the needle out further) to see if anything changes. That would give you a clue if its a jetting issue. If you were good at feeling how your motor is running I would also consider advancing the timing a bit...just don't advance it to where it will ping (pre-ignition usually under load)& pay close attention to the spark plug plug to make certain you don't got too far advanced & melt that thing! If you NOT good at sensing your motor, let someone who is make those changes. Old Yamaha parts are not easy to find.
Last edited:
- Thread starter
- #4
Well, it's getting difficult. I've checked everything you mentionned. All the settings are stock. The main jet is a 360, the low speed jet is a 60, the needle is the correct one and the clip was in the 3rd groove. Only the air screw was totally closed. I've put it on 1 1/2 turn out. The spark plug was a little black. The ignition was also correct (1.7 mm BTDC as specified for the 250YZ E). There is absolutely no blow by that I can see around the base gasket. I've done a cylinder compression test and it gives a 6 to 1. I haven't done the engine compression test as I don't have the right tools but from what I see behind the ignition and from the cases, it should be alright.
I know that bikes from this era are much pipier than modern bikes(that is, power valve equipped bikes) but when I compare it with a friend's 79 CR 250 or another friend's 78 Husqvarna 250, my bike is really really pipey. It's the transition from nothing to everything that makes it hard to ride. I always have to get it screaming in what seems to be one gear too low to get it running at reasonable speed.
Do you have any other ideas?
I know that bikes from this era are much pipier than modern bikes(that is, power valve equipped bikes) but when I compare it with a friend's 79 CR 250 or another friend's 78 Husqvarna 250, my bike is really really pipey. It's the transition from nothing to everything that makes it hard to ride. I always have to get it screaming in what seems to be one gear too low to get it running at reasonable speed.
Do you have any other ideas?
When you checked the ignition timing, any sign of blow by through the seals on the crank? Does it use Transmission oil? (Black plug usually means a richer situation. BUT sometimes the thing could be pulling transmission oil through a leaking main seal. A leaking main also hurts crankcase compression. Check for that).
I'm not familiar with the needle on those Yamaha's. When you say "third" position, are you counting from the top or bottom. Does it have 3 or five positions.
When you turned the air screw out what happened?
Having the air screw all the way in means someone else might have been fighting this situation as well.
Reed valves? Has that motor been ported by someone?
It should be in the same ball park as your freinds Husky's...certainly worth de-bugging.
Wonder if the ignition CDI module is OK.
I'm not familiar with the needle on those Yamaha's. When you say "third" position, are you counting from the top or bottom. Does it have 3 or five positions.
When you turned the air screw out what happened?
Having the air screw all the way in means someone else might have been fighting this situation as well.
Reed valves? Has that motor been ported by someone?
It should be in the same ball park as your freinds Husky's...certainly worth de-bugging.
Wonder if the ignition CDI module is OK.
- Thread starter
- #9
I've not noticed any blow by through the seals on the crank but I've also not removed the ignition. I will do it this afternoon.
The needle has 5 positions and it was on the 3rd from the top.
I'll also remove the reeds just to be sure and I haven't tried the bike yet with the new air screw position. As for the cylinder, it is completely stock (no special marks of any kind).
I suppose the bike has just been sitting around for 20 years (it's almost new). The crank seals could thus be a little shot. I changed the base and cylinder head gaskets when I changed the piston.
The needle has 5 positions and it was on the 3rd from the top.
I'll also remove the reeds just to be sure and I haven't tried the bike yet with the new air screw position. As for the cylinder, it is completely stock (no special marks of any kind).
I suppose the bike has just been sitting around for 20 years (it's almost new). The crank seals could thus be a little shot. I changed the base and cylinder head gaskets when I changed the piston.
Try a larger pilot jet. Or at least a new one. Even if it appears to be "open" it may be crapped out. You can get new pilot jets from the catalog house of your choice. Im assuming its a Mikuni.
Make sure the choke plunger is not leaking.
In my un-scientific tinkering, raising the needle position has helped me up top, but not so much on the bottom.
Heres another thing...check
http://yzworks.tripod.com
A friend of mine, Glen, who is part of that site had alot of issues with his old 250 being rough down low after he rebuilt the whole thing, but he's got it running really well now. He might have some ideas.
Im also thinking reeds as someone else did. Sticky, or inflexable? That is once the RPMs get going theres enough vacuum to open them up enough? Leaky boot?
The worst case of being "pipey" I ever saw it was an old IT175. Man, when I was a kid we rode that thing for a month before we found out that if you just leave the gas on long enough, it actually got up and went! It was a complete turd for the lower 80% of the throttle. :uh:
Good Luck!
Make sure the choke plunger is not leaking.
In my un-scientific tinkering, raising the needle position has helped me up top, but not so much on the bottom.
Heres another thing...check
http://yzworks.tripod.com
A friend of mine, Glen, who is part of that site had alot of issues with his old 250 being rough down low after he rebuilt the whole thing, but he's got it running really well now. He might have some ideas.
Im also thinking reeds as someone else did. Sticky, or inflexable? That is once the RPMs get going theres enough vacuum to open them up enough? Leaky boot?
The worst case of being "pipey" I ever saw it was an old IT175. Man, when I was a kid we rode that thing for a month before we found out that if you just leave the gas on long enough, it actually got up and went! It was a complete turd for the lower 80% of the throttle. :uh:
Good Luck!
original dirt
Member
- Aug 17, 2000
- 9
- 0
Sound suspiciously like bad crankshaft seals to me!. Do a quick check of the dry side seal. Remove the mag flywheel case cover. Look for any signs of wetness. Any sign other than absolutely dry is not good. Next grab the flywheel and try to lift it up and see if you detect any movement other than radial rotation ( if any sign up and down or left and right makes a slight click, it's very bad news). If the wet side seal behind the primary drive gear is bad the motor will smoke under load, and foul plugs with visible droplets of tranny oil. Bad seals are externally replaceable on that model, bad case bearings mean a total tear-down. Low/no power on less than 1/2 throttle is indiciative of leaky seals on a 250 mxer.
markthomps
Sponsoring Member
- May 27, 2000
- 255
- 0
Sounds like a slipping clutch to me, if the jetting all checks out. This was not a hugely pipey bike in its day.
original dirt
Member
- Aug 17, 2000
- 9
- 0
The higher the rpm, the worse the clutch would slip, and the higher the gear selected the more likely it would slip. Sonds like this thing is a terd at low to middle rpms. Not likely to be a clutch problem. Could also be bad reeds, piston skirt broken, modified, or damaged, leaking exhaust, restricted air intake, etc.
od
od
studmuff1228
Member
- Aug 23, 2005
- 1
- 0
i have the same bike and it will act like that if i run 89 in it doesn't it require 90+ if so is that what you are running? I'm not an expert but their are my 2 pennies
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