weak spark

[Cray]

ok i got an old 77yz250, recently rebuilt top end. has cdi ignition. it has a weak spark and fouls plugs left and right, and runs like a turd obviously. i suspect the stator is failing but i dont know how to check it?. i have voltmeters and a little no how but im no expert electrician. anybody got some advice on how to check this stator?..
Cray..

 

[KXKen]

I suggest you buy a service manual.

Also you might want to post this in a different forum like the basic maintence bike repair

 

[David Trustrum]

Also you might want to post this in a different forum

Are you trying to insinuate that a ‘77 YZ isn’t advanced technology itself?

 

[David Trustrum]

OK I’ll stop being a smarta%^*&

Do the simple stuff first. Sparkplug, Sparkplug cap, clean or replace airfilter (replace by now) Check that the coil is grounded properly. (clean plug contacts, check with meter etc).

Clean carb out properly esp pilot jet (never poke with wire though). Drain old gas out & check for water replace with fresh if unsure how old.
Wouldn’t hurt to repack the muffler & see how much carbon build up is in the pipe.

Then send the stator to someone to test rewind it. Most checks you do with a simple meter will probably only tell you if it is shorted or open, but the insulation breaks down under heat & causes drama. Check back pages of Dirtybike magazines for a rewinder.

 

[Danman]

Buy check with a meter he would mean to measure the resistance of the plug wire from the coil to the end were the plug goes. It should be very low. Resistance is the opposition to current. Low resistance = good current = good spark.

A quick check for spark would be to take the plug out and touch the bottom of it to the cylinder. Hold it buy the wire so you don't get a shock. Then kick it a couple of times. You should be able to see the spark. If it is a brite blue that not the problem.

Buy a service manual would be you best bet. It will tell you how to check for spark correctly and give you some good tips for troubleshooting.

 

[David Trustrum]

Hold it buy the wire so you don't get a shock

???????
Surely you are trusting that the wire is not breaking down, cause baby that’ll tickle you if it is!

Once a plug fouls it may not recover properly. Out in the open it may spark well but in a combustion chamber may just foul up again. I really would look at the HT lead & cap as well as they might have been the initial reason for the plug fouling. Use the recommended heat range.

Some ignitions are very weak at kicking speed. My 2000 GasGas is so weak you can barely see it, but it starts real easy & runs great at low speed.

& c'mon, where is he going to get a service manual for a 77? Besides in those days all they said was return to a Yamaha authorised agent for anything more than running out of gas.

 

[agitt73]

take your silencer off and ride to see if it runs like it should
the inside of my it250 fell apart and pluged $300 later
in electrical parts i figured it out

also could be jetting if it is stock your jett needle and main jett
is most likely worn out

 

[KXKen]

Originally posted by David Trustrum

& c'mon, where is he going to get a service manual for a 77?

I have no clue but I bet it is available. My 76 YZ80 had a darn good service manual.

 

[FenryHonda]

Service manuals are VERY available for this bike. They are on E-Bay constantly, plus Dennis Kirk (800-328-9280) has them for around $28.00.

I just picked up a '77 YZ 400 last month, and had the same problem as you. I found the problems to be:

a) deteriorated air filter- luckily the local Yamaha dealer had a NOS filter

b) plugged silencer- some moron had welded the silencer to the pipe so it was a good excuse to buy a DG National pipe and Type II silencer. The pipe was dented and the silencer was not the right one anyway.

c) very dirty, shellac filled carb- cleaned the carb and replaced all the jets, had stock jets and needed leaning out anyway, added Boyesen reeds also

At kicking speed, my YZ has a weak spark, but fires on the 1st or 2nd kick when cold, 1st every time when hot.

My advice would be to try these steps before you start disassembling the ignition system. ;)

 

[Danman]

Here is one pretty cheap

http://www.amazon.com/exec/obidos/ASIN/0892873299/chicagobiker/103-8488782-2651848

14 bucks used.

Surely you are trusting that the wire is not breaking down, cause ba

Yes, You should be O.K. if the insulation is in good shape. That is just a quick way to see if it at least has spark. Yes if its not in good shape it will light you up.

 

[David Trustrum]

Hey -I stand corrected, get a manual.

I never trust holding onto any part of the ignition system, I try to hook it under something to gnd it.

Actually ever since my brother tried to get me to hold the coil wire from the lawnmower. . .