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My 750 SE will neither start nor run properly

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My 750 SE will neither start nor run properly

PostAuthor: Mottek » Wed Oct 30, 2019 9:09 pm

Heyo Guys,
I've got an XV 750 SE 1982and while I normally love her, lately I had some problems.
It started out with the front exhaust leaking where cylinder and pipe meet, also my cylinder head gaskets are leaking for some time. The last two or three months she didn't start most of the time electrically and I had to bump start more and more. About three weeks ago I got a new battery, but that didn't help, around that time it started that I couldn't let her idle it felt like someone was chocking the engine. Last Sunday I was driving home from my GFs place and for the first time I felt a bit of a powerloss, but I still could reach 130 km/h easily. On that journey In also got a ton of misfires in both of the cylinders.
Something must have happened, because I spend the last two days trying to get her to bump start, without much success.
I tried re/adjusting the pilot screws to 2 1/2 revolutions, but no luck. A few times when I was running flat out with her I got a few seconds of engine running, but it was like she couldn't get enough air.
Shw is currently without air filter,as those get very dirty very quickly, almost as if she was backfiring into the filter, which can be felt sometimes.
The spark plugs are quite black and I never got them to turn a nice colour.

Could the months of running with no properly sealing exhaust have killed my engine?

Please help, I love that bike and need her for getting around, as I've got no car

Mottek
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Re: My 750 SE will neither start nor run properly

PostAuthor: Arjay » Wed Oct 30, 2019 11:10 pm

Sounds like you need a more systematic approach.

I apologize if this is already known.

1. check plugs. Tan is the correct color. Black -suity like from a chimney is too rich.
Gray/whitish non tan is too lean. Shiny wet black could be oil or gas.

Too lean causes backfiring as low fuel mix fails to fire in compression stroke but is later ignited in tail pipe on different exhaust stroke.

2. Check spark. A healthy spark is bright blue. Orange spark is weak. Coils go bad.

3. Check compression. Mine is arround 90 something, last time I checked. The compression levels shouldnt vary much between cylinders.I think the rule is not over 10 lbs difference. Not sure.

4. Check fuel flow.Open bowl drain screws. This is good thing to do every now and then anyway. It drains gunk that slips past gas filters. Be careful when tightening bowl screw back in. The metal is brittle so only screw in with mild strength. A gentle snug.

5. Check for airleaks. Some guys use brake cleaner and spray over carbs while engine is running. The idea is to detect a leak by listening to the engine react as brake cleaner is flammable. So if there is a leak the vapor gets sucked in and engine revs faster from combustion.I

6. Check your voltages at the stator connector, battery, batteries should have an amp test. Weak batteries produce weak spark. A battery amp test uses an instrument to create a dead short across the two battery terminals. The meter reads the amperage the battery outputs but more importantly if the battery has a bad cell, that cell will begin to boil rapidly revealing it is a bad cell. As to the stator wires you can unplug and test their resistance, that an pickup coil resistance as well. A bad rectifier/regulator will fail open or shorted. If it fails shorted then the main fuse pops. If it fails open then eventually your battery goes dead.

The best book for Virago repair is Clymer ISBN 0-89287-692-1

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Re: My 750 SE will neither start nor run properly

PostAuthor: chrismalm » Sun Dec 15, 2019 10:22 pm

:yup:
I have an idea that is more imagination than scientific conclusion, so please forgive me if I make false assumptions. I also suspect low compression. If your air filter gets black quickly, I suspect blow-by. If you have been running your bike in a dusty environment with no air filter, you may be wearing out your engine prematurely. Also your leaking head gaskets are contributing to the low compression. I am also guessing your oil is overdue for a change, contributing to cylinder and bearing wear.

You can maintain your bike to improve performance and minimize further wear, but restoring it to like-new performance would probably require an engine rebuild and cost more than it's worth.
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Re: My 750 SE will neither start nor run properly

PostAuthor: mark75 » Mon Dec 16, 2019 4:28 pm

Howdy!

Welcome to VTF.

I would make sure the battery is fully charged (1st)
:yup:
then get new spark plugs (Bosch Platinums, set @ 0.028" (WR9?))

then check the condition of the spark plug cables/caps

next I would make sure everything is snug/tight & well sealed (no leaks) much-as-possible.

Gen-I carbs are very susceptible to carb issues (especially if you're running with no filters) --- you may be dropping one cylinder as a result. (sooty plug would indicate carb issue)

I beg to differ about best service manual --- IMO the OEM (vintage 1982-1st edition) 920J one is the best. Yamaha LIT-11616-XV-91
if you can find one still intact, there is a very large (color- 4 page fold out) schematic that can't be found anywhere else [that I know of], plus it was written to educate their techs about the "new" Virago, so it has lots of good pics and illustrations.
[new '85-'99 S/M pics lack detail, like over-copied xerox pic]
"Over the Mountains of the Moon,
Down the Valley of the Shadow,
Ride, boldly ride" ... E.A. Poe


Time flies >>> whether you're having fun __ :bike: or not :fan: ---

Any day above ground is a good day.
so Laissez les bon temps roulez. just roll safely --- Semper Vigilans...always watchful

"I hear and I forget, I see and I remember, I do and I understand." ... Confucius*

If you don't read the news you are not informed.
If you read the news you are misinformed.
... Mark Twain


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