"86 won"t rev past 3K

[wayne billings]

Howdy She was running fine until the last time I rode it. Started and warmed up normal, hit the road and it would not rev past three K. I installd a new ignitec besause the vboost module was not working. Figured I might kill 2 birds. Well the vboost cycles as it should and it starts and warms up fine but still has the same problem. I do not think the ignitec is at fault. I tested the pressure sensor and got 2.3 volts on the bench. I live at 7500 ft. elevation for what it is worth. Any ideas? Thank you in advance. PS the fuel is fresh premium and the pump checks in at 4 psi.

 

[Fire-medic]

it would not rev past three K.

Sounds like something is seriously out of whack in the carbs or possibly the ignition box. However since you already replaced the ignition box, I doubt that's it.

I've found some neglectful problems on bikes.

• loose main jet off the jet block sitting in the float bowl
• slide needle not fastened in place
• pilot jet loose in the jet block floating around
• torn diaphragm
• misplaced slide diaphragm, intact but not properly located in the relief channel where it's supposed to be
• stuck slide, not able to move

These bikes do not need premium fuel. Regular will work fine.

 

[99YAMAHAVMAX]

Howdy She was running fine until the last time I rode it. Started and warmed up normal, hit the road and it would not rev past three K. I installd a new ignitec besause the vboost module was not working. Figured I might kill 2 birds. Well the vboost cycles as it should and it starts and warms up fine but still has the same problem. I do not think the ignitec is at fault. I tested the pressure sensor and got 2.3 volts on the bench. I live at 7500 ft. elevation for what it is worth. Any ideas? Thank you in advance. PS the fuel is fresh premium and the pump checks in at 4 psi.

They don't run well on premium gasoline----87-89 works best.

 

[wayne billings]

thanks Fire medic an 99yamahavma for your help. After I posted I Tidy"d up a few things and started it again. This time I played with the choke while revving. At some point about half way on it started revving higher, got to 5K+. Not pretty but alot better, must be in the carbs! Thanks. The forum rocks P.S. the air box was off. I will take a second look at the fuel pump and filter. when I get it right I will let ya"ll know.

 

[99YAMAHAVMAX]

thanks Fire medic an 99yamahavma for your help. After I posted I Tidy"d up a few things and started it again. This time I played with the choke while revving. At some point about half way on it started revving higher, got to 5K+. Not pretty but alot better, must be in the carbs! Thanks. The forum rocks P.S. the air box was off. I will take a second look at the fuel pump and filter. when I get it right I will let ya"ll know.

Most likely carbs clogging---rarely revs above 4 g's with airbox off.

 

[MaxMidnight]

In a post on your original thread I posted:
'Have you checked the throttle slide rubbers for tears or perforations? When was the last time the fuel filter was changed and is the fuel tank breather clear?'
Did you?

If you still have the airbox lid off also check that the slides lift as you open the throttle.
The first rule of diagnosis is start with the basics before diving in and looking at the more complicated stuff.

You say you have 2.3 v at the pressure sensor which is what's expected at ambient pressure. To ensure it is working as intended you would also need to confirm the output voltages between + & - 600mmHg.

 

[Fire-medic]

600mmHg.

23.62" Imperial

 

[wayne billings]

Thanks for the chart. I don't have the gauge to verify those specs, all i can verify is 2.2 volts I do have a mighty vac that reads inches of mrecury or cenimeters of mercury . Have a good weekend.

 

[wayne billings]

Thankyou maxM. what does 600mmHg thanslate to in inches or centimeters?

 

[wayne billings]

I removed the carbs in between posts and found the vboost cable disconnected at the manifold.? Other than the obvious of no boost @6K is there a problem?

 

[99YAMAHAVMAX]

I removed the carbs in between posts and found the vboost cable disconnected at the manifold.? Other than the obvious of no boost @6K is there a problem?

There is no "cable" to the manifold--there is a vacuum line from the servo to the #2 carb manifold---there are however 2 cables from the vboost motor to the transfer servo box and a 2" cable from the box to the vboost.

 

[wayne billings]

99 thanks

 

[99YAMAHAVMAX]

99 thanks

Why do I suddenly feel like Barbara Feldon?

 

[dannymax]

Why do I suddenly feel like Barbara Feldon?

Going way off topic here but pictured below is "99" my '99 named specifically after that very lovely lady mentioned above!

 

[Fire-medic]

"Oh Max!"

 

[MaxMidnight]

Thankyou maxM. what does 600mmHg thanslate to in inches or centimeters?

See the top of post #7.

I removed the carbs in between posts and found the vboost cable disconnected at the manifold.? Other than the obvious of no boost @6K is there a problem?

Engines need high intake speed through the venturi to ensure good atomisation of the fuel. A smaller diameter will give good atomisation at low engine revs but won't be able to flow enough air at high revs. Make it bigger and then low down power will suffer as the air speed drops.
Yamaha got round this by the carbs having smaller venturi to aid low down power but V Boost opening to allow sufficient air to be available at higher revs.

If the V Boost butterflies are not starting to open at the correct revs then the engine will not get sufficient air/ fuel and the power will drop off as you go past 6K.

Assuming the cable(s) are serviceable and V Boost works then connect them up and see what difference that makes. If that is not the case then you can still run the bike with V Boost open, you will loose some low end torque but you should notice a difference at higher revs.

However, as V Boost doesn't start to open until around 6K the detached cable wouldn't cause the problem that stops it revving past 3K.

I'm still curious about the advance unit. From the picture below you can see that it starts to advance rapidly at 1.5K and then tails off significantly at 3K where you hit the brick wall.



This may just be a coincidence but it would be prudent to rule this out as a possible cause to your problem.

Why do I suddenly feel like Barbara Feldon?

Could it be down to a change in your hormones?

 

[wayne billings]

Mr. yamahavmax Very possible. Maybe mine and not yours

 

[wayne billings]

MaxMidnight Thanks for the info. I could not find a new one from partzilla or yamaha. I just installed a ignitech, do I even need the pressure sensor? Had to ask.

 

[MaxMidnight]

Whist I don't have any experience or expertise on the unit it has inputs for Manifold air pressure (MAP) sensor so on that basis I would assume it would need a suitable device. Ignitech have them if required.
Wayne, what you need to read is a thread on Ignitech that will explain the options open to you.
Have a butchers here

 

[wayne billings]

Thanks again MaxMidnight This stuff is way over my head, and thanks for the option to see it. I will have to go with making sure the pressure sensor is functioning as it should. I assume the sensors rarely fail. Are there aftermarket sensors? Just in case.