Vacuum Advance/Boost sensor

[KIC]

I have been battling a MPG issue for awhile. After finding issues with my new diaphragms and such, the bike is running great, got rid of the infamous garage gas smell and return the bike to it's former fast and powerful self. BUT, I have not got it to return to the 42-45 MPG I was getting when I first bought it.

 

Background: New plug wires, New diaphragms, added the little white washers to the slides, synced the carbs ( many times) and they look real close, checked for vacuum leaks, looked, tinkered and toyed.

 

Now, my question:

 

How do you test/check the vacuum advance/Boost sensor ?

 

The line is hooked to the #2 carb base

I checked the little black and white thing (? what is that thing and what does it do ?? ) and it allows air both ways

When running, I disconnected it and tried to create suction to see if the RPM's would changed and nothing happens ( did not use a tool just my mouth and the suction stuck it to my tongue and did not release.

 

So, could that advance/booster allow the bike to run great but not be working at higher speeds and then affect my MPG ?

[Prairiehammer]

I have been battling a MPG issue for awhile. After finding issues with my new diaphragms and such, the bike is running great, got rid of the infamous garage gas smell and return the bike to it's former fast and powerful self. BUT, I have not got it to return to the 42-45 MPG I was getting when I first bought it.

 

Background: New plug wires, New diaphragms, added the little white washers to the slides, synced the carbs ( many times) and they look real close, checked for vacuum leaks, looked, tinkered and toyed.

 

Now, my question:

 

How do you test/check the vacuum advance/Boost sensor ?

 

The line is hooked to the #2 carb base

I checked the little black and white thing (? what is that thing and what does it do ?? ) and it allows air both ways

When running, I disconnected it and tried to create suction to see if the RPM's would changed and nothing happens ( did not use a tool just my mouth and the suction stuck it to my tongue and did not release.

 

So, could that advance/booster allow the bike to run great but not be working at higher speeds and then affect my MPG ?

 

The black and white thing is a pulse dampening orifice.

To check the Boost Sensor:

When I was diagnosing the Boost Sensor on my 1990, I used a MityVac. I could record the proper voltage output (2± volts at rest and about 4.9 volts when a vacuum was pulled on it.) but unless the engine speed was above about 1500 rpms, there was no increase in engine speed when a vacuum was applied. If the idle was set to 1500 or more when testing the Boost Sensor, then rpms did increase when vacuum applied.

[Condor]

When testing the booster you need to disconnect the hose from the manifold to see if there is any change in rpm. That inline plastic piece has a arrow on it pointing to the booster. Can't remember if it's an air check or not?? Been a while since messing with 1stGen carbs.... I think it evens out the manifold vacuum pulses by restricting the air flow?? You also didn't mention if you've sync'd the carbs yet??

[frankd]

Disconnect the sensor hose from #2 cylinder, then start the bike and have a helper keep the RPM at about 3000 RPM. Put the hose back on and the engine should speed up---enough to be obvious. If it does, your vacuum advance is working. When I bought my 89, the hose was cracked, and the vacuum advance inoperative. My gas mileage around town was just below 40 MPG. After changing the hose, it went up to 42-43 around town.

[jasonm.]

Disconnect the sensor hose from #2 cylinder, then start the bike and have a helper keep the RPM at about 3000 RPM. Put the hose back on and the engine should speed up---enough to be obvious. If it does, your vacuum advance is working. When I bought my 89, the hose was cracked, and the vacuum advance inoperative. My gas mileage around town was just below 40 MPG. After changing the hose, it went up to 42-43 around town.

 

electrically these don't fail. It's the "rubber/silcone " membrane inside that fails. Get a mighty vac type vacuum guage and test it. It can be tested on the bike. It can be repaired once removed. But it is involved and I think posted somewhere here how to do it. I have repaired mine.

[KIC]

Thanks for the information. If the booster is not working, what is the symptoms that would show ? Any type of symptoms while idling, running fast, decelerating or holding steady ? I will get a mighty vac to test it when I get back in town.

 

I do have another one on my parts bike if I need it.

 

Thanks again guys.

[Vonwolf]

electrically these don't fail. It's the "rubber/silicone " membrane inside that fails. Get a mighty vac type vacuum guage and test it. It can be tested on the bike. It can be repaired once removed. But it is involved and I think posted somewhere here how to do it. I have repaired mine.

I know this is old, but my original 87 Boost Sensor leaked so off to flea bay and I have a Booster that holds vacuum, but the volts stay at about 2V from 0 to as much pressure I can create. These are usually 15 bucks a pop, but it will get old quick getting a bunch of junked Sensors.

[Prairiehammer]

... but the volts stay at about 2V from 0 to as much pressure I can create.

 

I hope you were not applying "as much as I can create" POSITIVE pressure to the Boost Sensor. The Boost Sensor should show a voltage change when 600mm Hg (mercury) NEGATIVE pressure (vacuum) is applied and 600mm Hg positive pressure. Six hundred mm of mercury translates to about 11 psi. Too much positive pressure will damage the Boost Sensor. Six hundred mm of HG vacuum applied should indicate .5 volts. Six hundred mm Hg positive pressure should measure 4.9 volts on a good sensor.

 

Double check the procedure described in the manual. Confirm that you are checking the voltage properly. And make sure your digital voltmeter is set at the lowest range.

 

The Boost Sensor contains a strain gage/gauge. A strain gage is a device whose electrical resistance varies in proportion to the amount of strain in the device. In this case, the "strain" is the movement of the diaphragm within the Boost Sensor, movement caused by the varying vacuum applied to it.

Strain gages are very sensitive to varying pressures and to electrical over currents. It is entirely possible to "burn out" a strain gage. There is also an amplifying circuit within the Boost Sensor. It too can be defective.

 

Edited August 2, 2015 by Prairiehammer

[jasonm.]

let's get something straight here. Some people are saying pressure...there is NO pressure. Vacuum is measured typically in inches of Hg (mercury) ability to defy gravity. This a VACUUM device at the manifold...measuring vacuum and translating to VOLTS out.

Edited August 19, 2015 by jasonm.

[Vonwolf]

Sorry bad terms used, I applied as much negative pressure as my break bleeder could produce, not that much around 2.3 in.HG. The big thing is I start out at 2 volts and it didn't move from there and when I plug it in at idle the RPM jumps right away so I am apparently getting 2 v of advance right off the bat, but nothing changes from there. I was following the manual procedure as best I could, but I can't make very much vacuum I think it was enough to show a problem, but I'm not sure.

 

** I just noticed my "break Bleeder" is a Mighty Vac so I guess we are all on the same page

Edited August 2, 2015 by Vonwolf