Boost sensor vacuum restrictor

[CptKirk]

I finished a tear down to frame motor, wiring and reassembly of an 87 VR I bought on ebay.

The bike is on the road, looks and rides great :12101:but........ I was floored by the poor fuel milage.

 

When I reassembled the bike I replaced the boost sensor as it was fubar. I found somewhere along the line a line restrictor was put on the vac line to the sensor.I put it back in at the same time as the swap. Had never seen one on other VRs though.

 

Fuel mileage was about 20 mpg:confused07:. Thinking about the work done, couldn't find an issue. Pulled off the line restrictor and now am about 43mpg with better power curve. Why the radical change in mileage and performance with it out ?

Can't find restrictor in any parts listings so, for now it's in the box of parts I don't figure need to be there:soapbox:

Edited September 24, 2008 by CptKirk

[CrazyHorse]

I finished a tear down to frame motor, wiring and reassembly of an 87 VR I bought on ebay.

The bike is on the road, looks and rides great :12101:but........ I was floored by the poor fuel milage.

 

When I reassembled the bike I replaced the boost sensor as it was fubar. I found somewhere along the line a line restrictor was put on the vac line to the sensor.I put it back in at the same time as the swap. Had never seen one on other VRs though.

 

Fuel mileage was about 20 mpg:confused07:. Thinking about the work done, couldn't find an issue. Pulled off the line restrictor and now am about 43mpg with better power curve. Why the radical change in mileage and performance with it out ?

Can't find restrictor in any parts listings so, for now it's in the box of parts I don't figure need to be there:soapbox:

Interesting dont know why though. Did you put it back in and see if mileage drops again to confirm? Mine has a restrictor also and its a California model dont know if thats partial to those or not. I wonder if you had some sort of fluid in the restrictor. I wonder if any effect I would have if I removed the evaporative cannister?

Edited September 24, 2008 by CrazyHorse

[Neil86]

My 86 (Cdn spec) also has the fitting....I believe it protects the pressure sensor from rupturing if you have a backfire from the intake. My 86 is stored elsewhere...so can't check, but are you sure there isn't some internals...(ball, etc)...if so it may be unidirectional...perhaps it was installed backwards, not allowing vacuum to reach the pressure sensor?

If you look on the parts, go to Air Filter, I believe its part #11, Nozzle

[5bikes]

The boost sensor alone can any add 3-7 mpg. It's just like the old vacuum advances on 50-60's cars.

If you had pressure (not Vacuum) on it it could retard the spark, giving reduced power.

At 2k rpm remove the vacuum line under #2 carb, put your finger over the intake port, rpm should drop about 500 rpm. Put it back on rpm will go back up. It is working. Check restrictor can you blow thru it?

After rebuilding/cleaning my carbs (after sitting 5 years) mileage went from 37 to 47 in 500 miles.

[pegscraper]

I had thought that restrictor in the line was to smooth out the vacuum pulses to the boost sensor, the same reason why they are needed when using a carb sync gauge. That doesn't make sense why gas mileage should change so much by taking the restrictor out. Did you try putting it back in to see if mileage goes back down?

[Gearhead]

The boost sensor in my 87 was not working due to corroded electrical contacts. Repairing it was worth 3 or 4 MPG. I think mine has the restrictor, assumed the same function pegscraper just said.

 

Mine is also a CA model. I removed the canister, no change. I installed a little filter for the carb vents.

 

Jeremy

[CptKirk]

I had a close look at the restrictor. There is no clogging or obstrucion just a tiny spot of daylight. I did nothing to the carbs other than clean them with Seafoam though the fuel system, new fuel filter and drain the fuel tank. Engine compression is great on all cylinders. New plugs.Starts great Runs well. Carbs synchronized.

 

Tried a boost sensor from an 83 and an 87 donor bike. Both work and hold vac also check out with volt meter as good but mileage is poor with either installed.

 

It apeared someone jury rigged the restrictor in so that is why it was suspect. Mileage and performance better with it out so figured that was the issue. There is an arrow on it which was pointing in the direction of the boost sensor. Will try it the other way around. From the wisdom here :bowdown:it does belong on the bike. (US model) .

 

Have no fuel leaks but do get black sooty plugs after 200 miles. I check fuel levels in the carbs, if anything it is very slightly low on all four but very slightly. Pilot neeedles are out two turns each. No backfires or hesitations, plenty of power. In fact runs as good or better than my 83 VR which runs superb.

 

Carb slide diaphrams have no holes or tears but have not had a look at the coast enrichment diaphrams. Really puzzled as to why removing the resrictor improved the mileage so much but it did so I was happy. Prior I could get no better than 100 miles to a tank of fuel :think:and it really sucked the majority of the fuel on highway at 60mph (speedo is in MPH)

 

In fact the other day a 68 mile trip sucked a tank down 3/4 . So.................. a bit confused, surely the direction the restrictor is installed can't make much difference. Any idea which way the arrow is supposed to point? Without it in I ran 40 miles yesterday and the guage never left full and a Imp gallon topped the tank up full.(yes the guage works fine LOL)

[CptKirk]

The restictor is installed the same way I found it prior to tear down with a short length of tubing from the intake port of the (#2) front left (sitting on bike) forward cyl. (approx 3 inches long) then a 16 inch long length of tubing to the boost sensor. Could it be that it needs to be closer to the boost sensor as opposed to 16 inches away??

That could make a difference in timing lag perhaps???

[Squeeze]

As i've learned here, the Connection on a '83 is different than the Vacuum Port on the#2 Cylinder. I think it's direct on the Carb, above the Butterfly. If your Sensor is from a '83 Model, there should be something else different to compensate for that differing Vacuum Value. I think it's the TCI Unit which is different also. Maybe only because of that different Vacuum Value and it's computing to Spark Advance Value. The Sensor stayed the same, even with using the Port on the Manifold instead of the Carb Port.

 

Someone with better Knowledge of the '83 will chime in here soon.

[CptKirk]

I looked up the "nozzle" aka restictor on Bike Bandit .com per wisdom here:bowdown:.......... it shows under the airbox diagram as per that same wisdom BUT it shows the sensor tube taking vacuum from cylinder #1 not #2 so........ I'l change the boost sensor tube the cyl #1 spigot and see what happens.

I looked up the boost sensor part number for an 83 and an 87. Bike bandit shows the same part number for both years in their listings??

[Squeeze]

I've never seen that Line anywhere else than on #2 Cylinder, but it shouldn't make any Difference anyway, except the Lenght of the Hose. I'm sorry, but i first thought you're talking about your '83.

[Squeeze]

On a second Thought, if you have the Carbs synchronized, you have a Vacuum Guage. You could hook it into the System behind the Restrictor and read the Values on different Rpm and Throttle Positions. Now, take the Restrictor out and proceed the same test Values Rpm and Throttlewise. There shouldn't be a Difference in both Results, just the Time of Reaction should be faster without the Restrictor in the Line.

 

If there is a Difference, the Restrictor is somehow restricted. :D

 

It should have a Hole through it about 1mm in Diameter.

[CptKirk]

Great idea. Will try the synchrometer on it and see what shows. The lack of a lag did make a difference with it out of the line so sumthins up. More than likely it's the monkey (moi) on the tools.

 

You're right ,thought about it after posting, the ign signal comes from the pickup coils so I'm on a blind alley with the thought where vacuum comes from makes any difference at all.The inside dia of the hole in restrictor is 1mm as you note.:bowdown:

 

Wonder why they put an directional arrow on it as there are no checks in it , maybe just an SAE thing.

[Gearhead]

I've wondered about the arrow too. A hole shouldn't care which way it's installed. Also, assuming the carbs are properly synced, vacuum on all of them is equal so it doesn't matter which carb it's connected to.

 

Kirk, I just read this:

Without it in I ran 40 miles yesterday and the guage never left full and a Imp gallon topped the tank up full.

 

Is that your only mileage check since removing the restrictor? Where'd "about 43 mpg" come from? That would be 40 Imp MPG and 33 US MPG. Nevertheless, the change from ~20 to even 33 is remarkable.

 

The 83, I've read here, uses some form of ported vacuum (taken from above the throttle plate), while all others use manifold vacuum (taken from below the throttle plate). Above the throttle plate has no vacuum at idle and differs from manifold vacuum under other conditions as well. As you and Squeeze pointed out, the "boost sensor" (or vacuum sensor as it should be called) is the same for 83 and the others. Therefore, the TCI box must be different between the two models. All the boost sensor does is output voltage based on the vacuum in the line; it is the TCI's job to interpret that voltage and alter the spark advance curve accordingly.

 

At any rate, you've verified that the boost sensor is working, and since it's an 87 you need to connect to manifold vacuum (on the carb mounting stub).

 

Are you sure you didn't change anything else at the same time?

 

Jeremy

[Squeeze]

Great idea. Will try the synchrometer on it and see what shows. The lack of a lag did make a difference with it out of the line so sumthins up. More than likely it's the monkey (moi) on the tools.

 

You're right ,thought about it after posting, the ign signal comes from the pickup coils so I'm on a blind alley with the thought where vacuum comes from makes any difference at all.The inside dia of the hole in restrictor is 1mm as you note.:bowdown:

 

Wonder why they put an directional arrow on it as there are no checks in it , maybe just an SAE thing.

 

IIRC, those Hoses have different inner Diameter, the short one has around 4.5mm, the longer one is very tiny, 2.5 maybe. I don't know a Reason for the Arrow on that Thing.

[Neil86]

I believe that the 83 carb rack had one carb with the vac port for the sensor....ever since then the carbs haven't had it, and yes the pressure sensors are all the same and the 83 has a different TCI.

Even if it is only an orifice, the dampening would limit the pressure buildup during an intake backfire, as well as smooth out the signal. I don't think you would want the timing (via the pressure sensor) chasing the vacuum pulses at idle.

[Gearhead]

Even if it is only an orifice, the dampening would limit the pressure buildup during an intake backfire, as well as smooth out the signal. I don't think you would want the timing (via the pressure sensor) chasing the vacuum pulses at idle.

 

Neil, I agree with your premise 100%. But FWIW, the vac advance doesn't kick in until close to 2000 RPM.

 

Jeremy

[CptKirk]

Engine/trans got no more than a good cleaning as it was in perfect shape compression and otherwise (had a new clutch already). Electrics were not distrurbed other than volt/ohm checks of pickup coils, stator output and primary/secondary ign coils. Airhorn added with the correct relay properly installed according to Stebel instructions.

 

I did change the TCI as I made the mistake of using a pressure washer and cleaner to clean the engine and frame etc before disassembly DOH

 

I then ran the engine for a while and the steam caused the TCI to act up. Have learned my lesson and will never do that again. The vacuum sensor never did hold vac or pressure and by the look of the plugs then (sooty as they get) it had been run like that for a while.

 

I bought a matching vaccuum sensor and TCI off a low mile 87 VR fom Pinwall. The numbers off the TCI exactly matched the one I removed (although I did take the top off and dry out the origional and put back in for a test, does work now but not before I paid for and recieved the Pinwall items). The number off the vac sensor is close to the origional.

 

The bike has the matching set of TCI and sensor installed at the moment. I was messing around with the bike today as it's raining here and noticed that I can hear what sounds like a vacuum leak that follows one cylinder not a constant leak but one that follows the rythme of one cylinder and it is most noticeable on the right front side of the motor. The leak sounds to be above the valve covers and above the plastic shield near the water temp sensor beside the rad filler cap.

 

I checked every vacuum line I know of and all are connected and none have leaks. Short of pulling the airbox off no idea what might be causing the sound. It's not coming from the carb slide covers.

 

Carbs have not been removed or messed with by me other than flushing the bowls with Seafoam,checking the diaphrams for tears (also rechecked the slide covers for leaks with WD40 afterward), synchronizing, pilot screw check and checking float levels with a guage I made (slightly low as they are all right at midline of the slide covers) .

Edited September 25, 2008 by CptKirk

[Neil86]

Neil, I agree with your premise 100%. But FWIW, the vac advance doesn't kick in until close to 2000 RPM.

 

Jeremy

 

Jeremy....if you look in the electrical specs in a shop manual, you'll see 2 timing curves.....one at low absolute pressure (hi vacuum) and one at high absolute pressure (low vacuum), It shows extra advance right from idle speed, if the vacuum is high enough. The only variable is the manifold vacuum and therefore pressure sensor input.

[Gearhead]

Jeremy....if you look in the electrical specs in a shop manual, you'll see 2 timing curves.....one at low absolute pressure (hi vacuum) and one at high absolute pressure (low vacuum), It shows extra advance right from idle speed, if the vacuum is high enough. The only variable is the manifold vacuum and therefore pressure sensor input.

 

Yeah, you're right about the diagram. But I'm pretty sure I've measured it and in actuality the extra advance does not start at idle. I could be wrong though, should double-check. Maybe I'm thinking of the Virago, which I know does not vacuum-advance until 2k or so.

 

Jeremy

[Neil86]

My 86 isn't handy...guess one could cap port, apply, release vacuum and see if rpm change at idle.

[CptKirk]

Well, I'm embarrassed. The restrictor isn't the issue and no idea what is but have to confess I got ahead of myself. I ran the bike on a toy run after I removed the restrictor. Run was 43 miles and speed not over 40 mph. At the other end I fueled up and added a gallon of fuel estimated mileage 43mpg. Took a leisurely ride on back roads on return with speeds again not over 45 mph. Same fuel consumption so figured that I had fixed the issue of the bike being a fuel hog. Bad assumption on my part. I went out for a ride to see how far a tank would take me at highway speeds of 55 to 65 mph. Watching the fuel guage move toward empty is disheartening. It IS a fuel hog regardless of the restrictor and it's back in the line. The bike went through a full tank to reserve in 128 miles. So.............................. back to square one.It runs great, has all kinds of power and then some. Smooth acceleration but a gas hog.I'm wondering if the choke is partially on, any idea on how to check that angle? It's not leaking it out on the ground but might as well be for the poor mileage. Sorry to mislead, guess I was overly speculative on what I thought was a fix, bottom line I was out to lunch. Thanks for the attempts to assist.Any ideas of common places to look. Engine is in great shape whith a fresh valve adjust done by a shop, it is quiet and has tons of compression. Starts first time every time.

[Neil86]

So you are certain the installed pressure sensor has no leak and is advancing the timing?

Its firing on all 4 cyls.......exhaust hot (temp gun or fog water) on each cyl.?

The carb slides are all active?

Did you try carb synch....and observing vacuum on all 4 over the rpm range?

Front and rear wheel spin freely on mainstand before and after a ride?

[CptKirk]

So you are certain the installed pressure sensor has no leak and is advancing the timing?

Its firing on all 4 cyls.......exhaust hot (temp gun or fog water) on each cyl.?

The carb slides are all active?

Did you try carb synch....and observing vacuum on all 4 over the rpm range?

Front and rear wheel spin freely on mainstand before and after a ride?

 

I checked the sensor with a voltmeter and vaccuum. Holds vacuum, voltmeter shows an advance when vacuum applied. Voltage values checked against manual and are within spec'd values. Any stoppage and hold in vac keeps steady voltage until vac released or increased.All four cylinders are firing as all four pipes are hot. Used a spray of water on each which instantly sizzles and vapourizes. No wet plugs but do show a rich condition (sooty). No misfires through the entire rpm range loaded.Carbs synched using a four guage synchrometer. Idle at 1000 rpm. Reading steady 25kpa on all four. Visual check slides move freely and open at the same rate at all rpm to 4000 in neutral. (higest rpm taken to in neutral). WOT briefly in neutral and all four open equally.WOT briefly in neutral and equal vac drop on all four guages with steady smooth return to 25 kpa steady on idle return..New bearings in front wheel spinning freely. Rear axle oil changed , no material in the oil and rear wheel spins by hand with little to no effort. Tire pressures front and rear 32 psi front 40 psi rear.

[CptKirk]

Just so I have the right idea re float levels here is how I checked them.I used an adapter nipple to hook onto the drain tubes and plugged on a piece of clear plastic tubing. I held the tubing up beside the carb slide then opened the drain plug allowing fuel to run ito the drain line and plastic tube. Ignition on to run fuel pump. Sighted the fuel level in the plastic tube to give me the location of the fuel level in bowls. Showed that the level was at center of carb slide which I interperect to slightly low. I started the engine several times and let it idle then repeated the above on each carb. All levels are the same on each carb.Would this be correct or am I missing something?