Author Topic: Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling  (Read 2893 times)

Z13

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2004 breva 750 with suspected Subject issues. Found 2012 Archive topic on similar: http://wildguzzi.com/forum/index.php?topic=54146.0

Some comments and questions.
1. Throttle position sensor (TPS) was the culprit for me when symptoms were sporadic and momentary power loss at cruising speed.  Made correlation to an RPM range of 4000 to 4250. TPS is a potentiometer that physically drags a contact along a thin metal strip, with the motion of the throttle (located near RH injector) and thereby signals to the computer the throttle position. Understandably, there can develop a worn spot in this metal strip (common throttle position) ... then the signal is off spec. It'll run you about $140. It's very accessible to replace.

2. Suspected ignition coil problem... I first replaced the plugs, which resulted in better running.
My next step is to inspect fuel lines. ... scratch that, I'll just replace them, they're 10 years and 10's of thousands of miles old.  Regardless, I'll inspect for clues and pay attention to routing. I think coils are $150+ each so... other suggestions on testing and troubleshooting are welcome.

3. Fuel vapor lock. It was 95�F on my 34 mile commutes this week. 100�F once in town. She wouldn't idle at a stoplight. Pulled up for gas and heard a big vacuum suck when I opened tank. I'm stumped on this one because the cap itself is spring loaded for outward pressure release, and near the gas cap there are 1 or 2 small holes in the deck that appear to be vent related...I wonder if they are supposed to be vacuum breakers, with a rubber mouth on the inside acting as a one way valve inward. (?) If so this is clogged perhaps.

4. On fuel subject, the fuel light has only illuminated for me twice in 3 years, though I've ran outa gas a few other times. What the heck? Maybe I should check the connection to the dashboard. and the connection to the "thermistor" at bottom of tank?
« Last Edit: July 14, 2015, 11:14:37 PM by Z13 »

Offline Muzz

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #1 on: July 16, 2015, 08:38:03 PM »
Have yet to strike any of those problems.

The gearbox on the other hand........ :rolleyes:
Muzz. Cristchurch, New Zealand
03 Breva

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Offline Arizona Wayne

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #2 on: July 17, 2015, 01:06:17 AM »
Do you live by the ocean (salty air)?   Maybe this is why you have all these odd issues.  I  have 42K miles on my `04 750 Breva and have had none of your issues, but I live in Aridzona.   :undecided:     I've run my Breva on occasion @ 124F @ 65 mph uphill and it was doing better than me! 

Offline jackson

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #3 on: July 17, 2015, 10:22:46 AM »
Quote
3. Fuel vapor lock. It was 95�F on my 34 mile commutes this week. 100�F once in town. She wouldn't idle at a stoplight. Pulled up for gas and heard a big vacuum suck when I opened tank. I'm stumped on this one because the cap itself is spring loaded for outward pressure release, and near the gas cap there are 1 or 2 small holes in the deck that appear to be vent related...I wonder if they are supposed to be vacuum breakers, with a rubber mouth on the inside acting as a one way valve inward. (?) If so this is clogged perhaps.
The two holes consist of (a) a vent line so the tank can breathe (b) an overflow line to drain the excess gas if the tank is overfilled
One of following three items is causing this (or a combination of two or more)
1. Either the carbon in the evaporative canister is saturated or outlived it's lifespan
2. Or, the valve in the line that leads from the tank to the evaporative canister is defective
3.  Or, there is a kink in one of those lines OR a blockage in the vent line or the overflow line.
(You'll have to remove the tank to perform the following)
The first thing to look for is a kink and that's not there, you'll have to blow some compressed air from a can or a compressor through each hole in the holes that you see in the top of the tank and see if either one (or both) are clogged.
If they're not, you'll need to follow each line and unhook them from the canister (one at a time) and blow air through the top part of the hose to see if air is coming through.  Personally, I've had problems in the past (as have many other people) with these evaporative canister systems) so I remove the entire apparatus and vent both lines to the atmosphere.  Done it on a bunch of bikes and I've never had another problem.
Your choice as to how to handle it but one of the things I listed above is causing the vacuum in the tank.
NO longer can ride

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #3 on: July 17, 2015, 10:22:46 AM »

Offline cee2cee

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #4 on: July 17, 2015, 09:20:59 PM »
I live near the ocean and have 17K on my Breva 750 with none of these problems.  Good luck to you.

Doppelgaenger

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #5 on: July 17, 2015, 10:12:28 PM »

1. Either the carbon in the evaporative canister is saturated or outlived it's lifespan


I don't think that an evaporative canister can really outlive it's lifespan. It's essentially just a sponge that traps fuel vapors and it gets cleaned out when the engine draws air thru it. There is a chance that the activated carbon has broken down, but it'd be slim and even then you shouldn't have any problems. More likely, the fuel tank vent hose is kinked or the tank isn't venting due to something else

I would try doing a canisterectomy to see if that changes your ability to idle. Just pull the hose from the tank and block off the inlet to the canister. You can't leave it like this for any length of time other than what it takes to test the idle because you risk fuel vapor buildup under the seat. You can put it back on if your eco-sense takes over or remove the whole thing. Activated charcoal is cheap, you should be able to re-pack it yourself if it has broken down.

Offline jackson

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Re: breva 750 coil test, vapor lock, TPS sensor, high temp idling
« Reply #6 on: July 18, 2015, 05:48:57 AM »
I have owned several modern Vespa scooters with the auto (CVT) transmissions.  They have at terribly designed gas filler that is very easy to overfill.  Several people on the Modern Vespa forum overfilled theirs to the point where their canisters became saturated; their dealers (and in one case that I remember, the owner) removed them and attempted to dry them out with no success. They wound up buying new canisters.  I simply removed mine before the problem presented itself with our scooters.  The symptoms of a clogged canister are an engine that is hard to start, stalling and/or sputtering.  And when you release the gas cap, you'll hear a whooshing sound from the vacuum that's built up in the tank and lines.
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