symptoms of bad Presure sesnor/ spak advance vaccum

[chocking]

Hello,

 

Still experiencing an issue with my 89..Carbs have been professionally redone, new plugs and wires , sync is good, slides are working well, starts perfect, idles and purrs like a kitten, but has a lack of power, especially as I accelerate..tends to surge..especially around 4000 rpms..The bike is rideable, but my mileage has dropped from 40 to about 34.

 

I've been doing some research, and it appears that the spark advance could be the issue...which points me to the pressure sensor vaccum or a bad TCI.

 

Anyone who has dealt with a pressure sensor issue? What are/ were the outward symptoms....I've looked at the mannual on how to test it, but I'm not much with that skill..

 

Thanks

 

Clay

[cowpuc]

Got a little more time with the MK1's than the Mk2's like yours Chock - hope this is applicable, if not I sure one of the guru's will correct me and set us both straight.. If you look at the left engine cover - the one that your stator is under - its on the left side of the bike if you are sitting on it and is marked with "Yamaha - YCIS". Notice at about 10 oclock on that cover there is a small screw in plug that is has a flat screw driver slot cut in it? Unscrew that plug and you now have a window to view the fly wheel marks. On the flywheel you will see "T-1" and T-2 stamped in it if you rotate the wheel (can be done by bumping the starter). Those "T" marks are NOT for ignition timing - they are for setting the cam/valve timing when doing top end work.. They do stand for "Top dead center" though and the T-1 mark is stamped for TDC of the number 1 jug - the 1 jug is also the cylinder you ignition time from.. That said - if you locate the T-1 stamp and look beside it you should see a "H" stamp. That "H" stamp is suppose to be the 5 degree mark of which a timing light should shine on the pointer of the case locating it between the arms of the "H" when the motor is spinning 1 thousand rpm. Pretty easy to check with a clamp on timing light clamped onto the number 1 plug wire.

Once that checks out you should be able to spin the motor up and visually be able to observe a steady climb on the flywheel as the ignition advances. To my recollect - there are no actual timing marks showing full advanced - maybe cause the ignition timing is not suppose to be adjustable. May sound crazy but that is about the easiest way I know of to check for ignition advance.

Personally, over the course of riding these V-4's since 1987 I have had wayyyy more problem with carb diaphrams causing exactly the issues you are talking about than ignition advance problems BUT I am sure a vacuum leak, faulty pressure sensor, a loose pick up coil, a weak pick up coil, a faulty TCI could sure cause some mysterious issues.. I do think if it were mine, even though the carbs have been recently professionally rebuilt, I would spit check the headers right after start up and make sure she's hitting on all four - go ahead and check the spark advance but if it looked ok - I would pull those diaphrams and double check them and slide stickiness quicker than you can say Pucs Crazy..

[chocking]

Thanks...Yep..As always. the diaphrams could have an issue..I should double check them....just in case....any idea on where to get replacements..I know the factory ones..if available are super expensive, but have not had any experience with aftermarket ones.It's a bit of a mystery... have ridden these ventures for 20 years, and have not felt this surging sensation nor lack of power .....

[cowpuc]

I know there are a couple suppliers around Chock.. I have a like new set of after markets with slides sitting on the shelf that I would be willing to part with - not sure they would do you any good though as they are for the Mk1's.. Personally, I have found Yama Bond number 5 to work awesome for pin hole repairs on slides and I really really like stock diaphrams - they just seem more flexible which - at least in my pea brain equals out to more responsiveness so I just keep repairing old ones ..

I will say this - when you pull your slides you wanna check em for pin holes very carefully but you also want to stretch the area where the diaphram seals against the leading edge of the carb body - fine cracks along that edge are really easy to miss.

Gotta ask this,,, did the scoot ever run right after having the carbs rebuilt?

[chocking]

Ran perfect after carb rebuild....Ran like a raped ape...could not believe the torc and power it had...Its a crazy thing....I give it gas and it goes fair, but then suddenly surges and take off much faster...same thing in each gear....seems to kick in about 4000rpms..and the MPG have just tanked

[cowpuc]

Got cha,,, could be a slide hanging up - they get a little sticktion happening and that can do it.. Might not be a bad idea to pull the air box lid - take a gaze at the slide movement in each barrel - start it up, twist the throttle a couple times and see if they are each moving properly. It really doesnt take much to hang up a slide.

Bad plug also comes to mind,, even though they are new - pull the plugs and see if ya got one that shows fouling.. If your using iridium plugs - might try swapping back to standard NGK's - gap em on the tight side at 32 thou so they got room to stretch with time. Dont over choke it to start it - get it off `choke as quick as possible so the plugs dont have a chance to blacken (these smaller "D" plugs dont take much over choke to foul em).. See if that helps.. Also may double check the connection on the wires at the caps - get one that didnt get screwed in real well, been firing on a long arc and is now arc burnt - resistance there is a real killer of power..

 

You got a timing light you can toss on it? Not saying its not the spark advance - your idea to check it is worthy,, unscrew that plug, clamp on a light and peek in the hole - be REAL easy to see if the advance is working.. Just thinking out loud here with my thoughts about thinking its elsewhere and only from wearing out a few of these V-4's and hanging out with these lop eared VR varmints - havent heard much failure dealing with the advance system.. TCI = yes,, spark advance = not so much.. TCI will do some crazy things like we are talking about but usually the failure is easily noticable by the Tach acting up... IMHO of course...

[cimmer]

Clay, have you double checked the plugs on the vacuum ports on the carbs to make sure they are not cracked and leaking. I have an ignitech replacement ignition system we could try on your bike and see how it performs. It has a MAP sensor and doesnt use the stock pressure sensor. You might want to make sure the restrictor is still in line from the #2 carb vacuum port to the pressure sensor. It is located near the carb.

You might also want to check the conditions of the electrical connectors and make sure they are clean. A good thing to try is some contact cleaner on them to make sure they have a good connection. I would do this to the pressure sensor connections and the TCI box connections.

Let me know if you would like to try the Ignitech unit.

Rick F.

[bongobobny]

OK to check the pressure sensor it is not really that hard! You just put a voltmeter across the output terminals, and with the ignition on you should see about 2.5 volts. If you suck on the vacuum line (take a nice clean new hose and put it on the sensor) and then blow onto it. The voltage should be at near zero one way and close to 5 volts the other way...

[KIC]

OK to check the pressure sensor it is not really that hard! You just put a voltmeter across the output terminals, and with the ignition on you should see about 2.5 volts. If you suck on the vacuum line (take a nice clean new hose and put it on the sensor) and then blow onto it. The voltage should be at near zero one way and close to 5 volts the other way...

 

Do you have to take the fairing off to get to the vacuum sensor ? I have been wanting ot check mine for a while.

[yamagrl]

Do you have to take the fairing off to get to the vacuum sensor ? I have been wanting ot check mine for a while.

 

​Side covers and lower fairings only.

[cowpuc]

Still wondering if it would be prudent to just pull the timing plug and toss a light on it = if she's advancing properly the pressure sensor has to be working.. I know if you want full advanced timing marks ya gotta take a degree wheel and mark your own flywheel but you can still easily watch the advance movement and see if its acting properly.. May or may not be a good pretest before rippin er apart...

[bongobobny]

Yup!! The"Boost sensor" is located under the coil pack right next to the stock location of the TCI, but you can get to the connector to hook up the voltmeter without removing the upper fairing. It does make life easier to remove the false tank cover as well which is easy to do! The vacuum line for the boost sensor comes out on the left side and from what I remember it is the one with the little in line restrictor on it...

 

The procedure is in the Yamaha service manual, just suck and blow on the vacuum line to go from zero to 5 volts. Specifically pages 7-21 and 7-22..

[KIC]

The procedure is in the Yamaha service manual, just suck and blow on the vacuum line to go from zero to 5 volts. Specifically pages 7-21 and 7-22..

 

Thanks guys. So if it is idling, would just creating a vaccuum on the line cause a change in the advance ? Increase the RPMS? Just wondering.

[bongobobny]

Gee, never thought about that! Well, I guess if you disconnected the hose and blocked off the line going to the engine, then changed the vacuum going to the sensor either by increasing the vacuum or applying positive pressure, then I guess the idle would change slightly, but not sure of that. The advance is more of a dynamic thing but sure, if you are advancing or retarding the timing at idle it should change a little...