Fluctuating Rev's - Any Ideas?

Hey All.

First off, apologies if this has been covered before but I had a good 'ol look and couldn't find anything, so here goes...

My 5th Gen 1998 VFR800, which as you might expect has run like a Swiss clock throughout the time I've owned it has recently not been so sweetly purring.

At upwards of 3k rev's, with a constant throttle input, the rev's fluctuate up and down constantly - making for a less than smooth ride.
The fluctuation is of around 200 - 250rpm, and fairly rapid. It seems to lessen above say 5k rev's, but is still present.

It still pulls like a train, so power output doesn't seem noticeably affected, but it clearly isn't a happy V4...

I have found suggestions elsewhere that a fault similar to this could be as a result of a vacuum hose coming astray from the airbox, which seems plausible as I did have my tank up recently and it's always possible to miss these things on reassembly if you're in a hurry to get out for a spin, but I lifted it again today and so far as I can tell, everything is plugged where it should be, at least at the airbox end of the hoses.

Has anybody encountered this themselves, or have any suggestions on a possible faultfinding direction?

Any pointers would be much appreciated, as I'm dying to get it back running as sweetly as it should!






PS, having been unsuccessful in finding the source of the issue today, I bolted my tank back in place and went for a glorious spin around the southern half of Devon. 31 degrees here today, which for those of you accustomed to that sorta temperature let me assure you - it's a rare treat here!
I'm a little ashamed to say I wasn't ATGATTing it, but I couldn't quite face the prospect of a heavy leather jacket. Damn was it nice to be out in a t-shirt though!
*looks a little sheepish, expecting disapproving comments*

 

When was the last time you synchronized the carbs?

At such a high rpm it could be the fuel delivery as well, but if fluctuations stop at even higher rpm I doubt it.

 

Hi buddy.

Not sure if I'm misunderstanding you, but the bike in question isn't carburetted. It's a '98 VFR800fi-w.

Fuel injector balancing is I admit something I know pretty much zero about, so the answer to that is - never. So long as I've owned the bike anyhow...

 

Sorry for the confusion, I used "sync the carbs" loosely here The bike is not carburated, however you still need to synchronize throttle bodies and airflow through them. This is sort of the main point when synching the carbs as well, since in that case your jets are pre-set, and what you sync up is the air flow as well.

But to avoid confusion, let me ask again the correct question this time: When was the last time you synched the throttle bodies?

AFAIK there is no synching up the fuel injectors, that is done mostly on diesel engines via their flow rate and computer pulse, but for gasoline engines where fuel pressure is far smaller, the flow difference is negligible, so synchronization of fuel injectors is not really done/possible.

What I was referring to when thinking about fuel is pulsation of fuel pressure in the fuel supply, caused by faulty fuel pressure regulator or fuel pump that is on it's way out. I guess it could also be a dirty fuel filter as well. But, since the issue goes away at even higher RPMs, I am not really sure you have any fuel issue. You can check if you have fuel issues with a simple fuel pressure test, just to be sure.

 

Ah, ok. Thanks for the clarification.

The answer on that one would be the same - never done in the 15k miles or so that I've owned the bike. TBH I wouldn't be in the least bit surprised if it'd never been done in the bike's lifetime either...

Worth a shot by the looks of it. Has anyone any experience with homemade manometers and their reliability/ease of use? My riding is done on a budget unfortunately!

Forgive my ignorance, but am I right in thinking that the vacuum hoses run from the airbox to the throttle body assembly, with a further one for the MAP sensor? I want to make sure everything is in place and plugged as it should be before going any further, and thus far I've only been able to check them at the airbox end...

 

There are about 91 pages on troubleshooting in the fuel injection section of the Factory Service Manual, and it looks like your best tool would be a voltmeter. i think you can download the FSM from somewhere on this site.

The surging that you describe makes me think it is some sort of sensor fluctuation in the feedback loop (e.g. lambda or TPS) rather than any kind of mechanical issue. i don't think the synch of the throttle bodies would change unless you messed with the screws with white paint that the FSM says don't mess with. The position of the butterfly valves is factory set and normally should never be adjusted in fuel injected machines.

If your bike has O2 or lambda sensor that would be my first check as it has a large control authority on mixture and will cause surging when it goes bad.

 

Actually, 98 VFR does not have oxygen/lambda sensors, this was added in 2000 to improve emissions at lower rpm, since PAIR alone was not enough, and since model was unified for US market. Well, at least this is the case in US, I assume other countries have similar thing. I myself have 98 VFR as well

Now, for synching throttle bodies you use the air bypass valves, and you adjust these in order to synch the vacuum to reference TB (I think it is #2 TB). White screws that you refer to are for throttle body calibration, and this is done in the factory, and as pointed out, do not mess with these Each TB is calibrated in the factory, but they need to be synchronized together, and this is done via the bypass valves, which are also used by the manual choke for choke operation when starting.

The air bypass valves have plastic stoppers and "anti-spin" teeth, but over time these can turn a bit (vibration is a strong force), and even more, TBs themselves become dirty/clogged and need to be cleaned and synched from time to time. FSM has a very good description on how to do it with a 4-way vacuum gauge.

I got one from Amazon, I think it was by Motion Pro, with liquid. Personally, I think liquid is more sensitive, gauges can vary and need to be calibrated together.

If you want to make a contraption yourself, I would suggest a 2-way device, which consists of a clear plastic hose, shaped as "U", and filled to about 1/3 with heavy fluid, like transmission fluid or 90W gear oil. You could also use plain engine oil. You then attach one end of the hose to the reference TB vacuum hose, and other end to the TB you are synching. Idea is to adjust the given TB bypass valve until fluid level is the same in both sides of the "U" shape, which in essence means vacuum is the same in both branches. You then repeat the procedure for the rest of the TBs. One word of caution here, since vacuum in TBs is of pulsating nature, you may want to partially pinch both branches of the clear plastic tube in order to reduce the fluctuation, which will give you a better readout.

I have used this method before, with good success, but have since decided to upgrade to a real synch gauge

This is the picture of a tool I made. I used the ruler as a holder to get better and more precise read.


Note that you can use this contraption even if you do not have a reference TB, the caveat is that you synch carbs/TBs in that case 2x2, instead of synching all of them to the reference one.

Let me also say that I am not positive the issue with VFRnewbie's fluctuating RPMs is caused by out-of-sync TBs, I am just saying that it could be one of the culprits

 

Quick update to the post above, I just checked the FSM for 98 VFR, and cylinder #1 is the reference for bypass valves. FYI, bypass valves are called "starter valves" in the FSM ... same crap, different name

Also, vacuum is not set to the same value for all the TBs, so you will need to put a T (tee) into one of the lines and put a vacuum gauge, in order to measure vacuum drop for 2 out of 3 TBs. In any case, FSM has the magic sauce you want otstir: