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sacred screw's function

thats what i am saying. if the TPS is set correctly, and everything else is good but the stop is wrong, then then it wont run correctly. if the throttle bodies are open too much, it will return to idle very slowly, and idle a little high. if its under, it will over shoot idle.

so getting the sacred screw correct is as simple as setting your idle speed. because if everything is right, and your idle speed is right, then it means your TBs are at 4.7 degrees.

and if your TBs are balanced, the angle of the left one will be the same as the angle of the right one, within a very small tolerance.

if you dont believe me, you are welcome to try my machine. i adjusted things a LOT because they came WAY OUT, and it runs really sweet now. idles perfectly, throttles crisply, no strange behavior, and it took only two steps. if a novice can do it, it aint sacred.
 
Spaceclam

Well done!

Have you read what I posted about re-setting a tampered with stop screw under the thread Home-tuning a Breva V1100 etc? I think we are agreeing pretty much, but still for most folk the screw should be left alone.
 
i hadnt.

we werent disagreeing, as you were saying not to move it from 4.7 degrees, thus keeping it sacred. I was saying it is a valid adjustment, because if its out of adjustment, it needs adjusting. no sense in riding an un-tuneable bike because the proper adjustment has paint on it.
 
Spaceclam

The problem with your method and mine of resetting that stop screw is that we cannot be certain that the left plate and hence the true effect on the reset TPS is now at 4.7 degrees because of the effect on idle speed that the air-bleed screws have, even with a disconnected stepper-motor not adding to the confusion.

Guzzi advise using the air-bleed screws only to perfect idle vacuum-balance but they can also be used to control idle speed to some extent by leaning and enrichening the mixture without the ECU knowing it. Guzzi also advise having either both screws closed or just one slightly open, advice which I've found to be sound. So, if the screws are not as recommended and both have been opened to perfect idle vacuum balance, then althought the idle speed may be within range, suggesting that the stop screw is in the best position, the left throttle plate may not be at 4.7 degrees. The flow-on effect of that is that although the TPS is reading and telling the ECU that 4.7 degrees is correct now, it won't be.

You appear to know a bit about engine tuning principles, so you'll possibly remember that when engines had distributors, unwise mechanics would often set the ignition timing by rotating the distributor to achieve the fastest idle and then adjust the throttle stop to achieve the correct idle speed. That meant that the ignition timing was not correct in relation to the engine timing marks provided.

So, when needs must, we have no option but to use the desert-island methods, but they will never be as good as the text book ones.

My advice remains the same: Leave the throttle stop screw alone unless you have no other way to correct idle speed.
 
Hi All,
GrahamNZ said:
The problem is there is no way of knowing if the stop screw is setting the left throttle plate at 4.7 degrees.
Click to expand...

While helping my buddy Willie lower the idle on his B11 (see thread on BNS12) while following GrahamNZ's B11 tuning thread, the part on correcting a tamped with sacred screw, it occurred to me as I turned out the Left TP stop screw we watched the TPS reading on the VDSTS come down. That if all other parameters are tuned, then if that screw is turned till the reading stops moving, that should indicate a closed throttle (0 degrees ) for both TBs. Don't know if the VDST would continue giving a reading once the value passes 0 degrees, so starting with a high TPS reading would avoid testing that potential problem. Once the TBs are closed then just adding 4.5 to the value of the reading, that would a give 4.5 degrees physical opening on both sides. Turn in the stop screw to reach the summed value. If memory serves, 4.5 is the factory setting for the Left TP set screw.

Anybody see a fault with this method the setting the Left TP set screw? I don't think this procedure is necessary though, since the bike runs great by simply getting the idle back to spec., I just came across GrahamNZ's remark above.
 
Roper is a high priest in guzzidom to me, so that makes it a sacred screw... hey that sounds funny; call it what you want.

Anybody have opinion on the left stop screw resetting method I outlined above?
 
Hi folks I am after some avice about the sacred screw. I took my bike to a mechanic to check it before a big ride and after simply touching the return spring it would cut out at idle. He then of course looked at adjusting the idle screw and he changed to position of the stop screw on my 1200 sport. Started ok but would idle higher then down again. Dont know the best course of action
 
Hipp said:
Hi folks I am after some avice about the sacred screw. I took my bike to a mechanic to check it before a big ride and after simply touching the return spring it would cut out at idle. He then of course looked at adjusting the idle screw and he changed to position of the stop screw on my 1200 sport. Started ok but would idle higher then down again. Dont know the best course of action
Click to expand...

If there wasn't a throttle body balance done after that adjustment and a TPS reset performed. You may be able to get it back to the correct reading (4.6 degrees). If anything else was done, it is replace the throttle bodies.
 
There is a paint mark on the screw. You should be able to put it back to where the paint mark lines up.
 
Thanks folks for your help, the yellow paint is all but gone but i lined a bit of yellow on the under side of the screw with the stop arm that the screw touches, nothing else to match up with, the bike did ride smooth with only two ocassions of rpms rising by themselves at idle, wish me the best and is there any simple way to determine it is the right place, to check by a non mechanical person
 
Hipp said:
Thanks folks for your help, the yellow paint is all but gone but i lined a bit of yellow on the under side of the screw with the stop arm that the screw touches, nothing else to match up with, the bike did ride smooth with only two ocassions of rpms rising by themselves at idle, wish me the best and is there any simple way to determine it is the right place, to check by a non mechanical person
Click to expand...


No way to check that I know of. The throttle body is set up on a bench and as far as I know no one outside of the factory knows how it is set. If it is like the BMW throttle body setup, it is fully closed and an apparatus is used to set the degrees of opening based on a mechanical reading.
 
I'm a little skeptical about this "sacred screw" -just how sacred can it be? Sure, it might have been set up on a special jig that is not available to end users and service people, but it is an adjustment screw after all. If it was never meant to be tweaked, it would have been blanked off and made inaccessible.

If the valve clearances, TPS, throttle body balance, stepper motor temporarily disabled, etc, etc are correct and the bloody motor still idles too high, what choice do you have? Anyone tried a warranty claim for new throttle bodies because their ones have been set up incorrectly from the factory? I can just imagine the success rate.

In my case, I measured the amount of throttle body degree opening reduction required to get an acceptable idle using VDSTS, and then the TPS was reset back to normal. By noting the change, there is always the opportunity to restore the settings back to their previous reading. This ain't rocket science.
 
kiwi dave said:
I
In my case, I measured the amount of throttle body degree opening reduction required to get an acceptable idle using VDSTS, and then the TPS was reset back to normal. By noting the change, there is always the opportunity to restore the settings back to their previous reading. This ain't rocket science.
Click to expand...

Your method is sound since know where it was before and after movement. By knowing this, you can get back to the original position. In my opinion, if it is just moved, you are just screwed and have no way to return to the factory position.
 
I know this is an old thread and maybe I missed something, but I would like to ask this question. The position of 4.6 degrees, I assume is the postion of the throttle plate from a fully closed position? Assuming more, if the plate was at 0 degrees then no air/fuel would get to the engine,yes? So what is the problem in finding the plate position, we know the diameter of the plate and the fulcrum point, just measure the plate position, adjust the "screw" and go on about the business at hand. This talk of scrapping the TBs sounds like a great way to make money.

Please, someone correct my thinking if it is wrong.

Thanks
 
silvergoose said:
I know this is an old thread and maybe I missed something, but I would like to ask this question. The position of 4.6 degrees, I assume is the postion of the throttle plate from a fully closed position? Assuming more, if the plate was at 0 degrees then no air/fuel would get to the engine,yes? So what is the problem in finding the plate position, we know the diameter of the plate and the fulcrum point, just measure the plate position, adjust the "screw" and go on about the business at hand. This talk of scrapping the TBs sounds like a great way to make money.

Please, someone correct my thinking if it is wrong.

Thanks
Click to expand...

No one outside of the factory knows how to get a physical 4.6 degrees on the throttle plate. The electronics on the bike can't read zero degrees. There is no value given, like on the 15M bikes, for a fully closed throttle body in millivolts. The position the throttle is in when an electronic reset is done will read 4.6 degrees to the ECU, no matter where the throttle plate actually is. It could be an actual 2 degrees, or 10 degrees, but the ECU would read that position as 4.6 degrees. Also we don't know if your assumption is correct that the physical position of 4.6 degrees is the actual position of the throttle when it leaves the factory, only that the ECU map considers the value of 4.6 degrees to be the closed position. Do you have enough information now to know why you shouldn't touch that screw?
 
Guzzimoto, The paint wasn't helpful when I readjusted my friend's stelvio and B11 after determining that they had been tampered with. The screw just passed through the lump of paint.

John, guess that answers my question on page 3 of this thread - sort of.

Silvergoose, the reality is if some mechanic messes with the stop screw, it can be reset for acceptable performance. In both cases the bikes was idling high but would hunt downwards. We lowered the idle by turning out the set screw a quarter turn at a time, resetting the TPS each time, till the idle remained in spec. No problem.
 
I think that while that screw should not be turned under normal circumstances, it is an adjustment and there are times when it needs to be adjusted. That could be because someone unknowingly turned the screw for some reason and you are just trying to put it back where it needs to be, or it could be that your bike came from the factory with the screw mis-adjusted. What ever the reason, it is an adjustment and it can be adjusted. For most people that screw should be left alone. But the notion that once it is turned you will never be able to get your bike right again is crazy.
 
Rafael said:
Silvergoose, the reality is if some mechanic messes with the stop screw, it can be reset for acceptable performance. In both cases the bikes was idling high but would hunt downwards. We lowered the idle by turning out the set screw a quarter turn at a time, resetting the TPS each time, till the idle remained in spec. No problem.
Click to expand...

This is what I have found to be the best method of un-fucking a set of TB's that have been messed with.

Do the high speed TB balance and reset TPS. The classic signs of a screw that has been messed with is a cyclically changing idle as the stepper motor desperately cycles trying to find the correct idle. The sacred screw is almost always about .75mm proud of the casting. This can be used to judge whether it has been screwed in or, more usually, out to try and reduce the idle speed which some people perceive as 'Too High'.

If the idle is 'Surging' use a key to turn the screw towards that .75 mm spot by 1/4 of a turn. Re-set the TPS and see if he idle settles. If it gets better? Repeat until the idle stabilises. If it gets worse try going the other way.

No, its not perfect. Far from it. But it will usually get the idle back within acceptable parameters.

At the magneti marelli plant the original settings are achieved on a flow bench.

Pete
 
Pete,

Good information. Always worth a try before replacing throttle bodies. Thanks for the screw length, it does give a target to get you close.
 
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