sacred screw's function

[guzzi jon]

Greg, Todd(s) Pete, etc.
The sacred screw that shall not be touched on the new carc bikes like me griso, what is its specific function. Last year Pete and I were fucking with it and could get the bike to pop less on over run but idled a bit worse so we compromised. Now I want to fuck with it some more and a better understanding of its function would be really beneficial. I am currently experimenting with an open airbox and a fat duc, not near as good a solution as Todd's PCV, but with the wife unemployed, I am cheaping out. I have an exhuast gas analyzer that basically relates levels of carbon monoxide to AFR. To get 13.2 AFR, I need ~3.8% CO on the analyzer. I will be checking the fat duc to assure that I set it to prevent the overly rich readings Todd saw. I think the sacred screw may play into this as it changed the poping on over run, once I understand its function, I may be able to optimize the idle, and minimize the popping with the fat duc. so far to error messeges sent, but I've been keeping the fat duck set pretty lean ~13.8afr, it does seem to take a few minutes for any adjustments on the fat duc to reconcile with the ECU, but pulls cleanly at low rpm's with no lean surging and minimal popping
Thanks for any input you may have.
Cheers,

 

[GuzziMoto]

I am pretty sure it is a throttle stop, if you are talking about the screw I think you are. It sets the closed throttle position and could definitely effect popping on overrun but not really going to have much effect on mixture whenever the throttle is open.

 

[GrahamNZ]

Here we go, yet again.

The screw on the left injector body is factory set by the injector manufacturer using a flow meter to set the trottle plate at 4.7 degrees + or - 0.2 degrees. That is why when the TPS on the right throttle body is reset using Axone or VDSTS it reads the same 4.7 degrees + or - 0.2 degrees.

The trouble with altering the screw is that the left throttle plate will not be at the factory setting but when the TPS is reset it will be, making the two throttle plates out of synch.

Its the ECU and stepper motor which control idle speed and mixture.

Have my old posts on tuning these bikes and how to reset the sacred screw been lost in all the reincarnations of this site? If so I will re-post them if requested.

 

[john zibell]

Graham,

Just a fine point, you can move the screw and still have the TBs in mechanical sync, but the ECU will be getting bogus information as to the position (opening) of the TBs.

If you still have the material from the old board, it sure would be helpful to many if you could re-post that information.

 

[GuzziMoto]

The "sacred screw" is just an adjustable throttle stop. You should not play with it without understanding what you are doing, as it has been set to a predetermined value (4.7 degrees + or - 0.2) for a reason. But it is not what syncs the throttle bodies and playing with it will not put the throttle bodies out of sync with each other. If you reset the TPS after adjusting the sacred screw it would likely put them out of sync with the ECU at any throttle position, but until you reset the TPS you can go back by measuring the TPS voltage and setting it to what it was before you changed the sacred screw.

 

[GrahamNZ]

John

Graham,

Just a fine point, you can move the screw and still have the TBs in mechanical sync, but the ECU will be getting bogus information as to the position (opening) of the TBs.

Agreed 100%

If you still have the material from the old board, it sure would be helpful to many if you could re-post that information.

I'll put somehing together and call the thread Home tuning a Breva V1100 etc.

 

[guzzi jon]

Graham
Thanks for reposting your tuning thread, exactly what I needed,
cheers,
Jon

 

[Spaceclam]

Its only sacred if you dont have a way of resetting the TPS.

When i synced my throttle bodies for the first time, (at about 5k miles) they were SO out of whack, that my idle speed increased 600 rpm after synchronization, 'cause the cylinders werent fighting each other any more. Clearly, as the bike was delivered to me, the throttle bodies were not in sync at 4.7 + or - .2 degrees.

To reset after something like this? Since idle speed will be a combination of TPS reading (timing, fuel) and throttle body opening (vacuum pressure), you need to turn the sacred screw until the idle is correct, then reset the TPS. As soon as you do, it will throw the idle off again because the timing and mixture changes, but it will be much closer. Repeat this step once more, and you should get it really close.

 

[Mike.C]

As we are in Board info rebuild mode, in the interests of making the info in this thread as complete as possible - here is the official line on adjusting the sacred screw.

From Moto Guzzi Technical note No. 007-2007 dated 22/06/2007

Model: Breva 850/1100 Griso 850/1100 Norge 850/1200, 1200 Sport (and I am sure it applies to Stelvio)

Subject: Throttle Body Tamper seals

We would hereby like to inform you that from today onwards the warranty claims relating to replacement of throttle body will no longer be valid in case of an alteration of adjuster screw sealed with yellow paint is found.

We would like to point out that any alteration of the throttle body adjusting screws does not entail any advantage to bike dynamics. Throttle body correct adjustment can only be done by Magneti Marelli, the manufacturer of the part using an air flow meter.

This is the reason why, if the screws to maintain the adjustment are tampered with it is no longer possible to restore throttle body correct setting and throttle bodies must be replaced.

Best regards
Your Friendly tech manager
Moto Guzzi S.p.A

I post this not to say that you can't adjust the screw, obviously you can as several have done so. However IMO doing so is a last resort and in the normal course of events - don't.

 

[GrahamNZ]

Spaceclam

Are you sure it wasn't a tight pull-to-open throttle cable which caused the idle speed increase once you balanced the vacuum and reset the TPS? in an earlier version of the forum I posted a detailed procedure for adjusting the throttle cables. Of any interest?

MikeC

Good to post that official warning. In my recent post, Home-tuning a Breva V1100 etc I included the method we proved a while back to reset a sacred screw which had been tampered with. In that case it was a Guzzi dealer's mechanic who had altered the screw because he didn't understand the correct tuning procedure. Sadly that is unlikely to be an isolated occurance.

 

[Spaceclam]

i am 100% sure. If it were a tight cable, then loosening the screw wouldnt do anything, cause the cable would be holding it open, plus there was just a little bit of slack.

That warning is just more of piaggio's bullshit. Its not because there's anything wrong with altering that screw (hell, in my case, it was necessary since it was so poorly set up) its because, if the screw has been altered, that means the end user has done some tuning him/herself. Therefore, if he has a throttle return spring fail, it was clearly caused by the resetting of the throttle body idle stop stressed the screw and so it isnt covered under warranty. :angry:

This is exactly the kind of thing i would expect to hear from Vespa TO (out of business, now vespa SO).

 

[john zibell]

i am 100% sure. If it were a tight cable, then loosening the screw wouldnt do anything, cause the cable would be holding it open, plus there was just a little bit of slack.

That warning is just more of piaggio's bullshit. Its not because there's anything wrong with altering that screw (hell, in my case, it was necessary since it was so poorly set up) its because, if the screw has been altered, that means the end user has done some tuning him/herself. Therefore, if he has a throttle return spring fail, it was clearly caused by the resetting of the throttle body idle stop stressed the screw and so it isnt covered under warranty. :angry:

This is exactly the kind of thing i would expect to hear from Vespa TO (out of business, now vespa SO).

Of course, you are entitled to your opinion, but there is a very good reason to not touch that screw. That throttle position was set by the TB manufactures to a specific angle, 4.6 degrees for 5AM system, and 3.1 degrees for 15RC systems. If you mess with it and do a TB reset with software, the ECU will be getting incorrect information. Yours may have been way off as delivered, in which case it should have been replaced under warranty. But then the warranty service could spawn an whole new thread.

 

[GuzziMoto]

i am 100% sure. If it were a tight cable, then loosening the screw wouldnt do anything, cause the cable would be holding it open, plus there was just a little bit of slack.

That warning is just more of piaggio's bullshit. Its not because there's anything wrong with altering that screw (hell, in my case, it was necessary since it was so poorly set up) its because, if the screw has been altered, that means the end user has done some tuning him/herself. Therefore, if he has a throttle return spring fail, it was clearly caused by the resetting of the throttle body idle stop stressed the screw and so it isnt covered under warranty. :angry:

This is exactly the kind of thing i would expect to hear from Vespa TO (out of business, now vespa SO).

Of course, you are entitled to your opinion, but there is a very good reason to not touch that screw. That throttle position was set by the TB manufactures to a specific angle, 4.6 degrees for 5AM system, and 3.1 degrees for 15RC systems. If you mess with it and do a TB reset with software, the ECU will be getting incorrect information. Yours may have been way off as delivered, in which case it should have been replaced under warranty. But then the warranty service could spawn an whole new thread.

You are making several assumptions, the first being that where the factory specs the screw to be set is the optimum setting. You are also assuming that they are all (or even mostly) accurately set to the factory spec.
I do think that if you don't understand what the screw does and what the effects of adjusting it are then you should leave it alone. But to say that you should never touch that screw.... that's silly. it is as valid an adjustment for setting up the throttle bodies as any other. It is not the first place to start, but if the screw needs adjusting then as long as you know what you are doing then adjust it. It is no different the any of the other adjustments to the fuel injection system. It is not magic, it is mechanics.

 

[Spaceclam]

Lets pretend that the throttle bodies are perfectly balanced. And that the valves are correct, spark is good, and TPS reads 4.7 at idle, with the throttle body resting fully against the stop (the screw).

Idle speed is controlled by three things. Fuel, (controlled by TPS) spark (controled by the TPS) and air (controled by the position o the screw)

Now lets pretend, that in all of those conditions, the bike idles 600 rpm higher than it used to, before the TBs were balanced. The engine would fight itself, causing my idle to be correct with an improperly set screw. The balance was set for high speed first using the spring loaded screw, then idle was adjusted using air bleed screws, and the TPS reset. Idle balance is PERFECT.

Under these circumstances, the only way to get idle down to PROPER levels, is to adjust the air (screw) since all the other parameters, (spark and fuel, controled by the TPS) are correct.

ergo, the screw NEEDS ADJUSTMENT.

Taking it one step further, doing it myself voided the TB warranty, so that warranty notice is simply a tool to force me, the consumer, to take my bike to the DEALER for service, or face the consequences. messing with the screw will not damage the TBs in any way. It would make sense for a dealer to charge you to retune the bike after messing with it, but voiding the warranty is unjustified

 

[Mike.C]

It would make sense for a dealer to charge you to retune the bike after messing with it, but voiding the warranty is unjustified

I don't agree, if the bike as designed is unable to be re-tuned using the procedures designed into the system by MG after someone in direct contravention of the manufacturer's written instructions has fiddled with the screw, then IMO they are justified in refusing to fix the problem at no charge as it was caused by inapropriate tinkering by unqualified personel, not a manufacturing defect that is the purpose of a warranty.

Also, it is unreasonable to expect a service agent to retune the bike if the screw has been adjusted when MG themselves say that it is not possible.

Of course Graham's procedures for resetting the screw will help if there is no choice after the screw has been inappropriately moved, but they are not official recommendations and can only be viewed as a work around (albeit IMO a pretty good one).

If you think that the design of the fueling system is poor and needs improvement, by all means go yout hardest. But in the normal course of events it is clear that the adjustments provided by MG in their design are quite effective given the number of bikes around that run very nicely.

 

[GrahamNZ]

Hmmm.

What everyone is saying has validity to a point. But. For most folk it's best to leave that screw alone. Guzzi didn't coin the expression, "sacred screw", it happened on this forum.

The main message is to "leave the screw alone unless you really know what you're doing".

It took a lot of questions, answers and deep thought here to come up with a "desert island solution" to deal with a tampered screw, and it wasn't until a practical execution of the thoughts that it was proven to be effective. As MikeC points out, it may not be the perfect answer but in the absence of a realistic alternative.........

Then again, tampering with the screw could strike it lucky...........

 

[Spaceclam]

the reason it's a screw, and not cast into the TB is because screws are for adjusting.

all it is, is an idle stop. the only reason its called sacred, is because if your TBs arent at the correct angle, it will never idle correctly unless you compensate by setting the TPS low or high, but then it wont run right because the mixture/timing will be way off, in which case the bike will run like crap all the time.

however, if all other parameters are within spec, and the bike idles too low/too high, (as in my case, after balancing) then the "sacred" screw is nothing more than a throttle body stop. a quick adjustment, a TPS reset, and everything is exactly as it should be. It is however a last resort, because EVERYTHING should be set up properly before messing with it.

Its not that MG has a bad design, it's actually pretty simple and straightforward. the only reason it needs adjusting, is because MG does such a poor job of initial ADJUSTMENT at the factory, such that the TBs ARENT always at the 4.7 they are supposed to be. Mine were off by over 6 degrees.

And you dont have to be super confident to do it, either. I had never tuned a FI engine in my life prior to doing this, and at the time, wasnt aware the screw was sacred. I simply set all other factors, then set the idle, like i would have done on a carbed bike, followed by a tps reset because i understood that changing the angle of the TBs would change the throttle % the computer sees.

as long as you understand that changing the speed/power produced of an engine is simply a factor of air, fuel, and spark, it's really straightforward and simple.

voiding the warranty on the TBs is completely unjustified, since messing with that screw can not cause any kind of physical damage. engine warranty, maybe (poorly setup FI which already runs critically lean) but even that seems a bit harsh.

 

[GrahamNZ]

The problem is there is no way of knowing if the stop screw is setting the left throttle plate at 4.7 degrees. Yes, a TPS reset tells the ECU to treat the right throttle plate as if the right plate is at 4.7 degrees even if it isn't physically.

Relying on the idling speed is also an unreliable guide to the stop screw's position because idle speed is very much influenced by the stepper motor, and there have been many reports of that animal having a mind of its own. Gosh Todd and others have even figured out ways to isolate the beast once the engine is warmed. RPM dropping below the ideal figure momentarily when the throttle is closed rapidly, such as when coming to a dead stop and pulling the clutch, is a common trait of the stepper motor.

Frankly I have more confidence in the stop screw being correctly set at the factory than I have in the perfect performance of the stepper motor. While mine seems to work as intended 99% of the time, very occasionally the engine will stumble at the first cold stop of the day.

Then again, even Triumph are reckoned to get something wrong 2% of the time with their new bikes, so it is certainly possible for MM to be less than perfect.

What a lot of us would like to have is a really reliable way of checking that the left throttle plate is at 4.7 degrees but so far no one has come up with an answer to that.

 

[Spaceclam]

actually, the screw adjusts both sides. THe left side is mechanically linked to the right side. So if your TBs are balanced, that means, mechanically, that they open and close at the same rate, to the same point.

i have actually found the idle speed (as well as the speed at which it returns to idle) to be a VERY accurate indicator of TB position. of course, i also have the stepper motor removed, so for those of you who dont, what i just said may not be valid. but you should take it off, or buy todd's kit. cold starting isnt really an issue unless it's REALLY cold.

when i was adjusting it the first time, we adjusted the idle speed so it was a little high, (since it was way too high, and resetting the TPS would drop the speed cause of ignition) and reset the TPS. it idled about 100 rpm too high and it would return to idle very slowly, which indicated it was making too much power at idle (TBs open a little too much, higher than 4.7 degrees). so i brought the screw back just a smidge, (.3 degrees on the vstds) and todd reset it for me again, and now it idles/returns PERFECTLY.

The difference in night and day was only .3 degrees mismatch between the TB angle and the TPS angle, so its incredibly sensitive/responsive.

overshooting idle CAN be a function of the stepper motor, but mine did it, even after it's removal. this was before i balanced the TBs

 

[GrahamNZ]

Spaceclam
Looks like we're never going to agree on this but it's because the ECU thinks the left throttle plate is at 4.7 degrees and delivers fuel and timing to suit that base position that it's important that the plate is at 4.7 degrees. Vacuum balance can be perfect and the TPS can be reset to read 4.7 degrees but if the left throttle plate is not where the ECU thinks it is it will not be delivering the programmed fuel and timing to suit any diffent position.

What would be interesting would be to measure the clearance between the throttle plate and the injector body in one which has not had the stop screw altered. Remember how we used to set the balance on multis using a 3mm drill bit to ensure they were in synch at the point where a lot of the running was done? For a similar reason it's why the vacuum balance on modern Guzzis is set at 3,000 RPM and not just at idle. If I wasn't so lazy I'd make that check so at least I'd know what 4.7 degrees open is in a measureable way, not having access to a flow meter.