Idle RPM after tune up.

GRiSO 1200/8V. Tappets set at 0.1 & 0.15mm (I/O). Plugs @ 0.65mm. Clean Air filter. Fresh / quality fuel. Rollerised & with Beetle’s Map, a low mileage machine with <7000ks. ‘Sacred’ screw & butterfly linkage screws have never been touched....

Bike (still) runs beautifully, but since yesterday’s tune up idles around 1400+ RPM area.

Tune up done via Pete’s great write-up & Guzzidiag.. thanks! TPS confirmed at 4.8.

This morning I revisited the set up, and note that if I miss the final step of balancing the manifolds (at idle) via the highest Air bleed screw, the idle is indeed in the book 1250RPM ball park.

What’s confusing me is the first step versus last; ie balancing the manifolds via the L Bellcrank screw at ‘working’ RPM, and finally later matching the manifold pressures at idle RPM

.... if I complete the set-up as published, and then ‘revisit’ the 3500-4000RPM manifold readings, I find they are about 2cm Hg out

Q. Is it not better to have the manifold pressures matched at Cruise RPM versus matched at idle, given that the bike spends most of its time on the move?

Keen to learn the reasons.... prefer not the ‘just do it’ type answer!

(And Pete, you’re right, it is far easier (if not yet fully understood!) versus tuning 4 Carbs .)

Hello,

I run into the same problem with my 2015 Stelvio. I balance at 3500-4000 rpm, but then at idle speed the balance is not ok. I have learned to live with an unregular idle but I would really like to get it balanced better at idle too! With correct balance at cruising speed the engine is running much better of course!

Philip

Do you shut both air bleeds before using the bell crank screw to balance at 3,500-4,000rpm?

After the high speed balance do you recalibrate the tps?

Have you checked the paint on both the linkage rod ball joints and the throttle stop screws?

Yes I shut both air bleeds before balancing with the bell crank screw. And I reset TPS afterwards. The yellow paint is untouched . My Stelvio I bought from the local Guzzi dealer. It was his own personal bike. He sold it to me after one season with 9000 km's.
As I do al maintenance myself I have checked all these painted points.
At the moment the balance is perfect at 4000 rpm, but uneven at Idle. Both air bleed screws are closed now. If I open the left one a little bit, then the balance at idle is ok but re-checking at 4000 rpm I find a difference of 2 green blocks on my electronic balancing tool (Bought from HMB-Guzzi in Gernany). The tool works fine. We also have a Breva 1100 and that one I get balanced perfectly with the tool, at idle and at 4000 rpm.

Hi Pete, thanks for replying. Yep, both air bleeds closed < the process starts... and the sacred screws untouched.

In my case simply querying WHY after going to the trouble of balancing the manifolds at cruise RPM, you then (potentially) unbalance them for the sake of a smoother idle?

I don’t doubt the process, after all Balance screws are provided. I just wanna understand it!

There are always going to be compromises. Unless the discrepancy is large enough to be noticeable while riding I wouldn't worry too much. Things like valve clearance will change the valve timing meaning the cylinders will pump differently. What you are trying to achieve is an acceptable compromise.

Yes, some bikes will balance up better than others, like your Breva, but generally there will be some small discrepancy. Something like a leaky valve would be immediately obvious.

Cheers Pete. That all makes sense. Tbh I’ve not done the final (idle) balance; it seems smooth enough and more-or-less pegged at 1250RPM. And the engine runs just great... can’t fault it.

I'm about to have another crack at my 1100 to see if I can resolve the high idle speed issue.
At times it seems quite acceptable about 1300 revs but someone pointed out if I blip the throttle it doesn't come down straight away, might take several seconds even though I can clearly see the butterfly snap back.
The sacred throttle stop screw is still intact but I see we also have to check the ball joints as well.
I recently took the throttle bodies off for a good clean, I was wondering if it might have an air leak past the large "O" rings, is that something that could dry out with age?

I agree with Steve that getting equal valve clearances is critical if you want the best balance especially at tickover.

I was taught to always used to do the valves first on Triumphs Beemer and Guzzi and doing it that way you are seldom going to run into signivicant discrepancies (unless you have bigger pumping issues like piston and rings).

Personally I always preferred to synchronise the opening of the throttles as i love that smooth thrusty inital accelerate - but as i have yet to do the job on Snafu (my GRiSO) we will see if that is possible.

Roy, try some carb cleaner into the stepper motor. If that returns it quickly the stepper is like many, starting to age slightly. Mine has been disconnected (plugged) for years.

Thanks Tony, you seem to be suggesting that the stepper is changing position when you rev the motor, I wonder why it would do that.
How does the stepper know when to correct the idle speed, would that be based on TPS Voltage?
I have one of the plastic stepper valves, Pete sent me a replacement.
Q/ If I plug the stepper valve inlet then that should eliminate that, no?
Happy New Year
Lots of riding.

Plugging the stepper hose, takes the stepper out of effect. Should that make a difference, it's starting to get worn. I dunno what wears in them - mine is of a similar age to yours.

Pugging mine brought the idle back to 1200 +/- a bit (according to the rev counter). Before it was hunting around 2,000 rpm - hunting is a sign of severe wear.

the stepper is always trying to maintain a constant idle regardless of the conditions. Conditions are air temp, engine temp, air pressure, fuel mapping (warm up or normal), advance etc. etc. It simply adds more air or takes some away, when the ECU plays with the fueling.

However up your way in winter you might need to give it a bit of extra throttle, if the stepper is plugged, when starting on a cold day. Having another (presumably a metal bodies one) is another test. Plugging the stepper is a quick test without pulling everything apart around the throttle body assembly.