Power loss at 4000 rpm?

[milkman]

I have the same power issues atthe same range Trolle, 4000-4200 rpm
what was your solution in the end? (i read the posts bu think i missed it)

[trolle]

I did not solve that particular problem yet and having no  idea of the cause I can only guess:

Yesterday I made a roundtrip of 400 miles in strong winds (above 10 m/s and up to 17 m/s in the gusts) and showers. At some stretches of the road the wh8la2 had problems reaching 80 mph due to headwind and at other, where the wind came from the side or even from the back, there were no troubles at all. Now I measured the compression of both cylinders just before this trip and although the compression was the same at both sides, it is at the low end of the acceptable, ie. 123 psi.

My hypothesis is that the engine has  a small drop in power around 4000 rpm, maybe due to the working of the carburettors, as described in other postings. Under "normal" circumstances this drop is not perceptible but under extreme circumstances, like strong headwinds, the need for fast acceleration in fifth gear from 50 mph and up, steep inclines and the like combined with the less than optimal compression, the drop becomes perceptible and at times a nusiance.

My next project, therefore, is to put in new piston rings at both sides. After 60,000 miles the rings might be worn enough to make the compression less. Another  culprit could be the valves but as I have not done 10,000 miles on my new exhaust valves and seats yet and as the intake valves were judged OK by the shop who did the job, I tend to blame the rings. Having the nicasil walls of the post 81 engines I do not have to worry about cylinder wear and the last time I checked the walls were OK.

I have at this point checked ignition timing, torque of the head bolts, compression and for leaks in diaphragms.

The carbs have been fitted with new needles, jets and floats and new throttle shaft and butterfly on the right side.

The coils have been exchanged for a new set of high yield coils and the rectifier and voltage regulator have been exchanged with a new unit, comprising  both rectifier and regulator, which gives a much better power supply. The battery charging begins at 1100 rpm and the charging voltage lies appr. 1 volt higher than that of the old system.

greetings from a grey north with good riding conditions, ie light wind and dry roads.

trolle

[Justin B.]

Milkman, if you are "running out of steam", and not picking back up after that RPM, then it is likely a leaking carb diaphragm or two...

[milkman]

right, thanks
i used to pick up noticeably after that rpm, but now its laggin somewhat

hope to look at it when i can take teh bike apart

[wiemer]

Hi Trolle,

The problem at 4.0-4.5 I had too, when I fitted the heads with the larger valves.
Combined with rough riding in that RPM area.
Because the spark-plugs were too light I raised the needles -> problem solved. maybe you should fit the old needle jets: they are a bit bigger.
My experience with more or less worn bikes tells me that a worn engine loses power at low RPM's, but the high end is not so quickly affected, (when the heads are OK, and no bouncing valves in their guides)
IMHO this is a tuning problem.
So do not sink too much money in new part, but fiddle with the carbs, and pay some Euro's extra fuel.
I assume that the advance is finished at at most 3.500 RPM.
Our BMW's do not like a lean mixture! I am happy with 18 kms/liter in the summer.
Now she does only 16-17 kms to a liter, but I do mainly commuting 8.5 to, 8.5 fro. Not good for the bike. She suffers.
But even the battery of 5 years old let me down only once this winter!

P.S. My bike has lost power above 6000 RPM, since I changed the torn rubber tiny rings around the idle jets.
Too rich a mixture now? Those BMW carbs keep you busy.


wiemer.

[Ed Miller]

My bike has a power loss when it goes over 7K.  I think there's something wrong on the seat.

[wiemer]

Well, Ed.

Our not-so-strong bikes should have their max torque at 6.500 RPM, (book), so in real world the bike should pull OK at 6.000 RPM.
And that is only 144 kms, or 90 miles/hour.
Every now and then I test if the bike still does. And she doesnt anymore.  

Am I demanding too much?

wiemer.

[trolle]

Hi Wiemer

You have not been on for a while - nice to hear from you again.

Due to the rather harsh weather - snow and freezing temps - I have not been riding for the last couple of weeks so I have almost forgotten the feeling. Never the less your info on the powerloss between 4K and 4.5K eases my mind and, as you suggest, the rest is a tuning matter.

So now I will finetune the ignition setting and see if that helps. If it does, I will find other things to worry about  

greetings from a sunny, white and cold north with dry roads - good riding conditions

trolle

[wiemer]

I feel that the problems at 4.000-4.500 have to do with the bigger valves from 1981 on.
The guys (and gals) who have the pre 81 engines might confirm this!

Also, I feel (subjectively) that the intake sound at that RPM doesnt sound good, sort of bad interference.
Maybe there is some backwards going pulse when the intake valve is not yet closed.

It is not easy to make a bike have a dip-less torque curve.

wiemer.

[trolle]

Just the latest update: I had a suspicion that the ignition was a little on the premature side and changed it appr. 2 degrees. Quite an improvement in power at all rpm. The 4000-4200 as well although there still is a flat spot there but now it is acceptable. In the band from 5-7500 rpm it really flies.

Wiemer: My new Omega DICU stops at 3700 rpm
greetings from a grey north with good riding conditions

trolle

[wiemer]

Hi Trolle,

That is nice!
To feel a 2 deg change, you know your bike well!

Maybe you should go back another 2 degs! Those marking points are not sacred.
It is kinda strange that our CR 9.5 bikes have the same ign timing as an r 75 CR 9.0 and an R 50/5 CR 8.6.


I am going to check my ign also tomorrow.

What I experienced with my  benelli twin I rode for many years that too early an ign timing resulted in a less vibrating and more smooth working bike , but less throttle responsive and worse accelerating.
The Benelli also never pinked (detonated), nor does my R 65.

Wiemer.

[trolle]

I do not know much about the relation between  ignition time and  CR but never the less  I agree that it sounds strange.

The former ignition setting was set just at the beginning of the cycle - at the point where the diode on the Omega unit just lit up - and the new setting is at the middle of diode light. Maybe I was bragging a little there about the 2 degrees, it is probably nearer to 4 or 5 degrees.

The result is better starting, less use of choker, more power at all rpm and maybe slightly more vibrations. The last is hard to feel because vibrations in our engines IMHO are mainly due to faulty synchronization of the carbs. This is especially the case at idling; if the carbs are not properly in sync, the engine vibrates very much making the whole MC shudder, but when they are in sync there are no vibrations, even at rpm below 1000.

greetings from a showery north

trolle

[wiemer]

Yes, showery here too!

But there will be sun this afternoon, the meteo-site says.
When the CR rises, the flame will burn quicker, the ignition can start later.
The form of the burning chamber (head, squish band, piston) is very important too.

I dont understand a bit of the way you set up your ign. A diode light lights up, when the flywheel is in the S position??
What is the beginning of the cycle you mention? TDC?
You still need a strobe lamp?

I bet you advanced the ign. [smiley=behead.gif]



Wiemer

[trolle]

The red thing in the picture is the ignition control unit and as you can see, there is a rotating aluminium disk with an embedded magnet (visible in the picture).

When this magnet passes a feeler placed in the ICU a red diode in the ICU lits up and stays lit for appr. 10 degrees of the disk's rotation. At first I had set the lighting of the diode to match the S mark on the flywheel but this was to early and therefore I changed the position of the alu disk to match the midpoint of the diodelight's period with the S mark.

I have not checked with my strobe light yet but when I checked the old setting, both the S (at idling) and the Z (above 3700 rpm) where sitting too low in the inspection hole.

greetings from a sunny, part cloudy north with good driving conditions

trolle

[wiemer]

OOhh,

That is not simple to adjust! So small, one touch and the ign moves a lot of degs.
When the ign marking is low in the hole, it is late. the crank turns clockwise, looking from front.

You should check with the strobe lamp the actual setting, which MIGHT please you more than the factury setting!

What we learned now is that a late ign aggravates the 4000 RPM power dip problem.

Let us know!

Wiemer.