R25/2 Top Speed

[Kurt in S.A.]

After a full engine rebuild, I've been breaking the engine in...I have about 750 miles since the rebuild.  I have the tuning about as good as I can do...I get the full advance mark in the window when the engine is at full advance.  The carb has the stock jets and settings.

I recently tried a top speed run over a section of road that I've used before.  I laid down on the tank and got to only about 88-89 kph.  I could roll on more throttle but there was no more speed.  Before the rebuild, I could get to about 95 kph over the same section of road.

Some other forum discussions mentioned the possibility that the exhaust may be restricting the speed.  I'm sure it's not an original exhaust...but it has the swallowtail shape to it.  The holes in the aft end of the exhaust measure about 4.75 to 5.0 mm based upon a rod that I can stick into the holes.

How big should the holes be?  I've heard 6mm and people have drilled them out to 7 or even 8mm.  Does that seem reasonable to do?  Anything else I should think about checking?

Thanks...Kurt

[Kurt in S.A.]

[berndr253]

Hi Kurt,

before talking about holes in the exhaust - please change the main jet to one size above the size you've installed. If the maximum speed doesn't change we will see what to do

Best regards

Bernd

[Taz]

Hello Kurt.

I am not familiar with the R25/2, but regarding the silcencers you might want to have a look at this page: http://cms.bmw-einzylinder.de/index.php/baureihe/bauteilaufbauten-und-co/107-detailinfos-zu-endtoepfen.html

It's German text, but the links provide a lot of insight and might be helpful with identifying whether it's a original part or an aftermarket one...also you might want to visit http://monos.r25-bike.de/#auspuff_EN for info...there's a link to a longer german thread discussing the various exhaust pipes and how to improve with a drill...

Best regards, Taz

[Kurt in S.A.]

Bernd -

I think this might have been discussed quite some time back before I rebuilt the engine...sad to say I never got around to trying a larger size main jet.  My thinking is that I'm getting only about 55mpg on the engine and going to a larger jet will only make that worse I think.  As it is, 55mpg is pretty bad in my book.  Since normal running is close to running on the main jet most of the time, a bigger jet just makes it worse.  That would really make me scratch my head.   

Kurt

[Rütz]

Hi Kurt,
if you can see holes, it is likely, that you've got a rebuild exhaust like the right in these pictures:
http://cms.bmw-einzylinder.de/index.php/baureihe/bauteilaufbauten-und-co/111-endtopf-r24-und-r25.html
(R25/2 has the same inner construction)

These exhausts are really bad.

Solution:
5 holes á 8 mm  or
13 holes á 7mm  or...

...buying an new exhaust with originally rebuild interior (Hugget #18 12 4 090 004.F , i think)

[† Knut]

Kurt,

How fast can you get in the third gear and do you feel a reasonable power at the higher revs?
If I remember well, also a 25/2 revs more than 6,500 rpm and this should be more than 80kph


Knut

[Kurt in S.A.]

Knut -

I've never really tried that in 3rd gear.  As for feeling power...hmmm, it's only 12hp!!  I know that there are some hills that I ride up and I have to get some speed going downhill in order to make it reasonably far up the next hill.  I sort of use this one hill to let me know if the bike is performing well or not.  I must reach a specific point on the up hill portion before shifting out of 4th to 3rd...any sooner than that and the bike would be down on power for some reason...timing, tire pressures, etc.

Kurt

[rolf.soler]

Bernd -

I think this might have been discussed quite some time back before I rebuilt the engine...sad to say I never got around to trying a larger size main jet.  My thinking is that I'm getting only about 55mpg on the engine and going to a larger jet will only make that worse I think.  As it is, 55mpg is pretty bad in my book.  Since normal running is close to running on the main jet most of the time, a bigger jet just makes it worse.  That would really make me scratch my head.   

Kurt

you can't know without trying. It's a basic tuning tip; most engines do a bit better with main jet 1 or 2 sizes above stock. It's just that way, don't worry about the physics but go and try . Jets are cheap.

[Kurt in S.A.]

I definitely plan on getting a bigger jet.  Easy enough to try it and see what happens!   

Kurt

[† Knut]

In addition it's always worth to try a bit less of pre-ignition. Due the better fuel the burn (flame-front) is much quicker and this allows to reduce the pre-ignition as late as 35° before TDC. 42° could be too early and can lead to uncontrolled burn

Cheers

Knut

[Kurt in S.A.]

Knut -

So you're saying to time it so that the full advance mark is not in the window, but set to a slightly retarded setting.

Kurt

[† Knut]

That's what I wanted to explain.
Try an ajustment 6.0 to 11.0 mm (equal to 4° to 7°) before the "F" mark

Good luck

Knut

[Kurt in S.A.]

I ordered a 110 (I had to use a guitar string to unplug the current one, maybe it's deformed), 115, and 120.  Something to play with.  I'm going to go out and wait for the post now!   

Kurt

[Kurt in S.A.]

I installed the 115 main jet and took the bike out again today.  Slight wind-aided, the top speed was still in the 88-90 kph range...the bike even skipped a beat or two.  I rolled out and rode back to repeat the run...same...still only high 80s.

So, it would appear that the main jet is not the issue.  I haven't done a plug check to see if I'm over jetted.  When I got the bike, it had a 100 main jet.  I found that the Bing manual says 110 whereas the owner's manual I bought says 105.

Guess I'll have to look into other things...I do need to consider the plug color after running at high speed and then cut off the ignition.  I mentioned that the muffler has some pretty small holes...not sure I want to drill them out until I've checked other things.

Kurt

[BenW]

Kurt,

Provided you have installed the original carb for your R25/2, i.e. the model 1/22/44, then it should be equipped with a 110 main jet. Not sure which reprint of the owners manual you have bought, but the German version of the manual also shows the 110 main jet to be correct:


However, even jets with the same specs may behave differently, so it is always worth trying another new 110 main jet just to rule out a faulty jet.

You should also carefully check for any false air intrusion, for example in between the carb and the cylinder head. Also check whether your wheels are running freely, and that the brakes aren't limiting your top speed.

Lastly, you may want to try pushing down the ignition key with your left hand when trying to reach top speed as vibrations may cause the contact inside your head lamp fail and subsequently hinder the ignition system to function properly.

There may, of course, be a whole lot of causes for the issue...

Cheers and good luck
BenW

[4Taktix]

Hi Kurt, check if your air-filter-slide has become loose and maybe shut's the slots caused by engine-vibrations. If so, you could lock the slide
open with a piece of wire or tape for a test.
Then simply try a ride without muffler to see, if this is the limiting factor. Will save you lots of guesses and time also.
(make an educated guess what my bet is) Curious for your next report.
B.t.w: I had similar effects after my last engine rebuild (25/3) and the aftermarket-exhaust was just one of the root-causes.
Meanwhile with a "Power-air-filter" (modern wire mesh type like e.g. "K&N") I approached a 180!!! mainjet instead of stock 145.
Fuel consumption with big saddlebags in hilly countryside and "grab the flag"-style riding is about 4,5 litres / 100 km.
But: you should not waste a single second thinking about fuel consumption while chasing the best performance !
This will never ever lead you to the finish-line. It will work just the other way 'round 
Greetings to S.A. , Sascha

[Kurt in S.A.]

BenW -

The manual is an English reprint I got from Vech...inside the front cover the company/printer is "Nachruck...Schober-Verlag" out of Battenburg.  The 110 main jet that was in it before this switch was the same as before the engine rebuild.  The only exception is that somehow it got plugged after the rebuild and the quick fix was to run a proper sized guitar string through it to clean it up.  My thought was that didn't damage the jet but might have opened up some.  I will be going back to a new 110 jet and will likely do a plug reading next time.

I can check the carb-head bolts...don't think they're loose.  Wheels haven't been off the bike in quite some time...again same before/after the rebuild.  Hadn't considered the key issue but it seems to fit pretty snug.  Hmmm...do I want to take my hands off the handlebar at such breakneck speeds?  Not sure why it feels this way, but riding the R25/2 at 90 kph seems so very fast as compared to either of my twins.  Maybe it's because I riding with no fairing, lying down on the tank, and have the throttle wide open...that just has to be fast!!

Kurt

[Kurt in S.A.]

Sascha -

I've rechecked the filter opening...it's wide open.  I haven't cleaned/oiled it in a while...but I'm not running around in obvious dirty air.  Hmmm...probably should make sure it is indeed clean.  Interesting about removing the muffler...I wonder how "loud" it would be??

Kurt

[4Taktix]

Just Do it ! You'll probably need no longer the 2 miles to know, if this was the clue.
If you're concerned about getting a ticket, just tie your muffler onto the pillion-pad or rack, and say you've just lost it
and have no tools with you and have to ride home first to fix it there.
Concerning the filter cleaning - could be the reason for the "plug" in your mainjet. This filter is not too effective, thus it should be cleaned
and oiled the more often. (e.g. once a year)
Regards, Sascha

[Kurt in S.A.]

I'm back to the replacing main jet.  I was swapping the jet with the carb installed, I didn't feel that the needle jet up straight and/or far enough.  So I removed the carb and the needle jet...everything looked OK.  But in the process, the atomizer came loose and I want to be sure I know which direction to install it.  I strongly suspect that the slope on the top if it needs to be pointing to the rear of the bike...that is the air flow into the carb will be right to left in this picture.  The picture shows the needle jet screwed into the bottom of the atomizer.  Sound right?

Thanks...Kurt

[Kurt in S.A.]

OK, I figured it out.  There is a flat on the atomizer that mates to a flat in the bore of the carb.  So it can only go in one way!   

So, in fact, the air flows from left to right in my picture...I guess that is to create turbulence and help atomize the fuel??

Kurt

[Kurt in S.A.]

Did another run...seemed to be the same conditions in terms of wind, etc.  This time, with the new 110 main jet, I saw 93-94 kph...it might have gone a little faster but I had some traffic that I was catching so I rolled out.  I'll monitor this for a while...try the speed run the next time.  At some point, I'll try the plug reading to see what the current setup looks like on the plug.

Kurt