The New And Improved Unofficial R65 Forum V2

Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: dogshome on August 25, 2020, 04:22:21 PM

Title: 860 Kit
Post by: dogshome on August 25, 2020, 04:22:21 PM
My 650 has always ran like a 1980s 250 2 stroke. Except less noisy, no power band and better MPG! Apart from those things, she is light and easy to handle, not particularly fast and needs a gear change on hills. Similar performance (in a different way) to my old RD.

The engine is completely different to my friends R80. He describes it as deceptively fast. The 650 is nothing like that, more revs=more go, but rolling on and suddenly doing 90 is never going to happen.

So it's in. I found the upside down head gasket and dodgy pushrod rubber. The quality of the alloy and fasteners is nothing like my old RD, 39 years and 63,000 miles and the bores, rods and big ends are like new.

Some soft carbon buildup and totally dry exhaust ports. Perfect.

Just got to fix the exhaust and a test run is in order.
Title: Re: 860 Kit
Post by: dogshome on August 25, 2020, 04:31:14 PM
I can see the much discussed squish band is not perfect, but the design is clever and they squeeze in 2 more cooling fins.

I never achieved perfect squish on any of my bikes and ultimately performance is a combination of things.

Quality is excellent, although clearance on the back of RH cylinder to crankcase is only 8 thou. I thought I might have to Dremel some off, but it was OK.

My experiences with bike carbs is that the jetting matches the bore size and engine type (2 stroke or 4). Unless the engine is doing a lot of revs with modern cams, then the setup for one engine on a 32mm carb is pretty close to another, whatever the capacity.

I do know what rich and lean mixtures feel like, but insights into possible jetting changes are welcome. I have standard type  exhausts, standard paper air filter and absolutely no plans to change those things.
Title: Re: 860 Kit
Post by: mrclubike on August 26, 2020, 09:28:55 PM
After you assemble it turn the motor over by hand and make sure it does not lock up
I cant remember exactly but I think some R65 blocks foul the piston skirt or something to that  affect and need to be trimmed
Title: Re: 860 Kit
Post by: dogshome on August 27, 2020, 06:59:01 AM
First rule of comedy!  8-)

In gear and slowly by hand, plugs out. Then did on starter with plugs out and laid on head, then did with plugs in. All good, but new rings, assembly oil and bigger cylinders = hard time for battery.

I've rebuilt a few overhead cam engines and am always wary about that first rotation. Wasn't expecting any issue with this pushrod engine, just force of habit. My XT500 became about 560 and I put a huge Amal carb and a slightly modified speedway cam in it. I had to grind the rocker covers out and trim the tappet adjuster tops to get the big valves and lift inside. It changed the bike from soft into a bit of an animal. 

I seem to remember having to cut something out of the top of the case as well due to the forged piston shape - which wasn't Yamaha. Can't remember, it was in the 80s.

Just getting grinder to the totally rusted cross pipes now....
Title: Re: 860 Kit
Post by: dogshome on August 27, 2020, 12:51:36 PM
It runs. Hooting down with rain and I've been messing with the exhaust for a couple of hours. I bought some Keihan standard style stainless mufflers second hand for £60 and they are very quiet compared to the chrome ones that were on the bike.

So she sounds a lot slower  :D
Title: Re: 860 Kit
Post by: dogshome on August 28, 2020, 02:55:25 PM
We have unwanted interest!

Her big brother is happy sleeping.....
Title: Re: 860 Kit
Post by: Tony Smith on August 29, 2020, 06:35:40 AM
If you think it is good now, just wAit till it is run in.
Title: Re: 860 Kit
Post by: dogshome on August 29, 2020, 12:49:19 PM
If it EVER STOPS RAINING!!!!! Tomorrow looks good.  :D
Title: Re: 860 Kit
Post by: dogshome on August 30, 2020, 11:47:24 AM
It doesn't feel like it will need much running in. Already idles after 30 miles and a couple of deliberate stops and warm restarts. Less vibration and feels very turbine-like.

Keeping the revs between 2500 and 4500 and stopping after a few miles to cool. I've found with various engines that keeping the revs up, load light and stopping for a rest now and then is the best way. Iron bores and cheap pattern pistons take a while. These little forged beauties and plated 'liners' are not going to take long at all.

I am looking for 7th and 8th gear though. On my usual routes, even being aware of keeping light load, I did't think about changing down on the regular inclines. It all feels much more together. I didn't weight the pistons, are they lighter than the standard ones?

Is it worth E900? Same as it cost at the vet when one of my dogs got a very nasty infection, same to get my car through it's MOT a couple of years ago, same to help my daughter out now and again. Less than council tax council tax. Orders of magnitude less than a new BMW. A good holiday? Not really. Is it coming off and the 650 stuff going back on? NO CHANCE!  ;D
Title: Re: 860 Kit
Post by: dogshome on August 30, 2020, 12:04:50 PM
Going on a bit now, sorry!

RD250 plastic swing arm bearings. BMW Taper rollers.
RD250 Cheese crankcase screws. BMW Proper hard hex and cap fasteners.
RD250 Pretty poor castings and flaky paint. BMW Different alloy altogether and will last probably 1,000 years.
RD250 Very unreliable 2T injection and not long lasting (apart from the crank which was bulletproof). BMW Bulletproof all over.
RD250 Lively and tune-able engine (if you have time with a die grinder, which I did). R65 Lawn mower engine. R86 I'll let you know, but signs are good!
Title: Re: 860 Kit
Post by: mrclubike on August 31, 2020, 11:25:20 PM
The 860 pistons are heavier
But I cant remember how much it was

Without a side car you definitely want  a taller final drive  ;D
Title: Re: 860 Kit
Post by: dogshome on September 02, 2020, 11:23:39 AM
I should have checked the insurance first!!!  :o
5p per additional cc for the year.

Siebenrock confirmed standard jets all good with an unlikely incremental main jet increase. According to Haynes, mine should have one of the larger types already.

Title: Re: 860 Kit
Post by: BPT on September 02, 2020, 10:58:21 PM
I would never encourage anyone to flout the rules   ::) , and I do know that some countries can be quite strict, but I am curious  - the insurance company will know that the cc's have increased? Does this have something  to do with your (from what I've heard....) more rigorous inspection process?
Title: Re: 860 Kit
Post by: dogshome on September 03, 2020, 01:59:38 PM
If it had been an 800 or 900 upgrade, it would have been the same. It took a little longer because of the odd size, but we are talking 30+ year old bike with at best 50HP going to possibly 65. So the £10 yearly cost to me is very reasonable.

You could simply fit it and not tell anyone. However, if something really expensive happens then a simple count of cooling fins would identify the change. I doubt they would bother for written off bike and a bumper smash, if there was a fatality and multiple pile ups caused by me, then they might have a look.

If I were 30 yrs younger, the bike was a Hiabusa and I'd fitted a home brew turbo with nitrous, the premium might have been a little more.

I imagine there might be lots of undeclared mods, our bike inspection process is quite basic because it would be expensive and make no difference to accidents. A hot cam or performance exhaust is unlikely to affect the chances of SUV muppet pulling out on you.

Bike gets nicked, undeclared expensive mods? Still low risk to insurance company.

It's all down to risk and money. Which is the same as any government or corporate business model. Same in engineering.

I should charge a consultancy fee for this insight  ;D
Title: Re: 860 Kit
Post by: dogshome on September 06, 2020, 02:13:18 PM
She gets a wriggle on in 4th and 5th now. Front lifts up and hang on. I can't change gear quick enough at 4500 rpm. Happy at 75/80MPH indicated (70 then). Acceleration from that speed in top gear is very positive. Acceleration in second or third is lively, definitely in tuned RD400 territory now.

Pickup is clean, but I am trying to avoid any low speed lugging. Consequently I am looking like a novice at every stop junction. Stop, go, GO!, NO! go, Go!!!. Kangaroo fuel.

Actually becoming aware of power in the back wheel in corners now. Never an issue as 650. Unlikely to smoke the back wheel or become a problem in rain (we are talking maybe 65BHP instead of optimistic 50) and I feel has similar performance to my car at speed now. Saab 9-5 remapped 1.9 Diesel. Don't laugh, the Fiat engine with 9-5 petrol intercooler and swirl valves removed is easily 400NM.
Title: Re: 860 Kit
Post by: dogshome on September 19, 2020, 02:41:55 PM
Nearly 500 mile and she is loosened up. The starter can't whiz her over like 650, but it turns rather than coughing! Starting has never been an issue, but there is not a lot in reserve for this kit if there was a problem.

The suspension moves under acceleration now and if I was cruel, a wheelie might happen in second but I have some respect. Much nicer merging down the slip road onto the the A1 (interstate) and I don't feel harassed by the lorries. 650 was OK if revved, but a bit like my first car - not enough to be safe.

No pinging with 97 fuel and lead replacement/stabiliser. Plugs are white, but piston tops are black. No black smoke and no signs of lean 'hoover noises'. I will balance and set the idle mixture again soon.

7th, 8th and 9th gears need to appear. I really can't change fast enough to make full use of 650 gearing on the road and opening the throttle at 70 brings the front up to overtake very quickly in 5th at about 4000 RPM.

There is no more vibration than before. Fuel consumption is the same. No downsides apart from the cost  :P
Title: Re: 860 Kit
Post by: Bob_Roller on September 19, 2020, 09:08:14 PM
No replacement for displacement !!!
Title: Re: 860 Kit
Post by: dogshome on September 20, 2020, 01:31:54 PM
Balanced (CarbOnBal Arduino project) and mixture tweaked. A definite snarl above 4,000RPM and very 'revvy'. Clutch spline lubed, adjusted and idle set a little higher.

Unlike my XT500 capacity increase which made a thumper into a BIG thumper, this is more subtle. Yes "No replacement for displacement" but you would not notice in idling or very low down torque, just a big helping of extra from 2,000 RPM up. Which makes sense for a now very over-square engine.

0-60 I guess is about 6 seconds changing at 4,500 RPM. It would be 4 seconds if I didn't have to change gear  ;D
Title: Re: 860 Kit
Post by: wilcom on September 20, 2020, 01:55:44 PM
Both my R65's will upshift clutchless just like a Jap bike. Nothing to detect other than an RPM change. Pre-load and dip the throttle ((((BANG)))
Title: Re: 860 Kit
Post by: dogshome on September 22, 2020, 12:57:11 PM
A whole return trip to work (50 miles) with no crunches   8-)

If I don't think and just 'do' then all is good. the odd clutchless change happened - I feel all liberated now  ;D

Where are my rainbow gloves......

Engine (as posted above, thankyou) is properly zooming now run-in. The brakes (twin disc) are indeed 'adequate' when 860. Hitting 6,000 in 4th has the required 'millenium falcon' effect, hitting the brakes immediately afterwards is still exciting, but not in the same way  :o
Title: Re: 860 Kit
Post by: dogshome on September 26, 2020, 01:06:16 PM
500 miles and definitely run in. Oil change tomorrow.

There is a vibration at around 2,800 RPM but nothing below and no buzzing higher up. It's a power pulse/engine room sort that goes away on light throttle and is quickly gone-through in any gear.

The suspension reacts under power where there was nothing as a 650. A lot quicker and I am not worried about merging on the A1 with the 60MPH restricted HGVs now.

Tickover is now very positive and there is no sign of slowing, cutting or instability. Rock solid.

I'm not sure who's idea a 650 (or 450) was, but it wasn't a good one. Like your Escort 1100, Capri 1300 or Vauxhall Insignia 1.8 - Might look good on paper, but on the road. Eeek!. For my American friends, think Cadillac with 4 cylinder engine or any standard HD from the last 50 years to about 2000 :D

WHAT will we do with the gearing? Pulling away from cars at 70 is great fun, but the sufficiency of revs at 90 isn't!
Title: Re: 860 Kit
Post by: wilcom on September 26, 2020, 02:01:46 PM

WHAT will we do with the gearing? Pulling away from cars at 70 is great fun, but the sufficiency of revs at 90 isn't!

Tony Smith has the same conversion as you . He also has a taller rear drive. I'm not sure if he installed the taller rear drive, but he would be a excellent  resource if he has installed it.
Title: Re: 860 Kit
Post by: Tony Smith on September 26, 2020, 05:51:56 PM
No replacement for displacement !!!

Or, as Carol Shelby put it: "there ain't no substitute for cubes baby."
Title: Re: 860 Kit
Post by: Tony Smith on September 26, 2020, 06:02:36 PM
The final drive from an R75 significantly drops revs whilst retaining a lively nature. I have also (briefly) run the final drive for m an R100RS - it was not a happy motorcycle.

I think the final drive from an R80 might be worth looking at - I do not know the ratio offhand but provided it is close to the R75 ratio it should be happy.

Your Speedo of course becomes inaccurate, curable with strips of tape in the appropriate places, and/or a GPS Speedo.

Some time back I opened negotiations with Speed but to fit a GPS mechanism to an R65 instrument can. I recall they thought it was doable and I got as far as buying a dead Speedo for the project, but then other things intervened.
Title: Re: 860 Kit
Post by: mrclubike on September 27, 2020, 12:32:57 AM
You definitely want at least a 37/11
I know it works very well even with the 650cc barrels
The speedo isn't really off a lot but it is off.
I am using one out of a early R75/6
I like the early style finals better but just use what you can find in good shape
Title: Re: 860 Kit
Post by: dogshome on October 03, 2020, 06:59:05 AM
Finding one at reasonable money that isn't worn out is going to be a challenge. I can understand going too high will make it lug - which is exactly what is doesn't do now, but could as a 650 on fast road inclines.

I can cruise at 70 fine (85 indicated!) but a bit buzzy above that. The speedo reads 10 to 15MPH over at any speed over 25 anyhow, so that won't be a problem.

I'll keep my eyes open, I can't think of any reason someone would want to down gear (not many side cars about and GS isn't an off road weapon any more). So I expect there is more demand for higher ratios.

The engine is much transformed. I referred to it as a lawn mower (sorry!) at work as the response (and noise) was similar. She has a growl and gets up and behaves like a motorcycle should now  :)
Title: Re: 860 Kit Battery
Post by: dogshome on October 05, 2020, 05:33:28 PM
As 650, the starter would spin the engine over hot or cold. As a higher compression 850, it's more of a nudge than a spin, even hot. She starts, but I expect one frosty morning the nudge will not be enough.

Now I notice later machines have a much sturdier battery. Mine is typically 100CCA, the larger ones in the 300CCA range. The battery trays seem completely different, although only one dimension changes.

I had the tray out recently for spine lube and although solid it has little paint left. So I'm happy to chop it up a bit to make the larger battery fit. Before the tinsnips come out and it all goes pear shaped, any advice or previous mess up anyone wants to share  :-[
Title: Re: 860 Kit
Post by: dogshome on October 09, 2020, 12:53:59 PM
I measured and fitted a 220CCA battery which kicks her over properly. If I was doing a rebuild on 860, I would modify things to get the bigger battery in. The 20AH size with the conversion is a bit marginal. Several mods required to squeeze it in though!

The 650 barrels were in great shape, but are obviously smaller and lower compression by design. My mate reckons his 1000 was like two 500 singles having a dispute at idle, the 860 has some of that. Rock steady at 1100 RPM but the pistons are making their individual presence felt. His 800 he says is much smoother and probably the ideal size (more square ratio). 

I didn't balance anything, but felt no play at all in either the little or big ends. The cam followers were perfect. The bore and little ends were like new. Didn't grind the valves as the seats were also good. A couple of swipes to make them grainy perhaps, but it would have made no difference.

I'm on synthetic now 20/50W which she likes. The Keihan muffler I bought second hand are brilliant, but the chrome (Campbells?) ones I had on were much louder and would have been quite intimidating with the 860 :)     
Title: Re: 860 Kit
Post by: dogshome on December 04, 2020, 12:05:23 PM
Something I have noticed in the cold weather is the change in choke control. As a 650, full choke was required and flooding was almost impossible. With the added draw on the straw shall we say, she is easily overchoked. Unless freezing, full choke causes splutters and poor starting. No chake and a small amount of throttle is normal now.

I might make the jet for the choke smaller, but to be honest it's just a case of re-learning.
Title: Re: 860 Kit
Post by: dogshome on December 16, 2020, 01:53:57 PM
Scratch that. the dodgy plug was the issue, choking is fine  :embarassed:


The 1,000 KM break in is a little ambitious. With a few more miles/thrash under the bridge, the old girl is transformed. I rarely go near WOT or over 5,000 RPM but did last night to get around a lorry. The nose lifted, intake roar increased (in 5th) and I was looking at the lorry's lights in my mirrors in a very respectable time. The speedo said [SLOW DOWN] so I did.

As a 650 I compared her to a 250. That is about right. I can image the 450 is indeed comparable to a 125.

I understand squish, forged pistons, porting, carb sizes etc, but this conversion is absolutely spot on. Pulled the plugs (the replacements) tonight and they are perfect. I've avoided lugging the motor (and will continue to do so) but if you want that 'Harley' moment in town, it is there. I keep to premium gas and put small amounts of lead replacement in, but it doesn't seem to care in winter.

Forecast is 9C and no rain tomorrow, so off to work on the bike  :tekst-toppie:
Title: Re: 860 Kit
Post by: dogshome on February 28, 2021, 12:30:12 PM
Go faster stripes  8)
Title: Re: 860 Kit
Post by: mrclubike on February 28, 2021, 09:35:12 PM
The kit should come with those decals  ;)
I was wanting to do that to mine but I removed barrels  :wall:
Title: Re: 860 Kit
Post by: dogshome on March 02, 2021, 01:37:24 PM
Spurred on by shiney blue paint and stickers and against my better nature, I'm going to touch up the tank. The Faberge eggs have arrived (hopefully not egg shell). Wish me luck  :beehive:
Title: Re: 860 Kit
Post by: dogshome on March 19, 2021, 03:04:02 PM
The (Bosch) starter has been complaining about another 200cc and much increased compression. Seemingly increasing with every mile still. Cold weather not helping. A balmy 11C today and she was not playing. Several goes later and she finally kicked-off on one quickly followed by the other. Subsequent starts after only a minute or so, no issue.

So a bush and bearing kit is around £40 and a whole new epicyclic Arrowhead starter just over £100. I was going to strip the starter, but 40 years old and I'm thinking that the armature will probably want a proper fettle. Then I read about the very series wound current guzzling Bosch type and the geared ones. Seeing the voltmeter dip to 8V after a 2 second burst on the starter made up my mind. I'll clean up the old one and sell it if it just wants bushes.

The car just blew PS fluid out (my fault, didn't check all the joints on the new pipes) so the bike had better start tomorrow so I can fetch my 87 year old mums shopping!
Title: Re: 860 Kit
Post by: Bob_Roller on March 20, 2021, 07:45:31 AM
Most likely the starter has never been cleaned and lubricated since it was assembled 40 years ago .
I would disassemble it clean and re-lubricate it .
There are sintered bearings at each end of the armature shaft, they could be worn as well .
There used to be  a link in the FAQ section about overhauling a Bosch starter, the link shows a Guzzi starter it's the same as the airhead starter, just a different output gear, one more tooth than an airhead.
I've done four so far, not a difficult job at all .
If the battery voltage drops below 10 volts or so during starting, the electronic ignition doesn't work very well, if at all .
Title: Re: 860 Kit
Post by: dogshome on March 20, 2021, 02:09:56 PM
I was (not really) surprised that a new epicyclic starter was only £100 when a bearing and brush kit was over £40. You are probably right, I have no reason to think it is anything other than dirty with worn brushes and bearings. The price of things due to modern manufacturing and consumer preference is a bit off though. 3 carbon brushes and 2 oilite bearings shouldn't cost £40 (more than a week's shop for my mum). A whole starter costing about 2 weeks of my mums local shop (bananas, veg, sausages, bread, butter, chicken, yogurt etc) is also a bit off the the other way (too cheap).

Identifying the bushings and brushes and buying as misc parts would get the price down to maybe £20 plus postage, in which case it might be worth rebuilding. Epicyclic makes more sense with a high compression 860 in a 650 frame that can't fit a 1,000 size battery though (shorter swingarm).

I've noticed more rocking upon opening the throttle at idle with a few more miles and the ability to time-warp away from lights. Definitely nothing like the (perfectly good condition) 650 parts. The ability to outstrip lorries coming down the slip road has easily been surpassed! Equivalent modern 250 to probably 600 performance now.

Gratuitous power and (more importantly) torque chart below. I have no reason to doubt it.
Title: Re: 860 Kit
Post by: Bob_Roller on March 20, 2021, 03:03:42 PM
I was surprised to see the cost of replacement parts .

I got my last kit from Euro Motoelectric in the central US .
Title: Re: 860 Kit
Post by: dogshome on April 20, 2021, 12:11:48 PM
I got around to fitting the geared starter to replace the direct drive Bosch one. It has not been out at all in 40 years looking at it. I've not stripped it yet, but I think I will find a grubby armature and worn brushes exactly as you said.

The new one whips the 860 around like it should do, even when stone cold. I have to re-learn pressing the starter button now, jab and release - not press and hold!

Since I had the diode board out recently and didn't want to take all that out again, I left the Bosch support bracket in. Both 8mm nuts can be reached from the top with patience and the bracket itself just needs a tweak (gentle bend) forwards to let the old starter out and the new one in.

Luckily a combination of 13mm ring spanners, 1/4 and 3/8 drives meant the two big bolts could be removed and replaced without taking the airbox off or taking a chainsaw to the cases. They are poxy things and should have been capheads like the rest on the bike. I used the bolts that fitted the Bosch to fit the geared unit and they fitted fine.  :2vrolijk_08:
Title: Re: 860 Kit and Starters
Post by: Tony Smith on May 24, 2021, 07:20:03 PM
Bit of an omnibus reply to a number of posts.

I have just bought a brand new set of R-65 decals to put on my new sidecovers (more on this below). I see no reason to advertise the increase in capacity, I prefer that to be a big surprise. :-)

The reason I needed new sidecovers is that I have gone long wheelbase. At the cafe I go to on Sunday there is a resident expert on BMWs, the long wheelbase, different sidecovers and K100 front end has him flummoxed.

Starter motors

I hate Valeo starter motors, they are poorly made,l and do not last anywhere near as well as the Bosch starter.

I have two of them, albeit Chinese copies. The first was bought for my R1150, I was surprised that it is actually better put together than the OEM Valeo. But then that is no ringing endorsement as a 5 year old armed with a bucket of parts and flinging them in the same direction would result in "assembly" at least as good as Valeo's efforts.

I pulled the Arrowhead apart, greased it properly and resoldered a few things. It works, but I wish the GSA had a kickstart as I do to to some remote places from time to time.

For reasons I can't recall I also bought a Chinese starter for Airheads. At the time the Bosch starter I rebuilt in 2013 seemed to be struggling with the 860 kit, but that proved to be a poor choice of battery and a Motobatt cured that.

A Bosch starter works perfectly and frankly I would fettle it instead of buying a Valeo clone.

There is a problem with coil voltage robbing on startup - that is down to BMW's less than ideal wiring harness and is not the Fault of the Bosch starter.

For less the $20 you can fix that.

Take the hot lead off the coil and use it to trigger a simple 4 pin relay connected directly to the battery - use a fuse if that excites your.

The days of voltage drop while cranking are over. The wife's bike which hAs always been a hard starter now leaps into life on the first revolution.

Lastly, I took the plunge and bought an Australian made Wedgetail ignition system. This hAs a total rethink on coil driving and fully electronic ignition. I love it!
Title: Re: 860 Kit
Post by: BPT on May 27, 2021, 02:38:16 PM
How much is the Wedgetail system going for?
Title: Re: 860 Kit
Post by: dogshome on May 28, 2021, 05:49:22 AM
RE Starter: The Bosch one was in good condition, brushes still got life and bearings with very little wear. The commutator was still level and unscored, but was no longer copper coloured, even on the wearing surfaces. It took a little cleaning, but more than a quick swipe or two i've found on cars. The unit is cleaned, greased and marked 'good' for future use. The Arrowhead one was short of grease, but otherwise looked well-put together and hopefully won't (by design, it should not) consume the massive current the Bosch did when it gets down to 0C and below here again. I have Motobatt and all new power wiring.

I may look at the coil feed wire as it does meander around the bike. I'll fit a proper distribution box when I do that as I already have auxiliary supply for fairing lights.

RE:Wedgetail. I expressed an interest via the website some time ago with no human reply. The facebook page also seems to be cold. Hopefully it's just a Covid glitch and there is still a plan to make these to buy. I hope they have not got snagged by some Directive or regulation that needs 100 items to be proof tested / EMC / colour of box compliance. Or worse, lack of interest. In the RC field, auto-advance ignitions are cheap and plentiful. 28 degrees is a common advance value for 10cc - 50cc engines..... The RPMs are very similar to our much bigger engines. 7,500RPM being a typical 'redline'. We run them at that on the ground with the prop chosen for that sort of RPM. They may speed up or load a bit when moving in the air, but not greatly. tickover is usually a bit higher at 1,500 to 2,000 RPM depending on skill at tuning/prop weight/tuned pipe.