You're right about jet numbers and sizing, David. We're based at 4,300 feet elevation, and most of my riding is between 4,000-7,000 feet elevation. A 175 main jet is for sea level. Typically, you would gradually taper down, proportionately, especially for altitudes above 5,000 foot range. Ambient temperature is also a critical factor, so pay close attention to both altitude and temperature!
I've attached an older Honda workshop recommendation for my 1984 Honda XR350R that should clarify the goals here. (The later manuals are remiss about much of this logic, apparently too busy appeasing exhaust emission regulatory agencies.) 1984 was still an era when getting a dirt motorcycle engine to run right and last a long time held priority. Disregard the XR350R model-specific info, you're interested in the chart and the explanation of how to use the chart and do the math, which is actually quite simple:
Honda Carburetor Jetting for Altitude.pdf 732.23KB
In the real world, I'm loading my bike with gear. Just added TCI Products racks and skid protection with Nelson-Rigg bags, check out the magazine article and HD videos. I can find myself riding at the lower desert around Moab and Southern California events. (The Moab Area can provide a range of 4,000-6500 feet, possibly more at the La Sal Range.) My XR650R carries added weight and will soon have a larger fuel tank, too. I also have the Hot Cams Stage 1 camshaft, which eliminates the auto-decompression start mechanism on the stock camshaft and has been an absolute joy in real world riding gains.
All of that said, I strive for a balance here, plus realism about not having to change jets every time I go somewhere. For the main jet, I opted for a 172. For the pilot, I intentionally installed the less efficient straight 68 (not a 68S). I get the flow rate but not the "better emulsified" fuel. The needle clip is 3rd position from top. The engine is faithfully and thoroughly "uncorked" to Honda 'Power-Up Kit' standards as well. I had to complete this upgrade, as the previous owner did not, and the engine was suffering prior to the top end rebuild. Your European market engine (stock) should have each of these modifications in its original form. We can confirm. Here is the Honda "factory" N.A. Power-Up Kit (not the all-out racing HRC kit) parts list:
Honda Factory Power Up Kit Part Numbers.txt 432bytes
Why did I start with these specs? My reasoning: I'm not going to change jets for Moab, Utah (4,025 feet in town) or King of the Hammers at Johnson Valley, CA (2,300-4,600 feet elevation range). The 172 is conservative, slightly lean from a 175, rich enough for running at sea level without risk of holing a piston. (Much more considerate than Honda's draconian 125 main jet in North American engines, detuning the engine just to run with this ridiculously lean jet!) So, the remaining question is higher altitudes, and here's my experience to date. Accept this as my experience and equipment.
The engine runs very strongly from idle to redline, without blubber, hesitation or anything other than a fast ride. (See my Michelin road test 'Part 2' video for living proof.) It pulls incredibly well, with excellent transitions from curb idle through midrange to redline. The spark plug color looks great, a very good indicator of a single cylinder motorcycle engine's combustion process and air/fuel ratio.
As for starting, first let me emphasize that I use the manual compression release lever. Here's my approach:
1) I bring the piston to compression pressure (leading to TDC), then pull in the release lever and push over TDC just slightly with the kick starter before attempting to kick down.
2) Over TDC slightly, I firmly kick through (not with extreme force other than to overcome the needs of a 10:1 compression engine). I can get the engine to start most often on the first or second kick, at worst, the third kick.
3) Here is the capper: I avoid use of heavy choking. If completely cold, I either place the choke on half-choke or full choke for just a few kicks with the ignition turned off and compression release lever pulled in.
5) The throttle stays closed until actual engine firing, then gets blipped slightly, immediately, to let the engine breath and keep running. If cold-blooded, the throttle is held slightly open to let the engine stabilize.
4) If I fire the engine with the choke on full, it must immediately be stepped down to middle position or even off (warm ambient temps now around 75 degrees F when I start the bike). For a hardcore, subfreezing Nevada winter at 4,300 feet, I anticipate playing up the choke enrichment.
5) As soon as the engine is running, I open the choke lever fully. If I happen to get overly zealous with the choke and create a hard start or stall upon starting, I turn off the ignition, pull in the compression release lever, hold the throttle well open, and kick through 4-5 times to clean out the cylinder. Then I start over again.
This starting thing for the XR650R is apparently an issue. Many follow elaborate regimens to get the engine to fire. Personally, I believe they must be over-fueling or over-choking the engine. Choke opened immediately, I can get my engine to idle stably within a 1/4- to 1/2-mile of riding this time of year.
I do exert caution when the engine is cold while riding with the choke open. The engine is lean, and I can feel it wanting slightly more fuel, which I provide with throttle instead of the choke.
Keep in mind that the main jet size has virtually no impact on idle. The pilot jet does, however, and it also affects the lower speed throttle transitions. Of course, the idle mixture screw circuit has the largest impact with the stock carburetor design. I must say that other than the idle speed when cold with the choke completely off, my engine responds wonderfully, both hot and cold.
I compensate for the open choke until the engine warms. This is just my approach, I don't like over-fueling the engine during its warm-up cycle. Some don't mind. The tip-off for me is that the bike can be immediately ridden with the choke off, without hesitating, faltering or stalling—as long as I work the throttle slightly when coming to a complete stop and idle.
You may need or benefit from a leaner main jet at your altitude and engine tune, but this can only be tested at higher rpm with the engine's performance under open throttle. Lower speeds, you're more concerned with the pilot, and a 65, which is stock for North America, apparently is all Moose can provide. Sounds like they're emission constrained and not into the "Honda Power Up" kit approach, which is readily available through dealerships as "off-highway" use as you discovered. This is Catch-22, since the N.A. bike was not authorized for highway use, according to California and EPA, yet you can jet for better off-highway performance! And Honda is the guideline we use here, its Power-Up Kit part numbers include the 175 main, 68(S) pilot and the change in needle and seat.
The needle and seat change is not as crucial as the needle position, which should be 3rd clip position (from the top) for starters. This is an easy road test, as the transitions from a closed to open throttle, including abrupt throttle changes, will indicate the correctness or shortfalls of the pilot jet size and needle position. We can talk about this when you get the bike on the road.
For me, I took an educated guess and accounted for the improved breathing of the Power-Up part numbers, including the exhaust tip which you can see in the Nelson-Rigg/TCI Products coverage, and my camshaft change. I have the non-California, round inside intake grommet between the carburetor and the cylinder head, too. This was stock, as the bike, fortunately, was original sold in Texas. I have none of the California emissions pump paraphernalia, which pops up in distinctions between the XR650A and XR650AC parts listings.
AC is a California designated engine that deserves great sympathy. When you have the time on one of your online searches for part numbers, check out the distinctions and schematics for the AC versus straight A models. Here are the primary pieces you should not find on your European XR650R (not OEM on my cycle, either, fortunately) that California buyers will be chasing down for years to come: http://www.hondapart...uction-valve-ac.
The other key AC pieces were the extreme exhaust tip restriction, narrowed passage intake grommet, restricted air box and jetting restrictions. Some of these items carry over to the "A" models as well. After the complete 'Power-Up Kit' upgrade, my engine is like yours should be, with the stock, non-California round type intake grommet, larger exhaust tip piping, unrestricted air box (yours might have the restrictors still) and proper jetting. You might share photos of your stock Euro equipment. N.A. members and guests would find that interesting.
I am very curious what your stock jetting turns out to be, including the main and pilot plus the needle clip position. A light rebuild is a wonderful treat for these carburetors, and judging by your idle mixture screw, perhaps you'll be lucky, and nobody damaged the rest of the carburetor. The idle screw is readily available, on my used model that was an item I replaced. The very fine tip needs extreme care when installing the screw. If off-center, that tip will bend in a wink. Do not force the screw into position, you'd be guaranteed trouble! When you remove the idle mixture screw, the tip size will be clear.
As for removing the damaged screw, there are several approaches here. Since you'll have the carburetor off anyway, you can carefully drill the center of the screw head with a smaller bit. If you have a drill motor than can be reversed, a left hand drill bit will often spin a screw loose while you're attempting to drill the hole. If not, use care not to damage the threaded portion of the carburetor body, and spend some time with a drill and tiny easy-out, removing the mixture screw once you can get the easy-out into the drilled hole.
Let's keep this discussion going. I'd like to know your findings, your conclusions and your experience with both the jetting and the starting technique. I don't believe that any properly tuned engine should be "hard to start". These engines are sensitive only due to one cylinder and lots of compression. Make friends with your engine's particular starting preferences. Mine work for me, I can start the engine cold or hot on one to three kicks maximum. That's actually better than the Baja racing versions of these engines.
My favorite cult classic XR650R movie, "Dust to Glory", shows how race positions actually change during a pit restart. Andy Grider has just outpaced Johnny Campbell in an epic battle between pits and hands over his bike to the next Honda B Team rider. The B Team rider can't get the XR650R to fire promptly, and the A Team rider sails by...Good tale about the XR650R and hard starting, but don't let this get you down. With proper tuning and conservative use of the choke, anyone can share my experience.
Note: As for starting thumper motorcycles, this XR650R is not my first rodeo. I teethed on a '69 BSA Victor 441 and owned three Honda XR enduro bikes prior to the XR650R. Granted, the XR650R is the beast of the bunch, yet starting the engine should be the least of the rider's challenges.
As for setting the idle screw, 1-1/2 to 1-3/4 turns from lightly seated is a nice place to start at your altitude. Give that a go, you'll make fine adjustment with the engine completely warmed. Again, I go for just enough enrichment for a very smooth and stable idle, nothing more or less. I've only touched the mixture screw one time since initial adjustment. Here is the factory procedure, your best guideline:
Honda XR650R Idle Mix Setting.pdf 230.03KB
Many owners like to fiddle endlessly with the idle speed screw, adjusting it up and down regularly. I stopped following this myth by concentrating on stabilizing the engine with the hand throttle on the extremely rare occasions when the engine seems hesitant to idle. Create stability by using your hand throttle, gently blipping (no major rpm changes, please, this is not a premix two-stroke!) or holding steady at a slightly higher rpm than curb idle for a moment—until the engine cleans out and temperatures stabilize. With steady air and fuel flow, and coolant temperature stability, the XR650R will want to idle smoothly again—even with its stock ignition and OE carburetor.
As you gain confidence in the engine, its tune and your skillful carburetor rebuild and adjustment, this will get easier. If you set the carburetor to factory recommendations in each area, including float height, you should have a much less finicky engine. Don't race off to buy a $700 replacement carburetor just yet!