Greetings
I've been having a devil of a time getting my newly acquired 1999 BMW "Funduro" to run properly and I'm hoping someone can help me out.
The bike was not well-maintained, but I have done my best to clean it up and replace neglected parts.
In its current state, the bike will not run properly below 4000 RPM, so we can rule out any regulator issues.
It will start fairly easily enough (even when cold) but the idle is erratic. If I twist the throttle VERY SLOWLY, the engine speed will increase until about 2000 RPM's whereon it will stop increasing, or may even decrease a bit. As I continue to twist, the engine speed will eventually increase beyond 2000 RPM but there is another point around 3000 - 3500 rpm where it will become erratic again. Once it gets above 4000 RPM, the engine runs well and pulls strong.
When I bought it, the PO claimed that the choke cable was "sticky"...but in fact, it had broken off at the plastic hub and the choke plunger was actually seized inside its bore. Once I got that repaired, I also discovered that the screwdriver slot one of the pilot jets had been stripped, so I had to use a screw extractor to get that one out for cleaning. Since I didn't want to put a broken jet back in, I opted to replace them both but since they are an odd size (#41.3) I went to the next size up which is a #42.5
Float level is set as per the manual.
Mixture screws set to 3½ turns as per the manual.
Main jets are stock #132.5
Mileage is terrible...about 25 miles per gallon (about 10L/100km)
I have been across various forums and Facebook groups trying to solve these issues...to no avail.
One member mentioned that his mileage was bad until he replaced his spark plug boots, so I thought that I might be having an ignition issue, and I replaced ONE of my spark plug boots (I only had one spare).
The other day, I disconnected one of my ignition coils (so that only ONE spark plug would be firing) and started the engine. It ran roughly and needed choke to stay running - this was the "new" spark plug boot.
I repeated the test with the other spark plug (original) and it ran better. So I measured the resistance of the NEW boot and found it to be over 20K ohms...that's too much. The original boot was 13K ohms (closer to spec). I ended up replacing the NEW boot with its old counterpart, but noticed that its resistance was over 4 MEG ohms, so I had to clean it up to get it back to within spec. To make a long story short, I've ordered two NEW plug boots and they should be here within a week or two.
With the two original spark plug boots back on the bike, I was able to start it but it still ran quite rough, so I turned IN the mixture screw on the LEFT side carb, since I can't adjust the RIGHT side when the engine is running.
The further I turned it IN, the better the engine ran...until I reached the bottom of its travel. The LEFT side mixture screw is fully closed and the engine runs "better" than when it is set to factory recommendations. Right side is set to 3½ turns as per the manual.
This is what I find the most perplexing because the ONLY parts which are not OEM are the two pilot jets and I can't see how going up only ONE size would have such a devastating effect on overall operation.

I have two carb kits coming from Holland (hpefully, they will get here this month) which includes new pilot jets (supposedly the proper size), new needles and emulsion tubes, new float valve and seats, and new slide guides. If this doesn't get it working, then I am really at a loss as to how to proceed.
In summary:
Factory recommended settings make it run worse.
Only non-OEM parts are the pilot jets.
Floats are properly set and are NOT water-logged, each one weighs 7g.
Fresh fuel.
Fuel flow is good and strong...no blockages anywhere.
Engine speed will hang at 2000 RPM and dropoff, when gently accelerated.
Engine runs well above 4000 RPM but mileage is terrible.
No tears, holes or leaks in carb manifolds when installed.
Air filter is clean.
Valve timing has been checked and valve clearance is good.
SPark plugs are new, although the boots are suspect and will be replaced.

Wow, that's been quite the odyssey!
I know NOTHING about the F650, other than they're well regarded and generally very reliable, only issue I know of is water pumps fail.

I think you're on the right track, but would be checking coils and general ignition wiring while you're waiting for carb bits. I've gone down the wrong rabbit hole myself more than once.
Plug caps are only the (second) last thing in the electrical chain... Does the spark look FAT and GOOD? How's the starting - is the battery good?
Mixture screw - improving until it can't go in any further - hmmm, suspicious! These might be very susceptible to pilot jetting changes. Put the damaged one back in, just not tight so you can be sure to get it back out easily. OR just put the one good one you have in.

Does one carb do low speed only and the other cuts in at higher speeds? (Like I said I know nothing of these things) IF that's the case make sure the low speed one is 100% as best you can. It should then run clean to half throttle-ish. If they both work together all the way, scrap that idea!
Fascinating issue, I look forward to hearing the result.
And nice to see someone else from the 'Wack here, welcome!

__________________
Grant Johnson
Seek, and ye shall find.
------------------------
Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

What's the "Wack"?

These carbs (there are two) are neither staged nor synch'd...they both operate on the same shaft. Neither the manual, nor any online resources I've found (and I've dug through many) make any mention of synchronizing the carbs. There are two intake valves and it appears that each carb feeds a valve.

As far as I can tell, the coils are within spec and pass visual inspection in terms of cracks or arcing. But the spark plug boots are a known weak-link; the fact that one measured 4+ Megohms and could be brought to within spec with a light cleaning, attests to that.
I've thought about replacing the broken pilot jet, but I fear that I may ave damaged it during the extraction...I have it downstairs and will inspect it later...I only need to worry about the actual HOLE size, yes? It's a non-bleed type Mikuni jet as show here: https://mikunioz.com/shop/n151-067-p...v=13b249c5dfa9
But this is the perplexing part because many other models out there can withstand an increase (or decrease) in pilot jet sizes, in order to aid cold-starting, or improve mileage without creating problems in overall running...but I suppose this model could be sensitive to changes. As I said: This is the ONLY change made from the stock setup...so it is plausible.

In its own way, it IS a fascinating issue but all things being equal, I'd sooner be riding...and considering that I sold a perfectly functional, reliable and solid bike to get this one, is particularly upsetting.

Old time ChilliWACKers slang - not that I'm old time, though I do remember riding through in the '60's


Okay, good to know and no surprise with that setup. If there's no sign of any adjustment method, good enough. It's not like trying to synchronize two cylinders, which really does matter.
Note that some people have dumped the two carbs and gone to a single one! I wouldn't do it on a well running bike, but if the carbs are toast and the price is prohibitive to replace, it's a possible option.


And you've cranked it over with the plugs out and watched the spark? NOTE: Before you do that - some bikes - certain old BMW's IIRC - REALLY don't like that, electronics issues, so check if it's ok to do.


With that type, YES. As long a it's in there just very slightly snug it will work fine - it may not stay there long term, but useful for testing. You could even just chop off all the damaged part and saw a new slot in what's left - taking care on inserting and removing not to damage the carb of course!.


Plausible - and a starting point. I also strongly question the 3 1/2 turns out from fully in - very lightly - for the standard setting for the mixture screws - NORMAL is 1 1/2 to at most 2 turns out. I've NEVER heard of over 2. Doesn't mean it's not correct, but I question it.


Bummer! And I get it - my fun bike is in the throes of a massive rewire right now so I'm spending way too much time in the garage and none on the road. Well I confess garage time is fun too, but not when the sun is shining and the temp is perfect!

__________________
Grant Johnson
Seek, and ye shall find.
------------------------
Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

I believe I've read something about this is the manual...not a good idea!
Considering that the plug caps are known failure-points and are slated for replacement, I'll consider this as a MUST DO before going forward.



I've thought about using a Dremel to cut a new screw-driver slot...



I feel the same...3½ would seem to be too far out, but that's what the manual says!

A few thoughts I have:
3.5 turns seems a lot to me too but it's not critical , and if that's what the manuals say... It affects the tickover/low revs and your bad running implies deeper problems.
If it's running rich no surprise there's improvement with a mixture screw fully in.

Have you checked the diaphragms? Any hole/crack will affect pick-up. And the seal on the diaphragm cover.


Have you checked the inlet manifold(s) for cracks?
If the bike is running, spray some brake/carb cleaner in the area and see if the revs drop. If so there's an air-leak. Not necessarily the manifold of course. Maybe the choke bore?

Is there a little filter on the fuel input pipe? (like a DR)

Has a prevous owner been tinkering? Maybe an o-ring is missing, jets are wrong, needle is too low.
Worth checking a schematic to verify.

I expect a refurb kit will be a great improvement and a reassuring baseline. Apply lots of carb cleaner and let it soak. Then compressed air if you have it available.
Hopefully you'll be able to rule out the plug connectors soon.


[Edit] Maybe some heat from a soldering iron to help move the broken jet.

I'll just add something which is *probably* irrelevant:

A while ago I had a problem with my KLR which everyone, including me and every amateur or pro mechanic I consulted, believed was a fuel or ignition problem. Lots of carb/fuel line/filter/electric/electronic adjustments followed. Then one day in Texas, on a test run after extensive work by a dealership, I heard a "clunk" which seemed to come from beneath and behind me. It turned out the entire thing was caused by a loose muffler baffle--fixed by replacing the whole thing under warranty.

So: just a reminder that sometimes we get over-focused on one partial piece, forgetting that bike mechanics are a system and may benefit from systems thinking. Probably not what you're experiencing, but maybe....

Lots of good ideas there! A note on diaphragms - the hole can be TINY - stretch it and hold it up to the light. Easily missed.

I've also become a fan of ultrasonic cleaners, bought one a year ago, absolutely fantastic for small stuff like carburetors. Stuff comes out looking like new. A really big one would be nice but just can't justify it.

__________________
Grant Johnson
Seek, and ye shall find.
------------------------
Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

Forgot to add that to the list...diaphragms are whole and in good shape, still very pliable.

Repeatedly....

Interesting...I'll give that one a look-see.

There WAS...but since it kept getting kind of squished in there, I removed it as it presented a choke-point for the fuel flow.

I doubt it...by the looks of it, this poor bike never got anything more than basic maintenance, ie: oil changes.

Most pieces that should be there are present...jets are stock, so is needle-clip height.

I sure hope these "carb kits" are a bit more substantial than the ones I bought from Amazon...those were horrible; all parts had some sort of gold plating that was sure to flake off eventually...needle only had ONE clip position...main jet wouldn't thread into emulsion tube...just junk!
I bought these from Dellorto in the Netherlands and they claim their kits are made in Japan, so here's hoping I get some quality parts!

Just received confirmation that the spark plug boots are en route and should be here by July 7th...Canada Post is still sitting on my carb kits, so who knows when they'll arrive.
We are set to take off for another couple of weeks holidays on the 6th, so either way I'll miss both packages...they'll have to wait until we get back.
In the meantime, I've got an RV to prep so this "project" will be taking a back seat for a while.

I've been dealing with this for 4 months...I need a break from this headache.

(But still interested in any advice/opinions anyone is willing to share)...

Burn it!

Petrol + matches will save you polishing this Aprillia made @#$% before the water pump, head gasket, reg-rec, undersized chain, corrosion etc. etc. get chance to ruin your day further.

Assuming you enjoy pain, has https://f650.com/ anything useful to say in their FAQ'S ?

Used (I went for the petrol and matches solution after a little stroll in the desert courtesy of the chocolate teapot water pump design in 2003) to be the best resource.

Andy

Funny thing about that F650 forum...if I start a thread, my posts get truncated at about 150 words. This has happened more than once and it has aggravated me to no end.
...and I don't get a lot of replies on that forum either.

Believe me, if I had insured it for fire and theft, it would have been "stolen" and left burning in a ditch a LOOOOONG time ago!

Canada Post sent me a notice that they're holding something at their office...it's probably my carb kits, but since my spark plug boots aren't scheduled to be here until next Friday (when we're gone), I'm not in too much of a rush to pick it up. The Canada Day long weekend means that I'll probably have to wait until Tuesday, anyway...
So just to streamline things a bit, I thought I'd pull the carbs tonight and get them ready for my makeshift ultra-sound cleaner (bucket with orbital sander attached) tomorrow.
Removed the carbs from the bike (I'm getting REALLY good at this) and started prepping them for the bath.
I removed the choke plunger and for some reason, it appealed to me to compare it to the old one. As you may recall, the original was stuck in its bore by way of a "sticky choke cable".
This is when I noticed that the one I've been using does NOT match the stock part. It is about 3 mm shorter and the stud in the middle is shorter as well.
So could the "new" one have been the problem?
I'm thinking that if the plunger is not long enough to close off the port, it would allow an overly-rich mixture through all the time. This could account for the lousy mileage AND the rough running...and why the engine would run better when the mixture screw is closed off completely, yet run like crap in the off-idle position.
And if the stud isn't long enough to bottom out, or to do whatever it does, this could also account for some issues I'm experiencing, right?

Well the kits are here, so I'll install them when we get back...a carb rebuild never hurt.
And I'm still waiting for the plug boots...replacing a known weak-link is always a good idea.
So I guess I lied...the idle jets are NOT the only non-stock items in the carb...the choke plunger is too!

But could this have been the problem all along?
Hoping that it is...wouldn't that be wonderful, or am I dreaming?

Here are some pics for comparison. The STOCK plunger is the top one...the shorter bottom one is what I've been using.

Interesting! Always worth doing a careful comparison when installing new parts, even if it's a "genuine" part things may change.
It might be perfectly fine - or not - you'll need to inspect / measure the hole it goes into to determine that, but it's worth putting the old one back in and see what happens. Good luck!

__________________
Grant Johnson
Seek, and ye shall find.
------------------------
Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

So I tested my theory this morning by placing each of the plungers into the bore and it doesn't appear as though either of them block or unblock any port or passageway to a greater or lesser extent.
The only possibility would be that the shorter one might not seal the port completely, but the amount of travel in the plunger cable suggests that there is more than enough.

Back to the drawing board...