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Old 10-06-2010, 12:10 PM   #16
Steptoe
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I haven't used gauges to balance an airhead for nearly twenty years. I used to balance two or three almost every day for a few years and you soon learn how to do it by ear and feel.

On my own airhead i keep a screwdriver and 10mm spanner permanantly in the pannier and usually end up adjusting it every week as i feel it fall slightly out while riding it. It only takes a minute And i'm always fiddling/tweeking with friends airhead carbs whenever i see them.
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Old 10-06-2010, 01:32 PM   #17
Rapid_Roy
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I thought backfiring also meant the carbs could be out of sync, but I may be wrong.
Ear tuner here.
Last time I rode my '84 RT I increased one throttle cable gap to match the gap size on the other side and it ran awesome.I knew it was out of sync.

I do everything at the beginning of summer, and by the end of summer, I am lazy.
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Old 10-06-2010, 01:47 PM   #18
supershaft
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Mixture screw settings greatly effect idle and just off idle. It also greatly effects off throttle cackling I call it whether or not the bike has EPA air pumps or not. All BMW's can backfire on deceleration via the mixture screw. Some of them IMO run slightly better when they do cackle. If you don't like it, just a tad bit richer than optimum will stop it. BMW spec turns are JUST a starting point. With every thing being near perfect, the optimum setting can be far from the recommended setting and the other carb can end up with a very different setting. Sure, different settings from side to side can be a sign of something not being right but not necessarily so. A lot of times air leaks from loose carb spigots, sucked in enricher body gaskets, or leaking valves cause idle issues. Float level also greatly effects idle as do a lot of other things.

I think setting the mixture screw half way in between the stumble points works best. I call that a neutral setting. One carb might have a full turn from stumble point to stumble point and one carb might have one half a turn. This is common. An equally bit richer from both carbs' neutral setting would be 1 hour out on the carb with a full turn and one half hour out on the carb with a half turn. Does that make sense? Personally, I turn the screws quickly to help identify the stumbling points.

Very often the stumbling point on the rich side (out) is hard to detect. Sometimes it seems like the mixture screw has little effect at all. It's usually a sign of leaking valves that can be accurately diagnosed with a leak down test. In these cases, I resort to the fastest idle setting or one half turn out from the lean stumbling point until something further can be done.

Ear versus manometer or a gadget that mimics a manometer? A manometer is the ears BEST teacher. Period. (Even though mixture screws are best adjusted by ear.) The problem with a lot of manometers is that they are not damped properly. Yes, the damping on a manometer is adjustable just like on a Twinmax via different size holed plugs in the manometer's vacuum lines. Read the instructions!
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Old 10-07-2010, 12:50 AM   #19
bikecat OP
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Thanks for the overwhelming response and inputs.

As a guideline, how much deviation will be reasonable from the 3/4 turns for the mixture settings? Just so that if I get to that point I know that either I'm doing wrong or that there is something else to look at.

So my checklist:

1. Ensure fuel level in carb bowls correct; valves, timing etc good.
2. Start with stock settings, adjust after full warm up
3. Adjust and balance idle via idle screws first, throttle cables slacken
4. Adjust and balance via mixture screw, in-between stumble points
5. Adjust and balance off-idle via throttle cable adjusters.

Will go meddle and see how it goes this weekend. :)
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Old 10-07-2010, 03:42 AM   #20
Beemerboff
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When you check the fuel level take one bowl off when the other side is in place and full - that way the fuel doesnt drain out of the hose as much and you get a more accurate reading.

I use a colortune to set the mixture screw - the best mixture seems just out from inwards stumble, but mid point as previously suggested will be better that just in from out.

The popping could be a air leak in the joints in the headers, some have an aluminium gasket which eventually vanishes down the pipe.

The simplest balance check is to raise each throttle cable 1/16 th of an inch - the change in revs should be the same each side.

If the mixture is correct there should be just the slightest odelay before the revs move up too.
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Old 10-08-2010, 11:15 AM   #21
supershaft
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Quote:
Originally Posted by Beemerboff
When you check the fuel level take one bowl off when the other side is in place and full - that way the fuel doesnt drain out of the hose as much and you get a more accurate reading.

I use a colortune to set the mixture screw - the best mixture seems just out from inwards stumble, but mid point as previously suggested will be better that just in from out.

The popping could be a air leak in the joints in the headers, some have an aluminium gasket which eventually vanishes down the pipe.

The simplest balance check is to raise each throttle cable 1/16 th of an inch - the change in revs should be the same each side.

If the mixture is correct there should be just the slightest odelay before the revs move up too.
Correct jetting is way leaner than most people think. In actuality correct jetting isn't lean at all and what most people think is correct is WAY too rich. Bikes run pretty good way too rich except for making less hp, less mileage, and more carbon. I jet my mains like you jet your idle: just rich of stumbling. You can gain numerous hp doing that compared to many stock jetting setups. Nevertheless, idle and to a lesser degree midrange most always needs to be jetted a bit richer than optimum. Why the sacrifice? It's in order to better deal with the HUGE differences in vacuum and venturi size from cracking the throttle open. That a bit rich sacrifice is mostly to get through that just off idle lean spot that results from the big vacuum drop from opening the throttle. Starting that massive transition a bit on the rich side helps get through the lean times. Nevertheless, there is no need to go over board. More than just enough is too much!
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Old 10-10-2010, 01:21 AM   #22
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Tinkered with the bike this weekend. The needles from motobins came and they were the ones for the 40mm Bings. Sigh, ...... cleaned up the old needles and re-installed them. Will go after new 32mm Bings' needles later.

Surprisingly with these needles and stock BMW settings there's back-firing and the bike was unable to hold any idle even when warm. Rpm was like 200.

Followed the wisdom provided, managed to tune the bike fairly decently, now I appreciate the "stumbling" points for the mixture tweaking. I did the off-idle adjustments for 2,500 rpm and will try Snowbum's recommended 1,300 and 1,500 next time around.

The only area that I need more info and practice on will be the throttle cable adjustments; kinda difficult to ensure that both will engage simultaneously with the twist of the throttle. However, as things are now, I am really a happy puppy. :) Bikes runs really well, pulls hard and now I have to monitor the plugs and fuel consumption to see what they say.

Thanks to all the assistance and wisdom provided, really appreciate this.

Cheers
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Old 10-10-2010, 03:17 PM   #23
supershaft
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are
Quote:
Originally Posted by bikecat
Tinkered with the bike this weekend. The needles from motobins came and they were the ones for the 40mm Bings. Sigh, ...... cleaned up the old needles and re-installed them. Will go after new 32mm Bings' needles later.

Surprisingly with these needles and stock BMW settings there's back-firing and the bike was unable to hold any idle even when warm. Rpm was like 200.

Followed the wisdom provided, managed to tune the bike fairly decently, now I appreciate the "stumbling" points for the mixture tweaking. I did the off-idle adjustments for 2,500 rpm and will try Snowbum's recommended 1,300 and 1,500 next time around.

The only area that I need more info and practice on will be the throttle cable adjustments; kinda difficult to ensure that both will engage simultaneously with the twist of the throttle. However, as things are now, I am really a happy puppy. :) Bikes runs really well, pulls hard and now I have to monitor the plugs and fuel consumption to see what they say.

Thanks to all the assistance and wisdom provided, really appreciate this.

Cheers
I hope I helped as well bikecat. I always suggest looking up your own PN's before ordering parts. It's easy on the idiotnet!

If your original jet needles were worn in the area that they do wear right above the start of the taper for about 10mm, you need new needle jets as well. The needle wears itself out wearing out the jet. Does that make sense?

Don't get me wrong, I look up info on Snowbum's and Duane's sites sometimes but I know how to weed through the misinformation. There are TONS of it on both sites! I do agree with Snowbum about syncing the cables right off idle. With a properly set up manometer, you can watch the cables move around a bit under the tension of opening up the throttles. Just after the cables settle in is where I sync them. Remember that tightening down the lock nuts changes everything. Sync accordingly.

IMO, chasing some "cruise" rpm and load for sync is impossible. The load (manifold pressure) changes everything and it is always varying greatly.

supershaft screwed with this post 10-10-2010 at 03:43 PM
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Old 11-09-2010, 04:03 PM   #24
igormortis
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Interesting reading. It really does look like everybody finds their own way to getting these old bikes to run nicely.

Has anyone welded O2 sensor bungs into the header pipes to monitor Air/Fuel ratios?

igormortis screwed with this post 11-09-2010 at 10:19 PM
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Old 11-09-2010, 08:51 PM   #25
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Old 11-10-2010, 08:43 AM   #26
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Hiya,

Just an update. A pushrods seal leak became a piston rings overhaul operation upon the discovery of a broken ring. So hopefully after the installation of the rings my mileage will improve.

As they say .... Things can only get better ....
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