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Old 09-29-2013, 04:44 PM   #1
sandwich OP
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Another Carburetor Question

I just finished a frame up restoration of a 91 GS. It needed a motor and I put in a 1986 R80 engine and fitted it out with older rebuilt 32mm Bings. Not sure how old but they don't have the spring actuated choke returns. It was a California engine and I decommissioned the emissions plumbing. The crankcase breathers are still intact. The ignition system is new. The exhaust system is a new Keihan set up without the mixing box. I've got it to idle reasonably well - although it surges a bit and tends to idle fast when hot. The engine itself is a new rebuild with rings, seats, etc. The right cylinder is running cooler than the left and is running a bit rich. It's also vibrating quite a bit off idle and in general. I've synced it twice at low rpm but it it hasn't sorted out. If anyone can tell me the diagnostics steps I'd really appreciate it. I'd also love to know the best baseline set up is for the carbs. Is the 45/268/130 likely to be the best set up? I know this exists in a few past threads but I'd like to get as close as possible with this particular engine etc.. Thanks so much.
Jim
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Old 09-29-2013, 05:19 PM   #2
disston
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130 for the main jet seems too lean. I'd start with changing this to 145 and maybe 150. 2.68 for the needle jet should then be OK and 45 for the idles seems OK too.

Try clip position 2 first. if bogs on partial throttle try position 3.

The idea with balancing the carbs is to set the mixtures the same # of turns out to warm the bike and then take for a ride. After 10 mins ride the mixtures can be fine tuned. In till stumble then out till stumble, set half way in between this. Try to do one side then the other.

After you are satisfied with mixture you need some way to short the plug wires out so one side or the other can be turned off. Make a pair of tools from some long screws. The size of the screw is 4mm x .7. I've heard that a threaded rod size 1/8 may also work. But the screws can be found in better stocked hardware stores. Also get a pair of matching nuts. You are going to need the threaded ferule ends that originally came on the spark plugs, but we don't use on our bikes. The threaded ferule piece is longer than a standard size nut and will thread onto the end of the screw. It is held in place by one of the nuts. The head of the screw needs to be cut off so now you have a tool that will be threaded onto the end of the spark plug and the plug wire will attach to.

I'm going to try and post a picture of this later.

With the threaded rods attached to the plugs and the wires on top the ignition for one side can be shorted to ground with out causing harm to the ignition system. When one side is shorted out note the rpm held by the other side. Then short out the other side and note the rpm held on the other side. The two sides are balanced by adjusting the idle speed screws.

You will have trouble doing all this in a timely fashion at first. With the engine idling and not moving it can over heat. The ideal solution is to use a large box fan to cool the engine by blowing air from the front, like it would be if the bike was moving.

The whole process has to be repeated several times. And the end result is hopefully an even and proper idle.

Next we will cover off idle or above idle balance which also has to be set. It involves adjusting the cables so the carbs open simultaneously.
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Old 09-29-2013, 05:31 PM   #3
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Quote:
Originally Posted by sandwich View Post
I just finished a frame up restoration of a 91 GS. It needed a motor and I put in a 1986 R80 engine and fitted it out with older rebuilt 32mm Bings. Not sure how old but they don't have the spring actuated choke returns. It was a California engine and I decommissioned the emissions plumbing. The crankcase breathers are still intact. The ignition system is new. The exhaust system is a new Keihan set up without the mixing box. I've got it to idle reasonably well - although it surges a bit and tends to idle fast when hot. The engine itself is a new rebuild with rings, seats, etc. The right cylinder is running cooler than the left and is running a bit rich. It's also vibrating quite a bit off idle and in general. I've synced it twice at low rpm but it it hasn't sorted out. If anyone can tell me the diagnostics steps I'd really appreciate it. I'd also love to know the best baseline set up is for the carbs. Is the 45/268/130 likely to be the best set up? I know this exists in a few past threads but I'd like to get as close as possible with this particular engine etc.. Thanks so much.
Jim

Assuming you have avoided any air leaks in setting up the exhaust and mounting the carbs - check for air leaks first - you need to take another look at the carbs.

Surging and idling fast when hot - you say you have tried to synch the carbs - would suggest that you either have not correctly adjusted the carbs or you have some internal carb issues.

Best solution is always to start from the beginning.

Take off the carbs and check the various jets, needles etc and make sure they match your engine's needs. Clymers, Haynes etc.

If everything matches - clean and blow out the low speed jets and passages with air or aerosol carb cleaner. Check all o rings. Clean and blow through all passages and jets.

Check to make sure that the choke pieces are not reversed and that the choke gasket is intact and not sucked inwards. Check the diaphragm for leaks tears etc. Check float height.

Once you are sure everything is as it should be try once again to synch the carbs.

I assume you have looked this up but for your 86 R80 engine the carb set up is:
main jet: 135
needle jet: 266
jet needle: 46-251
needle clip: 3rd
idle jet: 45
air screw out: 3/4 turn

Good luck
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Old 09-29-2013, 05:48 PM   #4
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Quote:
Originally Posted by kaput13 View Post
Assuming you have avoided any air leaks in setting up the exhaust and mounting the carbs - check for air leaks first - you need to take another look at the carbs.

Surging and idling fast when hot - you say you have tried to synch the carbs - would suggest that you either have not correctly adjusted the carbs or you have some internal carb issues.

Best solution is always to start from the beginning.

Take off the carbs and check the various jets, needles etc and make sure they match your engine's needs. Clymers, Haynes etc.

If everything matches - clean and blow out the low speed jets and passages with air or aerosol carb cleaner. Check all o rings. Clean and blow through all passages and jets.

Check to make sure that the choke pieces are not reversed and that the choke gasket is intact and not sucked inwards. Check the diaphragm for leaks tears etc. Check float height.

Once you are sure everything is as it should be try once again to synch the carbs.

I assume you have looked this up but for your 86 R80 engine the carb set up is:
main jet: 135
needle jet: 266
jet needle: 46-251
needle clip: 3rd
idle jet: 45
air screw out: 3/4 turn

Good luck
Pretty much what he said.

130 mains very well might be perfect. The stock 135's are usually close. SO many people richen up the mains when that is the last place to richen them up. Most setups that came with 266 needle jets run better with 268's. They very often will rev quicker a bit richer there. Not always but don't blindly richen the mains. Richer there than absolutely necessary just slows them down AND waists fuel AND carbons them up which leads to pinging and people jetting them richer yet again. It's a long, slow road to getting slower yet. The later setups almost always run best with the needle jet on the third clip. Leave it there and adjust any off idle jetting with the needle jets, NOT the jet needles. All that being said and not wanting to sound too much like Plaka, it sounds like you need someone that knows what they are doing and talking about as far as setting up your carbs AND adjusting them. Especially the adjusting part. It is the toughest thing to learn. Don't feel bad. I have worked with long time pro's that were pretty much clueless. Especially adjusting the mixture screws. Anyway, hopefully someone knowledgeable can help.

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Old 09-29-2013, 05:49 PM   #5
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Thanks so much. There are no leaks at the spigots. I used vacuum gauges to sync. Is it really necessary to use the shorting method? I'll tear down the carbs and check the internals.
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Old 09-29-2013, 05:59 PM   #6
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Originally Posted by sandwich View Post
Thanks so much. There are no leaks at the spigots. I used vacuum gauges to sync. Is it really necessary to use the shorting method? I'll tear down the carbs and check the internals.
I would not use the shorting method. Vacuum gauges? I wouldn't use them either. Use a PROPERLY DAMPED manometer and let it teach more than just your eyes. Soon enough you will be tuning by ear and what other senses you haven't turned off. The Manometer is the best teacher. It's literally Mother Nature herself showing you what is really happening WHILE it is really happening.
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Old 09-29-2013, 06:03 PM   #7
disston
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I bow to SS as far as which jets to use but if I was doing it even if I tried to follow his directions I would probably end up on the rich side anyway. Old habits die hard.

The shorting method works. You need some method to balance the carbs. Any of the many tools such as Twin Max or Harmonizer or manometers work by telling you which carb is pulling more. The shorting method does this also.

I use a combination. Sometimes the shorting method and sometimes the Harmonizer.



This is what they look like.

If you have vacuum gauges and you are more comfortable with those they should work fine.
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Old 09-29-2013, 06:18 PM   #8
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There are too many good reasons why airboxes, carbs, manifolds, ports, and exhaust are designed and then very often redesigned on flow benches using manometers. How do you think the cylinders on a V8 are balanced? The answer is flow benches and manometers. That is when the heads and manifolds are designed. IMO it's foolish to think something else is better to adjust your carbs with. Manometers are the perfect solution. No electricity. Nothing mechaical. Just Mother Nature. Their only problem is that most people do not know to adjust their damping and that is easy and straight forward enough. Store bought manometers come with dampers and so many people never install them? That I can't figure?
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Old 09-29-2013, 06:39 PM   #9
Plaka
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Quote:
Originally Posted by sandwich View Post
I just finished a frame up restoration of a 91 GS. It needed a motor and I put in a 1986 R80 engine and fitted it out with older rebuilt 32mm Bings. Not sure how old but they don't have the spring actuated choke returns. It was a California engine and I decommissioned the emissions plumbing. The crankcase breathers are still intact. The ignition system is new. The exhaust system is a new Keihan set up without the mixing box. I've got it to idle reasonably well - although it surges a bit and tends to idle fast when hot. The engine itself is a new rebuild with rings, seats, etc. The right cylinder is running cooler than the left and is running a bit rich. It's also vibrating quite a bit off idle and in general. I've synced it twice at low rpm but it it hasn't sorted out. If anyone can tell me the diagnostics steps I'd really appreciate it. I'd also love to know the best baseline set up is for the carbs. Is the 45/268/130 likely to be the best set up? I know this exists in a few past threads but I'd like to get as close as possible with this particular engine etc.. Thanks so much.
Jim
That carb on that engine is jetted 135 mains, 45 pilot, 266 needles. You're careful not to say where you live so I can't comment on altitude corrections. But this can be wise too. When I find out where someone on the internet lives, I sneak over to their house in the middle of the night, lean a couple milk bottles of well aged boar urine against the front door, ring the door bell and run.

power balancing the carbs will give you a smoother running engine. Vacuum balancing will work well enough. But you still need to do it correctly and in the correct order. many people are very happy with that method. But like many very happy people, they don't know anything else. And Ambroce Beirces take on the fox and the grapes holds doubly true.
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Old 09-29-2013, 06:40 PM   #10
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Originally Posted by disston View Post

Next we will cover off idle or above idle balance which also has to be set. It involves adjusting the cables so the carbs open simultaneously.

No.
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Old 09-29-2013, 08:25 PM   #11
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Old 09-29-2013, 08:56 PM   #12
supershaft
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Power balancing? That cracks me up. Power balancing is what syncing the carbs with both cylinders running is. The shorting method is syncing your engine while it is dragging half of it behind for being dead. Most likely while the cylinder that is alive and doing all that extra work is still recovering from just coming back alive after having been killed itself. It makes no sense when you can tune a completely live engine while it is doing what it really does while it is all there running as if it really runs like that because that IS how it runs.
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Old 09-30-2013, 04:53 AM   #13
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I don't doubt that mercury sticks are more stable than gauges but aren't both doing pretty much the same thing? Aren't both measuring the vacuum at the carburetor? I used to have a car with dual Webers and used a air stick on that. Again - same thing but even harder to use. I also have a R100/7 that's highly tuned and has Dells on it - had vaccum ports installed in the spigots and it syncs reasonably well with the vacuum gauges. (Although not perfectly)
Is the problem with my GS the secondary circuit? The idle is not bad and the high speed running is not bad either. BTW short of making one yourself it looks like Motion Pro offers the only analog fluid stick contraption out there. Anyone used it?
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Old 09-30-2013, 05:08 AM   #14
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In a perfect world vacuum balancing would result in perfect balance but then there is the theory that vacuum doesn't tell the whole story and one cylinder for various reasons could still be pulling stronger than another.

What bothers me with shorting is if I first do a perfect vacuum balance at idle and above and then a shorting test results in one cylinder idling faster than the other what's that telling me ?

Sure it's telling me one cylinder is pulling stronger than the other but it isn't telling me why. I'm reluctant to believe that adjusting throttle stops is the appropriate corrective action.

Vacuum balancing may not be perfect but for me it's closer to perfect than my ability to get the mixture identical on both sides purely by ear so I'm more easily persuaded that one cylinder might be running weaker than the other and I adjust the mixture screw instead.
I've got to admit that if you set the idle excessively rich and many do then the above logic may not apply.

Back in the ideal world I want a vacuum tool that could stay attached while I ride around with the engine under load and observe the balance.
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Old 09-30-2013, 06:00 AM   #15
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In a perfect world vacuum balancing would result in perfect balance but then there is the theory that vacuum doesn't tell the whole story and one cylinder for various reasons could still be pulling stronger than another.

What bothers me with shorting is if I first do a perfect vacuum balance at idle and above and then a shorting test results in one cylinder idling faster than the other what's that telling me ?

Sure it's telling me one cylinder is pulling stronger than the other but it isn't telling me why. I'm reluctant to believe that adjusting throttle stops is the appropriate corrective action.

Vacuum balancing may not be perfect but for me it's closer to perfect than my ability to get the mixture identical on both sides purely by ear so I'm more easily persuaded that one cylinder might be running weaker than the other and I adjust the mixture screw instead.
I've got to admit that if you set the idle excessively rich and many do then the above logic may not apply.

Back in the ideal world I want a vacuum tool that could stay attached while I ride around with the engine under load and observe the balance.
Just do a compression check. Are both sides the same? Unless you have some very fancy head work done, the combustion chambers are not the same size. And then there is variation in carbon build up, ring sealing, valve stem leakage, intake and exhaust tract tuning, carb wear, etc.

You can set the idle mixture by ear because it doesn't matter much. You nail the idle balance with the throttle stops. And that isn't even real critical because at low speeds the rocking couple between the cylinders is shaking things so much smoothness isn't much of an issue.

The balance at cruising speed is another matter. Remember the compression difference. You can have exactly the same vacuum, that is exactly the same air velocity in the venturi, but one cylinder isn't filling as well. it doesn't have time***. So you give that side more velocity/vacuum to get it stuffed. The vacuum balance isn't equal, but the power output is. All the little factors also become more of an issue. Just how accurate are your valve settings? Are both plugs sparking equally or is one more worn/crudded? And so on.

The vacuum is a correlate of the power output, but a poor one. The world, and the engine, are not perfect. So looking at a pair of gauges isn't really telling you want you want to know. That isn't hard to do BTW, you need snubbers to even out the pulses so the needle is steady. The Walus differential meter had them. Simply feeling the engine vibration will do it. So long as you aren't getting a lot of vibration from unbalanced wheels (happens as the tires wear) you can feel it when the engine is right. You can also feel the sweet spot you tuned in with power balancing. it will be silky right there, but rougher elsewhere.


You can set your idle richer, and I've been playing with this, because it affects mixture in the low part of the throttle range---as observed at the handle bar. I'm finding that for most of my driving I'm in that low part of the range. So I have been paying more attention to the idle mix. I can diddle the screw a fair amount and get no change in what I hear the engine doing at idle, nor can I see it on the tach. So I can go a bit richer or leaner and the idle is the same. But I get more or less fuel in that whole low part of the range where the idle circuit is overlapping the needles.

Hot weather tuning is not the same as cold weather tuning. The air density is different. Fuel injection systems account for this with a mass air flow sensor. They can adjust for a low pressure front moving in! I have to do it manually using the tables in the Bing manual. I also have to adjust for altitude and re-adjust on tour when I go to a lower altitude. I'm on the eastern edge of a 5000 ft plateau. I go one state east and I have to retune. I can drive west to the California border before I have to retune that way. Plate tectonics are a bitch.

I'm not getting knocking despite the compression and the mileage is very good for high altitude. So I figure I'm lean enough. Now it's a matter of seeing what's happening with engine temps. I may richen up, and give away some mileage, to cool it down.
-------------

**Edit: I think this is right but I need to check it through with my brother. I set up the equations and ran into needing a whole lot of calculus in a hurry. Not my forte. Bro's one of the mathematicians in the family. (Though the last time I posed him a problem he bombed on it. But it may have been unsolvable. nasty bit of statistics. Still, I shouldn't have been able to spot his solution wouldn't work.)

Plaka screwed with this post 09-30-2013 at 06:28 AM
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