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Old 05-19-2014, 10:44 AM   #1
Clabedan OP
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Question Odd behaviour of carb synchronizer... Vacuum leak?

Hi,
Just attempted to synchronise my carbs after cleaning them. its the first time I do this on a BMW but frequently did it on a 2 cylinder 750 kawasaki.

When performing a carb sync on the kawasaki both needles on the carb synchronizer have an erratic behaviour until you reduce the pressure by tightening the vacuum gauges and the needles start stabilizing.

On my 1982 r80/7, only the left needle has the erratic behaviour you would expect, the right carb needle hardly moves at all as if little pressure was going through it, even with the vaccuum gauges barely turned in. I have tested the right cylinder with both sides of the carb synchronizer, and the same thing happened, so the carb sync works fine.

Might not be anything to worry about but I was just wondering if that said that something is wrong on the right side (vacuum leak maybe??)
Christophe

Addendum
Just turned the mixture screw in as much as possible on the right carb and its still going whereas the left one stalls when i do the same
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Old 05-19-2014, 02:17 PM   #2
JimGregory
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Sounds like that carb needs going through. But you knew that.
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Old 05-19-2014, 02:19 PM   #3
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Sounds like that carb needs going through. But you knew that.
Just been thoroughly cleaned in ultrasonic bath...
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Old 05-19-2014, 02:31 PM   #4
disston
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It sounds like a vacuum leak or a bad O-ring or other internal leak and possibly a blocked carb jet or passage.

The O-rings seem to sometimes go bad for no reason at all although we usually blame the current Gasohol we have to use. The two O-rings I have had the most trouble with are on the idle jet and the idle mixture screw. The IMS is one of the two you adjust to make the balance.

These two items use the same O-ring. It is an internal 3mm O-ring. The one shown on Max BMW is a total of 7mm wide. This would make it an M3x2 O-ring. Cross section is 2mm to get the total of 4 needed to have the 3mm O-ring make a width of 7mm. Got it?

This O-ring is too large. I think most of the dealer O-rings have this 2mm cross section. If you use this item grease it or use some Silicone Lubricant when assembling the idle mix and idle jets. They will tear when installed or when removed.

On Snowbum's Tech pages he talks about this and says to use an M3x1.75 or 1.70. I couldn't find any O-rings that size but have found some that are M3x1.5. I test fitted one of these and it seems to fit well enough. I tested it on a spare carb. I have not tried to install any of these on my bike yet. (sorry too many other things need to be done right now)
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Old 05-22-2014, 08:38 AM   #5
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I have cleaned the carbs thoroughly, replaced all gaskets and o-rings convinced that I had a vacuum leak and even replaced the rubber intake... But to no avail.
The only o-ring I haven't replaced yet is the throttle shaft o-ring, i was told it was complicated and risky to perform (can anyone confirm??)

The right hand side still behaves strangely
It will only run OKish if I totally turn the mixture screw in
At one turn out it backfires
At 1 and a half turn out it stops backfiring and keeps on reving up over 2000 rev (idle screw not even being active)
At 2 and a half turns it starts slowing down again

According to the tuning method, I should really set it in between 1 and a half and 2 turns out but it revs far too high.
The ideal set up I have is with the screw turned in. The right cylinder still doesn't die. The bike runs ok up to between 2000 and 3000 revs when it starts rattling and ok again if I go full throttle.

And if i connect my carb synchronizer again, after replacing o-rings and gaskets, the right reading still behaves as before. The needle doesn't have the expected flutter (the left one does) which, in my humble opinion, indicates a lack of pressure in the right carb.

Any idea where I should look for?
Could the throttle shaft o-ring be responsible for this? (This is the one I haven't replaced yet)

I'm lost ;)
Christophe
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Old 05-22-2014, 09:06 AM   #6
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You tune mixture to the highest idle speed, that simple.

Since at that setting yours is running up tp 2K rpm, its not the mixture that is the issue.

I'd check that you have the proper free-play on the throttle cables and that the chokes are fully off (push down on the choke levers on the carbs to confirm).

Once you have the idle speed right, then balance to two sides.
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Old 05-22-2014, 09:09 AM   #7
disston
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The problem with replacing the throttle shaft O-ring has to do with removing and replacing the butterfly plate. The plate is held on the shaft by two tiny screws that were peened in place back when it was manufactured. To begin with if you want to save the shaft you have to drill the screws enough to get them out without too much damage to the threads in the shaft. If you think you are lucky because the screws merely came out they probably took the threads with them. You may still have to buy new shafts even if you are trying to be careful like I was. I ended up cutting the old parts out and using new shafts which was a big time saver and less hassle.

Look carefully at the butterfly plate. It should have a small dimple punched into it and this mark belongs on the outside or visible side and in the upwards half when the plate is installed and closed. There are 4 ways the butterfly plate can fit in the shaft. Only one way is correct. Another feature you should be aware of is that the plate is beveled. It's edges at the top and bottom are cut at an angle so the plate fits the carb bore exactly when it is closed which is still at an angle to the bore.

The screws fit the shaft closely but the plate loosely. You have to put the plate on and close it to align the plate with the bore then tighten the screws. It is real easy to get the plate mounted crooked. The screws need some insurance. Many mechanics peen them again as they were when the carbs were new. I use Loctite. The mechanics that do the peening again are mostly the ones considered smarter than me.
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Old 05-22-2014, 09:57 AM   #8
Clabedan OP
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Quote:
Originally Posted by SOLO LOBO View Post
You tune mixture to the highest idle speed, that simple.

Since at that setting yours is running up tp 2K rpm, its not the mixture that is the issue.

I'd check that you have the proper free-play on the throttle cables and that the chokes are fully off (push down on the choke levers on the carbs to confirm).

Once you have the idle speed right, then balance to two sides.
I had already checked both free play in the cables and choke.
At highest idle, i.e 2 turns out, the engine is way too high with the idle screw being below minimum level (not active)

I'm afraid the problem is somewhere else...
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Old 05-22-2014, 09:59 AM   #9
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Quote:
Originally Posted by disston View Post
The problem with replacing the throttle shaft O-ring has to do with removing and replacing the butterfly plate. The plate is held on the shaft by two tiny screws that were peened in place back when it was manufactured. To begin with if you want to save the shaft you have to drill the screws enough to get them out without too much damage to the threads in the shaft. If you think you are lucky because the screws merely came out they probably took the threads with them. You may still have to buy new shafts even if you are trying to be careful like I was. I ended up cutting the old parts out and using new shafts which was a big time saver and less hassle.

Look carefully at the butterfly plate. It should have a small dimple punched into it and this mark belongs on the outside or visible side and in the upwards half when the plate is installed and closed. There are 4 ways the butterfly plate can fit in the shaft. Only one way is correct. Another feature you should be aware of is that the plate is beveled. It's edges at the top and bottom are cut at an angle so the plate fits the carb bore exactly when it is closed which is still at an angle to the bore.

The screws fit the shaft closely but the plate loosely. You have to put the plate on and close it to align the plate with the bore then tighten the screws. It is real easy to get the plate mounted crooked. The screws need some insurance. Many mechanics peen them again as they were when the carbs were new. I use Loctite. The mechanics that do the peening again are mostly the ones considered smarter than me.

You confirmed i was right not to touch them ;)
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Old 05-22-2014, 10:50 AM   #10
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Quote:
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You confirmed i was right not to touch them ;)
Unless the shafts themselves are worn, which can cause the problems you mentioned...
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Old 05-22-2014, 10:20 PM   #11
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I suspect the butterfly is open too far, or won't close all the way. Is it centered in the bore?

It's showing no vacuum because it's running the strongest - it needs to slow down and the high vacuum side needs to speed up.
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Old 05-22-2014, 11:22 PM   #12
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I suspect the butterfly is open too far, or won't close all the way. Is it centered in the bore?

It's showing no vacuum because it's running the strongest - it needs to slow down and the high vacuum side needs to speed up.
Thanks wirespokes ! I never thought of this and this vacuum problem with the sync was doing my head in!! Still, it's idle screw is below minimum level and it's mixture screw is turned all the way in so i guess the only way is to get the left side to run stronger

Both butterflies are closed, I did a visual sync to start with.

I'm gonna check float ball levels to see if that does anything... i understand from previous threads that they are very sensitive to this...
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Old 05-23-2014, 11:27 AM   #13
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Changed float balls, they had been slightly twisted and were not lying flat on the floor (one ball was lower than the other)
Changed needles, one was actually bent and snapped as soon as I attempted to staighten it
Put original BMW greyish diaphragms, a lot softer than the black ones I had...

Things have improved dramatically eventhough not perfect yet
I have set mixture screws to one turn out. Now if I screw it all the way in, engine dies which is a good sign !

The bike is still stuttering at low revs and Ok over 2000 / 2500 revs...
Any idea what may cause this? Starved for petrol at low revs but not at higher revs?

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Old 05-23-2014, 12:46 PM   #14
disston
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Stuttering at revs below 2500 rpm doesn't sound bad at all. My bike runs like that. I keep the revs up if I'm trying to move it.

Look at the plugs. A little brown is good but most plugs come out a lot whiter these days. I think it's because of Gasohol. Do you get Gasohol where you are?

BTW, your fix sounds terrific. Nice when parts show an improvement. You changed the needles? Was that the jet needles or the float needles? If you have change the jet needles then the needle jets may need a refreshing too.
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Old 05-23-2014, 12:54 PM   #15
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Quote:
Originally Posted by Clabedan View Post
Changed float balls, they had been slightly twisted and were not lying flat on the floor (one ball was lower than the other)
Changed needles, one was actually bent and snapped as soon as I attempted to staighten it
Put original BMW greyish diaphragms, a lot softer than the black ones I had...

Things have improved dramatically eventhough not perfect yet
I have set mixture screws to one turn out. Now if I screw it all the way in, engine dies which is a good sign !

The bike is still stuttering at low revs and Ok over 2000 / 2500 revs...
Any idea what may cause this? Starved for petrol at low revs but not at higher revs?
I would replace not just the needles but the jets they ride in. Mine were worn to about twice the original size. Make damn sure the starting jet down in the tube in the bowl is perfect. Make sure the tube itself is perfectly clear. Make sure everything is exactly to spec. I know you said the carb was clean but something is wrong.
Also make sure the timing is right and the spark is proper.
If you buy every jet new it will only be a few bucks. Well worth the peace of mind.
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