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TOPIC: BS34 idles down slowly

BS34 idles down slowly 2 years 9 months ago #505353

  • baldy110
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I am now finally tuning my ELR replica and ran into a problem today. I have a GPZ1100 engine with BS34 carbs. The engine has a big bore kit, stock cams, 4-1 Kerker, jet kit and Pod filters. I warmed the bike up, synced and set the idle mixture screws. It idles good and overall runs good. I noticed the carbs hang up when trying to idle down. When I blip the throttle it will not come down to where it started. If I load up the engine the idle comes right down, if I let it idle down on it's own it will take about 20 seconds. There are no vacum leaks, I am running new manifolds and I found no leaks. It is not the cable or linkage, removed the cable and it still does it. When I adjust the idle screw down to get the idle to lower it does nothing then all of a sudden idle down, same when I adjust he screw to idle up nothing then reaches a point and then suddenly idles up. It will not get out of control it will just idle about 500 rpms faster than it should be. Very weird. Any idea would help, any ides would help, I have run out of them.
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BS34 idles down slowly 2 years 9 months ago #505392

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If not already done, would assure the timing advancer is properly functioning, and especially that it's not sticking slightly before fully returning to idle position.

This may sometimes be caused by rust or lack of lubrication on the advancer weight pivot points.

Good Fortune! :)
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BS34 idles down slowly 2 years 9 months ago #505394

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Could also try enrichening the pilot adjustment screws by 1/2 turn each to determine whether that reduces a problem that might result from a too lean pilot mixture.

Good Fortune! :)
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BS34 idles down slowly 2 years 9 months ago #505401

Did you leave the synch device on the carbs as you adjusted the idle mixture screws? My experience is that after you adjust the mixture you have to adjust the synch again and then re-adjust the mixture, then the synch again and so forth until everything balances. It's what I had to do to eliminate the idle speed hunting on my BS34's. I don't know if it's because the slides are vacuum operated or what but that's how it ended up working out.
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BS34 idles down slowly 2 years 9 months ago #506033

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OK I have spent about 10 hours on this issue and no fixes yet. Here is what I have done so far.
Checked the ignition advance, working properly.
Checked all valve clearance, all within specs.
Checked the intake boots, no leaks all sealing as they should.
Replaced all o-rings in carbs including the shaft seals.
Checked float levels using the clear tube method, all within specs.
Checked the rubber diaphrams, all in very good conditon look brand new.
Synched carbs they are spot on.
Replaced IC ignitor box with a known good one, no change.
Disconnected the throttle cable, no change.
Replaced the idle jets to #40.
Put the #37.6 idle jets back in.
Turned the idle mixture screws in and out on both the 37.5 and 40 idle jeta, the engine reacts to this but does not effect the hanging idle
None of the above made any changes on the the way the engine idles down. It still hangs around 1500 RPM after blipping the throttle no matter what I do. It will idle high for about 20 to 30 seconds then suddenly drop. Other than the weird idle the carbs perform well. I am about ready to take a sledge hammer to these things. I really need some help on these. Thanks
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BS34 idles down slowly 2 years 9 months ago #506058

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Couple of random thoughts --

Possible leak in vac hose where attaches to petcock?

Tried disconnecting the petcock vac hose at carb, capping the nipple, and test run with petcock in Prime position?

Any clean-air paraphernalia still fitted?

Reportedly, some pod designs may partially obstruct entrance to a carb passageway.

Good Fortune! :)
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BS34 idles down slowly 2 years 9 months ago #506065

Does it only do this when the engine is nice and hot? When it's cold or only just warmed up does it not do this? The reason I ask is because that's how mine behaves, I've just learned to live with it.
Sounds like you have gone through everything pretty thoroughly.
I wonder if it has something to do with the relation of the slide spring and the hole in the slide and the fact that you are now using pods. I'm sure some carb guru has figured this out and knows what spring to use to close the slide quicker.
Here's an interesting article on CV carbs, the BS34 especially.
www.motorcycleproject.com/motorcycle/tex...cv_carbs_work_v3.swf
Last Edit: 2 years 9 months ago by DiamondSkyBlue1000.
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BS34 idles down slowly 2 years 9 months ago #506101

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In my experience, out of sync tend to be the most common cause of this. Have you tried to switch intruments between cylinders to detect if one is off?
1977 KZ650B1
1980 F1 engine
B1 3-phase alternator
B1 Points ignition
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BS34 idles down slowly 2 years 9 months ago #506309

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I again took carbs off seperated them and again went through them, for the 11th time. Found no issues again, put them back together, mounted on bike, synched and set idle mixture screws. Bike runs great expect for the damn idle, still hangs up. I am starting to think the previous owner of these carbs messed them up or there is an internal crack in a casting causing a vacumn leak that I cannot find. I have no idea how the carbs performed before I got them. There has got to be someone here who has dealt with this.
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BS34 idles down slowly 2 years 9 months ago #506318

  • mark1122
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does it act the same when its hot or cold ? a motor will naturaly run lean and richen as it warms.
does adding choke help?
this can help u detect wether it is lean or rich.

it sounds like a lean issue and an air leak could be a posible culprit. i know u said u checked for leaks several times. i hope u dont hit the roof when i ask, but r u spraying all areas with a fluid, like wd40 ? i find this to be a very good leak detection method.
as mentioned, maybe the slide springs are the wrong weight.
did u say the needles were not stock ? maybe try the stock 1's ?

otakar has had sucsess with these carbs on his gpz. give him a pm.
76 kZ, frame gusset work,1200CC.Ported by Larry Cavanaugh,This email address is being protected from spambots. You need JavaScript enabled to view it., 1.5mm.over intakes,.ZRX12 rear end,and seat,96zx9 front end.
01 CBR600F4i Track bike.
Cobourg,Ont.Can.

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~ (k) / (z)
Last Edit: 2 years 9 months ago by mark1122.
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BS34 idles down slowly 2 years 9 months ago #506356

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Yes I have checked for leaks about 20 times, there are no leaks. It occures hot or cold makes no difference. I spent another 10 hours on this today with no gains. The springs and needles do not effect idle, that comes from the idle circuit. All other functions work great, bike starts with ease hot or cold pulls really hard and cruises great so I know the needls and slides are fine.
These came off a police bike so I am assuming they have lot of miles on them. I am starting to think something is worn internaly. I am going to just buy another set of see what happens. To bad because these are very clean and purty.
And no Mark I won't go thru the roof you are just trying to help. I am a fan of the BS34 carb and have bult and tuned a lot of them. This is the first time I have had a problem I cannot figure out on a set, very frustrating and a hit to my ego.
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BS34 idles down slowly 2 years 9 months ago #506374

I'm glad you're not giving up on the BS34's, I like mine. They perform great, they idle smooth, the acceleration/deceleration is smooth with no popping or surging even with the pods and Kerker. I went on a long ride (about 350 miles) last weekend and was averaging over 40 miles a gallon with no issues even in the tight twisties. I have never been able to solve the slow return to idle issue when sitting still on mine either but because the carbs are so awesome everywhere else, I just ignore it. It's only 200 to 300 RPM's. If you do happen to figure it out, please post it. You have my curiosity piqued.
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BS34 idles down slowly 2 years 9 months ago #506380

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Do you have another set of carbs (any kind) that you could use for a test? I'm thinking if you do you could put another set on, without hooking up the cables, start the engine and blip the throttle at the carb. If the idle quickly returns to normal you could for certain rule out any ignition. Then if you put the BS34's back on without the cables and blip the throttle and the idle hangs high the problem must be with the carbs. On the other hand, if the problem persists with the other carbs you may need to look a bit closer at the advancer to ensure it isn't slow to react due to weak springs, stickiness, etc. Of course if neither carb set hangs with the cables unhooked it may just be a cable or throttle tube issue (although this seems highly unlikely). Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
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BS34 idles down slowly 2 years 9 months ago #506381

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Another swag --

:unsure: Possible over-zealous cleaning of pilot air jet whereby it's been reamed out too large.

Good Fortune! :)
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BS34 idles down slowly 2 years 9 months ago #506459

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it may be posible that there is too much ware on the slides ,and u are getting air bypassing the slides??
76 kZ, frame gusset work,1200CC.Ported by Larry Cavanaugh,This email address is being protected from spambots. You need JavaScript enabled to view it., 1.5mm.over intakes,.ZRX12 rear end,and seat,96zx9 front end.
01 CBR600F4i Track bike.
Cobourg,Ont.Can.

~ ~ ~_@
~ ~ _- \,
~ (k) / (z)
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BS34 idles down slowly 2 years 9 months ago #506531

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Just tossing random thoughts --

Perhaps one or more of the throttle slide springs has weakened, and slow to restore the slide to its idle position.

Good Fortune! :)
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BS34 idles down slowly 2 years 9 months ago #508937

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Got a different set of BS34. Cleaned them up and put them on, guess what same thing so it's not the carbs. I again re-checked the manifolds, no leaks. I am at a total loss here.
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BS34 idles down slowly 2 years 9 months ago #508939

  • MFolks
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Checked all vacuum lines? Also the rubber sync plugs will dry rot letting in air.
1982 GPZ1100 B2
General Dynamics/Convair 1983-1993
GLCM BGM-109 Tomahawk, AGM-129A Advanced Cruise Missile (ACM)
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BS34 idles down slowly 2 years 9 months ago #508940

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There are no vacumn leaks.
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BS34 idles down slowly 2 years 9 months ago #508943

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Possibly weak springs on the timing advancer. Or pivots needing lube.

Good Fortune!
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