Author Topic: Secondary Venturi  (Read 1381 times)

WaterMover77498

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Secondary Venturi
« on: March 11, 2017, 06:21:21 PM »
I'm having trouble getting my '82 to run reliably. I know nothing about carborators, and after five times taking them off cleaning, and putting them back on, I've noticed something has hasn't changed the whole time.

So, the bike will start and idle very well. It will rev, but as it warms up, the bike starts to rev uncontrollably. When the reva go crazy, the secondary Venturi seems to be putting out a lot of gas compared to the front. If this is in fact "the problem", i haven't been able to figure how to change it.

Any ideas?

Thanks team!
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Rikugun

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Re: Secondary Venturi
« Reply #1 on: March 11, 2017, 07:19:46 PM »
I'm unsure how to say this delicately but if you know little about carbs and after 5 cleanings they're still not right, it's possible they have been assembled incorrectly.

Revving uncontrollably almost sounds like a vacuum leak but I'm not sure how that may relate to the increased fuel from the one venturi. I'm guessing you have the tank and air box off while running the engine and can witness this phenomenon? Can you post a pic with an arrow pointing to where the fuel is coming from?

Did you bench sync the throttle plates before installing the carbs? Despite your contention it starts and idles fine, is it possible one throttle plate is opened further than the other and it is feeding sufficient air to let the engine "runaway"? This could explain why one carb is drawing more fuel than the other into it's venturi.
« Last Edit: March 11, 2017, 07:22:50 PM by Rikugun »
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WaterMover77498

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Re: Secondary Venturi
« Reply #2 on: March 11, 2017, 08:03:48 PM »
Rikugun, don't worry, I won't be offended if you say the carbs are put together wrong. I wouldn't mind being called an idiot right now if it helps me get to the bottom of things. Can you think of anything people get wrong during reassembly? Any check balls or other small parts that might fall out and not make it back in?

The yellow arrow in the attached photo shows where the fuel is coming from. There is enough fuel coming out to wet the throttle butterfly when the throttle is in the closed position. Almost no visible fuel comes out the secondary venturi of the front carb.
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fiat-doctor

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Re: Secondary Venturi
« Reply #3 on: March 11, 2017, 09:42:57 PM »
I'd guess vacuum leak or carbs out of synch. 

With them off, shine a strong flashlight at the throttle plates and see if they both "bleed" a similar amount of light.
This should get you close enough to use a vacuum synch tool on the running bike.

Plugged idle passage can make fuel dribble from the venture as well.

Steve

WaterMover77498

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Re: Secondary Venturi
« Reply #4 on: March 11, 2017, 09:47:47 PM »
Steve, can you give me an idea where the idle passage is?

Thanks,

Paul
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fiat-doctor

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Re: Secondary Venturi
« Reply #5 on: March 12, 2017, 10:56:16 AM »
It's complicated :)   Carb needs to be spotless...   the idle circuit includes the idle jet, idle mixture screw, the tiny transition holes next to the butterfly valves, an air bleed jet and all the passages that connect them. 
Carb cleaner, compressed air, ultrasonic cleaning are your friends here.

Don't forget the check the basic synch of the butterflies, make sure the carbs are seated in the intake boots, the condition of the boots themselves, condition and routing of the vacuum lines.

Steve

WaterMover77498

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Re: Secondary Venturi
« Reply #6 on: March 12, 2017, 03:00:21 PM »
Thanks for all the suggestions. I think I'll swap my carb boots for a set on a spare engine and see if that helps. They look rough, and I haven't done anything in that regard yet.

One more thing I noticed: air seems to be coming out of the rear carb. Is this more evidence that I need to work on synchronizing the cards, or do I have some valve shim work ahead of me in the future?
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fret not

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Re: Secondary Venturi
« Reply #7 on: March 13, 2017, 03:02:38 AM »
Valve clearances are the first thing you need to check.  If they are out of tolerance you can't really tune the carbs effectively.  Once you know the clearances are within specification you can check for air leaks with the engine running.  Spray WD40 or carb cleaner around the intake boots and around the carbs, if the RPM changes you have a leak.
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Rikugun

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Re: Secondary Venturi
« Reply #8 on: March 13, 2017, 09:13:41 AM »
One more thing I noticed: air seems to be coming out of the rear carb. Is this more evidence that I need to work on synchronizing the cards, or do I have some valve shim work ahead of me in the future?
This comment and only one throttle plate wetting seem significant but first things first. Regarding valve lash check, as fret nut said, it's the right place to start. If this bike is new to you I'd go a step further and perform a leak down test or at least a compression test too.

I'm really questioning if this bike does idle well now. If this is your first XZ, you may not know how it is supposed to sound when running properly on both cylinders. I wonder if it starts and eventually idles on one (or primarily one) cylinder. The idle speed screw would have to be turned in a lot to achieve this. Once warmed up, the available fuel atomizes better and that same idle speed setting would cause the engine to race when revved or not come down or to come down slowly mimicking an air leak.

From a cold start, monitor the heat of the front header pipes and rear "Y" pipe to see if they heat evenly.
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Prophet Of Doom

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Re: Secondary Venturi
« Reply #9 on: March 13, 2017, 02:52:51 PM »
All good advice. 

Yes, to do the whole job properly the lash is the first place to go, but for a quick diagnostic the first thing I would do is a carb sync.  Since you have had the carbs apart for cleaning you are guaranteed that will be mis-set now.  It's a low cost and easy thing to do, a well known source of a hanging idle, and you will need to buy or make a gauge anyway.  Sync is not an optional step, whereas lash can be a fair way out and still run reasonably well.