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TOPIC: keihin cv34 - vacuum sync

keihin cv34 - vacuum sync 03 May 2014 04:57 #631180

  • undiablo
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I have bougt the four vacuum gauges with restrictors but... I am confused.

My bike has te vacuum petcock from carb #2 and also vacuum from #1 and #3 that join a tee to the clean air valve.

I can forget about the petcock because I will be running gas through a bottle.

What about the emission system? Can I run the engine blocking the two hoses that join at the tee?

Bike is kz750 h2 with cv34 keihins.

Thanks in advance guys!
Kawasaki KZ 750/4 LTD 1981
Kawasaki KLR 650 2011
Argentina - Buenos Aires

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keihin cv34 - vacuum sync 03 May 2014 05:04 #631182

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You can cap the small clean air system vacuum hoses if you want to, but I don't think it's necessary. Without vacuum I think the clean air system would be disabled & inactive.

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keihin cv34 - vacuum sync 03 May 2014 05:27 #631184

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undiablo wrote: I have bougt the four vacuum gauges with restrictors but... I am confused.

My bike has te vacuum petcock from carb #2 and also vacuum from #1 and #3 that join a tee to the clean air valve.

I can forget about the petcock because I will be running gas through a bottle.

What about the emission system? Can I run the engine blocking the two hoses that join at the tee?

Bike is kz750 h2 with cv34 keihins.

Thanks in advance guys!


The emission system should be disconnected when syncing, with each vacuum nipple from the carb holders being attached to a separate gauge.

Where an unused additional vacuum nipple is provided to serve a vacuum operated petcock, it should be plugged/capped in order to prevent a vacuum leak.

It's okay to remove the clean-air system.



Any unused vacuum nipple should always be plugged/capped, and should never be left "open" allowing ingestion of outside air (a massive air leak).

Good Fortune! :)
1973 Z1
KZ900 LTD
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Last edit: by Patton.

keihin cv34 - vacuum sync 03 May 2014 05:27 #631185

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Saludos, mine is a Canadian model, 1980 KZ750 LTD and it does not have the clean air system, the carburetors are the same as yours and I have #1 vacuum open, #2 to petcock, #3 and #4 plugged.
I have a 1980 Kawasaki KZ750 Ltd. I bought new. I recently managed to get it out of my garage after 28 years and put it on the road again (2010). I feel like a kid all over again. Since I have acquired 3 78 KZ1000 Ltd, 1 1981 KZ1000 Ltd, and another 1980 KZ750 Ltd. Love the LTD's.
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keihin cv34 - vacuum sync 03 May 2014 05:36 #631186

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mopguy wrote: Saludos, mine is a Canadian model, 1980 KZ750 LTD and it does not have the clean air system, the carburetors are the same as yours and I have #1 vacuum open, #2 to petcock, #3 and #4 plugged.

You should cap the #1 vacuum port, same as 3 &4. Otherwise, you would have a vauum leak there, unless it's clogged.

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keihin cv34 - vacuum sync 03 May 2014 05:40 #631187

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Thanks, Martin.
I have a 1980 Kawasaki KZ750 Ltd. I bought new. I recently managed to get it out of my garage after 28 years and put it on the road again (2010). I feel like a kid all over again. Since I have acquired 3 78 KZ1000 Ltd, 1 1981 KZ1000 Ltd, and another 1980 KZ750 Ltd. Love the LTD's.

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keihin cv34 - vacuum sync 03 May 2014 06:24 #631188

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Thanks to all of you guys. I was just worried about the emission system getting bad. I will try to synch the carbs this afternoon.

I originally bought this set from Wired George and never sync them because WG already did with his "on bike tuning".

Now that I have the gauges, I will give it a try.

I could never get ride of the hesitation between 2000 and 3500 RPMs.
Kawasaki KZ 750/4 LTD 1981
Kawasaki KLR 650 2011
Argentina - Buenos Aires

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keihin cv34 - vacuum sync 03 May 2014 10:15 #631199

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Yes, it's ok to pull off the vacuum lines for the emissions device during testing and syncing (and capping the ports or connecting them to vac gauges), but if the hoses are left unconnected, the air-suction-system will get fresh air all the time into the exhaust. That will cause big backfires in the exhaust when you close the throttle when riding. So if you leaving the vac lines unconnected after syncing, you will want to plug the ports on top of the valve cover.

The vacuum lines are there to close the diaphragm valve during times when you are coasting and the throttle is closed (or anytime the vacuum is really high in the intake tract), so that no air can get into the exhaust at that time. That's when the mixture is very lean anyway, and extra air is not needed in the exhaust.
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keihin cv34 - vacuum sync 11 Jul 2019 08:56 #807369

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loudhvx wrote: Yes, it's ok to pull off the vacuum lines for the emissions device during testing and syncing (and capping the ports or connecting them to vac gauges), but if the hoses are left unconnected, the air-suction-system will get fresh air all the time into the exhaust. That will cause big backfires in the exhaust when you close the throttle when riding. So if you leaving the vac lines unconnected after syncing, you will want to plug the ports on top of the valve cover.

The vacuum lines are there to close the diaphragm valve during times when you are coasting and the throttle is closed (or anytime the vacuum is really high in the intake tract), so that no air can get into the exhaust at that time. That's when the mixture is very lean anyway, and extra air is not needed in the exhaust.


I removed emissions devices and plugged my carb valves, but the plug that I put over the reed valve intake is a little too big. I think I put a 1/2" rubber cap and it needed to be a 3/8" cap. Do you think that could be causing me to run lean, since this input is not fully plugged?
Current: '19 Harley Roadster, '71 XLCH
Past: '84 GPz 550, '82 KZ750 LTD

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keihin cv34 - vacuum sync 11 Jul 2019 10:46 #807373

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No. The passages from the reed cages go directly to the exhaust ports and have nothing to do with the incoming mixture.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough
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keihin cv34 - vacuum sync 11 Jul 2019 10:46 #807374

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The air going into the reed valves on top of the valve cover does not affect mixture. That air goes into the exhaust. It mixes with burning exhaust fumes in the exhaust header.

If air gets in, it will cause popping in the exhaust since that gas may still be burning, but it does not affect what was burning inside the cylinder.

Popping can be a lean condition in the mixture, but it can also be from air leaking into the exhaust.
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Last edit: by loudhvx.

keihin cv34 - vacuum sync 11 Jul 2019 13:51 #807385

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loudhvx wrote: The air going into the reed valves on top of the valve cover does not affect mixture. That air goes into the exhaust. It mixes with burning exhaust fumes in the exhaust header.

If air gets in, it will cause popping in the exhaust since that gas may still be burning, but it does not affect what was burning inside the cylinder.

Popping can be a lean condition in the mixture, but it can also be from air leaking into the exhaust.


Ahh okay, I misread this. I was thinking the popping was in the carb, not the exhaust.

I'm trying to figure out why my carbs are popping. I have yet to check the valve clearance, but the carbs only started popping after I increased the jet size (due to pod filters and exhaust) so I'm quite confused why it's popping if that relates to a lean condition. Before I increased the jet size (to 140 and 70 per the recommendation of many on this site) the bike was lagging significantly under load.

Now the carb pops and it's super difficult to start.
Current: '19 Harley Roadster, '71 XLCH
Past: '84 GPz 550, '82 KZ750 LTD

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Last edit: by dpivas7.
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