Help with the marble mod - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 7 (permalink) Old 05-27-2019, 08:39 PM Thread Starter
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Help with the marble mod

I threw a marble in both of the big hoses attached to the engine since I’m having popping in deacceleration. It wasn’t backfiring at all until the bike heated up and it lost a ton of power and started hesitating. I pulled over and popped the marbles out and it was back to normal. Did I miss something? My air mixture screws are at 2.5 turns out and am struggling to find a screwdriver to adjust them while in the bike.
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post #2 of 7 (permalink) Old 05-27-2019, 11:30 PM
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Ok, if your bike is completely stock, you need to remove the entire PAIR (pulsed air injection reactor) system. Remove the seat and tank for easy access. Remove the hoses where you put the marbles. Leave the connectors that the hoses connected to on the heads. They can be replaced with "coasters" but I just capped mine off with rubber caps I got from the auto parts store designed for capping off car heater hose fittings. They worked fine for over 100,000 miles. Now, notice that the two hoses that go to the front and rear cylinders both go to a valve under the front of the seat. There will be two more hoses connected to that valve. One large one, and one small one. The large one goes into the surge tank (what I call an airbox) through a rubber grommet that seals it. I removed the hose and plugged then hole, but if you don't want to go that far, just disconnect that hose from the valve, and put a marble, bolt, or something in it to seal it up. The small hose goes to the rear carb. Remove it, and either replace it with a vacuum cap, or do it the same way as I suggested doing the hose that goes to the airbox. Remove it from the valve, and stick something into the end of it to plug it up, tie it up out of the way, throw away the valve itself and the two hoses that went to the cylinders. If you have any more problems they are not caused by the PAIR system.

2 1/2 turns is about right for the pilot air screws. I had to fabricate a screwdriver to fit them. Everything I had was too long. If you still have the stock intake and exhaust system, the stock jetting will be fine. If you have modified either one, you will need to rejet the carbs.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike
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post #3 of 7 (permalink) Old 05-28-2019, 09:58 AM Thread Starter
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Thank you so much for the detailed instructions! Any idea what size diameter the rubber caps are that you used?
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post #4 of 7 (permalink) Old 05-28-2019, 10:13 AM
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https://www.amazon.com/Dorman-Help-02254-Bypass-Cap/dp/B000CO8ZL0/ref=asc_df_B000CO8ZL0/?tag=bingshoppinga-20&linkCode=df0&hvadid={creative}&hvpos={adpositio n}&hvnetw=o&hvrand={random}&hvpone=&hvptwo=&hvqmt= e&hvdev=c&hvdvcmdl={devicemodel}&hvlocint=&hvlocph y=&hvtargid=pla-4584138857988657&psc=1

They come in 2 sizes, 3/4" and 5/8" I believe it is the 5/8" that fits, but not positive. Measure the outside diameter of the fitting where the hose goes. Copy and paste the link.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike
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post #5 of 7 (permalink) Old 05-28-2019, 11:00 AM
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The nipple on the reed box is 1/2".

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post #6 of 7 (permalink) Old 05-28-2019, 11:16 AM
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Quote:
Originally Posted by VN750Rider/Jerry View Post
https://www.amazon.com/Dorman-Help-02254-Bypass-Cap/dp/B000CO8ZL0/ref=asc_df_B000CO8ZL0/?tag=bingshoppinga-20&linkCode=df0&hvadid={creative}&hvpos={adpositio n}&hvnetw=o&hvrand={random}&hvpone=&hvptwo=&hvqmt= e&hvdev=c&hvdvcmdl={devicemodel}&hvlocint=&hvlocph y=&hvtargid=pla-4584138857988657&psc=1

They come in 2 sizes, 3/4" and 5/8" I believe it is the 5/8" that fits, but not positive. Measure the outside diameter of the fitting where the hose goes. Copy and paste the link.
I just did this to my VN750 because the air valve was making hissing noises. Any auto store will have these 5/8" caps and you'll also need a vacuum port cap. Dorman sells both in little variety packs. I removed the air valve from under the seat, discarded the vacuum and 2 reed valve hoses and capped the hose running from the air box with a wine cork (fit perfectly).


Chris Glennon - Portland, OR
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post #7 of 7 (permalink) Old 05-28-2019, 03:03 PM
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Replace petcock with tw200 mod. Bet that solves your starting hot issue.

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