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Carb tuning issue

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Carb tuning issue

PostAuthor: Motorpickle » Wed May 23, 2018 8:07 pm

I restored the original exhaust on a 920r that had been run with open pipes. I’ve been trying to tune it but I can’t get the exhaust hiccups out. It doesn’t sound like pops more like subdued misses with a sucking noise. Anyways I’ve adjusted the pilots to no avail. The front cylinder is close to all the way screwed in and the the back is a couple turns out. I have a stock air box and exhaust. While I was fiddling with it I noticed if I covered the intake of the air box the rpm shot up so I taped over 70% of the opening and it runs MUCH better. I rode it up and down the street and it pulled naturally. My question is: what does this mean??
1981 XV920R X 3
1981 XS650
1981 XT250
1978 CB750K
1975 CB550K
1974 TX500
1967 TR6

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Re: Carb tuning issue

PostAuthor: Hellgate » Wed May 23, 2018 8:12 pm

Are your carbs set to stock settings? If not I'd recommend going back to bone stock to develop a baseline. I suspect the main jets are larger than OEM and the pilot screws are back out too much, maybe? The pilots should be at 2 1/2 turns out from seated.

Stock is the ideal place to troubleshoot from. Anything else is a guess.

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Re: Carb tuning issue

PostAuthor: Hexrod » Sun Jul 22, 2018 1:32 pm

I would be very interested to learn what happened here, as I have much the same situation. Replaced open exhaust with an original ( also pulled the heads, replaced valves and seals, new gaskets etc) and since I got it back running a couple weeks ago, I have had the same "hiccups." Last Fall, with the carbs cleaned and running the open exhaust, the bike did not have that hiccup at all. Doesn't really do it when choke is on, but gives that hollow hiccup,once every couple seconds, while it is warming up. Does it much less when fully warmed. The bike idles fine, but starting out, it "lags" a bit, feeling like it's fighting itself. Get up to about 1/4 throttle, and it shoots like a rocket on up from there. I suspected maybe a pilot jet was clogged, so I pulled the carbs and cleaned and blew out the passages last night. Everything seemed clean, but you never know. The original exhaust I put on had a small leak left side, so it is off getting welded. The carb appears bone-stock: 41 pilots, 126 (front) and 124 mains. The brass cap over the pilot screw has not been messed with. Any advice, once I get the exhaust back on? Thanks, Lee

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Re: Carb tuning issue

PostAuthor: bstig60 » Mon Jul 23, 2018 8:18 am

The rear cylinder is #1 on Virago engines, so it gets the larger main jet in most cases to aid in engine cooling. On the RH and RJ, They are as follows: Main jets should be 126 Rear and 124 Front.
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Re: Carb tuning issue

PostAuthor: Hexrod » Mon Jul 23, 2018 7:37 pm

OK, thanks Bill. What's interesting about this bike is that when it had open pipes, the front plug was dry, ashy gray, while the rear was sooty black. Unless I got things mixed up when I first pulled the carbs , they were jetted then 126 front, 124 rear, so that's the way I left them. Now, after the valve re-do, both plugs have looked about right. FWIW. Thanks again for the response.

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Re: Carb tuning issue

PostAuthor: Hexrod » Mon Jul 23, 2018 9:06 pm

BTW, have you ever encountered the odd "hiccuping" that motorpickle and I spoke about?

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Re: Carb tuning issue

PostAuthor: Hexrod » Tue Jul 24, 2018 8:46 pm

Well, I swapped the main jets today, so the 126 is in the back. Once the exhaust is back together, we'll see where things stand. :-)

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Re: Carb tuning issue

PostAuthor: nanno » Wed Jul 25, 2018 12:59 am

With regards to those hiccups, it sounds a lot like a bit of the old synchronisation routine is due...
http://viragotechforum.com/viewtopic.php?f=12&t=43227 - Nanno's accumulated findings (and blatant show-off) - infos on my TR1's, my performance and reliability mods and a bit of show-casing of the stainless steel exhausts I build

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Re: Carb tuning issue

PostAuthor: Flyingdog » Wed Jul 25, 2018 6:08 am

:yup: :;: Yep sync'n as a last step is always a good move. But (IMO) I've never subscribed to that "cooling" theory. That sounded good when I first heard it, back in the eighties. But that all changed when I started tearing these things apart. Removing this or that and started experimenting with anything concerning how these things use fuel the most efficiently. First thing I found out was the #1 carb never needs the larger jet. If that was the case, be curious as to why 97% of the Virago engines out there, use a larger jet on the #2 carb? Cooling? Not hardly.....simply put and JMO, it all was to do with air density between the front and rear carb...and...keeping the A/F ratio correct. Look into any inline four setup. Same reasons. You'll usually find the inner two carbs with the smaller jets, cuz they run hotter than the two outer cylinders. Same principle as our V-twins. Heat and air density dictates what pilot screw setting, main jet, emulsion tube, jet needle... goes in what carb to obtain that perfect A/F ratio.. But that subject as been beat to oblivion.

What happen to this thread's OP?
Last edited by Flyingdog on Wed Jul 25, 2018 2:20 pm, edited 6 times in total.
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Re: Carb tuning issue

PostAuthor: Hexrod » Wed Jul 25, 2018 6:50 am

Thanks for the thoughts, guys! I will report back once the exhaust is back on and it's running again.

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Re: Carb tuning issue

PostAuthor: bstig60 » Wed Jul 25, 2018 7:51 am

If you mean the engine is occasionally popping back thru the exhaust, the syncing the carbs will resolve it. You will need a Manometer to do the sync properly. Google search on how to make one...
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
It is easier to help you if we know where you are.


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