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Carbs and Fuel System
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post #1 of 13 (permalink) Old 06-29-2009, 02:17 PM Thread Starter
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Unhappy Highway Miss

Sorry I haven't been involved more on the forum, but the bike has been down for a while with charging problems. Replaced the R/R (twice) and the Junction Box and checked all the wiring to no avail. So I brought it into the shop for a new stator. And while it was there, I had them rebuild & re-jet the carbs for the new pipes (V&H) and de-eared configuration.

Got the bike back after almost two weeks and the charging system is great. However, the carbs are not so good. At first after a highway drive and a stop, the front cylinder wasn't firing. Rev it up, and it was good to go. Pulled, cleaned, and regapped the plugs and solved that. But now the engine misses intermittently at highway speeds. Starts around 50 mph and gets progressively worst up to 80 mph. If I punch it she goes without a hitch. If I slack off a bit when it misses, she smooths out.

From stop to 50 mph, she has no problems except I was expecting a bit more pull with the re-jetting (138 mains). But I may have lost a bit.

I'm bringing it back to the shop Tuesday. Hate to lose the bike again.

Any and all suggestions would be appreciated.
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post #2 of 13 (permalink) Old 06-29-2009, 02:28 PM
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keep us posted.. myself and 1986vn750 are having similar problems...

'99 VN750 with 8900 miles and growing (3,111 when I bought it 03-22-08)
Ear shave with Unipods
Drilled mufflers and degoat
TOC springs
N/C Street shield EX windshield
Jumbo Saddleman Highway Studded saddlebags
Willie and Max Saddlebag mounting kit
License plate turn signals
Home made sissy bar
Luggage Rack from Scoop
http://youtube.com/user/99vulcan750

Last edited by 99vulcan750; 06-29-2009 at 02:33 PM.
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post #3 of 13 (permalink) Old 06-29-2009, 05:46 PM Thread Starter
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Trying to tune the mixture on the carbs, I find them them 10 cm hg out of sync. I can peak them out at 18.5 and 25 cm hg but still 6.5 cm hg off. I thought the shop would have balanced the carbs.

Would this cause the miss at highway speeds?
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post #4 of 13 (permalink) Old 06-29-2009, 06:03 PM
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hmmm.... that could be doing it...

'99 VN750 with 8900 miles and growing (3,111 when I bought it 03-22-08)
Ear shave with Unipods
Drilled mufflers and degoat
TOC springs
N/C Street shield EX windshield
Jumbo Saddleman Highway Studded saddlebags
Willie and Max Saddlebag mounting kit
License plate turn signals
Home made sissy bar
Luggage Rack from Scoop
http://youtube.com/user/99vulcan750
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post #5 of 13 (permalink) Old 07-05-2009, 10:29 PM Thread Starter
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Well I tweaked the air/fuel mix, balanced and that helped, but it only mellowed out the highway miss. Find it doesn't miss until after I've driven 10 miles or so. Maybe heat is related. Replaced my ignition wires today with some NAPA high performance metalic core white silicon by the foot wire. Haven't had time to go for a long ride on the highway today. Tomorrow we'll see.

Oh, I thought about starting all over with the carb adjustments. I want to start by balancing the carb butterflies using a wire as a gauge. Then readjust the air/fuel mix. And finally adjust the balance/sync with a manometer I'm building.

Thoughts?

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post #6 of 13 (permalink) Old 08-24-2009, 11:02 AM Thread Starter
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I've adjusted a little, cleaned a little, rode a little. Kept getting better and worst each time. Added fuel filters, rebuilt petcock and rerouted carb breather. The last I did was adjust the needle clips. First to the third and then to the forth. The miss is pretty much gone, but I don't think it's running as good as it could. So I'm ordering new jets. Shop installed 138 mains, not sure about pilots. I'm going for 40's and 140/142's as recommended. Will that be enough or should I go with 142/145?

Here's what I'm ordering from http://www.motorcyclecarbs.com/Keihin_Main_Jet_99101-393_C78.cfm

KEIHIN MAIN JET 99101-393 #140
Item #674 Your Price: 6.95

KEIHIN MAIN JET 99101-393 #142
Item #675 Your Price: 6.95

KEIHIN PILOT JET N424-22 #40
Item #552 Your Price: 7.95

If this doesn't look right to anyone, please let me know soon.

Not looking forward to removing the carbs for the first time. Got many questions about that. Any new tips, shortcuts, and recommendations will be more than welcomed.

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post #7 of 13 (permalink) Old 09-19-2010, 01:38 PM Thread Starter
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Talking

After probably a hundred different experimental tweaks, I may have solved my highway miss problem. I finally got the guts to split the carbs and check the floats. The shop that couldn't fix the problem replaced the float valves, so I figured ... Anyway, I set the floats at the high end of their allowable setting and voila, flutter at cruising speeds gone.

I still may have other problems, but I can now ride to work without the exasperation I had before.

Put new clamps on carb boots today. Tomorrow I'll tweak and balance.

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post #8 of 13 (permalink) Old 09-19-2010, 02:57 PM
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138 is the stock size for the main jets I am running #42 pilot and 150 main 2.5 turns out on the air fuel screw with degoated and debaffled stock pipes and unifilter pod filters @ sea level


34k so far
VN750.com member #613
Keep the shiny side up
coastered
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jetted 42-150 2 shims
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inside of tank coated
spline lube
long Beach Blue Pearl Paint 4/07
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post #9 of 13 (permalink) Old 09-19-2010, 03:06 PM
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Wow, over a year since the last post. Good to hear from you again, and glad you finally solved the problem with the stumbling engine.

You might like this piece of writing from Daniel Meyers and his "Life is a Road" series.
It is entitled "Wrenching", or alternately "Why I do my own work."
http://lifeisaroad.com/wrenching.html

It sort of describes your experience with the shop not really being competent in fixing and adjusting your carb float level.

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/11-vn750-general-discussion/9127-top-ten-items-you-would-suggest-new-owner-do-his-new-ride.html
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post #10 of 13 (permalink) Old 09-20-2010, 01:24 PM Thread Starter
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Yeah, it's been a pretty depressing year or two. Try and fail and try again. Lots of learning, lots of new parts. Two years with her and only 3000 miles. With a nice 22 mile ride to work, this doesn't add up to much. Hoping for a long autumn and a mild winter to make up for lost time.

Oh, nice read. I seen worse.

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