poor running - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
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post #1 of 12 (permalink) Old 07-06-2008, 11:06 PM Thread Starter
 
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poor running

Hello to the group. I have a 85, VN700. It has been running fine but now has developed a strange problem. It starts and idles with the choke fine. Turn off the choke and it idles for a short time then starts missing on the rear cylinder. I have changed all the electrical end parts, coils, pickups, plugs and CDI. I have a spare bike for parts. Anyway what I have noticed is that if you rev the engine to 4000 rpm or so it starts to sputter and backfire. Also noticed the exhaustpipe starts to turn red. If I cover the carb intake a little it starts to run better. I cleaned the carbs but still does it. Also the idle is a little erratic I have to turn it up to 2k or it idles for a while then starts missing and stalls. Anyone have any ideas on whats going on here?
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post #2 of 12 (permalink) Old 07-07-2008, 12:03 AM
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To me the pipe turning red means that it is running lean. You may have a slow fuel feed to the carb. Maybe the rear carb float is sticking up. Try tapping the carb with a light tool to try and free the float. If it helps it is only a temperary fix.

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post #3 of 12 (permalink) Old 07-07-2008, 10:13 AM
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I agree with the lean running diagnosis. However, to me, it sounds more like an air leak somewhere in the intake system. Either a carb boot has come loose or gotten misaligned or the diaphram in the carb has holes in it allowing more air in. I wouldn't run it like this very long as you can also burn the valves if you haven't already since the exhaust pipe was glowing red.

To test this, you can squirt some WD 40 around the boots while the engine is running. If the idle speeds up, you have a leak around the boot(s). If not, the leak is elsewhere.

Jim W
93 VN 750 "Ursula"
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post #4 of 12 (permalink) Old 07-07-2008, 07:40 PM Thread Starter
 
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Well, Thanks for the advice, I tried the WD40 trick and no change. Idle is still erratic and at 4000 rpm it starts cracking and popping on the rear cylinder yet. I had a set of jets here so I rejetted both carbs. went from 38 to 40 on slow jet and from 135 to 140 on the main. Still does this. What the ^$#%$. I'm going to leave it alone for a day or 2 and see if anything else comes to mind on what this problem could be.
Larry
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post #5 of 12 (permalink) Old 07-07-2008, 11:41 PM
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Well it's a little late to say don't change the jets! You are just masking the problem since your jetting didn't change by it's self. Lean means too much air. Air leak somewhere. Are your filters falling apart? Something has happened to allow too much air in and you need to find that. I'd go back to the original jets and work through your system to find what has changed.

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post #6 of 12 (permalink) Old 07-08-2008, 08:41 AM Thread Starter
 
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Fergy, Last night (late last night), I kept thinking about this thing and put the original jets back in. I agree, they just didn't change themselves. I wanted to see it it made things better. I'm going to try something today and see if it has an effect. I will post the result here later. I'm going on a camping trip for the next 2 days and the wife won't let me bring the laptop. I might sneek it in somewhere, hehehe.
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post #7 of 12 (permalink) Old 07-08-2008, 11:06 PM
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Strange and untimely things happen sometimes, like the time I pulled the carbs on my KZ1000 to clean them and when I put them back on the bike, it ran like crap. I went through everything, including pulling the carbs again, checking the manifolds (boots) for leaks, retracing my steps, trying to figure out what I had done to cause the sudden change in state. Turned out that my #4 plug wire was split right where it goes into the plug boot. I hadn't touched that when I pulled the carbs, but for some reason, it decided to crap out at that time, throwing me for a three or four day wild goose chase. I understand how easy it is to start second guessing, and get off on the wrong track! Hang in there, and enjoy your camping

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post #8 of 12 (permalink) Old 07-09-2008, 03:42 PM Thread Starter
 
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Well, I was able to convince the wife that bringing the laptop was a good idea. We can keep an eye on the weather radar with it. I have an aircard for internet access. When I get back home here is my plan. I'm going to swap the 2 carbs, front to rear, and see if the problem moves to the other cylinder. Only going to move the carbs and not the boots. If the problem moves then it has to be the carb. Any thoughts anyone?
Larry

PS... Campings been great till this afternoon. Started to rain and we are in a tent. Thank God I sprayed everything with silicone before we left
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post #9 of 12 (permalink) Old 07-09-2008, 04:21 PM
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Not sure if you can just swap the carbs like that. I don't think they will line up properly when reversed.
I still think something is letting more air into the system than needed.

Jim W
93 VN 750 "Ursula"
Moved R/R 08Sep06
R/R rewire 17Feb07
New R/R, Installed 14Jun08
New Stator installed 10 Jun08
Maintenance Free Battery 12Jan08
21480 Miles
VROC #8542
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post #10 of 12 (permalink) Old 07-09-2008, 04:59 PM
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I'd say check the air filters too.


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