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I have a 93 vulcan 750 and it just doesn't want to run right. It seems to run rough at low speeds and it backfires frequently. Anybody have any advice. I thought maybe something to do with a vacuum switch valve, but valve checked fine. Going to check for vacuum leaks, spark plugs, and fuel lines/filter today. Please help!!!!!!
 

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Definitely check for a vacuum leak somewhere - check your vacuum lines to your carbs, and also check that your air filter ducts and boots are on tightly. Can you tell which cylinder is backfiring?

Sorry I can't be of help on your turn signal issue - I'm guessing a relay, but that'd only be a guess.
 

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Check your plugs for black and wet electrodes. Sounds like you have a rich condition, which would cause sluggish running and backfiring. Have you dosed her with seafoam? Did this just start, or has it been doing this a while? If it just started, you might have gotten some bad gas or something like that. Air filters clogged? Give us more history on it if you can.
 

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Also could be choke not turning all the way off. I didn't mean to imply it wasn't vacuum leak related, but I'm leaning (no pun intended) more toward a rich condition. Vacuum leaks generally cause a hyper idle usually rising, but rich sounds more like what you are describing.
 

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Also could be choke not turning all the way off. I didn't mean to imply it wasn't vacuum leak related, but I'm leaning (no pun intended) more toward a rich condition. Vacuum leaks generally cause a hyper idle usually rising, but rich sounds more like what you are describing.
Can you say more about that, Fergy? Just out of curiosity (you know me), but why the hyper-idle and rising? (That also may have been my problem with McKnight, and not a stuck float valve at all...)
 

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Cindy, you know I'm no expert, so this is the way I understand it. Some of it from experience, since I had carb problems with both of my KZ's and had to work through them, with help from the carb guru, Wired George. I learned a lot from him, but certainly haven't achieved and am far from expert level!

As an engine warms up, the fuel to air ratio has to change. Cold engines need more fuel to run, thus the choke. But as an engine warms up, it needs more air/less fuel to run properly.

On a warmed up engine, too much fuel will bog it down, slow down the idle and make it run sluggish. Because so much fuel is left unburnt, when it hits enough air, it can cause backfiring through the exhaust. You usually will see black exhaust, and smell gas in the exhaust.

When a warmed up engine gets too much air, it will run lean, hotter, and idle will increase. If it's just badly adjusted, idle will be more stable, but will still rise as the engine heats up. If there are vacuum leaks, the idle will be more variable, as vacuum increases, and sucks more air into the intake. Also, vacuum leaks can cause a piston to get a hot spot and literally melt, if left un repaired.

To adjust a carburetor to the right mix at idle, you should warm up the bike, then while it is idling set the idle adjust knob so that it is idling at 1100 rpms, turn in the idle mix screw on carb one until the idle starts to drop, then turn it out until you achieve maximum rpm, go back in 1/4 turn and adjust the idle adjust knob back down to 1100 rpms, then repeat the process with carb two. This should be the correct idle adjustment, not taking into effect any adjustments for popping on decel from marbling etc.
Of course, none of this works right if there is a vacuum leak, or your choke is not turning off all the way, or if there are other carb problems, like clogs, stuck floats, etc.

That's all I know about it!
 

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Cool! I mean, interesting! But can an overly lean condition also cause backfiring? I only ask because my better half's s40 used to backfire like a mother until we decreased the air/fuel ratio (um, less air, more fuel)- they come out of the factory waaaaaay lean, to the annoyance of many owners. So I was thinking that backfiring is due to overly lean conditions, but, in theory, it's just the wrong mix altogether that can be the culprit?
 

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The reason a rich condition causes some backfiring through the exhaust is because it hits air and ignites. This is actually a "lean" condition although the unburnt fuel in this case is because of too rich a mix. The lean condition occurs in the exhaust or on the exhaust stroke as unburnt fuel hits the massive amount of air.
Lean conditions cause the popping we hear on decel. Those are caused from the rolling off of throttle, where the pilot jets are only allowing a small amount of fuel into the chamber with a very high amount of vacuum because of the high rpms, causing backfiring. So, you are right, really, lean conditions cause the backfiring in both cases. But in the first case, it's too much fuel, way too much to burn in the combustion chamber, causing unburnt fuel to get ignited when it becomes a leaner mix, when combined with all that air as it escapes through the exhaust.
So, it's ultimately the wrong mix in all cases. Hope this is clearer than I think it is...:hitanykey
 

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In all cases of backfiring, though, the issue is the same? That is, unburnt fuel hitting hot air in the exhaust chamber and igniting?
 

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That is why we all install the coasters or marbles. That system adds air into the hot exhaust to help burn un-used fuel. And lower emisions.
 

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That is why we all install the coasters or marbles. That system adds air into the hot exhaust to help burn un-used fuel. And lower emisions.
Got it, thanks. So here's a question (not to hijack the thread, but related): McKnight was marbled, but I wanted to go back to the stock set-up. Am basically there, but the A/F screws appear to be turned all the way in - and one of 'em I can't get to budge (unless I pull the carbs again....rrrrrr). So, if you're marbling to reduce the air available to the unburnt fuel, do you bottom out the A/F screws to get more air into the combustion chamber?
 

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Most have to open out the pilot screws some to help get rid of the popping after marbling, which would open the pilot circuit to allow more gas in at idle. Turning them all the way in wouldn't give it "more air", but just "less fuel" at idle. Of course, technically, less fuel means more air in the ratio, but the pilot circuit is a fuel adjustment at idle and decel.
 

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Discussion Starter #13
Haven't tried seafoam yet. Its not my bike so I haven't had a chance to look at her. My friend owns it and had asked me if I knew what the problem was. I thought vacuum leak, but wasn't sure. The rich condition sounds more likely though. I will give him the advice and keep u updated. I will also try to find out more from him what is going on. Thanks guys!!!!
 

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Discussion Starter #14
A little change in situation!!!!! Sorry

The problem with the rough running occurs when the bike is warmed up. At cold start the bike runs fine... After starting it up and going for a short run the bike starts to sputter from what I was told and then it dies. It has been in storage for awile so I told my friend that it could be bad gas or a filter problem. What I am unsure of is the backfire which causes me to think it is running a little rich. Anybody have any advice on what I should tell my buddy??? thanks
 
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