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1992 VN 750
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109 Posts
Discussion Starter #1
I have not been able to reduce the popping from the front cylinder even after coastering and then setting the air/fuel mixture screws to 2 3/4 turns out. The rear cyclinder is just about right...no popping on decel at all at about 2 1/2 turns out. How far out is it "safe" to turn the air/fuel mixture screw out before I risk it coming out altogether or negate its purpose in the first place?

Is it normal for one cyclinder to be this different from the other? I have V and H Cruzers...would it make sense to put a larger slow speed jet on the front carb or would this not provide the desired effect? I need to find a way to stop the popping/backfiring pretty soon...with the Cruzers and having to slow down quickly from 70 to 30 to go through these quaint little German towns the tranquility of these peaceful mornings are being shattered by the ever-present Kawasaki Vulcan pop/backfires! :)

KM...I enjoyed the reply on the purpose of the right ear hose and its associated effect on carb performance! Good stuff...
 

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Premium Member
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2,850 Posts
You could probably safely turn it out some more, maybe up to a whole turn. Try it a quarter turn at a time and see.

Or, screw it all the way in, and then count the turns out til it falls out, reinstall, and take it from there.

In the year and a half I've been hanging around here, I've seen a variety of numbers mentioned as to how many turns is safe until it will fall out....
 

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The Professor
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3,147 Posts
The popping is caused by an exhaust leak or excess fuel igniting when it hits the hot exhaust.

It is normal for one cylinder to be different, normaly you would run the front cylinder leaner than the rear.

Turn the pilot screw in and lean the pilot air/fuel mixture on the front cylinder. If that does not help you may have an exhaust leak. :beerchug:
 

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1992 VN 750
Joined
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109 Posts
Discussion Starter #4
OK...I will look at the exhaust and see if there might be a leak. I used new gaskets but perhaps they are not torqued properly or theymight be binding somewhere and not able to seat correctly. The V and H pipes snug up pretty well so I will look at the left side again and see if I can find something or see if the HD gaskets will work better. The ones I bought from Kawasaki seemed a bit on the "lean" side anyway! Ha.

SO..pipes loosened, reset, and retorqued and see if that makes any difference in the popping during decel. MTF...
 

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1992 VN 750
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109 Posts
Discussion Starter #6
I know... :)

I used the cigar trick after I installed the V and H pipes last week but found no leak. The popping is still there but after a week and several hundred miles I can always do it again and perhaps cigarette smoke will work better...at least until the Government changes the blend and prevents any of those chemicals being added that make it taste just oh so good! Ha.

But I will readjust them and see if I can loacte a leak. They sound so good on acceleration I find it hard to believe that there is a leak...but I will continue to attack this methodically. Before I change out jets or turn the screw out 4 turns or so! It think 5 1/4 is about the limit but at 3.5 I figured it would have been cured. Maybe time to go back to the basics on this one and take them off and use another gasket. Thanks Fergy!
 

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1992 VN 750
Joined
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109 Posts
Discussion Starter #7
Update...

Loosened the right side pipe and reset it and then torqued it to specs. NO change. I think I will check the carb boots and see if they might be loose and tighten if necessary to see if that might be the issue. Then time to try the HD exhaust gaskets and if nothing improves after then I am at a real loss.

Would rejetting to 135 make any difference? Make it worse? Heck...maybe I should simply enjoy the rumbles and snaps and snarls and take her to see The Professor when I move back to the states! :)
 

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The Professor
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3,147 Posts
Update...

Loosened the right side pipe and reset it and then torqued it to specs. NO change. I think I will check the carb boots and see if they might be loose and tighten if necessary to see if that might be the issue. Then time to try the HD exhaust gaskets and if nothing improves after then I am at a real loss.

Would rejetting to 135 make any difference? Make it worse? Heck...maybe I should simply enjoy the rumbles and snaps and snarls and take her to see The Professor when I move back to the states! :)
The H-D gaskets are smaller diameter than the KAW, after trying them I went back to the KAW ones.

If adjusting the pilot screw out does not change try turning the screw in, it may be getting too much fuel.

Check your front coaster and make sure you have a good seal.
 

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Premium Member
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4,054 Posts
Good points Lance. I was thinking surely he's tried turning the screw in through the process of elimination. When you coaster (I marbled) is there a gasket that goes under it?
 

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Linkmeister Supreme
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7,960 Posts
fergy, I pulled the factory coaster plate off the front clyinder on my bike yesterday, just to take a look under it. There was no discernable gasket on the Canadian version of the bike.
 

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1992 VN 750
Joined
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109 Posts
Discussion Starter #12
Hmmm. I haven't thought that the coaster plate itself might be the problem. I will check that tonight to see if the gasket has a defect or there might be some other leak. On my bike there is a pretty good sized gasket that came off the original coaster plate but it might need either 1. replacing or 2. some gasket sealant. I will try the gasket sealant first though! :) Then, as ya'll have suggested I will keep using the air fuel mixture screw at various locations to see if it improves. I didn't think that it would improve at less than 2 1/4 turns as that has seemed to be the "sweet spot" after coastering or marbling, but every bike is different. So...at least I get to play with it some more tonight...wrenching is a good pastime these days! BTW, the popping only occurs during a hard decel...if I roll the throttle off it has no discernible "carb farts" or snapping and popping!

I might have made the poor assumption that the front cylinder just needed more fuel since the front cylinder is the one that runs lean when compared to the rear. The left side pipe has what I consider proper exhaust deposit and residue after riding while the right side has almost no deposit or residue at all inside the exhaust tip. Maybe the rear is too rich?

This bike is a joy to ride, the V and H Cruzers sound awesome, and now I have a few other things to try over the weekend. Thanks to all for the suggestions...I will let you know when I find the culprit.

BTW, both of my my boys are taking the MSF Basic Rider Course this weekend and are having a great time...more importantly, they are learning something at the same time that will serve them well in the future. I know that when you can get a 20 year old and 18 year old on summer break from school to get up at 0600 to go to class without having to wake them up yourself, it must be enjoyable for them...I rented them a Yamaha YZF 6 for the course...seems like it turned out to be a good choice too as they are handling it well and mastering the friction zone exercises and u-turns with little effort. Those U-turns on the VN 750 are a bit more delicate but I took the Experienced Rider Course last Monday and after a couple of attempts got the U-turns down pat in the 18 foot box...nothing builds your confidence more than those slow speed riding exercises. I wholeheartedly recommend them to anyone. Plus, the HD riders in the course simply could not believe how good the Vulcan sounded with the Cruzers when ridden side by side with their bikes....Metric Pride...I will put my $2000 bike up against their $20000 bikes anyday! :smiley_th

MTF...thanks again.
 

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1992 VN 750
Joined
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109 Posts
Discussion Starter #13
Update...

Got home last night and sure enough, the last couple of threads might have hit the nail on the head! Checked behind the right ear and the top bot on the coaster was loose...it wouldn't tighten up either! So...time to do some garage wrenching!

Found the original bolts which are about 1/4 inch longer than the ones that came with the SCP coaster kit, got some trhead locker, gasket sealant and went to work. Bolt is now snug, gasket is coated and reinstalled (did the left side while I was at it) and the fuel/air mixture screw is now reset at 2 1/4 turns out. I am only waiting for the magic hour of 1100 on a quiet SUnday morning to fire her up and see if this was the issue. For some reason I do not mind the "German quiet laws" on Sunday, except when I want to fire up my V and H pipes! :smiley_th

I will let you know if this rectifies the issue but after this I do not know what else might be the issue. I don't think rejetting is required since the bike pulls strongly all the way up to as high as I care to rev her and idles and sounds like a charm. Maybe the little buzzing in the hand grips could be caused by improper jetting, but that is a stretch.

If this fixes the popping and snapping and slight backfiring I will let you know.
 

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1992 VN 750
Joined
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109 Posts
Discussion Starter #14
Update...

Sweet! Looks like a 90% solution! Now just a matter of finetuning the air/fuel mixture and I think it is solved. Thanks to Professor, Fergy, and Ol Hoss for the suggestions...:beerchug:
 

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1992 VN 750
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109 Posts
Discussion Starter #15
Done! :)

After a major epiphany yesterday while driving my car to Frankfurt I finally realized why I was not able to reduce the "popping" from the front cyclinder.

For some reason, when you adjust the air/fuel mixture screw for the rear carb it does absolutely nothing for the front cylinder! Ha. :doh:

Since the popping was from the right side pipe it just seemed natural to adjust the air/fuel mixture on the right side of the bike which is why the rear cylinder kept getting richer and the front cylinder popping never improved! But now all is well in Deutschland and the popping and backfiring during decel has been virtually eliminated. Thanks again to all for their imputs!
 
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