Popping during decel/idle and poor gas mileage - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 21 (permalink) Old 07-16-2016, 01:48 PM Thread Starter
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Popping during decel/idle and poor gas mileage

Hi all. I bought my bike used a couple years ago. I know it has aftermarket pipes on it. I also added K&N air filters shortly after getting it. It has had a pop during deceleration and idle since I got it. The pop at idle is minimal, which is why I've never corrected it. The pop during deceleration is pretty noticeable though. I've also always had poor gas mileage on this bike, maybe 30-32 mpg. My question is whether these issues can be fixed by adjusting the screws on the carb? Would I need to adjust the pilot screw only or the main as well? Would this also cause poor gas mileage? Would a rejet be preferable over tinkering with the screws? A rejet might be a little difficult being that I only know the manufacturer of the aftermarket exhaust and no other specifics about it. Any help is appreciated.
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post #2 of 21 (permalink) Old 07-16-2016, 02:24 PM
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Hi, there is lots of info on the forum, look up marbling, coasters, ear shave and re jetting.

A lot will depend on weather you've replaced the stock filters for like for like K&N standards or pod type,
as for jetting with K&N's and aftermarket pipes (doing away with the goats belly) the general consensus is 140 or 142 main jet and a 42 pilot along with two needle shims

Here's from an earlier post:

Quote:
Originally Posted by Hexadecimus View Post
40, 140 with a coaster/marbling and 3.5 turns is probably closer. 42 pilots might be okay, but it will suck a lot more gas and you'll need to dial back the screws to find best lean idle. "Best lean idle" first (CARBURETOR SET UP AND LEAN BEST IDLE ADJUSTMENT), then mains at WOT and a plug chop to set your top end, then shims if necessary to cure lean surging at cruise speeds and tune the midrange. If 4 shims doesn't do it, go to 42's and zero shims, retune from the beginning. Midrange is still mostly pilots, so a bump up there can help.

I run 38/140 and 3.5-4 turns with pod filters, coastered, and wide open slip-on Screamin' Eagle "mufflers". I put that in quotes because they're empty pipes.
I have no lean problems, starts and runs great, and probably the best mpg of anyone on here.

Excerpt from my patented super duper nifty neato jet cross-section charts:

# area % change
38 0.11 1.00
40 0.13 1.11
42 0.14 1.22

# area % change
132 1.37 1.00
135 1.43 1.05
138 1.50 1.09
140 1.54 1.12
142 1.58 1.16
145 1.65 1.21
148 1.72 1.26
150 1.77 1.29

If anyone is going past about +15%, they either have forged internals and a turbo, they're masking some other huge problem, or they're copying bad advice from all the people who like to massively over-jet.

As you can see, there's a huge % change when you swap pilots. That's why we need to play with the screws to fine tune it. Each size bump begins to drop in larger sizes because it's based on diameter, while fuel flow is based on cross-sectional area. We really need about a +15% option on pilots and #138 mains.

Think about how much extra air you're getting and power you're making. Most intake/exhaust combinations won't get more than 5-10% gain, 8-12% if you're doing lollipops, torque cones, etc. to really tune it right. That's the increase in jet size you should shoot for.
hope this helps

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post #3 of 21 (permalink) Old 07-16-2016, 02:35 PM
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Your gas mileage is correct; that's what these bikes get. Nothing to correct there imo.

Use the search bar at the top of the site and search 'popping on deceleration' which will lead you to numerous threads that describe how to back the idle air screws out on the carbs, and to 'marble' the bike aka remove the smog stuff.

You don't really need to rejet unless you change intake too, in which case going up one size (and adding a shim under the needle) is suggested.

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post #4 of 21 (permalink) Old 07-16-2016, 04:50 PM Thread Starter
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Thanks guys. I will attempt to adjust the pilot screw first and go from there. If I need to rejet, I'm thinking of doing the intelajet.
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post #5 of 21 (permalink) Old 07-16-2016, 06:39 PM
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Your mpg is quite a bit lower than it should be.

I get 42-44 mpg, and consider that a touch low since some others report higher. Riding habits can make a difference there.

With the open pipes, you'll need a re-jet. Your plugs should be looking pretty lean right now. Reading the plugs is the way to tell if jetting is correct.

What brand of pipes?

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post #6 of 21 (permalink) Old 07-16-2016, 09:11 PM Thread Starter
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Quote:
Originally Posted by Spockster View Post
Your mpg is quite a bit lower than it should be.

I get 42-44 mpg, and consider that a touch low since some others report higher. Riding habits can make a difference there.

With the open pipes, you'll need a re-jet. Your plugs should be looking pretty lean right now. Reading the plugs is the way to tell if jetting is correct.

What brand of pipes?
Pipes are Jardine
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post #7 of 21 (permalink) Old 07-16-2016, 09:40 PM
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Quote:
Originally Posted by thtanner View Post
Your gas mileage is correct; that's what these bikes get. Nothing to correct there imo.
Strange but there are many forum members that would and have disagreed with your opinion as to the correct gas mileage including myself. When my bike starts to get less than 42 mpg then I know I have to use some Seafoam, check the air filters, etc. because that is a sign of a problem. It is very rare that it does drop below that as I try to keep up on things the best I can. Now I do know that some modifications will decrease things a bit but it should not be this dramatic of a drop with just an exhaust change.

Firewirl, when you get a chance please give us the benefit of a introduction under the Newbie Check-In section with a bit about your riding/wrenching experience and your location. Just a little something we like to see usually before sharing the wealth of knowledge that is free here.
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post #8 of 21 (permalink) Old 07-16-2016, 11:38 PM
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I get about 40 if I ride like normal peoples.but I've been told I don't always ride like normal peoples.

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post #9 of 21 (permalink) Old 07-17-2016, 09:46 AM
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I get 40-45, all city driving, very little 'throttle twisting'..

that makes a huge difference in the mpg you get. the faster/harder you accelerate, the more gas it drinks..

Also, rider/passenger weight have huge impact on mpg. you put 50% of the empty vehicle weight on its back, its working harder to move it

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post #10 of 21 (permalink) Old 07-17-2016, 01:37 PM Thread Starter
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Well that's probably where I go wrong. I don't ride with a passenger often, but I do love to go 0-60 as fast as I can. I'll take that over a mechanical problem any day
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