VN750 Backfires & Pops on Deceleration - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 10 (permalink) Old 12-07-2011, 05:57 PM Thread Starter
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Talking VN750 Backfires & Pops on Deceleration

Hey everyone, I just reassembled my 92' vn750 and the rear cylinder pops and backfires badly on deceleration. I've shaved the ears, coastered it, rejetted the carbs with 42 and 142 jets (as per Phils ear shave procedure), and installed straight pipes. the idle screws are let out 3 and 3 1/2 turns. should i let the rear carb screw out a little more? i dont want to crack a piston (expensive!). is it backfiring due to a total lack of backpressure due to the straight pipes (literally a set of steel pipes welded to the headers) any help and or advice is much appreciated!
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post #2 of 10 (permalink) Old 12-07-2011, 09:27 PM
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Originally Posted by leonard3rd View Post
Hey everyone, I just reassembled my 92' vn750 and the rear cylinder pops and backfires badly on deceleration. I've shaved the ears, coastered it, rejetted the carbs with 42 and 142 jets (as per Phils ear shave procedure), and installed straight pipes. the idle screws are let out 3 and 3 1/2 turns. should i let the rear carb screw out a little more? i dont want to crack a piston (expensive!). is it backfiring due to a total lack of backpressure due to the straight pipes (literally a set of steel pipes welded to the headers) any help and or advice is much appreciated!
It sounds like you have done most everything we usually recommend after an earshave. 3 1/2 turns out of the idle fuel/air mix screw is about the max you can go. 5 turns out is all the threads, and the screw falls off.

The idle speed screw is the one with a black knob on the left side of the engine.

I am suspecting the wide open pipes and lack of backpressure may be a large part of the cause of popping on deceleration, but will let some of the more experienced wrenches weigh in on the topic.

Gordon

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post #3 of 10 (permalink) Old 12-08-2011, 07:43 PM
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Completely open pipes, which have almost no backpressure, cannot be tuned, at least with carburetors. If there is one benefit to FI, it is that you can map it to deliver an exact air/fuel mixture throughout the rpm range, from idle to redline. There is no way you can do that with carbs, they have an idle circuit, pilot jets, and main jets. Without backpressure, you cannot get a consistent air/fuel mixture over a wide rpm range. With set jetting, it will be too rich in some spots, and too lean in others. There will only be a very narrow rpm range where they will be right on. Many Harley riders have seriously shortened the life of their carbed engines by running open pipes. I suggest getting some real motorcycle exhaust pipes before you destroy your engine. Or look into that FI conversion idea someone posted about.

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post #4 of 10 (permalink) Old 12-08-2011, 08:18 PM Thread Starter
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Thank you for your advice, much appreciated!
I will get more money in Jan and plan on buying some pipes now.
Would you recommend something? I prefer it loud and sort of the drag style (no frills), but I really hate that backfiring!
Please tell me what you think,
Thank you
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post #5 of 10 (permalink) Old 12-08-2011, 08:32 PM
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I like the V&H Cruzers myself, but they are most likely not what you are looking for. They exit on both sides and look close to stock. You can get staggered pipes that exit on one side, I think they are made by Jardine. If you are looking for those long narrow pipes that exit on the same side, and end at the same place, I don't think you can get them for the Vulcan 750. There is not much in the way of aftermarket stuff for the Vulcan, and now that it has been out of production since '06, I expect what little there is to start disappearing.

Here is a link to some I just found that I've never seen. They do not look like the Jardine pipes I have seen before, and might be more along the lines of what you are looking for. http://tinyurl.com/6uwfc3e

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2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
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Last edited by VN750Rider/Jerry; 12-08-2011 at 08:38 PM.
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post #6 of 10 (permalink) Old 12-09-2011, 08:01 AM
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I tried this past summer to use some aftermarket pipes, got the same pop noise. So I put another set of aftermarket pipes and had no problems.

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post #7 of 10 (permalink) Old 03-13-2015, 06:42 PM
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I just finished installing the Vance and Hines Cruzers yesterday, took it on a couple rides, very loud ( which is awesome) but backfiring and popping on deceleration. Very annoying, I've tried several different throttle speed settings, still backfiring. Running KN air filters, and Denso iridium plugs. Thinking about putting it in the shop?
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post #8 of 10 (permalink) Old 03-13-2015, 08:06 PM
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Might try opening the mixture screws a little. If it's lean, it can pop.

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post #9 of 10 (permalink) Old 04-01-2015, 07:43 PM
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Thank you so much, I really have no idea where the mixture screw is . I just took my wife for a ride and it's very annoying rightnow!
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post #10 of 10 (permalink) Old 04-01-2015, 08:54 PM
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