Two Wheels Good: Advice Needed

"cazzyodo" (cazzyodo) 08/15/2014 at 10:36 • Filed to: Helplopnik

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I bought a 1995 Suzuki VS1400 Intruder a few years ago. It was in pretty good shape overall but I've had issues here and there, nothing major. Unfortunately, I have not put as many miles as I have wanted to on it. Was going to sell it but it's liberating and I own it completely.

Here's the thing...it has always had a little bit of a "pop" to it when I pull off the throttle and revs start to lower. It would sound like occasional gun shots at stop lights when idling.

The past couple times I have taken it out, including this morning to work, idle has been buttery smooth. Sounds great!

But it sounds like I'm setting off firecrackers when I downshift or engine brake. Really, it has gotten bad.

Those pipes are straight...no baffles. I think the bike was dropped at some point and rather than take dents out of the stock pipes these were put on. I plan on repainting them and wrapping parts in that thermal wrap stuff (puts off some serious head on the left side).

I will use my lunch break today to see what I can do to address the popping but as I have no idea what I am doing (as I like to point out in anything car related I post) I could use some help.

So, Oppo, whatcha think? I'll address specific questions best I can but could really use some advice.

Lumpy44, Proprietor Of Fine Gif > cazzyodo 08/15/2014 at 10:43

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Straight pipes introduce all sorts of strange noises. Not sure why it does when the revs come down though.

McMike > Lumpy44, Proprietor Of Fine Gif 08/15/2014 at 10:56

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Burn Baby Burn

The last thing I want to address today is the subject of deceleration backfire, or "popping". This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:

Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

No ifs ands or buts, that's what causes it. But the bigger question is how does gas get there in the first place, and that's a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there's a kicker, and something you should understand before we go any farther:

A motor in perfect tune will exhibit deceleration backfiring.

Therefore, just because your motor is banging it up, doesn't mean there's anything wrong. And consequently:

Getting rid of the noise means de-tuning your motor.

Yup. If you've jut got to eliminate that popping, you'll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn't necessarily a good thing. So lets talk about what causes the problem.

Ok, so you're riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an "overrun" – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to "quench" the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.

2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a "programmed air injection valve" (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it's also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it's there, even when you can't hear it.

So the bottom line, is: That backfiring is perfectly normal and expected. If you've just got get rid of it, that's up to you. You're entitled to set your motor up the way you want, and your goals are your goals. But don't refer to it as "fixing" the popping. Rather, the correct way to think of it is "de-tuning a bit to get rid of the popping".

There are a few ways you can do this.

First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.

Can't credit the link since it's dead, but it looked like a Honda VTX message board.

It could also be a clogged idle/pilot jet.

JEM > Lumpy44, Proprietor Of Fine Gif 08/15/2014 at 10:56

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I'm not sure either, but with an off-road cat and free-flowing exhaust on my Jeep, I have similar things happen (to a lesser extent, and I like the little "pops") when I come off throttle sometimes, especially blipping the throttle on a downshift.

I assume it's some sort of mini sonic boom that happens to due a sudden change in back pressure or something. For an engineer, this is a shitty non-technical assessment I know but there you go.

I imagine if you put on mufflers, you'd get rid of the problem.

EDIT: I typed this before McMike's post which makes a lot more sense.

McMike > Lumpy44, Proprietor Of Fine Gif 08/15/2014 at 11:00

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Yeah, straight pipes let you hear EVERYTHING. the popping is common with carbed bikes. If it's bad or just got worse with no modifications, it could be an exhaust gasket leak, a clogged jet.

McMike > cazzyodo 08/15/2014 at 11:23

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I had an edit, and ran out of time to add it to my other post.

I had this problem with my old Honda. (1100 vtwin, twin carb, straight pipes)

-Picture of my old motorcycle-

It always popped a little bit on overrun, but one day it just decided to backfire on overrun, and struggled to idle. Everything else was perfect except for when it was at idle or on overrun.

I re-torqued the manifold bolts, thinking it was a leak. - nothing.

Then I disconnected the fuel line from the tank, added a 12" hose with a funnel to it, filled it up with Seafoam, and started the bike to let it run dry.

Once the bike ran through the fuel and started to run on the Seafoam (I could tell my the exhaust note and the smell) I gave it a good rev, and shut the bike off. That meant that the bowl, the float, flow valve, and the jets were "soaking" in the seafoam.

I let it sit for a few days.

Then I reconnected the fuel line, and tried to start it. It sputtered and coughed while it was working the fuel back into the motor, but after five minutes it was idling on gas.

It got rid of it.

I realize this is a ghetto way to diagnose and clean a carb, but it beats the hell out of taking the tank off, and removing motor mounts to remove two carbs to rebuild them. It was a "free" repair since I had everything lying around. - and it worked.

cazzyodo > McMike 08/15/2014 at 13:24

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A neighbor suggested I try something similar...high octane and the entire thing of seafoam...run it through. Not sure where that ranks along the good or bad idea train of thought. I used some seafoam last year and it helped but not in the quantity he mentioned.

Friend of mine who I went riding with a couple weeks ago brought up getting to the carb and we both agreed it would probably be a good idea to tinker during the winter if I could.

Thoughts?

McMike > cazzyodo 08/15/2014 at 13:34

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I would re-torque the exhaust manifold(s), then try that concentrated seafoam carb soak.

I got lucky that the soak worked and I didn't have to take anything apart.