Downhill, No Throttle, Backfire

[MotorPlow]

Beautiful day for a ride.  Met the guys in Blacksburg and then off for a hindred or so miles of twists and curves.  The Vision did pretty good except....

I noticed that as I would decellerate, going down hill in 5^th gear (sometimes 4^th gear), I would get some slight backfiring.  Not real loud like gunfire, but certainaly enough for me to hear it.  The bike has never done this before.

I get back home to see what I could see... I found the vacuum hose to the rear cylinder from the YICS, still connected, but looped up into the throttle linkage.  No big deal, wasn't affecting the throttle (rerouted it).  What I did notice was that I didn't have those small hose clamps on the vacuum hoses going into the cylinders from the YICS and I had a little bit of play in the throttle cable.  Throttle cable was a real easy fix and I have small hose clamps on order.  Could the lack of hose clamps be causing enough of a vacuum leak to cause the backfiring????

I made my yearly Denis Kirk order the other day, so when that gets here I will be doing my Carb Sync (don't think it's that far off), oil change, coolant change, final drive oil change, etc...

Can anyone think of any other reason why I might be getting this slight back fire symptom other that the YICS Vacuum line leak?  The idle doesn't hunt around like the actual YICS is leaking.  OH!....  My battery is about due to be replaced too,  charges ok while riding but only keeps about 11.8 - 12.2 while at rest.  It's been this way for a while, just gotta get off my butt and get a new one.

Help,
Chris F.
Callaway, VA

[Paul_Jungnitsch]

Leak in the exhaust will do it. Decelerating means rich mixture in the pipes, any leak (can be really small) upstream in the exhaust introduces enough oxygen to ignite the unburned fuel.

[louthepou]

I'd add also that an exhaust that isn't really restrictive (i.e. The Macs with the see-through baffles...) will allow backfires in the same situation. Pow! In my case, pretty much anytime I descelerate from high RPM with the throttle closed.

Louis

[MotorPlow]

A problem with the exhaust is not what I wanted to hear.  Could a vacuum leak be causing the extra fuel in the exhaust pipes?  (I really want this to be a simple vacuum leak  :-/)

[Brian Moffet]

before looking for leaks anywhere else, make sure the lines to the YICS do not have splits in them.  If they do, it will cause a helatious backfire like you're seeing.

assuming you still have the YICS.

Brian

[GA_Wolf]

Actually, it is not a problem...the slight backfiring you here is like Lou said..more like due to an unrestricted exhaust if you have an aftermarket set on.  Also if yoiu are running even a little rich, the engine deceleration could cause the same thing...Think F1 or tuner cars..when they engine brake you hear them exhaust firing..on F1's and CanAm cars with short pipes you actually see the flame...My MAC's pop on deceleration, also.  Does not affect performance or cause any harm as far as I can tell...now if it was the loud gunshot type of back-fire..then that's a whole other issue.

[MotorPlow]

Well, I am not going to worry too much about it then. ?The YICS lines are relatively new. ?Rick rebuilt my carbs and checked the YICS a while ago and put all new fuel and vacuum lines on everything. ?I will check the underside of the exhaust and see if I have any rot there. ?There was some surface rust down there, but nothing to cause a leak. ?When I do my Spring PM, I will re-sync the carbs and set the fuel/air mixture.

I was under the impression that this bike has a preset air/fuel mixture setting. ?I've read where this set screw is capped off by the factory. ?Anyone have a picture of where this cap is located so as I can pop it off and start playing with the air/fuel mixture?

Again, your help is appreciated,
Chris Froeschl

PS: The exhaust is stock.

[Superfly]

What is the issue with the gunshot backfire.... I have been getting them on my macs....

[Paul_Jungnitsch]

My Macs are prone to leaking in the front where they bolt to the head. 2 gaskets solved that (seemed to be a slight misalignment compared to stock), although even then they need to be retightened once and a while. On the rear cylinder they can leak around the sealing rings, which I replaced once. The hex clamps can also loosen and it can be a bear to get a good solid connection in this area.

[Superfly]

I was having a problem with the front left side attachment to the head.  When I start the bike in the morning, it sounds like a gunshot goes off when it catches & I give her some juice.... I even ducked once ;D

[Walt_M.]

Back in the last century, off throttle backfire was a sign the valve clearances were a little tight. Checked them lately?

[Superfly]

No I havent, It is on the my new bike.  Did not notice a problem untill I put the Mac's on.

[MotorPlow]

Okay, I started pulling things apart to see if there is anything a miss and this is what I found:


Both sides of the YICS Box leak, but don't leak side to side.  It appears that someone had already repaired the box once.  One side leaks a lot while the other only leaks a little.  I wonder if this could be a reason for my back fire issues.  Well, off to search forum posts to learn how to repair the YICS Box...

Thanx,
Chris, Callaway, VA

[h2olawyer]

Chris -


I think you can epoxy the seam without tearing it apart.  As long as you are sure it doesn't leak between the chambers.

Here's the method I used to repair mine - leaking internally but not externally. ?I think an easier way has been found, but mine's held great for about 2 years now.

//www.xz550.com/YICS.html

Depends on how confident you are in using a band saw. ?Also, make sure the edges of both halves are perfectly flat & true. ?I also used aviation form-a-gasket when I put it all back together. ?A very thin coating on the gasket works best, hence the need for flat & true mating surfaces.

This hasn't worked for some people so others will probably have easier, proven techniques.

H2O

[Lucky]

for those who don't have acess to a band saw, Jason swears by the 'crack it open with a hammer method': place it on a wood block and *Smack* it with a hammer at the corners. be sure not to place the mounting hole on the block, leave it hanging off the wood.

smaking the corners will eventually weaken the seams.  Jason says it'll pop open, but I had to pry it open with a screwdriver as well.

You can make a gasket if you saw it open. if you crack it open you'll have a 'toung & groove' you can fill with epoxy or form-a-gaskett.

if you use any other method to saw it open, be sure to sand both edges flat (on a piece of glass) so you have a nice even surface to mate.

if your just leaking around the edges there, try stuffing it with epoxy. you might have to do it again though.

--Lucky

[Superfly]

I did the hammer method.... works great.  
Did the saw method.... threw away that YICS!

[MotorPlow]

Okay...
I used the hammer method to seperate the YICS.  I wacked it once, then twice, then somehow my finger got in the way on the third wack.  Damn that hurt.  Looked down and the YICS was seperated, perfectly.  As it was already seperated at some point, no tongue and grove, but does have RTV gray gooped al over the place.  I also found globs of brown slime in there...  I wonder what that is/was.

The auto parts store had three types of gasket material.  One was cork, so didn't consider that one.  The other two were, Rubber Fiber Material (Black) and one that was more of a Paper Fiber Material (Green).  I bought the black one as I already have some of the green in the shop.  I know the green is okay with fuel as I have used it in the past to make a gasket for my tractor's carburator, but don't know how good it will hold up under pressure (or vacuum).

What do you think for the YICS?  Rubber/Black or Paper/Green?  I did find the appropiate red silicone.

Thanx for you continued help with this,
Chris
Callaway, VA

[Extent]

I think that black goop is just from the gas.  I had quite a bit of carbon in mine the first time I tried to fix it, I just cleaned it out with carb cleaner spray while I had it open.

[ProfessorRex]

I think when I fix mine I'll just use JBweld to glue it together, no gasket, I think that'll make a better seal, and it'll last longer.  Thoughts? Experiances?

[Extent]

I don't think JBweld is fuel safe.  I JB'd it once and it started to discolor and seperate after a while.  I didn't have two halfs of the YICS on tho, one side of the bond was an acrylic sheet.  I would find a fuel safe epoxy to use.