Pipes popping

[garyS-NJ]

Last year I "opened up" the muffs on my '84 to get better sound and tone (I like loud pipes). I cut off the muffler tips and punch holes In the muffler (see pic). my carbs and airbox are stock with mixture screws set 2 & 1/8 turns out.. ). With the exhaust mod I did not rejet because I thought the intake would govern how it breathes.. However, with the exhaust mod, I got a noticeable muffler pop (sounds like unburnt fuel being ignighted in the collector) at idle and at low engine load cruising between 1500 and 2200 rpm. Letti g it idle warm for a while greatly minimizes it but a few wackos on the throttle bring a it back for a little bit and then riding up and down on throttle does it.. Sounds like unspent fuel being ignighted in pipes collector. Perhaps less scavenging with mufflers opened up.. Otherwise this appears to be a rich condition in the idle/intermediate circuit which I guess might be leaned out with the mixture screws, pilot jet, or a main jet needle shim but one would think that with the exhaust flowing more air, if anything it would be running more lean. Its not popping out the carbs and not leaking air at the intake). I do have a slight exhaust leak at one of the rear head connections. And I have an ignetek ignition with Gary's timing curve. Perhaps somewhere I'm getting a miss which ends up dumping an unburnt charge into the pipes... Thoughts? http://uploads.tapatalk-cdn.com/20161030/8656c72cffa654db30502445d97d24ee.jpghttp://uploads.tapatalk-cdn.com/20161030/eb481ef1fd1996878c6d09ae03a44fea.jpg

 

 

 

 

 

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[CaseyJ955]

General experience with CV carbs vs flatsides here.

 

Uncorking the exhaust often means you increase jetting to keep up with the added flow, on flatside carbs and this seems to be intuitive. I have experienced it both ways on CV carbs, but they are adaptable and sometimes will take that added flow and run to rich. Some of the Vmax guys end up dropping jet sizes with free flowing exhaust. Im running two sizes smaller than stock with a full Kerker on my own vmax but im at 6300' to. Vmax have a quality Im not sure is shared with Venture, and that is they like to be a touch lean.

 

Does it smell rich? You could read the plugs and see if they show rich running, you could also expirement with running in the AF screws in 1/8 turn increments making a note of their current position in case the changes are counterproductive, you can at least get back to where you are now. This should lean out idle and off idle throttle positions. I think (I think, but not positive) that the needles dont start in until around 4000 rpm on stock carbs so shimming needles for an issue in the 2000 rpm neighborhood may be futile. A smaller main jet leans it out but a smaller pilot air jet enriches, sometimes dramstically. Besides shimming the needle outwards enriches the mixture. Im assuming carbs are synched up properly. A lean mixture can cause exhaust popping to as a result of elevated exhaust temps.

 

I hope this gives you something until someone fluent in Venture tuning chimes in.

Edited November 4, 2016 by CaseyJ955

[cowpuc]

Hey Gary,, I have played with scoot motors some thru the years.. Built a number of em with Valve overlap in the camming, high compression stuff where being piped for scavenging was essential and, knowing the valve timing of our Ventures (no overlap) - I really dont think that is an issue. I have also ran (before getting home for test n tune) Ventures with carbs plugged, diaphrams torn, air filter holed from mice, muffler missing on one side and on and on and generally speaking,, what you are describing just doesn't "fit" for some reason..

Tell you what does fit though,, having an exhaust leak up near the head.. That'll do it IMHO.. If it were my bike and I was chasing what you are - I think the first place I would begin is making sure the exhaust system is sealed, especially back to the muff joints.. If its loose on the muff seals it will probably snort fire when ya wack er up and chop er BUT, because its pulling fresh air AFTER the collector all the cracklin will come out the chopped muffs ya got on er instead of internal of the collector...

[syscrusher]

This seems like a good opportunity to ask about something that's been bugging me for a while. I don't have popping on decel but just idling the exhaust is louder, throatier on one side. Put another way, one side is quiet and the other can be heard a little better. Overall it's not loud while riding it. Assuming that both mufflers are still properly stockish, what should I be looking at?

 

I am using Mark's crossed over collector replacement where right front and left rear are the cylinders feeding the louder side. All of the heads get warm pretty much equally and the spark is provided by coil over plug on the OEM digital TCI. The last time I checked the carbs were balanced, but that was before changing collectors, COPs, and #3 throttle cable, all of which shouldn't have affected the balance, or should it?

 

So, have I got a bad cylinder, a really different idle mixture in one cylinder, or should I make sure both muffs are in same condition and any ideas how to do that? Or something I'm not even thinking about? What would rule out idle mixture, etc? Should I do a leak down test?

[CaseyJ955]

I know this marvelous Yamaha V4 will run shockingly well with a cyl not pulling its weight. Did you get a visual on the slides through the airbox? are they all doing the same thing when you blip the throttle or rev it some? I had a binding slide once and I could hear a change in the rumble. If you have COPs it would only take a moment to swap the R and L coils to RO a weak coil.

 

Im not sure if this works but if you slide the mufflers off and listen... Assuming compression and valve lash are within spec.

[snyper316]

Hey you know maybe some jokester put popcorn in there. On a serious note I have a question I am wanting to make sure I remember correctly. When you all turn the screws on the mixture how do you count the turns? I usually turn the screw twice and call that one full revolution. But when I was playing with my spare carb I was messing with the hole thing I took everything out of it. Anywho's I was playing with that screw a little bit and it don't seem like it would be for fuel or air. Maybe for vacuum on the diaphragms. Also as I was turning the screw in and out I did kinda strengthen up not much but I could tell when it bottomed out. On the bike however I just turn it in until it is snug, Because I was affraid of screwing that screw up or worse the carb body's. I wasn't out to destroy it but I did notice what I consider half a turn which is would be me one turn that fine point in the carb throat did noticably move. Not much but did move, so anyways just checking I would suggest maybe if you can tell what side it is on is turn the screw out just a hair.

[CaseyJ955]

Hey you know maybe some jokester put popcorn in there. On a serious note I have a question I am wanting to make sure I remember correctly. When you all turn the screws on the mixture how do you count the turns? I usually turn the screw twice and call that one full revolution. But when I was playing with my spare carb I was messing with the hole thing I took everything out of it. Anywho's I was playing with that screw a little bit and it don't seem like it would be for fuel or air. Maybe for vacuum on the diaphragms. Also as I was turning the screw in and out I did kinda strengthen up not much but I could tell when it bottomed out. On the bike however I just turn it in until it is snug, Because I was affraid of screwing that screw up or worse the carb body's. I wasn't out to destroy it but I did notice what I consider half a turn which is would be me one turn that fine point in the carb throat did noticably move. Not much but did move, so anyways just checking I would suggest maybe if you can tell what side it is on is turn the screw out just a hair.

 

One turn is a full 360 degrees. I look at notches in my screwdriver handle and when it goes around and comes back to the same position thats one full turn. Good to be precise as even 1/8 turn can make a difference. I count the turns in until it is GENTLY seated and write it down so I can always undo it. If you have an AF screw seated than that would be an issue fur sur.

 

I once hydrolocked a CR500 and when I pulled the head it was full of popcorn, so I feel your pain!

[garyS-NJ]

I screw the mixture screw in all the way and mark a dot in line with the screw slot. Then I put a dot on my screw driver in line with the other dot and rotate counterclockwise 2 and 1/8 turns

 

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[CaseyJ955]

I screw the mixture screw in all the way and mark a dot in line with the screw slot. Then I put a dot on my screw driver in line with the other dot and rotate counterclockwise 2 and 1/8 turns

 

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Thats a good way to do it. Although I just found out about a colortune plug, Im going to play with one of those for sure.

 

I usually start at 2 or 2.5 then take my lil' screwdriver and head up to Mt Rushmore. By the time I come down the other side and get back to Rimrock I've stopped several times and inched them in by 1/8 turn increments until I get some lean hang or que... er popping through the carbs then I go 1/4 turn back from there. I know when she snaps back to idle with no hang, chuffing or wavering and the idle/off idle performance is crisp and true without flat spots up to when the needles take over, they are golden. I adjust the front bank and rear bank individually and ususally end up aprox .5 turn further out on the rear bank to get it just right.

 

Thats my back yard-trailer dwelling-mom and dad are first cousins-old school- seat of the pants adjustment method, but it seems effective with some patience.

[snyper316]

One turn is a full 360 degrees. I look at notches in my screwdriver handle and when it goes around and comes back to the same position thats one full turn. Good to be precise as even 1/8 turn can make a difference. I count the turns in until it is GENTLY seated and write it down so I can always undo it. If you have an AF screw seated than that would be an issue fur sur.

 

I once hydrolocked a CR500 and when I pulled the head it was full of popcorn, so I feel your pain!

 

Yeah was referring to 360 degrees just making sure on that. I haven't hydrolocked anything but I have done the other. Vapor locking sucks to specially when you need two more tree's and the damn chainsaw does it... anyways thanks.

[bongobobny]

Yup, as pointed out in several sources, the 2 1/8 turns is a ROUGH adjustment!!! To fine tune you have to analyze the exhaust gas emissions and/or the color of the explosion! No two carbs are EXACTLY identical...

[CaseyJ955]

Yup, as pointed out in several sources, the 2 1/8 turns is a ROUGH adjustment!!! To fine tune you have to analyze the exhaust gas emissions and/or the color of the explosion! No two carbs are EXACTLY identical...

 

I have long adjusted in until idle drop or stumble than back out a turn or so after steady idle returns, then road test adjust. I have never seen a colortune before, which is embarassing considering how much time I have spent with cars and bikes. I just watched a bunch of youtube vids on it and im jazzed, it looks like an ingenious device, I must have one! How cool to be able to get AF dialed in but get the jets and needles close w/o all the road testing, which has been effective for me but monotonous and time consuming.

 

I feel like a 4 yr old that just discovered Legos.

 

For those of you that mentioned and use it, has it been useful on the CV carbs for jets and needles as well as AF?

[snyper316]

I have heard of people doing this many of ways, One being using a temp laser, Of which I do not have but there like $40 at harbor freight. Now that being said I bought my voltmeter a while back it has an inferred volts for something or other haven't figured that part out just yet. Back to original thing about temperature that voltameter also came with a white wire and a temp setting and just found out exactly whats going on. I have 3 cylinders running right around 160-170 ish the back left one however I knew was hotter because I could feel the difference on my legs. Its running about 205. Someone told me I believe you want it 185 ish; I don't know I just assume get the Morgan tools and do it the closest to the right way.

Edited November 6, 2016 by snyper316
miss spelled words

[garyS-NJ]

Hey Gary,, I have played with scoot motors some thru the years.. Built a number of em with Valve overlap in the camming, high compression stuff where being piped for scavenging was essential and, knowing the valve timing of our Ventures (no overlap) - I really dont think that is an issue. I have also ran (before getting home for test n tune) Ventures with carbs plugged, diaphrams torn, air filter holed from mice, muffler missing on one side and on and on and generally speaking,, what you are describing just doesn't "fit" for some reason..

Tell you what does fit though,, having an exhaust leak up near the head.. That'll do it IMHO.. If it were my bike and I was chasing what you are - I think the first place I would begin is making sure the exhaust system is sealed, especially back to the muff joints.. If its loose on the muff seals it will probably snort fire when ya wack er up and chop er BUT, because its pulling fresh air AFTER the collector all the cracklin will come out the chopped muffs ya got on er instead of internal of the collector...

It does have a small exhaust leak at the rear right (#4?)head to pipe clamp but that leak has always been there. The popping (which is not loud ass backfiring from bad timing or igniting a bunch of fuel or either, but rather softer bulb bulb like a small amount of fuel burning) didn't start with the first level of muffler uncorking (I did a post I think about process, I cut off the reverse cone ends and progressively drilled the rear baffle plate and punched through inners) but came on with the final baffle holes (I was like oh ****). I have a bunch of miles and she seems to run ok without noticeable rich/lean on the plug color (ill recheck) but I never tried to sync the carbs nor have I played with the mixture screws.

 

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