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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> rejet required??? /cgi-bin/YaBB.pl?num=1259603306 Message started by Bubba on 11/30/09 at 09:48:25 |
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Title: rejet required??? Post by Bubba on 11/30/09 at 09:48:25 Hi all, I need a little feedback. I just took the stock muffler off and installed one I picked up off e-bay. First thing I noticed is the sound (way louder and lower but not ear-splitting). Wasn't even sure I liked it but now that I've ridden it around a bit I think it's growing on me! It is pretty much a straight through but does have baffles/resonator? built into it. They look like this: http://i823.photobucket.com/albums/zz155/onelunger/IMG00224.jpg I've done nothing to the carb except turn the air mix screw out 2 full turns from all the way in. Runs strong right now but when shutting down the throttle I get some backfires. I live a mile high and the stock setup ran fine with 1 1/8th turns out. My questions are: 1) If I reduce the white spacer size would that possibly give me enough fuel to eliminate the backfire? Can it be done without removing the tank? 2) It's an '06 so I think the Main is 145 and the pilot is 52.5. If I need to move up a jet size should I go to a 150 as a starting point? and leave the pilot as it is? 3) can the jets be changed out w/o removing it from the bike? Any thoughts would be welcome. Thanks |
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Title: Re: rejet required??? Post by Serowbot on 11/30/09 at 10:03:38 With a baffle that open, on a big thumper, my guess is, that you're going to get pops on decel no matter what... But,.. if you're up for it,... no harm in tryin' a bigger jet, or spacer mod... Tuning tips... A good indicator of overall jetting is the weather...if popping/backfiring increases on humid days, or at higher elevation, that indicates rich....less popping at those times indicates lean. To test the main, accelerate from 30 or 40 mph, in 4th or 5th gear at full throttle for several seconds, then reduce throttle by about 1/8,...if power increases for a second, you are lean on the main jet. Go up one jet size and test again. ;) |
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Title: Re: rejet required??? Post by Bubba on 11/30/09 at 14:32:46 Thanks serowbot...I think I'll try the white spacer mod first. If that works then great, if not, I was wondering about a dial-a-jet. Anyone have experience with a dial-a-jet on our bikes? Thanks |
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Title: Re: rejet required??? Post by Serowbot on 11/30/09 at 15:33:43 Personally,.. I say dial-a-jets are an expensive pain... one more thing to go wrong.. Maybe good, if you are a circuit racer and need to re-jet every weekend for different conditions... Seems to me,... you'd have to be set up lean, and allow the DAJ to constantly correct... what can it do if you're rich?... Maybe I'm just old fashioned... |
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Title: Re: rejet required??? Post by Boule’tard on 11/30/09 at 16:36:14 I can tell you from direct experience that the white spacer mod reduces backfiring. I had NO backfiring after taking the spacer completely out of the stock carb. With another carb I was running a DAJ and it didn't help with backfiring no matter what the setting. Pilot jets, the mixture screw, and the needle height are your best bets. |
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Title: Re: rejet required??? Post by bill67 on 11/30/09 at 17:22:45 I would go up one on the main and pilot and leave the needle stock,You just need to take off fuel bowl to get to them. |
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Title: Re: rejet required??? Post by Routy on 11/30/09 at 22:17:52 Trimming the white spacer 50% made mine run perfect, but it didn't change the backfire one bit. If you are not rough (lean) running in the intermediate 1/4-1/2 throttle range, I can't believe you would touch the spacer. And if you are smooth running in the other ranges, I wouldn't touch the jetting either. From some posts I have read, I think some have gotten themselves in real trouble messing w/ jetting while not really knowing what does what. Remember that running at 1 mile high elev, has already richened your jetting quite a lot. |
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Title: Re: rejet required??? Post by LANCER on 12/01/09 at 03:23:39 1F283F3F3C6C6D656E5D0 wrote:
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Title: Re: rejet required??? Post by Bubba on 12/01/09 at 07:24:47 Thanks for all the input everyone! This is the greatest forum on earth! Woohoo, my first real wrenching on this bike! I worked on it last night. Removed the tank, then, fiddled around to remove the screws that hold the spacer in the diaphram (they were tight! stripped the head on one but still managed to remove it...*tip-I replaced them with 2 itsy-bitsy screws from an old hard drive!!!), then inserted a #4 nylon washer in it's place. For now, everything is back in place and (thank god) it still runs! I plan to try Lancer's recommendation of turning out the air mix a bit more and see where I am from there. If that doesn't work then I'll follow his advice and bump the pilot jet up a size. I do have a very, very small air leak at the muffler/header joint but I've shimed it with some HVAC sheet metal and I plan to put a (small) amount of sealer there to close it up. I still have the old spacer and have documented everything I've done up to now so if I screw up I should be able to get it back to where it was. The reason I mentioned the DAJ is because even though I live at a mile high I still ride up into the mountains and can see a gain or loss of 5-6,000 feet in 30-40 minutes of riding. If I understand correctly, the higher I go the richer it runs??? Is that right? Thanks again BTW, this pipe I'm using is really starting to grow on me...sounds like a real bike now but the wife thinks it sounds broken... :P |
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Title: Re: rejet required??? Post by justin_o_guy2 on 12/01/09 at 07:53:17 Yes, higher = richer, cuz the air is less dense. BUT,, what delivers the fuel to the charge air, as its headed for the intake valve? Isnt it the venturi effect? Wouldnt a less dense air pull less fuel into the mix? |
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Title: Re: rejet required??? Post by Charon on 12/01/09 at 14:54:30 Carburetors respond to volume flow, not mass. The air volume at altitude is the same, but at higher altitudes the air mass is less. The carburetor meters the same volume of gasoline to the same volume of air, but the mass of the liquid gasoline doesn't change, so the mixture goes rich. Electronic fuel injection actually measures mass air flow and adjusts fuel flow to match. There is still a power loss, but at least the mixture stays correct. |
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Title: Re: rejet required??? Post by justin_o_guy2 on 12/01/09 at 15:16:32 The air volume at altitude is the same, Thats the part that answers it, RAT Thayer. Thanks. |
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Title: Re: rejet required??? Post by Serowbot on 12/01/09 at 15:42:53 In the old days,... you used to flip over the lid on your car's air cleaner, to lean the mix in the mountains... Man,... I'm old.... :-? |
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Title: Re: rejet required??? Post by Boule’tard on 12/01/09 at 20:02:24 527970637E7F110 wrote:
Yeah, but he is running the stock CV carb, which is supposed to correct for air density to some degree, right? (bear with me, this is at the edge of my understanding). I thought the CV carb's diaphragm was designed to set the height of the slide/needle so that at high altitude it opens a lesser amount, matching the lower mass of air available and lowering the needle, which keeps the mixture from going rich. Of course it can only correct as well as a needle and needle jet can.. no change to the pilot circuit and main jet. So to get the mixture perfect at high altitude, Bubba would have to change those, and to get all his power back he'd have to uncork the airbox. |
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Title: Re: rejet required??? Post by Charon on 12/01/09 at 20:34:17 The CV carburetor, sometimes called a "Constant Velocity", or "Constant Vacuum", or in Brit-speak a "Constant Depression" carburetor, cannot really correct for air density. That is because it controls air flow (with the slide) and fuel flow (with the needle), and the two are rigidly fastened together. The idea behind the CV is that it won't allow the engine to stumble or die if the throttle is whacked open suddenly from idle. With a manually controlled throttle, suddenly opening the throttle at low engine speed causes the venturi vacuum to drop (the venturi gets bigger and the airflow is still slow) so the metering signal goes away and the fuel doesn't get sucked into the airflow. That's why accelerator pumps were invented - they squirt a shot of fuel into the airflow to make up for the temporary leanness until the airflow speeds up. The CV uses the airflow to control the slide. Unfortunately, no matter what you do to the airbox, you are going to lose somewhere around 3% of the power for every thousand feet above sea level. That's even with a perfect mixture, because air density drops about 3% per thousand feet. The amount of power the engine produces is almost a direct function of the amount of air it burns. The only way to get the power back at altitude is a supercharger of some sort. |
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Title: Re: rejet required??? Post by Routy on 12/01/09 at 20:38:55 But you can't get all the power back at altitude. |
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Title: Re: rejet required??? Post by Boule’tard on 12/01/09 at 21:56:44 4A61687B6667090 wrote:
Ah, I think I got it. I was thinking the throttle butterfly was the bottleneck on airflow, and the slide/diaphragm assembly was there to sort of "measure" airflow and adjust needle height accordingly. But that doesn't fully make sense because the butterfly is downstream of the slide, making it the bottleneck on air/fuel vapor, not just air. Plus I'm neglecting the venturi effect of the slide on the needle jet.. bleh, maybe one day I'll fully get it. I like the Lancer VM carb :D Quote:
I still think reducing the intake restrictions will help at high altitude. The pressure drop, thus loss of density from one side of the filter to the other has the same effect as a change in elevation. Of course the effect changes dynamically with airflow.. drag caused by the intake tract/filter etc. is negligible at idle but goes up exponentially with engine speed. This can be remedied with a turbo or supercharger like you mention, or to a lesser degree an intercooler or simply uncorking the airbox. |
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Title: Re: rejet required??? Post by Charon on 12/02/09 at 06:07:06 The topic of improving airflow by making changes to the intake system has arisen before. However, I have seen no documentation on this forum of any measurable effects. One or two people have posted dyno charts, some of which have vanished. As far as I know, no one actually posted a dyno run of, say, a stock bike followed by a dyno run on the exact same bike with the air filter removed. Or for that matter, with the entire airbox removed. At least one of the posted dyno runs did not have the exhaust gas analyzer connected, so it was impossible to see whether the mixture changed with engine RPM, an effect I would expect if there is airflow restriction. Some of the posted runs have had incorrect RPM measurements, which have led to incorrect torque and/or horsepower measurements. In short, all we seem to have are "seat of the pants" measurements, and we all are susceptible to the placebo effect. If it's noisier, it must be more powerful. By the way, unless the high-altitude airbox openings are reversible, the leaned high-altitude mixture will remain to plague the engine on the return to low altitude. Airplane carburetors have a pilot-operated mixture control, which changes gasoline flow to compensate for air density (altitude) changes. If I remember correctly, my old Trail 90 had a carburetor with an altitude control on it - a two position control for above or below 6000 feet. It made no difference that I could tell, but I never had the chance to take it over 6000 feet. As a matter of interest, it seems to me it would be possible to convert a CV carburetor to non-CV operation. Block the air passage from the venturi to the top of the diaphragm, open the top to atmosphere, figure out a way to connect the throttle control to the slide, and block the butterfly wide open. Especially at first, I think I would want to remember carefully where the kill switch is. |
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Title: Re: rejet required??? Post by Bubba on 12/02/09 at 07:06:57 Holy smokes! You guys know WAY more than I do. I took the bike out last night (before the snow starting flying!) and turned the air mix screw incrementally out to 2 1/2 turns. Guess what? It quit backfiring on decel!!! I only managed to ride her for about 20 minutes but it seemed to work. It still pops/gurgles/rumbles if I leave it in gear and roll up to a stop but it doesn't backfire when shifting up or when the throttle is shut down fast. One thing I did notice is that when I'm at idle and I rev the engine and shut the throttle down quick I get a pretty blue flame shooting out the back of the pipe...is that normal? From what I understand in reading Charon and Boule'tard's comments is that going back and forth in altittude is going to always be a problem (and I'm by no means an expert!). That's why I was curious about the DAJ. From what I've read, it will smooth out any variances in a lean running condition by filling in the lag between the various overlaps in jetting. Apparently snowmobilers swear by it. |
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Title: Re: rejet required??? Post by Boule’tard on 12/02/09 at 07:39:12 Agreed.. to get back to an optimal mixture as you return to a low altitude, it would be necessary to re-cork the airbox to match your jetting. This has been thought of and little adjustable airbox doors (http://thunderproducts.com/tpi_valve.htm) have been invented. I would not take any stock in dyno runs where the only change was airbox on/airbox off.. you have to re-balance both the airbox and jetting. The "airbox off" run could easily yield less power because now the mixture is too lean, and without the EGA connected, who knows which side of optimal the mixture is on? The point was that you can compensate for high altitude by making the airbox less restrictive, keeping the mixture more or less the same as you had at low altitude. Bubba, congratulations on getting your bike running right. Mine has a little burble-pop on deceleration too. Think of that and a little flamethrowing as not bugs but features 8-) |
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Title: Re: rejet required??? Post by Charon on 12/02/09 at 07:57:43 With no attempt to "flame" you, I still think that some testing needs to be done on the dyno. My thought is that the dyno, using the EGA, can tell us whether the airbox is restrictive in the first place. If the airbox proves to be restrictive, then changes may help. If it turns out to be well-designed and non-restrictive in the first place, improvement from that end may be difficult. Few of us have the financial resources for unlimited experimenting. If one of us does, I think it might be interesting to put the dyno on a truck, haul the bike and the dyno to high altitude, and see just what happens. When bike and car magazines publish dyno tests, they make their measurements in whatever ambient conditions exist at the time, then apply standard atmospheric corrections to correct the readings to sea level standard temperature and pressure. Those corrections are usually also applied to drag strip testing as well. |
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Title: Re: rejet required??? Post by Moofed on 12/02/09 at 08:00:35 Maybe you guys need something like this: http://www.thunderproducts.com/tpi_valve.htm Lancer mentions it as an optional component of his performance carb kit. EDIT: I see Boule'tard beat me to the punch. Quote:
That's pretty much creating a ghetto VM carb. I say ghetto because I've read that the angled bottom of the slide on a VM is actually an important detail (for off-idle mix I believe). Also, doesn't the CV slide not fully block the throat? That would make it hard to idle. :o |
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Title: Re: rejet required??? Post by Boule’tard on 12/02/09 at 08:27:47 567D74677A7B150 wrote:
Oh no offense taken at all, and I appreciate your thoughts. Agreed that the placebo effect is going to be there when you get done working on your bike, expect an improvement, and really want it. Race pipes are the worst for that.. I'm guilty of thinking my dirt bike was faster when the exhaust/jetting configuration was way off.. but it was LOUDER dammit! One way to test power output cheaply is to find two flat sections at different altitudes, and do timed 40 to 60mph roll-ons. That way the modder can't fool himself. Someone should try that, Bubba ;) |
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Title: Re: rejet required??? Post by Bubba on 12/02/09 at 08:39:09 I may have to do that little experiment...next spring. Right now we have 4 inches of snow down here at 5280, up about 5000 more feet there's going to be snow till spring... ;D |
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