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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Stock Carb Slide Mod /cgi-bin/YaBB.pl?num=1523577809 Message started by DragBikeMike on 04/12/18 at 17:03:29 |
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Title: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:03:29 The stock carburetor receives a lot of discussion in this forum. It almost feels like it is connected to the twist grip with a rubber band instead of a braided steel cable. The response is poor to say the least. In addition, the low speed, transition and mid-range circuits are grossly lean. All sorts of surge, hiccups, poor response and afterfire. It improves a bit once warmed up but overall, it’s anemic. I have seen lots of jet kits that utilize softer slide springs and enlarged vacuum ports in the slide. I haven’t seen those mods discussed herein so I decided to give them a whirl. I started by cleaning up the lean spots using mods frequently discussed in this forum. I will elaborate with additional posts with pictures. |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:06:49 I started by cleaning up the lean spots using mods frequently discussed in this forum. I uncovered the idle mixture screw so it could be adjusted, I enlarged the stock #52 pilot jet (.019”) by drilling out with a #75 drill (.021”), and I raised the slide needle by installing a .060” washer in place of the stock nylon washer (.113”). Here is a picture of the pilot jet drill work: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:10:08 I know that many folks frown on drilling a jet, but I have drilled dozens of pilot jets with absolutely no adverse effect. Now a main jet, I don't drill those. Here is a picture of the needle spacer replacement: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:13:26 Best idle was achieved at 1-3/8 turns out. As expected, it was a quantum leap from the as-delivered condition. Easy starting, good smooth idle, no more surge, almost no afterfire. Everything’s better….except the twist grip still felt disconnected from the carb. Turn, wait, go. I figured a softer slide spring would result in quicker response. I tied off several of the coils with thin nylon chord to shorten the spring & reduce preload. Stock spring free length is 6-15/16”. With the coils tied off the free length was reduced to 5-3/4”. I installed the spring with the tied off section down in the slide. Here is a picture of the shortened spring: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:15:42 Results: It sorta kinda felt like it might have improved. Hard to tell. Let's try some more. I reduced the free length to 5-1/8”. Here is a picture of the even shorter spring: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:22:08 Results: Not good. If anything, the response was a little worse than the stock spring. After giving it some thought, I concluded that softening the spring is the wrong approach to better response. The variable slide adjusts the size of the venture to the optimum position necessary to maintain velocity. The vacuum signal ported to the area above the diaphragm is proportional to the velocity. The spring opposes the lift force applied by the diaphragm. If you soften the spring the slide assumes a higher position at a given air flow, and in turn results in lower velocity, which in turn results in a less responsive system. Let’s try the vacuum port concept. The stock slide has two vacuum ports in the bottom. A #40 drill bit just fits into each port so the ports are .096” diameter. A new slide costs around $125 so I was reluctant to just hack away. I checked the screw holes for the slide needle retainer plate. A #38 drill just fits. I left one of the screws out to act as an additional vacuum port and tried again. Note: Make sure both screws are tight. Then remove one while leaving the other tight. The single screw is more than adequate to hold the retainer in place for a quick test. here is a shot of the screws that hold the plate in place: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:27:46 Results: My seat-o-da-pants evaluation says that the response is significantly improved. The twist grip now feels solidly connected to the throttle plate. The roll-ons are very nice. Disassembled and drilled a #36 hole in the bottom of the slide. I installed the slide needle retainer on the bottom of the slide to use as a template. There is one unused notch in the retainer so that’s the location I used for the new hole. By drilling with the retainer used as a template, you will be sure that the new hole lines up correctly with the retainer when everything is reassembled. I strongly advise that you use a drill press so that the hole is exactly perpendicular to the bottom of the slide. This well ensure proper alignment with the retainer plate notch. I think this mod works pretty good. Here is a shot of the bottom of the modified slide: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:28:46 Here is a shot of the top of the modified slide: |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/12/18 at 17:35:17 Keep in mind that you can test these modifications without doing any sort of permanent alteration. I didn't like the shorter spring, so I simply removed the nylon cord and it was back to the stock configuration. I liked the results of the additional vacuum port created when I left one screw out, so I reinstalled the screw and drilled a permanent hole. I can still reverse the hole by simply tapping to 6-32 and installing a third screw. If any of you have tried either of these mods, or if you try them in the future, LMK if you like the results. |
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Title: Re: Stock Carb Slide Mod Post by Dave on 04/13/18 at 02:56:10 Glad you gave this a try and the results are promising. Drilling jets doesn't scare me - the only issue is that most folks don't have jet drills or reamers or gauges. Your mixture screw ended up very near the 1.5 - 2 turns that is specified as the range that indicates the pilot jet is the correct size. Since you are qualified to experiment - can you figure out a way to make the TEV valve provide a bit more fuel? That will help to eliminate the popping/banging when folks close the throttle between shifts or when compression braking. |
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Title: Re: Stock Carb Slide Mod Post by batman on 04/13/18 at 11:00:49 Well I have done all the mods you listed ,except drilling the pilot jet (the stock jet being good at my elevation ) Dave mentioned a mod to the TEV valve , I have removed about 1/3 of a coil on the spring at a time until the proper valve action was attained , (keeping the cut end outboard to avoid damage to the diaphragm ) opening the valve a bit earlier and closing it a bit later, tends to reduce afterfire from the muffler. Looking at the intake as a system, you may want to tune intake runner length ,I've boosted EV by 4-7% in some rpm ranges and these boosts are rpm dependent and occur in all gears. You may want to check out Gadgetman grove (which I just completed -my version as yet untested) and Singh groves and their possible improvement to performance. |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/13/18 at 22:56:52 I will take on that TEV valve (I call it the coasting enricher system). My understanding of that assembly is that under normal steady state and acceleration the spool valve is parked and air to the pilot and transition circuits is provided via the #230 pilot air bleed and #45 air bleed jet, located in the dome below the diaphragm. When manifold vacuum spikes during deceleration, the high vacuum acts on the TEV diaphragm to overcome the spring and shuttle the spool valve over the port that connects the #230 pilot valve to the pilot & transition circuits. Under the high vacuum condition, the pilot and transition circuit receives air only via the #45 air bleed jet. When I had the carburetor apart, I measured that #45 air bleed. It was greater than .016" and less than .018". That's pretty small. I didn't measure the threads on the jet. Does anyone know if these jets are available? Installing a smaller one will make the mixture richer on hard deceleration. I think Batman's spring mod probably has merit, but also imagine it would be easy to go a bit too far and end up with that #230 air bleed covered up under normal driving conditions. I will test the spring and determine what the preload is with the TEV valve in the parked position. Then can compare to diaphragm area and figure out how much vacuum is necessary to shuttle the valve. I will also do an operational test with a vacuum gage & damper to determine average driving vacuum and deceleration vacuum. We can get a better idea of where the spring should be set. As I mentioned earlier, deceleration mixture can be made richer by installing a smaller air bleed jet (maybe a #40 or #35) if they are available. I have noted that on earlier models (86-88 & 95) the illustrated parts breakdown only shows one air bleed in the dome (a #70) and the TEV valve is incorporated. Those older models may have used a system where the TEV closed off ALL the air bleed. Maybe I will try plugging the #45 entirely just to test it and see how it works. Wish me luck. Hey Batman, in your opinion, how did the additional vacuum port in the bottom of the slide work? Do you feel that it improved the way the throttle responds to the twist grip? Does it feel more connected? BTW, I checked out the YouTube vids on Gadgetman's groove. I am absolutely reluctant to turning a die grinder loose on that $525 carb. Do you happen to have any drawings or sketches that show exactly how that groove is configured? You know, depth, width, angles, etc. Have you incorporated it into your carb? If so, was it hard to do, did it work good? |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/13/18 at 23:11:44 Oooooooops! Sorry Batman, I missed the comment that stated you had completed, but not yet tested, the groove. Let us know how it works. Best regards, DBM |
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Title: Re: Stock Carb Slide Mod Post by batman on 04/14/18 at 09:04:52 Yes, opening the vacuum ports made the throttle response seem much faster, but I caution anyone to go very slow when enlarging them ,you can easily make them to large , at the same time I trimmed the spring on the TEV valve to compensate for the faster slide movement, again going slowly 1/3 of a turn at a time . If you measure the TEV spring before cutting and go to far you could add a small washer to restore spring length.I have also done a mod to the bottom front edge of the slide facing the air filter slightly easing it's rather blunt edge ,hoping to increase air flow speed at part throttle. One might also ask themselves why the leading and following edges of the throttle plate and the shaft (and screws that seem to long)and the plate it's mounted to have such square edges ? and if eased might help produce a more laminar flow.I have also reduced the throttle return spring by one coil to lower wrist strain at steady highway speeds |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 04/15/18 at 12:09:43 I finished an evaluation of the TEV and came up with some interesting results. I will provide all the details in a separate post. Bottom line, you can completely plug the #45 jet with no adverse effects. Mine ran fine with the jet plugged, but mine hardly exhibited any tendency to afterfire before I plugged the jet so it was difficult for me to determine if there was any improvement. I left it plugged since it runs real good. The spring is right on the ragged edge. If you shorten it by 1/8" it will only leave about 0.5" Hg margin over entering idle vacuum zone (i.e. the pilot air bleed might be shut off at idle). If you shorten by 1/4" it will most likely close off the air bleed when the engine is idling. I believe closing off the #45 jet should greatly improve any afterfire problems under normal driving conditions. Shortening the spring a bit should improve any afterfire issues when blipping the throttle. Will elaborate in my TEV post. |
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Title: Re: Stock Carb Slide Mod Post by LANCER on 04/15/18 at 16:08:42 Did some similar tinkering with the vacuum ports in the slide years ago with some success. I had read an article on mod's for the carb on a sporty and used that as a guide. I drilled a couple of sets of holes, both a bit smaller than suggested for the sporty and could feel an improvement in responsiveness. I added a dial-a-jet and left it like that. I did not have backfiring issues to speak of mainly because of the exhaust system I used, so did not do anything to the TEV. I may still have the article, but would need to dig through boxes if someone wanted to take a look. |
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Title: Re: Stock Carb Slide Mod Post by batman on 05/15/19 at 19:23:51 I did not drill a third hole in the slide but increased both of the original vacuum ports. the Gagetman Groove ,was done mostly to improve fuel mileage , and increased mine by 3.5 mpg /9.8 miles/per /tankful (2.8 gallons) in mixed riding . A side benefit seemed to be a smoother idle ,in spite of less fuel usage , this occurs because the fuel at idle slides down the back of the throttle plate ,encounters the grove ,bounces off the front of the basically V shaped groove ,which both further atomizes the fuel and directs this flow upward in the intake passage. This allows backing out your throttle speed adjustment screw and provides most of your increase in mpg. Without the groove fuel is less atomized , because it merely tries to follow the floor of the intake passage, the opposite of what happens at higher throttle settings. |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/19/19 at 04:23:40 I did some quick calculations on this... Stock air bleed area = 0.0145 sq-in (2 x .096") Your air bleed area = 0.0226 sq-in (2 x .096" + 1 x 0.1015") Therefore total air bleed hole area was increased 56% over stock. Another alternative would be to drill the two stock bleed holes to a #31 yielding total bleed area of 0.0226" or 56% increase in area. A 56% increase is quite a bit so I can see why the change would have been very noticeable and why we must be very careful. If we use a 3/32" for the new bleed hole area will be 0.0214 giving a 48% increase in bleed hole area. If we use a #41 for the new bleed hole area will be 0.0217 giving a 50% increase in bleed hole area. Planning on doing this later today and will start with an additional 3/32" bleed hole... |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/19/19 at 10:34:05 Fun read on CV carbs: https://www.baggersmag.com/hot-cv-carburetors?fbclid=IwAR2mwoIy-lw7R07k4BQC9eYA0Nw6ef1lKY9XIDsYuP0MmxVoTYcmjCC_m2M#page-15 |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/20/19 at 13:45:28 Just finished this up with an extra 3/32" hole in the slide. I let the engine warm up in the garage a bit and it takes throttle no problem and sounds a bit snappier but it is meaningless until I go for a ride... So funny story... I drained the tank because the last time I had to remove the petcock to get the tank off. I found if I place the speedo cable under and parallel to the tank bottom the tank WILL come off without touching the frame. Get the job all done pour the gas back in and fire it up. Starts right up and takes big throttle input. A minute or two later off choke no longer takes throttle and sputters. WHAT??? Turns out the tank was right on the ragged edge of reserve and it was no longer feeding in the ON position. Now it's all good. That was a bit scary for a while... |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/20/19 at 14:09:45 Ride Test Done... The bike has a MAC exhaust, 55 pilot, 150.0 or 152.5 main, and idle mixture screw between 1.5 and 2.0 turns for best fast idle. The needle is still a tad too low as it is just slightly lean on inputs but it is magnitudes better than it was in bone-stock trim. The engine pulls hard and strong and throttle feel and response was MUCH improved but slide opening was a bit lazy. This simple mod really works. The carb responds as before under normal part-throttle inputs -- Feels no different at all. However, if you whack the throttle now you immediately hear intake noise and get an instant change in power delivery. Before the engine would respond and spin up very lazily. Now it is right there when you hit the throttle. I've got to say... This simple little engine when jetted and uncorked really comes alive. It is still a bit flat on top and I assume that is due to the short/small cam but who cares? This little gem makes GOBS of TQ in low-mid and that is where it needs to live. For those of you wanting that last little bit out of that dull carb this is what you need. Total bleed area was increased from 0.0145 sq-in to 0.0214 sq-in for an increase of 47.7%. Just do it... |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/20/19 at 14:43:20 0E08077B797E7A4A0 wrote:
LOVE the results and thank you SO MUCH for sharing! |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/21/19 at 07:40:58 I am going to go for a longer ride later today and/or tomorrow. One thing I would like to mention is that I experienced none of the fluttering I have so often read about in other forums. This likely happens when the slide hole(s) are already large and you increase them further and/or add additional hole(s) and/or the spring rate is already low. I can definitely imagine this happening on a Hi-Po bike that already comes from the factory with a pretty good state of tune. However, with respect to this mild machine my guess is the spring rate and slide holes are on the conservative side (i.e., small) so we will not have such an issue unless we drill way too much. This is a very common mod for the Kawi KLR650 which has a similar Keihin CV carb and riders never have issues after doing it. Also... I run the bike with the right-side airbox lid removed. Would not be too good in the rain but it makes some intake noise and my wife likes it. Have any of you done the same? And... My SRX600 has a Dynojet Stage 3 kit with slide drilled and new slide spring and the secondary CV carb responds instantly - I might even call it explosively. The Yamaha engine has a comparatively light flywheel so that plays into it as well. |
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Title: Re: Stock Carb Slide Mod Post by DragBikeMike on 05/21/19 at 09:05:49 Cool! Thanks for the feedback. Looks like we have 3 or 4 of us that have tried the drilled slide with some degree of success. Seems to be a decent mod, and super easy too. |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/21/19 at 09:45:28 0F09067A787F7B4B0 wrote:
100% agreed and since so many of us do "The White Spacer Mod" this is the sensible thing to do while we are in the carb. Add to that the fact it can be done by simply removing the seat and tank and you may be a little nutty NOT to do it! I have ridden bikes with CV carbs and I've got to say this is THE LAZIEST one I have ever been on top of. Pretty happy for now... |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/22/19 at 13:29:02 Another test ride... Pulls like a freight train and makes me wonder if the stock slide EVER found its way to the top. I also put the right-side airbox cover back on and while it does not sound as cool it pulls harder on top than it did yesterday which leads me to believe it was a bit lean. If an airbox is too open you lose pressure drop and signal across the mains so I am fairly sure this was the case. The needle is still just a tad low but it is close enough for my wife. It may also be the fuel/float level is too low causing the carb to meter a bit lean. Throttle response is nearly instant compared to what it was and I think this is all a stock engine needs - A larger bleed hole is not necessary. I got it out on some twisty curves and it exits corners nice and hard now. Good power everywhere at any engine speed in any gear. No issue at all whacking the throttle WOT in fifth at 40 MPH. No hitches, no fluttering nor stuttering... No issues at all. This is definitely a MUST DO modification in my opinion... |
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Title: Re: Stock Carb Slide Mod Post by VortecCPI on 05/22/19 at 14:59:18 I just checked the fuel/float level with a length of clear line and it is just below the gasket so it looks pretty good. Either the needle is too low or it is just too fat. A larger slow jet would help but I am already between 1.5 and 2.0 turns with the #55 out so it is what it needs. I'm a throttle feel/response junkie so I may looking for a little too much out of this thing but the next time I'm in there the white spacer is coming right out. Does anybody know if the older needles were richer? |
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