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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Evolution of a Hotrod - Part 2: The Airbox /cgi-bin/YaBB.pl?num=1621150483 Message started by DragBikeMike on 05/16/21 at 00:34:42 |
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Title: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:34:42 This is the second in a series of reports intended to document the results of progressive modifications to the LS650 engine. Part 1 outlined the project, set the rules, and established the baseline performance values for a box-stock LS650. Part 2 examines the airbox, air filter, and stock carburetor jetting. The carburetor jetting is included because almost any significant alteration to the airbox requires a jetting adjustment. An internal combustion engine needs air to produce power. Most modifications intended to increase the power output of an engine are intended to get more air into and out of the cylinder. High performance air filters, larger carburetors, velocity stacks, high performance camshafts, ported cylinder heads, headers, and free-flowing exhaust systems are all focused on moving air and combustion products through the engine. Superchargers and turbo chargers have one function, cram more air into the cylinder. Larger displacement engines generally make more power. A bigger engine can induct more air on each intake stroke. Hmmmm! More displacement = more air. It’s all about the air. Additional air permits the introduction of additional fuel. It’s an unbeatable combination. If you can get more air into a cylinder, then you can add more fuel. Seems to me the logical place to look for more air is where it first enters the powerplant, the airbox. What I am about to describe worked good for me. If you decide to try this stuff on your own, you assume responsibility for the outcome. If you don’t have the skills, don’t do it. If you don’t understand something, STOP and get help. Get a manual. Read up. Comply with ALL the safety requirements outlined in the manual. Make sure you know what you are doing before attempting any of this stuff. Let’s get started. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:36:06 The LS650 airbox is very restrictive. Air enters the airbox at the snorkel, located under the seat. There’s a bunch of junk in the way. The seat, wires, connector plugs, etc. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:36:45 If you remove this decorative side cover (I call it a “tin”), |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:37:22 you will find the plastic airbox cover underneath the tin. Remove the single screw that holds the plastic cover in place, |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:38:03 and you have access to the air filter element. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:38:53 Look into the airbox and you can also see the bottom of the snorkel. Lookin bleak already. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:40:01 If you want to see if your bike will run better, just leave that plastic airbox cover off. Put the tin back in place. Give it a whirl, but be careful, it’s gonna run lean. Don’t go wide open throttle (WOT) in any gear higher than 3rd. With the cover removed, my 3rd gear acceleration time dropped by a little over 1-second (10.48 to 9.35). Don’t run your bike like this for anything more than a quick test. It will be too lean. You can hurt it if you get carried away. So, a simple test reveals that there’s some significant performance lurking in that airbox. I contend that the only modification necessary at this point is to leave the plastic cover off, put the tin back on, and rejet the carburetor. The tin provides adequate weather protection. You can’t turn a hose on the thing, but under normal pleasure riding conditions everything works fine. I’ve spent about 2 years riding around without the airbox cover. No problem whatsoever. The only parts needed are #150 main jet, a #55 pilot jet, and a suitable washer (.27” OD x .12” ID x .06” thick). The washer is used to raise the slide needle. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:40:46 As I mentioned in Part 1, the brass plug that covered my idle mixture screw has already been removed. I left the screw at 2 turns, which is exactly where I found it when I removed the plug 3 years ago. If you haven’t already removed the plug you might want to do that. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:41:35 Remove the float bowl. I suggest you convert the float bowl screws from pan head to socket head. These are M5 x 0.8 pitch x 15mm long stainless-steel socket head cap screws. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:42:13 If you still have the stock battery box, it might be a bit tricky to remove the float bowl. A smaller battery makes things a lot easier. This is a Sportster battery. Lots more room with this setup and replacements are easy to find. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:43:02 Change the main jet. A #150 works good. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:44:13 Don’t loose the washer for the main jet. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:44:56 The pilot jet is optional, but I prefer the #55. It’s a bit fat, but afterfire is greatly diminished. Overall response and cruise are better too. Once the jets have been changed, reinstall the float bowl. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:46:03 The slide needle adjustment is a bit harder. It’s important. You must do it. If you don’t adjust the needle, you will run too lean in the range where you spend the most time, part throttle. I remove the seat and fuel tank to gain access to the top of the carburetor. Remove the top of the carburetor. Again, converting to socket head screws makes your life a lot easier. These are M5 x 0.8 pitch x 10mm long stainless-steel socket head cap screws. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:46:57 Carefully remove the spring. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:47:53 The slide diaphragm is fragile. Handle with care. Gently work it out of the groove in the top of the carburetor, and then pull the slide up and out. Note the orientation tab in the lower right corner of the picture. Those two small screws you see can be difficult to remove. They lock the slide needle in place. Don’t attempt to remove the screws until the slide & diaphragm have been pulled out of the carburetor. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:48:40 The slide assembly has a black dry-lubricant bonded to the outer surface. Keep it clean. Don’t scratch it. Don’t try to clean it with chemicals. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:49:20 To adjust the slide needle, you have to remove those two small screws that I mentioned previously. I find that an impact driver with an extra-long bit is the best tool for the job. It’s very easy to round out the head of the screws. The impact driver helps to prevent that. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:50:06 A short piece of plastic pipe allows you to support the slide while using the impact driver. You don’t want to damage the slide or the needle. They must be properly supported while you remove the screws. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:50:41 It’s best to use an assistant to hold onto the slide while you lightly tap the impact driver. You need like four hands to wrestle with the thing. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:51:20 Once the two small screws have been removed, the lock plate and needle assembly can be removed from the socket in the bottom of the slide. Pay attention to how the small parts stack up. White .110” thick nylon washer at the top, then the e-clip, plain steel washer under the e-clip, then the spring at the bottom. Like this. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:52:03 Raise the needle about .05” by replacing the .110” thick white nylon washer with a .06” thick washer (.27” OD x .12” ID). You can find all sorts of suitable washers at the hardware store. Just make sure that the ID fits over the needle and the OD isn’t larger than the original white nylon washer. It must fit into the socket in the bottom of the slide. This shows the original .110” thick washer replaced with a .06” thick washer. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:52:38 I find that it’s a lot easier to install the needle assembly back into the slide if you use a surgical clamp or similar implement. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:53:14 Install the assembly with the slide on its side so that the spring and lower washer don’t fall off. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:53:47 Then capture the needle assembly with the plate. Make sure that the two smaller notches in the plate align correctly with the two orifice holes in the slide. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:54:23 Like this. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:54:58 A long magnetic screwdriver is the hot ticket for installing those two small screws. Pull down on the needle to compress the spring. Then install the plate, align it with the orifice holes, and install the screws. Lightly tighten the screws. No need to make them super-tight. They’re hard enough to get out as it is. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:55:36 There’s a groove in the top of the carburetor that accepts the raised lip on the periphery of the diaphragm. When you install the slide assembly, you must make sure that the lip on the diaphragm fits nicely into the groove all the way around. You also must pay attention to the orientation tab on the diaphragm. It goes in the notch (circled in red). |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 00:56:21 It should look like this. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 01:01:39 Reinstall the slide spring and the carburetor cover. Be careful not to pinch the diaphragm. Then reinstall the fuel tank and seat. Ready to rock & roll. The motorcycle runs so much better. You won’t believe it’s the same bike. It has loads more power. The obnoxious afterfire is almost completely eliminated. No more KaBoooms. It idles good. The lean surge and hiccups at part throttle are gone. It runs freely past 6500rpm in 1st through 4th gear. I did a noise test with the NIOSH app. Noise level at 1300/3000/4000 rpm is still 57/74/77 dB. I was surprised. I thought it would increase more than 1 or 2 dB. Acceleration tests were impressive. Second Gear 4K to 7K: 4.32 seconds Improvement: 0.28 seconds Third Gear 4K to 6.5K: 8.04 seconds Improvement: 2.44 seconds Fifth Gear 3.5K to 5K: 7.78 seconds Improvement: 1.69 seconds Fifth Gear 3.5K to 5.5K: 13.62 seconds Improvement: 8.81 seconds Now, it will pull all the way to 5700 rpm in fifth gear, where before it would not pull past 5500 rpm. On the freeway, it has reserve passing power. You can merge onto the freeway without fear of being run over. Fuel mileage was good. I averaged 55 mpg over several tanks of gas. Fuel economy dropped 2 mpg. That’s fine with me. The dramatic improvement in performance is well worth 2 mpg. Chalk one up for the #55 pilot jet. It seems to run a bit cooler. On the freeway at steady state, it hangs in at about 310F. Highest I observed was 319F. I attribute the reduced temperature to the rich part throttle mixture. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 01:06:08 For under $20 bucks you can make dramatic improvements to drivability and acceleration. Nothing has to be permanently modified. It still uses the stock air filter element. It can be restored to the completely stock condition in a few hours. It’s not noisy. Fuel economy is still good. Almost 9 seconds off the 5th gear pass. What’s not to like? I know this rejet mod is well documented. There are several posts on the forum that give you the step-by-step. But I have never seen any posts that test the modifications and report measurable data. I wanted to do the mod starting from scratch, and then test it to show what you can expect. You can see from the acceleration test data that this simple modification makes a big difference in performance. It really is a lot of bang for the bucks. Next, I want to move on to the exhaust system. I think that will force me to take a hard look at the stock carburetor. I can tell that it’s fuel delivery is grossly restricted by the stock needle. Some simple measurements and calculations show that the stock needle limits the maximum main jet to about a #155. Jets larger than #155 don’t have much effect on the air/fuel ratio. As I amp up the air flow through the engine, I think it will shine light on the carburetor and force me to address the slide needle. I hope some of you find this project informative and can use the data I collect to help make decisions on your own project. If you have suggestions or comments on my test methods, post a reply so we can discuss. As mentioned earlier, if you have a particular component or modification that you would like to see included, let me know and we can collaborate. Best regards, Mike |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by Dave on 05/16/21 at 09:55:11 DragBikeMike: Since you are located at Sea Level.....your #55 Pilot Jet might be an acceptable level of "rich:". For those of us who live at higher elevations - it likely would be overly rich. On most of the bikes I have worked on a #50 works fine - only on one bike did I feel that the #52.5 ran better. (The elevations where I live range from 500MSL down at the Ohio River - to 825 MSL at the hilltops, and I have never needed to use a #55 Pilot Jet). |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by zipidachimp on 05/16/21 at 14:48:24 This seems to be an appropriate time to ask: what is the purpose of an extended manifold between carb and cylinder: http://4.bp.blogspot.com/-HIQW94ZRtdg/TjlLwWPjITI/AAAAAAAAAe0/-k7UQSKJsrg/s1600/Manx+Peirce+CaraibiRockers.jpg 50 years ago I raced a 250 Ducati single with a 3" manifold. No one could explain what it did, but it ran better with it than without. Would an S40 perform the same? Cheers! 8-) ps: https://www.eurojamb.com/products/amal-gp-carb-manifold-webco-norton-bsa-350-441-500-goldstar-manx |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 14:54:15 Great comment Dave. I’m glad you brought up elevation, and also glad you shared your view on pilot jet size. My neighborhood is 627 feet above sea level. I do my acceleration tests at 1014 feet above sea level. My riding is limited to a range of about 5 feet above sea level to 1168 feet above sea level. You’re correct, I’ve got a pretty good column of atmospheric pressure and can probably tolerate a little more jet than the folks in Denver. I was curious why you recommend a pilot jet smaller than 52.5. Then I remembered that the early model carburetors were set up a lot different from the later models. The parts fiche shows that the carbs from 1986 through 1995 came with a #47.5 pilot jet. Only one pilot air bleed is shown. It’s piece 12 and is identified as a #70. The factory service manual lists the pilot air bleeds as a 2mm and a #67.5. That tells me the early model carbs had a fixed 2mm air bleed (probably just a drilled hole) and a replaceable pilot air bleed (#67.5 or #70). I have a 2016. My as-delivered pilot jet was a #52.5, and my pilot air bleeds were #45 & #230 (pieces 12 & 40, both removable jets). That thing ran about as lean as you could get and still be able to operate. As delivered, it ran terrible. It didn’t care if it was at sea level or all the way up on the Pali Lookout 1168 feet above the ocean. It took forever to warm up to a point where it could be ridden, and even then, it ran terrible. I can’t imagine putting a smaller pilot jet in that thing. Note that my air mixture screw remains at 2 turns, exactly where I found it when I drilled out the plug. My idle starts to drop when I turn the screw in ½ turn. I think that’s just about right. There’s almost no afterfire. I can get a puff of black smoke if I start continuously blipping the throttle (it starts about the fourth or fifth blip). That’s just the muffler loading up. Idle is smooth and steady. Response is excellent. Do you think it’s possible your experience with the stock carburetor is associated with an early model carb, the one with the different pilot air bleeds? Do you think I’m overlooking something? Most comments I’ve seen on the forum report extremely lean as-delivered mixtures, so switching to a smaller pilot jet doesn’t seem logical. I’m not trying to be a jerk; I just want to make sure I’m not missing something. This is a good discussion, and it deserves a little more exploration. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/16/21 at 15:00:05 Zipidachimp, I have some reference books that discuss the intake tract length. It's not something I have studied much, but I know Batman was big on that particular modification. I believe he felt that he achieved some good results. As far as I know, it doesn't make too much difference whether or not you install a spacer between the carb & head, or you just extend length by adding an airhorn or velocity stack. It's a tuning type thing that can alter performance over a specific range of operation. I'll look over my books and see what I can come up with. Thanks for the suggestion. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by verslagen1 on 05/16/21 at 15:49:15 In the case of the stock carb, I believe it's important to have the 'air horn' at least the length of the rubber duckie between the carb and airbox to straighten out the air flow. But it maybe needed to make the 'eyebrow' port work correctly. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by Dave on 05/16/21 at 16:07:26 07010E72707773430 wrote:
All of the carbs I worked on had a #47.5 pilot jet installed when I took them apart to clean and re-jet. The increase to a #50 Pilot, 3 washers in place of the white spacer on the needle, and a #150 main was a drastic improvement in how they ran. On one bike it was necessary to bump up to a #52.5 pilot, and on my own bike the #50 was fine but I had to take out one washer and only run two before I could correct the mid throttle surging (I tried a larger pilot jet but it didn't cure the issue and a #50 pilot and two washers was the cure). |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/17/21 at 09:42:22 Dave: It sounds like those carburetors you worked on were all pre-1996. By any chance, did you take any photos? I think it would be a valuable piece of info to capture and share. Seems to me, the older versions (pre-1996) need a #50 pilot jet, while the newer versions (post-1996) need a #55 pilot jet. That would be good info to share. Since the factory service manual and parts fiche aren’t perfectly in agreement, photos would be the best reference to differentiate between the old & new versions of the carb. This is the pilot air bleed arrangement for the late model carbs. Two removable pilot air bleeds, a #45 and a #230. If anyone has photos of the pilot air bleeds on an older carb, please post so we can all see what it looks like. |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by Hiko on 05/17/21 at 21:01:08 My bike has a 150 main jet and a 52.5 pilot jet The pilot screw is open 2 turns A dyna catalyst muffler is fitted. The carb needle has a half thickness washer Today I stripped the carb slide out to confirm the washer thickness It measured 1.52 mm I reassembled the carb and removed the airbox door and went for a test ride' First thing I noticed was the different sound slightly louder but deeper The difference in performance was quite noticeable and if all these mods were done at once to a stock bike I believe the difference would be dramatic. I am very pleased with the result. I may have to change or clean the air filter more often but well worth it I think Thanks again to Drag Bike Mike for his efforts |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/19/21 at 22:06:02 Hiko, I'm glad the mod worked good for you. Sounds like you ended up with a .06" washer (1.52mm). Can you tell us what year your bike is? Do you have two removable brass pilot air bleeds under the diaphragm, or only one pilot air bleed? Can you tell us what size your pilot air bleeds are? If you have an older carb (pre-1996), can you post pictures of the area under the diaphragm? Is your slide needle the older style 5C16 with three grooves? If so, what groove is your e-Clip installed in? Thanks for the feedback. Best regards, Mike |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by Hiko on 05/20/21 at 22:06:15 My bike is a 2006 but was not registered until 2012 and then hardly used at all until I got it in 2018 when it had 2800 km on it There is only one groove in the carb needle and the washer fitted is a plastic one I made at 1.52 mm to replace the original plastic one I did not take much notice of the air jets under the diaphragm so not sure what they are like one or two I played around with the pilot screw tuning today but to be fair with the bike hot and the idle slow there did not seem to be a lot of change from one turn out to nearly three so left it at about 21/4 |
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Title: Re: Evolution of a Hotrod - Part 2: The Airbox Post by DragBikeMike on 05/21/21 at 10:24:38 Thanks for sharing Hiko. |
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