Air Box and Air Filter
Slide Cut Away
I started out to present two articles that I assumed would say the same thing in two different ways and maybe make it a little easier to understand. While agreeing on most things, the authors part ways on a couple topics. Since I installed a White Brothers 38mm Mikuni on my SR and have not done any of the modification or jetting changes on the stock carburetor, I will simply present the views of each author and let the reader determine there own course of action. The two articles are entitled "Once Over Lightly" by Joe Minton, in the August 1986 issue of Motorcyclist Magazine and a two part article in FSSNOC/Thumper News # 38 & #39, the "SR Odd's and Ends" by Bill Vonderhey.
If anyone has any experience with tuning the stock carburetor on the SR500 and wants to comment on the information provided, I would be glad to include it here. Send any comments to email@example.com
Check out the Guided Tour page to see where these parts are located and what they look like.
Vonderhey: Before evaluating your carb's performance, be sure all other systems are working properly, I.E. ignition-timing, spark plug, valve settings, etc.
Air Box and Air Filter (TOP)
Minton: The stock airbox is actually very good, with a large intake horn and a carefullly shaped tube leading to the carburetor. The stock filter, though, is terrible and should be replaced with one from K&N. There is no substantial gain from removing the airbox unless you are planning extensive engine work.
Vonderhey: I decided that the airbox was an obstruction that had to go. It didn't apear to me to be of high quality anyway, and it's removal provided on of those previouly mentioned opportunities for a little extra "free" horsepower.
Carburetor Size (TOP)
Minton: The SR's head doesn't breathe well enough to justify a larger carburetor. I tried both the 32mm carb that was stock with my '81 and the earlier 34. There was no noticable power difference, but the 32, because of its internal shape, works better at part throttle.
Pilot Jet (TOP)
Vonderhey: I do not recommend increasing pilot-jet size over stock; the stock jet seems to work fine and we don't want to start fouling plugs ath the worst possible time - when you're idling in traffic at a red-light. The pilot-jet affects the carb's mixture mostly in the idle to 1/8 throttle range and seems to perform O.K. as-is.
Slide Cut Away (TOP)
Minton: The K&N filter's low restriction has a severe effect on the stock-carb tuning. The restrictive stock filter acts as a built-in choke, and the carb was tuned to run with quite a pressure drop built into the system. Most who have tried to correct the stock jetting to accommodate either the K&N replacement or a larger filter that entirely replaces the airbox have been frustrated because raising the needle or replacing the idle jet doesn't help very much. It is necessary to modify the stock slide to tune it for the less-restrictive K&N. The stock throttle-slide cutaway is 4.5, large for a muffled engine. It needs to be about 3.5 for the K&N and Kerker header pipe. I have found that .05 inch removed from the bottom of the slide corrects the stock carbs tuning. This material removal lowers the slide and corrects the severe hesitation just off idle. You can do this with a sharp file and, if you are reasonably careful, can save the price of an aftermarket carb you really don't need.
Vonderhey: I am not aware of any richer slides (with smaller cut-aways) available for the stock SR carbs, so a modification is in order.... File .050" off the bottom of the slide, making sure that the bottom edges are square with the slide and that all sharp edges are slightly bevelled and smooth. Don't file the cut-away area, just file the bottom of the slide where it contacts the carb-body. The mixture should now be improved from idle to 1/4 throttle and the spitting-back and hesitating should disappear.
Note: In the Minton article, Joe refers to the stock slide cut away as being 4.5, Bill Vonderhey refers to the stock cut away as 3.5. My manual for the 78 and 79 agrees with Bill Vonderhey as being 3.5. I would also note that they agree on the amount to be removed from the bottom of the slide (.050"). This may be a typo in the Minton article.
Jet Needle (TOP)
Minton: In addition to the slide modification and the larger main jet, you need to raise the needle 1mm and maybe go down a needle jet or two. If you have an early SR, you can simply move the needle clip down one position from stock. The later carb does not have notches in the needle, and you have to thin the plastic spacer on top of the needle to raise its position. A file does the job, but be careful not to take off more than 1mm.
Vonderhey: On '78 and '79 SR's, this mid-range mixture can be enriched by raising the needle in it's clip. On '80's and '81's, you will have to file .040" (1mm) off the thickness of the needle's little plastic washer. This will have the same ettedt as raising the needle one notch (the notches on jet needles are 1mm apart).
Note: Here I think both Minton and Vonderhey are saying the same thing. Move the clip from the 3rd (middle) slot to the 4th slot (the 1st slot being the top one). Having never seen the 80 and 81 carburetor, I'm not sure how the plastic washer can be made thinner and raise the needle.
Needle Jet (TOP)
Minton: The needle jet rests on top of the main-jet stand and can be gotten out by removing the stand. Most SRs come with a P-8 needle jet, which my be too rich at part throttle. Mikuni makes both P-6s and P-4s which are leaner. After you have run your motorcycle with the other modifications for a while, you might try on of these smaller jets to improve milage.
Vonderhey: If after raising the needle the midrange is still too lean, a change to a bigger needle-jet will be necessary. When first wrestling with this problem, a quick survey of dealers indicated that the stock P-8 needle-jet was the biggest available size. To increase fuel-flow through the stock jet, either it's hole has to be made bigger or the needle made smaller. Adapting a different needle with it's different thickness, curves and taper would be a "shot-in-the-dark"....... A lot of time on the calculator finally showed that drilling the standard P-8 jet with a No. 36 drill would increase the jet's hole size from .104" to .106" and would provide the proper increase in fuel-flow. This was done and proved to be just the cure for the mid-range "blahs". Months after doning this, brother Tom and I found out that there is indeed a bigger size needle-jet for the SR; it shows up on either Mikuni or Yamaha (cant't tell you which because I didn't write it down!) parts manuals as "Next-larger-size", with no number designation, such as P-10.The hole in this jet appears to be exactly the same size as the drilled-out P-8.
Note: Here is where the major difference in the two articles appears. Minton is calling for leaner needle jet to get a leaner mixture in the midrange, which seems inconsistent with raising the needle to enrichen the mixture. Whereas Vonderhey is drilling out the stock needle jet to get more fuel flow through the jet and a richer midrange. I was just rereading this section and was wondering if Joe Minton is using the 32mm carb and Bill Vonderhey is using the 34mm, could this account for the difference in opinion on richer or leaner jet needle? Drop me a line and tell me what you think. firstname.lastname@example.org
Main Jet (TOP)
Minton: The K&N requires you to increase the main jet three sizes (up to 330 for the 34mm carb and 290 for the 32).
Vonderhey: If you remove the stock airbox and filter and use an after-market exhaust, I would go up two sizes and work from there. If you keep the stock filter and exhaust, I would recommend starting with the stock main-jet.
Accelerator Pump (TOP)
Minton: The stock accelerator pump is fine, and I don't recommend fiddling with it.
Here are some questions I have received about SR500 carburetor modifications.
From: Roberts Vernon <ROBERTV@NTSB.gov> via the Thumper
Q: Are the Minton Mod's from the SR500 Web Site still the gospel or has Vonderhey Mod's been more accepted. Or, does one set apply to the 34MM carbs on the 78's and 79's and the other to the 32MM carbs on the 81's and 82's???
A: The major difference between the Minton and Vanderhey articles seem to be in the air filter and exhaust as well as carburetor size. Minton uses the 32mm (80-81) carburetor with the stock air box and K&N filter. Minton exhaust modifications consist of a larger ID head pipe and stock muffler.
Vanderhey on the other hand appears to be using the earlier 34mm carburetor and removes the air box and uses a slip on Uni open cell foam air filter. Vanderhey doesn't say what, if any, exhaust modifications were made.
Since both parties agree on the slide modifications I think this is a good place to start. The less restrictive intake and exhaust modifications you make will probably require richer setting in the upper throttle settings (needle and main jet). I'm afraid it's a trial and error thing, with all the modifications and combinations of modifications the best you are going to get is a starting point. If your going with a new air filter in the stock air box, start with the Minton Mods. If you change the exhaust and pitch the air box, start with the Vanderhey Mods.
Hope this helps, Ron