Ascot Forum
First Generation Ascots => Tech Section => Topic started by: scottly on May 07, 2014, 10:34:59 pm
-
Hey folks, I've made a few pulls on the dyno lately, trying to sort my carb out. All numbers are with a large-by-huge K&N filter, a free-flowing muffler, and a 60 idle jet, which has proved to be too rich. The next pull will be with a 58 idle and a 160 main.
147.5 main: 25.7 HP, 23.09 ft/lbs
152.5 main: 25.835 HP, 23.815 ft/lbs
157* main: 27.4 HP, 24.95 ft/lbs
*Drilled jets are like a box of chocolates: you never know what you get. ;)
-
Looks like she wants more fuel. You are very fortunate to have a dyno available.
Do you have an exhaust sniffer or exhaust gas temperature gauge to help tune with?
Good luck & please keep posting up your jetting & tuning info.
Thanks
J.
-
Yes, I do have a sniffer, but found that it didn't work well with the stock muffler, hence the swap to the Muzzy. I had the best pull yet, today, with a main drilled to "167": 28.81 HP, and 27.47 ft/lbs. More importantly, the A/F graph stayed within the readable range, instead of mostly flat-lining on the lean side (top of the bottom graph).
-
Here is a larger copy of the graph. It's a bit fuzzy, but still readable for the most part.
Do you think that 30HP is possible?
-
I was hoping to report 30HP tonight, but it didn't happen. :( Even with the main drilled to "178", the sniffer was still leaner than optimum for max power. The next step is to modify the slide needle height, which, as I'd already looked into a while back, is a major PITA, compared to changing idle and main jets. I can drain, drop the bowl, swap both jets, reattach and prime the bowl in about 15 minutes these days. ;D
-
Can you use a small washer to shim the needle up like on a regular slide operated carb?
Good luck & keep up the good work.
J.
-
I rode the bike a bit today with the 178 main and a 60 idle, and I have to say it ran pretty darn well, but I still believe the stock carb is worth more HP. ;)
I've found some .023" thick washers to use as shims for the needle. Here is a pic of the stock assembly:
-
I made some dyno runs with the needle shimmed .046", and the improvement was very slight.
Does the factory manual give any theory of operation on the "air cut" valve located on the left side of the carb?
-
I played a bit with the "air cut" valve; it appears to control when the slide is allowed to rise. With no spring, the motor wouldn't idle at all, but ran OK at 3K or more. With a weaker than stock spring, it would idle, although poorly, and seemed to pull better at lower RPM than the stock spring. Not much, if any gain, can be found there, so I would not recommend messing with it.
The bike is currently jetted as follows:
idle 58
main 185
needle shimmed .046" (not sure if this is worth messing with, either. ;))
-
I have not had one of these carbs apart other than removing the float bowl.
Does the throttle slide have a vent hole in the bottom, next to the hole that the needle drops through? I just installed a DynoJet kit on a VT Ascot. Part of the kit is a drill bit, sized to enlarge that throttle slide vent. This (along with a lighter spring) is supposed to allow the slide to respond to throttle input faster. It's kind of a leap of faith type of thing, taking a drill to your parts & permanently changing the part. I have some more work to do on the VT Ascot, but initial impressions of the DynoJet kit are good.
Keep up the good work
J.
-
I didn't notice a vent hole in the slide, and I gave it a fair amount of attention...
-
You would most likely notice & remember if the slide had the small vent hole in the end. I wonder if drilling a slide vent might be something you need to enhance the throttle response.
Here is a link to the DynoJet "Tech Support" page: http://www.dynojet.com/jetkits/contact_support.aspx
You might consider giving them a phone call & seeing if they have any information that you can learn from. I had to call their Tech Dept. with a question that I had & they were very helpful.
I just don't know very much about how these CV type of carbs work.
Please keep up the good work & documentation. I'm very interested in following your progress.
J.
-
Time to update this thread! I found a PDF of the manual for the XL 500, and it has the same air cut valve, even though it doesn't have a CV type carb; once again, don't mess with it. ;)
These last two runs were with the 58 idle and 185 main. I should add that little gain in HP was recorded after going larger than 167, but there was a slight increase in torque, and more importantly, a much better throttle response. My dealer has been very kind, allowing me to swap jets after I bought two each of idle and mains, but after a point I started just drilling the original main, as it was easier than a trip to town. I'll probably trade one of the new mains for a 190, but I'm in no rush. ;D
-
Here is a bigger picture of the Dyno graph.
J.
-
So if I understand correctly, you have uncovered 5 horsepower with just jetting changes? That's great! Keep up the good work.
How about some pics of your bike, modifications & exhaust pipe. Have you tried the ThumperStuff header yet?
J.
-
Unfortunately, I never made a base-line run with the stock air-box and snorkel. The modification that resulted in the huge increase in main jet size the most was the K&N filter:
http://ascot500.com/index.php?topic=214.0
It would be nice to be able to post a 500K size graph; at least it would be easier to read, not to mention much easier for me to size. ;D
-
I believe that a box stock FT produces about 24-26HP. So you are looking at a sizeable increase.
Do you know how much difference the muffler swap made?
Do you have a K&N in the OEM airbox, or did you remove the airbox & install the filter onto the carb inlet?
I used to run K&N filters when I lived in the Arizona desert. I found that the real fine dust would get past the filter & on into the intake tract, carb, etc. Next time you have the K&N out, take a white rag & try wiping the inside of the filtered/clean side. If the rag shows dirt, it's getting past the filter. I swapped to UNI filters & have not had the problem since.
J.
-
I can't really say how much difference the muffler made; looking back at my notes, there was no difference between the stock muffler and an open header, but the bike was still very lean at that early stage of testing.
I removed the stock airbox, and adapted the K&N filter to the stock rubber tube that attaches to the carburetor. I did apply some filter oil to the inside of the tube, and now that I've had the bike off road in dusty conditions, I should check to see if grit is making it's way past the filter.
-
Are you still running the original exhaust downtubes?
Have you ever had them off & looked down inside the head pipes? I was surprised by how much the inner tubing narrows down the flow path. Here are some pics & measurements that I took from the OEM head pipes & the ThumperStuff header.
http://ascot500.com/index.php?topic=138.msg1464#msg1464
I would like to see a back to back dyno testing of the header. Some folks feel that the stock FT runs better off the OEM head pipes with the smaller tubing I.D. Maybe you can wrangle a deal from Mark at ThumperStuff for testing purposes.
Good luck
J.
-
It would be nice to be able to post a 500K size graph; at least it would be easier to read, not to mention much easier for me to size. ;D
Some wishes can come true. At least once the Admin guy figures out the right screen to make the changes. :-[ ;D
As far as re-sizing pictures, this program is free & I really like using it. You can do a whole batch of pics at one time. It's called: "Image Optimizer" at: http://xat.com/io/index.html
Keep up the good work
J.
-
Yes, I'm running the stock header, but I'd like to try the Thumperstuff one; unfortunately, the budget is kind of tight right now. I saw your post earlier about the ID of both pipes at the cylinder head end, so I measured the stock '82 XR pipe: 1.00".
This graph is the first one I posted, with a 167 main, overlaid with part of the second graph with the 185 main.
-
It would be nice to be able to post a 500K size graph; at least it would be easier to read, not to mention much easier for me to size. ;D
Some wishes can come true. At least once the Admin guy figures out the right screen to make the changes. :-[ ;D
J.
Woo Hoo!!!! That works great!!! Thanks! ;D ;D
-
Wow thats pretty good changes. Is your engine stock other than the KN air filter and the free flowing exhaust?
Have you done anything else since?
-
Yes, the motor is stock, other than the filter and silencer. I have a used White Bros WBX-1 cam, which will probably be the next modification.
-
The bottle-neck in the stock carb appears to be the needle jet; with an ID of .101", and the needle OD about .071" at the point it enters the restriction in the jet with the slide fully open, the combination will only flow as much as a 1.824mm main jet.
-
You sure your I.D and O.D isn't mixed up ?
-
The needle has an OD of .071", at the point in the taper where the needle meets the ID of the needle jet, with the slide fully open.
-
Wide open throttle tests on the dyno with the drilled needle jet showed an improvement in the air/fuel mixture from 3200 RPM up, but now the bike is waay too rich at lower speeds and small throttle openings. Next up is a Mikuni 36mm carb that was fitted to the same XR500 that the WBX-1 cam came from.
-
I put the bike with the 36mm Mikuni on the dyno today, and found that the air/fuel was actually rich at small throttle openings, when I thought it was lean. The carb has a #35 pilot, while others have reported using a #25. The #220 main jet appears to be very close to perfect.
Just because you can snap the throttle wide open at 2700 RPM, it doesn't mean you should, as the motor can't take that much carb until about 4000 where the Mikuni starts overtaking the stock carb. It's interesting that the HP basically flat-lines from about 5600 RPM to about 6850, while the stock carb started dropping off.
Just for grins, I'm posting two versions from the same two dyno runs; the first is the stock carb in blue compared with the Mik in red, plotted against RPM, and the second is plotted against MPH. As usual, tests are run in 4th gear, WFO.
-
Nice ... that mik has an accelerator pump? .... this is good stuff to know.
-
After swapping the #25 pilot in, the hesitation was even worse! At 1/8 throttle, the bike ran poorly, even in neutral, and the spark plug was sooty. I happen to have a VM 32 on hand, and comparing the two carbs I found there was a brass washer missing on the VM 36 between the main jet and the emulsion tube/needle-jet/main jet holder, which may have allowed for fuel to leak past the tube. Sorry, no pics of the washer, as it's now on the carb on the bike, but I've pointed out the shoulder where the OD of the washer would seat, drawing the tube firmly down.
-
With the missing washer replaced, the bike ran much better, enough to ride it to a bike show last Saturday. ;D
Yesterday I ran it on the dyno with the #25 pilot, and found it to be lean compared to the #35, but the tests with the #35 may have been skewed by the missing washer.
I made 3 runs back-to-back; the differences at the right side of the graphs are due to the rate of opening the throttle. Run 189 in blue was with the throttle opened somewhat rapidly, run 190 in red was with the throttle gradually rolled on, and run 191 in green was with the throttle snapped open as fast as I could. The motor bogged, then recovered, which is why both the HP and torque curves on that run start above zero; it confuses the dyno computer.
-
Pulled the carb off the bike today to replace the original #35 pilot. In the 2nd pic, with the jet removed, you can see a gap between the bottom of the washer and the tube, which simply dropped from gravity. I suspect that when I took the carb apart last summer to check the settings the washer fell out and I never noticed it.
-
I tested my new pipe today. :) :) The graph shows the new pipe compared with the best previous run:
-
Congratulations you passed the 30HP level. 8) This illustrates how much a free flowing exhaust can release more power.
Have you noticed any change in MPG, or range on one tank of fuel?
Please keep up the good work & documentation.
J.
-
I've tested three different exhausts now. The first, that resulted in the 2+ HP increase over the stock pipe had primary tubes 26" long, measured from the sealing surface at the head, around the outside of the bends. The second used the straight lengths from the stock outer pipes to extend the length to 34". The third had 22" pipes. The 22" pipe had a slight advantage over the 26" from about 3700-5100 RPM, while the 26" had a slight advantage from about 5600-6700. The 34" pipe had a noticeable drop from about 4300-5100, but interestingly all three pipes showed the same peak torque.
-
This is the next pipe up for testing:
(J, you asked about MPG; all I can say is the dyno tests of the 3 previous pipes appear to have consumed over 1 gallon of gas. ::))
-
After testing a few different configurations of pipes, I finally settled on a design that seemed to be the best compromise of usable power band and weight. As far as being a "tuned" exhaust, I started it up without the muffler to heat up the header paint, and it rattled my ear drums! I did a couple of dyno runs with the stock muffler removed from the stock header, and it was no where near as loud as just blipping the throttle with this pipe. It's even loud when shutting the throttle; it sounded a bit like the scene at the beginning of "The World's Fastest Indian" when Burt wakes up his neighbors. ;D
-
As Chuck has raised the question of pipe size, I thought I should update this thread. ;)
I did try an XR 500 pipe, adapted to the FT, and found it caused a loss of both torque and HP compared to any combination of primary pipe lengths with an ID of 1.270". The XR pipe dyno run is 223, plotted in green.