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Post by blacksmithjk on Jun 22, 2008 18:39:10 GMT -5
36MM Mikuni Flat Slide Pumper Carb –
I have a 1978 SR 500 that I have been working on as a project for the last couple of summers. I am having a problem or perhaps two that are driving me crazy and keeping me from enjoying and trusting the bike. Last fall on my last test ride of the season I ended pushing the bike two miles to my house. I don’t ever want to do that again.
I had problems with the original stock carb that made it not worth saving so I bought a 36MM Mikuni Flat Slide Pumper Carb from Thumper Stuff. I was also having a week spark so I had the stator rewound (low RPM circuit didn’t test properly) from a place that came highly recommended in New England.
I am currently having two problems that I am hoping one or a number of you might help me solve. The first is the fouling of the plug with serious black coating after a short ride.
The second one has to do with starting after I turn it off. It will restart immediately after the engine has stopped but then there is a period from 5 to 30 minutes where there will be no indication that anything has happened when you kick it over. It will start on the 1st or 2nd kick when it is cold with no chock needed in 45 degree weather. I appear to get a good spark during this period as it is white and jumps a quarter inch gap I have on my tester.
The carb is clean (was brand new with very few miles on it. Fifty miles or less). Fuel is fresh and of good quality. The jetting is 25 on the Pilot and 137.5 on the main. The adjustment screw gives it’s highest engine rpm from full in to one turn out. In first and second gear at about 3000 rpm there seems to be a minor surging, only noticeable through minor feelings through the handle bars.
A couple of things that could factor in are a lightly oiled K&N air filter and a HyperPak ignition module that is sold on the Internet. The valves are properly adjust and have been checked after being run and the gap measured. The same is the case with the timing.
I have tried the whole jetting thing but never got past the problems I have listed.
I would appreciate any suggestions on what I can try to get this bike to a trustworthy running state.
Thanks
Carb
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Post by stevec5000 on Jun 22, 2008 19:13:03 GMT -5
It's WAY too rich! You need to put in smaller jets and/or adjust the tapered needle and the float level. I would start by putting everything in the carb back to stock. The best way to adjust it is on a dyno but if you don't have access to one of those you can use a Colortune to setup the mixture at idle and then do some medium and high speed test runs with an exhaust gas analyzer strapped to the tank. Or you can just guess but that takes a lot longer with lots of trial and error but keep making the jets smaller one size at a time until it runs the best and doesn't foul the plug.
BTW, a bad coil will cause hard starting and Yamaha's always seemed to have weak coils so I would test the coil output and the plug wire and cap to make sure you have a good spark.
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Post by blacksmithjk on Jun 22, 2008 20:07:41 GMT -5
Thank you. I was told that this is stock but I had my doubts. I can't remember gut is a 125 leaner than a 137.5 or is it the other way around.
As per the coil, do you know of any aftermarket that would work. I know that the SR coil is a bit of an odd ball in it's rating.
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Post by stevec5000 on Jun 22, 2008 20:34:18 GMT -5
Thank you. I was told that this is stock but I had my doubts. I can't remember gut is a 125 leaner than a 137.5 or is it the other way around. As per the coil, do you know of any aftermarket that would work. I know that the SR coil is a bit of an odd ball in it's rating. By putting it back to stock I mean the stock sizes for the SR, not what came in the carb, whatever that was, but it might be hard to determine since the stock carb uses an oddball jet that isn't measured by jet size but by flow. Smaller sizes are leaner but I wouldn't worry about the main jet yet unless you drive all the time at full throttle. I'm currently using a #140 jet if you measure the hole size but it's still a bit too rich at full throttle when I get up to 70 MPH. It's a CDI coil so any coil suitable for a CDI ignition system will work with a low primary resistance of approx. 1 ohm but will be hard to mount on the bike. I'm using one I got at Walmart for a Ford that's mounted back under the seat. |
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Post by stevec5000 on Jun 22, 2008 20:57:17 GMT -5
You'd better get the Tuning Manual for that carb and read it over and over until you understand how to tune it for each range of engine speed, at idle, full throttle and mid range because if you are going to adapt a different carb besides the stock one you will have to be the one who figures out how to tune the jets to make it work properly.
You'll also need a bunch of tools and test equipment to do it properly unless you pay someone else to do it for you.
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Post by davedunsboro on Jun 23, 2008 6:34:04 GMT -5
I've had a similiar problem before & when I went to buy a new plug ,the guy in the yammy shop took one look at the old plug & said "your running it on 98aren't you" I just gave him a dumb look & said "why not" . His theory was that carbied bikes cant handle it as they can't burn it properly & to go back to 92 .Anyway I put in a new plug & changed my fuel & I haven't had a problem since & the plug has been OK & no problem starting either ! 98 seems to work on EFI & it adjusts the mixtures to suit the fuel type & it will burn it OK whether it be 98 or 92 .A carbie can't do this but when these bikes were built in OZ we only had super(leaded) & standard (unleaded) & I don't actually remember putting standard into anything as it used to make it ping & lose power! I hope this info is useful to your problem cheers Dave.
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Post by davedunsboro on Jun 23, 2008 6:55:51 GMT -5
I also seem to remember standard being a much lower octane as well possibly into the mid to high 80s . I also know of guys that swear that 10% avgas is the go too but its hard to get? I've got 4 SR500s & they all ping if in too high a gear going uphill ! If you use that lead replacement stuff it just loads up the plugs as it settles at the lowest point(carbies float bowl) I found the only answer was to buy a bike with EFI to beat the problem(SRs are fun but you cant sit on 180kph for too long) Not that I do that all the time but if I go for a ride with mates with Ducati's etc they get real pissed waiting for me to turn up ! Good luck with it ! Cheers Dave.
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Post by jdbrees on Jun 23, 2008 7:01:31 GMT -5
I have to agree with Steve, you're running too rich. If it starts in 45 degree temperature with no choke or throttle the pilot or idle circuit is supplying too much fuel. The flatslide 36mm pumper carb that Thumperstuff sells is technically a Mikuni TM36-68. Factory recommended jets are as follows: mainjet 130, pilot jet 12.5, needle jet P8, needle 9dzh6-50, air jet 1.0 . The first thing I would do is replace that pilot jet - I feel it's the source of all your problems. The 25 you have in there is just too big. I hear of others using a range of 11.5 to 13.5, but nothing near 25. Also, note that the long screw with the large white knob is an idle speed adjustment - it controls the open height of the slide at closed throttle. I use it as my "hot start" fix - I turn it in 3 turns and she starts first kick every time, then 3 turns back to a 1,000 RPM idle. The pilot or idle circuit adjustment is the knurled brass screw at the front of the carb, just underneath where it attaches to the engine. This screw controls fuel, not air! Turning it in will will make the idle circuit leaner, while turning it out will make it richer. I've not been able to find a manual for the TM36-68, but there is online a fine tuning manual for the Mikuni HS40 carb at the Mikuni America website. www.mikuni.com/pdf/hs40_manual.pdf. This is an excellent guide for tuning your TM36-68. |
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Post by stevec5000 on Jun 23, 2008 7:47:56 GMT -5
I'm using a #17.5 pilot jet in my HS40, it had a #15 jet at first but that was slightly too small and caused it to pop a lot just off idle.
The jet has to be customized for the kind of air filter and muffler he has but you are right, jdbrees, if he has a #25 it would be much too large.
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Post by jdbrees on Jun 23, 2008 10:57:56 GMT -5
Hey Steve,
How is that HS40 working out for you?
What kind of exhaust are you running?
Porting work?
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Post by stevec5000 on Jun 23, 2008 11:30:05 GMT -5
The HS40 works great! It passed with very low emissions, is easy to start now and gets about 60 MPG.
The engine is all stock except for the Supertrapp chrome muffler and pod filter.
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Post by jdbrees on Jun 23, 2008 20:33:27 GMT -5
Wow - 60 mpg is great! I've got the TM36-68 from Thumperstuff, a 1 3/4" down pipe, and an Emgo Dunstall replica muffler and I'm getting about 46.5 MPG last I checked. My plug says I'm running a bit rich - black and dry. I'll have to experiment a bit to see if it's the idle circuit or the needle circuit. I know my main is on the small side, but I rarely hit WOT.
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Post by caferacercarl on Jul 3, 2008 3:43:45 GMT -5
All my race engines burn a just off white plug on 98 octane under load/wide open throttle at under 13 to 1 air fuel ratio, measured on state of the art gear, these bikes have the advance to burn the fuel. go to a Dyno and have it sorted, ask curly or wizz954 on this forum, or stew ross for that matter. its quick, accurate and cheap, by the time unleaded registers a colour, its WAY WAY too rich.
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