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Post by Bronson7 on Apr 28, 2005 18:14:31 GMT -5
What rpm are folks running at 55-60 mph? I'm trying to sort out my 75 mph top end. Mine is turning at around 3800-3900 rpm @55 mph. I finally was able to install the stock pipe, had the carb apart and replaced some diaphragms, rebuild kit etc, but still, only 75 mph. All jetting is stock. It winds through the gears pretty quick. I'm begining to wonder if I have an undersized front sprocket. Just curious as to your rpms vs. speed. Thanks.
Bronson7
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Post by canucksr on Apr 29, 2005 9:37:05 GMT -5
Hi Bronson
The stock SR500 should have a 16 tooth countershaft sprocket and a 44 tooth rear sprocket. It's odd that your bike won't pull past 75mph. My bike is revving at about 5000rpm at 65mph in 5th gear and will pull up to about 90mph. It's got a single walled header pipe and universal Supertrapp with about 14 discs in it. The carb is a 34mm TT500 Mikuni with a 280 main jet and the needle raised one notch. Also a K&N air filter with the airbox cover discarded.
Maybe your bike has low compression or is running too rich. Is the sparkplug the correct colour (tan brown) after you run it? Is there some sort of restriction inside the exhaust? They tend to rot from within sometimes and that would slow you down.
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Post by StewRoss on Apr 29, 2005 17:15:47 GMT -5
Hi,
Yep, one of mine, an H model ('81), had a problem where it wouldn't go much faster than 130 kmh or a VERY grudging 140 down a steep road. I put that down to the standard muffler.
I had admittedly done about 95,000 kms or so, however I bought it from a friend at about 55,000 and it didn't really change in performance over that time. I imagine that there is probable internal problems inside the decaying muffler.
It is now slowly undergoing a transformation and if it doesn't reach over 200 (or more) I'll be disappointed.
The first thing any SR owner should do is to change the front sprocket to 17 teeth. In its low state of performance it probably won't go as well up top but it extends the usable rev range somewhat.
If you do have the original muffler fitted you could always replace it with something less restrictive (louder) and see the difference.
I'm still convinced about replacing the original carby with something like a 36/38 Mikuni, unless you are trying to keep it standard. I have one that I am keeping standardish and I guess I wouldn't replace the carb on that one. My cafe racer however, has a 38 mm Mikuni from a 350 TZ and I have a 38 mm Kei Hin smoothbore for the other one, once it's ready.
SR
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Post by Bronson7 on Apr 30, 2005 8:18:34 GMT -5
Well, I counted all the teeth and I'm running stock (16/44). When I bought the bike it had a Supertrapp exhaust and would only go to 75 mph.I had planned on rejetting but in the interim, The muffler developed a crack half of it's circumferance. I was finally able to fit the stock exhaust back on and clean/rebuild the carb. A previous owner had a makeshift shim under the clip of the needle which I removed. This is an '81 SR500H with only one notch on the needle. I'm really bummed about the top end speed as occasionally I'll need to get on the interstate with a bit of speed to spare. The stock main jet is a 260. The only thing I can think of now is to change out main jets. I have a 280, 290 and 300. I'll do a compression check also. I doesn't burn oil but I guess I could have a burned exhaust valve. The bike really pulls well going through the gears but it quits at 75. What do you all think about trying a larger main with the stock exhaust?
Bronson7
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Post by canucksr on Apr 30, 2005 10:24:04 GMT -5
Hi Bronson
I don't think it makes a lot of sense to increase the main jet size unless the exhaust is also changed to a less restrictive one and the air cleaner too.
Have you checked the float level in your carb? Maybe the bike is starved for fuel at higher speeds.
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Post by Bronson7 on Apr 30, 2005 10:32:55 GMT -5
Canucksr, I've got a (K&N Stock Box) on it. I've just put it all back together but I'll re-check the float level. Maybe the main jet increase is a bit of stretch at this time as the bike should run fine with the stock pipe and jetting.. I've been looking all over for what the compression should be but havn't found anything yet. Do you know? Thanks.
Bronson7
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Post by canucksr on Apr 30, 2005 11:20:17 GMT -5
Bronson
Compression should be 135 - 150 psi. If it's lower than 135, squirt some oil in the spark-plug hole and check again, if it goes up then the rings are worn.
If compression remains low after this, then the valves may not be seating correctly.
Have you checked the adjustment of the decompression lever cable? If it's not right, the exhaust valve could be partly open.
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Post by canucksr on Apr 30, 2005 11:52:41 GMT -5
Is the valve clearance correct? .004 intake .006 exhaust.
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Post by Bronson7 on Apr 30, 2005 13:47:15 GMT -5
Just read the plug and did a comp test. The plug looked brand new. Pretty good indication of a too lean condition. Probably explains a very slight surging at idle. The compression was 120 psi. It wasn't a good test as I could barely kick it through the compression stroke so I imagine I lost some pressure trying to do this. I couldn't get the kick lever down fast enough, as a matter of fact, it felt like it was flat locking up. I must me turning into a weiny in my old age. I didn't do the oil thing but I'm on my way now to do it. The compression check was done hot and with full throttle. Compression not withstanding, it's way, way too lean.
Bronson7
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Post by Bronson7 on Apr 30, 2005 13:49:15 GMT -5
Oh, forgot. Valves are right on at .004/.006. I set them yesterday before the test ride.
Bronson7
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Post by Bronson7 on Apr 30, 2005 15:01:36 GMT -5
I just did the compression check again. This time I got 130 psi.
When I gave the cylinder a shot of oil, I could only go to 60. I think there's a leak in my tester. Exactly how do you go about doing this test? Do you kick it the same way you would start it? I know one thing for sure, the carb has to come off and I'll recheck the float level as per your suggestion. The ceramic shouldn't be white. Thanks.
Bronson7
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Post by canucksr on Apr 30, 2005 16:07:33 GMT -5
130psi is plenty of compression. As far as I know your procedure is correct, full throttle and kick it like your starting it. So you've got good compression, and a good spark, that only leaves fuel. It's very puzzling about the 75mph top end. Maybe you are right about a too lean mixture. I gave up on the stock SR carb years ago but I believe there is a fuel/air mixture screw on it that has to be turned out a certain number of turns. Can't think of anything else.
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Post by Bronson7 on Apr 30, 2005 16:24:12 GMT -5
Canuckus, I may follow you and StewRoss down the road and put a Mikuni 36 on it and do away with the airbox. For the time being I'm going to pull the carb again and recheck the float level. If that doesn't fix it, I'll play around with the jetting. No fuel/air mix screw on this puppy, not even under a brass plug. This carb, IMO, is waaaaaay more complicated than it needs to be. I'll let you know how I fare. Thanks for everyones help. I'll post results in the fuel forum seeing that this is turning into that kind of problem.
Bronson7
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Post by canucksr on Apr 30, 2005 16:28:20 GMT -5
I don't think there is an air/fuel mixture screw on post 1980 SR500's. They only sold SR's in 78 - 79 here in Canada
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Post by StewRoss on May 1, 2005 4:23:07 GMT -5
Yep, come to think of it, I had carby problems on both of my H models. I ended up selling one and the problem went with it (...should've kept that one...). The other one I put the larger E model carby on and that fixed it up...I'd pull the carb apart and then it'd run good for about 1 km and then back to RS again...I have that and another similar H model carb sitting in my shed collecting dust. Can't recall whether they have pilot adjustment or not???
That could be another option, to get the E (F) model carby. They're 34 against 32mm...seem to operate better as well.
Mikuni wise there are a couple of options. I used a 38 on my cafe racer. It would be easier to get a kit already sorted, however, Sudco in the US should be able to get you pretty close to the mark. Sudco Mikuni's are pretty cheap to buy. They may even jet the carby for you before you buy it if you buy a new one.
You'll also need a breather filter as well if you ditch the airbox. K&N?? That's what I used anyway.
The throttle is the other thing. (Heh, heh...Nothing is ever as simple as it sounds...) You will either have to find a cable to suit the standard throttle or get rid of the standard throttle and use a different one altogether...
It is likely that if you use a standard SR cable and have it modified to accept the Mikuni type cable end it will work. I've not done it myself, but I'm sure it can be done. You're going from two push-pull quadrant cables to one that pulls up directly on the slide...in case you didn't know that...
As for a pipe, I have a standard header with a Staintune stainless replica Conti megga on my 'normal' bike...see the pics section. Sounds nice, a little loudish, but others seem to like it. I wear ear plugs so it's OK for me too. You need to fabricate a bracket that mounts to the standard rubber mount to suit.
SR
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