jeffz
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Posts: 215
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Post by jeffz on Apr 8, 2008 16:40:18 GMT -5
I've had a strange problem with my '78 now since last year sometime. While i'm cruising around town, around 30-45mph, with the throttle steady, I can fell the bike surge a little. Almost as if i'm twisting the throttle back and forth ever so slightly. Doesn't matter what temp it is outside either, if that matters. I know it runs a bit rich, too. I can't recall what jetting i'm running, but it's set up with a K&N and Suppertrapp muffler. Any help is appreciated.
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Post by strick on Apr 8, 2008 19:06:53 GMT -5
I am not familiar with the vacuum side of these carbs/tank setup yet, but that would be something to check. I come from the dirt bike side of cycling, so here goes: Take some carb cleaner and spray it on the intake and any gasketed closure on the carb with the bike running. If the engine picks up idle/rpms you have a leak, which can cause surging at any throttle setting. Second, I would ask how clean is the carb? A surge can be as simple as a piece of 'something' in the carb being sucked up into the main jet. Finally if the other two check out o.k. I would say, based on the throttle setting, I would shim the needle UP, I know you say that you think the bike is running rich, but with an open air box and exhaust a bike should require MORE fuel. I have read the would Minton/Faireheit/Kramer (whoever) article, and that really needs to be edited by someone because they talk in circles. Oh, but a word of advice: fiddling with carbs can become addictive!
There is my 2 cents - please post back with what you find. There of course can be worse things like rings, valves, blah blah blah, but it may be really simple.
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Post by colinjay on Apr 8, 2008 22:06:58 GMT -5
It might sound strange, but, what is the condition of your chain and sprockets?
A worn chain or one with tight spots in it can cause a "cyclic" surging effect at constant speed. A chain that is a fraction too tight will also do similar as the rear suspension moves over road irregularities/bumps. Worn sprockets can also have a similar effect. It is worth looking at before going through the pain of stripping the carbie.
CJ
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Post by mattb on Apr 8, 2008 23:17:19 GMT -5
As a first simple check, + 1 to the suggestion of checking your chain - mine needs tightening about once a week and if it exceeds the 2cm play it starts to do this, and quite noticeably too (and regardless of tension, it will do this if worn out, as mine was until recently). What rpm are you cruising at? Under 3000 and I get this, and if the chain is loose then often under 3500rpm (which is a shame - the bike feels nice at 2500rpm :-(
Matt
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jeffz
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Posts: 215
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Post by jeffz on Apr 9, 2008 12:22:24 GMT -5
It seems to happen anywhere below 4k, since I usually don't cruise around town with it cranked wide open. The chain would be a good thing to look at, as I never really gave it a good once-over after I bought it. I do know that the little plastic "idler" for the chain is so worn that it doesn't do anything right now. I was going to replace it along with the swing arm bearings. As for the carb being clean, when I pulled it out of the shed after 5 months, I kicked it about 15 times and it fired and ran great. I drove it 5 blocks to the shop and cleaned the carb anyways and it seemed to run the same. One more thing on my mind is my throttle cables. After I put the drag bars on, there was some strain on the throttle cables that made the engine rev if you turned it to the left. But the surging happens when i'm going straight. I'd like to go out and ride it some more to see if I can pick up on any more small details but it's supposed to be cold and rainy all week.
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jeffz
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Posts: 215
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Post by jeffz on Apr 14, 2008 16:44:01 GMT -5
Took it out today and rode for a while. Still doing it. I checked the chain and sprockets and, although they were a little on the dry side, they aren't worn. I'm going to go check my cables out and if that doesn't help I think i'll start digging into the carb.
On a side note, why does it seem like my SR makes absolutely no power above 4.5K? After doing some reading on this site, it seems as if a lot of you can run your's up to 6k or more and still be pulling. I have plenty of low-end grunt, as do most SR's, but mine just doesn't seem right. This is the only SR i've ridden, so it's hard to compare or get a base-line feel for what its supposed to be.
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Post by StewRoss on Apr 14, 2008 17:18:08 GMT -5
The bike should run to over 7,000RPM...especially if you have a free'er flowing exhaust. Standard the power will begin to drop off a little over say 6,500...make sure the pipe has no internal blockage, although a Supertrap should be fine, more discs equal more noise and a free'er flow; ensure that the ignition timing is OK, did it start to run worse after some maintenance? Cam timing? But that shouldn't change unless it is played with...carby problems can make it run rich...is the accelarator pump linkage intact? SR  |
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jeffz
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Posts: 215
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Post by jeffz on Apr 15, 2008 12:26:52 GMT -5
I don't remember it ever running any other way than it does now. Valve's were set a while back, so I could go over those again. Compression is fine, FWIW. Linkage for accelerator pump is there and working as far as I know. I'm starting to lean towards the carb being set to rich. I had a XT carb on it for a while (no acc. pump), but I can't recall how it ran or what jetting I used. I think we used that carb to tune, and then replaced it with the stock carb with the same jetting. My dad is very knowledgeable when I comes to older Yamahas, as he was a mechanic back in the '70's and 80's. But for some reason he doesn't thing there is much wrong with it. Maybe I should get him do drive it around.
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Post by joneill4 on Jun 4, 2008 18:58:54 GMT -5
I see I'm a little late on this thread, but my power doesn't really come in until 4k. You are missing out on a lot of fun IMHO. If traffic permits, I ride between 4k and 6k.
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Post by wotavidone on Jun 15, 2008 6:50:54 GMT -5
My bike might just pull 6500 rpm in top, on a calm day on flat ground, with 17/42 gearing.
Today I cruised home at between 5000-5500 rpm and my mate on his 900 hornet claimed he was doing 110-120 km ( about 68-75 mph) to stick with me. It had a bit of throttle left and responded if I twisted it, so I guess if I really caned it, I might make 140km (88mph), if I lay on the tank. But I would not know, because while he was doing 120km, my speedo was showing 85!!! In any case the bars and seat encourage sitting upright so that's what I do.
I run the original carb, stock airbox, with a stock filter frame with the original foam removed and half inch aftermarket foam fitted. The muffler is an aftermarket thing that is considered too quiet by most of my mates. It sounds good to me, till someone starts his Harley, then I can't hear mine running. Really the bike does not pull strongly above 5500 in top, though through the gears it steps out nicely all the way to the redline.
I occasionally toy with the idea of going down a tooth to 16 on the front, but what the heck, I like only doing about 4500 to 5000 rpm at the speed limit. In any case, since carl joined the forum, I've concluded the real limiting factor is probably the exhaust.
Mick
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Post by chew652 on Jun 15, 2008 8:38:43 GMT -5
On a side note, why does it seem like my SR makes absolutely no power above 4.5K? After doing some reading on this site, it seems as if a lot of you can run your's up to 6k or more and still be pulling. I have plenty of low-end grunt, as do most SR's, but mine just doesn't seem right. This is the only SR i've ridden, so it's hard to compare or get a base-line feel for what its supposed to be. When I bought my SR it would not run much more than 4K. After digging around awhile ,I found a previous owner had done the Minton Mods to the carb and added a K&N filter.The bike also came with a large stainless steel Supertrapp using 4" discs.For some reason there were only 4 discs in place.It was like someone stuffed a potatoe up the pipe. I added 4 more discs and it improved greatly. Ithen added 4 more for a total of 12 and upped the main jet to a#330. The thing runs great all the way to redline now. I would check the bike for any intake or exhaust blockage....or play with your new street tracker. ;D |
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jeffz
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Posts: 215
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Post by jeffz on Jun 15, 2008 19:55:14 GMT -5
Well, i've been waiting for a 34mm Mikuni to show on my doorstep, which I was going to put on the '78. But now that I have the "new" SR, that needs a carb adjustment anyways, I'm going to just put it on there. I rode it down the block yesterday and immediately could feel the difference from my '78. In good and bad ways. Good, as in it's faster and handles better, but the brakes and clutch need adjustment, and I need to find some screws for the headlight, since it decided to fall out in the alley. Can't believe it didn't break.
Anyways, per the topic of the thread, the '78 still has a surge too it. I've noticed that it gets worse as I close the choke more. I leave it open slightly and it seems to drive smoother. Anyone have any thoughts on that?
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jeffz
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Posts: 215
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Post by jeffz on Jun 18, 2008 17:37:34 GMT -5
Today I decided to take the discs off the Supertrapp to see if anything was clogging it. That was the case, sort of. There were 9 total discs. 5 right after the muffler, then there was a block-off disc and then 3 more regular discs and finally the end cap. I had no idea that there was any sort of block-off disc in there since they all looked the same when they were bolted on. So I stacked all 8 right after the muffler, then the block-off plate, and then the end cap. It woke right up! Actually pulls above 4.5K and will knock off 70mph without breaking a sweat. It's a little louder but that doesn't bother me. But, it still has a stumble/misfire that I can't figure out. Can anyone offer any suggestions?
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jeffz
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Posts: 215
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Post by jeffz on Jul 10, 2008 17:44:34 GMT -5
I think I may have figured it out. At least it doesn't do it anymore. Negative battery connection was corroded and the positive connection was broken, but still making some contact. I'm thinking it may have been getting a bad connection.
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