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Surging at full throttle...

Old 10-10-2013, 03:42 PM
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Surging at full throttle...

I'm hoping someone has some ideas for me - my '98 SH recently developed a running condition, where it seems to "lag" a bit off from idle, "lags" a bit as I accelerate hard and at full throttle, surges noticeably. Earlier in the year, I had made some jetting changes per the carb write-up here - 48 pilot jet, raised the rear needle a touch, etc. I rode it like that for awhile, but felt like I'd lost some overall top speed, so I put everything back to stock and ran it like that for the last half of the summer. A month or so ago, I started noticing the condition I mentioned above, along with the fact that the bike is once again "hiccuping" back into the carbs and dying at stops and what-not. One day, the bike died as I came to a stop and when I hit the starter button, there was a hesitation, which made me think maybe my battery was taking a dump. The battery that came with the bike appears to be a Walmart special and who knows how old it was when I bought the bike last spring, so I went ahead and replaced it with an OEM Yuasa and installed a MOSFET R/R. I also recently changed the spark plugs, but to no avail. This afternoon, I opened her up and when she started surging, I applied full choke. Nothing changed. I then scoured the interweb for awhile, including posts here and decided, based on what I read, to check my ignition coils. I removed the rear coil and tested it with an ohm meter. Everything checked out well. I cut about a half inch off the end of my spark plug wire and threaded the boot back on. I did NOT remove the front coil, but the check I did do on the secondary side of the coil says that coil is good as well. I also removed about a half inch from that plug wire and reinstalled the boot.

Is there anything else I need to check? I adjusted my TPS last fall and had the carbs synched when I did my initial rejet. The bike seems to charge fine at all RPMs, but does "hiccup" quite a bit more than I'd like it to. I think I will go back to the 48 jets (from the stock 45), as although I felt like I'd lost some top end speed, it never seemed to hiccup and it sure as hell never died at a stoplight!

Just as I was about to submit this, I remembered it has gotten cooler and windier in my neck of the woods. I don't know if the blustery conditions could or would contribute to my problems or not - if the gusty winds change the pressure in the airbox enough to eff with my bike.

Help?
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Old 10-10-2013, 03:52 PM
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If you've had the carbs on and off, go through all of the hoses and carb boots and make sure everything is sealed correctly. Sometimes a vacuum leak (especially at the carb boot) will create surges like you are describing. Check for cracks as well... wind should not affect your jetting. Cold maybe, but that could also exasperate a vacuum leak if one were present.
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Old 10-10-2013, 05:55 PM
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I agree, a vacuum leak somewhere. Hey, where do you live in Wisconsin? It's been getting down to the upper 40's around by me. Just a matter of time before the snow starts flyin'...
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Old 10-10-2013, 08:18 PM
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Would a vacuum leak come on all-of-a-sudden? I will pull everything apart tomorrow and see what I can find. I have to admit, I'm really sick of tinkering with this bike. I'd have to figure out an exact number, but I figure I've owned around 12 street bikes since the early 90s and I have NEVER had to tinker with a bike this much. When it runs well, it's a blast, but when it doesn't, man, it SUCKS!

Thanks for the responses.

I live in Neenah, WI. It might as well be a damn wind tunnel...
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Old 10-10-2013, 08:57 PM
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Funny, my wife & I were just up in Neenah almost two weeks ago to buy a SUV at a Bergstrom dealership (wifes family is friends with the Bergstrom's). You are about an hour and 15 minutes north of me.

A vacuum leak can just happen. A hose could crack from age or come loose from vibrations.
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Old 10-11-2013, 08:55 AM
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Okay. So I'm looking at the carb boots, fuel hoses, vacuum hose to the fuel petcock, right? All the PAIR bulls*** had been removed and capped appropriately.

Also, I do remember awhile ago a tech telling me that he had found the 'Hawk's carb boots had a tendency to get egg-shaped if overtightened. His solution was to use different clamps. He did not specify what he used, but I assume it was the ol' stainless radiator hose clamps. Has anyone heard of this problem before? If that's the case, do you buzz the little locating tab off the carb boot or do you cut a hole in the clamp for the tab to fit into?

Yes, Neenah is nice. I grew up here. I've moved away a few times - Florida, South Carolina, Arizona - but I always end up back here. I saw John Bergstrom at a gas station in town one time, driving his brand-new Ferrari 458 Italia. I approached him to tell him what a cool car it was and realized it was him - sitting with his wife and eating an ice cream cone at the gas pump!
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Old 10-11-2013, 09:16 AM
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I'm looking at a fiche of all the hoses and what-not that run in between the carbs. I assume this is where I should be looking, correct? Check and/or replace all the O-rings in the fittings?

Thanks, gang!
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Old 10-21-2013, 01:45 PM
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So, I pulled the carbs and went though all the hoses, snipping the ends of any that I could and completely replacing the hoses that ran from the air cut valves (#29 on the carb fiche). I cleaned and lubed any O-rings I found and put everything back together. I did not find anything that would have been problematic.

What I did find, just before reinstalling the carbs, was that two jetting changes ago, I put some black RTV on the tabs of the slides that hold the jet needles in place. Back when I went to the #48 pilots, and adjusted the shims on the needles, I'd read that someone else had done this, so it seemed like a good idea at the time. At any rate, that RTV had eventually blown loose from each holder and lodged itself in the vent holes at the bottom of said slides. Essentially, the front slide then had NO holes and the rear slide was down to one. I cleared the RTV completely out of each slide, physically verified that each slide moved up and down freely and reassembled everything. After taking it for a test ride, I was able to verify this was indeed the problem.

All is well and the 'Hawk now runs like a beast, with no lag in throttle response off idle, no surging at full throttle and zero carb farting! I spent a lot of time on this thing this year - wheel and steering bearings, carb tuning, front and rear tire changes, MOSFET R/R change, battery replacement - and I'm really looking forward to just RIDING it next season!
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