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miss at high RPM, don't know what else to check!

Old 06-01-2012, 06:55 AM
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miss at high RPM, don't know what else to check!

okay, STOCK 98 vtr. Runs real strong in the low RPMs but starts missing, up around 5-6000. If I stay in it, the bike surge once it reaches the 5-6000 range. I can ease into the throttle and bring it up to around 8000 rpm with no miss. I have cleaned the carbs thoroughly and synced them. The big rubber boots on the slides have no tears or pinholes in them. The cans on the bike have been gutted so I first thought it was a lean issue. Bought a jet kit and went with bigger jets but that didn't help. I went one jet size at a time, up to 188's. Also tried adjusting the needles that came with the kit. In jetting the bike, I've had the air box on and off several times and all the hoses were reinstalled correctly every time. I don't think the airbox is leaking. The air filter is new. The plugs are new. The ignition timing is good, as is the cam timing. The valve lash is within spec. I bought used coils, an ignition box, and a computer and installed them, all with no improvement. The throttle position sensor is operating correctly, as per a factory Honda service manual. I tested the ignition pulse generator with a craftsman voltmeter and the max reading I got was 1.0 volts, well above the .7 volts minimum the manual specs. I don't have a peak voltage adapter but I believe using one would just result in a higher reading.

My compression test yielded 150 psi on both cylinders; the book specs 164. I feel this is acceptable.

Finally brought the bike to a dealer, and their diagnosis was that the carbs need cleaning. Unless there is an invisible port that I missed while cleaning the carbs (which I have done three times), that is not the problem... Feel like I paid 85 bucks for them to test drive it around the block and then tell me it's the carbs!

Any suggestions would be greatly appreciated!

Last edited by 2404life; 06-01-2012 at 07:13 AM. Reason: more info
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Old 06-01-2012, 07:07 AM
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Side-stand switch? On some of these bikes at high rpm's the side stand vibrates/bounces enough that it very briefly cuts out. Try bypassing it and see if it happens- 5 min check.
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Old 06-01-2012, 07:10 AM
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will do, thanks for the input! Bike shop wants $720 to clean the carbs and perform a "premium service" which I assume is fluids, chain adjustment, etc. All of which I have done. They don't call it a Stealership for nothing.
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Old 06-01-2012, 08:15 AM
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You've done quite a bit- probably more than the dealership would want to try.

Do you have anything strange going on near the intake of the airbox? Wiring in the way perhaps or anything not stock in between the forks or fairings? My next guess would be the coils, but you've replaced those already...
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Old 06-01-2012, 09:29 AM
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Choke

I'd pull the choke out while it's missing and see if more fuel helps. If not then it's ignition.
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Old 06-01-2012, 10:00 AM
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What jet kit did you buy? If you've still got the 188 jets in it then you're WAY to fat. For what you've got done to the bike the stock mains should probably work great. I'd go back to teh 175f and 178r mains. What do you have for pilot jets? I'd go through the carb set up thread on this site and set your carbs up accordingly and then go from there. I know you've said you've cleaned your carbs several times but try going back and putting the jetting where it needs to be. Did you install the jet kit the first time you tore the carbs apart to clean them?
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Old 06-01-2012, 03:22 PM
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The carbs are back to stock, 175 and 178 mains. The slow jets are stock as well. I did not install the jet kit the first time I cleaned them. I went real rich on the carbs to be positive it wasn't running lean; I burned a hole in a piston on my old CX500 running lean! Pulling the choke out doesn't help at all. As far as the ignition goes, I don't know what else to check. Possibly my test on the IPG was faulty based on my lack of a peak voltage adapter. Before using my junk craftsman multimeter, I used the MAX function on a borrowed fluke and came up with the same reading of 1.0 volts; in my opinion a passing test. I tested the coils and leads via resistance readings based on a post I found on another VTR message board and the were okay, plus swapped them with a good used set. It's my understanding that Ignition modules and ECUs either work or don't, but I swapped them w/ good used ones as well. Checked the stator via a resistance test as directed by my manual. Another test I was unable to perform was a peak voltage test on the coils. Any thoughts on testing the ignition further?

As far as I can tell, everything looks stock near the airbox. The fuel lines aren't kinked, and I loosened the fuel bowl drains after attaching rubber tubes to them, draining them into individual bottles. Filled up the bottles pretty quick and evenly.

Wonder if there is a wiring problem. Bad ground, broken wire somewhere. Found wiring harnesses on ebay for around 30 bucks. May be easier than trying to trace the wiring. At this point, I don't care about my time, just want the damn thing to run right!

Again, thanks for the input!

Last edited by 2404life; 06-01-2012 at 03:30 PM.
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Old 06-01-2012, 03:39 PM
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Do you have access to a timing light? If so try hooking it up and see if when it's cutting out if the light cuts out. That would tell you if it's ignition or not. It's starting to sound ignition related. I assume that you've checked the air filter for being dirty through this process and there are no restrictions in the air intake?
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Old 06-01-2012, 04:26 PM
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How long have you owned this bike? Did it run well for you previously? Any and all history may be of help.
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Old 06-01-2012, 05:18 PM
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What Now?

Now I would test each ground on the frame and check the charging voltage as you rev up the engine. A bad ground that causes overcharging will send the ignition module into chaos.
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Old 06-01-2012, 06:16 PM
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I have had charging system cause high RPM miss as well as plugs and plug wires. Like reported earlier, pull choke when it is missing to see if fuel has any impact on the issue. Charge battery really good over night and also see if that has a short term impact on the issue.

Your problem sounds hard to find so a dealer is likely to take advantage of you and still not fix the issue.
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Old 06-01-2012, 08:14 PM
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Have you checked the filter screen inside the tank or taken the petcock apart to see if it is clean and the diaphragm is in good shape?
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Old 06-02-2012, 05:09 AM
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GTS- I do have a timing light, great idea! Air filter is factory Honda and brand new. I've inspected the airbox so many times; surely there is no restriction there!

VTRsurfer- I cleaned the pilot jets as prescribed; I also tried the pilot jets that came w/ the factory pro kit- no improvement.

Beerhunter- I have owned the bike for a few months. It seems I learn everything the hard way- test drive a bike before you buy it; don't take the seller's word that it runs good! Seemed like a reputable dealer but I'm learning that I can trust no one! It has run this way for as long as I've owned it.

zxbud- I will try that when I get the bike home from the dealer today, thanks for the help!

Thumper- the plug wires and coils are from ebay, said to be from a known good running bike. I have tested the continuity in the wires and coils and they are exactly the same on all four ( the old set and new). I would assume all four wouldn't be bad. The plugs are brand new. Pulling the choke does not clear the miss. From all the help I have received on this board, that does point to ignition! I will try charging the battery and see if that helps.

8541Hawk- The filter screen inside the tank is clean and I have disassembled the petcock and it is in good working condition.


this damn thing has me stumped, I don't even care if I ever ride it again, I just want to figure it out! I've been working on cars, bikes and boats for the better part of 10 years and this is surely the most frustrating problem I have ever had. Again, I really, really appreciate all the input and will keep y'all posted!
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Old 06-02-2012, 06:00 AM
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It is possible that someone has done previous engine work, and may have timed the cams to open the front cylinder's valves after only 90 degrees of crankshaft rotation following the rear cylinder.

When you checked the valves and cam timing, did you verify that the rear cylinder valves open 270 degrees after the opening of the front valves?
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Old 06-02-2012, 08:25 AM
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I still dont understand where the problem is. Are you saying it misses all along after 5000 unless you baby it along? Or once you get past a certain point you can go full throttle up high? Are the needles stock also or shimmed? Is the air filter stock? Does not sound like the low speed jets. Can you give a better description of where in the range and under what throttle conditions?
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Old 06-07-2012, 05:04 AM
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Sorry for my delay in re-posting. Beerhunter, I didn't check that the rear cylinder valves open 270 degrees after the opening of the front valves. Will recheck that.

Cliby, The carb is bone stock. Maybe I will take video and post it up. Got it back from the dealer and it now runs real well until around 6500 where it doesn't fall flat on its face but definitely loses a lot of power. I doubt that I could get it to rev to redline.

I will post up what I find out, and maybe a video. Its usable as of now; maybe I should just enjoy it for the summer and tinker with it this winter.

Thanks again all for the input, I now have another list of things to consider.
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