Bike hits a wall at 5000 rpm

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cross
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Re: Bike hits a wall at 5000 rpm

Post by cross »

Stock air filter
Sasha

'82 Honda CBX
'99 Triumph TBS
'01 Honda Valkyrie

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steve murdoch icoa #5322
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Re: Bike hits a wall at 5000 rpm

Post by steve murdoch icoa #5322 »

That is exactly what happened with my '79.
Slight stumble starting around 4500 rpm with DG, stock jetting and single K&N air filter.
Returned to the original stock set-up of metal screen and foam filter and the stumble went away.

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CopperCollar
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Re: Bike hits a wall at 5000 rpm

Post by CopperCollar »

Have you put the stock air filter back in?

Have you tried running the bike without the filter in ?

Have you tried obscuring the filter with a piece of newspaper?

These will help with the diagnoses.

Runs great without the filter you may be to rich.

Runs great with the filter partially obscured ...you may be to lean.

Do the simple troubleshooting first, pulling carbs and re jetting is not a lot of fun, especially on the CBX!

STG III kit has a really funky needle that is extremely rich.
Compare it to the stock needle and you will see what I mean.

Unless you have ported the head, opened the valve pockets, gone with bigger valves, different cam profiles etc... I see no need for the STGIII kit on an otherwise stock CBX. The STGIII just over fuels the engine.

Mike
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Re: Bike hits a wall at 5000 rpm

Post by daves79x »

With the stock intake and airbox, you're right Mike. However, the installation of pod filters on CV carbs is a whole other animal. The Stage 3 kits are a good option for those, as the air volume increase is tremendous. So you have to jet way up to compensate or you will be extremely lean. For example, if you are trying to jet with stock needle and stock Keihin jets, here's what's close: '79 carbs with stock #98 main - #120 is a good start. '80 carbs with #110 stock main - #125-130 is close. '81-'82 with stock #105 main - low #120s. Exhaust really doesn't matter.

So you see what pods do. The Stage 3 kit's needle, as you said, is drastically different from stock as are the jet sizings, but generally setting the clip to the leanest position and using the smallest main jet supplied will be close with pods. As you said, it's crazy to use a Stage 3 with stock intake.

Set up correctly with pods and the much larger jetting, CBXs will still get in the low-30s mpg and run quite well.

All that said, I still hate pods.

Dave

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Re: Bike hits a wall at 5000 rpm

Post by Bird_Of_Prey »

Hi Guys I am new to the site.

I have just bought a CBX and it has this exact same problem zero power after 5000 rpm, top speed is 70 mph, early days for me as I havent checked the obvious.
It has the after market air filters, 6 into 2 exhaust but megga style (loud) silencers. But engine sounds good so I am not thinking mechanical issue.

I will be checking shim clearences, and compression, then syncing the carbs, check timing.

My impression is carburation, but I dont rule out ignition, seems to run hot after a run so will fit temperature gauge.

Any Advise warmly recieved, meanwhile I will trawl through the forum and internet.

Cheers An Optamistic 6 owner.

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Re: Bike hits a wall at 5000 rpm

Post by NobleHops »

Bird_Of_Prey wrote:Hi Guys I am new to the site.

I have just bought a CBX and it has this exact same problem zero power after 5000 rpm, top speed is 70 mph, early days for me as I havent checked the obvious.
It has the after market air filters, 6 into 2 exhaust but megga style (loud) silencers. But engine sounds good so I am not thinking mechanical issue.

I will be checking shim clearences, and compression, then syncing the carbs, check timing.

My impression is carburation, but I dont rule out ignition, seems to run hot after a run so will fit temperature gauge.

Any Advise warmly recieved, meanwhile I will trawl through the forum and internet.

Cheers An Optamistic 6 owner.
Nice to meet you!

I think you may be the first person here from Thailand - if the spirit moves you then maybe post up an introduction in the new member forum, and by all means, avail yourself of the info in these forums. And then ask away! Everyone here loves to see a bike get sorted out and running great.

Welcome!
Nils Menten
Tucson, Arizona, USA '80 CBX, sort-of restored :-)

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mac.
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Re: Bike hits a wall at 5000 rpm

Post by mac. »

Hi All,

I have a similar problem that happened quite suddenly. I have had the carbs synched (200km completed before problem occurred) as suggested on the forum :text-thankyouyellow: and the bike sounded and ran very nicely. I had taken the revs up to 8000 rpm and sounded really nice.

On today's ride I noticed the problem initially at around 6000 to 7000 rpm, but as the ride continued it would get to about 4500 rpm and start to breakdown. No matter how much more throttle input the speed does not increase. I also noticed a thin layer of oil on the top centre section of the engine cover once I got home. The centre portion of the "H" and seems to come from around the 4 bolt heads in that centre section. Easily wipes off. (I topped up the engine oil last night?) so don't know if these issues are separate.

My initial thoughts before I saw the fine oil deposit was to check the ignition systems (ie) change the plugs, leads, caps and capacitors as they probably need changing anyway.

Thanks in advance.

Mac
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steve murdoch icoa #5322
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Re: Bike hits a wall at 5000 rpm

Post by steve murdoch icoa #5322 »

Have you checked your oil level?
It sounds like it might be over-filled.
Have at a look at the air filter and make sure the crankcase breather system isn't blocked.

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