8000RPM "sag"....

[Mikep328]

When I was out on my "lost gear selector" ride I had noticed that the engine, which had always revved enthusiastically to red line under load was not doing that.  Going through 6k it came on as usual but at around 8k is essentially fell off.  It wouldn't go much past an indicated 8k RPM on the same road/conditions where it had previously hit red line with no strain in the same gear.

I tried again yesterday, putting fresh fuel in and being careful to keep my boot against the shift lever (I'm waiting for the circlip) so it wouldn't fall off (it did anyway).  Same result at WOT under load - a bit over 8K and the engine wouldn't accelerate beyond that.

When I got back to the garage I pulled all the plugs (ouch, ouch, OUCH!) just to note any difference but they all looked good AND identical!

 I have done no work whatsoever on the engine, other than squirting some carb cleaner into the #3 airscrew hole a few days ago.  The air cleaner is new as of late last year - with maybe 30 miles on it when I was out and discovered the 8k limit.  Spark plugs look great.  Ignition timing was spot on when I last checked it with a strobe last November.  It's the DSS E-ignition so no reason it should have changed timing.  I pulled the cover to check that the screws were tight and the unit could not have shifted/retarded the timing.

The carbs were bench synched last November and, as mentioned, the engine went to redline easily and I don't really see how anything could have changed within the carbs to affect that. 

Obviously something has changed...   

[McCabe-Thiele (Ted)]

My first thought is try squirting some cleaner in the other three jets. Do you use an online filter - if not is the strainer working as it should or is fine debris getting past.

It's either running out of fuel or spark - next suspect might be the coils?

[TrickyMicky]

Whilst I appreciate what you have said re. your checklist, the only time I have ever encountered this was many years ago on a CB750. It turned out to be that in my keenness to get it serviced as quick as possible, I had set the ignition timing (points) on the "T" mark instead of the "F" mark meaning it was grossly retarded and gave exactly those symptoms. You say that you checked it last November, definitely worth checking again to remove it from the suspect list, especially if it's not actually misfiiring, just gutless. Good luck.

[McCabe-Thiele (Ted)]

Interesting post Mikey - If it's that retarded I would expect it to take more cranking to start plus possibly needing choke for longer.

[Mikep328]

I'll check the timing!

Another thing I've been meaning to check...and I've learned over the years that "meaning to check" usually means "never gets done"... is the fuel delivery.  I have been wanting to pull the fuel petcock assembly and check the strainer.  The tank itself looks very clean internally from just looking in with a flashlight but I can't get an angle to actually see the strainer in the tank.  I also have one of those small plastic in-line fuel filters from DSS in the fuel line.   But maybe the strainer and/or filter is now partially obstructed, providing insufficient fuel at WOT under load. 

None of the shop receipts for parts when the bike was put back in service by the previous owner show a new strainer though there is a receipt for a new petcock Oring.  So I don't know if the petcock was ever removed/strainer checked or if the Oring was replaced with the petcock in position.  Wish I hadn't just filled the tank!  Well, I can check the fuel flow from the tank, both with and without the in-line filter so I'll at least do that.

I hadn't really thought about it but at the moment wondering, why add an in-line filter if the strainer in the tank is in good condition?  Honda saw no need for an additional filter...

[Athame57]

Could this be the throttle cables? If they are not winding it all on.......

[Mikep328]

Thanks!  Yes, that's another thing I'll be checking!

[McCabe-Thiele (Ted)]

The strainer on my 400 had pretty much  fell to bits when I removed it - as though it had gone brittle over time. That said your in- line filter should stop debris but if partially blocked due to strainer being holed it  could affect fuel flow rate.

[Mikep328]

 I found the problem! 

Was it ignition timing?  Nope, spot on with the strobe per specs.

Were the carbs not fully opening?  Nope, opening fully, no issue with the cable or synchronization.

Was there insufficient fuel flow from tank due to the strainer and or inline filter being clogged?  Nope, plenty of fuel flow.

Then what the heck was it?                 

 It was (drum roll)...



The tool tray cover! 

Yep, the cover had slid back so as to obstruct part of the air intake!!!  I removed it and the bike once again blasted through 8kRPM with no hesitation!  The cover is probably original and has lost all its "rubbery-ness" so it doesn't grip the sides of the tool tray.  Whether it slid back on its own while I was riding or I inadvertently moved it when I had the seat up, I don't know.  I'd get a new one if they were available but since they don't appear to be, the tool tray cover will remain at home.

[McCabe-Thiele (Ted)]

I think these are 3D printed versions he mentions the lid in this listing.

https://www.ebay.co.uk/itm/301295576149?_nkw=honda+cb400+tool+tray+cover&itmmeta=01J5JWEQW0W51QDK9QSRJ55K50&hash=item46269da455:g:dz8AAOSwJnxgKDb4&itmprp=enc%3AAQAJAAABAHoV3kP08IDx%2BKZ9MfhVJKmteozDXKolEkQZX--gie0yGR%2BARdx8DT51IKJXwxBW6L4saYKJSEITs6nHCBu37vUYOpHR43YTOMg4c6HVEqcGSekotShE9PnL0aktT2ZuqIAE%2F79xXLuBGlcnOrH%2BMweyMpBUjMbWp5zLxDmZQu7WhcALGS%2FuchFv699CNd7tQqmFjjqLsJMBgpQaL9LfMryhXB6D6jj2zZSUTxRh5b9afoBtPhV5W2c54Rt9FEyxN4STEal%2BLgowqN4t1ia1T%2Fh%2Fq0MxCwNLiYY%2BX6ZE4ZuIw586sN8reAQDnSIqh%2Bj7e%2FgJlmcTogFw53eCKWtdcsM%3D%7Ctkp%3ABk9SR47-utysZA

[Bryanj]

Reminds me of the customers 500 that wouldnt pull when he had it but was fine when we had it in, one day i was early and he was late dropping it off and i caught him opening the seat and removing his waterproof trousers! Sold him a bungie cord to hold them outside the seat and all problems cured

[Mikep328]

Yeah - I'll sort of hijack my own thread...  I have a friend who worked in a repair shop and liked to tell the story of a Porsche 911 turbo that was not producing the boost/power that it should.  They confirmed that on the dyno and worked on the car for two hours before discovering the problem was that the floor mat had bunched up under the accelerator pedal!

[Mike_Berkshire]

Yep, the cover had slid back so as to obstruct part of the air intake!!!  I removed it and the bike once again blasted through 8kRPM with no hesitation!

Serendipity - in a separate post I was asking how much of a problem it would be that I was missing certain parts from the airbox assembly including the tool tray and the perforated grill that sits in it - clearly no issue!

[Mikep328]

I tried to order a new lid for the tool tray from the eBay site Ted listed but he said he is out of the lids and no idea when he would get more.  

[McCabe-Thiele (Ted)]

I tried to order a new lid for the tool tray from the eBay site Ted listed but he said he is out of the lids and no idea when he would get more.

That's a shame I suspect whoever is printing them will make some more, I've seen the 3D tool tray on e-bay via a different seller.
If I come across it I will post a link.

Found this old post on the top mesh part.https://www.sohc.co.uk/index.php?topic=21223.0