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Discussion Starter · #1 ·
Hello all, new here and need some jetting confirmation. I have just bought a CB400TI (new gen with the disc brakes). Mods are stock looking mufflers but must have some baffles removed or look-a-like after market with a nice throaty growl. Air box stock but have 6-8 holes drilled into top with stock foam filter. Bike has some blueing at first 3" of exhaust pipes. Bike starts great, idles great, pulls good up to about about 5-6 grand but then power floats around there and will not get past 6 grand and will not go over 60-65 MPH!! Been reading a lot here so jumped in and took carbs out to see what up- to my surprise saw 105 jet where stock 70-75's should have been then has 135 mains. Bought an assortment of jets to get this running right and plan to try a 75 or 78 and a 110 or 115 to start/ also plan to try 1-1/2 turns out on mix screw
;)-crappy day so will try tomorrow.

Any first thoughts for the good combo??? Oh, live at 5280 altitude and ride at 5200- to 8000 ft but mostly 5200-7000 ft!!

Thanks
 

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Check your stator specs. There's a sticky in the electrical section of the SOHC 400/450's/Rebels. Your ignition timing may not be advancing fully.
 

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Discussion Starter · #3 ·
Any carb related feedback from the group based on how far from stock the existing jets are set up i.e. that should account for the way it is running etc?????
 

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Moved to SOHC Fuel/Carb section
As Dan mentions you need to check the ignition system first. This is a classic symptom of no advance in the timing which may be due to a bad stator or the CDI unit. This thread will help you out there. http://www.hondatwins.net/forums/64...400-450-cb450sc-manual-trans-1978-1986-a.html
Next up is your jetting AFTER the ignition is solved. Your already in the rich run area due to the elevation. For lower altitudes we recommend increasing the primary main jet one size. In your case you're probably in good shape there.
 

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Discussion Starter · #5 ·
Thanks for the replies. So I put in the 78 and 115 jets and it runs really strong up through 8500-9000 rpm's (that's all the room I had on the expressway) felt like it would rev some more! I felt a small flat spot around 3500-4500 depending on what gear etc-not that bothersome but would be nice to sort that. Seems to rev about a 1000 rpm/per ten mph-so at 80-8grand etc-think I will check front sprocket to see if I can get better top end.

I am not ignoring ignition suggestions but with this greatly improved running after jet install I tend to want to stay with carbs to get all smoothed out. Tried to sync carbs with vaccum cages but had trouble with silly placement of adjust screw and lock nut- but will try again when I invent a better tool for that. Feel about 90 percent good-more time to tinker next week.
 

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The Jetting the bike had ( 105/135) would drown the motor in fuel with not enough air and then at high altitude would make things even worse, good to hear going to closer to stock helped!.

The Hawks always cook the down pipes, I owned a brand new off the dealership CB400T in 1981, and within 100 miles the pipes were dis-colored. Even with re-jetting and NOS down pipes on the bike I have now did then same thing, they just run lean.
 

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Discussion Starter · #7 ·
Ok, Sorry for the delayed update of how my 78 -CB400 is running after re-jetting while on a longer test run. Seems to run well on secondary streets, good pull through 1-4 gears with almost no noticeable flat spot at low RPM’s. When in 5th gear on open road it pulls strong up to about 7000 RPM (again at about 10 MPH/per 1000 RPM-more on that later). It seems that it does not want to pull much more over about the 7000-7500, does not feel flat or breaking up on acceleration just seems to not have more fuel entering to allow higher rev’s-so will not go faster than about 70-75 MPH. Now I am going to get a higher tooth front sprocket to knock down the rev’s at higher speeds but I but feel that maybe I should try a 122 main jet that I have purchased to see if I can get the rev’s available to be closer to the 10,000 redline?? Any feedback on that assumption would helpful. [/I][/B]
I may try the front sprocket mod first to see if the higher speed/lower rev fix may be just what I need for around town riding-I am always leery of making a change like bigger jet and loose how decent it is running up through 7000 RPM etc.
Thanks for any replies to get this fined tuned without too much messing around.
JH
 

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Good to make one change at a time. If you changed both the jets and the sprocket you wouldn't know which one helped (or hurt).

Adding a tooth to the front sprocket will lower your rpm relative to speed but may not increase top speed. I'll be interested to hear how it works out for you.
 

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Discussion Starter · #9 ·
Thanks, If not looking at the sprocket and just looking at the" not pulling after 7000 RPM", does that in fact show the need for a larger jet???
 

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This would be a great time to do a series of plug chops. This would tell you how your mixture is at various throttle positions and give you clues as to the need for jetting changes. If you google "plug chops" you will find a bunch of suggestions on how to go about it.
 

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The usual problem with rpm flattening out is in the timing advance but that seems to so up more at the 5K rpm range. Check that you are getting full advance. Spec is 43* +/-2* at 4500-5350 rpm
 

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And to add, the CB400T will do well over 100 at sea level, In know as I was did it when I had my new 1981, in 1981 ( no interest in doing it again ). You should not need to do anything but stock for the performance, it's 43hp, plenty of go. You won't get all that top end though is your at altitude, just physics, less oxygen, less hp.
 

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Discussion Starter · #13 ·
Thanks, the "plug chops" is very interesting indeed.
New update, I just installed the 17-tooth front sprocket (no new jetting done/agree on the one mod at a time) got it back out and WOW, glad I tried this first!! This little bugger really ran quite differently. It pulled nicely through all gears with not having to shift so quickly as with the 16-tooth sprocket! And best part was in 2nd through-4th gears on open road I could hit 10K red line, in 5th it pulled good through about 8K-9K and I got it up to about 85 mph until I ran out of road to test. Other best part was the 17-tooth knocked down the rev's about 1K to =60mph at 5K instead of 60 mph at 6K etc.

I am getting quite smitten with this new friend and it is really fun to drive now!! Will try more this weekend but as it stands-for what I want -don't think I want to mess with the jets at this point-no flat spots anywhere through the range- but will keep my eye on the plug colors to confirm safe lean/rich fueling.

Now I know it's not fuel related but next will ask for feedback on a rough front fork ride over medium to hard bumps. The last owner installed Hagon front springs listed on receipt as "214MM 10 weight oil with 140MM air gap". I'm not a suspension guy, so new info to digest. Would like to know if this was set up to be this firm/rough or is there room to explore a smoother ride???

Although it handles good - but a less rigid ride would sure be nice to complete this fun machine. Any help or re-direct to a post that addresses would help.

Thanks in advance!
Having fun in Colorado, yeah!!!!
 
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