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Bike Losing Power

TheZimmermaniac

New Member
Joined
Apr 17, 2023
Total Posts
8
Total likes
0
Location
Alliston, Ontario, Canada
Hey all I’ve recently purchased a Honda cb400t(as a complete amateur to motorcycle service) and got it out a few weeks ago and found that it was not running correctly at about 75-80km/h and 5500-6000rpms. After doing lots of research I found my timing was off so I replaced both my stator(from a bike salvage near me but ohms out fine) and cdi(Rex’s speed shop). Took it for a test ride yesterday but the problem still persists. So I wanted to run by some of my next steps to try.

1) I took apart the carbs yesterday as I haven’t done that yet. Will finish it tomorrow and have it synced up

2) I haven’t swapped the ignition coil yet begin still producing spark but from my reading it might be worth changing. I did do the ohms test on it yesterday and it was good. But certainly would be interested in swapping to the gm coil upgrade

3) and lastly I don’t have one but could get a leak down test gauge

Let me know what you think and I can certainly get more information if you need it. Any help would be appreciated.
 
After the stator and CDI replacements have you checked the timing advance?
I'm guessing that you're using new NGK D8EA plugs and have checked that the plug caps are still at 5K ohms resistance.
Compression readings? Hot engine, throttle held wide open, both plugs out and cranked until gauge stops rising.
Valve adjustment done?
If the ignition and compression check good then it's in the carbs, there's a sticky post in the SOHC Fuel section on rebuilding these carbs, just takes time and patience.
 
Hey thanks for the suggestions I have put those spark plugs in brand new and I will check the end caps to confirm resistance. The compression test I will try once my carbs are back together and reply my values. Maybe fingers crossed it was the carbs :p.
 
Check compression as soon as you can. 150psi is the minimum. Anything lower you're just hacking around the issue. You can do a compression test with the carbs off the bike. It actually might be easier because normally you want the throttle wide open when you do the test.

If it loses a lot of power after warmup to the point that it stalls out it may be the coil. Usually when it's the coil it might run great for a few minutes and then stalls out and won't start again for a few minutes after the coil cools down.
 
Just to give a little update I was just about to sync the carbs up tonight and noticed that my valve cover gasket was just leaking a little so I went to tighten up the cover screw and went just a little to much by hand and the bolt snapped. So I luckily got it out but now I need to source a bolt. Unfortunately not many suppliers ship to Canada for a reasonable price.
 
Those shoulder bolts tend to snap off when tightened past the stop point.
Call you local Honda dealer and order 1 bolt 90082-413-000 and 2 grommet washers 90441-413-830
The remaining threads should unwind out of the cam/rocker holder with some encouragement
 
Hey so here is an update. On Friday my buddy and me we synced the carbs and because it was a little rainy my friend took the bike out he is a much more experienced than myself. When he took it out he did say that it started to some what sputter but after a few seconds and opening the throttle it cleared right up and after about ten minutes of him riding it was running great. I haven’t had a chance to have a go on it yet as the weather has been pretty terrible here but looking forward to testing it out again. As a side not we did check compression with engine warm but maybe not the strongest battery because we did it after syncing carbs and we were getting about 170psi which seems to be just in spec. Thanks for all the suggestions here :).
 
Good to hear, and let us know how it goes when you get a chance to ride it yourself in good weather.
 
Good deal. 170 compression is actually doing quite well considering the age. Since there was a stumble that cleared with riding the carbs may have some fuel deposits that just need cleaning out so add some SeaFoam to the fuel or another good brand of fuel system cleaner.
 
Got to take the bike out yesterday and it was cruising really well at at about 80-90km/h I did notice on deceleration and hovering around 50km/h I was getting some backfiring and popping. After reading I was curious about my exhaust and if there is supposed to be a baffle plate inside and with it gone could it be creating back pressure? If there is supposed to be one any ideas on what might fit in there or if there is a part number for what I need? Pretty sure from a little research I have done the previous owner probably removed whatever was there.
Also if I should start another thread in a different section I certainly can do that.


This is a link to what my exhaust pips look like
https://photos.app.goo.gl/pVbhoM6jWpReUbeP9
 
Got to take the bike out yesterday and it was cruising really well at at about 80-90km/h I did notice on deceleration and hovering around 50km/h I was getting some backfiring and popping. After reading I was curious about my exhaust and if there is supposed to be a baffle plate inside and with it gone could it be creating back pressure? If there is supposed to be one any ideas on what might fit in there or if there is a part number for what I need? Pretty sure from a little research I have done the previous owner probably removed whatever was there.
Also if I should start another thread in a different section I certainly can do that.


This is a link to what my exhaust pips look like
https://photos.app.goo.gl/pVbhoM6jWpReUbeP9

Deleted duplicate post.

I'm not familiar with your model like Jim is, but the air cut valves on the carbs could also be playing a part in the deceleration popping. Not so sure the removal of any baffle would cause it, but Jim knows these bikes inside and out and he'll be along later (he's on the west coast)
 
Unfortunately the PO removed all the baffles. Only real cure is to replace the mufflers. The air cut valves Tom mentions enrich the mixture on decel, high vacuum condition, to minimize the afterburn popping/backfire. Might check the left carb at least to see if it's in good shape, right side requires carb removal and splitting the carbs to access it.
Air cut valve location
201_4642.jpg

Here's the baffling on the earlier CB400T1
GEDC2387.jpg GEDC2388.jpg
 
Ok I can check the air cut off valves I know when we cleaned up the carbs a few weeks ago the air cutoff valves rubber was in rough shape so we swapped with better spares my friend had. For the exhaust is there a good aftermarket option or am I looking for a basically the best original one I can find online?
 
So I think we can fully put this thread to bed :p. Last weekend I was able to get a full on parts bike for 250$ with the stock exhaust. So I stripped it all down to the frame last weekend and got a new gaskets for my exhaust. I spent this weekend cleaning up the old exhaust it needed some polishing also. Slammed the pipes in and just finished taking it for a ride. What a crazy difference in sound for one and the ride is much smoother. Thanks to everyone for your input and help looking forward to the coming summer. Ride safe y’all.
 
Awesome! Yes, I always forget these bikes are a bit picky about the exhaust being in good shape. Some bikes really don't care, but apparently these ones do. Glad to hear it is running well and thanks for the update.

Also, forgot to mention, 170psi is great. Pretty close to brand new. How many miles are on this? If you keep up with doing the valves every season and checking the oil frequently you should be able to keep it on the high end of the compression range for years to come.
 
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