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manual tach on '81 CM 400A?

redlotus

Veteran Member
Joined
Jun 28, 2022
Total Posts
125
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Location
Columbus, OH USA
Hi guys. I was wondering on my 400A I'm working to get back on the road if it's possible to install a manual tach instead of or in addition to the gear indicator panel. I know there are aftermarket digital tachs which require calibration, but I prefer an original one. Is there a gear drive already there and I would just need a manual cover, cable, etc? Thanks for any input. Cheers, Ron
 
No cover or any fun like that. You'll have to use a digital one. Is there any reason why you want it? I believe it's already configured so you can't really redline it a dangerous way on the hondamatics anyways so the only advantage I could see is for fine tuning idle adjustments, which you should do with an external tach anyway.
 
400A tach

No cover or any fun like that. You'll have to use a digital one. Is there any reason why you want it? I believe it's already configured so you can't really redline it a dangerous way on the hondamatics anyways so the only advantage I could see is for fine tuning idle adjustments, which you should do with an external tach anyway.

I was thinking more of the visual (and informative) fun factor :) Digital it is then I guess. Thanks, Ron
 
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I mean half the fun of shifting is watching that needle rise, yes? :) I was just thinking I might occasionally rev it out to 5 or 6 K or so. Safe enough?
 
You only have 1 and 2/Lo and Hi. But, the advance kicks in around 4K to 5K, so 5K should be fine. I looked in the owner's manual and the FSM and see nothing about what the redline limit is specifically for the Hondamatics. Maybe Jim can chime in on that, and if there's some sort of governor. But, I will say top speed is somewhere around 80-85mph.
 
It's a pretty fun bike, I have had mine for over 12 years now put a good amount of miles on it. I don't really like going over ~70mph except in some short spurts here and there. When I had a windshield on the bike originally 70 wasn't too bad. But, without I'm a bit of a wuss. :lol:
 
yeah shields make a difference, even smaller ones which I had on a couple of Aprilia big wheels scooters (250cc) that would comfortably do 75 if I got into a tuck with no bad cross winds. reminded me of my road bike racing days :) fun watching the Tour de France with that mix of bikes and motorcycles tearing around beautiful little twisty roads in France and Spain, etc. someday :)
 
I was thinking more of the visual (and informative) fun factor :) Digital it is then I guess. Thanks, Ron
I agree, I like my old tach on my 400T, first time learning of the N,1,2 gauge on the A, I was thinking, hmm what is that, where is the tach. Now I have heard that these old tachs are not exactly accurate or reliable. Maybe you can find an add on digital that is to your liking. I'll bet you could find one that has a graphic display.
 
I agree, I like my old tach on my 400T, first time learning of the N,1,2 gauge on the A, I was thinking, hmm what is that, where is the tach. Now I have heard that these old tachs are not exactly accurate or reliable. Maybe you can find an add on digital that is to your liking. I'll bet you could find one that has a graphic display.

They're reliable if you get them open and regrease them. Sometimes it's just the needle gets warped a bit and pushes up on the glass. But yes, they are a bit inaccurate and it can vary between the ranges. I don't know if they were accurate when they were new. If you use them for adjusting the idle, do it with a grain of salt. I'll use the tach to get it in the ballpark and then verify with an external tach when I think I'm done. You also just kind of get used to the sound it makes after you've done it a number of times on a particular bike.

I took the 400A out today for an errand, and for kicks (and "educational purposes" <g>) I did get it up to 80mph in a small spot. The bike didn't sound like it was overloaded and ready to explode. Had a bit more to go, but I don't like going that fast so I left off the throttle and went back to 65mph. So, I'd say you shouldn't need to worry about any of this unless you just want the digital tach for the fun factor.

I like that the Hondamatic's are fast enough to pass stuff on the highway, and fast enough to cruise without it sounding like a total chore, but they aren't balls of fire that can tempt you to get into trouble.
 
Time for a big bore kit and a higher stall converter!

:lol:. I still laugh at people who want to mod the Hondamatics for performance. There's really no point in doing so. There's not as many of them out there as other models. It's not the rarest bike ever, but the more people that keep doing bad mod attempts on these just makes one less bike out there which always frustrates me. Especially for a bike that was never intended to do so. I'm not saying you couldn't do it, but I'd only trust someone like Jim who has worked on these types of bikes literally thousands of times to come up with reliable and safe (emphasis on these two) solutions to do so.

Getting a bit OT, but the only real mods that I could see is swapping to a 450A head (maybe I don't know if things are the same, Jim would know for sure) and another possibility is making the ID large on the intakes to accept the larger and more common 400T carbs. I don't know if the 400T carbs will line up the same with the airbox boots after that and you'd probably have to play around with jetting a little bit. I've never done it or heard of anyone who has. I doubt that would even give any real performance boost, but could make it easier to source carbs since there was more manual versions out there.
 
:lol:. I still laugh at people who want to mod the Hondamatics for performance. There's really no point in doing so. There's not as many of them out there as other models. It's not the rarest bike ever, but the more people that keep doing bad mod attempts on these just makes one less bike out there which always frustrates me. Especially for a bike that was never intended to do so. I'm not saying you couldn't do it, but I'd only trust someone like Jim who has worked on these types of bikes literally thousands of times to come up with reliable and safe (emphasis on these two) solutions to do so.

Getting a bit OT, but the only real mods that I could see is swapping to a 450A head (maybe I don't know if things are the same, Jim would know for sure) and another possibility is making the ID large on the intakes to accept the larger and more common 400T carbs. I don't know if the 400T carbs will line up the same with the airbox boots after that and you'd probably have to play around with jetting a little bit. I've never done it or heard of anyone who has. I doubt that would even give any real performance boost, but could make it easier to source carbs since there was more manual versions out there.

I only mentioned it because I once worked with a guy who did those things to a CB750A and it did work pretty well for what it was. And of course, it wasn't really a serious suggestion LOL
 
I only mentioned it because I once worked with a guy who did those things to a CB750A and it did work pretty well for what it was. And of course, it wasn't really a serious suggestion LOL

I never been on a 750A. I used to ride with an older fellow who owned one back in the day and he regrets getting rid of that bike now that he is older and isn't as interested in going fast like he used to.

I have heard from him and other people that they were nowhere close to the performance of a regular 750 and coupled with the heavy size of the bike and how high it is you had to be a tall, strong person to really ride it so they ended up not selling as well as they hoped. They canned the 750A, and put the CB400A the next year and that did much better as a beginner's bike.

One advantage of the 750A vs the 400A that I like is the points instead of CDI. However, now that the bikes are getting older the points are starting to become an issue. Not getting points and condensers, but specifically the springs on the advance mechanism. It's a problem with the 750s and 550 SOHC4s for sure. A lot of talk on those forums about trimming back a coil or so to fix it, but that's a hack fix I don't see holding up in the long term. So far, nobody has sourced new springs and I fear that in another 10 years or so people will just either accept erratic idle issues as the norm with these bikes or just swap it over to modern electronic ignition.
 
Though the 400A engine is set up more for torque, it would surprise me if it wouldn't turn 8000, possibly a little higher, since the manual engine turns 9500 IIRC. A quick check online for specs shows a few errors (pretty sure it does not have 32mm carbs and does not make 52 hp at 10,500) but most agree the A model has about 27 hp at 8000 rpm

I thought I read an official road test somewhere that it has 44 hp. 27 sounds a bit slow, esp considering the weight. I'll have look that up. Thanks for the input!
 
As already determined there is no provision for a cable drive tach for the A models. I have seem electronic analog tach's offered on the web, can't find anything suitable on a quick search.
The redline for the A model engines is the same as the manual trans engines, 10K. I have heard of a couple CM400A's that legitimately hit 100 mph, one had a ticket to prove it.
Changing the carbs has been done, using the smaller VB22's with 30mm venturi's, and it was a fail. Loss of torque. Now, I can't say for sure this is not a way to go because the people who did it weren't well versed in carbs and IIRC one used pod filters.
I do know that a head swap to a manual head doesn't work because the combustion chamber is larger which lowers compression. I don't think anyone has tried a cam swap using the manual trans cam, that is a possible experiment someone can try.
One simple thing for anyone is port matching and intake port clean up. If you look inside the ports you will see the casting seams that protrude into the port, grinding those away can hurt. Port matching is just a case of mounting the insulators and grinding away any of the head the protrudes so there's no ridge.
You can see the ridge at the end of the red straw
201_4052.jpg
This is after some mild grinding
201_4550.jpg
Don't bother smoothing out the rough grind marks, those actually help creating turbulence which keeps the air/fuel mixture atomized.
 
wasnt there a mod somewhere that used a 400t right cover and someone said the gear was already in there? I thought i read that somewhere. Anyway, too much messing with the engine IMHO is bad on these so I just added one for fun. I think i have it set at 1.5 (wasted spark) and my idle shows around 80-90. Looks cool though.
tach1.jpg
 
wasnt there a mod somewhere that used a 400t right cover and someone said the gear was already in there? I thought i read that somewhere. Anyway, too much messing with the engine IMHO is bad on these so I just added one for fun. I think i have it set at 1.5 (wasted spark) and my idle shows around 80-90. Looks cool though.
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You got a analog fuel gauge too! That is so cool. I like how you worked the digital tach readout into the original gauge.
 
wasnt easy but a dremel and some hot glue do the trick. The fuel gauge works ok, but as with any gauge, as the bike moves, so does the reading.
 

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wasnt easy but a dremel and some hot glue do the trick. The fuel gauge works ok, but as with any gauge, as the bike moves, so does the reading.

Did you add a fuel sender/float to the inside of the fuel tank on your bike?
I think there's Lots of bikes that would be improved by a gauge (y)
 
wasnt there a mod somewhere that used a 400t right cover and someone said the gear was already in there? I thought i read that somewhere. Anyway, too much messing with the engine IMHO is bad on these so I just added one for fun. I think i have it set at 1.5 (wasted spark) and my idle shows around 80-90. Looks cool though.
View attachment 15478
That only applies to manual transmission models that didn't come with Tach's, the CB400T1 and CM400/450E.
 
BTW, forgive the basic question as I don't have a manual yet, but on an electric tach where would I pick up the signal on my 81 400A? Is there a coil where you usually tie to the negative terminal? I think I've found one I like but the ebay seller hasn't responded. Thanks, Ron
 
BTW, forgive the basic question as I don't have a manual yet, but on an electric tach where would I pick up the signal on my 81 400A? Is there a coil where you usually tie to the negative terminal? I think I've found one I like but the ebay seller hasn't responded. Thanks, Ron

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