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1978 cb400a Hondamatic. What kind of fuel mileage should I get? Bike is mechanically reconditioned, starts right up idles fine. Have taken it out for some long rides. To this point I'm only getting between 25 and 30 mpg. I'm 6'2 and 250 pounds, but my xs400 yamaha does over 40mpg. Just curious about the fuel range of similar bikes.
 

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I'm far from an expert on the 400 series engines, but in my mind the fact that it is the Hondamatic probably has a lot to do with it. Your other 400 is likely a 5 speed manual and think about the difference in mileage on manual trans cars vs automatics back in the day when most automatics were 2 and 3 speed and manual trans cars were mostly 4 speed. The Hondamatic uses a torque convertor arrangement so it revs more and possibly with some intended slippage to get you to speed instead of using multiple ratios to get you there more efficiently
 

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They also used a different cylinder head with smaller inlet ports (and valves??). I believe it was de-tuned to produce less HP than its (manual trans) brothers (26.8 HP vs 43 HP). This would make it work harder to 'pull its weight', along with only two gears going through a torque converter (not direct drive which transfers a bit less power to the rear wheel). On my CM400 manual trans I get between 45mpg (hard accelleration/high rpm riding) and about 55 mpg for light riding/accelleration.
 

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Possibly, a member with a Hondamatic hasn't seen this post just yet.
 

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hondamatic fuel consumption.

PM sent to jetcash and steve9e
hi guys,:D

Just seen the comments regarding fuel consumption of the CB400a. I ride in some very hilly areas mainly where the engine is worked hard and without a doubt I ride 130-140 miles before reserve which gives somewhere near 55-60 mpg. That is UK gallons not US tiddlers.
Yes the engines are re-tuned to move the max torque down the rev range ( with a reduction in bhp) but still retaining the approx .max torque level of the manual engines. Torque is what you want.

Incidentally no body should be tempted to fit the manual pistons & engine top end on the Hondamatic - It does not work - I know.:D
 

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1978 cb400a Hondamatic. What kind of fuel mileage should I get? Bike is mechanically reconditioned, starts right up idles fine. Have taken it out for some long rides. To this point I'm only getting between 25 and 30 mpg. I'm 6'2 and 250 pounds, but my xs400 yamaha does over 40mpg. Just curious about the fuel range of similar bikes.
hi, I think that the way you drive the 400a has a lot to do with the fuel consumption you get. Ride it like a manual using the 2 spd gearbox to the 50 mph cut off will in my opinion use far more fuel. I hardly use 1 st gear apart from the initial moving off and snick it into top at about 15 - 20mph and leave it there. I ride in some very hilly country and will only drop down into first gear on say very steep hairpins or part of the way up long steep climbs, just to save the torque converter a bit as they get very hot anyway. Sure the Hondamatic can press on if it is required but you get the best out of it riding steady away. Hondamatics are pretty rare in the UK and quite a few of my friends initialy would question it's on the road performance but I get more comments now about it's pulling power and fuss free riding. The reduction in bhp does not seem to be a problem in real life, certainly on the majority of UK roads.
 

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The UK gallon is .83 US gallons so 60 mpg for you is equivalent to 49.8 mpg in the US, 50 = 41.5mpg
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Thanks for the info on trying to use a manual top end on the A model. Question has been asked before and while it will fit no one knew if it actually worked or was a disaster.
 

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I have a 1979 CM400A Hondamatic, I am getting around 39 mpg on a good few days. I do let the bike warm up a lot before going on a longer rides. I put Shell mid grade(89 octane) gasoline in Canada. I am also changing gear a bit earlier than normal
 

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hi jim,

I too could not understand why the manual top end could not be used on the 400a, but the short answer is that the characteristics of the torque converter has to match the torque curve of the engine. Obviously the detuned motor revs much lower than the 42 bhp engine at about the same torque so having more bhp at the wrong period was no good. I could not get the bike to exceed 50 mph and trying to do a stall test failed miserably. When I accumulated the correct H2 head and smaller carbureters It just transformed the bike at a stroke made the performance you would expect from the 400a. One thing you can get away with is using the larger choke carbs off the manual BUT you have to fit weaker springs in the dampers. I covered this discovery in my Winter Rebuild a couple of years ago. Interesting isn't it? ;)
 

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The different heads and cams plus smaller venturi carbs will change the engine performance completely as you've documented. Low end torque is what torque convertors live for. I suppose that one of the automatic shops that modify toque convertors for drag racing could make the appropriate changes, don't really know what they do internally to them.
hi jim,

I too could not understand why the manual top end could not be used on the 400a, but the short answer is that the characteristics of the torque converter has to match the torque curve of the engine. Obviously the detuned motor revs much lower than the 42 bhp engine at about the same torque so having more bhp at the wrong period was no good. I could not get the bike to exceed 50 mph and trying to do a stall test failed miserably. When I accumulated the correct H2 head and smaller carbureters It just transformed the bike at a stroke made the performance you would expect from the 400a. One thing you can get away with is using the larger choke carbs off the manual BUT you have to fit weaker springs in the dampers. I covered this discovery in my Winter Rebuild a couple of years ago. Interesting isn't it? ;)
 

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I ran out of gas! I only got 164 km and reserve was empy. i was doing around 100km aka 62mph most of the time. i was coming to red light and dead out of has. lucky i have caa. is that normal ?
 
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