1978 Honda CB400A (Hondamatic) fuel/air problem? - Motorcycle Forum
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Old 08-17-2010, 03:22 PM   #1
bobsmith
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Default 1978 Honda CB400A (Hondamatic) fuel/air problem?

Hi,

I am not a mechanic, but have reasonable mechanical ability, so am hoping to fix this problem myself if someone might be able to point me in the right direction. 2 years ago, we fully serviced (at a bike shop) my dad's hondamatic (cb400a), including carb rebuild, new plugs, etc., etc. This bike only has 600 original miles on it, so had sat for most of its life. I kept taking out every once in awhile after that, but then let it sit again. Fast forward to last week, and to get it running again, I cleaned the carbs thoroughly again and put all back the way it's supposed to be, (I think - referring to the Clymer manual), gave it new plugs and new gas, cleaned out the lines, new battery. I started it up, and it seems to idle fine, but only with the choke flaps closed, even when I've let it warm up idling that way for a while. As soon as I let the choke go, the bike dies. Similarly, if I try to give it any throttle at all, it dies. I attempted to adjust the throttle adjust screw, but all that did was give me the ability to allow the bike to idle without having the choke flaps closed. Again, in this situation, any throttle would kill it. I am guessing this is something simple, but I don't know where to start. Any help you can give is much appreciated. thanks.
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Old 08-17-2010, 06:35 PM   #2
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I am guessing this is something simple, but I don't know where to start. Any help you can give is much appreciated. thanks.
Major vacume leak or the idle (low speed) jet is still plugged (not clean).

You are testing with the air cleaner fully attached, aren't you ??
On some models, the stock air filter is important for getting the mixture right.
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Old 08-17-2010, 08:47 PM   #3
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Also check the carb intake boots for cracks.
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Old 08-18-2010, 10:23 AM   #4
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I have had the same issue with my 78 hawk. I am able to ride her once she warms up but have to play with the choke when i stop (pain in the ass in traffic) i have been looking for the intake boots but they seem to be part of the air box. i hope to find some at the meet this weekend.. let me know if you find a fix and i ll do the same.
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Old 08-18-2010, 12:05 PM   #5
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Thanks for the advice. Yes, it the air cleaner was hooked up, but that is one item I did not even check -- I will take it out, inspect it and clean it and see if that helps. I can't think of where a vacuum leak would be originating...all the rubber seemed good, and no visible leaks when I put the carb back together, all hoses tight and not cracked, etc. And I think all the jets should have been clean...dunked in the carb solution for at least 45 minutes and then blew out with compressed air. I'll try to check the air cleaner first and see if that does anything. thanks again.
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Old 08-18-2010, 04:55 PM   #6
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i have been looking for the intake boots but they seem to be part of the air box.
The most important intake boots are on the OTHER side of the carb....between the carb and engine. Sometimes they are not really boots but big, thick fiber gasket blocks.
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Old 08-19-2010, 07:43 AM   #7
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Those ones seem to be good on mine. But I have cracks in the ones from the airbox to the carbs.
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Old 08-19-2010, 08:26 AM   #8
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If you have already done this,then please excuse.. To adjust the carb,there is the idle adjustment screw that sets the idle for both carbs by turning one screw-Honda manual calls it the throttle stop. Also,right in front of the bowl,there is a screw on each carb that the manual calls the pilot screw,and that screw sets the rich,lean mixture for the bike for each side.It could be that someone messed with these before you bought the bike and that is part of the problem? Turning the screw in makes the bike run leaner BTW
You should remove the screw and squirt carb cleaner in there and blow it out to clear out the passage,if you have not already done so. Then put the screw back in till it bottoms, and then back it out from 1.5 to 2.5 turns ,or as the bike dictates. You may want to find out the present setting of the screw,by counting the turns by turning it in gently until it bottoms. Another thing to be careful about is that there is a spring and possibly a little washer under the screw. If they do not come out with the screw, you could blow them out and lose them.
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Old 08-19-2010, 08:32 AM   #9
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Thank you i will try to look at those.
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Old 11-20-2010, 11:03 PM   #10
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IAnother 78 honda hawk 400t owner, just wanted to say my pilot screw has a small rubber washer but no spring for both carbs don't think the hawks have a spring here
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