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Discussion Starter · #1 ·
I have a 1978 Hawk CB400TII. This is a recent purchase and was running very poorly, but could reach about 50-55mph when pushed. I took off the carburetor and did a thorough overhaul of it but I didn't replace any jets. I blew through everything, set the floats, put all the springs back in the right places and set the pilot jets to spec. After I reassembled the bike it started at first touch and was running much more smoothly. However, when I get it up to about 40mph in 4th gear (the top gear on this bike I think) the revs just fade. It feels as if the engine is flooding, but I can't find any mixture adjustment anywhere.

I'm suspicious about how easily it started. That suggests a rich mixture too.

Any ideas?
 

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OK, this is a 5 speed transmission and when right will approach 100 mph if not exceed it.
The "pilot" screws are the mixture adjustment. The listed turns out is only a starting point. After the carbs are sync'd ride for 2-3 miles to get everything real hot and set on the center stand. Adjust idle to 1200 rpm and then adjust one of the mixture screws for highest idle while keeping the idle down at 1200. Then repeat on the other carb. Repeat all of it one more time and ride it a couple of miles to clear it out to recheck. At 1300 rpm the second idle mixture port starts to be uncovered negating the mixture adjustment so keep the idle speed down.
 

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Discussion Starter · #3 ·
OK, I'll give this a shot. I haven't been able to get into 5th gear yet; another thing to look at.
 

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At 40 mph in 4th, your revs are too low as well. Just to be nice to the neighbors, I only use second at 30 mph. These don't have a lot of torque. Try shifting to 3rd at 40 mph and I think you will notice a difference.
 

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Discussion Starter · #5 ·
Syncing the carbs the way the Honda shop manual says to do it seems to need specialist equipment. Do you have a way to do this that doesn't involve that kind of an investment?
 

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Syncing the carbs the way the Honda shop manual says to do it seems to need specialist equipment. Do you have a way to do this that doesn't involve that kind of an investment?
With Jim's wealth of experience (especially with the 400s), I'd say he does qualify as Specialist Equipment. I'd also bet that he would agree that it did involve quite an investment. ;)
 

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Discussion Starter · #7 ·
Tuning the carbs as described above has helped a lot - but there's still a problem, and I'm thinking it may not be carburetor tuning. The bike revs well in gears 1 and 2 - I've gone up to 6500 revs no problem, and there's plenty left in the bike. In gear 3 it's not so good, and in gears 4 and 5 I can't get up past 4000 revs. Am I just not riding this bike properly? My usual ride is a 1700cc kawasaki tourer, a totally different animal. Any ideas as to what might be choking off the revs in high gears?

J
 

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6500 RPM is right where the engine is REALLY starting to make good power..... You should still be well below red-line if you go to 8500 RPM.....
At 40 mph, you should have JUST BARELY shifted into third gear....

Try this.....
0 to 25...1st gear
25 to 38/40 .....second gear
40-ish to 55/58....third gear
60-ish to 65/68 ...fourth gear
70+....5th gear......

I rev my little 175 to these speed numbers ALL the time....No problem....and it has less than half your engines HP and torque......
 

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6500 RPM is right where the engine is REALLY starting to make good power..... You should still be well below red-line if you go to 8500 RPM.....
At 40 mph, you should have JUST BARELY shifted into third gear....

Try this.....
0 to 25...1st gear
25 to 38/40 .....second gear
40-ish to 55/58....third gear
60-ish to 65/68 ...fourth gear
70+....5th gear......

I rev my little 175 to these speed numbers ALL the time....No problem....and it has less than half your engines HP and torque......
Its taken me 3 years to get used to the buzz these twins generate at those RPMs... truth is, once you get there you can tell the engines love it.
 
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