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Discussion Starter · #1 ·
Hello,

This is a follow-on thread Post top end rebuild. http://www.hondatwins.net/forums/62-engine-discussion/102730-cb400tii-78-top-end-rebuild.html
CB400T MII 1978
VB31B
Cylinder head rebored & honed
Oversize pistons and rings fitted

Bike runs great, however, the Idle sounds and feels "lumpy jumpy". see video.

Everything feels smooth on the upper end, 3rd => gear.
I'm guessing this is a carburation issue with an oversupply of fuel.
The tacho is too unstable to road sync the carbs.
Plugs are showing black sooty.
Idle/Mixture screws have been wound in from spec to 1 to 1.5 turns out. To reduce fuel
Some small amount of smoke is present when revving engine
I have pulled these carbs about 3 times to check float height and cleaned them well as per Larry's book.
My crystal ball is indicting carb removal …again.

Question: Downsizing the idle jets from spec to reduce fuel flow at idle. Good Idea?

Feel free to throw in your opinions
Thanks
 

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Since the carbs were built/delivered in a lean run configuration downsizing the jets is the last thing to think about. It's running rich for some other reason. Jets swapped? Missing the rubber plug between the jets? Float level too high? Float needles sticking open or not sealing? Air cut valves not seated fully?
 

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Discussion Starter · #4 ·
Hi Yes,

Jims tutorial is impressive and I plan to go through it with a fine tooth comb. Thanks for the link.
If I did have the jets in the wrong place wouldn't the symptoms be more aggressive ??

I will definitely double check the jet placements.
Check rubber plugs & float level
I have checked the air cut valves and polished them clean with an earbud but I'll check again.

I loosened the choke cable in case it was open when it shouldn't be.

I have had some improvement by winding the idle mixture screws in.
Plug image attached
View attachment 231177


I will concentrate on the slow jet circuit as everything else seems OK. + what is mentioned above.

View attachment 231185
View attachment 231193

You don't happen to know spec jet size for the VB31B ?? and where they go ? I can't find it in the manual.

Thanks
 

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Discussion Starter · #6 ·
So I ended up winding the mixture screws in further in and this made some improvements.
The Idle has smoothed out quite a bit.:D
Spark plug Auto part Automotive engine part Fastener Automotive ignition part
Right-hand screw is almost all the way in
I'm having trouble getting to the 1200RPM mark without the engine stalling however it sits at 1500RPM reasonably well.
I'm still going to pull the carbs to investigate the oversupply of fuel..... (after I've been for a few rides):wink:

Thanks Jim for helping me out with tech support. This is my first bike/project and this site is an invaluable source of information.
 

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Discussion Starter · #8 ·
since my last post, I've had those carbs on and off the bike 4 times.

1 : stripped them back to bones and checked, jet placements, float heights, float valves, air cut valves, slow jet caps.
No Improvement.

2: replaced all jets, Emulsifier tubes. This resulted in the rich running state becoming far worse with black smoke and wet plugs.
On inspection, the new main jet emulsifier has two variations, one with a combination of 4 holes to 1 hole the variant has 4 to 2 holes. Images attached.
The original set up had one of each. Also, I noticed that the diameter of the main jet emulsifier tube seemed larger than the original.
Brass Ammunition Metal Wood
Finger Thumb Hand Fashion accessory Nail
3. Reverted back to the original emulsifier tubes. downsized jet 110-105 and 75-72. resulting in ok idle but massive bog down under load.

4. On inspection I realized I had mixed the jets up on one side lol grrrrr

I feel like these carbs are playing mind games with me and they are winning and my wife thinks I've lost it!
 

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I can't say which of the old emulsion tubes is correct but they are far better than the new ones which are wrong.
Don't feel bad about popping the carbs off several times, it happens to all of us at some point.
 
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