[Hawkman]

I have a 1978 Honda Hawk 400 twin that I’m working on for a friend.
When he let me at it, it would run and ride but it was low on power and barely made it to 35mph in first and would go basically nowhere in the other gears.
I took the carbs off and cleaned em up a bit, I found a clogged jet or two and the accelerator pump had the spring on the wrong side of the diaphragm so that wasn’t working at all. I put them together best I could with the same just a just cleaned out.
The bike runs good now but only for a minute or two. I started it up after putting the “brand spanking new” carbs on there and went to ride it around and it got to 70mph quite well but then after a little bit the bike would hardly accelerate even at full throttle, popped through the exhaust sometimes, and even died whenever I tried taking off, so a significant loss of power.
I have a feeling it’s a fuel supply issue but this is the first one of these style bikes that I’ve worked on so anything helps 😁

 

[DD23]

35 in first !

Does this bike when at running temperature seem to "surge" , at about 4 to 5 k revs ?

Anyways it's had carb problems as stuff was blocked/incorrect in the carbs. Perhaps that was an attempt to fix the engine problem by the last owner.

To clean the carbs, start at the fuel filler cap.

So, does the engine seem to surge just as if it's got a fuel problem ?

Over.....

 

[Hawkman]

35 in first !

Does this bike when at running temperature seem to "surge" , at about 4 to 5 k revs ?

Anyways it's had carb problems as stuff was blocked/incorrect in the carbs. Perhaps that was an attempt to fix the engine problem by the last owner.

To clean the carbs, start at the fuel filler cap.

So, does the engine seem to surge just as if it's got a fuel problem ?

Over.....

In the “low power mode” I get brief glimpses of power, split seconds of acceleration every 10is seconds while the throttle is pinned. It doesn’t seem to surge smoothly up and down as I think you’re describing, tonight I’ll just let it idle for a while, hopefully when it gets warm it’ll start making things obvious.
I drained bad gas out of the tank when I got it and I cleaned the petcock. The line from petcock to carb could stand to be replaced though, it looks like it’s shrunk in diameter from heat or something.

 

[DD23]

So the tank filler cap vent is clear then ?

By surging I mean it's as if a rev limiting device has kick in, engine does not rev past 5 k when up to temp ?

A reduced diameter fuel hose would decrease the fuel flow, so carb empties more quickly than it can be filled, to yes replace that hose and try again...

 

[Hawkman]

So the tank filler cap vent is clear then ?

By surging I mean it's as if a rev limiting device has kick in, engine does not rev past 5 k when up to temp ?

A reduced diameter fuel hose would decrease the fuel flow, so carb empties more quickly than it can be filled, to yes replace that hose and try again...

Yes it does have a rev limiter of sorts, doesn’t wanna rev a lot. I didn’t check the cap vent, I didn’t really think about there being a passage in the cap, makes sense though, you could have too much vacuum in the tan and then no more gas comes out or whatever. I’ll try cleaning that tonight.

 

[3hunner]

You might want to fully strip the carbs and dip them or clean them with an ultrasonic. Make sure you have the right size jets in the right places...primary main jet (smaller size) goes into the carb body, and the secondary main jet (bigger size) goes into the brass needle jet. Make sure your float heights are within spec too. If its still having issues after that, the next place I would start looking into is electrical, the CDI and ignition coils. Electrical issues with these bikes can feel similar to carb issues. Sputtering at certain rpm then picking up again. Another thing too is I'm not sure the 78 CB400T model came with an accelerator pump. For that year and model you should have VB21 carbs. Those have air cut diaphragms but no accelerator pumps. Good luck!

 

[Hawkman]

You might want to fully strip the carbs and dip them or clean them with an ultrasonic. Make sure you have the right size jets in the right places...primary main jet (smaller size) goes into the carb body, and the secondary main jet (bigger size) goes into the brass needle jet. Make sure your float heights are within spec too. If its still having issues after that, the next place I would start looking into is electrical, the CDI and ignition coils. Electrical issues with these bikes can feel similar to carb issues. Sputtering at certain rpm then picking up again. Another thing too is I'm not sure the 78 CB400T model came with an accelerator pump. For that year and model you should have VB21 carbs. Those have air cut diaphragms but no accelerator pumps. Good luck!

It has some kei hin carbs on it, or some brand with a similar name. I was kind of trying to avoid having to dip them. :/

 

[3hunner]

The model number should be stamped on the side of the carbs. When you dip/ultrasonic the carbs and do a proper rebuild that pretty much eliminates the possibility of it being carb related you know…then after that you can start to check off electrical issues…pretty much a process of elimination with old bikes in general.

 

[Hawkman]

The model number should be stamped on the side of the carbs. When you dip/ultrasonic the carbs and do a proper rebuild that pretty much eliminates the possibility of it being carb related you know…then after that you can start to check off electrical issues…pretty much a process of elimination with old bikes in general.

The carb that I can read the number on says vb22a.

 

[Hawkman]

So the tank filler cap vent is clear then ?

By surging I mean it's as if a rev limiting device has kick in, engine does not rev past 5 k when up to temp ?

A reduced diameter fuel hose would decrease the fuel flow, so carb empties more quickly than it can be filled, to yes replace that hose and try again...

Update, this doesn’t seem to have a vented fuel cap. I replaced the fuel line and it doesn’t seem to have helped.
Started it up just now, cold, and it idles at 5k, no response to the idle screw. Then it dipped back down to 2k and responded to the idle screw and then jumped back up. That repeated over and over again.
I tried a starter fluid leak test and I found that if I spray starter fluid at the right side intake (between carb and motor) then the bike revs up, but if I spray it at the left side, then the bike idles down and dies.
this is so confusing.

 

[Hawkman]

Going to take the carbs off once again and make sure they’re spiffy. Woohoo go me.

 

[3hunner]

Sounds like you have a serious intake leak. Get new intake boots and o rings for sure

 

[DD23]

Yep, that will do it, so some more fueling issues found.

There will be a tank vent else fuel flow will "hang"...

If I had a penny for every time I offered the following I would be a rich man...

Research and get some of those red neon plug caps that flash as HT passes thru them but you will have to make an adapter for them to fit inline between oe plug cap and plug. A loss of HT can clearly be seen even when riding. A cheap real world bit of kit for ign problem identification and will pay for them selves the first time you use them. Never lone them to anyone as you will not get them back !

Also a multi meter.

Increase revs, increased rate of change, increased ac volts output.
Check AC volts output of ign related wires, you may find one that decreases AC volts output with revs, down to about 28 volts AC, when the misfire starts to happen. I have come across a "leaky" handle bar kill switch, so to illuminate that just disconnect the cdi black/white wire.....but you will have to stall the engine to stop it, it turn off the fuel and wait.

 

[DD23]

Yep, that will do it, so some more fueling issues found.

There will be a tank vent else fuel flow will "hang"...

If I had a penny for every time I offered the following I would be a rich man...

Research and get some of those red neon plug caps that flash as HT passes thru them but you will have to make an adapter for them to fit inline between oe plug cap and plug. A loss of HT can clearly be seen even when riding. A cheap real world bit of kit for ign problem identification and will pay for them selves the first time you use them. Never lone them to anyone as you will not get them back !

Also a multi meter.

Increase revs, increased rate of change, increased ac volts output.
Check AC volts output of ign related wires, you may find one that decreases AC volts output with revs, down to about 28 volts AC, when the misfire starts to happen. I have come across a "leaky" handle bar kill switch, so to illuminate that just disconnect the cdi black/white wire.....but you will have to stall the engine to stop it, it turn off the fuel and wait.

 

[DD23]

You know if CMSNL com ?

Find your exact bike, exploded views and component pictures are really usefull...

Post a link to your exact bike in CMSNL so we can refer to it...

You have fueling issues and may have ign issues as well.

You have checked valve working clearances, cam and balance chain, and air filter.....

Again, no rag under saddle, it happens...
.

 

[Hawkman]

You know if CMSNL com ?

Find your exact bike, exploded views and component pictures are really usefull...

Post a link to your exact bike in CMSNL so we can refer to it...

You have fueling issues and may have ign issues as well.

You have checked valve working clearances, cam and balance chain, and air filter.....

Again, no rag under saddle, it happens...
.

Update:
I thoroughly cleaned the carburetors and chem dipped the jets and air bleeds and bowls and whatnot. Got new intake boots, slapped everything together.
The bike started up great after a week or so of sitting and idled good after some messing with the idle screw.
I went to go ride it and and it started off great but the bike won’t rev past a certain throttle percentage.
In any given gear it will rev up to the same amount of throttle (~70-80%) and then just sit at that rpm (8-9k).
Would this be indicative of an electrical issue, something keeping spark from happening correctly perhaps?
Side note: the left hand carb’s bowl screw doesn’t go in all the way or something so it won’t seal properly, causing a consistent leak from the drain nub, it doesn’t seem to have an effect on anything as more gas just flows right into the bowl from the tank.

 

[Hawkman]

https://www.cmsnl.com/honda-cb400tii-hawkii-1978-usa_model7218/partslist/#.Y4mWzdM74lQ

My bike

 

[Bill H]

Does the bike have the original air intake ,filters, power chamber and exhaust on it ? Any change can cause the air flow thru the carbs to change and give you a lean condition on the upper end. Just a thought.
Bill H

 

[Hawkman]

Does the bike have the original air intake ,filters, power chamber and exhaust on it ? Any change can cause the air flow thru the carbs to change and give you a lean condition on the upper end. Just a thought.
Bill H

The bike looks to be completely factory besides the intake boots that I just replaced

 

[3hunner]

I would fix that leaky float bowl asap…probably just need a new screw/o ring. As for the the bike cutting out, it could be electrical. I had a similar issue on my bike. Wasn’t happening at throttle position but rather at a certain rpm (7500rpm to be exact) no matter what throttle position/gear I had it in, whenever it reached 7500rpm, it hit a flat spot. So i swapped the old oem coils for different ones and the issue went right away. Kind of a process of elimination since I KNEW the carbs were perfect and compression was good. Hope you made sure your float height was in spec on both carbs. Might want to start with electrical from here. Try changing out the coils and maybe even the cdi. I am using Rex speed shop cdi