Project Hawkthorne, or how to stumble your way into getting a rideable '78 CB400TII Hawk II

Also just did the compression test, both cylinders tested @~100-110psi with the engine still somewhat warm from my ride back about an hour ago.
Here's some photos of the spark plugs:20251007_214932.JPG
1759893231444.jpeg
Sparkplugs are in their respective sides.
 
What is the compression? Has anyone done a leakdown test? Another possibility here, assuming the jets are in the right places and they are genuine Keihin jets, is the clutch pads are burnt and need replaced.

EDIT: Did not see the compression test post. Throttle wide open, choke off. Does not need to be warm to do the test. If done when warm it only increases the PSI by about 5PSI reading anyways. 110 is very low for these bikes, BUT if you are using a cheap Amazon gauge it's probably not going to be accurate so borrow a real one from someone if you must.
 
Well, did the compression tests again, engine cold and at WOT:
Both were around 160psi (pictures in order respective of their sides)
1759959841976.jpeg 1759959993045.png

ancientdad said:
I'm just surprised there still places that sell RC Cola, haven't seen it in Florida since I was a young'un. :)
Click to expand...
You can find it in any grocery store up here, just not at every convenience/gas station store.
 
Last edited by a moderator:
OK, compression isn't that bad after all.
I would verify the plug end caps have 5K resistance.
Carb sync followed by mixture adjustment for highest idle speed.
If still a problem then go after float levels next.
 
Well, I will continue to follow your progress, but I will need to refrain from providing any further advice. You may be entering expert territory, so I will leave it to the experts.
 
LongDistanceRider said:
OK, compression isn't that bad after all.
I would verify the plug end caps have 5K resistance.
Carb sync followed by mixture adjustment for highest idle speed.
If still a problem then go after float levels next.
Click to expand...
I'm thinking it's the carbs still, when they were "cleaned", I don't think they were properly cleaned. Just partially taken apart and dumped into a ultrasonic for an hour or two and blown out. I'll check the caps, though I would be surprised if frank gave me bad caps, they are NOS NGK caps from what I recall... But things do break down even in they sit, this bike is a testament to it! 🤣
esmith250624 said:
Well, I will continue to follow your progress, but I will need to refrain from providing any further advice. You may be entering expert territory, so I will leave it to the experts.
Click to expand...
Any advice is good advice for me, and I appreciate that there are others watching this thread. I can never state enough that I don't have much technical knowledge, more-so when it comes to carburated engines bigger than a weed whacker.
 
Fizzbuzz3000 said:
I'm thinking it's the carbs still, when they were "cleaned", I don't think they were properly cleaned. Just partially taken apart and dumped into a ultrasonic for an hour or two and blown out. I'll check the caps, though I would be surprised if frank gave me bad caps, they are NOS NGK caps from what I recall... But things do break down even in they sit, this bike is a testament to it! 🤣

Any advice is good advice for me, and I appreciate that there are others watching this thread. I can never state enough that I don't have much technical knowledge, more-so when it comes to carburated engines bigger than a weed whacker.
Click to expand...
They are new NGK caps that I just purchase from DSS and charge the same cost. They were definitely made in the past few years.
 
Here is how to do it:


You can also do it by ear if you've done it enough. Basically, just turn the screw all the way in slowly until it seats. Bike should die/bog out or be very close. Turn it back out until it does about the same. In between those two extremes is where you want.
 
My method is to adjust the mixture screw of one carb to get the highest rpm, where turning in either direction lowers the rpm.
The rpm must stay between 1100 and 1300 which in some cases means readjusting the idle speed, usually slower. If the idle speed has to be adjusted you readjust the mixture again. Repeat on the other carb.
Note if the 2nd carb requires idle speed adjust then you've got to go back to the 1st and repeat.
When done you'll notice that there's @1/4 turn rich/lean that makes no change, set 1/2 way.
Frequent riding a few miles after changes and rechecking is advised.
 
Hey all I've pulled the carbs back off, and gonna get them a proper clean. I decided to try the "pull the airbox out" method to get the carbs of and can say, it's just easier to do it the way the FSM tells you to do it. Hopefully I can get this issue with the carbs solved. Untill next.
 
What is the FSM way? I was pretty sure they want you to do the same. You can pull the box back just enough to sneak the carbs out of the way.
 
Maraakate said:
What is the FSM way? I was pretty sure they want you to do the same. You can pull the box back just enough to sneak the carbs out of the way.
Click to expand...
You pull the tank off, and remove the bolts holding the connecting plates for the upper engine block. You loosen and move the rear clamps that connect the airbox and carbs, and loosen the bolts that hold the front boots on. You then pull back on the carbs, and twist down and pull to the left. The carbs will come out fairly easily. pull off all of the cables and the carbs are free. And you just do the reverse to put them back on, no picks needed.
 
I personally don't like doing it that way because it's very easy to strip the threads on the head for the intake insulators and you should replace the o-rings once disturbed. Removing the tank and part of the engine holder bracket does make things easier and should be done.

Glad you found an easier way to get the job done.
 
Hey all, back with another update: The carbs were plugged, and plugged bad. They were sat in the ultra sonic for 3 hours and spewed rust, dust, grime and water from the previous cleaning! The guy who cleaned them (who has nearly half a century of experience in rebuilding carbs and cleaning them) also told me that the diaphragms and o-ring for the air-cutoffs need to be replaced, so I'll get to ordering that. Might have a running bike by the end of the week!
 
If you read the whole VB carb clean thread and do pull the pressed in slow idle jets, it will run good, but if you skip that expect a dead stumble in the midrange.

How to Rebuild the VB Carburetors

This is dealing with the VB22J carbs in particular but most of it applies to the VB21 and VB24 carbs. As everyone knows that has done these they are problematic to get right the first time at best so hopefully this will help you get there. In this case I'm dealing with VB22J carbs that have had...
www.vintagehondatwins.com www.vintagehondatwins.com
 
Hey all, back with another update, carbs are on and synced. It has a very slight erratic idle, though that might be due to me slightly over/under adjusting the mixture screw. I was able to get the bike up to 7-9k rpm until I hit 5th, then it for sure stumbles back down to the 50mph/5k range. Had me thinking:

What if the aftermarket 3/16" petcock the PO put on the tank is restricting fuel flow? I got a proper sized 5.5mm one on hand was gonna wait until I got the gas tank empty, but maybe that would be a better idea. And as a last note, I seem to be having an oil leak out of a bolt that sits right above the the power chamber, is this a common leak spot?


Until next time!
 
Mixture screw is not as mystic to adjust as it appears. Set them to 2 1/4 turns out to start, or flush with the casting. When bike is warm turn one side in until it stumbles or dies. If it fully seats and does nothing or very little you have a dirty carb circuit or another issue. Diagnose and come back. Go out until the stumble is gone and then a bit more. The idle may raise. Usually about here is where it needs to be. Honestly, most of these bikes seem to be around 2 1/2 turns out. Setting it there without further adjustment is probably good enough if you cant get it dialed in.

Need a picture of the leak, but probably from the gear shift lever seal if its coming from thr left side.
 
The idle circuit is clean, the screws do affect the idle when pulled in or out. I have the carbs set at the stock 1 1/2 turns (at least as far as I could eyeball it) and it idles fine, I just might be paranoid about it and be falsely hearing it skip. I might do another quarter to half-turn on them to have it a bit richer side. I'll send a photo of that bolt tomorrow.
 
Ahh yeah, forgot the 78 CB400T is 1 1/2. Probably could just set it to 2 turns out and it would be fine if you're unsure.
 
well, I got the cabs back on again and it still just flat-lines at 5k rpm in 5th. I genuinely don't know what to do at this point, the only thing left is to go and pull the slow jets out, I suppose.
 
Tape part of the air duct off with furnace tape or duct tape. About 1/4 to 1/2 and let me know if things improve.
 
Just taped the air ducts 1/2 off and it just made it worse. I couldn't snap the throttle at all and couldn't get the throttle past 1/4 turn with out it almost dying.
 
Only tape off part of it. If you tape it all then it will be like riding with the choke on. Get us a picture of how you taped it.
 
I have taken the tape off already, but here's a crude rendition of what I did with 1/2 of the air intake taped off:
1761436231603.png
 
Fizzbuzz3000 said:
well, I got the cabs back on again and it still just flat-lines at 5k rpm in 5th. I genuinely don't know what to do at this point, the only thing left is to go and pull the slow jets out, I suppose.
Click to expand...
Doing it on my 78 Tii made a huge difference. Mid range flat spot gone and better high end too.
 
Maraakate said:
Try it like this and let me know.
Click to expand...
I did it like this and the result were it began to stumble and flatten on the throttle out sooner than without the tape on.
1761516414990.jpeg
also here are some videos, first one is just after the bike has started and ran, second one is after a short 3/4 mile turnaround.
streamable.com

Watch 20251026_150733 | Streamable

Watch "20251026_150733" on Streamable.
streamable.com
streamable.com

Watch 20251026_151238 | Streamable

Watch "20251026_151238" on Streamable.
streamable.com
 
Check the spark plugs. I know I gave you new ones, but look at them and check the gap. I just had one that ran very poorly at various ranges and it ended up being the one plug was pushing on the electrode causing a misfire. If in doubt, reset the gap to 0.025"-0.028". I know I gap them to 0.040" before sending them out (usually), but just check and set in case something isn't up to snuff.
 
Fizzbuzz3000 said:
You know what drill bit size+ tap size you used? for future reference.
Click to expand...
#3-48
20251026_181750.jpg20251026_181801.jpg

Amazon.com: Musiclily Ultra UNC#3-48x1.1 Inch Stainless Steel Humbucker Pickup Mounting Screws for American Made Electric Guitar, Nickel (Set of 20) : Musical Instruments

Buy Musiclily Ultra UNC#3-48x1.1 Inch Stainless Steel Humbucker Pickup Mounting Screws for American Made Electric Guitar, Nickel (Set of 20): Pickups & Pickup Covers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
 
Maraakate said:
#3-48
View attachment 51493View attachment 51494

Amazon.com: Musiclily Ultra UNC#3-48x1.1 Inch Stainless Steel Humbucker Pickup Mounting Screws for American Made Electric Guitar, Nickel (Set of 20) : Musical Instruments

Buy Musiclily Ultra UNC#3-48x1.1 Inch Stainless Steel Humbucker Pickup Mounting Screws for American Made Electric Guitar, Nickel (Set of 20): Pickups & Pickup Covers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Click to expand...
That is probably better. I used the 4-40 and did need to drill about 2-3 mm. Don't drill too deep, only enough for tap to engage and cut a couple threads.
Here is my effort.

VB21B pressed in idle jet orifice depth and other leaks and glitches

My 78 CB400T2 has been sidelined for a year due to carbs leaking like a sieve. It was 2 years since following LDR's rebuild thread and I must have used some HF nitril Orings on the fuel cross over tube. They had shrunk so bad the tube rattled in there. The McMasterCarr Orings that Frank...
www.vintagehondatwins.com www.vintagehondatwins.com
 
hey all, I'm back for a small update: I got a bit too agitated and ended up crashing the bike on some slick gravel at ~1-15mph earlier this week. I'm fine, other than a sore shoulder and my dignity broken. The bike itself is fine as the crashbars did its job and got somewhat bent. Probably can bend them back and keep using them.

Anyways! Shrinking the gap down to OE specs did not improve the 5k RPM issue! So I'm thinking it's the slow jet is still plugged to hell, or I have an air leak somewhere in the air box. I'm thinking the former more than the latter.
Maraakate said:
#3-48
Click to expand...
While I think it's the same, but is this size also OK for VB21B carbs? I do not have VB21A ones.
 
Hey all, I am back! After taking a breather, I now have the secondary 42 jets out (which look excellent) and have yet to blast some carb cleaner into the carbs; While it may seem a bit odd, but would there be a way to keep the carbs mostly together while pulling out all of the o-rings to I can go to town with the carb cleaner/wire? I would assume not but will ask anyways. Couldn't get a good picture of the channel area past the jet, apologies.

20260415_210259.JPG20260415_210229.JPG
 
You must log in or register to reply here.
Back
Top Bottom