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Discussion Starter · #1 ·
Hi y'all,

I am a new rider and I'm learning a lot about my 1978 Honda Hawk cb400TII.

I just installed a new petcock, the old one was leaking badly. However, now I have a situation where the fuel isn't flowing fast enough - it drips through the fuel hose. I'd like to see a steady flow of fuel when I'm riding and not little drops! I wonder if I installed the petcock incorrectly? There was a plastic tube over the intake valve when I got it new - was I supposed to keep that part on???? It didn't look like a filter or anything. I thought it was there to protect the valve before installation. Ugh.

Secondly, I know that I could use some engineering 101 classes, but I'm having a hard time imaging how the gas gets to the top of the intake valve in the gas tank once the gas level is below the top of the valve???

And lastly, the fuel hose from the petcock to the carbs has been changed out as well. Does the line have to be straight or can it loop and turn upwards, making an s shape?

Thanks in advance! This is such a great forum!
 

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Hi y'all,

I am a new rider and I'm learning a lot about my 1978 Honda Hawk cb400TII.

I just installed a new petcock, the old one was leaking badly. However, now I have a situation where the fuel isn't flowing fast enough - it drips through the fuel hose. I'd like to see a steady flow of fuel when I'm riding and not little drops! I wonder if I installed the petcock incorrectly? There was a plastic tube over the intake valve when I got it new - was I supposed to keep that part on???? It didn't look like a filter or anything. I thought it was there to protect the valve before installation. Ugh.

Secondly, I know that I could use some engineering 101 classes, but I'm having a hard time imaging how the gas gets to the top of the intake valve in the gas tank once the gas level is below the top of the valve???

And lastly, the fuel hose from the petcock to the carbs has been changed out as well. Does the line have to be straight or can it loop and turn upwards, making an s shape?

Thanks in advance! This is such a great forum!
I myself am Unsure about YOUR specific Bikes specs, But if it is a VC (Vacuum Carb iirc) it dont matter as long as it is not kinked. I feel you should check your Carbs for Vac leaks, and most likely that is Your Low/No flow issue.

honda-cb400a-hawk-hondamatic-1978-usa-fuel-tank_bighu0113f7013_ffe8.gif

As for the Plastic tube, You need that. it is a "strainer" or screen and probably microperforated so you didn't see the holes.
As for fuel levels, position 1 is primary supply.(taller brass tube side) Position 2 is into the reserve, at this point You should keep in mind You are into roughly the last 1/8 of a tank and should hit a Gas station real soon as to not putter out stranded.

tank-illus.png
The yellow arrow points to the petcock intakes (or "straws"). The blue area is the "fuel reserve," and is accessed by the short tube.
The red area is unusable fuel.
The white area above the blue, inside the tank, is the "normal" fuel, and is what the longer tube can reach. Each tube has an opening at the end, preventing it from reaching the fuel which is lower than its upper end.
Your tank has less red area this example was of a Ninja 250 tank.However it was the best example Google tossed Me that had all of what you needed to See to understand easily and clearly based on you question.

I alway refer people to CMS, especially if they Do not have Shop Manuals yet.
Mainly because they Do have crude assembly diagrams and all OE Honda part#s provided You can find your bike. I use it myself for my bike to faster find OE part #s.

Being that CMS is based in the UK, it is NOT cost effective to order parts from them to the US, typically speaking when counting for customs and shipping or whatever else. I get the #'s and run them through ebay and as some have referred to it the Google Monster, lol.

Do Yourself a favor, Get a Shop repair manual for Your Hawk. Do Your Hawk Justice and stay away from Chiltons and Clymer.The only thing those two are going to teach You is how to rebuild your Honda Hawk as a Howda Wounded Duck!

I am sure someone on HT has some form of 78 Hawk and related Shop Manual in PDF Adobe Acrobat form.......Pita to use without a laptop since you would really need to print the whole thing, and still awkward with a laptop.

I know because I cave a PDF Manual for Blue Jay (bike in sig/avatar--[SUB]got dang I need to update that stuff[/SUB]) and when things get serious, Out to the shop I go Laptop underarm,charger in one hand, and a big cup of "Don't Hulk out on it"......err uhm ye I meant Coffee in the other.

seriously though CMS will save you for now, Hope you find that plastic piece in unharmed condition
 

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As Mayhem describes you do want the plastic filter screen. It should look like this. With this in place there's no need for any additional filter.
201_4144.JPG
The fuel line should make a gentle arc from the petcock to the carb inlet similar to this
B84ZTGH.jpg

PM sent about the Honda FSM
 

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Discussion Starter · #4 ·
Thank you for the info! The diagrams are very helpful.
Yes, I have the original manual as a PDF, it's very helpful - as is this forum. I did keep the filter and reinstalled the petcock with it on. I had to really work at getting the old one out as it had probably been in there for a long time.

I am still having fuel starvation issues, but it's intermittent. When the bike has been running for a while, the fuel flow goes back to a dribble.

For now, I'm on to reinstalling the choke cable. I am missing the plates on the carbs that they attach to, and wonder if I'm calling them the right thing when I do a web search. Choke cable plate? Choke cable bracket? Is there a cam wheel too?

I seem to suffer from a lagging throttle and poor fuel consumption (80 mi/tank). But perhaps these are questions for a new thread. I'm also looking through archives to see what I can learn.

Thanks to you both!
 

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Pictures of your carbs will be very helpful so we can see what may be missing. Yes there should be choke plates, a bell crank, joining linkage, springs, etc. You should see most of this stuff in the carb rebuild stickies
+1 on the gas cap restriction.
To remove the petcock filter in the future use a screw extractor, should be removed and cleaned yearly.
 

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Discussion Starter · #7 ·
That website is very helpful, thank you! It's funny, though, the part I'm looking for is the only piece in the diagram that isn't numbered (between 25 and 26).

I'll try opening the fuel cap. If that does help, what's the fix? I can't ride with the fuel door open, right?

I'll take photos! Thank you!
 

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That website is very helpful, thank you! It's funny, though, the part I'm looking for is the only piece in the diagram that isn't numbered (between 25 and 26).

I'll try opening the fuel cap. If that does help, what's the fix? I can't ride with the fuel door open, right?

I'll take photos! Thank you!
New Gas Cap is the only guaranteed fix, You can try Vinegar soaking to try pulling the rust out....no guarantees it will do the job.

I wouldn't recommend riding with a loosened cap, You could lose it altogether,but it is a given you will be sloshing fuel all over the top of the tank, which will eventually ruin the finish on even the best of "fuel proof" enamels.

Just think though, after it has sloshed up and out, where is it going to be prone to run Down to, Front edge of the seat, exhaust headers, the cylinder head, frame......ect.
 

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Discussion Starter · #9 ·
I have a couple photos of the carb, I took them when I was having petcock issues. They dont' show the side, but I'll take more later. Is that brass bracket behind the petcock for the throttle cable?
petcock2.jpg petcock.jpg
 

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That website is very helpful, thank you! It's funny, though, the part I'm looking for is the only piece in the diagram that isn't numbered (between 25 and 26).
That part is the throttle cable mounting bracket that only came with the left carb. It holds the pull, push and choke cable in position.
"Push" cable is closest to the head.
201_4030-002.JPG
Green circle is your pull cable, Red circle is your push cable and the Blue is the choke cable. It's easiest to install the cables on to the carbs and linkages before installing the carbs on the engine.
 

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Discussion Starter · #11 ·
Hey guys, I just wanted to give a quick update. The petcock is fine, now that it's installed correctly. There is a bracket missing for the choke cable, and I'm having a local shop help me with it. Boy, I wish I could say that's where the complications end. But, it's an old bike and I'm learning a ton. I know that I can post here if I hit a wall. Thanks for your help!
 
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