Need help with 1979 CT70 rebuild

[vcarangelo]

New to bikes, but I've been a gearhead all my life and worked on all kinds of cars and trucks. My son and I have been rebuilding this bike for him which was handed down by my dad (before my son was even born!... long story). Anyway, we have it running but I still have a couple of things that don't seem right to me:

Carb - I'm not sure that I installed the spring on the choke shaft correctly because it only runs with the choke in the closed position. As soon as I open it the engine dies if you don't keep blipping the throttle. Can anyone post a picture of how the end of the shaft looks under that black cap?
Also on the carb, there is a hose that should connect to a nipple just above the idle screw to somewhere..I didn't make good enough notes about this before we took the carb off. Is this something anyone can explain or post a picture of to see where the hose should go?

Electrical - There is a round connector-half that has a number of wires going to it and contacts on the flat side like it mates to something. It's sticking out of a hole in the right-side of the frame near the handlebars. Can anyone explain what this is? I've found a download of a user manual from a 1973 CT70 that shows the engine kill switch mounted on the same end of the frame but on the left-side. This switch has a day time and night time run position, which I assume turns on the headlight. I'm wondering if this is the headlight switch (broken) for this bike, since the headlight will not come on.
None of this info is shown clearly (or at all) in my Clymer service manual. I really appreciate any help you folks can offer.

 

[69ST]

Congrats...welcome aboard...let the madness begin! Don't worry, it's a benign disorder:35:

Sounds like the pilot (idle) jet is clogged. Pretty common, since most went into longterm neglec...umm...storage with gas in the tank & carb. The carb will have to be thoroughly cleaned and the fuel circuits cleared. I'd plan on either rebuilding or replacing the carb. This vintage carb is not the easiest thing to find or to source rebuild kits for. You'll have an easier time sourcing the latter. As for the extra hose, it's probably the crankcase vent tube - primitive emission control. Worst case, you can always use a more readily available OEM replacement carb originally intended for an earlier model. Won't be "concours correct" but it will run normally.

The `79 has some one-year-only electrical parts. It was the last year to have the keyswitch in the traditional location in the frame and the only year with the keyswitch in this location AND fixed handlebars. Sounds like the unit with the turn signal switch is your DOA. I kinda doubt that the OEM part, with the original part number, is still available. However, Honda has been pretty kind over the years, using the same part over a number of different model bikes. Thus, once you figure out which unit is trashed, it will almost certainly be possible to source the same, or nearly identical, part under a different PN. Most likely, a late `70s/early `80s vintage CT90/110 or other small, road legal model will have a usable part and still be avaialble as new OEM or NOS.

 

[vcarangelo]

Thanks for the help. I take it from your info on the keyswitch that this bike should have the keyswitch in the frame as well as the kill switch on the handlebar (or am I misunderstanding)?

 

[69ST]

Thanks for the help. I take it from your info on the keyswitch that this bike should have the keyswitch in the frame as well as the kill switch on the handlebar (or am I misunderstanding)?

Correct, ignition keyswitch in the frame + kill switch on the bar. Something that the earlier models should have had.

 

[CT70Longhorn]

Also on the carb, there is a hose that should connect to a nipple just above the idle screw to somewhere..I didn't make good enough notes about this before we took the carb off. Is this something anyone can explain or post a picture of to see where the hose should go?

The original '79 OEM carburetor does not have a hose on that nipple. Suppliers like CHP and dratv.com sell a replacement carb for the '79 that shows a hose connected to that, but I've never been able to figure out what the deal is with that.

This is what the original setup looks like, sans the fuel filter of course.

 

[Flash]

The original '79 OEM carburetor does not have a hose...but I've never been able to figure out what the deal is with that...

Believe its an emissions thing that came on later models. Honda won't sell carburetors anymore without it due to current laws.


Damn niiiice Yeller '79 CT70 BTW!!!:rockon:

 

[vrodsss]

I'm sort of fond of the yella two ! :common001:

 

[Flash]

I really like that color/model. Coulda bought a local low mileage one here with title for...$400! Still kicking myself...

 

[chee001]

Please Help Me!! Those pics are the first ray of hope I have seen since I got my 79 minitrail. I bought it for 600.00 with what i think is a 24mm flat slide mikuni carb. The bike idles great but runs like crap. If I ease into the throttle it runs ok but if I try to gun it it floods out. I tried changing the pilot jets and mains with no success. I thought I needed a new exhaust so I bought a lifan pipe but its worse. Is there anything else I can do to make this large carb work on my bike? Please help!!!

 

[motodevo]

that carb is too big for anything under 125cc, way too big for 72cc.

 

[chee001]

So a smaller or stock carb is my only option? Is there anything else i can do to make this work?

 

[motodevo]

even if you jet it down for the motor, it will never be right at WOT, i'd just buy a ct70 style carb, you can get them for bugger all on ebay, the c70 passport style ones can be had new for under $50 and will work great with no fiddling

 

[69ST]

So a smaller or stock carb is my only option? Is there anything else i can do to make this work?

No carburetor that's larger than than the intake port is ever going to work optimally. Even if you custom-machine an intake manifold to smoothly transition from 20mm at the port interface up to 24mm at the carb outlet, the motor won't draw enough air at low-to-mid rpm to generate sufficient airflow velocity inside the venturi. A 24mm carb has 225% of the venturi area of the stock 16mm unit; it's also 44% more cross-sectional area than the port itself. Any increase in venturi size is accompanied by a matching decrease in airflow velocity at any given CFM. Slower airflow degrades fuel atomization and makes fuel circuits lazy; the result is lean bogging, weak throttle response and a carb that's damn near impossible to tune.

Do yourself a favor, sell the 24mm carb to someone with a larger engine and use the proceeds to offset the cost of an OEM Keihin replacement. Unless your plans include more displacement and a race head, you'll be a lot happier with the results.