Thread: air leak between carb and manifold?

Question, the symptoms are that the bike starts but won't idle (even with the adjuster screw played with significantly and the fuel air mix screw adjusted) unless the choke is on full. Once I open the throttle wide it's OK but I have also changed the throttle needle to the middle slot so its richer, at this point i can turn the choke off and it seems to run ok from about 1/2 to full throttle. I believe I may have an air leak between the carb and intake manifold. I'm not sure what the correct stackup is for the gasket/isolator to curved tube between the carb and manifold. Could someone clarify this for me also is there a good test I could perform to verify the issue (a friend mentioned firing up the engine and then spraying starter fluid where the suspected leak is. If there is a leak it will be noticeable by the reaction of the engine). My understanding of the gasket/isolator/tube set up is as follows (CT70 KO w/ standard carb):

carb
-not sure ???
curved tube
gasket?
isolator?
gasket?
manifold

as always any advice is appreciated.

I've always stayed away from ether for this purpose because of how flammable it is, and it's a bit dangerous for an engine. If you want to diagnose the problem this way, use carb cleaner. It will make the engine stumble if there is a leak.

Is the carburetor slide in backwards?
Have you checked the screen in the carb petcock? They are often clogged.

Thanks Trainwreck, my thoughts exactly on the ether, especially since I would be sitting on the bike or very close to it when spraying such a flammable liquid. I believe the slide is in correctly (to clarify - slide being what the throttle cable connects to then "slides" down in the top of the carb) i believe so, the side of the slide that has an angled slot on it is what mates up with the idle speed screw so the further the screw is in the higher up the slide goes. I disassembled the carb a few weeks back and throroughly cleaned the screen then but it may be worth a second look, I haven't been riding the bike since that time just working on it and occasionally firing it up.

When its running and I lean down towards the head it sounds like an air leak but there is a lot going on and a lot of noise so its hard to say for sure. Also, all I have between the curved tube and manifold is a gasket (thin one at that) and up between the curved tube and carb is what I believe would be an isolator piece, its black hard plastic and about a 1/4in thick. This setup seems incorrect based on my reading.

Do you have an oring on the front of the carb where it meets the spacer?

yep the oring is on the carb face that would meet the spacer (in my case the black plastic piece that is about 1/4in thick) then the face of the plastic spacer mates directly with the aluminum tube. from there its the tube with a thin gasket between it and the manifold, its this union where I suspect the leak though it could be at either spot.

You can use silicone seal for the gasket but I think your slow jet is plugged up {the small one next to the main jet}. Clean out the center and side holes with a wire off a wire brush. Make sure jet hole is clean too

Hi you are missing the gasket that goes between the insulator and the intake at the carb from what you described. i just looked at my parts manual and there is a gasket that goes there . also be very careful cleaning out the jets with wire you can mess them up by enlargeing the orifice to much, there is a jet cleaning tool you can buy if you need to depends how things are clogged up. Good luck.. Eric.

Im sure there are better tools but it dont run anyway and that will tell you if thats the problem. Ive been doing 50's, 70's, 90's & 110s for about 21 years without a problem using carb cleaner and a brush piece cut off a wire brush. That and countlesss set of KZ1000, KZ900, & Z1 carb sets that use tiny slow jets. I fail to see how a jet file will do less damage then a piece of wire brush if youre careful. Works for me anyway, do whatever you want, its yours

FWIW - here is my experience - I'm working on a 1975 CT 70 K5.

On my bike, between the manifold and head, there is a gasket, an insulator (a thick piece of plastic) and another gasket. The insulator and top gasket are smaller diamter than the gasket next to the head. I notice that later models only had the o-ring, and I believe 1970 had this same 3-part setup.

I have a weak mounting hole on the left side of the head and tried to run it anyway. Made a whistling sound - like putting a blade of grass between your thumbs and blowing.

I also have advice from an additional long-term honda-fixer-upper-guy - that a single wire from a wire brush is the best thing for cleaning out the carb jets. In my single experience so-far (at least in this decade), cleaning the carb with carb-cleaner, and the jets with a wire, has made a huge difference in my little bike. (And clamping down the loose mounting bolt). Idles well, runs much quieter and smokes much less. Just getting the accumulated junk out of the carb helped the most.

Cheers!

thanks for all the advice, this weekend the problem was fixed. though the carb looked very clean, i cleaned it again this time i removed the idle jet, couldn't see through it and after giving it the wire brush treatment and thoroughly cleaning everything else again with carb cleaner and blowing it all out with the compressor. I put it back on and it fired up second kick. the kicker still slips a ton but i think i have the feel of it so can gauge it enough to just deal with it for now.

it runs great and had it topped out at about 32 in my neighborhood but i have stop signs every other block so i don't think i actually got it up all the way, and i don't have plates for it yet either so i don't want to press my luck.

it does seem to possibly ping a little so i need to double check the timing but it idles like a dream now.