carb issue

E

emso

Member
It would explain the poor power and the bogging. ..looks like I'm taking the head off
 
6

69ST

Well-Known Member
Yes, especially if you're getting reversion on the intake side. That would dilute the incoming air/fuel charge, probably degrade atomization quality, too. Look at the bright side, R&R-ing a head is fairly easy.
 
E

emso

Member
racerx said:
Yes, especially if you're getting reversion on the intake side. That would dilute the incoming air/fuel charge, probably degrade atomization quality, too. Look at the bright side, R&R-ing a head is fairly easy.
Click to expand...
Looks like it was the intake valve leaking. Got it all back together and doing a compression test show's it at 147 psi. Now all we need to test it on the trail is some dry weather.... thanks for all the advice
 
E

emso

Member
Daughter and I finally had a chance to test out the bikes. The '73 ran great but imI still having issues with the '77. It still has no power. Compression is showing 150 psi, checked when engine cold and with full throttle. The spark plug is black and sooty...best I can describe it. Could it be a weak spark??
 

Attachments

  • IMG_20180705_103948579.jpg
    IMG_20180705_103948579.jpg
    1.5 MB · Views: 135
  • IMG_20180705_103933113.jpg
    IMG_20180705_103933113.jpg
    1.7 MB · Views: 104
6

69ST

Well-Known Member
Overrich...only a question of what the cause is. First things to check are the air filter element and the inlet side airbox boot. Foam filters can deteriorate, those rubber boots can get pinched, or collapse.

If those check out, time to diagnose the carburator.
 
E

emso

Member
Air filter element is new and I've removed the rubber boot that the air gets drawn in from. Everything has been replaced in the carb. I'll try it out without the filter and see what happens, maybe its too restrictive . Could it be a weak coil?
 
6

69ST

Well-Known Member
Coils tend to be all-or-nothing, either they work or they don't. Points are mostly that way, but, too narrow of a gap, or fouling can result in an in-between (poor running) condition. A fouled plug can also give unpredictable results. A failed, or dying, condenser can also cause running problems and the easiest way (for most of us) to diagnose one is by replacing it.

Still, I'd want to verify that the carb isn't the issue. You've given the impression that the engine is, essentially, flooding.
 
E

emso

Member
Latest update. Swapped carbs and checked spark plugs, looks like the carb from the 1977 is over fueling ...but why? Carb had been rebuilt to factory spec. Any suggestions ?
 
6

69ST

Well-Known Member
emso said:
Carb had been rebuilt to factory spec.
Click to expand...
Clearly, that isn't the reality of the situation. This is where virtually everyone goes off the rez, at some point during the process of learning these machines. Most simply take it for granted that carburetors are all the same. So, why wouldn't the same carb cleaner that works so brilliantly with a 4bbl automotive carb, or a 40mm bike carb do the same thing with a 16mm Keihin...after all, they're all just metered air & fuel leaks, right? Understand why that mindset doesn't translate and things become clearer. The reason: scale. The smaller the engine (and the carb that feeds it) the less margin for error. These little carbs are high-precision pieces. Thus, as you've seen, a slight change can have a seemingly disproportionate impact.

Since you're getting an overrich condition, we can probably rule-out the most common issue...a partially-clogged emulsion tube (which is also the main jet holder). Assuming that the carb body is as squeaky-clean as you make it sound, the first thing I'd check is the main jet. Was that replaced with a new one? Main jet sizes, right from the factory, varied over the years. While Keihin's numbering system has been consistent since day one, there are other parameters...emulsion tube configuration, needle jet internal taper, jet needle profiles and carb body configurations - including the airbleeds. Reformulated gasoline hasn't made life any easier. Most of the rebuild kits out there have larger main jets than the carburetors originally had installed. Sometimes that works out well, other times not so much. The point, here, is that there's more than the main jet controlling fuel flow. CT70 carbs have come with mains as small as #58 and as large as #65. Every one of these carbs, regardless of subtle flavor variations, had to deliver the same air:fuel mix...because that's what these engines need. This is a long-winded way of saying that you may have an oversized main jet and you cannot, necessarily, use your other bike's carburetor as a reference guide.

The first thing to try is adjusting the jet needle. Move the C-clip to the top groove, then go for a test ride and see if anything changes. It should be lean. If it's still rich, time to pull the float bowl. Check the float height. Check the float for leaks & pinholes. Shake it, if you hear liquid sloshing around inside, it's leaking. Place the float in a freezer then, once cold, submerge it in hot water. Any pinholes will issue air bubbles. Make sure that the inlet needle and seat assembly is complete, with the spring & plunger inside the needle and the gasket on the brass inlet seat...between it and the carb body. Check the airbleeds. Spray solvent through each. You should see roughly the same amount of solvent flowing through both the main & pilot. You'll have to remove the pilot airbleed screw first, to perform this check. If all of this checks out but the engine is still running rich, time to source a smaller main jet. I'd go for the next two smaller sizes, for the sake of expediency...you'll have the second, if needed and the postage will likely exceed the price of a single jet.
 
E

emso

Member
Thanks for the information, I've completed all of the steps mentioned and it's still running rich. The main jet I have is a 58, it was bought from dratv. I'll check their website and see if I can get smaller main jet from them. If they don't carry smaller, any suggestions on who would? Thanks again
 
6

69ST

Well-Known Member
AFAIK #58 is the smallest available, for this carb. It's gotta be something else. If you can, try swapping the emulsion tube from the other carb, with whatever main jet is installed in it, at present. Let's rule-out the emulsion tube.. or in...if it's the problem. One step at a time...
 
wanrep

wanrep

Active Member
Did they replace the needle jet set when they rebuilt the carb?
Over the years, that needle and little tube that sits on top of the emulsion tube will wear against each other, causing a too rich mixture.
I've seen tubes with grooves worn in them.
If it hasn't been done, I'd replace the needle jet set IF you can find one.
Honda has discontinued it.
If you can find a Keyster rebuild kit, I think it may include the tube but I'm not sure.
 

Attachments

  • s-l500 (1).jpg
    s-l500 (1).jpg
    11.4 KB · Views: 104
E

emso

Member
finally had some time to play with the ct70 and I'm still having issues with the carb. I've replaced everything in the carb except the emulsion tube. Done with this carb....I'm looking at the carb's that dratv sell's any input?
 
hrc200x

hrc200x

Active Member
There are emulsion tubes out there that have more and larger holes than what came stock on a 70cc motor, if I recall maybe from atc90 or ct90's or maybe newer ct70's. Possible one of these is installed in your bike? Can you swap the tubes between your bike that runs good carb and this one? Have also seen pilot and main jets drilled out causing over fuel, but sounds like jets are new, unless by chance you got someone elses returned item after they drilled it?

When swapping the carbs did you keep the slide/needle with the carb that ran good, or left the slide/needle on the bike it belonged to? There are different slide lengths, biggest change was early KO to late KO. Late KO might be the same up to '77.

Had two KO's one ran great, other did not, swapped carbs between the two and the problem followed the carb. Took all the "guts" from the good carb and put it in the bad carb with no change, its like something was bad internally, but this wasn't a rich issue.
 
Last edited:
E

emso

Member
Thanks for the reply. I put the bad carb, from the 1977, on my 1973 and the problem followed the carb. It's a very odd issue, the 1977 will run perfect on idle, I can rev the engine way up with no bogging but put it under load, me sitting on it, 1 gear no problem, shift it in 2 and it starts bogging, no power...almost like it's running out of gas but check the spark plug and it's black from unburned fuel. I've rechecked the timing and point gap both are good. I think the carb is junk, any experience with aftermarket carbs ?
 
6

69ST

Well-Known Member
What you're describing sounds to me like emulsion tube clogging. I ran into exactly what you talking about, on a `76. That emulsion tube was challenging, almost beyond reason, to get completely clean. Ultimately, I found a jet file small enough to do the job and the difference was night & day.

I've run across very few carb body castings that were damaged, either mechanically, or by oxidation, to the point that the carb was good as a parts donor only. That said, it is possible that some P.O. cobbled yours together from mismatched parts. Not really sure how that might play out. The biggest difference I've noticed in 1970s vintage Keihins is the emulsion tube...mainly the number of orifices and the positioning of them. That, along with the needle jet, could have a substantial effect on jetting. IOW, it's possible that these differences could result in one 16mm carb delivering a richer mixture than the other, despite being fitted with a smaller main jet.
 
Top