bogging bike in 2nd gear

I'm fixing up a 1984 CT70 for a friend (I believe they stopped selling the CT70 in 1982 in the States, but continued until 1984 up here in Canada) and have encountered some bogging issues.

Basically, the bike idles just fine with no choke, and putts around in first gear with no choke, but when I drop it into 2nd and give it some gas it needs the choke on or it dies/bogs.

Here's what I've checked/done so far:

1) First off, this bike had it's head replaced with the empty head from Beatrice. The original head ran fine on the bike, but, the exhaust stud snapped off of it and when trying to drill it out I basically ruined the head by drilling too close to the muffler port and drilled right through that thin wall that separates the stud from the port. That wasn't a good evening.
With that head you have to use the larger valves, which I did.

2) After the head rebuild the bike ran, but bogged. So, I swapped out the stock Keihin carb with the 16mm Keihin replacement carb from Beatrice that I had on another CT that ran fine. The bogging issue remained.

3) Adjusted the needle jet height. Still bogs.

4) Kicked dirt and swore. Still bogs.

Basically, I've concluded that it is not the carb. Any idea what else it could be?

I've seen 1980s models as late as 1987, actually. Unfortunately, there's F/A for documentation covering the details unique to any of them produced between `83 & `91. So...educated guesses are as good as it's going to get, until such time as someone digs into the motors of those models to verify what's inside.

I'm assuming that you've verified point gap. Next step is to verify cam timing. It may be off by one tooth...easy to do during reassembly, especially if you're inexperienced with these motors.

If the problem persists after verifying those two parameters, next step is making the main fuel circuit richer. Start by dropping the jet needle C-clip to the bottom groove. If the engine runs better, then you're on the right track. If there's no difference, you may have lowered the static CR with a head/piston dome mismatch. Not knowing which piston crown and combustion chamber dome configuration of the original head makes this a guessing game, for the time being.

A fresh set of points and condenser could be what you need. At least that will help you narrow the trouble shooting and they could be near the end of their life cycle anyways.

Unlikely to be points gap as the bike ran well before the head change over, but it's easy enough to check.

Cam timing is something I'm very aware of and usually get it right, but again, it's easy enough to re-check, so I'll do that too.

Will also try setting the c-clip to the bottom groove.

Thanks for the advice guys. I'll let you know the results.

we had a guy on the forum (no names mentioned) who had an obscene bogging problem. if you're sure it's not the carb, i'd lean towards timing. one tooth makes a BIG difference when there's only 26/13. that's a guess but i don't think it's off by much.

Even on finer teeth systems, timing off 1 tooth makes a difference. on a V engine, if you have it off one tooth on one side, you might never notice, but if you have it off one on a single pulley 4 cylinder etc. it runs like a bag of rocks. either idles awesome and dies with RPMs or won't idle for anything and screams on the top.

something to consider. Totally agree with above info, btw.
keep in mind you COULD have a fuel delivery issue to both the old carb as well as the new… not saying it's plausible, but possible...

btw totally loved your step 4. i've done that a few times myself. i find flipping it off to be the best method.
hope this helps...
[video=youtube;rryQfAnQs3M]http://www.youtube.com/watch?v=rryQfAnQs3M[/video]

Ha! Great video. I saw the Kids In the Hall do a live recording for their show in Toronto back in the day. They were great live, and they even had The Shadowy Men From A Shadowy Planet playing at the show.

I'm going to give that timing sprocket a toothy turn. The head was indeed off so it's not a bad place to look.

Well, I moved the timing sprocket one tooth and the bike wouldn't start at all so I changed it back. Next I cleaned the points which made no difference. Finally, I cleaned the jets of the carb and the thing started and ran (which it did before the head swap) but only with the choke on. The carb on this 1984 has no adjustable needle but a friend gave me a tip: take the little metal washer from an old set of points and slip that over the needle jet to raise it. I did that and the bike works better but still a bit lean.

At the end of the day that's going to be good enough. It runs and idles fine now, just need the choke on to do it.

steampick said:

Well, I moved the timing sprocket one tooth and the bike wouldn't start at all so I changed it back. Next I cleaned the points which made no difference. Finally, I cleaned the jets of the carb and the thing started and ran (which it did before the head swap) but only with the choke on. The carb on this 1984 has no adjustable needle but a friend gave me a tip: take the little metal washer from an old set of points and slip that over the needle jet to raise it. I did that and the bike works better but still a bit lean.

At the end of the day that's going to be good enough. It runs and idles fine now, just need the choke on to do it.

Click to expand...

Looks like your issue is carburetion-related, that's a good thing imo. As much as I dislike the non-adjustable carbs, they should still work better than what you're now willing to accept. Trust me on this one, what's "good enough" at the moment...won't be for long, once the novelty (of having it running kindasorta okay) wears off. Plus, there's no way the motor will be able to make full power with the choke partially "on".

My guess, at this stage of the process, is a clogged emulsion tube (or/and other blockages). I'd expect to see a big difference from rodding-out the minuscule orifices with a thin wire. However, that, alone, may not be sufficient. That carb was designed & tuned before the current era of piss-like pump gas...and it was jetted on the lean side. It may be necessary to go one step larger on the main jet. Being a Keihin metered fuel & air leak, jets should be available - if not a complete rebuild kit.

OTOH, if you're looking to minimize the bucks, it may be worth trying one of those cheap & cheerful PZ19 carbs that OLD CT likes. He's made a very convincing argument for them for the non-purists & budget-minded among us. The bottom line remains the same, there's no arguing against what works. If the solution turns out to be cheap, so much the better, if it works well...you'll be cheerful. :4:

I've got a 60 main jet coming. I agree that riding the bike with the choke always on is a bit of a compromise, but the guy I was fixing it for wanted it yesterday as he's at camp with his son, so...he accepted that the choke being on is a bit of a compromise. To him, the bike has never worked better, which is more a testament to how crappy it ran before. I told him that right now it just needs to be ridden a bit to find out what's what. There's always a dial-in period after these re-builds.