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Discussion Starter · #1 ·
First off, I'll start with the disclaimer that I'm a noob to all this bike work stuff...I apologize if a similar question has been asked previously. I didn't see it turn up in my searches.

I'm working on a 78 CB400.

Here's where I'm at, and I'll probably take it in to a shop because I don't know if I have the patience / skill to get in to too much more.

I recently cleaned and rebuilt the carbs (new o-rings, floats, float needles, etc.) b/c they were leaking from the overflow tube. I finally got all that worked out and have managed to get them to stop leaking (thanks Bill for the carb assembly). I got it all back together, and she started right up with no hesitation, and idled nicely.

I took her out for a quick spin and there was very very poor acceleration. No matter how much I opened the throttle, the acceleration was poor, and little power. But at higher RPM (like above 6K) she took off and responded as I would expect with plenty of acceleration and as much power as I might expect out of a 400.

Something clearly needs to be adjusted, but I don't quite know where to begin. I know the things to look at are carb synchronization (this was done recently, but before I tore into the carbs). When I feel the pressure from the two exhausts, they feel pretty similar. If they're different from each other, it's not noticeable to me. I haven't checked the timing, but it was fine before I started down this road. I pulled the plugs. The left one looked pretty much ok (maybe just a little bit on the white side, but I think mostly ok). The right plug, on the other hand is a bit disconcerting. It was certainly fouled with wet oil. As far as I can tell, this is a fairly recent development, and there was no mention of this when I had the bike in for a tune up just over a month ago. There's no smoke though, not on start-up, not on acceleration. I'm pretty sure the floats are adjusted properly. but it's hard to know for sure as I find the credit card guide to be a little arbitrary depending on where precisely it's placed to make the measurement. The choke seems to be operating properly. At least when I pull it out I see the choke thing (sorry, don't know the names of all the parts yet) that's attached to the carb moving, BUT when I first started it, I had the choke out, and pushed it slowly in immediately after it started up and I heard no change in the idle. In that sense the choke didn't seem to be doing anything. A buddy rode it, and he thought it felt like the choke was stuck open (but I don't know how much he knows).

I guess what I'm asking, is:
1. What could this be due to?
2. What would the sequence of events to troubleshoot this include?
3. How do I know when I have the air/fuel mix adjusted properly on each carb?
4. What else could I be missing?

If it's straight forward, I'll dive into it (maybe). If I take it in to a shop, I'd like to be a knowledgeable customer.

Thanks in advance!
BaileyMan
 

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With the 400 carbs, it would be easy to "swap" the primary and secondary jets when re-installing.... Once everything is in play, (higher revs to WFO) this doesn't make much/any difference...Initially though, the difference would show as lack of power as the cylinders are actually fighting against each other (one trying to run faster, one slower)..... At low throttle openings, the carb with the transposed jetting will be too rich and wet the plug... Another member had this problem, so those posts should be able to be found with the search feature.....
 

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Discussion Starter · #3 ·
Thanks for the suggestion. I just moved and the new garage doesn't have any decent light. I'll check the jets when I get some lights in there, hopefully today. I'm pretty sure that I didn't transpose the jets, because I actually didn't take the secondary main jet out of the jet holder. At the time I didn't realize they were two separate parts, and I just cleaned them as a single unit. It's possible though that the previous owner transposed them when he had them cleaned (and I'm not convinced the guy that did them did a great job). But it was hesitating a little when I first got it...but not in the same way it is now.

The secondary main jet is the 105 right?

I'll update after I check out the situation.

Thanks,
BaileyMan
 

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Discussion Starter · #4 ·
Well, now that it's after the holidays, and I'm back home, I took the carbs off again. It seems that I didn't put the slow air jet rubber plug back into the right side carb.

Could this be the source of my problems?

I'm still learning about how carburetors work and what all the jets do when...

I thought I'd ask before buttoning her back up again.

Thanks,
BaileyMan
 

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It COULD be a good reference, if 1/3 ACTUALLY WAS smaller than 1/4....LMAO!
 

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thats worthy of posting my my shop area.... right next to the exploded engine posters i have for the early yama engines. thanks GB!

EDIT! oh shite, 1/3 is bigger than 1/4... maybe i'll wait on that poster size printing.....
 

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You're right Steve but I just never paid attention to the 1/3rd part.. I believe the rest of the diagram is acceptable.. I'm trying, with no luck so far, to find my other diagram..

I "think" that whoever made that chart meant for it to be 1/8th instead of 1/3rd... Typo?





GB :mrgreen:
 

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Discussion Starter · #10 ·
It seems that I didn't put the slow air jet rubber plug back into the right side carb.
This was the source of my problems. Put that plug back in and she runs like a dream...

Thankfully it was so simple...
 

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Ahhhhh....The satisfaction of figuring it out for yourself.... (See, you knew more than you thought you did, and successfully fixed it!)... :D :D :D :D Steve
 

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Discussion Starter · #12 ·
It is quite satisfying, especially as this was my first foray into a carburetor overhaul. :D :D

Thanks for the suggestions, and the charts. I had a number of mornings the past week waking up with a busy mind thinking about how carbs work and the roles of all the jets...

Cheers,
BaileyMan
 
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