Topic: CB400 backfiring and hot running problem

[myawesomebike]

Hello everyone.  I've been poking around this forum for the past month looking for little bits of technical knowledge on the CB400s.  I picked up a '75 CB400F in October with about 12k miles on it.  I've put a little bit of work into it, mostly cleaning it up, but also some carb stuff.  I ordered a carb rebuilt kit on eBay which was a big mistake, few of the parts worked.  Regardless I also got the carbs taking apart and cleaned, there was a lot of grime on the 1 and 4 emulsifier tubes.  The bike ran MUCH better after that, but I found some problems on the highway.

I've talked to a couple of guys I've run into around here (Philadelphia) who have CB400's, and they've been able to get them going up to at least 100mph.  After the carb rebuild I gave it a shot, but had a tough time past 80mph.  The engine starts backfiring around 8k RPMs.  This seems to get worse as the bike gets hotter, sometimes even as low as 6k RPMs.  Yesterday I pulled the plugs and they looked ok, but I threw some new ones in anyway.  This smoothed the running out a little bit better and, when running cold, easily got the engine up to 9k RPMs without any backfiring, but then after an hour long ride (mostly highway) the backfiring started back up.

The bike was in good shape when I bought it and I didn't do a lot of work, but I did do the following:
-Pulled and replaced spark plugs
-Cleaned up points
-Tore down, cleaned, and rebuilt carbs

I didn't sync the carbs, and I'm sure that could help, but this seems to be an issue more with temperature rather than JUST carb syncing.

Any insight or advice would be greatly appreciated.  Thanks again, this is a great forum!

[KB02]

When I first read the title I thought, "It's running lean." After reading your post, I think I picked up on a clue:

I did do the following:
-Cleaned up points
-Tore down, cleaned, and rebuilt carbs

POINTS:
My suggestion would be to go back and double check your work and make sure that the points are spaced properly and that you didn't knock the timing off in the process. Both are easy things to knock out of wack.

CARBS:
There is nothing wrong with a good cleaning. So do it a few times.  The more gunk in these carbs the worse they are to clean. It wouldn't be a bad plan to pull them again and just make sure they are nice and cleaned out. Gunk likes to find hidey holes and hang out there. The points sound more likely the issue, but if you're working on it, it's not a bad thing to do.

Those are my first thoughts after reading your post anyway. For what it's worth...

[myawesomebike]

Thanks for the reply.  I'm pretty sure I'm going to keep the bike pretty stock, but I was considering an electronic ignition instead of points.  Has anyone done this and had any major problems?  Any suggestions for brands?

-Thanks

[specweld]

if you are still using the standard suppressor caps on the sparkplugs, try changing them for a new set.
They have resistors inside the which break down and cause the problem you describe.
I am speaking from experience, I once had this problem and spent many hours trying to fix it, gave in and took it to a honda dealer, the first thing he did was to remove the suppressor caps, measured the resistance and then binned all four of them, replaced them with new ones, bike was like new. simples.
Considering the cost it's well worth a try.