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Discussion Starter #1
I recently got a '78 CB400 and it's been running a little jenky since day one. I've cleaned the carbs, did a quick oil change and slapped some new shorty's on there (I figured the back-pressure loss might be to blame as it was straight pipes when I got it) but it sounds as if the left cylinder is running a bit sporadically (sputtering and backfiring). Here's my thought process...

Possible problems:
Spark plugs: I swapped them and it didn't change things
Air leak in carbs: I am missing a screw on one carb - is that enough to cause a significant leak that would throw the cylinder out of commission?
Coil: Would a faulty coil cause only one cylinder to work improperly and would swapping the spark plugs trump that issue?

Also, I had the bike running in the driveway earlier to heat-seal some muffler patch that I used to connect the header pipes and an extension that I put on to feed through the footpeg/rear brake lever (I'm okay with a patched up bike. This is my <$800 junkyard bobber). After about 20 minutes of idling, I started seeing fairly significant smoke coming out of the carburetor area, but I really couldn't pinpoint a specific cause of the smoke (no, it wasn't just from the muffler patch). I had previously run the bike with no breather hose (there are pods on the bike now) and it was spitting all over the bike. Could the smoke just be that crap burning off as the bike was getting hotter or do I have a serious problem on my hands?
 

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Well let's start with the basics, Compression? Valves adjusted? Correct NGK D8EA plugs gapped at .026"?
The coil itself won't be an issue if one plug is firing however you could have one wire bad, unlikely since they are steel solid core, or one of the plug end caps open, supposed to have 5K ohms resistance.
One screw missing from a carb? The only ones that can be missing would be the bracket screws. Every other one is important. Which screw?
Yes air leaks are a possibility, How are the carb insulators? hard and cracked or soft and pliable. You mention cleaning the carbs, were the air cut valve diaphragms in good condition? were they sync'd? mixtures set correctly?
 

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Discussion Starter #3
Bear with me as I'm learning as I go...

I'm not sure about the valve adjustment - that's something beyond my ability level and I don't want to start tearing apart without someone by m side who knows what they're doing. I've got a neighbor with a compression gauge so I'm going to borrow his so I can check the compression. THe plugs are correct.

The carb (same side as the sputtering) is missing one of the 3 float bowl screws. I do intend to get a new one but the local bike shop guys are dicks to me because they're all Harley heads and I'm trying to putt around on a CB400. I cringe at the thought of paying shipping for a screw though...

The carb boots are soft but do have a couple cracks. They're clamped pretty well though, and I really don't think that would be the leak point... maybe I'm wrong. When I cleaned the carbs all the visual checks looked good and the mixture seems to be on point, however I'm not sure about them being synced. How can I check/sync them?

Thanks for the direction thus far
 

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The screws there are 4mm x 16mm. I replace those with SS socket head (allen head) screws so if I need to drop the float bowl off I can do it w/o removing the carbs. I get those at Ace Hardware.
The boots are probably the originals and cracks mean they are failing. I'd go ahead and replace those. Look for the CB450SC boots since the originals are no longer available. Partzilla has had the best price recently. Also be sure the carbs are fully inserted in them, the carbs actually bottom out in the boot.
The valve adjustment and carb sync are covered in the FSM and Clymer manuals. If you need a manual PM me. I don't suggest relying on the Clymer since there are multiple errors in it.
 

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Discussion Starter #5
Thanks for the tip on the screws, I'll pick up new ones tomorrow! I have the Clymer manual hard copy but I know I stumbled across the original in a digital format somewhere before. It should be in my bookmarks.

With that being said, I think I've found the problem... or at least I hope I have because there is a very simple solution if it is in fact the case. I popped open the gas cap this morning and saw that rust had started forming in the tank - that wasn't there a few weeks ago. I'm assuming that some rust has jammed up in the carb and is attributing to the sputtering cylinder... Now that I think about it, that carb has been dripping a little more gas from the overflow than I'm comfortable with - float needle might be stuck with rust too. I'm going to drain/flush and soak the tank with vinegar (we used to do salt and vinegar on tractor tanks when I was a kid living on the farm) overnight, clean out the carbs as best I can without doing a full bath and hopefully that will be the end of the sputtering. I'll keep you posted on the results.
 

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Regarding the Allen head screws, it's a good idea and it should be done anyway to prevent stripping but if you want to remove the bowls without pulling the carbs and you don't have the Allen heads yet, I've found that a finger bit driver works for the stock screws, they have bins of them at most hardware stores.
finger.jpg
 
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