Topic: 73 CB500 - Not Running - Backfiring

[sk8er0918]

Hi all,

I have a 73 500 four which stopped running and is now backfiring out of the exhaust. Last weekend I replaced the oil and spark plugs and the bike ran great for about 10 miles. While riding it, I started to hear a hiss or psst noise every minute or so, which in my experience indicated low gas (it was getting low in the tank). I got to a gas station, put fuel in the tank (ethanol free – which I always use) and after that the bike pretty much lost a lot of power in the lower gears and 'limped' home. When I got home, the bike stalled in the driveway and then wouldn't start after that. Now when I try to start it, it will backfire once every dozen starts. I read through most, if not all of the articles in the forum related to the backfiring issue and did my best to troubleshoot the issue. Things I did were…

•   Pulled the carbs and cleaned them out (they were already pretty clean as I did it two years ago)
•   Adjusted the cam
•   Checked the connections on the spark plugs to ensure they were coming from the correct coils (one to 1 &4 and the other to 2&3). The wires are a set length, so it really would be hard to mix them up.
•   Checked the spark plugs for spark (they were arcing pretty good)
•   Shot carb fluid around the boots to see if there were air leaks

I adjusted the tappets back in January, so there should be no issue there as well. I also rebuilt the petcock last year and don’t see any issue with gas getting to the carbs. One of the PO’s of my bike had already replaced the system with Dyna Coils, so I no longer have the original point system in the bike.

Some other info on the bike… everything is pretty much original. The bike came with  a 4 into 1 exhaust and I replaced the OEM air filter for one of the UNI foam ones a year ago.

At this point, I believe I checked almost everything and was hoping for some input/ feedback. I am a novice mechanic and have pretty much learned everything from this forum and the Clymer manual. I appreciate any help I can get.

Thanks,
Matt

[flybox1]

List the intake and exhaust you are running.
What are your carb jet sizes? brand?
Post a closeup picture of your spark plug tips (all in one picture is best)

[Smudgemo]

Are you sure the gas you bought is good?  Could your fill-up have introduced debris that got past whatever filtering you've got?   How thoroughly did you clean the carbs?  Those jets are pretty small.  I had an aftermarket petcock on my 550 that lost its sock somehow and wouldn't have done a thing had I started using it that way.   

It just seems that most of the stuff you've checked wouldn't suddenly be an issue after a fill-up as opposed to introducing a fuel problem.  Things would be fine until the bowls started filling with the new mix, and things go downhill from there.  I'm a novice too, so I'm just using logic as opposed to experience.

[calj737]

One of the PO’s of my bike had already replaced the system with Dyna Coils, so I no longer have the original point system in the bike.

From your detailed description, I would focus on the ignition system. Depending upon the age of the Dyna ignition, they can be susceptible to failure. If you can, swap that ignition with another and try to fire the bike. It sounds like the bike was losing power due to failed spark and now your plugs are possibly fuel soaked and not firing.

The Dyna coils do not require an electronic ignition. And the Dyna ignition does not require Dyna coils. So, to be clear, which components are "Dyna"? The ignition or the coils or both?

If you are using the stock coils, then you should also inspect the plug caps/boots for their resistance value. Unscrew them each, use a meter, and probe both sides of the metal crimp inside. You want a value below 5.5K Ohms on each of them.

[Deltarider]

From your detailed description, I would focus on the ignition system.

+1

[sk8er0918]

Thanks for the feedback. I’ll do my best answer what I know now and get back to you after I am able to look at the bike and get photos (Sunday).

Flybox1 – The intake is the OEM rubber (ish) boots. They are similar to what you see here on the 4 into 1 site (http://4into1.com/reproduction-rubber-intake-insulator-16211-323-000-honda-cb500-cb550/). Mine are in decent shape, don’t see any rips or tears. I am pretty confident the exhaust is a MAC 4 into 1 megaphone. I am not sure about the jet sizes… I assume they are probably stamped on the jets? I wish I knew to take photos of them before putting it back together and on the bike (the carb system is a pain to get off and on). I’ll get shots of the plugs to you on Sunday… they are a little dirty (I didn’t realize I was running it with the choke @75%for those few miles), it wasn’t until I wiped them down Wednesday night was I getting a really good arc. Prior to cleaning them the spark was weaker… maybe I have bad spark plugs? I didn’t really think they could be the issue since they were new.

Smudgemo – I am pretty confident the gas is good. I think the issue I was having started coming on a few miles before I got the gas. I filled the bike with the hopes it was that. The only filter I have is the stock screen in the petcock, and based on my clear fuel lines, gas seems to be feeding fine.

Calj737 – The bike has Dyna coils and ignition system. Do you know where these systems fail and how I would be able to troubleshoot them. I have a multi meter, but really don’t know much about setting it up and using it appropriately. In general I know very little about electrics.

[calj737]

First, focus on insuring you've got good clean plugs. While doing that, check the caps too for good measure.

The Dyna ignition was prone to failing on one side 2/3 or 1/4 when hot. It seems there was a bad batch of these a few years back. May be that yours is among them. To test that, there is a Technical document on their website for how to verify working condition of their electronic ignitions.

[sk8er0918]

calj737 - I replaced the spark plugs and got the bike running. The plugs in the engine were pretty new, but you couldn't tell looking at them. I think they went bad because I was running the engine with the choke on and also realized I had a little extra oil in the engine from my recent oil change. Again I am a novice so I can be wrong. Thanks for your help.