Ignition/electrical questions '86 vf500

I am trying to diagnose an issue, but I'm a bit stuck. The bike will start and idle fine for a few minutes. It will rev and drop back down between 1500 rpm and 1100 rpm. Sounds decent and I don't hear any valve ticking. After a few minutes, the idle starts dropping and any throttle to save it will either kill the motor or it will sputter up to about 4000 rpm and no further (it runs rough at this point). Once the motor stops, it will not start back up (After few tries I stop until the next day).

My initial once over after buying the bike yielded acceptable compression. #4 was the lowest just under 150 psi. The rest were in the 150's. All cylinders fire after the 1 and 3 coil replaced. Spark plugs were replaced with NGKs the Honda manual suggested and gapped to .035. The carbs are getting fuel and the petcock has the gravity mod done. Battery shows increase voltage when it is running just over 14 vdc. For my own piece of mind, I got the battery tested and as a result of that test, had to replace the battery (lead acid). Same issue, but ran considerably longer than previously. I cleaned the ground at the rear of the bike and did a hot skin test, no voltage leaking out to the frame anywhere. Lastly, oil and filter.

After a two days with the original battery, I forgot to plug the battery tender in. The bike wouldn't turn over the next time I tried. Even after I removed it and left it on charge a few days. That battery got tested and again, replaced. I think the regulator/rectifier may be bad, but would that cause the bike to run like that then shut off? It appears the engine heat might have a say in the issue, but the erratic rpm climb when I try to save it from stalling sounds like an ignition problem to me. CDI's maybe? Would the pulse generator cause these symptoms?

I've thumbed around the Honda manual, but I'm stumped what to try next. I've spent the last few days browsing the forums and haven't found anything yet. I found a receipt in the manual the previous owner had the carbs serviced earlier this year, March I think it was and at a Honda dealer too. That said, I'm hoping I don't have to pull the carbs. Please don't say pull the carbs..... I'll do it if I have to though. Begrudgingly.

 

make sure the choke isn't sticking ON, and inspect the plugs when it stalls.

 

Plugs seem ok after. They’re usually slightly wet with gas. I have not pulled them directly after stalling because I’ve always tried to start it again. As far as I can tell the choke seems to be working. It hasn’t fired up until I push the choke down. I usually forget to until it’s turned over a few times, I haven’t had a carb bike in a few years.

 

if the plugs look wet, they're not OK. could be a carb problem or weak spark.

 

There are so many possibilities. Just a couple comments:

Just as you have forgotten to engage choke on a carb bike, so have many "technicians" forgotten, or never truly learned, how to work on them. Especially a set of V4 carbs. "Serviced" can mean a lot of things. Typically the lead techs aren't the ones working on a 86 VF500F that the shop probably didn't even really want to work on.

Your initial problem has many characteristics of an ignition system trying to run on a bad battery / bad charging system. But after a new battery and seemingly good charging output, that doesn't seem likely now.

Heat can always be a game changer. Pulsar coils (the pickups that are triggered off the end of the crank) should be on a list of suspects. I have found you can't completely trust resistance and continuity checks to absolutely diagnose them though.

So, again, MANY possibilities. Including carbs. Yeah I said it.

Good luck.

 

I was thinking pulse gen as well. Figured I try here before dropping the oil I just put in. Say it is the pulse gen, do you know where I could find a replacement? A quick google search isn't promising and I prefer to not have to rely on used eBay parts.

 

Put the bike on the side stand and there is no reason to drop the oil. Maybe roll up on some 1" boards for some "lean" insurance.

Gonna have to roll the dice of replacements. Try to find a vendor that broke apart a "running" bike and offers returns. You'll get some eyeballs here, but you can start a "wanted" thread too.

 

try reducing plug gap from .035 to .030. the wider gap needs alot more voltage to fire, and that could strain the electrickal system.

 

Dang. Well I scout eBay for one then. I work this weekend, but I will report back my findings. Thank for the lean tip.

 

I’ll give her a go before I pull the pulse gen.

 

no need to remove pulsers for testing, just go to their plug connectors.

 

Ok, late to reply my apologies. Holidays, family, work, etc. it was the pulse gen. I checked it at the plug as squirrelly dan said. Ohms we’re around 400 for both sides. I started the bike and waited for the inevitable stall. Checked the resistance at the plug again. I can’t remember what the exact measurement were but one was really high compared to other by thousands. I would assume they’d be closer together even hot. So I browsed eBay for a seller that does returns for faulty parts as Cap said. I don’t know what bike it came from, but it did not have the oil sensor wire. I cut the loom from the original and pulled the pin from the connector to use with the replacement. It’s resistance reading was 360 and 361 ohms. Leaning the bike on the side stand worked fine, no oil spilling issues. The replacement wiring was slightly shorter, but it still reached to the plug.

It would not start at first. Like back to square one when I first got it not starting. I started checking the wire connections and ensuring they were plugged together properly. Pulled the plugs one at time to make sure I still had spark. Went ahead and gapped to .030, suggested by squirrel. The bike started right up after that. So I’m not sure if it was the plugs or wiring. The bike idled for awhile and seemed like it had no plans of stalling. So for the first time owning the dang thing, I rode it. It was awesome.

Somewhere in the neighbor hood of 5-6k rpms, it back fired on the left side and was hesitating with throttle. Felt like it was missing. When I got back from the maiden lap around block, I felt each jug to make sure they were all hot and they were friggin hot. Pulled each plug again and the back left spark plug was way cleaner than the other three. Very odd. So that’s where I’m at right now. Oh and the fan was on when I got back thank god. So it was running at temp. When I get time, I’ll start on this new issue. Sound familiar to anyone or have suggestions? I’m just gonna follow the manual for now to troubleshoot.

 

Few things...

WTF ?! I'm gonna assume it was listed as VF500F. I always look at "seller's other items" to see if they have other parts for the correct bike and also see if they happen to be selling the Gauges to verify mileage.

The left rear carb is usually the one that has the worst issues from being stored. It's the first one I always check after I have pulled a set. So clogged jets means less fuel and a clean (lean) plug.

Or running hot from running lean.