VFR 93 rides fine for a while but then suddenly dies and won't throttle

So i've been having a bit of trouble with my VFR 750 3rd gen from 93. It started out with the bike dying on the highway after riding for about an hour. I quickly realised that this might be the R/R being fried, and sure enough, after getting the bike brought back home, it turned out that the rectifier was dead. I charged the battery fully and replaced the rectifier. I also changed spark plugs and oil at the same time as it was about time. I also changed the battery just to be sure. The bike started right back up and ran great until about an hour or so of riding, when it then died again. This time the bike died in a different way though. First time it died i was not able to get it started again. This time i could start the bike, but only with the choke on. If i tried to give it throttle it died and same if i let off the choke. Still suspecting electrical problems i went tested the different parts:

Battery shows 13.19V when disconnected and 14.00V connected with engine running at 3kRpm.
All sockets on the rectifier shows the right resistance.
All sockets on the stator connector shows 0.5ohm
When running at 3-5kRpm all stator sockets show 60V AC.
After dying the battery also still showed >13V when disconnected

So this should pretty much eliminate any electrical problems right?

The weird thing is, if i leave the bike standing for a day or so after dying it starts right back up.

After reading some online threads i suspect it might be a problem with vaccum building inside the tank after running for a while? How can i check this without going out and potentially getting stranded again. Or is it maybe some other cause?

Hope you guys can help out here, im pretty lost!

 

I don't have the honda manual in front of me, if you tested the stator readings and they jibe up with what the manual says it should be your stator should be ok. I cut all the white plugs out and hard soldered all the wires with solder and shrink tubing. Same goes for the R/R. If you have vacuum issue in the tank, after a ride, open your gas cap, if there is an issue you'll hear a Whoooosh or something. I don't that is the problem though.

What do your carburetors look like? You can shine a light through the left side mid fairing and look at the base of the V. Maybe your fuel tubes are leaking, fouling out your engine? How does your engine oil look? I had a bad o-ring and fouled out a cylinder on my 93. I ended up replacing the fuel tubes along with new o-rings and problem solved. Have you done a carburetor service on this bike? Just throwing stuff out there for you.

 

could be the fuel pump relay crapping out when warm or pickup coils, which can fail after reaching operating temperature. check for sparks next time it quits with the spare plug you carry for that purpose.

 

The way I understand the fuel pump is this: its designed as an assist pump when the fuel reaches a lower level than the petcock, (because honda designed the tank with fashion in mind over function, - my GPZ500 has a gravity feed petcock and has no fancy stuff.) Its easy to check that part, with the bike running it should have light clicking when you touch it you can feel it too.

What are the pick up coils? Pulse generators? Have you seen coils go bad on bikes before? not too often but shi* does happen. Easy enough to use a heat gun set at a sensible temperature to check it right?

 

^^ yes on one of my '86 VFRs, it ran fine until it reached operating temp then lost sparks.

 

Hi Kastrup and welcome to the MadHouse

Your forum name suggests you may live not far from friends over in Odense, in which case you have plenty of brilliant riding roads nearby including the amazing StoreBelt link. Hopefully we can help you get your bike running well again before winter arrives.

When you get time please visit the Introductions forum and say Hi to the rest of the folks on here tell us a bit about you and your riding history and add a photo of your VFR - its easy click the "Upload a file" link and follow the prompts to select and upload pictures from your computer.

As for your question "So this should pretty much eliminate any electrical problems right?" the answer is possibly NO.

The voltmeter results look OK - but were they done twice? Once with the engine cold and then again after taking the bike for a 20minute ride by which time the engine and electrical components will be hot - and when heat related issues tend to appear.

My suspicion is that the earlier RR failure may actually be linked to the Stator starting to fail - it is approaching 27 years old!

When it failed the second time could you still hear the fuel pump running? If not it may explain why you could only get it to start with the choke.

All four elements of your charging system(Battery, Stator, RR, Loom) need to be in good order, and a fault in any one element can damage some or all of the others. The insulation on the stator windings can over time start to degrade, meaning it is still working fine when it is cold, but once the bike gets up to temperature the stator windings start to get hot and potentially allow a short circuit between phases. Suddenly instead of charging the bike, if there is a short developing in the stator, it can quickly drain power from your battery enough to shut down the electric fuel pump and once the bowls in the downdraught carbs are empty you will end up stranded.

If you have had enough measuring with a voltment then the quick check is to simply remove the stator cover and have a quick look. If you see crispy burnt sections on the stator windings or the windings now exude the smell of fried electrical windings you will either need a new one or find a specialist equipped to rewind it. Hopefully you can still get a stator cover gasket from Honda or CMSNL.

Let us know how you get on.



SkiMad

 

Could be many things... charging system seems OK, but I'd ditch that OEM R/R someday and put in a newer tech mofset, soldered connections, etc. Perhaps put a permanent voltmeter somewhere you can monitor all the time.
Old relays can act up, etc. Keep checking stuff and you'll find it.... dodgy grounds, etc.

 

Next time you get stranded I would pop the fuel fill cap as mentioned above. Sounds to me like the vent is plugged and the tank is under a vacuum after riding for awhile and fuel doesn't flow. Of course if that isn't the problem you are still stranded, but i think that is probable from your information.

 

Thanks for the input everyone! What a great community on this forum I went for a ride yesterday and after about 30 minutes it died on me again, i was able to ride the last 500 meters home with full choke. After getting home i tried popping the fuel cap, this made no difference so it wasnt a vaccum problem. It was late so i didnt have time to do full checks on the charging system. I checked the battery voltage = 13.19V so dosent seem like the battery was drained? Also it had plenty of power to turn over the starter motor. Could it still be that the fuel pump did not receive enough power? I even tried switching the battery out for a new, fully charged one, but the bike would still not run without choke until it cooled down.

What exactly are the pickup coils and where on the bike can i measure them? I read other places that they might indeed cause the bike to die when getting warm, but if theres still plenty of juice in the battery would they really be the problem?

Considering driving it to the local shop and let them do their thing as this might seem a bit out of my abilities.

 

Have you tried jumping the fuel pump relay. You have a fuel flow problem as it will run on the choke, but there isn't enough fuel to run normally.

 

look in the service manual to find the plug where you can measure and check pick up coil resistance. the best and easiest check is to see if the spark plugs are sparking. if you haven't checked yet (after it quits), you're slacking off. remove one of the rear cylinder plug wires and test with a spare plug.