1984 VF750F eventually stalls

Hi all,
I acquired my '84 vf750f last year and I adore it but I'm having some issues that I have not seen after looking through many many pages in this section(where I presume my answer surely lies under an unassuming post name).
Bike when purchased was by no means perfect but ran ok and didn't seem to have any major problems. Just before storage last year and now it has a strange issue.

At idle the bike runs okay and run run for upwards of 30 min without issue,however as soon as the bike is ridden about 20 min or so it stalls and will not restart unless you wait a couple hours upon which the whole process repeats.
Have taken it to shop who claimed to have experience with these bikes and assured me a fuel pump was most likely the culprit fast forward they had the bike for almost a month and call and inform me that the carbs are the issue and go on to tell me it isn't worth it to repair we can trade you and on and on.
So my question is have (the community) are they right or were they just trying to get a headache out of their shop? Might the fuel pump still be the issue? Lastly if the carbs are at fault here are the people I saw offering rebuild services still around?

Thanks sorry for long first post
Dylan

 

PM squirrelman for the carb rebuild if you choose to go that route but the fuel pump still sounds like the culprit from here.

 

If the bike can be ridden for upwards of 20 minutes I would lean more toward some type of electrical issue instead of a fueling issue. What you are describing can often happens when the coils are going bad. Next time it happens and the bike won't start you should check for spark at the plugs. You might also want to check if it is related to a venting issue. Pop the gas cap open and try to start it the next time if stalls.

 

Thank for the reply very much appreciated now that is interesting I had not considered that since how quickly the bike went from running well to stalling seemed a lot like a fuel delivery issue but it's my first bike and I'm learning as I go. I'll admit I was/am clueless on this because of its consistancy despite different intervals of warming up and how fast I would go while riding and have it either idle 10 min or 30 min and drive same route (few hours in between of course) and wind up stalled in the same damn parking lot. I'll be getting the bike back from the first shop here Monday was supposed to be today but they are out racing and will purchase a Clymer manual and test the coils and hopefully the fuel pump.

 

Classic pick-up coils(pulses generators) are bad, (not the coils that drive the sparkplugs). The ohm test is in the shop manual. My R did the same thing, started fine, ran fine, then after a bit, acted like 2 cylinders cut out. It would die and not restart till it cooled down. They tested bad, swapped for a different set and haven't had a problem since. Basically, they get warmed up and the resistance changes for the worse and they don't send a proper signal to fire the coils. My money's on that. You can find a manual online or someone here can shoot you the Ohm range for the test, cold-warm.

Fairly common problem.


For your bike they should test at 480ohm cold between white/yellow and yellow for cyl. 1-3 and white/blue and blue for cyl. 2-4. Your manual should tell where to locate the plug to test, if not, there is a pdf. manual here...
https://drive.google.com/file/d/0B6uU39dq308cNTUzODE4NmYtMDIxNy00MzAwLWFkZDUtNDNkMWUxYTVmYWQx/view?ddrp=1&hl=en&pli=1

Section 18 of the above manual will show you.

 

Thank you very much for the information and the quick response. As soon as my bike is back in my hands I will see that is going on. Thanks again

 

First off don't listen to your mechanic. I doubt they really know all about the VF750F's. You just can't swap carbs from any VF bike and have it metered right. 50/50 chance at best.

Squirrelman can set you up there. Your problem could be fuel filter, bad ignition box (2), bad coil and most likely seeing it fails when hot are the pulse generators. Their trait is to fail when warm to hot. Not a big deal to change. You just need to ebay the parts.

 

Does it stall suddenly or does it sputter like it is running out of gas?

 

It seems to sputter but only for a very short time(maybe I get 30 seconds from when I know it's wrong to motor not running but I'm not sure it has been awhile after all) like one other ppl said almost like for that little bit it's only trying to run on 2 cylinders

 

AH, check your connections to your ignition coils (triggers, not high voltage). I had a problem after get my 83 all back together. the pins on the coils and the female connectors were corroded and were intermittent. Each coil fires a pair of cylinders, firing in both at the same time. So if one coil is intermittent, then you are running on 2 or 4 cylinders. My problem was just the opposite of yours, I set the idle with it running on 2 cylinders, then suddenly it would jump up to 3k rpm. After cleaning and burnishing contacts, I reset idle and all is well.

 

i concur with those who've suggested an electrical component that fails when hot: CDI (you have 2), pulse generators, ignition coil.:sorrow:

 

Okay wow thelse responses are great thanks a bunch. I know what the pulse generators and coils are and do and found the test but you mention cdi? What are those maybe I am just missing it in the service manual...I'll look again

 

CDI units are in the tail, near the tail light, again, check connectors. I have posts where guys have opened the up and fix cold solder joints. CDI is Capacitor Discharge Ignition, they are the units that trigger the coils. Google "CDI motorcycle", lots of info.

 

A little follow up, decided to fix a few problems with the gas tank last week. Drained it, took it off, fixed problems, put it back on. fill about a gallon of gas. Went out today to go get it filled, and ran for a little while and quit. Didn't think I had enough gas, went and got 2 gals more, dumped in tank, same thing. I recognized sound of it running on 2 cylinders, so loosened tank, turned it 180 degrees and rested it on the tail. Started engine, then wiggled connector/wire to right coil, varoom, off it went. Turns out blue wire had fatigued at the crimp on the connector. Opened up connector, stripped back 1/4 more wire and re-crimped, then soldered. Again check all YOUR connections, doesn't take much.

 

Only thing here is the motor stops doesn't sound like it is dropping onto 2 cylinders which all the above mentioned failures should cause. Could be as simple as bad connection on the ignition switch dropping power out. Every connection could be a cause of power loss

 

Good find and easy fix.:thumbsup:

 

Agree with the pulse generators. Classic issue when they go bad.