Help Please. am lost to what to look at

Hay VFR family my 83 vf750 f runs for about 20 mins then the rpm needle drops to 0 then she dies but if i keep the revs up i can still ride but it`s like it is going to stall & will if i don`t keep the revs up she only dose this when @ normal running temp.http://vfrworld.com/forums/images/smilies/confused.gif has anyone had this same proplem if so what did you do.Any help please as i would like to keep her but toying with getting rid of her.http://vfrworld.com/forums/images/smilies/custom/crazy.gif
Thank`s to all
Dave

 

Fuel pump

My first guess if fuel pump. Necks bike did this a few months back . The fuel pump would get hot and stop pumping, just an age thing.

 

Hi,
I don't know that much about 83's but the general symptom seems to be fuel starvation. Whether this be from the suggestion above or a blocked fuel filter or fuel tank breather. Either of these and you would suck fuel until it just couldn't any more.
Change or bypass the filter to eliminate it.
Does the fuel tank sucks in air when you open the filler when it wants to stall?
If people have had dodgy pumps then bypass the pump if you can and see how you get on.

 

Thank`s a lot i have got an other pump i`ll try that i`ll keep you posted.
Thank`s again TOE CUTTER & gh53821

 

It is not the pump

Hey

If it was the pump then you wouldn´t be able to give it gas to keep it running.But check every fuel filter if every thing is okay there check the spark plugs.If all checks out good fix your carbs.I have no fuel pump just filter and a line to the carbs.On these bike you don´t really need a fuel pump.Just take a hose straight from the tank and hold it up then turn the petcock on you will see there is enough pressure from the tank.

Best regards

Gunnar

 

Thank`s gunnarf06 checked fuil pump yesterday it is working fine going to check spark pug`s today if have time. but what is pluzzeling me is when the bike warms up after 20 mins i lose my rpm`s the bike will still run when revs are up but if i don`t it will die. Do you think that i could be my cdi boxes???
Thank`s
Dave:

 

Hey

There are two possible choices here.1. The fuel pump works enough to pump fuel into the carbs and lines that is enough to keep your bike going for 15-20 min.2 The only object that changes with heat is pulse generator should be 480 ohm when it is 20 celcius degrees hot.If that is not right then when you engine gets hotter and the pulse generator becomes maybe 490 ohms at 20 Celcius then you will get a wrong ignition time.Have you tried setting the low rpm.Hope this helps

Best regards

Gunnar

 

Gunnar how do i test the pulse generator`s,?? is there a way to do this & what would i need. sorry i`am new to all sorts of testing electrical always gets me.
Thank`s
Dave

 

Also check for loose rust in the tank.

Regards,
Rollin

 

Hey

No problem.Okay you need a ohmmeter.Then you need to open the right cover on engine that clutchs and oil filler side.On that side you will also see harness coming from the engine exact 5 wires in a 6 pin connector.You begin to unplug and take the ohmeter between white/yellow and yellow.Then you need to heat up the coils (two black coils sitting above the starter clutch) and heat them up with a hairblower to 20 degrees Celcius.Then you should get 480 ohms.Then you set ohmmeter between white/blue and blue then do the same.One question for you does you tachometer go crazy when you try to start?

Best regards

Gunnar

 

Gunnar, clarify to the rider that he only needs to access the plug for an initial test......he does not not need to remove the clutch cover to initially test coils at room temp. i suspect that just taking the ohm meter on a test ride and examining resistance after motor quits would be LOTS easier and simpler than removing the whole cover with the gasket and multiple different length bolts. there could also be an issue with the possible UNavailability of the coils from Honda?

i believe that a coil that's bad will show abnormal resistance whether hot or cold.

 

Gunnar no my tachometer does not whack out on start up. i will be checking my spark today after the bike warms up to see if i`am getting spark when warm, do you think it could be a short some where or some other wiring problem??????
Thank`s
Dave

 

Hey

When I did this test first I tought also just mesure it and if I am anywere close to 480 ohms then they are fine.That cost me new coils and used cdi units also changed the spark plugs and wires but that needed anyway.It runs on 1-2 ohms maybe my issue was abnormal but I know now that is better to be safe then sorry.

Best regards

Gunnar

 

Hey

Okay that is good then your 1&3 cdi unit is probarly okay.Your 2&4 cdi unit controls the fuel pump.

Best regards

Gunnar

 

ok my bad this is what happens ( reworded ) start the bike runs fine 20mins later the Tachometer dies as if it was unpluged bike still runs but runs rough. what i have done checked fuel pump it pumps fine even when bike is hot checked the spark when bike is cold, have not checked spark when bike is hot today after lunch will check. Thanks for your response so far & sorry for the mix up.
Dave

 

Hey

It is okay nobody is perfect.But I would get a used CDI box 1&3 and try changing it out.I am almost sure that is your problem.Do one thing for my have a extra spark with you and start the bike and when it starts to unbehave take one of the spark wire (while the bike is running) and set it on the extra spark plug and see if you get a spark.Begin with the rear cylinders 1&3 that is.Then you will finally get your answer what is wrong.

Best regards

Gunnar

 

Well Sounds like you have the problem almost figured. I will say that my 85 did the same thing, and it was my fuel pump. Something was wrong with it and after about 20 minutes it would overheat. Easy way to test it was we just bypassed the pump while it was malfuncitiong ( straight gravity feed ) and the bike ran fine. Luckily TOECUTTER had a spare back at his garage and hooked me up. If you troubleshoot your current avenue and come up empty handed, spend 5 minutes and bypass the fuel pump ( when its malfunctioning ) and see if the problem goes away immediatly.
I am no mechanic, just someone that had the same symptoms on the same generation bike. Luckily the guy I was riding with WAS a mechanic lol.

 

OK update ran the bike till hot ( 20 mins ) let her die results i found was no spark to back two cylinder`s which is 1 & 3 which also would explain the rough running after the tachometer drops. Of to see about new coils.?$$$$$$$$$
Thanks
Dave

 

good luck dude, hope it wokrs out. dont forget about us when its fixed cant wait to hear about her back on the road!

 

one for the road

Hey Dave

Before you change CDI box take the High voltage coils (the black ones attached to your air box) and switch them just to be sure that is not them.

Best regards

Gunnar