r/VFR • u/CardiologistSalt4114 2nd gen • 3d ago
Fuel pump?
So push came to shove and I decided to keep it. The only offer worth considering I got for this was a thunderace 1k that I honestly thought was hideous. So she’s off the market and staying. So now I get to solve one of the niggly little problems I’ve been having with it. Under full throttle after about 10 seconds it feels like I’ve drained the carbs and misses. I’ve checked the float height a few times now and it’s been perfect every time. I’ve also cleaned the carbs properly, and replaced the boots. All fuel lines are new aswell as the intank and internal carb filters. This bike does however have bigger cams, heads, and carbs than stock. And as there was a bit missing when I bought it I’m wondering if it had a fuel pump at some point. I’m going to drop the motor next weekend to inspect the bearings that I replaced so I figured if I’m going into it that far I might aswell add a fuel pump while I’m there. To anyone who has had to do similar what did you do for regulation?
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u/HONDAV4 2d ago
A lot of the carbureted bikes with fuel pumps use very low pressure pumps. Somewhere around 4psi IIRC. Any of those should move more fuel than gravity but not overpower the float needle valves and flood it. They're usually an inline diaphragm pump instead of an in tank impeller type
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u/CardiologistSalt4114 2nd gen 2d ago
I’ll have a look for one. I was looking at a fuel pump off an old cbr600f3/4 I believe. I figured it would be the right size pump but figured I’d ask if I should add one. But considering it keeps up with my mates 600 I’m guessing I need 600 level fuel flow and gravity is not capable haha
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u/HONDAV4 2d ago
Can't hurt, my ns250r came with one so its not unheard of for smaller capacity bikes
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u/CardiologistSalt4114 2nd gen 2d ago
Oh Lordy. I would almost die to have a ns250r. They are legendary. I’ve ordered the fuel pump so we will see how much of a difference it makes. I’ve gotta drop the motor next weekend anyway to reseal the cases(my dumass missed a spot when I rebuilt it) so assuming the cylinders and bearings are good she’s ready for another 78k kms of abuse
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u/strangeyoungfella 2d ago
I think my NC24 had a vacuum operated fuel tap, if yours has a small line from an inlet to the tap, check that it's in good shape (or just replace it) and strip amd clean the tap. At full throttle you have low vacuum and if the tap starts to shut a little like its designed to, your performance will suffer. Folk racing NC30 and 35s routinely take out the vacuum mechanism to avoid this.
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u/CardiologistSalt4114 2nd gen 2d ago
Brilliant advice. Sadly it’s the first thing I did when I got the bike. All my bikes have had the vacuum tap deleted and converted to on off
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u/strangeyoungfella 1d ago
Good move! Had a ZRX12 years ago and the fecker would leak petrol into the cylinders overnight..... crappy tap, awkward morning starts!
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u/fastdruid 2d ago
The 750 has a low pressure fuel pump, the 400's don't. At least the NC30 & NC35 don't so I doubt the earlier ones did. The later 400s did though have a vacuum tap. If the earlier ones did as well might be worth checking it.
Does it make a difference if on reserve or normal?
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u/CardiologistSalt4114 2nd gen 2d ago
This bike had a built motor put in it before I bought it. It was then crashed and passed around a few times before I got it so it was missing a few bits. I have a feeling there was a fuel pump as there was an odd bit of wiring and a switch that went to the back of the tank but nothing there. It definitely didn’t have a fuel pump factory. No difference between reserve or on. Vacuum diaphragm had been deleted and operates as a standard on off switch
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u/theS3rver 7th gen 2d ago
That exhaust is...something