Hey guys,

First of all a disclaimer and a thank you. I don't have a CBR, but I bought a '00 CB600F (hornet) a couple of months ago. I know it's a CBR-forum, but the info on the CB600F's is hard to find, and it got most of it's components from a CBR600F, which is why I've been lurking around here a lot, which has helped. Thanks for the information so far

Onto the issue: It turned out the bike was running rich, and I had black smoke coming out of the exhaust at full throttle that made me look like I was taking part in Wacky Races. I was getting about 12km/L, whereas others had 15-20km/L. I had the carbs clenaed, replaced the air filter & spark plugs & gave it a vacuum sync. I'm getting 15km/L now with very conservative riding, not great but ok. Still some black smoke at WOT. It's fine to ride though, no other complaints.

I was just going through some things from the service manual, decided to the the Throttle Positioning Sensor. The manual said it should be between 4-6Ohms, it was (5.4), and the resistance should increase when I opened the throttle. Here's where I got curious, because nothing happened. It doesn't matter how much or little throttle I give it. I even took the sensor off and manually moved it, but the reading didn't change. Is this indicative of a broken TPS? It might explain the lackluster fuel economy bc I don't get any benefits from the timing, but information about TPS on carbed bikes is quite limited so I was wondering if someone could give their 2 cents worth.

Thanks in advance, I hope I'm forgiven not asking about a CBR (yet) .

 

With carby bikes, TPS is only used for ignition, doesn't affect fuel. Unless it has some kind of injectors in addition to carbs? Also make sure you are measuring correct wires for variable-output signal. There should be 3 of them.

Most likely previous owners drilled out jets in carbs or installed larger ones in mistaken belief that would give them more power. Actual opposite is true, remember that '60s hot-rod saying?

Get carb-rebuild kit with all OEM parts to replace unknowns in your carbs. Also set float-level to stock with external wet-test to confirm. Then sync carbs.

 

Thanks for the reply. I'm not sure it's running rich anymore after the vacuum sync, I've just done it so I'll check the plugs after a couple hundred km's. I checked them a week ago and 1 was fine while the others were black, hence the vacuum sync.

I know it the TPS is used for ignition, but I was mainly wondering whether that could be the reason I'm not getting great mileage, as it would be worth fixing if it's broken. Not a 100% sure yet if it is.

The jets looked fine, so if they drilled it out they mustve done it really carefully. I was hoping it wasn't that though, because they're not cheap, edit: nvm, they're not expensive at all, might be worth a shot..

 

You can use soft copper wire of different gauges to measure existing jets. Slowly increase size until it barely fits. Then measure with micrometer to confirm size. How does that compare to stock?

 

Hi @dannoxyz , thanks for the replies.

For some reason the cb600 manual states the resistance of the TPS must go up at WOT. I checked the cbr600f2 manual and instead it gave a formula for the range of the sensor output in Volts. I checked that and it seems to be in spec, so the TPS is no longer suspect for me.

The trick with the wire is smart. I still have the bike partly apart so I'm going to pull the carbs and give it a shot. I also realised the only plug that looked clean last time I checked them was from the cylinder with the only carb I replaced the idle jet from. Not that it would make sense to me that someone would drill out the idle jet, but then again the bowl screws were stripped to hell so maybe the previous owner wasn't the wisest anyway.

 

Most people don't realize that there's no Philips screws anywhere on entire bike. JIS only!

 

@dannoxyz Bingo Bango! Just measured the other pilot jets. .46mm wire is going through. The new pilot jet doesn't even let .41 wire through. Pilot jets are supposed to be .40. Unfortunately I cannot check the main jets as I don't have accurate wire that size, only 1.19 which (fortunately) doesn't go through. They're supposed to be 105/108. Anyway, off to buy some jets. Thanks for pointing it out, may just be this simple.

 

Here's good site with large selection of jets for different carb models: https://www.jetsrus.com

 

Thanks. Unfortunately in this regard, I live in the Netherlands. I say unfortunate because I just had to pay over a 100 euro's for 7 jets, but it is what it is. Should get it back in fuel savings soon enough .

 

Hey @dannoxyz . I fitted the jets and it worked like a charm. The first tank it seemed I got over 20km/L, which is really nice. I'll look at the plugs in a few hundred KM's to make sure it's not running lean, but otherwise, everything seems right.

Thanks again for the help