Hey guys! So i got my bike up and running and around town handles excellent, no surging, stalling, burping... then i take it out to flog her a bit and bog city!! She doesn't want to get up there without a fight. When i go up to fifth and let off the throttle she acts like she's going to die. Its gotta be a fuel problem right? I replaced the in tank filter and the gas line filter. The air filter is pretty old but i can still see through it.

I want to post a pic if I can, 90 CBR600F1

 

So I've been reading a lot of past threads here and seeing that I have "ram air" and probably no rejetting I believe I have an issue with the intake. I read this:


I've posted it before but the best way to fix the ram air effect on a naked bike is a way I found a long time ago on custom fighters

Originally Posted by Mustapha dump Click the image to open in full size.
The main cause of issues when the fairings are removed is the float chambers seeing a slightly lower pressure than the airbox sees when moving at speeds above 45mph or so.
The ZX series of Kawasaki sportsbikes suffer from this problem very acutely.
What you really need to do rather than restricting the amount of air entering the airbox, is to enable the float chamers to see the same pressure reference as the airbox.
The easiest way to accomplish this is to find the two Tee pieces between carbs 1&2 and carbs 3&4 which are the float chamber vents, on some banks of carbs on the Hondas it is one large Tee piece between the 2&3 carbs as all 4 carbs are linked across the top.
Once you have determined the float chamber vents, attatch a short length of rubber hose to the Tee piece, and then run it into the airbox through a hole drilled in the underside just large enough to pass the hose through tightly.
Face the open end of the hose towards the front of the bike under the airfilter, if needs be slash cut the hose at 45 degrees and face the cut upwards and forwards.
By doing this the float chamber vent will be fed by the air entering the airbox which then ensures that the pressure in the float chambers is equalised with the airbox pressure, once this is achieved the air passing through the venturi will suffer a slight pressure drop, as this happens the air pressure in the float chamber becomes slightly higher than that in the venturi directly above the needle jet/jet needle, and this higher pressure in the bowl then forces the fuel up the needle jet/emulsion tube and introduces it to the airflow through the venturi where it is then atomised and carried into the cylinder.

What do you think?

 

If your running naked (not literally) LOL, this could be the problem.


Also you could be fuel starving. There have been issues with petcock flow from the tank. Fuel filter. Does your bike have a functioning fuel pump? Have you verified it's working properly? If you don't have a pump the engine could be running out of fuel.


You don't want to run with it in this condition. Fuel starvation can lead to major engine damage due to lean burn.

 

Thanks for your reply! Well I haven't yet checked the fuel pump relays and such. I noticed in another thread that a senior member said he deleted the fuel pump and it isn't an issue until sustained 11K rpm? If its not the air issue I thought I may just get rid of the fuel pump as I don't want to invest $170 for a replacement!
The other thing that interested me from your reply was the petcock comment. What in it would fail? Its not vacuumed, I replaced the internal filter and o ring, and cleaned it out.

Thanks for your advice!

 

The other options besides using the "t" vent method are:
1. To completely block the frame air inlets and drill ~24 1/2" holes on the top of your airbox in the low pressure area under the tank ala dynojet airbox mod.
2. Rotate the snorkel tubes away from the frame air inlets and make sure they do not have a ram-air effect going on and are located in a low pressure area behind the frame.
3. Partially cover the air frame inlets with duct tape or aluminum tape. About 1/2 to 2/3 covered does it.
4. Build covers that block ram air from occurring and pick up air behind the headlight or near the triple tree facing up.

If you do a search you will see I went ram air on purpose back around 01-04 and fought it for 2 or 3 years before ripping it all off and going to the dynojet airbox mods. The dynopet airbox mods have worked perfectly for years. The ram air I fought depending on ambient temperature, barometric pressure, elevation, etc. I could dial it for one temp, elevation and pressure, but if it changed more than 5* either way the tune was off and I fought it like crazy, tearing out carbs, adjusting jets, and dialing up or down the restrictor size I ran on the t-fitting. I tinkered more than rode those years.

 

You can run without a fuel pump but given the issues you describe (cutting out at higher RPM) this was suggested as a possible reason for the issue. When the CBR first came out in 1987 the early ones did not have fuel pumps. Customer complaints resulted in Honda adding the fuel pump. It's your choice if you want to run without one.



Yes another suggestion that it looks like you have addressed. Other possibilities are partially plugged float vent tubes, floats shutting off too soon causing fuel starvation, partially plugged fuel cross over tube(s) or tee. Pretty much anything relating to fuel delivery.

 

Hey thanks a lot you two! I really appreciate the sound advice based on experience here. Im going to try the easy thing first, partially block the frame inlet and see if it improves. Next if it does i will try the airbox mod as that makes sense to rich it up.
If it can run fine without a fuel pump then I may just do this as less potential parts failure sounds good to me. I have no idea why its even necessary as fuel is not restricted other than the inline filter.

Thanks again for your help. I noticed none of you commented on the looks of her, whatya think??

 

Drakito,
Did you say 24 half inch holes?
thats a lot of drilling! why not just block the frame vents and take the airbox cover off? Just saying, not much left to it after that much drilling if thats not a mistype.

 

That's not what we said. You can run without a fuel pump BUT it can run out of fuel at higher RPM. Which means it won't run FINE. Maintaining pressure on the fuel to the carbs keeps the float bowls full. A lot of fuel is flowing at higher RPMs. Since you describe bogging at what seems to be higher RPM, then it doesn't make sense to remove the fuel pump unless you know it's bad. My suggestion if it is bad would be to replace it. Changing something JUST BECAUSE is not a good trouble shooting method.

 

It looks pretty good for an upside down bike. Are you from Australia? /wink.

On drilling the holes 24 holes is only like 1/3 of the air box lid. Complete removal actually ends up with some high pressure ram air happening again. The holes are near the front of the bike and around the 2 intake tubes inside the box. DO NOT DRILL the holes into the section of the airbox facing forwards, only up.

Dennis is correct on the fuel pump. You can temporarily bypass the pump to see if it is causing the issue, but I would always run a pump. Fuel starvation mid corner on a deep long turn would suck and possibly cause a collision.