'93 CBR900RR Performance Swings from 100hp to 5hp

thomprd · 09-17-2006, 8:30 PM

I struggled to find a good title for this post. The situation is complicated, so please bear with me if you are good at diagnosing problems. First, the background. My bike came with significant body damage, so I rebodied it. The rebody kit from eBay came with 49 state carbs and an fiberglass intake that went to a port in the front facia. It seemed like it might provide more airflow even though I understand the physics of "ram air" enough to know that you can't compress air in such a way and you won't increase compression by using this system.

If I remember my numbers right, the new carbs were jetted with 165 mains, and the ones that came with the bike were 135s and stock is 125? With the new carbs and intake the bike freakin' hauled, BUT it wasn't very polite anymore. The power band came on very abruptly.

Well, a while ago.. the bike started misbehaving and cutting out and having problems. Ignoring anything that was happening, we put the original carbs back on with the new intake and the bike didn't seem like it wanted to ever wind up above 8k rpm's. We said, hmm.. maybe the intake actually provides more air (?) and so we upped the jets a size or two.

Result? The bike winds to 12k the first time off the line, but you grab second gear and it stammers and stutters and belches and doesn't like second gear. It's like the bowls have emptied and there's no fuel left. Fuel problem? I did the fuel pump test removing the feed at the carbs and shorting the 2 wires in the connector like the manual says and it could drain the tank in no time. No fuel pump problem. That takes the FP relay out of the equation, so maybe that relay is making the fuel pump not work. I don't know if this is advisable or not, but I tried riding it with those 2 wires shorted, I think that should run the fuel pump 100%. No change in performance. Perhaps the FP is overcoming the floats somehow and flooding it, but that doesn't really make sense to me.

The fact that it winds up fine the first time would indicate that the timing is OK, it is capable of getting fuel, and I have spark. I tested the battery thinking maybe I had voltage droop. 12.5 voltage standing and 14.5 voltages rev'd up.

The bike rev's like an animal standing still and if I run it through 2nd gear, squeeze the clutch and let it idle for a minute, it will come back to life (again, like the bowls have emptied and take a minute to refill)

One more detail. There are 2 breathers that come out of the top of the carb assembly that are shown Y'd together in the manual. With the new carbs and air box, there were holes for them to run into the airbox. I've tried this setup with them into the airbox and with the holes plugged and the breathers open-air. I do not understand what the diaphram does in carbs nor do I know why with the other carbs these needed to be plummed into the airbox. The bike did not run right with these open-air with the other carbs. I'd like to understand them and know where they should be, now. It's quite possible that they let the carbs fill with fuel and it's just not opening up. I'll try some more internet research just to understand carbs better and see if I can understand what affect these breathers have.

Thanks for any advice you can give,
Ron

JBODIE · 09-17-2006, 11:41 PM

On my 94, i recently had full exhaust, ignition advance, spark plugs wires etc put on at the dealer. Swing by your local shop and let them do a once over or get the manual and trouble shoot.

thomprd · 09-18-2006, 4:15 AM

Did I post in the wrong thread? I guess I didn't look for a "mechanical problems" thread. I'm not taking the bike to a dealer, or else I would have done that a month ago. I have many more things that I'd rather spend $600-1000 on. I only paid $2200 for the bike.

japanese900rr · 09-18-2006, 5:03 AM

Have you thought perhaps there's a loss in pressure in the fuel pump? Try converting it to either a gravity feed or replacing the pump with an automotive pump sold at auto stores. You are technically pumping a lot more fuel with the bigger jets, the bike it over 10 years old, whichever the reason, you can't go wrong by changing the gas pump.

I advise changing all fuel filters too. The inline filter and cleaning + spraying air through the filter in the tank (the one on the petcock stem)

thomprd · 09-18-2006, 6:54 PM

I thought about disassembling the petcock and seeing if there was anything wrong with it. The fuel pump test worked great, though. I don't think there is a fuel pump problem -- AND I forgot to mention that I just replaced the fuel pump with a brand new OEM fuel pump because I thought it may have been the problem. Once I get it fixed, I'll probably put the old one back on, make sure it still works and either keep or eBay the almost-new-one.

I guess I'll break down and read about diaphrams online somewhere..

Ron

jetbdude · 09-18-2006, 7:12 PM

It might sounds stupid but it happened to me once so.... make sure the float in the bowls are not up-side down. When I put the float back in my dirt bike I've put them the wrong way and experienced the same problem you're having... The difference in the float was not really noticeable, took me a while to figure it out...

abtech · 09-18-2006, 7:15 PM

I would check the integrity of the rubber diaphragm covers in the top of the carbs. My 750 had a similar problem and 2 of these rubber accordian like covers had pinholes in them and would lose pressure once underway.

I also had a tank of gas with a fair amount of water in it at one time and the bike acted exactly the same. Revved to the moon in first and then fell flat on it's face after shifting to second.

You may also want to look at the needle settings once you resolve the erratic running problem, as this can help make the power a bit more friendly (no big surge at the higher RPMS). Usually raising the needles (by lowering the clips) is the correct direction if it runs good with the bigger main jets.

jetbdude · 09-18-2006, 7:20 PM

Quote:

Originally Posted by abtech

I would check the integrity of the rubber diaphragm covers in the top of the carbs.

Had the same problem on a boat in Mexico this winter.

onevcs · 09-18-2006, 7:30 PM

I am amazed at the amount of knowledge in this site. Or perhaps at my lack of knowledge.

Good luck with your problem.

Ted Edwards · 09-24-2006, 11:14 PM

Thomprd'
I suspect that the problem may possibly be with the ram air to the stock carbs. I had a similar problem when I fitted a ZX900R engine into a ZX750R chassis and forgot to refit the float bowl air pipe from the airbox. result was it revved to kingdom come when idle and ripped through 1st until 45 MPH then hit a wall and felt like it was out of gas. If I changed through the gears gently it would get to the same speed and die again. After a few days of trying every thing I concluded that the higher air pressure in the venturi was overcoming the atmospheric pressure in the float bowl and therefore stopping the fuel from getting up the emulsion tube. I reconnected the pipe and it howled through all the gears all the way up to well over 160 MPH. Try riding to see if it is speed related or remove the ram air and use the normal air box.
Regards
Ted

abtech · 09-24-2006, 11:23 PM

Quote:

Originally Posted by Ted Edwards

Thomprd'
I suspect that the problem may possibly be with the ram air to the stock carbs. I had a similar problem when I fitted a ZX900R engine into a ZX750R chassis and forgot to refit the float bowl air pipe from the airbox. result was it revved to kingdom come when idle and ripped through 1st until 45 MPH then hit a wall and felt like it was out of gas. If I changed through the gears gently it would get to the same speed and die again. After a few days of trying every thing I concluded that the higher air pressure in the venturi was overcoming the atmospheric pressure in the float bowl and therefore stopping the fuel from getting up the emulsion tube. I reconnected the pipe and it howled through all the gears all the way up to well over 160 MPH. Try riding to see if it is speed related or remove the ram air and use the normal air box.
Regards
Ted

Did you read his complete post? He said it ran fine until recently, so unless one of the hoses became plugged, I doubt it's the problem. He even mentioned that he tried it both with and without the float bowl equalizing tubes to the airbox and it made no difference.

I agree, the float bowls need to be maintained at the same pressure as the intakes, but fail to see how it ran fine and then developed a problem due to something that would have been there all along . . .

meepers · 09-24-2006, 11:28 PM

had the same problem on my hawk after it sat for 6 months, one of the diaphrams had deteriorated and was letting air by. but if your having the issue on 2 sets of carbs, might be vac hose issues.