Issue with RPM range 5-6.6k, Gears 2 and above! 2005, Carb

Old 07-18-2018, 03:46 PM
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Exclamation Issue with RPM range 5-6.6k, Gears 2 and above! 2005, Carb

Hi all,

I have a CBR125 2005, carb model, which I have just done a full engine rebuild to. My issue was there before I rebuilt the engine, or made any other modifications.
Before the rebuild I was running a Mikuni VM28 carb, 240 jet, I have put stock (CBR150) back on and this has made no difference.

My issue is that when I go from first to second, third etc in high revs, I have no issues. However, when I am coming down through the gears, or just going up the gears in a slow mode and it reaches 5000 RPM, it struggles to get past 6k. When it reaches 6k, it slowly starts to gain pace and I am into 7k RPM.

One scenario would be that If I've been on a dual carriageway, I have come down the gears, I am suddenly back into 2nd/3rd to move out from a round about, and I just can't move anywhere. I find myself needing to go down to 1st! By doing this, it's put me around 11k revs, but allows me to then change gear into second - but I am at around 7k (bypassing the 5/6k range) - This is very dangerous if I have cars behind me.

Things that I have tried so far:

Changing various jets in Mikuni Carb
Switching Carbs & settings
Switching Airfilters
Using Baffles for all of the changes above - as well as taking them off
Changing Plugs CR8EIX & CR8EIA from standard and CR9
-Porting had been done at the same time as the rebuild, this made no difference-
Stator replaced
CDI Replaced
New Piston & rings

Race Camshaft was present /250 angle, original replaced - NO major improvement, but loss of performance in acceleration.

Timing is TDC.

Something may be pointing towards electrics, but I am a little stumped on this one. If anyone can help, I would be most appreciated.

Old 07-19-2018, 07:03 AM
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Location: Lancs ,UK
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Hi , just a check, what jets are you running now, as stock should be
Main 118
Pilot 38
Pilot screw 1 3/4 out
Float level 13mm

Any problems with PAIR
Old 07-20-2018, 05:25 PM
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Apologies for the delayed response. I have been too busy in the spray booth - the home made one!

So, as I switched back to standard carb, I still had modifications to that carb too.

Main 127
Pilot 38 and the rest approximately the same.
I run a Pipercross air filter on it, although I did run a K&N Induction FO 21C6 on it, yes, a brick, a car induction air filter and it sounded ace This was only with the Mikuni though. I only switched back, just in case it was the carb causing the problem.

However, having switched the cam back to standard, which I am terribly disappointed about, this seems to have helped a little. But, I was thinking of other changes that I made, but not modifications, repairs perhaps. There was one... The pipe which is on the EGR valve, coming down onto the water pipe, which has a couple of turns in it, was faulty, or split. I replaced this for a radiator pipe, but the diameter is or was roughly 15mm vs 8mm. I think this was my problem, with too much going through this pipe, with no real seal around it. I tried an old crank breather pipe out and the problem seemed to go away.

Touch wood!
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