Forum

[ric]

I recently fitted a Mikuni conversion to my stock B5 EFI but despite having a larger gearbox sprocket I find I am hitting the rev limiter upright along the flat. I intend to do some head work over winter which will undoubtably improve the capability of the engine over stock but ultimately improvements will still be hampered by the ECU limiter.
Any ideas on how to remedy this short of bolting on a PCv just for its ability to extend the rev range?

[Bullet Whisperer]

My money is on the valve springs, try fitting better ones.

[ric]

Certainly wouldnt hurt when I’m in there as I’ll be upgrading the valves, but the bike exhibits exactly the same symptoms at a gps measured 88 on the larger 18 tooth gearbox sprocket as it used to at 83 with the stock 17.

[Bullet Whisperer]

Sounds more and more like the dreaded valve bounce then, ric. If you have access to a dyno, run it with the air filter hose off and I bet when you hit the 'limiter', you will see a fog blowing back out from the injector body, just as I saw coming out from the carb on an Electra X some years ago. The inlet valve shuts, compression builds up very briefly, then the inlet valve bounces open very slightly and the mixture, or some of it gets forced back out and engine speed hits a wall. I was able to hold a running Electra X engine at full throttle in neutral and it hit this 'limiter', which a change of cams and valve springs cleared up. Regards, Paul.

[Adrian]

Hi ric,



it sounds like you've found a way to achieve Electra-X performance with an EFI Bullet!



My old Electra on its standard 18T gearbox sprocket would definitely run out of puff in top, even with a ported head and a big fat Mikuni carb doing its thing. 18T was too high and dropping down to 17T helped, though a set of cams borrowed from Bullet Whisperer finally unleashed the missing top end. I don't know how UCE/EFI cams compare with the Electra-X AVL profile where rev-limiting bounce sets in, but I gather that most of the successful tuning exploits for EFI and former EFI Bullets have involved using performance cams.



One other point that might not be on your radar is that our hosts' carb conversion kits have changed a bit since they were first launched. Early ones replaced the EFI alternator rotor and ECU with and Electra-X type rotor and the performance (ignition only) TCI black box (ex-Suzuki GS125), while current versions appear to retain the ECU and stock alternator rotor with some sort of jumper cable arrangement to bypass the redundant ECU functions. It would be interesting to know how much of the ECU is actually bypassed.



I'm assuming that the EFI's hydraulic tappets aren't an issue.



A.

[ric]

Hi Adrian, the carb was fitted as a cheap(er) solution to an EFI problem. However the bike has only recently woken up at the top end after using the carb support bracket I’d made to reposition the carb outlet from pointing at the port wall to directly at the valve head. Fitting the AVL parts would probably be a good solution.

[Dan H]

Hi Ric, in my experience with the EFI engines, it is always the valve springs which are the restricting component for the rpm limit (as Bullet Whisperer has mentioned). The rev limiter on the ECU is usually not that far behind, but you will extend the revs by a good few hundred rpm by fitting the up rated valve springs (all though the standard cams may not give much performance at these higher rpms).

[ric]

I'm certainly not in disagreement about the valve springs. However having just checked out all the other costs required to evolve this bike to just a slightly higher level than it currently stands, I'm thinking these monies would actually be far better being put towards upgrading my T120 or towards replacing the B5 with one of the new twins.