YZF750R won't rev past 4000

ftang · Post by **ftang** » Tue Oct 04, 2005 2:49 pm

...sometimes. And sometimes it's fine.

Hi everyone. A greenhorn's first post here. So: it's a 1994 YZF750R, 20k miles, well looked after, recently graced with new plugs. After being caned through the gears a couple of times recently, it suddenly wouldn't go past 4000rpm, in any gear or none at all. Splutters when it hits it, and sounds different - more raw, kind of - up to those revs.

Then, half an hour or half a day later, all is suddenly fine again. First thoughts turned to fuel starvation or pollution, but I don't think it's that - there was no water in the bowls and they filled up again very fast.

It's such a specific fault that I wondered if anyone had run into this before - before I settle down and check the timing, coil resistances etc. Trouble is, I'd have to make sure the fault was happening before I start... I did ring up the nearest Yamaha specialist here in London, and they said they wouldn't touch anything that old! Stunned.

Further background: it's got a Dynojet kit, noisy exhaust and free-flow filters, though it's not been properly dyno'ed. Generally it runs well with a mild flat spot around 7Krpm.

Intermittent faults, eh? Love 'em.

dragracer1951 · Post by **dragracer1951** » Tue Oct 04, 2005 3:26 pm

I can see a number o fpossible reasons for this
Pull the plugs. Have a look and I'll bet it's running rich
Let me know and we'll go from there

creed12r · Post by **creed12r** » Wed Oct 05, 2005 12:00 am

Sounds like dodgy electrics to me.
If you have a bad connection from the digital ignitor to the exup valve motor, it would do exactly what you describe.
chris

ExupElvis · Post by **ExupElvis** » Wed Oct 05, 2005 7:20 am

noisy exhaust and free-flow filters

Questions here ftang - Are the EXUP valve and airbox still in place? Individual pod filters on the carbs will do strange things to driveability sometimes, and the effects of removing the EXUP valve are well documented on this forum as having detrimental results.
Just wondered, but if you have a Dyno Jet kit in there you may well have to look no further than that......and yes, the dealers are useless for the most part.
Welcome to the 'Hood BTW!
~B~

Duncan Hines · Post by **Duncan Hines** » Wed Oct 05, 2005 7:45 am

I would check your EXUP servo. I had to replace mine, it would flick in and out, short and i had the same issue untill i replaced it. of course check your wires in and follow them back to see if there is a break some where.

ftang · Post by **ftang** » Wed Oct 05, 2005 8:01 am

Thanks for your interest, all. The Exup's still in place as is the airbox. I had it rejetted a while back. It got three sets of main jets until a big flat-spot due to over-richness was reduced to acceptable levels. I know I should get it properly set up on a dyno but the kids ate the money...

But it usually runs fine, as it is again today. Come the w/end I'll make sure the exup's working and pull the plugs. I'm keeping the revs in the lower half now because if anything seemed to provoke the complaint, it was when I thrashed it. Even managed it doing exactly the same thing, on the same spot, two days in a row.

And I can tell you that showing a bunch of cars your tailpipe as you head for the horizon, only to be passed by them all two minutes later as you stumble along at 40, makes you feel like a complete idiot.

ExupElvis · Post by **ExupElvis** » Thu Oct 06, 2005 12:24 pm

Sounds like Duncan has the winning advice here, Yup - check the servo.

ftang · Post by **ftang** » Mon Oct 10, 2005 10:42 am

I think you may have found the culprit, folks. The exup servo tries to turn the upper pulley on its start-up rotation but can't move it more than a mm or two. I guess the valve or the cables may be jamming. Which leads me to...

There's a hidden 8mm bolt, one of three holding on the exup valve cover, which, being an original part of the exhaust system, is more-or-less welded in place. I can't get on it straight because of the sidestand plate, and that's held on by a long engine-mounting bolt. If I take out the bolt should I support the engine or will it hang there ok for a while? Or what's the usual solution here?

Cheers all.

dragracer1951 · Post by **dragracer1951** » Mon Oct 10, 2005 10:55 am

Pull the pipe and drill out the screws

creed12r · Post by **creed12r** » Mon Oct 10, 2005 12:59 pm

I can usually get straight on the bolt you are talking about by just removing the sidestand switch mounting bracket (two bolts), but might be different on the yzf 750.
If you need to remove the complete sidestand bracket, (by removing the lower rear engine mount bolt) the engine will still be adequately supported by the other 5 mount bolts so no need to worry about supporting the engine.
If you want to know if your exup valve or motor are stuck, disconnect the cables by loosening them at the adjusters and remove them from the motor, pull the cables and see if the valve moves, if it does then it is your exup motor at fault, if it is seized and the bolts are difficult to get out, do as dragracer says and take the downpipes off, you will find it easier in the end.
chris

KontoBoy · Post by **KontoBoy** » Mon Oct 10, 2005 2:13 pm

...and I've got a replacement header with EXUP valve if you need it --just $50 plus shipping.

Pics are here:

http://members.cox.net/kontoboy/parts_for_sale.htm

Kontoboy

ftang · Post by **ftang** » Tue Oct 11, 2005 4:11 pm

Ok, I've learned a bit now. The whole valve outer - pulley, allen bolts, cables etc. - is like the surface of Mars: red and dead. The cable ends have welded to the pulley and the valve was stuck. So I may be up for a replacement. Checked the new prices, and when I'd recovered from the shock, sent kontoboy a pm about his header.

Oddly, when the central bolt is loose, the valve rotates fairly freely...

As to the servo: it behaves oddly. It's getting some kind of power and it operates continuously, counter-clockwise, when wired directly to the battery as suggested in the manuals. But it won't go through its start-up rotation when I turn on the ignition; it just kind of twitches. The cables are now disconnected. The wiring to the black box is fine, showing continuity on all 5 leads. The question is: should it do its little back-and-forth routine whatever position it starts from, with or without being attached to the valve? And what makes it go clockwise - reverse current in the same wires or current down the other ones - ie how do I check its reverse operation?

BTW, I presume the cables go top-to-top from the valve pulley to the servo (ie counter-clockwise rotation of the servo pulley opens the valve) - cos like a fool I forgot which one went where...

EXUP Worldwide

YZF750R won't rev past 4000

YZF750R won't rev past 4000

spluttering

removing valve cover

Won't rev