GPX750 won't rev

Rusty · Post by **Rusty** » Wed Apr 11, 2007 12:42 pm

Idle jet settings?

Sounds like what my old Honda did when the idle jets were set stupidly rich.

BladeBoy · Post by **BladeBoy** » Wed Apr 11, 2007 12:46 pm

OK, This is what I would try, I will probably get shot down in flames, but here goes, Problem is you don't know if it is a fuel or electrics problem so I would pull a spark plug lead off slightly and see if the spark dies at the revs indicated, given that if your problem is fuel I would think the spark would continue until the motor stop completely, if not you have a fuel issue.

MickLC · Post by **MickLC** » Wed Apr 11, 2007 1:29 pm

I'm making a couple of assumptions but...

I'm pretty certain the GPX750 had a diaphragm type fuel tap, but if you are testing the fuel flow on the CB500 tank and it is pouring out, then I assume it is an old style tap that doesn't need a vacuum line to operate?

So has the vacuum line from the carbies for the original fuel tap been blocked off properly? Big source of air in the wrong place if it hasn't.

Now this is working off what I've been reading about Z650 carby's but it also sounds like it's happening when you are coming off the idle jet and onto the main jet. Is the needle height set correctly? What happens if you open the throttle 100% quickly instead of slowly working up to it?

Barclay · Post by **Barclay** » Wed Apr 11, 2007 3:27 pm

Idle needles are set to standard.

Fuel tap is not being used, and yes I have blocked off the vacuum point correctly.

I agree, it is happening when I come onto the main jets. I noticed especially with the old carbies, it started dying as soon as the diaphrams started to rise. They didn't rise cleanly, instead they stuttered and jumped around.

What do you reccomend, raising or lowering the needles (they're adjustable in the second set of carbies)?

If you open the throttle flat out from nothing, the engine dies, doesn't rev at all.

MickLC · Post by **MickLC** » Wed Apr 11, 2007 3:57 pm

At full throttle I don't think the needles are doing much at all, it's just main jets and a lot of air going past them. So you'd think that they're either putting too much fuel in or not enough...but then reading back over earlier posts, it's the same with different sets of carbies, so you wouldn't think it would be the settings on them...

Ahh bugger, I don't know, I'm out of my depth again. These things are so much easier when you can see what's going on.

Barclay · Post by **Barclay** » Wed Apr 11, 2007 4:06 pm

Yeah, I agree with that, forums do make it a bit painful to diagnose things.

I remember seeing a post about a month ago on another forum of a guy describing the exact same problem, do you think I can find it now?

The main jets aren't getting a chance to open really, as the needles come up the engine dies. I'll try raising them a notch, see if getting a bit of extra fuel when they open helps. Failing that i'll have to stuff around withthe main jets. I just find it hard to beleive that i've got two sets of carbies, that have both been set up wrong, and no ones fixed them. I don't know if they've been on running bikes before, one was off this bike (not running when i got it) other set from the wreckers.

Barclay · Post by **Barclay** » Wed Apr 11, 2007 4:28 pm

I think I may have it. I'll find out when i get home in a few hours (don't you love having the internet at work?)

GPX's have highly restrictive airboxes. I've pissed this off, and put on a set of K&N's. Both sets of carbies perviously ran with airboxes.

The K&N's will have a much lower pressure drop, so as a result, i'm getting a higher airflow for the same fuel metering. Therefore I need to richen the running mixture.

I can do this to a certain degree by raising the adjustable needles, but I might need to bite the bullet and increase the jet sizes.

See, forums are useful for thinktanks..

Cross fingers!

Neka79 · Post by **Neka79** » Wed Apr 11, 2007 5:59 pm

Barclay wrote:Everything points to fuel starvation, I know, but with the fuel line pulled off the carbies, fuel gushes out of it. I'll dot point the situation:

when u have the fuel line pulled off, theres not gunna be any kinks, or a air tight seal...however when u plug the hose back into the fuel tank, u may be pressurising the system, or not enuf fuel getting thru.....

.... when u plug it back into the carby its nice and str8??

just read ur post mate..u could be right..didnt i mention that before?? lol...

Phil · Post by **Phil** » Wed Apr 11, 2007 7:03 pm

Gosling1 wrote:If Phil spots this thread, he could ask his mate Tim, who will know the problem - Tim has one of the bestest most modified GpX750's I ever seen........ its a cracker.

just noticed, i'll get tim to read it tomorrow.....he's busy at the moment, he's just got his bike back on the road after getting new (read bigger) valves and some more work - the smile on his dial yesterday indicated the work was a success

Neka79 · Post by **Neka79** » Wed Apr 11, 2007 7:19 pm

mwahahahah Barclay- i hope ur willing to spend $16mill on ur gpx now..lol

Barclay · Post by **Barclay** » Wed Apr 11, 2007 9:07 pm

We've spent about 5k so far. It's got an R1 subframe and ducktail, Honda CB500 tank (although that's being changed to a Z1000 tank for handlebar clearance), streetfighter front fairing, we completely remade the wiring harness and dashboard, it had a ZZR600 rear wheel but that didn't fit, etc etc. Will post up pics when we're done.

I'm building the bike for the girlfriend, I'm on a Triumph Daytona myself (traitor, I know, but I started with a ZZR250). Will be using it to ride down to Phillip Island this year.

Phil · Post by **Phil** » Wed Apr 11, 2007 9:34 pm

Neka79 wrote:mwahahahah Barclay- i hope ur willing to spend $16mill on ur gpx now..lol

he has just spent another couple of mil on it as well

well over 20 now methinks

Neka79 · Post by **Neka79** » Wed Apr 11, 2007 10:45 pm

Barclay wrote:We've spent about 5k so far. It's got an R1 subframe and ducktail, Honda CB500 tank (although that's being changed to a Z1000 tank for handlebar clearance), streetfighter front fairing, we completely remade the wiring harness and dashboard, it had a ZZR600 rear wheel but that didn't fit, etc etc. Will post up pics when we're done.

I'm building the bike for the girlfriend, I'm on a Triumph Daytona myself (traitor, I know, but I started with a ZZR250). Will be using it to ride down to Phillip Island this year.

sounds interesting.... ive seen a early cbr with R1 ass end..didnt look too bad i guess...

u running gpx front end?? so other than the motor and frame wot is GPX??

Barclay · Post by **Barclay** » Wed Apr 11, 2007 11:49 pm

Most of the bike really. Well, the mechanical side of things.

Okay, pulled the jets out of the second set, turns out they were 105's, not 110's like they're supposed to be. Replaced with 110's, made no difference.

Changed the spark plugs to the right ones, no difference.

Raised the needles, and viola. They are N59J Needles, they were on the second rung. I moved them firstly to the third, then to the forth. Can almost get WOT now, and alot more power. Am going to move it to the fifth notch on the needles tomorrow, and try that. After that the bike will be going to dynobikes at Wacol to get it sorted properly. At least it's rideable for now.

One more question, I don't suppose anyone has any experience with putting motorcross bars on these bikes? The R1 seat is a touch further back than the original, and I want to make the seating position more upright.

Smitty · Post by **Smitty** » Thu Apr 12, 2007 8:57 am

Barclay wrote:Most of the bike really. Well, the mechanical side of things.

Okay, pulled the jets out of the second set, turns out they were 105's, not 110's like they're supposed to be. Replaced with 110's, made no difference.

Changed the spark plugs to the right ones, no difference.

Raised the needles, and viola. They are N59J Needles, they were on the second rung. I moved them firstly to the third, then to the forth. Can almost get WOT now, and alot more power. Am going to move it to the fifth notch on the needles tomorrow, and try that. After that the bike will be going to dynobikes at Wacol to get it sorted properly. At least it's rideable for now.

One more question, I don't suppose anyone has any experience with putting motorcross bars on these bikes? The R1 seat is a touch further back than the original, and I want to make the seating position more upright.

.cool
glad perserverance paid off re the carb

the bars?
having had a GeePeeX myself
I have to say they have to be among the
most difficult bikes to do anything with re the bars
no-one makes anything for them
you will probably have to end up fabricating
mounts for the bars yourself if you want MX bars

cheers

Smitty