GPX750 won't rev

[Barclay]

I've got an 86 GPX750 that's being rebuilt. The bike runs fine to what i'm guessing is about 5000rpm (no tacho), but if you try to open the throttle any further, the engine stalls. It's as if it's running out of fuel. It doesn't matter whethe the bike is stationary or being ridden.

I know this isn't the case, there's plenty of fuel supply. I've tried two different sets of carbies with varying float levels in both, exact same problems. For now i'm going to rule carbies out.

The spark plugs aren't the type specified, this may be causing it, i'll replace them tomorrow and see. As I said the bike runs fine at low revs though.

I've got the Haynes manual, and i've tested the ignitor module following the procedure listed in the back. I'm coming up with some matching readings, and some different ones. I would of thought if the ignitor module failed, it would generally just stop. Am I wrong?

When I rev the bike, just before the engine starts to fail, the ignition appears to be fully advanced, and not moving any further. I'm sure the advance isn't meant to top out so early, as top advance is listed at 10,000rpm.

Does this all sound typical of a faulty ignitor unit?

[Gosling1]

... as top advance is listed at 10,000rpm.

Does this all sound typical of a faulty ignitor unit?

I think the fault is in the book. Most motors have reached full ignition advance well before 10k !!

To me, it sounds like your ignition advance unit has failed, and you are not getting any ignition advance at all - this would explain why the engine stops revving at 5k.........

Having said that, diagnosing problems via forums can be hit'n'miss

I would have a *real* good look at the ignition advance unit, double-check timing etc, use a strobe light.

[mike-s]

that and also doublecheck the jets to make sure that they are stock (105) or maybe 106.5, which is what i used when i had a full system on the bike. made it *just* burbly at WOT .

but yeah, advancers usually kick in by about 4-4.5

[Barclay]

I stand corrected, i was reading the wrong part of the book, I was reading for 600's. The 750 reaches 40 degrees at 3500rpm. Which is what my engine does, the ignition timing is spot on. Using a strobe light, it's on the 'F' mark at idle, and at 3500 it reaches the top mark. About 500rpm after this the engine craps itself, but the timing does not move from this point until the engine dies.

Wrong plugs cause this fault maybe?

The jets are 110's i think, they weren't much different from standard, and the engine acts the same independent of which of the two sets of carbies are on it. Main jet's supposed to be a 112 according to the manual.

I am confused with the ignitor module, in that all of the readings are pretty much correct, bar pins 9-10, they're about double the top value for the range, but otherwise all the readings are inverted. Where the book says connect '+' lead and '-' lead, I'm getting the exact opposite readings, as if i've connected the leads the wrong way. Obviously I've checked i'm not doing this..

[plane]

Is the airbox sealed correctly?

Take the lid off, give it a clean and reseat it so that it seals properly.

Cheers,
Pete

[mike-s]

cant help with the readings, what idoes the manual indicate the function of pins 9-10?

Oh and trust me, going up to 110 makes a huge difference. i had rebuilt a carby with brand new freaking everying and i put 4 new sliders in (from a good carby), I put a dynojet kit in according to the instructions and made it a stage two with mains of 118 as i had put a full system in the bike.

The only problem was that it choked above 2/3 throttle, which got me a max of 9krpm if i was lucky. I dropped it back to 112.5's and it would go to redline, but still wasn't running too fantastically at the dynoday we had.

Putting OEM 105 sized jets in (i bought these new, as f*ck knows what size the jets in the bike were) solved my woes with the top end. As the carbies were such a pain in the arse to put in and out (i had done it over 15 times while testing diff settings), i left the slight stutter when cold at just off idle.

[Gosling1]

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.

Well, from the sounds of things, you have sorted the timing advance issue, thats one less problem. Not sure if the 'readings' on the ignitor pins will make any difference, MikeS is on the money, jetting could be the issue......

[Neka79]

i was gunna suggest air starvation..blocked hose sumwhere?? even fuel hose blocked?? fuel type lock?? air lock..sumthin in that order??

sounds stupid...but try running the bike with the fuel cap open...

once saw a bloke spend 2 days trying to get a race bike going, only to realise the cap was causing a fuel lock..

[Barclay]

There is no air box, it's running a set of K&N pod filters, could this be the problem?

I've taken the fue line off the carbies and held it in a bucket, fuel gushes out at a stupid rate, so i'd say fuel is getting to the carbies okay.

I'm not sure whatthe functions of pin 9 and 10 are, but i'd say the ignition unit is working okay.

I feel your pain aout pulling carbies in and out, i've lost count now.. I'll have another look at the jets, I think the second set hasa dynojet kit (enlarged float needle, adjustable intermediate jets, etc etc) but i'm just a bit sus in that both sets of carbies with different setups do the exact same thing.

[Barclay]

Actually probably a good place to ask, the bike's a streetfighter, currently it's got a honda CB500 fuel tank, although I've got an '04 Z1000 tank to go on it, fits remarkably well. No one has a fuel connection piece they want to sell (the bit that bolts onto the left side of the tank)

On that note, what do they look like? the fuel tank is bare, so I don't have any of the fittings.

[Smitty]

Is the airbox sealed correctly?

Take the lid off, give it a clean and reseat it so that it seals properly.

Cheers,
Pete

GeePeeX7fiddys don't have an airbox
they have a filter housing thingy under the seat

hmm....
I just had a thought
one thing to try is...REMOVE air filter assemblies
and see if problem still occurs

a blocked or restricted air filter will cause exactly
the symptoms you are getting

hth

Smitty

[Barclay]

Have been doing most of the trials without the air cleaners, no difference.

I've got the correct plugs in my hands, will fit them when i get home, cross fingers that fixes it. I'm not holding myb reath though...

What about an incorrect air gap between the 'distrbutor' and the pickups?

[Smitty]

Have been doing most of the trials without the air cleaners, no difference.

I've got the correct plugs in my hands, will fit them when i get home, cross fingers that fixes it. I'm not holding myb reath though...

What about an incorrect air gap between the 'distrbutor' and the pickups?

hmm..unlikely me thinks
coz the wrong airgap would be there all the time
and the thing would not rev...at all

plugs?
maybe , so give 'em a try

another thought
with the strange tank ( or the normal tank for that matter)
are the fuel lines straight? no kinks?

based on yr description .....fuel starvation is a possibility


hth

Smitty

[mfzx6r]

fuel supply to the carbs

[Barclay]

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:

- Bike is an 88 GPX750
- Fuel tank is Honda CB500, with fuel line straight from tank to carbs. With the line removed from the carbs, fuel gushes out. There are no kinks.
- Airbox removed
- Two sets of carbs, one seemingly original, the other appears to have a dynojet kit (the manual doesn't list what the original jetting should be, just says 'not specified') Same problem with both sets.
- Float level has been adjusted from minimal to overflowing. Increasing the float level did make it run better but far from perfect.
- Standard ignition, timing advances to 40' (i think) at 3500rpm fine, stops advancing at that point.
- Engine starts to stall at 4000rpm
- Wrong spark plugs (trying correct ones tonight)
- Engine runs great below 4000rpm
- It's a very sudden stop, at one throttle position the engine is fine, open it up just a touch more and the engine stalls. It doesn't hunt and splutter, it just slows down and dies.
- Manifold pipes (the rubber tubes joining the carbs to the motor) are in the wrong order, I didn't think there was an order until I saw numbers on them, but i doubt this would make a difference.
- IC Igniter box readings seem to match what's in the manual