Problem with wide-open throttle (loss of power) SOLVED

[konna]

Hello!

I'm having a weird problem where my GS runs truly great except on wide-open throttle. If I open the throttle fully, the bike loses it's ability to pull. The power seems to be varying quite a bit and the bike struggles to run. It feels like it is choking.

If I let the gas back just a bit, the bike will again pull very hard and run great. It feels like a deadzone at the end of the throttle range. The bike idles great, throttle response is really nice and overall it has never been better. I suspected the the main jet is too small and have been experimenting with larger jets but it seems to have no effect on the problem. All carb adjustments have noticeable and logical effects on other parts of the throttle range, as expected.

Some things that I have tried to remedy the problem:

- Carbs have been opened and cleaned multiple times, I suspected blocked main jets.
- There are no vacuum leaks, the boots are brand new OEM suzuki parts
- Carburettors have been serviced with Tourmax branded rebuild kit (at about 61000 kms)
- Tried adjusting the needle position (Tourmax needle with 5 positions)
- Tried main jet sizes 120 (Tourmax), 130 (EBC) and 137.5 (EBC)
- Checked the diaphrams, I believe they are okay. I suspect they are original. They are not torn and are quite elastic. I've not seen any other carb diaphrams ever, so I'm not 100% on this.
Edit:
- checked fuel level in carbs, ok


Other info, may or may not be relevant:

- 1999 Euro spec GS500E
- 68500 kilometers
- Top end has recently been rebuild with new gaskets (original pistons and rings)
- Air filter is HiFloFiltro -branded in original airbox
- Spark plugs are almost new, showing light brown color
- Fuel tank cap has been serviced and is working great.

Ask away if I'm missing any info that may be of help. Thank you so much in advance for any possible insight on this issue.

-Konna

[Bluesmudge]

Have you checked the fuel/float height? With the clear tube method it should be level with float bowl gasket. You can also measure float height but that's just a less accurate proxy for float height.

It's possible you don't have enough fuel flow to keep up with the amount required for full throttle. Does the bogging happen over time or as soon as you hit full throttle?

[konna]

Thanks for the reply!

The problem hits instantly when certain throttle position is reached. There is no gradual decline. It also goes away when throttle is gradually released, the bike suddenly gains a lot of power and starts pulling. If the throttle was limited as to never go to the deadzone it would be very hard or impossible to know that there is something wrong with the bike.

Edit: To clarify, the problem exists also near the wide-open position. Around 90% of the throttle area is usable, the last 10% is unusable. In my original message it may seem that the problem only exists when the throttle is fully open.

I've only checked that the floats are in spec while upside down and open. Around 14mm from the rim, as instructed by the service manual. I will do the clear tube method next and see that they are ok.

[konna]

The problem is not fuel level in bowl, tube method shows good levels on both sides.

Old problem of fuel starvation when driving on the highway (110km/h+) is gone also after servicing the fuel cap.

One thing is puzzling me, the valve clearances were okay before the rebuild but have not been checked since. The machine shop changed valve seals and cleaned the head and valves. They also said that they "lapped" some surfaces. Perhaps I ought to check the clearances again. Could out-of-spec valve clearances cause such an issue?

I want to stress that the bike runs really great, stable idling, really responsive and pulls great. It's just the end of the throttle range thats not working.

[herennow]

Got a cheap air filter, or proper Suzuki OEM?

My guess is running too rich at WOT.

[konna]

Got a cheap air filter, or proper Suzuki OEM?

My guess is running too rich at WOT.

The filter air filter is not OEM. It is manufactured by HifloFiltro which I believe are regarded as good quality. Not particularly cheap either. They are widely available in Finland. Does anyone even run OEM air & oil filters?

I will have to try original 115 sized Mikuni main jets to rule out rich AFR.

I know the symptoms can be very similar, I just assumed that it is more likely to be lean and went from there.

Can you elaborate on how you arrived at the guess that it is too rich? I thought that 120 is very reasonable main jet size. There would have to be a problem with the air filter if the engine ran rich with 120 main jet wide open throttle. I've run the engine with the same and similar filter without this issue too, so it seems unlikely.

[Armandorf]

if it is too rich you could partially block the filter and try a test run again.
this symptom is better or worse as it heats up?
i know nobody goes should go full throttle after 2 minutes of choke but...

maybe you need to change the main jet after changing the filter but i imagine the difference will be subtle. 1 o 2 sizes max

if you still have the old filter change it again.

[herennow]

Sorry, I know nothing of the pre 2000 2 jet carbs. What does the workshop manual say for stock main jet size?

Carbs , especially CV ones, are black magic, you have no control over throttle opening, it's all done by the vacuum that depends on engine revs, load on engine, throttle position, and the restriction from filter/ airbox.

If you search my posts you will find a looooing thread where I spent too much time and money chasing loss of power at WOT, especially at 5 to 6k revs.
Problem went away with OEM air filter. Note that Suzuki supplied air filters with a built in restriction for early 2000 models to get this right...

In the process I discovered that even one size up on the main makes things too rich.i also knocked my head going richer and richer before realising.....
Buddha disagrees with me on this. He also says that your problem is common and we should just avoid full WOT and learn to ride just under that throttle opening.

I suggest going back to stock jets, needle and air filter first. Hopefully you can avoid all my hard work.

Remember these bikes are old, the jet the needle sits in wears also, this leads to richer running in the first place. A little bit of wear makes a big change.

[herennow]

Ps, cmsnl is your friend for spare parts like the OEM filter for right year.

[konna]

this symptom is better or worse as it heats up?


maybe you need to change the main jet after changing the filter but i imagine the difference will be subtle. 1 to 2 sizes max

if you still have the old filter change it again.

There is no noticeable difference when it heats up. I find it difficult to believe that such radical symptoms could be present if the mixture was off by a little (1-2 jet sizes)...

I never had the original suzuki air filter, the filter I took out from there was aftermarket. The bike ran beautifully before the top end rebuild, so the problem is quite new. Before the rebuild, I ran the tourmax jet (120) and new needle - and it was fine. I'm at loss here. I just cleaned the carbs since they were out of the bike anyways during the top end rebuild and the problem appeared

What does the workshop manual say for stock main jet size?

115 is stock main jet size, which I found in the bike when I did the carb rebuild. They have "square" font numbers. After the carb rebuild the bike has been driven about 8000km with 120 jet and the hiflo filtro without problems...

I suggest going back to stock jets, needle and air filter first.

I'll start with new diaphragms, then I'll try stock jet (115) with original needles. After that I'll go for OEM air filter which is very expensive compared to aftermarket. Propably have to check the valve timings also. Thanks for suggestions. I'll go read your thread and see if I can learn something more from there.

[konna]

I found this in another thread that I find very interesting. I'll swap the diaphragms to check.

Hey I ran into another scenario which might cause this problem, and I have sorta struggled with it through a gen 1 and gen 2 carb on my 95.

The 2nd possibility and if this is the case the problem will get worse and worse, and in a gen 2/3 carb it will hit the right cyl, but in a gen 1 left cyl is that you're getting a small amount of fuel into the vacuum line due to a tiny hole in the diaphragm.

That would effectively cause the cyl to choke on its own fuel. It is initially characterized by a bike that's easy to start and almost runs fine without choke and as vacuum drops when the revs rise, it actually works well enough till WFO where there's enough fuel getting in via the vacuum port that it kills that cyl.

Now I diagnosed it in mine this week, I am yet to fix it and get my theory proved right. You might get to it before I do at this rate. But please try and let me know.

Cool.
Buddha.

[Armandorf]

top end rebuild and you dont know if your valve cleareances are correct? i would check that first.

probably something will break while you rebuild the carbs, my suspects are rubber parts, orings, diaphragms.
if you open the carbs you should be prepared for some gaskets to break.
Its like the first twist of the idle adjuster in years breaks the dirty grime seal and it leaks.
and the obvious clogged jets.

[konna]

Update! I got it working. Here is what I did:

1. Checked valve clearances. Right exhaust valve was too tight, I ordered a shim and replaced it. Right intake valve was too loose, I replaced the shim. I also finally bought the tool, I should've done that earlier instead of jamming screwdriver there for at least 10 times already. The tool was not perfect but it was better than the screwdriver method.

2. Replaced the carburetor diapraghms. The right diapraghm was somehow damaged, it did not fold 100% correctly. The carb had some hesitation in slide movement, I suspect because of the incorrect folding. Only by comparing slide movement and diapraghms to brand new diapraghms I was able to tell that it was damaged enough to matter.

3. Installed main jet number 120

These fixes corrected the main problem which was engine losing power in wide-open-throttle conditions. Now some slight hesitation in 5000-6000 rpm appeared instad. The engine also seemed to lose some of the "pep" it had previously with the large 137,5 main jet. But at least the WOT full gas power loss was gone! After testing it some more I took it apart again did some more fixes:

4. Raised the needle 1 position. This brought back some of the happines in the engine, it responds to throttle better now and seemed to gain some power at low and mid throttle positions.

5. Installed a "choke pipe" to the air filter. I manufactured a choke to mimic the extra length that seemed to be installed in OEM suzuki filters as well as another filter made by HiFLoFiltro. My local parts shop had two filters marked as compatible, the other has the pipe while the other does not. I had the one without so I used some plastic, gorilla tape and scissors to manufacture an extension. It was easily visible that the part where the pipe should be in was blackened while rest of the filter was very clean.

With pipe (HFA-3501):


Without pipe (HFA-3503):


After modifying the filter, the hesitation at 5000 is gone! There is some slight hesitation at 6000 but I think I'll leave it at that, it is barely noticeable - much less than ever before.

Thank you all for your suggestions and help! I would've never guessed the air filter could affect it so much. I hope that some day someone with similar problem finds this thread and gets rid of it!

[herennow]

GREAT, took me a year to figure it out.

feels great when it rocks!!!!

[Armandorf]

This is excellent, you combined all the user suggestions, solved and reported it.

It's not my intention to steal the thread but as we are almost on the same topic,

Btw, how hard would pulls the bike in, let's say:

6th at 5000rpm? I usually downshift but if not it takes ages to gain speed. Yes I can cruise with a little bit of acceleration and speed builds up.
Another thing I noticed is that in certain throttle position (60%+) it grunts a lot but not much more power.
Do you feel a "switch or step" in sound if you give it generous throttle?

5th at 7000rpm should be fairly obvious pull?

And finally
4th at 7000rpm, it's the more peppy gear for slow highway (around 100kmph)

Because in my bike I noticed that the last quarter throttle does nothing, but this became fairly obvious when hot and in 6th on a highway. Then I started to feel it more clearly even in 4th.

I'm using all the torque and runs great below 6k.
But I am missing all the "real" power.
Sometimes I imagine there is a lot more power to unleash and other times the GS is not as powerful as the "50hp" and 40Nm suggests.

It's fairly obvious I have to do a carb tuneup, I have my own thread and  will continue there.

[konna]

Btw, how hard would pulls the bike...

Your description sounds familiar. I think the bike is just not powerful enough to pull away at highway speeds in top gear. Even my Honda CB 750 with around 40% more power and torque is not pulling very hard without downshifting. These are bikes without fairings, I think the drag is just too much. Downshift allows the revs to go higher which increases power and the bike feels more peppy.

The change in sound when opening the throttle is familiar too. I believe it is the sound of the intake, coming from the airbox.

Now after over a 100km of testing on highway, gravel and small twisties I think it is obvious that the GS truly comes alive after 8000rpm. However it pulls fairly linearly starting from 3500, with small dip around 5700rpm and gaining power after 8000 all the way to redline.

[Breno sueth]

Good evening gentlemen, I've been struggling with a similar problems. I'm new to the forum and I got no idea how to actually post something on my own so I came to this post with similarities of my problem.

A year ago I bought a 02 gs500 the bike was running rough, and later on I got deployed and just now I started working on it.

I have a straight pipe exhaust, no muffler.
Also got some cheap air intakes pod from Amazon.

I rebuilt the carb with stock settings and bike would stall after pulling the throttle fast  (50%) when on neutral.

Changed the jets settings for:
Pilot 25
Mid main 68
Main 150

No neddle adjustment nor diaphragms

Also replaced the boots.

I've fuel/air mix at 3turns out now.
During idle black smoke comes out with choke on.

When choke is off, no smoke and the bike revs start to go up and won't come back down.
Sometimes fire will come out of the exhaust during these events.

Initially after installing these new jets, I have the fuel/air at 2 turns out and the bike would bog and stall after giving 50%+ throttle on neutral and when taking off if with choke on it would be a rough start, the bike would take off fine but then bog down and I needed to give more throttle with the clutch on and let go of the clutch, when doing the same thing with the choke off the bike would simply shut down.

Now it doesn't do it anymore after messing with the air/fuel ratio, even with the choke off BUT if I'm riding I.e at 40 mph and get ready to stop, whenever I press the clutch either to down shaZam! it just the bike roll, the revs will go up and won't come down, if put the choke on it goes slightly down and but not back to 1.5k ish stays around 5k (note: at this point the bike had enough time to warm up plus I live in Lemoore CA and it's very hot here), if choke off it won't come down at all, so I have to throw some gear 1st, 2nd, and let go of the clutch a bit so it comes down.


I also have suspected vacuum leak, that's why I put new boots on, I've also sprayed wd40, carb cleaner and other inflammable products around the carb but no difference on the rev.

This is a cafe racer project.

I'm thinking about go wider on the jets but I have no more ideas of what can it be.


Someone help me please lol

Best regards, Breno Sueth

[Armandorf]

Pilot 25 seems too much, did you try with less than 2 turns?
20 is usually plenty.

[Breno sueth]

Hey Armando, yes I've tried, I started with 1 1/2 turn but when taking off it would bog and stall (with the choke off) with the choke on it was losing power but after giving some throttle it would go.