33BHP Restricted GS Fuelling Problems?

[Meuryn]

Hi everyone,

Have been on a 33bhp 2001 GS500E for a few days now, and am having some issues with the bike. Essentially, once you hit 55-60mph upwards, the bike starts to 'cough', the exact same feeling you get when the bike starts to run out of fuel. Giving more throttle once the coughing begins seems to make the problem worse. I'm assuming the 'coughing' I'm experiencing is a fueling problem, but it could be something else? The coughs are sometimes associated with a surge of power a few seconds afterwards. The bike refuses to go faster than 70mph indicated - so about 65 real speed - despite the fact that others with 33bhp restricted GS's have reported speeds of around 90-100mph as a maximum. Acceleration, compared to my 125cc bike, is great up to about 50, although I'm not sure it's as good as it could be, having never ridden another 33bhp bike. The problem seems to be present regardless of rpm or gear when in the 60+ zone, although I'm almost always in top gear, at 5000-6000rpm plus at the 60mph+ stage.

I'm no expert with bikes, but I'm guessing it could be a carburetor problem, so is pulling the carbs out and giving them a good clean my first port of call? Would really appreciate anyone's help, as this problem is ruining my enjoyment of the bike at the moment 

Meuryn.

[Paulcet]

Yes, I would suspect a clogged jet. 

[sledge]

I wouldn't suspect there is any problem  Sounds to me like the restricter kit is doing exactly what it is supposed to be doing ie: limiting the power of your bike to the legal max of 33bhp and its speed to the legal max of 70mph

What you are saying is....I want my restricted GS5 to perform like an unrestricted GS5........Fact is you are somewhere between 15-20hp down from stock........ it is not going to happen.

The answer is simple, do what everyone else does, pass the test and then remove the kit 

[Docker]

mine had the exact same problem and it was due to the mixture being too lean (possibly something to do with the 33bhp restriction? im restricted as well)
An easy check to see if thats what is causing it is to put a sock over the air filter and a couple balled up in the air filter, if it runs better then you know its running too lean and you need to increase the jets, might also be worth checking float heights etc while you have the carbs open.

[Meuryn]

I wouldn't suspect there is any problem  Sounds to me like the restricter kit is doing exactly what it is supposed to be doing ie: limiting the power of your bike to the legal max of 33bhp and its speed to the legal max of 70mph

What you are saying is....I want my restricted GS5 to perform like an unrestricted GS5........Fact is you are somewhere between 15-20hp down from stock........ it is not going to happen.

The answer is simple, do what everyone else does, pass the test and then remove the kit

Hi Sledge,

Thanks for your input, but I don't think you quite understand how the UK motorcycle test system works, nor how air/fuel washer type restrictors are supposed to work. In terms of the UK bike test system (if you passed your test before Jan 2013) if you're aged 17-19, you pass your full test, yes the same test older riders will do with DAS, on 125cc bike, and enjoy all the benefits of a full motorcycle license. The only catch is, because of your age, you're limited to a bike that only produces 33bhp at the crank, with an associated power to weight limit. Put simply - I've passed my full test, but the government limits me to 33bhp for 2 years because of my age. After two years, the restrictors can come off - I don't even have to do another test - it's simply a case of building up experience, and don't get me wrong, I'm all for the pre-Jan 2013 system! Hope that clears a few misunderstandings up.

In terms of how the restrictor is supposed to perform; the washer is placed in between the carbs and the engine, and restricts both fuel and air intake. In theory, the washer is supposed to keep the mixture within 'specification', but in my case, at least this is what I think, not enough fuel is being added to the mix by the carbs, or maybe there is too much air being added. That would explain the coughing. With a properly working restrictor, there should be a loss of power up in the rev range as the washer prevents fuel/air entering the engine. There should certainly be no coughing, nor any surges of power. The bike should also be able to do more than 65-70. My 125cc Honda CG125 could do that on a good day, and that produced about 10bhp. The Gs has triple that power with only, say, 90kg more in weight. Hope you can see what I'm getting at.

Thanks!

[Meuryn]

Sounds to me like the restricter kit is doing exactly what it is supposed to be doing ie: limiting the power of your bike to the legal max of 33bhp and its speed to the legal max of 70mph

Almost forgot; it's impossible for a washer type restrictor to limit a bike's speed to a specific value. I'm fairly sure that could only be achieved on a fuel injected machine, with a specific map installed.

[Meuryn]

mine had the exact same problem and it was due to the mixture being too lean (possibly something to do with the 33bhp restriction? im restricted as well)
An easy check to see if thats what is causing it is to put a sock over the air filter and a couple balled up in the air filter, if it runs better then you know its running too lean and you need to increase the jets, might also be worth checking float heights etc while you have the carbs open.

Ah, that's an interesting point! I'll definitely try that out once I've had a clean of the carbs. Thanks for your help!

[Meuryn]

Yes, I would suspect a clogged jet.

Nice to have some confirmation I'll probably clean out the carbs later this week. Will report back, and thanks for your help!

[sledge]

Hi Sledge,

Thanks for your input, but I don't think you quite understand how the UK motorcycle test system works, nor how air/fuel washer type restrictors are supposed to work.

I did understand the test procedures back in 1982 when I passed it but yeah....seems they change on a regular basis and as they don't really apply to me I don't pay much attention these days 

I have bought and sold (at a reasonable profit) 5 restricted GS5s now. 2 Each fitted with the plate type FI international and Alpha Technik kits and I couldn't believe just how bad they made the engine perform over stock, all of them had symptoms such as the ones you describe even after a thorough service and valve adjustment. However the one fitted with the OEM Suzuki kit, (as fitted as standard to the GS500EU) was nowhere near as severe. Probably due to the fact it works by limiting carb slide movement and altering jet sizes as opposed to restricting the amount of mixture the engine can draw in per cycle. I considered rejetting but decided against it as I was looking for profit not performance.

Go ahead, clean the jets out, maybe even consider  rejet with a plug chop. You may see a slight improvement in how the engine runs but you wont see a significant improvement in performance 

BTW....if someone is getting a genuine 100mph from a restricted GS5 on the straight and you can provide proof I will drop my pants in the foyer of my local Tesco and then post the pics in here for all to see 

[Badot]

BTW....if someone is getting a genuine 100mph from a restricted GS5 on the straight and you can provide proof I will drop my pants in the foyer of my local Tesco and then post the pics in here for all to see 

Hmm... the old Ninja 250s with ~35 hp would be able to hit 100 gps IIRC. Perhaps a GSF with a small rider and a good tuck could win us a good laugh.

[sledge]

Are you talking about the mid 80s EX250E or......on this side of the pond the GPZ250R?

86/87 models had a quoted top end of 94mph, later years slightly less. And we all know how optimistic quoted figures are 

[Meuryn]

Hi Sledge,

Thanks for your input, but I don't think you quite understand how the UK motorcycle test system works, nor how air/fuel washer type restrictors are supposed to work.

I did understand the test procedures back in 1982 when I passed it but yeah....seems they change on a regular basis and as they don't really apply to me I don't pay much attention these days 

Makes sense The largest displacement you can learn on these days is a 125cc, although you can ride a larger capacity bike if you're over 19 with an instructor. It's not so much a performance gain I'm looking for - it looks like the average top speed for a restricted GS is 80 to 90mph, it's the coughing and surges of power that are the real problem, because they effectively prevent me riding at any more than 60, which kind of defeats the point of passing the full test and moving up from a 125cc 

I'll let you know when I've stripped/cleaned the carbs what happens 

[snOhio]

what are they going to do, confiscate the bike and dyno it?  lose the restrictions and then let's talk.

[john]

What you are saying is....I want my restricted GS5 to perform like an unrestricted GS5........Fact is you are somewhere between 15-20hp down from stock........ it is not going to happen.

First of all, a jetted GS with sport exhaust, jet kit, ignition advancer and tuned on a dyno makes 43HP on a dynojet.  So 33HP should not be that much worse!  That's like only 5HP less than stock!

[sledge]

what are they going to do, confiscate the bike and dyno it?

Possibly 

http://www.bikechatforums.com/viewtopic.php?t=239302

[sledge]

What you are saying is....I want my restricted GS5 to perform like an unrestricted GS5........Fact is you are somewhere between 15-20hp down from stock........ it is not going to happen.

First of all, a jetted GS with sport exhaust, jet kit, ignition advancer and tuned on a dyno makes 43HP on a dynojet.  So 33HP should not be that much worse!  That's like only 5HP less than stock!

But its not all about power, you also need to consider torque. Torque is the amount of work that can be done, power is the time in which that work can be done.

Take a 2001 model and manufactures figures. Suzuki claim a max of 47.7hp @ 9200rpm, do the sums and that gives us 27.2lbft at max hp.

Now drop that to 33hp @ 9200rpm and torque drops off to 18.8lbft. A loss of about 8.5lbft or.....about 30%

So it follows that a restricted GS5 will only do about 2/3 as much work as an identical but unrestricted GS5. And because the difference in power between 47.7 bhp and 33bhp equates to a loss of about 30% it will take an extra third of the amount of time to do this work as an unrestricted one would. This will reflect in a reduced top speed and an increase in acceleration times.

[mitch79]

Simple test: Apply the choke when the bike coughs and plays up at speed. If it improves - your lean.
If it is lean enough to cause stuttering, and you've checked your jets are clean and float levels correct, then I'd suggest going up two sizes to a 135 main jet as a starting point.

[Meuryn]

Simple test: Apply the choke when the bike coughs and plays up at speed. If it improves - your lean.
If it is lean enough to cause stuttering, and you've checked your jets are clean and float levels correct, then I'd suggest going up two sizes to a 135 main jet as a starting point.

Will definitely try that out tomorrow, good idea! Thanks for the info on the jets btw

[angusgst]

But its not all about power, you also need to consider torque. Torque is the amount of work that can be done

I don't mean any offense, but this statement about torque is completely wrong. The relationship between torque and power is commonly misunderstood, so I will explain. There are a number of concepts which need to be understood: force, distance and speed; torque, rotation and angular velocity; energy (work), and power.

When a force pushes on an object over a certain distance, the work done is equal to force * distance. The power is equal to the work done divided by the time it's performed over: power = work/time = (force*distance) / time = force * speed.

The rotational equivalent of force is torque;  torque can be thought of as rotational force. The rotational equivalent of distance is rotation. In a rotating system, the work done is equal to torque * total rotation. The power is equal to the work done divided by the time it's performed over: power = work / time = (torque * rotation) / time = torque * (angular velocity). [Angular velocity is commonly measured in 'revs', ie revolutions per second.]

Solving the above for work gives: work = power * time = torque * angular velocity * time.

So, the amount of work performed is actually determined by power, not torque by itself. Torque is simply one of the two components which go into creating power.

[Badot]

To continue what was said above -- Since we have the relation power=torque*angular velocity, we can regear to get however much torque we want at the expense of angular velocity. This is exactly what a transmission does.

So assuming your engine is running at peak power, that also means your rear wheel is at peak torque for whatever gear you're in. Any two vehicles maintaining the same power, regardless of the produced torque, will push the vehicle forward just as hard due to the regearing done in the transmission.