cannot rev over 5000

[Craig_ulysses]

I have an interesting problem with a 1984 Suzuki Katana 750. It won’t rev over 5000 and if you hold the throttle more than half open it splutters and loses all power. This is how we received the bike and we don’t know its previous history or what might have been done to it.
- Idle is good
- It has good power up to 5000 rev’s.
- Starts easily
- Doesn’t blow smoke
- I think it has correct carb’s.
- Constant Velocity BS 32SS Mikuni.
- The main’s are 117.5.
- Raising or lowering the needle has little effect. They are currently set to the middle position but I have tried top and bottom positions.
- I have tried the air screws in multiple position with little effect from half a turn out to 3 turns out, they all have the “O” ring fitted.
- I have tried raising and lowering the float level with little effect. It possibly seems a bit better if the level is higher.
- Carb’s seem clean inside with no evidence of varish or died fuel. I have sprayed CRC carby cleaner down all orifices.
- I can see the diaphram piston lift about half way on each of the carb’s but no further. They move freely and can be lifted by hand but don’t open further when the throttle is twisted.
- The diaphram’s look to be in good condition and seat properly.
- The butterflies fully open with the throttle.
- It has pod filters rather than the original air box.
- Timing light shows that advance moves about 10 degree’s when I rev it. Does anyone know if it should get more advance?
- Spark seems fine on all cyclinders.
- I have tried another coil.
- Pulse generators are approx. 400 ohms.
- If I put my hand over each carby inlet in turn (without pod filter) I get plently of fuel on it. So main jets are passing plenty of fuel.
- Extra earth’s have been connected to coils to ensure they are ok.
- Power is good at coils and igniter (12.7 volts)
-
Any idea’s would be appreciated.

[Purplehaze]

I have an interesting problem with a 1984 Suzuki Katana 750. It won’t rev over 5000 and if you hold the throttle more than half open it splutters and loses all power. This is how we received the bike and we don’t know its previous history or what might have been done to it.


- Raising or lowering the needle has little effect. They are currently set to the middle position but I have tried top and bottom positions.
-[/FONT] I have tried the air screws in multiple position with little effect from half a turn out to 3 turns out, they all have the “O” ring fitted.
- I have tried raising and lowering the float level with little effect. It possibly seems a bit better if the level is higher.
- Carb’s seem clean inside with no evidence of varish or died fuel. I have sprayed CRC carby cleaner down all orifices.
- I can see the diaphram piston lift about half way on each of the carb’s but no further. They move freely and can be lifted by hand but don’t open further when the throttle is twisted.
- The diaphram’s look to be in good condition and seat properly.

- It has pod filters rather than the original air box.

- If I put my hand over each carby inlet in turn (without pod filter) I get plently of fuel on it. So main jets are passing plenty of fuel.

Any idea’s would be appreciated.

The pod filters scream clearly to most KatRiders as it stands now. Very few claim to be able to tune them appropriately, and very few less have been able to prove it.

Firstly, leave the float heights at stock (14.6mm) and the fuel screws/pilot screws at 2.5 turns out. Do not bother changing this until you have her revving reliably.

Placing your hand over the inlets (IMO, the airbox is crucial for this so you can get a even restriction across all cylinders and not just one at a time) can let you get and idea if a more rich mixture will do the trick. Also, the issue is that the pod filters may be flowing too much air, so the restriction with your hand IS what gives it plenty of fuel.

If you haven't sync'd the carbs, now would be the time.

Ensure those diaphragm springs are fitted to your carbs, ensure they slide evenly and with little pressure under a finger's force.

Dirty carbs can also lead to this issue, so if you haven't given them a proper cleaning (Carbs 101 here on KatRiders.com) and eliminated that possibility you may be missing the main point. The carbs can appear clean but those tiny passages can be quite dirty.

From what I'm reading, you may also have needles that do not match your main jets. Stock needles have one setting. Verify that your main jets and needles work together well (i.e. From a kit). If you find it is sourced from a kit ensure all the jets match and PROPERLY set up the kit for YOUR EXHAUST and YOUR INTAKE. Issue being that Pod filters do not play well with our CV carbs. Once again I recommend ditching them. You gain so little, and Katanas need the back-pressure on their valves regardless.

[Craig_ulysses]

Thanks purple haze, I'll continue to try things. If its as critical as you say then it might be difficult. The bike also has a aftermarket exhaust and none of the fittings for the airbox. I'm not even sure the frame is original as there arn't mounting points for the ignitor etc, they are just zip tied on. But its quite interesting.

[Kreylyn]

If it's a pretty hard 5k rpm issue that does not change when ridden in any gear... I'd probably want to test the ICM to make sure it's not a faulty box.


Krey

[Craig_ulysses]

Thanks Tone and Kreylyn.
I'll ensure that the floats are exactly 14.6 and try again.

The ignitor on it is a bit questionable as the box has been opened before and the circuit board is a bit messed up with water ingress or something. But having said that, the bike is clearly running on all 4 and the timing light shows it advances about 10 degrees. Do you know what advance it should you up to?

My current thoughts that I'll investigate further on Monday are
- I wonder if I should replace the diaphrams even though they look ok, maybe their age means that they don't lift as easy as they should, I am going to lift them manually with the throttle open and see what happens.
- I have a CDI of a CBR of similar age that I think I can cobble up, and see if that works better.

Regards Craig

[Craig_ulysses]

Hi Tone, No luck. It doesn't appear possible to set the float level to 14.6. I think the BS32SS cards needs to be at 22.4mm which is what I've set them to.

It's a bit hard to tell but I think that when I look down the carb when the bike is running, once the carb pistons get half way it they fully open and a heap of fuel goes in and floods the motor.

Do you know a good place to buy carb kits with all jets (air and fuel) including the diaphram and needle jet.

Regards Craig