Won’t rev past 7000 rpm

[xxenzor]

Hey everyone,

After months of working on my 1980 Suzuki GSX400E, I finally got it running and idling steady, and it rides great. But there’s one issue I can’t figure out—it won’t rev past 7,000 RPM, whether it’s under load or just revving in neutral.

Here’s what I’ve done:

• Set the ignition contacts to spec,
• Adjusted the valves,
• Installed new carb diaphragms,
• Synced the carbs and cleaned all jets,
• Replaced all O-rings on the intake boots, carbs, and throttle shaft seals,
• Running the stock air filter.

The bike is in great shape now except for this RPM issue.

Electrical Specs:

Battery Voltage:
• Key Off: 12.61V
• Key On: 12.04V
• Idle (1500 RPM): 15.04V
• 2500 RPM: 15.2V
• 5000 RPM: 14.9V

Voltage Drops:
• Lead Drop: 0.22V
• Negative Drop: 0.11V

Coil Resistance:
• Primary: 4.0 ohms
• Secondary: 40.8 ohms

Stator:
• Between Stator Wires: 1.4 ohms
• Stator to Ground: 0 ohms

• R/R Diode Test: 0.924V

I’m wondering if the electrical system might be the culprit, especially since I’m seeing 15.2V at 2500 RPM and 15.04V at 1500 RPM. Could this overcharging be what’s limiting my RPMs, or is there something else I’m missing?​

 

[Grimly]

There should never be as high as 15V DC anywhere on the charging system - 14.4 to 14.8V absolute max - how accurate is your meter?

 

[salty_monk]

I'm generalising here as I haven't worked on one of the twins....

It is a bit high. I would look to install an SH775 unit RR. I doubt that is causing any sort of limit in revs though. I assume you have an ignitor box for ignition but with a manual advance mechanism? You should double check your timing setup, check the advance mechanism is working smoothly & also check for any damage to the springs etc. You might also check that the springs haven't been changed out or messed with.

Also check out your HT wires from the coils. Make sure they are not worn or cracked (run it in the dark & move the wires around, sometimes you can see it "losing" spark through cracks in the insulation to the head).

Lastly the Clymer manual lists some tests for the ignitor box. When I had a similar issue on a 1980 GS1000G it turned out to be the ignitor box that was at fault. From memory the test where you put some power into the box with an AA battery was the test that showed the fault. - Another point to make here is that high voltages caused by a bad RR can easily kill the ignitor boxes.

One of the tell tales for a broken RR is that the headlamp fails from the over voltage.

Question: What do you mean by ignition contacts? Are you just talking about plug gaps?

 

[xxenzor]

i adjusted the contact points on the ignition advance and lined up the timing so that the Fire mark is lined up when running

 

[unfocused]

Actually, over 15.5 volts is where stuff starts to get damaged. His voltage isn't causing an issue from his current readings. However, install a guage and watch as your riding. This can tell a whole different story. I use to rebuild 300 amp alternators (50DN on buses), dual voltage Leece Nevilles (emergency vehicles) and more. I had extensive training in the 12/24/36 volt industry and wiring issues do not scare me.

Make sure your voltage at the votage regulator is also battery voltage and zero or less than .1 volt of loss...


I blew a CDI box or more on my 88 FZR1000 years ago and discovered that the shunt style alternator over-charged to 17+ volts. It blew my headlight bulb so bad it stuck to the inside of the glass lens... I never ride without an external volt gauge anymore becuase this cost me a fortune and those CDI boxes took the path of the DoDo bird...

 

[KiwiAlfa156]

i adjusted the contact points on the ignition advance and lined up the timing so that the Fire mark is lined up when running

You need to check the ignition advances from the 'F' mark to align the second fully advanced mark at around 2750 rpm using the strobe. This is actually the more important alignment as essentially below 2750 rpm is idle. If the ignition timing is too advanced or retarded at full ignition advance, power will be negatively effected

 

[Brendan W]

Stator to Ground: 0 Ohms? Is that right? Did you mean infinite.
I would expect the older shunt type R/R to be in the range 14 - 15.5 Volts.
Meter batteries need replacing. Generally as they die they over read voltages.
Has the valve timing been checked?
One tooth out could produce the symptoms you describe.

 

[xxenzor]

KiwiAlfa156 The right lines up good with Fire and the left lines up too but will shake back and forth before the F and after the F. I have the points set at 0.014. Both have good static timing

 

[xxenzor]

KiwiAlfa156 I can get it to 8500 Rpm in neutral on the centre stand but as soon as i’m in any gear it maxes at 7000 with WOT. The exhaust also sometimes Pops when giving throttle. Not quite a backfire. No leaks in exhaust and the pilot screws run best at 2.5-3 turns out

 

[KiwiAlfa156]

Just to confirm, your bike is stock and its a TSCC 4-valve per cylinder twin?

 

[xxenzor]

KiwiAlfa156 Yeah it’s called GS400 in canada but it’s the same as GSX400 in the usa. I also just tested the spark plug caps detached from the coil wire and was getting 8.8k on the left cap and 9k on the right cap

 

[xxenzor]

KiwiAlfa156 and i’m getting 125psi on both cylinders with WOT and bike warm enough to run without choke

 

[xxenzor]

Took her for another rip and i can get it to 8k in 2nd but every other gear the bitch starts bouncing the needle all shaky at 7k. I can barely get past 100km in 5th

 

[KiwiAlfa156]

OK. Compression is a little low, but even across the cylinders. ( around 128 - 175 psi is the spec range). You say it has ignition points? Can you post a photo of the points assembly? Also have the plugs been changed?

 

[xxenzor]

KiwiAlfa156 Seems weird compression would be so low with only 22xxxKm. I was having horrible idle problems for months that got solved by adjusting the points gap. I set the static timing and the dynamic timing is good except the left cylinder timing bounces around the fire mark a bit. Plugs are changed. I am also 3200 above sea level if that helps

 

[Buffalo Bill]

Maybe the valve springs are worn out, so the valves are floating already at 7000?

 

[Nessism]

The compression is fine. And valve springs don't cause problems like this. New carb diaphragms throws a red flag. Where did you get them, and how did you go about the process of changing them. Mind you, changing diaphragms is doable, but it's not an easy task.

 

[gustovh]

Just a thought that ou may have 1 pin too many between the cam chain. Ie 21 instead of 20 pins between cam marks. This would delay intake and limit rpm. Worth a check, yes happened to me and 7000 seemed to pretty much be the limit until I corrected the issue, ran flawless after that.

V

 

[xxenzor]

Nessism I got the diaphragms from JBM, and I also have spares from eBay. The bike was having the same issue before I replaced them. The old diaphragms were in good shape with no holes, but I must have pinched one during reassembly because the left diaphragm blew up as soon as I started the bike.

 

[Nessism]

Put your finger inside the carb throat and lift the slide up to the top of its travel, then let go. The slide should move up smoothly, then when you release, it should move down with a damped motion, and clunk as it reaches the bottom.