1980 GSX400 - Still Stuck at 7k RPM After More Work

[xxenzor]

I’m still dealing with the same issue on my 1980 GSX400—it won’t rev past 7,000 RPM under load, and it’s hard to even get up to 100 km/h. This bike was my dad’s, and it sat for 15 years before I got it running again last August. Everything on it is stock, and here’s what’s been done so far:

• Cleaned the carbs (6 times).
• Synced the carbs.
• Replaced throttle shaft seals, carb O-rings, and carb boot O-rings.
• Replaced carb diaphragms (the old ones were destroyed), and both throttle slides come down smoothly.
• Adjusted the valves.
• Installed a Boyer electronic ignition with a new coil.
• Installed an SH775 rectifier (charging at 14.52V).
• Checked compression: 125 PSI on both cylinders.
• Running the stock air filter.
• Checked the exhaust: no blockages.
• The petcock flows fuel good. (sucked on vacuum line and fuel comes out of the gas line and not the vacuum line)

It feels like it hits a rev limiter at 7k RPM under load, but it will rev to 8,500 RPM when not under load (on the center stand). I’m fed up with this issue—any advice on what to try next?​

 

[Nessism]

Does the centrifugal spark advance move freely?

 

[xxenzor]

Does the centrifugal spark advance move freely?

Yeah the whole ignition works good. I followed the boyer instructions and it’s direct on the Advance mark at 5000 rpm

 

[Nessism]

Did you grab the rotor, twist it against the spring, and release? Does it move freely?

BTW, why did you clean the carbs 6 times? If you followed the tutorial available here, it shouldn't take more than ONE.

 

[xxenzor]

Did you grab the rotor, twist it against the spring, and release? Does it move freely?

Yes it does. i’ve also cleaned it and the surface rust off


BTW, why did you clean the carbs 6 times? If you followed the tutorial available here, it shouldn't take more than ONE.

Ive only soaked it once but the 6 times means more if i’ve taken it off and sprayed the jets and holes to make sure it wasn’t clogging up

 

[KiwiAlfa156]

I know this sounds silly but is the air filter new?

 

[Nessism]

You never answered about the mechanical advance unit. Maybe the aftermarket ignition is binding up the advance unit.

And cleaning the carbs 6 times means you didn't to it right the first time. Is the gas tank clean and rust free. And no, a filter is not a proper way to eliminate rust from entering the carbs.

 

[KiwiAlfa156]

You never answered about the mechanical advance unit. Maybe the aftermarket ignition is binding up the advance unit./QUOTE]

To be fair, the xxenzor did say, " I followed the boyer instructions and it’s direct on the Advance mark at 5000 rpm"

Boyer ignitions are (and I may be wrong), electronically controlled spark advance and the bob weights etc. are redundant. If the timing is correct at full advance as per the Boyer instructions (and the only way you could confirm that is by dynamic timing with a strobe). And assuming the correct Boyer ignition kit is being used (the correct full advance in degrees is set in the circuitry) if the advance aligns at the stated RPM, its sounds like the ignition is working correctly. ​​

 

[KiwiAlfa156]

Hmmm, how old is the Boyer unit? Was it new when you fitted it? Are there any identification numbers on the controller box? Do you have the kit number?

 

[xxenzor]

Hmmm, how old is the Boyer unit? Was it new when you fitted it? Are there any identification numbers on the controller box? Do you have the kit number?

Brand New Boyer, I’ve talked to boyer when installing it to make sure everything was right. Kit 00291.

It’s weird cuz when it hits 8000-8500RPM not under load it just stays there as if it was the a rev limiter. And then when its under load it barely wants to get up to 7000rpm.

I soaked the air filter foam in degreaser and made sure it was super clean and then reoiled it

It seems like it would be air/fuel related but i’m not sure what else to try

 

[gustovh]

If I were you I would check to make sure the pin count on the timing change between cams is correct. Had the same problem on a new to me 850L and it took me a couple months to figure out that there was 21 pins between the cams not the correct 20 pins. When that happens the intake valve opens late and restricts the top end. Shouldn’t take you long to find out. Just a thought.

V

 

[bren]

what do your plugs look like? maybe check your float height? sounds like maybe it's running lean up near wot?