Any 1980 GS400S owners out there?

[Oldspooneye]

Your Bike is really a GSx400E (=4 valves per cylinder and TSCC (twin swirl combustion chamber) with a fairing -see suzuki.org linked above.

You can get part numbers to take to your supplier at this European site

https://www.cmsnl.com/suzuki-motorcycle_model13053/

I think this is exactly the info I needed. Thank you so much.

[John Park]

There seems to be a reflex to go after the carbs if anything isn't running right. But if the valves aren't properly adjusted, the intake boot o rings are hardened up and leaking or the fuel tap is wonky, it can seem as though the carbs need 'rebuilding'.

The fuel system rubber bits will either have been replaced by this time or will need to be replaced. But if the bike runs fine after the valves and intake boots have been serviced, you might consider enjoying the summer riding season and leave the carbs as a winter project.

These motors also are rather fuel sensitive as they are jetted very lean stock, which was fine back when gasoline was the old tradition brew; today's mix with ethanol pushes the mixture even leaner. Plus, with a 10:1 compression ratio you need to run 90 or better octane, and preferably ethanol free if you can find it. Mine would sound like a bad night at a flamenco cafe on regular.

[Rijko]

There seems to be a reflex to go after the carbs if anything isn't running right. But if the valves aren't properly adjusted, the intake boot o rings are hardened up and leaking or the fuel tap is wonky, it can seem as though the carbs need 'rebuilding'.

agreed, like the word 'reflex'.
Simple test i always do with old intake boot rubbers is start the bike cold,
let it run for a minute to settle so it runs evenly without choke,
and spray brake cleaner on the intake boots where they meet the cylinder head.
If the rpm changes, the boots and/or o-rings are bad.

Don't take off the boots to check old o-rings if you do not have new ones
to replace them, they will most likely not seal properly if you
put the boots back on.

[Oldspooneye]

There seems to be a reflex to go after the carbs if anything isn't running right.

Those are some wise words. You guys are probably going to laugh at this noob, but as I was going through the bike doing some general maintenance I went to check the air filter and it turns out the last owner didn't really believe in them. Who knows how long he ran it without one. As soon as I pooped in a filter the engine ran way better. I'll probably clean the carbs but like John said I'll probably "leave the carbs as a winter project."

You guys have already been a great source for info. Thanks again!

[Rijko]

nope, won't laugh at something like that..
po's can sure throw you a curve ball.
cool you found the solution !

I can relate to your amazement when you discovered
the missing airfilter - some time ago i bought a GS1000
non-runner and wanted to change oil and oil filter.
There was oil in the engine but no oil filter ???

[Oldspooneye]

Looks like I spoke too soon. The engine idles better but I just went to take it for a spin and there's no power and the bike stalls every time I stop. This wasn't the case when I ran it without the filter yesterday. I think I might be in over my head. Any suggestions?

*Edit* - Okay here's what's happened so far. I picked up the bike last weekend. It ran okay and had plenty of power but one side sounded like it might need a carb cleaning. I changed the oil, oil filter and spark plugs. It still ran okay but wouldn't start yesterday. The battery is shot so I thought it just didn't have the power to turn over. I put in a fresh battery yesterday and found out that the last owner didn't use an air filter. Today I put in an air filter and it started up right away and idled fine. I took it out for a spin and it has no power and stalls when I stop and backfired a couple times. I pulled the air filter just to see if that was causing it for some reason. No change. I drained the gas and put in fresh gas. Still no change. No power and stalls when I slow down / stop. I'm ready to light this bike on fire. Does anyone have any suggestions?
.

[John Park]

I have zero confidence in the OEM fuel tap that came on that bike. That said, there are those who still run one successfully. I'm running one off a '78 - the old original version with two diaphragms and a check valve - but those are getting old and scarce.

There may be a 'prime' position on your tap lever. If so, try switching to that position after the bike is running and then see how it runs. Essentially, this opens the tap wide open so you can fill the carb bowls if you've run them dry without cranking until the battery is dead. You can't leave it there because the vacuum shutoff feature is bypassed and the carbs could overflow and drown the motor. Some taps had a screw that you turned - often seized or stripped by now - for priming. There's also a [small] hose that runs from the tap to one carb for vacuum to hold the tap open. If that's leaking, the fuel delivery will crap out under load.

Assuming the filter screen in the tank is not clogged, you should get plenty of fuel flow on Prime, which you could verify by connecting a hose to the tap and draining it into a container.

This has all the symptoms of insufficient fuel flow from the tank; that said, as soon as you have that fixed, I'd recommend checking the valve clearance adjustments. Running the valves too tight will either burn the valves and/or damage the cams and rockers. You'll need to source a valve cover gasket first and Suzuki Canada may have them in stock….or not.

These bikes also had rather funky charging systems prone to burning out the alternator stator and regulator/rectifier module. If the headlight doesn't brighten when you rev it, put a voltmeter across the battery and see if you have about 14 volts at, say, 4000 rpm. If it's still twelve something, you're looking at working on the electrics. The good news is that the newer style regulators [Shindengen SH775] are leaps ahead of the original and solve the problem.

Before you light it on fire, consider that you're working on an almost inevitably neglected, 37 year old motorcycle which is still attempting to run anyway. It's a given that you will have to learn things and fix things, and find out how to source parts and information. These are great bikes, so hang in there.

[Gorminrider]

I'm ready to light this bike on fire. Does anyone have any suggestions?

Yes. Give it to me !
seriously though, try what JohnPark is saying...test fuel tap by disconnecting from carbs and sucking on vacuum line...BY The Way, is the Vacuum line attached? likewise check flow on Prime.
Also make sure tap isn't dumping gas through the vacuum line . (smelling oil and check for oil level change is another symptom of failed tap diaphragm) ...inside the tap is a very thin diaphragm that can dry up and crack over 37 years, though out of three bikes, I have only seen this once and the others are fine.
Still, Fuel taps are a common replacement but your bike wants one that has an outlet facing forward or it interferes with frame. (others Can be made to work however)

A heat-gun is worth having to soften the rubber before pulling carbs.

Put your hand near headers when starting to test for a cylinder not firing. Usually, yes, I'd guess a balky carb.
Stopping at low speed suggests idle circuit maybe on that one side...if pulling enricher knob (the "choke") makes it work, that'll be the place I'd suspect. Taking the carbs apart is no big deal.

Main jets (the emulsifier tubes) are also very commonly plugged after sitting around.

IF you have a spare car battery around, you can run bike with that rather than "deep-discharging" your shiny new battery while you get the bike sorted. Motorcycle batteries hate being stressed with too many fruitless startups.
I haven't ever taken MY intake rubbers off. They work until they' re proven not to. The risk of snapping screws corroded into the aluminum block off is a very real one....

[Oldspooneye]

Well I tested the fuel tap and it seems to be working fine. So the next thing to do is pull the carbs? and clean them out?

The headlight didn't brighter when I revved the engine so I guess I'll have to look at the electrical as well. That wouldn't have anything to do with my immediate problem though, would it?

[John Park]

The fuel tap operates on engine vacuum. If the intake valves aren't closing properly the vacuum to the tap will be who knows what. Unless you KNOW the valves are adjusted properly, taking the carbs off is maybe premature. Have you tried running it on Prime yet?

At what point are you going to get to the #1 service operation in resurrecting one of these bikes? You can't 'tune' carbs to an out of spec motor. Sure, it's probably some crud in a carb jet or two but a tight intake valve will make for all sorts of variable maladies, and tighten up over time is what they do. I even have a spare gasket you can have - don't need it anytime soon - if you get stumped for one.

The headlight is not encouraging, but without a voltage reading it's too early to tell for sure. If there's enough juice to start it, it should still run okay.

[Suzuki Mad]

It was called a GS(X)400E in the UK and was similar to the GS(X)450E and the GS(X)250E that we had here. As to parts interchange I've never worked on one to say. Sorry

[Runeight]

I just finished a 1982 GSX400E Euro model. Most parts were available from Suzuki using Babbits on line. Several parts were ordered outside of the US. Build thread in Project section. I used a 1982 GS450 as a US reference.

Using Babbits and CMSL you can find what you need. Fleabay worked well also. Every part number was cross referenced and of course great advice right here.

I have odds and ends from my build so let me know what you might need. I'm looking for some engine guards if you run across any.

Remember patience is a virtue. My build took 14 months to complete. I'll subscribe to this thread.

[Oldspooneye]

The fuel tap operates on engine vacuum. If the intake valves aren't closing properly the vacuum to the tap will be who knows what. Unless you KNOW the valves are adjusted properly, taking the carbs off is maybe premature. Have you tried running it on Prime yet?

At what point are you going to get to the #1 service operation in resurrecting one of these bikes? You can't 'tune' carbs to an out of spec motor. Sure, it's probably some crud in a carb jet or two but a tight intake valve will make for all sorts of variable maladies, and tighten up over time is what they do.

Okay, so I've now adjusted the valves. The exhaust valves on one cylinder were a little tight as was one of the intake valves on the other cylinder. I'm still having the problem of barely any power. Right now it rides almost as bad as my old 50cc scooter. I've ordered a carb rebuild kit off ebay so that will probably take at least a week to get here. Any other suggestions to try in the meantime?

[Runeight]

Since you are waiting on carburetor kits pull the carbs and get everything ready to go. Replace the O rings on the manifold boots. Getting those screws to break free with a impact driver will keep you busy enough.

Boots and O rings are available from Suzuki.

[RichDesmond]

Order a set of JIS screwdrivers. There are no phillips head screws on your bike.