Help with troubleshooting GS450

[Arek]

Hi All,

Thanks for a great forum!

I have hit a wall and need some pointers to get on with my build.

Some years ago I bought a really cheap GS450 (85 model). It was completely taken apart, including the engine. Now I have finally put the engine together and put it in the frame together with the harness.

I was able to start it twice but after that it simply will not start. Engine turns fine but it never fires. The two times it ran the idle was doing high revs, but I have a feeling it is electrical.

The below list is what I have checked / done to trouble-shoot:
- Compression is good (130+ psi on both cylinders).
- It gets fuel as I can smell it out of the exhaust.
- Put in a new rectifier / regulator.
- There is a spark on both plugs, although not super fat and blue.
- I have changed the coils out with used GS500 units, as the old ones looked really worn.
- Battery is good / new fully charged.
- Carbs taken apart and cleaned.

There is a K&N pod filter on it now (one big unit with two holes for the carbs). Although this could make it run rough, I still reckon it should start as a minimum.

I have a GS500 ignition box I’m considering putting on instead of the old GS450 unit, but since I’m getting a spark I’m thinking that is not the problem.

With the story goes that the harness is a real mess! Homemade connections and corroded open ends and connections. I’m planning to re-do the whole thing later but again, as I’m getting a spark I’m thinking that is not the issue – or could it be?

Can someone tell me what should be the next thing I look for?

Thanks,

[pete]

Welcome along!

One key thing I see missing off your list there is checking valve clearances. Clearances not being right will contribute to hard starting. Not a guaranteed solution by any means but it's something you need to ensure is right as part of the troubleshooting process.

How certain are you that the carbs are 100% clean, especially the pilot circuits? Have you tried adjusting the air or fuel screws to richen the pilot circuit up?

You need air, fuel, and spark for it to fire. It sounds like you have plenty of air, for spark you can validate that with a timing light to check the timing while cranking it, and even though you're getting fuel you need to ensure you're getting enough.

That's my 2 cents for the moment anyway...

Incidentally when getting the VM33s working on my Kat there was a point it wouldn't fire and the pilot circuit just wasn't rich enough, wouldn't even fire up on the choke. Upping the pilot and turning the air screw in to richen the circuit did the trick.

[Arek]

Thanks for the answer.

Valves should be OK, as I was really carefull with the clearance and shims. But both the valves and seats have been treated and I was told they can get a bit tighter with time as the surfaces will "find each-other". Will have to check.

I cannot guarantee the pilot circuits in the carbs are 100% clean, and I have not tried the screws. Will have to give it a go.

Thanks pete - will keep my busy for a few hours.

[Redman]

Welcome to TheGSR.

You say have marginal, or at least not optimum spark. An often overlooked cause of marginal ignition on these bikes are the spark plug caps. Some folks do not know they deteriorate over time and usage, and are easily replaceable.

I am not entirely sure about what the values should be on a 400, but can check the ohm values with the caps in place on the wires (reading the cap and the wire and the coil). OR can twist off the cap from the wire and check just the cap, should be about 5K ohms. With time and usage and heat they can go to lot higher ohms and maybe even complete open (infinite resistance).

Tell us more about "have cleaned the carbs".

[LAB3]

I'm with Redman on this one, check out those caps! That's one of things I overlooked and finally got to after putting a few thousand miles on my bike, swapping them out was cheap and made all the difference in how well it ran.

[Arek]

Welcome to TheGSR.

You say have marginal, or at least not optimum spark. An often overlooked cause of marginal ignition on these bikes are the spark plug caps. Some folks do not know they deteriorate over time and usage, and are easily replaceable.

I am not entirely sure about what the values should be on a 400, but can check the ohm values with the caps in place on the wires (reading the cap and the wire and the coil). OR can twist off the cap from the wire and check just the cap, should be about 5K ohms. With time and usage and heat they can go to lot higher ohms and maybe even complete open (infinite resistance).

Tell us more about "have cleaned the carbs".

Thanks - the part about the caps I didn't think about. Will get new ones and see how we go.

Regarding cleaning the carbs; Well, took them apart but not all the way. Did open them up so there's access to all parts and gave them approx. 30 min in an ultrasonic cleaner. Did not remove the jets, but let them sit on the unit as the cleansing fluid did reach them. Maybe that was wrong?

[LAB3]

Thanks - the part about the caps I didn't think about. Will get new ones and see how we go.

Regarding cleaning the carbs; Well, took them apart but not all the way. Did open them up so there's access to all parts and gave them approx. 30 min in an ultrasonic cleaner. Did not remove the jets, but let them sit on the unit as the cleansing fluid did reach them. Maybe that was wrong?

Take them all the way down, be sure to drill out the caps and pull out the pilot screws as well. I've never used an ultrasonic cleaner but can say that it takes at least overnight in a dipping solution to get them all the way cleaned out.

[Steve]

Regarding cleaning the carbs; Well, took them apart but not all the way. Did open them up so there's access to all parts and gave them approx. 30 min in an ultrasonic cleaner. Did not remove the jets, but let them sit on the unit as the cleansing fluid did reach them. Maybe that was wrong?

Please remember that any shortcut taken is merely an(other) opportunity to do it over and do it RIGHT.

It is not just the jets that need to be cleaned, it is the passages beyond them, deep inside the carb body.

I have not (yet) used an ultrasonic cleaner, but I would suspect that the size of the jet would severly limit the effefctiveness of the ultrasonic action in the passages beyond the jets. If you don't have chemicals available, at least remove the jets and run the ultrasonic longer to see if that helps.

Even with soaking in chemicals overnight, there might still be some residue, so it is advisable to backflush with carb cleaner spray and/or compressed air to make sure the passages are CLEAN.

.

[Arek]

Alright - seems like I need to roll up my sleves and get working. Will start with the valve clearance, plug caps and then move on to the carbs.

Will post in this thread how I'm doing. Might take a few days.

[pete]

I had forgotten about the spark plug caps, good call Redman! Definitely worth checking out (I replaced mine years ago with generic NGK variants, can't remember the details I'm sorry).

With the carbs, the pilot circuits are not exactly large, and neither are the pilot jets. The smallest amount of emulsified fuel is going to cause you much grief, so yes most definitely tear the carbs down completely and give them a proper overhaul and clean. For new O rings, go to http://cycleorings.com/, he's a member on here also Hopefully he has the right ones for your carbs.

[Arek]

So here's an update.

I went out and got new spark plug caps (Generic NGK 5 ohm) and it gave me a nice fat blue park this time (big improvement). I had a feeling it would start, so I tried that before getting to the carbs - and it fired up right away (hurray!). But it ran really "un-clean". It started with the choke down and idled OK, but would not take any throttle at all! The slightest touch of the throttle kills it. Pushing the choke down gives it some RPMs, but in what I would describe as a "gallop". Am I correct in thinking it is getting too much air? Some days ago I took out the air-screws, and I'm almost 100% sure they were 2 1/2 turns out, but seems I'm wrong. Is there a guide for getting them to their optimal position?

I will still take the carbs all the way down as suggested here, but I'm just trying to understand the above symptoms.

Thanks so far...

[pete]

2 1/2 turns out is probably a good starting point... but are they air screws or fuel mixture screws on those carbs? The general rule of thumb is if they're on the inlet side of the carbs, they're air screws, if they're on the outlet/engine side they're fuel mixture screws. Big difference there!

If they're air screws, screwing them in will richen the pilot circuit. Conversely if they're fuel mixture screws this will lean the pilot circuit out.

I'd really highly recommend ensuring the carbs are good before doing too much more as any adjustments you make to compensate are likely going to be nullified if dirty carbs are indeed your problem.

Good progress getting a better spark and getting some life happening though, that's a great start!

[Arek]

Hmm, OK, so they must be fuel mixture screws as they sit on the engine side. Thanks for the explanation.

Next step, taking the carbs apart and cleaning them all the way!

Will keep you guys posted.

[LAB3]

Hmm, OK, so they must be fuel mixture screws as they sit on the engine side. Thanks for the explanation.

Next step, taking the carbs apart and cleaning them all the way!

Will keep you guys posted.

Two turns out is where mine where when I drilled out the caps, I've played with them since then and I always seem to return back to two turns out.

[Steve]

The screws are, indeed, MIXTURE screws. Suzuki calls them "pilot screws".

When the carbs were new, they set the screws with some fancy machine at the factory and called it "optimal". That setting was more for pollution regulations than it was for operability, so were set a bit lean. Over the years, the quality of fuel has changed a bit. Here, we have Ethanol in most of the fuel and it requires a bit more liquid to provide proper fueling, so the screws might need to be turned out just a bit more than stock.

When I rebuld a set of carbs, I start with the screws at three full turns out. Yes, it's too much, but it acts a bit like a choke, enrichening the mixture. The cold engine will run better with a richer mixture, so it at least will start and run. When it warms up, you can slowly turn the screws in, while listening to engine speed. As the mixture leans out, it will approach a "happy spot" where it runs best, then will start to fall off. When the speed falls off, turn the screw back out 1/8 to 1/4 turn and do the other carb. They will likely end up between 2 and 2 1/2 turns out.

When you are using the "choke", keep in mind that it does not operate like a choke on a car's carburetor. Those would have a plate that would almost completely cover the intake (choking it) to raise the engine vacuum to the fuel circuits in the carb, richening the mixture. Your motorcycle carbs move a plunger that opens separate circuits for fuel and air, providing the rich mixture you need. It relies on the high vacuum created by a CLOSED throttle. If you crack the throttle open at all, you destroy the vacuum and you are asking the engine to run on the stock, LEAN jetting.

You don't always need full "choke" to start the bike. Apply full "choke", start the bike. Slowly back it off to the 'run' position, noting where the engine is running the fastest. That is about where your maximum "choke" should be. Try starting with just a little less than that. As the engine warms up, it will speed up, so reduce the amount of "choke" to keep the engine speed around 2000. When the engine responds to blips of the throttle, it's ready to ride. If your valve clearances are set properly, carbs are cleaned and adjusted, that will only take less than a minute.

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