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GS300L starts and idles but bogs with throttle

cdahl383

cdahl383

Forum Apprentice
Helping my neighbor with his 1985 Suzuki GS300L. It’s a nice clean bike. Only 3600 miles. It sat for many years and wouldn’t start.

We pulled the carbs off and cleaned them, replaced the needle and seat assemblies, pilot jets, and put new diaphragms on the slides. Float level was adjusted to spec. We ran new fuel line and vacuum line to the petcock. He also replaced the petcock assembly. Tank was flushed and cleaned out.

We got it to start and idle. He put it in gear and it takes off like it should. But as soon as you start to open the throttle, it starts to run crummy. It sort of bogs, sputters, misses, etc. Sometimes if you open the throttle slowly it will rev up and run pretty good. Eventually it will start running goofy though.

The spark plugs are new. The carb boots looked okay, no visible cracking anywhere. It ran the same with and without being hooked up to the airbox with air filter and airbox lid in place.

He’s looking to sell it and wanted to get it running good first. At this point he’s frustrated with it and doesn’t want to really buy anymore parts for it if we’re just guessing at what’s wrong.

It seems to me the bike is starving for fuel. It’s clearly running well on the idle circuit. The needle jet and main jet were cleaned and had no obstructions.

We did not touch the fuel screws. I was thinking of adjusting those, but I’m not sure those will cure the issue as it seems more like a mid throttle or full throttle issue.

His other buddy replaced the slide diaphragms. Not sure if they were OEM Suzuki or aftermarket.

Any suggestions on what to look at next?

Adjust fuel screws? Check for vacuum leaks? Adjust float higher?

Appreciate any insight!

https://youtu.be/6pfVO7vykok?si=MPobQ6ucQoBlWaj-
 
Seems odd that runs same with or without the air box connected, but I've not done much with the smaller GS's. I do know, on the larger bikes, there is a rubber plug that goes in behind (under) the pilot jet, leaving those little rubber plugs out will cause the appx. problems you describe.
 
Generally speaking, if you used an Amazon or ebay rebuild kit, the sizes aren't right.
Many have tried, and found the best way was to properly clean their old jets and re-use them.
 
rphillips said:
Seems odd that runs same with or without the air box connected, but I've not done much with the smaller GS's. I do know, on the larger bikes, there is a rubber plug that goes in behind (under) the pilot jet, leaving those little rubber plugs out will cause the appx. problems you describe.
Click to expand...

I thought that as well. But it idled great either way. You can shut it off and it fires right back up and will idle all day long nice and smooth. As soon as you crack the throttle a little too much though it starts running all stupid. Sounds like it wants more fuel but it's just not getting it for whatever reason. Slide not coming up, blockage somewhere in the carb, etc.

I looked up the diagram for the carburetor for that model and did not see any plugs under the pilot jet for these carbs. I believe my GS550L has those, but these carbs appear to not have those plugs. I didn't see anything in there when I took them apart either.
 
Grimly said:
Generally speaking, if you used an Amazon or ebay rebuild kit, the sizes aren't right.
Many have tried, and found the best way was to properly clean their old jets and re-use them.
Click to expand...

That's the thing, I don't know what they used. My neighbor had two other guys trying to help him with the carbs before I got to them. One guy put in a rebuild kit and some new needles/seats. Another guy put in new diaphragms. I'm not sure what parts they put in there. The jets were all stock looking though.

The pilot jets were so bad you couldn't even see through them. I cleaned them several times and just couldn't get them to clean up good. I finally bought new pilot jets for him. They are genuine Mikuni jets (40 pilot jets). I think they're fine as the bike idles great.

The needle jet and main jet I just cleaned with chem-dip and then carb cleaner and compressed air. They looked good to me and all the holes were clear.

I really think it is something to do with the slides not coming up, or not coming up fast enough. Maybe those diaphragms are different from OEM? Maybe they're not sealing up well around the edges? We could try putting a little grease along the edges of those to help seal them better.
 
Most likely, the pilot circuit is plugged up. The jets may be clean, but the passages inside the carb body may not be. Shoot carb spray into the pilot circuit, and make sure it's coming out the small holes in the carb throat. I'm not sure about those carbs, but there are typically several small holes that must be open. Also, remove the pilot screws and springs, and shoot carb spray in that way, again, looking to see where it's coming out.

See how the exhaust pipes at the head are turning dark blue? That's because you guys are running this bike too long while parked, without any air blowing over the engine. Get a big fan and use that if necessary.
 
Nessism said:
Most likely, the pilot circuit is plugged up. The jets may be clean, but the passages inside the carb body may not be. Shoot carb spray into the pilot circuit, and make sure it's coming out the small holes in the carb throat. I'm not sure about those carbs, but there are typically several small holes that must be open. Also, remove the pilot screws and springs, and shoot carb spray in that way, again, looking to see where it's coming out.

See how the exhaust pipes at the head are turning dark blue? That's because you guys are running this bike too long while parked, without any air blowing over the engine. Get a big fan and use that if necessary.
Click to expand...

How can the pilot circuit be causing the problems though? It starts and idles great. The bike runs on the pilot/idle circuit while idling, which is does great. It seems like the problem is the transition from idle to part throttle, as in the needle, or slides not rising, etc. Isn't that the case, or am I misunderstanding something?

The carbs were soaked in chem dip, blasted with carb cleaner, and then blasted with compressed air. I suppose it's possible something could still be clogged up, but I'm not sure how after all that.

The pipes were like that years ago when he first started messing with the bike. That is not a recent occurrence. I'll mention the fan idea though if we end up messing with it again soon. He has a pretty big fan somewhere in his garage that we could use I think.

You mentioned the pilot screws. I did drill out the little covers so you could adjust them. I took them out when I cleaned the carb, but maybe they aren't adjusted right. What is a good starting point for those? Maybe 2 turns out or so? I can't remember where I set them. Maybe they're set too lean.
 
Just because bodies were dipped doesnt mean they were dipped long enough to sufficiently clean internal passages. I do 24 hours regardless of how they look when taking off the bowls. Jets the same thing. Also pluck a wire from a wire brush and poke and scrub all jet holes with it then rinse in carbs spray and air.
As for MIXTURE screws and pilot screws. General wisdom is 2 out from gently seated for CV carbs and 7/8 out for VM carbs. . And why they affect transitioning to the main circuit. They may pass enough fuel for it to idle fine. However they need to pass enough fuel to fill in the momentary gap in the engines needs between the idle and main circuit. Not enough fuel and you get a stumble or it outright dies.
Another factor is the air intake structure. Im an old school believer that it was engineered with the stock airbox for optimum performance and it should stay that way.
Take out the slides on CV carbs and backlight the diaphragms to check for pinholes that will stop them from functioning correctly.
 
My neighbor called me last night and said he went out and started the bike, let it warm up for a minute, then attempted to rev it up, and it ran perfect! No clue what happened or why it suddenly runs better. Only thing I can think of is maybe some sediment from the tank got into the carbs at first and was blocking a passage in the carb or maybe the intake boots weren't fully sealed and by running the engine maybe it expanded something and now it's sealed up? He said he shut it off and came back out again later and it did the same thing, ran just fine.

One of my buddies suggested looking at the rubber o-rings that go between the cylinder head and the intake boot. He said if those were old and dry rotted, they might be leaking and pulling in some unmetered air. But it's possible they swelled up a bit after getting hot and now it's sealed up again. Maybe we can spray some carb cleaner or something around the intake boots next time we're looking at it just to verify.

I'll double check the fuel mixture screws as well. Maybe we can back those out a little more if necessary.

I know on my GS550L they were initially around 2 turns out when I bought the bike. I ended up going to about 2.5 to 2.75 turns out and the bike seemed to run better like that. I tried going to 3 turns but didn't really notice a difference, so I left it at 2.5 turns.
 
Great it's fixed, but would have rather known what the problem really was instead of just guessing what it might have been. Now we're back to the orig. question with no answer. Main thing congrats it's fixed.
 
It's 'fixed,' but was it ridden (roded? :bi_polo:)? Roaded?

Just because it performed well while standing still, doesn't mean it will when under load. Under way.
 
Geez you guys haha! I can't just unfix it to find the problem haha!

It's winter here in Michigan, snow and ice on the road, so no we have not ridden it and don't plan to until the roads are dry. We'll see how it runs once the weather is warmer and roads are dry again. Might need some fine tuning then as engines sometimes run fine with no load but then with a load they exhibit some issues.
 
I m in Oklahoma but im a native Michigander. Hazel Park just off the 9 mile and I75 exit. Theres a 1 mile stretch clear enough for a jaunt.. MAN UP GANDER!! LOL
 
chuck hahn said:
I m in Oklahoma but im a native Michigander. Hazel Park just off the 9 mile and I75 exit. Theres a 1 mile stretch clear enough for a jaunt.. MAN UP GANDER!! LOL
Click to expand...

Haha! Actually we did get some warmer weather today. It got up to 45 degrees today. But now the roads are all nasty and wet with salt, etc. I don't think my neighbor will want to take his bike out in that crap. We'll take it out for a spin as soon as the roads are decent for sure.

I've been to Hazel Park a few times. There used to be a horse track there in Hazel Park, went there a few times. I drive through there often going up to Auburn Hills for work. Oklahoma is a long ways from Michigan! How did you end up way over there?
 
Tracks long gone. Got tired of what youre going through right now. I sure dont miss that weather for a second. Sold my 35 acres up in AuGres and got out
 
I see. That's cool. I've never been to Oklahoma before. Do you get a lot of tornadoes there in your area?

My neighbor took the bike out for a ride yesterday! I couldn't believe it. I didn't think he would. He just went up and down the street a little bit. He said it started right up and ran well for the most part. Only thing he noticed is that in higher gears it wouldn't take full throttle or much more than 1/2 throttle or so. It would sort of bog or hesitate, etc. Makes sense as the higher gears put more of a load on the engine, so if there's a problem still that's when it would show up. Sounds like it's still not 100% right.

I ordered a set of O-rings for the intake boots. We're going to swap in the new o-rings and check the mixture screws on the carb and see how it goes after that.
 
Sometimes carb problems are actually ignition problems.... just saying :) I had an issue similar to that on a GS1000G where the "ignitor" box was shot. There is a test in the Clymer manual for it. I think you'll have electronic advance but if it's mechanical check that it is free & the springs are still functioning etc.

I think the O rings will help & yes you can test by spraying carb cleaner around if you like.

Now you have it running I would run a high concentration of Seafoam through it regardless. My guess is that the heat opened up some passages but they are still partially blocked. Make sure the Airbox is also sealed in all the right places. Sometimes you need to get new boots & clamps, run Silicone round the seams & replace the foam around the door & where the filter goes. If it's the type with a foam filter insert... replace it with generic Uni stuff (cut your own) and make sure it's oiled properly. The original stuff will slowly disintegrate.

The fact that it will run the same with airbox on/off is a big no no.... You must have airleaks somewhere for that to be the case.
 
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