GT380 poor idle/low RPM metering

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Boise
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Suzuki 2-Strokes: GT380 1974

GT380 poor idle/low RPM metering

Post by Boise »

1974 GT380 all-stock restored, fully-rebuilt crank and top end, good compression, perfect timing checked and rechecked by dial gauge and timing light, sparks consistent all 3 cycls.

Looking to see if my story is familiar and to learn what anyone has done about it. I've heard these carbs are pretty marginal, to start with.

Ever since rebuilding this project, I've never been able to get it to carburate right at idle and low RPM off-idle. Starts easy on choke, idles nicely on all three until warm-up dictates dropping out choke. Once warming up and off choke, I get a poor idle and gutless bottom end, in great part because after a bit it drops a cylinder, which will come back if I blip on the choke momentarily (but will load up shortly). Runs great everywhere else 3-for-3 once the revs finally climb and I'm on the throttle. Standing starts require a high rev and a lot of clutch abuse to not bog.
The only way it stays running at idle, barely, is with the idle air screws almost all the way in, settling best of 1/4 on outer carbs and 1/8 turn on center. Not 1 and 1/4, 1/8; actual 1/4, 1/8. And the idle opening is raised to compensate for the lack of flow through the idle circuit. Therefore it's into the pilot at idle all the time and thus has poor transition into the bottom of the needle, just feels lean all throughout the bottom end transition.
Have fully disassembled and basic-rebuilt carbs twice before, and just tore em down again to make sure the idle circuit bypass ports are actually open. They are, it's spotless in there.
I suspect the needle jets and needles have had better days, so I'm hunting down a jet kit that has the harder-to-find replacement for 164-0-2 needle jets. But such wear should just mean a richer than ideal mid-throttle, yet the bike indeed runs happily along at cruising speed. I do not imagine the condition of the NJ/N has anything to do with the idle circuit, but maybe???
B8 plugs read ambiguously from brown electrode, to brown-to-white/clean porcelain.
So I am just not getting why this thing seemingly won't pull fuel through the idle circuit at low revs, which I believe is what is going on.
One off the wall thing I tried was to plug the crankcase oil drain return lines, as though they could feed so much air back through as to affect idle mixture. No-difference.

What dumb thing have I missed? Or are these just truly horrid carbs that can only reach an uneasy truce with all other conditions being optimal?

Thanks in advance. I know these discussions are minefields. Hope to gain some wisdom...
Boise GT380
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grumps
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Re: GT380 poor idle/low RPM metering

Post by grumps »

Hi

I have just had my carbs cleaned and rebuilt. Before it would drop a cylinder and not idle well/ Now it starts great and will run all cylinders after choke but has an erratic idle. It's too early to point fingers but the guy who did the carbs says the balance of the 3 carbs re idle settings is very critical.

In my manuals the method of balancing the idle is to cut out one cylinder at a time and adjust each carb. That sounds very clumsy and I'm looking at using a flow meter on the carb intakes to balance them to each other. There are some screws that look like vacuum takeoffs at the engine side of the carbs but the manuals do not refer to them. If they are I can use a carb balancer kit to set them all to the right and equal airflow.

So, basically, yes, they are tricky.

My carbs are the set with the actuating rod across the top, not the ones with the 3 cables operating each carb separately.

I will watch this thread with interest
If money can't buy happiness - explain motorcycles and beer!
GTandcbr
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Re: GT380 poor idle/low RPM metering

Post by GTandcbr »

I guess you know that all the symptoms point to blocked pilot circuits. That's why you have tried cleaning them twice already. I know you say the circuit and bypass are clear but it's impossible to see in the internal channels. Have you had them in an ultrasonic cleaner? These carbs work fine but have to be spotlessly clean inside.
Just in case you don't have it this may help
http://www.3cyl.com/mraxl/gt/manuals/su ... /cover.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
Couple of other observations. Please don't buy Keyster jets or needles. They are useless and will make it impossible to set up. Blocking the SRIS valves is okay and commonly done by owners not bothered about keeping the bike original.
Last edited by GTandcbr on Thu Jun 28, 2018 2:10 pm, edited 2 times in total.
98 carb Blackbird and GT550j
sportston
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Re: GT380 poor idle/low RPM metering

Post by sportston »

GTandcbr wrote:Have you had them in an ultrasonic cleaner? These carbs work fine but have to be spotlessly clean inside.
+1 on that.
You have a lean mixture (as indicated by the necessity to screw in the pilot air screws so far).
So the question is why?
As previously mentioned, a clogged pilot jet circuit is one possible cause. Or it could be....
Inlet air leak (e.g. Cracked rubber inlet tract, poorly sealed cylinder base gasket, crankcase leak etc.)
Needle too low (clip in wrong position)
Incorrect pilot jet size
Incorrect float level setting
Stale fuel

Probably not the main cause of your problems, but certainly capable of making things harder, is badly synchronised carbs.
Make sure you have an inline fuel filter fitted. If the problem is caused by dirt in carbs, you will keep having the same problem if you don't have a proper filter.
Good luck
Boise
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Suzuki 2-Strokes: GT380 1974

Re: GT380 poor idle/low RPM metering

Post by Boise »

Thank you grumps, GTandcbr, and sportston!

Appreciate the confirmation of symptoms and advice on direction to proceed, and that great manual link. I am so behind the curve on finding such resources, for having these bike for nearly 10 years now! Super community here, I'm most grateful.

Is there a passage that cannot be seen nor probed between the holes in the idle-pilot circuit that necessitate ultrasonic cleaning? I don't see anything I can't access nor blow compressed air through, based on the diagram in the manual.
With LED light shone through the pilot jet hole, jet removed, I can clearly see through both holes fore and aft ("2" and "3") of where the slide cuts off and they are clear as day in the high desert. But I'll certainly seek out somebody to ultrasonic clean the bodies.

In order to get new Needle Jets to completely baseline these slugs, I ordered a jetting kit from a company in NY (http://www.ivansperformanceproducts.com/GT380.htm" onclick="window.open(this.href);return false;). So that will give me benefit of fancy multi-taper needles and some tuner advice that comes with the price of admission for a pile of brass.

I'll certainly have to set the balance, since I have to disassemble each slide to get at the needles (replacing the famous melted bellows too). And I broke one of the bellcrank lever thingies that raises the slide rod, so that is going to slow me down hunting that down... Anybody got a line on one a half-dozen?

Again, much appreciation to the panel.

Boise
GTandcbr
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Re: GT380 poor idle/low RPM metering

Post by GTandcbr »

Can you get the part number from here to give us an idea which part you are after?
https://www.partzilla.com/catalog/suzuk ... tor-gt380l" onclick="window.open(this.href);return false;"
Although both exit holes may appear to be clear the narrow channel in the pilot bypass circuit that exits near to the front of the venturi can still restrict flow. It's far better to use an ultrasonic cleaner than to stick anything in there that may damage the integrity of the circuit.
98 carb Blackbird and GT550j
sportston
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Re: GT380 poor idle/low RPM metering

Post by sportston »

Boise wrote:I broke one of the bellcrank lever thingies that raises the slide rod, so that is going to slow me down hunting that down... Anybody got a line on one a half-dozen?
Boise
I have one of those I think. I'll try to have a look in my garage tomorrow. Not sure how much to post to USA. Might be cheaper to order one from ebay.com
Boise
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Suzuki 2-Strokes: GT380 1974

Re: GT380 poor idle/low RPM metering

Post by Boise »

sportston wrote:
Boise wrote:I broke one of the bellcrank lever thingies that raises the slide rod, so that is going to slow me down hunting that down... Anybody got a line on one a half-dozen?
Boise
I have one of those I think. I'll try to have a look in my garage tomorrow. Not sure how much to post to USA. Might be cheaper to order one from ebay.com
I found a new part in the US much to my surprise. Should have soon. Suzuki 13564-33110 - THROTTLE ARM
I found a good e-fiche to use at https://www.cmsnl.com/suzuki-gt380-1973 ... partslist/" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

And for new needle jets, I found a vendor in NY with a jetting kit that includes them plus a multi-taper needle:
http://www.ivansperformanceproducts.com/carbs.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

If I get the lever tomorrow, I should be able to button up the bits and report on my results.
sportston
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Re: GT380 poor idle/low RPM metering

Post by sportston »

Cool! Hope your bike is making a ringading sound soon.
Boise
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Suzuki 2-Strokes: GT380 1974

Re: GT380 poor idle/low RPM metering

Post by Boise »

sportston wrote:Cool! Hope your bike is making a ringading sound soon.
Success!
Too many variables at once to say what exactly did the trick, but after 10 years of poking around the edges, now it's running right with a comprehensive carb rebuild.
My bet is on the new needle jets and needles that came with the jetting kit. And I wonder how much those boots at the top that like to melt have an impact on metering if air could flow in from above the slide. (I replaced these boot with new, aftermarket and perhaps better rubber I think, as the ones on there were shot.)
As-set by recommendation of the jet kit, the bike is running well all-around, quieter, smoother acceleration, I'm very pleased indeed.

I did idle and air screw setting by ear only, as I could not find a place to hook up my rev counter: I forgot that I replaced the points setup with a electronic trigger kit years ago.
But it seems okay, happiest at 3/4 turn each, and all the same now.

Thanks again for everyone's advice.

3 second vid of running bike...
https://photos.app.goo.gl/PdDNcAoFHddjsmU48" onclick="window.open(this.href);return false;

Next projects:
Replace petcock, previously repaired, still leaky.
Delve into why oil tank bleeds down mainly into middle crankcase while sitting.
Repair/replace defunct speedo and tach.
sportston
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Suzuki 2-Strokes: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S

Re: GT380 poor idle/low RPM metering

Post by sportston »

Boise wrote: Next projects:
Delve into why oil tank bleeds down mainly into middle crankcase while sitting.
Replace the CCI banjo fittings. They have ball valves in them, which leak with age. You can get new ones from Kawasaki. There is a post or two on here somewhere with the relevant part numbers
Boise
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Suzuki 2-Strokes: GT380 1974

Re: GT380 poor idle/low RPM metering

Post by Boise »

sportston wrote:
Boise wrote: Next projects:
Delve into why oil tank bleeds down mainly into middle crankcase while sitting.
Replace the CCI banjo fittings. They have ball valves in them, which leak with age. You can get new ones from Kawasaki. There is a post or two on here somewhere with the relevant part numbers
Brilliant! Thank you!!!
GTandcbr
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Re: GT380 poor idle/low RPM metering

Post by GTandcbr »

https://www.cmsnl.com/products/valve-ch ... z2l2cso80N" onclick="window.open(this.href);return false;
There you go
98 carb Blackbird and GT550j
Boise
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Suzuki 2-Strokes: GT380 1974

Re: GT380 poor idle/low RPM metering

Post by Boise »

GTandcbr wrote:https://www.cmsnl.com/products/valve-ch ... z2l2cso80N" onclick="window.open(this.href);return false;
There you go
Thanks very much. Found three on this side of the pond, on their way.
GTandcbr
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Posts: 590
Joined: Tue Mar 10, 2015 12:35 pm
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Re: GT380 poor idle/low RPM metering

Post by GTandcbr »

Read this link it will help you
https://www.oldjapanesebikes.com/blog/2 ... ion-lines/" onclick="window.open(this.href);return false;
98 carb Blackbird and GT550j
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