Still not right, FFS.

[bob808]

Gs750 78. USA Import

-Never ridden with original exhausts/ set up cos the pipes were rotten carbs seemed to be set stock.

-I have

Rebuilt, rebored,1 size over,
New cam chain,
Dynatek ignition,
V and H 4-1 exhaust.
115 main
Clip up 1

-Pretty sure,

Cam timings are right
Dynatek timing is good
Petcock is working,
No air leaks
Airbox good
Sprayed brake cleaner to find leaks, found none.
Plugged 2 vacuum tubes on carbs, use other one for petcock
Been at this bike for 5 years and had the carbs off at least 40 times.

-AAArrrrrrrrgggghhh.

-I will check these again, of course if there are no new ideas.

Top speed WOT wont go above 60mph
1,2,3, gears seem fine.
4 and 5 gears seem to be un responsive.

-I have tried different main jets and clip positions, and these do make small differences but the same problem is always there, more or less.

Is there a fuel flow amount per minute I could check?

-I am considering,

Checking advance.
Riding with open airbox,
Checking I have the correct Dynatek,
Exhaust air leak,
Cleaning petcock.

Runs well in lower gears.

Any help gratefully received, thank you.

[Nessism]

Plugged 2 vacuum tubes on carbs, use other one for petcock

This is wrong. You blocked the float bowl vents. Likely starved the engine for fuel.

[bob808]

s-l1600.jpg So the ringed parts I blocked. Middle one is used for petcock vacuum. Is this correct?

[zed1015]

s-l1600.jpg So the ringed parts I blocked. Middle one is used for petcock vacuum. Is this correct?

The "ringed" vents MUST be left open to atmosphere.
The other on #3 carb is vac line to the fuel tap and must be capped if not in use for a vaccum tap.
Put your needle clip position and main jet back to stock .
The rebore and exhaust should need no jetting changes.
Open airbox would require air corrector jets and up a size on the mains.

[Nessism]

Zed's air correction jets are the "cheat code" for guys running pods, or non-airbox applications. A Dynojet kit is another option, but it's more expensive.

Edit: 5 years? Still not running right? What took you so long to find this site?

[chuck hahn]

Also note that there is a throttle stop screw on the gang plate. The manual will tell you to set that stop so the BOTTOM EDGE of the slides go no more than 1MM above bore of the venturi. This is necessary because any more spoils the venturi.

[unfocused]

This is based on my experience with 2 GS engines and 40 years of tool collecting and greasy fingernails...

Your mains are way to big and you NEED the "air correctors" unless you are running a stock air box. However, read below because a stock air box without a filter or a non-stock insert will cause issues.

Boring an engine doesn't automatically require bigger main jets. In some cases you need to go smaller.
Case in point - 77 GS750 runs a 102 main, 78 GS1000 runs a 100 main jet - factory spec'd.
The GS100 engine is more efficient running the VM26's than the GS750 is. In layman's terms - it pulls more full through the same carb venturi becuase of the volume of air it moves.

Blocking those vacuum ports hurts nothing.

My scoot - 78 GS1000 engine mounted in my 77 GS750 running single screw VM26 carbs. Manual petcock - non-vacuum. Modified air box, Ebay electronic ignition, Katana coils, stock plugs, 4-1 unknown exhaust (possibly a MAC system based on looks) with a Super Trapp insert. Engine is stock, worn out, oil leaks. Valves adjusted, timing adjusted and manual T-chain tensioner... Just an oldie but goodie... like me :-)

I have air correctors installed and run stock main/pilot jets and clip height... Bike has been well over 100 mph with zero issues. I had the same issues as you before the air correctors. I tried everything within my knowledge and plenty of research across the WWW. 2 different sets of VM26 carbs (single screw [Kawasaki]and dual screw [Suzuki]) Different plugs, coils, ignition, exhaust... blah blah blah... ugh.

And if you're wondering, all of this happened within the last 6 months because I too was fighting my issue for way to long, but not 5 years.

Modified air box - this was my main problem. This setup allows to much air to pass the emulsion tube (main jet holder) drawing in massive amounts of fuel causing a seriously rich condition from part throttle to 3/4 throttle. If you can get it to get past 3/4 throttle - hold on as she run like a beast.

Air Box 2.jpg

[unfocused]

Air correctors installed in my single screw VM 26's.
Very simple to install.
However, you need to remove the carbs again - ugh.

After installing these, I am a firm believer that if you mod an engine (any mod) with VM26 carbs, you need the air correctors as this prevents changing clip height, mains, pilots again and again and again...
After everything is done, my scoot starts, runs, drives with zero issues however the choke starting procedure needs rethinking LOL...

Resized_20240217_132757.jpg

[bob808]

Thank you guys.
Glad to have something new to try.
Can't remember where I heard about capping the vacuum tubes.
I was about to go all the way round the houses again.
Going to sit down tonight and read up on Air Connectors. and if I have time tomorrow before work, will take the bike for a ride.
The thought of taking the bike out and it behaves as it should, after all this time, makes my head spin TBH.

https://triumphmotorcycleforum.com/t/suzuki-never-ending-story/446

[Rich82GS750TZ]

zed1015 knows all you need to know about the air corrector jets.

https://www.thegsresources.com/_forum/forum/technical-forums/carb-fuel-exhaust/1749274-still-not-right-ffs?q=Air%20Corrector%20Jets

[zed1015]

Going to sit down tonight and read up on Air Connectors.

You only need air correctors if running open carbs, pods, stacks or an airbox that has been cut or modified and offers virtually no restriction.
They cure the off idle bogging / hesitation and need no other carb changes except a small increase of the main jet.
In some cases such as the Yamaha XS1100 the main jet is actually reduced in size from stock.
In the case of the GS750 your stock main should be 102.5 and increased to 112.5 with corrector jets and no stock airbox.
Your current spec with airbox should need no jetting or needle clip position changes from stock and your 115 main is too big which with lose you top end power..
You should put all carb settings and jetting back to stock before going any further and uncap the carb vents.
The carbs overflow pipes in the float bowls do double as vents to a degree but at high speed their small bore can offer a restriction leading to fuel starvation .

[KiwiAlfa156]

All good advice above.

You are right to look at the ATU and ignition timing. Probably a good idea to do this first as this results in poor responsiveness as overly advanced or retarded ignition timing lowers reduces combustion efficiency. Adjusting carburetion against a mistimed ignition won't fix the problem.

The most important important timing marks are the ones at full advance which is achieved at 2,750 rpm. The ATU has to be operating smoothly for it to advance to 32? BTDC.

The vent tubes are crucially important, as the float bowls have to be at atmospheric pressure. It's the air pressure differential between the atmospheric in the the float bowls and the low pressure of the air flow through the carb venturi, that 'pushes' the fuel through the jets and creates the mixture.

[bob808]

I am going to update this, Ive been working overtime and now Ive got some kind of virus (deadly perhaps, I dont know) possibly Rabies.

[Datsa Noydb]

Along with uncapping the vent ports, be sure to install proper tubing on them and route the tubes to a non-turbulent area. Not sure about factory routing on your bike, but some of the later bikes would route the tubes over the back of the airbox or to a spot under the seat. STILL air gives the best atmospheric reference that is necessary for proper running.

[Mary Hinge]

I am going to update this, Ive been working overtime and now Ive got some kind of virus (deadly perhaps, I dont know) possibly Rabies.

Well first off and most importantly I hope your get rid of whatever it is you've got PDFQ Bob.
You don't mention how the plugs looked during your various tests, I'm guessing they must be very rich or very lean.
Rob/Zed1015's help was invaluable when I has similar issues as after years of old 2T's I was lost with the GS.
Steve at Motocarb provides an excellent service along with competitive pricing on all genuine Mikuni spares.
Can remember who sent me the attached Suzuki carb specs doc.

Bike model					Carburettor Type					Jets sizes and Adjustments
Prefix	Model	Year	Country	Type	Model	Carb Type	Total	Bore mm	ID	Idle rpm	Idle +/-	Fuel Level mm	Float Height mm	Main Jet #	Main Air Jet mm	Jet Needle #	Needle Jet #	Pilot Jet #	Throttle Valve	Bypass mm	Pilot Outlet mm	Valve Seat	Starter Jet #	Pilot Screw	Pilot Air Jet #	Cable Play mm
GS	750	83	US	E	BS32SS	Single	4	32	31300	1150		5.0 +/-0.5	22.4 +/-1.0	117.5	1.7	5D12	Y-6	37.5			0.7	2.0	45	pre-set		0.5 - 1.0
GS	750	76			VM26SS	Single	4	26						105	1.1	5F21-3	P-1	22.5						1.25	1.6
GS	750	77			VM26SS	Single	4	26						97.5	0.7	5F21-3	O-6	27.5						1.75	1.6
GS	750	77		B	VM26SS	Single	4	26	45010			3	26	105		5F21-3	P-1	22.5						2
GS	750				VM26SS	Single	4	26	45020					105				15
GS	750	78-79	US	E	VM26SS	Single	4	26	45030	1050	50	2.5 - 3.5	23.0 +/-1.0	100	0.7	5F21-3	O-6	15							1.6	0.5 - 1.0
GS	750	80-81	US	LT/LX/EX	BS32SS	Single	4	32	45400	1050	100	5.0 +/-0.5	22.4 +/-1.0	112.5	1.7	5C28	Y-4	42.5		0.8,0.8,0.8	0.7	2.0	50	pre-set		0.5 - 1.0
GS	750	80		L	BS32SS	Single	4	32	45410	1050	100	5.0 +/-0.5	22.4 +/-1.0	112.5	1.7	5C32-3	Y-5	42.5		0.8	0.7	2.0	50	3.5	150	0.5 - 1.0
GS	750	82	US	EZ/TZ	BS32SS	Single	4	32	45420	1050	100	5.0 +/-0.5	22.4 +/-1.0	112.5	1.7	5C28	Y-4	42.5	135	0.8,0.8,0.8	0.7	2.0	50	pre-set	170
GS	850	77 -79	US		VM26SS	Single	4	26	45060	1050	100	3.0 - 5.0	24.0 +/-1.0	102.5		5DL36-2	O-6	15		0.8	0.6			"5/8	1.2	n/s
GS	850	80	US	GT	BS32SS	Single	4	32	45110	1050	100	5.0 +/-0.5	22.4 +/-1.0	115	1.7	5D50	X-5	40		1.0, 0.8, 0.8	0.7	2.0	32.5	pre-set		0.5 - 1.0
GS	850	80	US		BS32SS	Single	4	32	45120	1050	100	5.0 +/-0.5	22.4 +/-1.0	115	1.7	5D57-3	X-6	42.5		1.0,0.8,0.8	0.7	2.0	32.5	3.5	200	0.5 - 1.0
GS	850	82	US	GZ	BS32SS	Single	4	32	45160	1050	100	5.0 +/-0.5	22.4 +/-1.0	115	1.7	5D50	X-5	40		1.0,0.8,0.8	0.7	2.0	32.5	pre-set	180	0.5 - 1.0
GS	1000	79	US	S	VM26SS	Single	4	26	49010	1000	100	3.0 - 5.0	24.0 +/-1.0	95	1.5	5DL36-3	O-4	15						1		1.0 - 2.0
GS	1000	79		C	VM26SS	Single	4	26	49020	1000	100	4.0 +/-1.0	24.0 +/-1.0	95	1.5	5DL36-3	O-2	15						pre-set	1.2	1.0 - 1.5
GS	1000	79		S	VM26SS	Single	4	26	49020	1000	50	4.0 +/-1.0	24.0 +/-1.0	95	1.5	5DL36-3	O-2	15		0.8	1.5			pre-set	1.2
GS	1000	79		L	VM26SS	Single	4	26	49050	1000	50	4.0 +/-1.0	24.0 +/-1.0	95	0.6	5DL36-3	O-2	15		0.8				pre-set	1.2
GS	1000	80-81	RSA	G/GL(X)	BS34SS	Single	4	34	49160	1050	100	5.0 +/-0.5	22.4 +/-1.0	110	1.7	5D57-3	X-8	40	140	0.9, 0.8, 0.8	0.7	2.0	45	pre-set	160	0.5 - 1.0
GS	1000	81	US	G/GL(X)	BS34SS	Single	4	34	49160	1050	100	5.0 +/-0.5	22.4 +/-1.0	115	1.7	5D57-3	X-8	40	140	0.9, 0.8, 0.8	0.7	2.0	45	pre-set	160	0.5 - 1.0
GS	1000	82	US	SZ	BS34SS	Single	4	34	49300	1050	100	5.0 +/-0.5	22.4 +/-1.0	110	1.2	5D58	X-1	45		0.8, 0.8, 0.8	0.9	2.0	32.5	pre-set		0.5 - 1.0
GSX	1000	82	UK	SZ	VM32SS	Single	4	32	49310	1050	100	5.0 +/-0.5	22.4 +/-1.0	95	1.5	5DL82-3	O-6	20		0.8	0.8	2.3	45	1.25		0.5 - 1.0
GS	1000	83	UK	SD	VM32SS	Single	4	32	49320	1050	100	5.0 +/-0.5	22.4 +/-1.0	95	1.5	5DL82-3	O-6	20		0.8	0.8	2.3	45	1.25		0.5 - 1.0