Please help

[CrazyOtte]

I can't get this 1980 Suzuki Gs750E running correctly. I'm a decently accomplished mechanic. I've never failed at a project like this before.

All the carb parts are oem mikuni, stock jet sizes, and 100% clean, I'm positive. Slides move smoothly with the butterflies, I bench tested with a shop vac. No vacuum leaks, compression good, spark good, Valves in spec and cams timed correctly, running it off a fuel bottle and the float bowls are full to the correct level. I'm using the stock Airbox with a K&N replacement filter that's very similar to the OEM part. I can start the bike on Choke and it'll idle around 3k. I can coax the throttle on Choke and get it to rev to the red line with awful throttle response.
Off Choke, I backed the mixture screws way out (rich) and it'll start and run for a few seconds before dying. If you open the throttle, it kills it.

I've dumped about 25 hours into this problem and I'm at the end of my rope.

[LAB3]

All the carb parts are oem mikuni, stock jet sizes, and 100% clean, I'm positive.

Let's start with the basics, how did you go about cleaning the carbs?

[Runningdog]

How old is the gasoline?

[Suzukian]

Did you sync the carbs? How did you set the initial butterfly opening in the carbs, you can use the shank of a very small drill, then adjust from there. It sounds like fuel starvation, how did you set the carb floats. Make sure they aren't leaking, or sitting too low..

[CrazyOtte]

Let's start with the basics, how did you go about cleaning the carbs?

Total disassembly, ultrasonic cleaner for 1 hr at 50C. Solution is 75% Berryman Chem dip, 25% WD40.
Ran a jet cleaner wire through every hole and sprayed carb cleaner through every passage. I have disassembled and cleaned them 6 times now.

Gas is less than 2 weeks old.

Butterflies are synced using a 0.05mm (TINY) drill bit, synced to number 3. Bike doesn't run well enough to vacuum sync.

Carbs are being fed from a bottle. Floats set to spec on the bench, and verified by pulling the drain plug and measuring the fuel inside. They're all being filled properly.

[gustovh]

A shot in the dark here, is the fuel line connected between carbs 2/3?
It happened to a fellow that I know, very similar symptoms when his line was connected to the vent T between carbs 1/2

V

[Rich82GS750TZ]

Running it off a fuel bottle as you are means there is no vacuum hose that is normally there from petcock to nipple on the front of # 2 Carb body.

Have you capped the nipple on the front of # 2 Carb body?

[LAB3]

Total disassembly, ultrasonic cleaner for 1 hr at 50C. Solution is 75% Berryman Chem dip, 25% WD40.
Ran a jet cleaner wire through every hole and sprayed carb cleaner through every passage. I have disassembled and cleaned them 6 times now.

Gas is less than 2 weeks old.
.

Had to ask, we've seen no shortage of folks who have done a "thorough cleaning" with a spray can on the outside of the carb.

Rich brought up a good thing to check for sure.

Many of us have had "carb problems" turn out to be ignition related, specifically spark plug cap resistance, an often overlooked item. My last three bikes all had at least one cap that was beyond spec, should be 10k ohms on yours. Not sure I can add anything more on this one.

[tom203]

Your pilot system ain’t delivering..bike needs this perfect to have decent idle and smooth transition to higher speed.
The fuel bowl has feeder tube that sticks onto carb body…..this is small along with the small hole in bowl bottom that feeds it….this is easily clogged and prevents fuel from reaching the pilot system above the throttle plate area. The ”choke” system has its own fuel and air delivery setup, so bike can start with “ choke” , but idle will be erratic if pilot system ain’t delivering.
check your mixture screws…should be pointy, not blunted. Lightly seat them then back off 3 full turns

[Suzukian]

This is way out of left field, but make sure your valves aren't adjusted too tight.

[CrazyOtte]

What should the secondary resistance (measured across the 2 plug wires) be? I have one at 60k and one at 31k ohms.

[LAB3]

Can't say I know for sure but 15k SEEMS to be in the ballpark, you'll need to ask/dig/Google further on that. Did you pull the spark plug caps off the wires? They're removable, just hold the wire firmly and twist the cap counter clockwise while pulling on the cap.

The cap itself should be 10k ohm, supposedly they can be disassembled and the resistor removed and replaced with a piece of heavy wire, screw, etc. Even if they test good you'll probably want to clip about 1/4" of wire off to get to clean metal for a better connection.

Since non-resistor plugs are getting scarce these days you might want to consider going with non-resistor caps and resistor plugs which are easy to find. Got my last set of caps at AutoZone for $2 each. NGK part #LZFH They're a simple 90° bend that does what it needs to.

[Chuck]

I had a similar problem after dipping, rinsing followed with ultrasonic cleaning. Bike (1981 GS1100E) would run on choke but off choke one or more cylinders would drop out. The Home header pipe was cooler so I knew that was the problem cylinder. I ran the carbs through ultrasonic again and changed the way I cleaned the pilot jet circuit (Tom was right; that was my problem). I had been trying to blow gumout carb cleaner through the pilot circuit holes visible when you open the throttle/butterfly but that was nearly impossible to do. I tried putting the pilot jet and rubber cap back in the carb body and then blew gumout through the air jet which I had left in while holding the butterfly partly open and putting a finger over the pilot fuiel mixture screw hole (the screw was out). I could see fluid squirting out all the small holes in the throttle body (I had already cleaned the brass inlet tube with small carb cleaning needles so that wasn't the problem). Next time I fired it up it idled and ran fine off idle with the choke off. I hope that helps.

[Chuck]

Now I know I need to escape the "#" symbol as in "#1" cylinder...

[Rob S.]

Too bad you don't have a whole bank of good carbs off another bike to positively rule out anything other than carbs. Do you?