GT750 Left Cylinder Again

[Striker1423]

I'm stumped.

This bike has the CV carbs at stock settings:

• 110 main on the left cylinder, 107.5 in the center, and 110 on the right.
• 110 butterfly's, and 0.8 air corrector jets installed.
• Timing was done per Uwe's instructions for the Accent Ignition and set at 3.41 almost 3.42 on the center cylinder (as close as I could get it).
• I bench-synced the throttle blades.
• I also checked the diaphragms statically and running. No observed flutter they all drop the same when the vent is thumbed over.
• Fuel screw appears to make no change on the left cylinder (except to kill it completely.
• Center and Right cylinders running as normal as I would expect.
• Compression is 120 on the left, and 119 on the right. Center wasn't checked (runs fine).

The Problem:

When I start the bike, it takes choke for an extended period. The RPM's go up to about 2500 or so, and all three cylinders seem to run ok. Choke comes off once the RPM's drop and the bike struggles to idle. Notably, the left cylinder barely runs. It sputters and is noticeably weak with each exhaust puff. Plugs when pulled are oily and look wet on the center and right. Almost nothing on the left. Also, very low oil seen on left plug or out of the exhaust compared to the other two.

Things I've tried:

• If I adjust the fuel screw out, the cylinder dies completely (more fuel).
• Adjusted left carb throttle blade open outside of sync (picks up cylinder, RPM's stay high.
• Felt intake pulses of all three pots (all feel similar).
• Cleaned carb without complete disassembly (bowl, jets, passages) all appear clear and flow well.
• Changed spark plugs more than once.
• Checked for spark on head (appears consistent).

Things I haven't done:

• Checked/swapped coils against each other (Not sure how, unless I swap the actual wires in the plug, and then swap the plug cable to another cylinder.)
• Taken the carb rack apart to dip the carb.
• Checked fuel height (only float, may try to make a fuel height gauge out of a main jet plug).
• Rechecked the timing.
• Rechecked for an air leak (no leaks when tested in the bike prior to running).

I'm leaning into it being a fuel issue or possibly the one coil, but the lack of visible oil on the plug also has me worried a bit. However, it does smoke normally on choke and the compression checked out fine. The Accent ignition plate isn't completely centered, from my point of view it appears to favor the center and right cylinders in proximity to the pickup. But, I don't think that would be an issue since it appears to spark ok and is just a touch off-center (slots allow for this a tad).

Anything else I'm possibly missing?










 

[m in sc]

raise fuel level, float level, see if it helps.

[Simmons1]

While you are poking around at the coils check what voltage they are getting.

I was chasing some carb and charging system issues on my Buffalo and discovered by accident the wiring had a big voltage drop and the coils were only getting 8.5 volt. I wired power to the coils straight from the battery thru a relay.

The difference it made was astounding. Much smoother idle and low RPM throttle response before I even looked at the carbs.

[Striker1423]

Raising the float height seems to have fixed it. I also bench synced the carbs again. I'll double check everything once the airbox is back on it.

Idle is much better and more even.

[SoCal250]

Raising the float height seems to have fixed it. I also bench synced the carbs again. I'll double check everything once the airbox is back on it.

Idle is much better and more even.

[teazer]

I usually check FUEL level with a small adapter.  It is surprising how often the float level looks OK but fuel level is wrong.

It's a good idea to check ignition timing with a strobe lamp, though those motors don't seem to care if they are a few degrees out - even ones with high compression and heavily ported.

Unfortunately I cannot access my FB account for the moment.

[Striker1423]

I usually check FUEL level with a small adapter.  It is surprising how often the float level looks OK but fuel level is wrong.

It's a good idea to check ignition timing with a strobe lamp, though those motors don't seem to care if they are a few degrees out - even ones with high compression and heavily ported.

Unfortunately I cannot access my FB account for the moment.

Oh ok, I thought you were mad at me.

I will try to buy or make an adapter. Evans Ward?

[teazer]

I had my local machinist make a slightly longer drain bolt as a banjo bolt and used a spare oil feed banjo and some plastic tube.

A simpler solution would be an adapter with the correct fine thread on one end and a barbed other end to slip clear plastic tube over. Z1 enterprises had a suitable one IIRC or see if Evans Ward can machine up a few. I did buy one from him a while ago, but I think that was for an RD with different thread.

[Evans Ward]

This may be the last one I have on hand for stock CV carbs 74-77 GT750 and yours Striker1423 for the cost of shipping if you want it.

[Striker1423]

I think I actually ordered one from your eBay Evans, haha.

[teazer]

If he didn't take that last one, please let me know and I will take it...

[Evans Ward]

If he didn't take that last one, please let me know and I will take it...

It's yours Richard!  PM me your mailing address and will get it out to you.

Evans

[Striker1423]

OK, so here's an excerpt I read on the kettle club forum. I'd like to hear opinions on the accuracy of this.

- Basically, unless your throttle butterflies are virtually closed tight on a fully closed throttle, you will experience surging to one degree or other and yes, there are certain other variables that can exacerbate this, but let's concentrate on the carbs. So getting the butterflies somewhere near right is achieved in the first instance, on the bench. The valves need to be bench set as per the Suzuki manual but, what the manual doesn't tell you, is that the No.3 carb is the master and should be adjusted first, using the idle adjustment screw. with No.3 set, you can adjust the other 2 via their screw and locknut set up or whatever set up your carbs have.

- Once that is done and the carbs are reinstalled, you need to then set the pilot screws at the factory base setting of 3/4 turns out. You then need to employ the services of some make of vacuum gauges but, unless they are specifically designed for 2 strokes, and I don't know any mechanical types that are; then they are extremely difficult to use correctly in order to obtain the optimal set up. I you are persistent, like I was; you'll get the balance there or there abouts. But, then there is the art of setting the pilot screws of each carb so that you actually get the bike to idle at about 1050rpm by just pilot screw adjustment i.e. without use of the idle speed adjuster, which should in essence, be backed off to the point where it is not being used. I found that about 1 turn out was ok for my bike.

and further...

- What is critical here is as stated earlier - the throttle butterflies must be aligned on the bench to open at exactly the same time, and any adjust after that is by adjusting the pilot screws ONLY - do NOT re-adjust the butterfly angles once set. Although not very well described in the Suzuki new-type carburetor service manual from 1974, this is the procedure stated by the factory. These pilot screws do NOT adjust the air/fuel mixture, they are in fact miniature throttles designed to run the engine at tick over with the butterflies as closed as possible.

What I'm reading is that the throttle blades need to be aligned perfectly and NOT adjusted again? That only the pilot adjustment is necessary to achieve proper idle? Well, so far I can get it close, but once the carbs are in the bike, I can't see the throttle blades anymore and the idle knob absolutely needs turned in to make the bike run, so this time, I'm trying to make only extremely slight adjustments (less than an 1/8th of a turn to see what it does on both the blade and pilot screws.

I can get the middle and left carbs to "mostly" agree, but the right carb is always the best runner, and the left is mostly the weakest of the three. Usually I get within a few mmHG of each other on my Carbtune and
the Digisync is even closer, but since it's only numbers on a screen so it's much harder to see what's what with the blip of the throttle. What is constant is the right carb is always the highest vacuum reading. I can't seem to bring the other two up to meet the master carb without really cranking on the throttle blade adjusters. When I do, the idle hangs and is slow to respond, etc. 

I think my sync process is off or out of step somehow. Thankfully the bike mostly idles OK, but it's apparently a mystery to me to get a nice crisp throttle response and run-down after closing the throttle. No air leaks and float heights should be fine. I'll check the fuel height as soon as Evans adapter gets here on Monday. 

[Simmons1]

Thankfully the bike mostly idles OK, but it's apparently a mystery to me to get a nice crisp throttle response and run-down after closing the throttle. No air leaks and float heights should be fine. I'll check the fuel height as soon as Evans adapter gets here on Monday. 

You just described pretty much how my Buffalo runs. That said, my bike is sporting Higgspeed chambers, and foam pods.

I would tend to believe what you posted above from the Kettle Club that it is virtually impossible to have zero surging.

[Striker1423]

Got another reply to another poster, but we both have idle issues:

Bench setting is not sufficient for these carbs, particularly with their age and wear. Dynamic setting for "your" engine with "your" carbs will be required. Pilot screws are best set nearer to 2 turns out (these are "volume" screws, not mixture/air screws) they give you a good tickover, so you can turn the throttle stop (knurled adjuster) right down. It's having the volume screws AND throttle stop controlling tickover, that causes surging. Pilot screws do not cause rich mixture, and smoking is not a sign of that. 2 stokes do smoke. Briefly that is the reply now. Dynamic setting with "very" careful carb balancing is the answer.