77 GS550 Carb Help

[Guest]

I am at the end of a complete rebuild of a 1977 Suzuki GS550 which has pretty much consumed all my free time for the past 8 or 9 months now. Partial build thread can be seen here:

http://www.dotheton.com/forum/index.php?topic=69759.0

I've consulted the BassCliff site quite a bit during this build and try to research as much as I can before asking for help but I'm stuck. I'm in the process of tuning the carbs for 4-1 MAC exhaust with shorty pipe and K&N filters and am currently running 15/50/100 jets and the needle is 2 slots from the bottom (I believe). Pilot air screws are out 2 turns and pilot fuel screws are out 1-1/4 turn. It starts right up and idles good. The bike rides pretty good if I increase the throttle slowly to get going. The bike seems to run really good at high speeds but more specifically, higher RPMs. A hard throttle bogs the engine down until the RPMs get going. I had 25 pilot jets in and switched this afternoon back to stock 15 and it seemed to help but still isn't running right under hard throttle. I'll do more testing tomorrow hopefully.

Here's my question, what would be the primary causes of this low/mid hesitation until RPMs get going? I'm thinking needle adjustment is necessary but I can only raise 1 additional notch before I'm maxed. Any suggestions?

[JPM]

One more thing....What's the trick to get the tank off without fuel going everywhere? This vacuum petcock with no off position is driving me crazy!!!

[2BRacing]

One more thing....What's the trick to get the tank off without fuel going everywhere? This vacuum petcock with no off position is driving me crazy!!!

The only time a petcock in proper working order should let fuel through when the engine is not running, is on "Prime". On "Run" and "Reserve" no fuel is supposed to come out if the engine is not running.

If yours does not do that, it is faulty.

[Steve]

The easiest way to tune the carbs is to get real friendly with your spark plugs.

The stock jetting on that bike is 15 pilot and 80 main. I don't understand your 15/50/100, what's the "50"?

ANYTHING that you do should only be ONE step at a time, or else you won't know which change is affecting your readings.

Step back and think for a moment. Why do you need to change the jetting? Because you are increasing the airflow through the engine by removing the (slightly) restrictive exhaust and the somewhat more-restrictive airbox and filter. How much to the exhaust and airbox restrict airfow AT IDLE? Since the answer is NONE, why do you need to increase the fuel? Stock pilot jetting will be fine, until you increase the displacement of the engine. Your setting of 1 1/4 turns is quite a bit, try backing them off to about 3/4 to 7/8. Turn the air screws in to about 1 1/4 to 1 1/2. And don't assume that all the carbs will take identical settings.

For the MAIN jet, yes, you will need to increase a bit. You say you have 100s, that might be a bit much, but your spark plugs are the ones that will tell you. Do a full-throttle run. With the throttle still held full open, hit the kill switch, pull the clutch, coast to a safe spot where you can pull the spark plugs and check the color on them. Repeat as necessary. Do the same process at HALF-throttle to check the needle settings.

.

[JPM]

The 50 I am referring to is the little jet/screw on top of the pilot jet. It screws down on top of the pilot jet and says 50 on it.

Went on some plug chop runs this afternoon trying to identify why it's not running quite right.


Half Throttle Chop - I'm not an expert by any means but they looked a little lean to me due to the smokey greyish coloring. Left to right is cylinders 4,3,2,1





Full Throttle Chop - Again, looks lean but the bike actually would not really accelerate or run well at all under full throttle. Bogged down so I may not have gotten the best readings. This time, left to right is cylinders 1,2,3,4.





Thoughts? Lean at half throttle tells me needle needs to be raised. Lean at full throttle tells me main jets need to increased also. I'm 'leaning' towards increasing the mains 1 and retesting.

[whiterabbitt]

Your way too rich at full throttle and a bit rich at half throttle. The plugs should be lighter than your half throttle chop. Also if you have ethanol in your fuel it will make the plugs look greyish. It makes tuning a pita

[Steve]

I agree, those plugs are looking plenty RICH, not lean.

Did you ever adjust the pilot screws? Back in post #4, I suggested that 1 1/4 turns was too much, especially since you have larger pilot jets in there. They might not affect your mid- and high-throttle readings much, but they DO contribute to the overall fuel delivery at all throttle openings.

Drop the needles one notch. Drop the mains a size, maybe two. Do these changes one at a time, or you will not know which one did the trick.

.

[JPM]

Thanks for the replies. I am currently waiting on a new bowl drain screw and new petcock because I'm tired of dealing with gas drips. I've switched the mains down to 97.5 and will do another full/half chop test. Since I'm waiting on parts, it'll will be a couple days before I'm back together. I re-adjusted the pilot fuel screws down to 1 turn out instead of 1-1/4 and also triple checked float heights and confirmed needle position. I am currently 1 notch above the bottom slot. I went ahead and ordered 95 mains in case I need to take another step down.

[JPM]

Update: I've been playing with the carbs for a while now and have found that 95 mains and the needle raised to the center position is having the best results. Still some hesitation at initial throttle but mid through full is much better. It's idling rough & inconsistent so next step is to re-sync and play with the pilot fuel and air screws a bit more.

[whiterabbitt]

A color tune will help immensely on pilot tuning. I can get mine super close in just a few minutes with one.

A digital tach helps too when setting the air screws.

[JPM]

Carbs have been sync'd and I've messed with the pilot and air screws to the best of my ability. Bike is running pretty good at the moment EXCEPT off of idle. If the bike is idling at a reasonable RPM (sub 1k), cracking the throttle results in major bog down. When I feather the throttle and rev up the RPM a little, if I crack the throttle before the it settles back down to idle, it revs great so I am assuming there is some additional tweaking needed in the pilot jet or pilot/air screws? Any suggestions?