I had to pull the OEM carbs becase she was behaving like I had an air leak. Under acceleration it was slow slow slow and then when the RPM finally hit 7krpm, she would start to accelerate hard.
I had to pull everything out to check for leaks and I did find a few holes in the airbox which I plugged up.
I also found a few of the slides on the OEM carbs a tad sticky due to small burrs that I sanded out of the slide shaft/body. Hopefully these few items and better sealing carb insulators will resolve any issues next time the OEM carbs go back on.
I had also wanted to try out a modification to the gixxer carbs that I read about which should help me with my rich idle problem. Basically I had to drill out the pilot airjet 1 size larger (based on my harbor freight drill bit set) at the pod side of the carb intake to allow a little more air in. It definitely has affected the carb's state of tune because I'm running too lean now even at 3.5 turns out
There is a flat spot around 4k rpm depending on what gear I'm in and how hard I wack the throttle open but it goes away as the engine warms up (indicates Lean condition).
I found the #4 plug to be sooty black and this is an indication of running rich (or poor spark) so I backed off the mixture screw a half turn and went for a 30 mile ride. Acceleration is decently strong through midrange throttle but as I continue to open the throttle I'm getting some missing above 7-8k RPM and a slight acceleration when I back off the throttle which indicates the main jet is a bit rich.
After the highway run she was idling rich and wanted to die out almost like it still had extra gas that needed to be burnt so I had to feather the throttle to keep her running at the red light.
Right now, it seems like I'm running lean in the pilot circuit and rich in the mid-main circuits .... probably I will need a smaller main and a larger pilot (or solder the pilot air jet and re-drill). Unfortunately I'm not in the mood of dropping more money on these carbs so I will take them off and try the stock carbs and solder the pilot air jets to bring them back down to the original size (they are pressed in so they cannot be swapped).
Not sure if I can get my money back on these carbs. I'm not likely going to find a buyer for $200+ but we'll see how the OEM carbs go.
Right now I just want a reliable runner that I can let my wife use if she needs.
I did order a set of igniters from Procom Engineering due to all the raving reviews on the forums and because I'm getting a misfire at high RPM on both sets of carbs. I suspect the GM coils I ran for a season might have damaged the igniters due to their low resistance.
With the OEM Carbs reinstalled, I went out for a few quick shakedown runs and found a slight stumble in the 4k RPM range and only under acceleration. Although I was still accelerating through the rough spot, it felt like there was more to be had if only I could smooth out the acceleration. I eventually found a clogged accelerator pump passage in the #2 float bowl and I tried to clear it out with an overnight soak in pinesol followed by spraying brake cleaner and compressed air. I managed to unclog the passage but not confident that it's as clear as it could be. Also the accel pump diaphragm still looked to be in decent shape so I put the bowl back on and went for another shakedown run. It's smoother but it's still present.
The new igniters are making a solid difference though. smooth consistent acceleration well above 8k RPM where I used to get misfires. The engine sounds and feels much more even and smoother too. If I could fix that one 4k rpm stutter I would be golden. I think I need to just go out and ride now. I suspect it will work itself out with some seafoam and working that accelerator pump more.
Part 1
Part 2
Part 3
Part 4
I had to pull everything out to check for leaks and I did find a few holes in the airbox which I plugged up.
I also found a few of the slides on the OEM carbs a tad sticky due to small burrs that I sanded out of the slide shaft/body. Hopefully these few items and better sealing carb insulators will resolve any issues next time the OEM carbs go back on.
I had also wanted to try out a modification to the gixxer carbs that I read about which should help me with my rich idle problem. Basically I had to drill out the pilot airjet 1 size larger (based on my harbor freight drill bit set) at the pod side of the carb intake to allow a little more air in. It definitely has affected the carb's state of tune because I'm running too lean now even at 3.5 turns out
There is a flat spot around 4k rpm depending on what gear I'm in and how hard I wack the throttle open but it goes away as the engine warms up (indicates Lean condition).
I found the #4 plug to be sooty black and this is an indication of running rich (or poor spark) so I backed off the mixture screw a half turn and went for a 30 mile ride. Acceleration is decently strong through midrange throttle but as I continue to open the throttle I'm getting some missing above 7-8k RPM and a slight acceleration when I back off the throttle which indicates the main jet is a bit rich.
After the highway run she was idling rich and wanted to die out almost like it still had extra gas that needed to be burnt so I had to feather the throttle to keep her running at the red light.
Right now, it seems like I'm running lean in the pilot circuit and rich in the mid-main circuits .... probably I will need a smaller main and a larger pilot (or solder the pilot air jet and re-drill). Unfortunately I'm not in the mood of dropping more money on these carbs so I will take them off and try the stock carbs and solder the pilot air jets to bring them back down to the original size (they are pressed in so they cannot be swapped).
Not sure if I can get my money back on these carbs. I'm not likely going to find a buyer for $200+ but we'll see how the OEM carbs go.
Right now I just want a reliable runner that I can let my wife use if she needs.
I did order a set of igniters from Procom Engineering due to all the raving reviews on the forums and because I'm getting a misfire at high RPM on both sets of carbs. I suspect the GM coils I ran for a season might have damaged the igniters due to their low resistance.
With the OEM Carbs reinstalled, I went out for a few quick shakedown runs and found a slight stumble in the 4k RPM range and only under acceleration. Although I was still accelerating through the rough spot, it felt like there was more to be had if only I could smooth out the acceleration. I eventually found a clogged accelerator pump passage in the #2 float bowl and I tried to clear it out with an overnight soak in pinesol followed by spraying brake cleaner and compressed air. I managed to unclog the passage but not confident that it's as clear as it could be. Also the accel pump diaphragm still looked to be in decent shape so I put the bowl back on and went for another shakedown run. It's smoother but it's still present.
The new igniters are making a solid difference though. smooth consistent acceleration well above 8k RPM where I used to get misfires. The engine sounds and feels much more even and smoother too. If I could fix that one 4k rpm stutter I would be golden. I think I need to just go out and ride now. I suspect it will work itself out with some seafoam and working that accelerator pump more.
Part 1
Part 2
Part 3
Part 4
followup: a new accelerator pump diaphragm and that 4k rpm stumble is 90% gone. I suspect the passages might be a little clogged, but she's good enough to run now.
ReplyDeleteNext thing to fool around with is muffler options.
I know its been a while since you posted this nice little blog, but did you go back to your std. carbs, and sell your GSX model?
ReplyDeleteI am curious since i want to put pods on my 79' CB750 DOCH. On all the forums, the treads are flooded by people saying, stick to the standard filter and carburetor. However, if you want the clean open look frame, there is only one way, remove the filter.
That's why i liked your blog! You did the job, and listet the pro's and con's. I think i need to go for a jetkit, as i have heard that this should help sort some of the problems you experienced.
To me it also sounds like your carbs were adjusted to cold settings. Wouldn't it have been possible to make the fuel mixture more lean in cold settings, knowing that you would then have to use the choke?
I believe that changing the carbs is a matter of, what is the bike used for. I want this to do small trips around the city, so i can accept the challenges and hours of fine tuning that come with this change. If you use the bike every day for work, keep it standard!
Last question for you, if you should do it all over, would look for a renovated carb. with a jetkit, or would you do it as you did and renovate some old "junk"? (or would you consider a more expensive RC carb?)
I got caught up in a relocation and my life has been on hold until I get settled in a new house. I have not ridden in over a year but all my bike parts are in storage at the moment with no place to tinker. I have a new bike but haven't had the yearning to ride in my current limbo.
Delete-I have gone back to standard on my 750 but I'm running aftermarket emulsion tubes from a carb clean kit that seems to be messing up my acceleration.
-A jet kit is important if you plan on doing the carb swap, there is bound to be some fine tuning required to get it running right. If you've got the patience and a spare bike you should definitely go for it.
-If I were doing it over again I would not consider an expensive pre-configured carb since they run more than what these bikes cost. I think I would purchase a stock set and a jet kit and go from there. I may still go this route since I still have my gixxer carbs. We will see what happens.
July 2019 - I've resisted the urge to go to pods on the original carbs but I think I have the bike performing optimally this year. I'm very happy with the acceleration with the stock carbs and the airbox. The 4-2-1 exhaust gives me a solid improvement over the oem 4-4 exhaust and also gives me a significant weight savings since I've also removed the center stand. I would like to try pods next year but everything is just working so good after getting it back on the road this year.
ReplyDelete