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Post by davidn88 on Oct 9, 2021 12:26:39 GMT -5
Hey guys  I decided to replace the 12mm stock Weber with a 17.5 Dellorto, but I'm having some issues with this! The carburetor was a bit faulty, and I decided to replace all the gaskets and replace the auto choke for a manual one. This 17.5 is the ID version, which has: - A22 Needle - 53 Main jet - 40 Idle jet I also replaced the Sp1 stock air filter box, with the Sp2 air fiter box + Malossi red hiflo, and I'm running the bike on premix (20ml/L). I knew the 53 main jet was way short than I actually needed, so I started with a 60 main. Before I replaced the carburetor, I warmed up the engine with the 12mm to avoid some cold issues with the new carburetor once I start it for the first tie. So, on the first time, I snugged in the idle air screw, and I tried to work from there, but just wasn't possible. The engine was clearly running lean, so I decided to go with the larger main that I had (76). With the 76, the bike runs fine, but the idle air screw is just a half of a turn out, so I will bump it a size up. But even with the cold engine, I can't switch the manual choke ON, because the engine just boggs and dies right after, and once I start it, the engine stays on higher Rpm's and after a minute or two, it just decreases the RPM, which is driving me nuts, once I can use the manual choke, because it looks like it's running on a auto one! One more thing that is annoying me, is while on half of throttle, the engine can't stay steady because it looks like he "wants more". And once I give it a little more, it goes crazy and keep pushing really well, but I feel a stuttering when I want to run on a medium pace. Any tips here? |
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Post by 90GTVert on Oct 12, 2021 10:09:07 GMT -5
What's the engine setup? Some time ago you posted about a C16 + 70cc BBK setup that you were contemplating. If that's what it is now, some of that part throttle action where it stutters and then goes hard could be par for the course. More race oriented pipes will have a dead zone where they don't make much power and then they'll suddenly pickup and go when RPM increases. You kinda have to keep them sputtering rich to some degree when they're out of their powerband or they'll be too lean when they're in the powerband.
My first suggestion is to get a main jet kit. A 53, 60 and 76 is not a good tuning supply. You need to take smaller steps with the main jet. Everyone has their own process to some degree, but I like to try to get the main jet right or close before I worry about other areas.
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Post by davidn88 on Oct 13, 2021 4:24:52 GMT -5
What's the engine setup? Some time ago you posted about a C16 + 70cc BBK setup that you were contemplating. If that's what it is now, some of that part throttle action where it stutters and then goes hard could be par for the course. More race oriented pipes will have a dead zone where they don't make much power and then they'll suddenly pickup and go when RPM increases. You kinda have to keep them sputtering rich to some degree when they're out of their powerband or they'll be too lean when they're in the powerband. My first suggestion is to get a main jet kit. A 53, 60 and 76 is not a good tuning supply. You need to take smaller steps with the main jet. Everyone has their own process to some degree, but I like to try to get the main jet right or close before I worry about other areas. First of all, thanks for you help  I haven't done those mods, because I still need the scooter on my daily basis, so for now, it remains with the engine stock + air filter box from Sp2 with Malossi hiflo filter + 17.5 Dellorto. I tried the 76 jet, because with the 60, I couldn't even start the engine because it would rev like crazy, even with the idle screw completely snugged in. So I tried to start with a larger main jet (because I didn't have pilot jets in hand to try), and the there were some massive differences. I have 60, 62, 64, 65, 66, 68, 70, 72, 74, 75 and 76 main jets, that's why I haven't tried anything larger, because with this one, the engine runs well, even considering that the manual choke can't be activated because the engine just boggs out once I start it. |
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Post by 90GTVert on Oct 13, 2021 6:43:02 GMT -5
Have you tried changing needle clip positions to see if you can get an improvement?
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Post by davidn88 on Oct 13, 2021 13:34:50 GMT -5
Have you tried changing needle clip positions to see if you can get an improvement? Yeah, I did it. I was running on the 2nd position from top to bottom (has 5), and today I swtiched to the 4th position. Still have the stuttering on half throttle, but it seems a bit smooth. Tomorrow I will test with a new spark plug. Btw, in which situation I could change the needle? |
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Post by 90GTVert on Oct 13, 2021 17:36:16 GMT -5
Needle position should have the most effect from roughly 1/4 to 3/4 throttle, but it can definitely change idle properties and may make small differences in WOT performance.
Raising the needle (moving the clip toward the bottom grooves) will richen the mixture. Lowering the needle (moving the clip toward the top grooves) will lean the mixture.
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Post by davidn88 on Oct 14, 2021 4:34:45 GMT -5
Needle position should have the most effect from roughly 1/4 to 3/4 throttle, but it can definitely change idle properties and may make small differences in WOT performance. Raising the needle (moving the clip toward the bottom grooves) will richen the mixture. Lowering the needle (moving the clip toward the top grooves) will lean the mixture. Yeah I know, but in which conditions people replace the needle? As I said, yesterday I changed the needle position and by the evening I took the scooter for a ride...Well, after pushing hard for a bit, I noticed that the engine was bogging a bit... One thing that I noticed lately, is when I push hard for a while, the engine looks a bit sloppy after that, and it doesn't run with the same power...This was even clear yesterday. |
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Post by davidn88 on Oct 14, 2021 7:24:49 GMT -5
Quick update here. So today I placed a new spark plug after warming up the engine. After going flat out, I stopped the engine to check the spark plug and I got surprised with this:   I don't understand this...On the 12mm stock carburetor, I was running de 75 main jet. In this new 17.5mm the 76 main that I got in it, it's way lean!!! |
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Post by 90GTVert on Oct 14, 2021 10:59:13 GMT -5
Yeah I know, but in which conditions people replace the needle? Most commonly, when you can't get 1/4 to 3/4 throttle right with needle position changes, assuming the pilot and main jet close. I don't understand this...On the 12mm stock carburetor, I was running de 75 main jet. In this new 17.5mm the 76 main that I got in it, it's way lean!!! That's how it typically works. The bigger the carb, the bigger the jet. You also said you replaced the airbox and air filter. Less restrictive inlets are sure ways to require larger jets. |
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Post by davidn88 on Oct 14, 2021 11:44:32 GMT -5
Yeah I know, but in which conditions people replace the needle? I don't understand this...On the 12mm stock carburetor, I was running de 75 main jet. In this new 17.5mm the 76 main that I got in it, it's way lean!!! That's how it typically works. The bigger the carb, the bigger the jet. You also said you replaced the airbox and air filter. Less restrictive inlets are sure ways to require larger jets. Even considering that it's a full stock engine, is it really necessary a jet that big? |
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Post by 90GTVert on Oct 14, 2021 13:05:13 GMT -5
It's not always necessary, but generally a larger carb will need a little larger jet. The simple way to think about it is more airflow needs more fuel. If you don't think you need more airflow, then what's the bigger carb on there for? You can look at the way manufacturers sell carbs to see it. Here are examples :
Naraku 17.5mm PHBG : 70 MJ
Naraku 19mm PHBG : 75 MJ
Naraku 21mm PHBG : 90 MJ
SWR 26mm : 122 MJ
SWR 28mm : 125 MJ
SWR 30mm : 142 MJ
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Post by davidn88 on Oct 14, 2021 13:51:50 GMT -5
It's not always necessary, but generally a larger carb will need a little larger jet. The simple way to think about it is more airflow needs more fuel. If you don't think you need more airflow, then what's the bigger carb on there for? You can look at the way manufacturers sell carbs to see it. Here are examples : Naraku 17.5mm PHBG : 70 MJ Naraku 19mm PHBG : 75 MJ Naraku 21mm PHBG : 90 MJ SWR 26mm : 122 MJ SWR 28mm : 125 MJ SWR 30mm : 142 MJ Absolutely, you're right! The thing is, this 17.5 PHVA, came with the OEM 53 MJ. Honestly this shocked me! Today I made some tests... I thought the problem could be the larger carburetor + the airbox (because I changed it was well, and it allows a bit more airflow). So I tried to change the box for the OEM one, and the spark plug was running exactly the same color! I already bought some jets, but they only arrive next week...Despite this situation, I tried the 12mm stock carburetor again, just to read the spark plug, and this was the result:  Near perfect, isn't it? |
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Post by 90GTVert on Oct 14, 2021 14:20:03 GMT -5
The plug looks OK. I don't use the plug color for much other than ballpark though. I do test runs back to back with different main jets and record results for WOT tuning. Part throttle is pretty much just feel without CHT and/or EGT gauges, so make sure it responds well and if in doubt let it sputter because rich is more safe. Idle is mix screw and speed screw adjustment for a good idle speed and quality and no hanging idle (hanging may also indicate lean needle setting), unless they can't get where I want without changing a pilot jet or something else. www.49ccscoot.com/faq/carbtune.html |
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Post by davidn88 on Oct 18, 2021 19:04:26 GMT -5
The plug looks OK. I don't use the plug color for much other than ballpark though. I do test runs back to back with different main jets and record results for WOT tuning. Part throttle is pretty much just feel without CHT and/or EGT gauges, so make sure it responds well and if in doubt let it sputter because rich is more safe. Idle is mix screw and speed screw adjustment for a good idle speed and quality and no hanging idle (hanging may also indicate lean needle setting), unless they can't get where I want without changing a pilot jet or something else. www.49ccscoot.com/faq/carbtune.htmlI'll have a read on that! Thanks |
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Post by repherence2 on Oct 22, 2021 19:12:16 GMT -5
Have you tried different idle jets yet?
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