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Carb Jetting Weird!

netwurk13

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I understand that every bike has different requirements for carb jetting, but I feel this is super weird. Let me know your thoughts.

I just finished a build and now I'm trying to get the carbs in line. I changed to UNI air filters and a 4 into 1 exhaust. I expected a significant increase in air flow, but my pilot jet seems to like it the best at the stock 35 jet 1 1/2 turns out. When reaching around 4000 rpms it bogs down. I heard major exhaust popping, but NOT on decel. I currently have a 140 main jet. I've gone up to a 160. I thought for sure it was lean, but it seems like even with the mods, it might want a 118-120 main jet. Does this seem too lean considering the mods?

I've been battling a high idle hang, that seems to get worse the richer the mixture. Even though I'm running 35 pilot jet. I've checked for vacuum leaks a million times, rebuilt carbs to the T including the felt throttle shaft seals, new intake boots, completely tight, etc. Float height tested 10 times. Carbs have been vacuum synced three times. Throttle cables smooth as silk. No binding. When the pilot jet is at 1 1/2 turns out im able to stop the high idle by pushing the throttle closed, but when releasing the throttle you can tell the throttle shaft is returning perfectly back to the idle speed adjustment screw. When I make the pilot circuit richer (thinking its a lean high idle), it gets worse, and I'm not able to stop the high idle.

Almost seems like she wants to be at completely stock settings. Could this be the case. I find it weird considering the mods.

supp.jpg

*straight from k8 supplement of shop manual
 
What type of filters are the UNI filters? As far as getting the jetting correct you need to ride it....when you get to the trouble spot start closing the choke. If it starts to pick up as the choke closes then its lean....if it make no difference or makes it bog down more then it is rich already. You also didnt mention anything about adjusting the slide needle jet......the needle has a bigger affect on over all jetting then any of the jets in the carb. It is a pain to adjust in the keyhole carbs as there is only one c clip slot in the needle so you have to use shims. Air filters and exhaust I would not expect much more than one or two jet sizes different. I dont even need 160 jets on 970cc engines so you are way rich using those on a stock displacement 750.

About half the time a hanging idle is caused by the ignition advancer. They either get sticky, worn out or the springs get weak or a combination of worn advance weights and weak springs. On top of that if the timing is already off or the idle is set too high, the worn out advancer will cause the idle speed to be all over the place never wanting to settle down.
 
I agree completely. I have an aftermarket points plate and advancer. I took the advancer apart and lubricated the cam, etc. Seems like their is no binding. The ignition timing is spot on during my test. Doesn't stay stuck advanced.

I agree about shimming the needles. I need to do that, but I'm trying to get as close as possible with pilot and main jets, then fine tune the needles
 
The needle goes hand in hand with the other jets, is more important than the main jet in fact. Main jet is wide open throttle only, needle position determines drivability across the whole rpm range other than absolute idle and wot.
 
Good to know. I didn't know that. So if I put shims it will make it richer, does that mean in my case when it hits approx 4k rpm I'm running lean? You can't make the midrange leaner by adding shims correct?
 
Yes adding shims makes it richer. Like said, use the choke to determine if you are rich or lean then adjust accordingly. You need to go back to a a sensible main jet 115 to 120 range at the most.
 
So far, neither with pilot circuit nor main circuit does choking the engine cause improvement. Btw, the uni air filters are like pod filters, but foam, and use oil.
 
Dont like foam filters, to easy to over oil and cause air restriction and other problems. Foam filters need foam filter oil and need to be oiled very lightly. Most people grossly over oil foam filters. This causes everything to be overly rich as it restricts airflow as well as having oil sucked in while running. If the choke makes no difference or worse then it is overly rich.
 
Yes, I agree. Oiling can be very subjective altering the amount of air allowed into the cylinder. Thereby, altering A/F ratio. I'm super frustrated. Today I went to stock 110 jets and 35 pilot as a start. Had to turn the pilot out more than three turns for it to behave nice, but even then not enough. Threw in a 38 pilot and had to have it at 1/2 turn out. While reving at stand still it works great, but when you ride her its not enough. I'm convinced during all these trials that some fuel at idle is affected by the main jet. I'm going to try 35 pilot and 112 or 114 main. For some reason the main jet definitiely affects my idle. Just ordered the shims for the needle, but it's incredible how everything needs to be in a perfect balance.

keep in mind I am using the choke to assess if im lean or rich. Both at speed and idle. During cranking and running.
 
i must add this . i have a JDM 750FZ and when re building the carbs all the info said 102 main jet so i took out the 75's and put them in. it bogged out at 5000rpm and everything pointed to electrical faults, cracked coil , melted cdi's etc. i found by trial and error it went like stink with the rectangular air filter cover off. i then found out my JDM bike had a smaller intake on the air box than the bikes made for europe and possibly america. too much fuel was killing it. to save taking the carbs off and re fitting the 75's i have left off the air filter cover and now it pulls more air in around the side panel. these bikes are so sensitive to carb settings and air boxes
 
Just an update. I have a feeling all my efforts are for nothing. In the middle of all my carb tuning i had an ignition coil go bad. I swapped it out and just continued. However, today I decided to double check points gap and realized that their is constant arcing on the points from BOTH points. I tried resetting the gap to the upper and lower range of the gap indicated in the service manual. Arcing continued. Not sure why but I'm assuming BOTH condensers went bad (aftermarket).

I had already ordered a dyna s electronic ignition. It should arrive in a couple of days. Planning to pop that in and see what happens.

Btw, I have an electronic regulator/rectifier.
 
Ok so I put in the Dyna S ignition. What a difference. Everything super stable, etc. Much difference with carb tuning in general.

Still have a dilema though. I'm down to the factory 35 pilot jet and a 115 main jet. I'm at two turns out with the pilot jet. Could running a stock 35 pilot with UNI pod foam filters and a 4 into 1 exhaust still cause a hanging high idle when the engine is warmed? I have checked all linkages, seems to be returning great. Throttle shaft comes back to original position with a snap. When the engine is warmed up and i accelerate it comes back down to a high idle. Only way of bringing it down is to push the throttle forward momentarily or put it into a higher gear and slowly let out the clutch. This also brings it back down.

I'm convinced the high idle is not a LEAN high idle. I've checked for vacuum leaks, etc. I just think shes getting too much fuel at some point.
 
WOW. Points arcing - bad condenser. I have seen this problem before causes misfires backfires all kinds of weird looks like a fuel related problem.
Glad you put in a electronic ignition will make bike much more reliable and no fiddling with points.

next step to help trouble shoot -

I would remove the air filters and see how it runs.
 
Ok, so a high idle all the way down to reducing pilot jets #35 (stock) to one turn out. Otherwise great. The only thing i can think of is a high float bowl level that is too high that facilitates fuel getting into the cylinders. The way that I set the float bowl level was to cut a piece of tubing to the stock height. If i remember correctly 14.5 mm. I setup all the carbs right side up, and as soon as fuel started to flow at this level I set it. Could it be that my float height is too high causing the engine to intake more fuel than normal using stock pilot?
 
high float would make it rich causing it to bog down. Lean makes it high. Carb sync, vacuum leaks, lean mixture, hanging ignition advance and or weak advance springs, bound linkage, too tight throttle cables all cause hanging idle.
 
Ok, so a high idle all the way down to reducing pilot jets #35 (stock) to one turn out. Otherwise great. The only thing i can think of is a high float bowl level that is too high that facilitates fuel getting into the cylinders. The way that I set the float bowl level was to cut a piece of tubing to the stock height. If i remember correctly 14.5 mm. I setup all the carbs right side up, and as soon as fuel started to flow at this level I set it. Could it be that my float height is too high causing the engine to intake more fuel than normal using stock pilot?
The above advice is good :agree:. Once everything else in the tune-up is verified, then on to the carbs. You have made several changes in jetting and now are back in the ballpark. Next adjust the accelerator pump.

I've not heard setting float height described exactly that way; it's either upside down, float resting on valve, top of float bulb 14.5mm above gasket surface - or it's with a sight tube; assembled, mounted and fuel level a few millimeters below the top of float bowl. Anyway, I doubt that's the original issue.

I've not read where you have properly "bench synched" the carbs - but more importantly - vacuum synched the carbs while running. These motors are very sensitive to carb synch. A hanging idle, or inability to lower idle to spec, can easily be caused by a slight out-of-synch condition. That condition is often characterized by one or more slides sitting too high (like it's opened by the throttle). Your ability to get a lower idle by pushing on the throttle cable pully indicate the cables have insufficient play or a slide(s) sitting too high.
 
I made sure the accelerator pump was on the money...and confirmed and decloged all the passageways....i confirmed float height with the clear tube method...actually i found it super useful to set float height like that..i cut a piece of tubing 14.5 mm high....then with each carb individually, in the right side up position, and fuel attached, i made sure that the float would barely allow fuel to come in with anything above 14.5 mm....and vice versa...i would make sure that it would shut off fuel when the float level would reach the 14.5 mm tubing.

I absoluetly bench synced the carbs initially, and bench synced the carbs multiple times while running...to the extent that I bought fittings to use only one gauge to rule out calibration issues between gauges..

What I did notice was that my high idle cam settings are not right...i follow the manual to the T. Pull out choke, screw in high idle screw until it rests on the cam, then push choke in and turn min 2 1/2 turns....it does bring up the RPM, but immediately causes crazy high idle...meaning....if I leave it adjusted to those settings...and push the choke in....I cant bring it to a normal idle...I've literally had to do away with the high idle system....so far I have it under control at 1 turn out on the pilot jet and making sure the base idle speed adjustment screw is not too high....any ideas appreciated.
 
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