After aprox. 10 sec at WOT engine starts to severely bog like float bowl is running out of fuel. Fuel delivery to the carb checked, everything ok. Checked float level, keihin value is useless here because it is impossible to set such a low value (16 mm from top of the float to the carb mating surface). Tried to set it for a higher fuel level but overlfows immediately. O rings inside the carb checked, one was damaged, fixed it with a sealant but problem is still there. What else could cause it to run out of fuel?
problem with 28mm OKO
- Thread starter narx
- Start date
The standard float hight (for Keihin PWK 28 ) is NOT 16 mm but 19 mm!!!
mopedadenyou
island rider
main jet too small???
Damn, you are right, I forgot where I found 16mm value. Anyway when I checked it was around 20mm so that was near correct height. I tried to set it a bit higher and carb was flooding then I set it back to stock value. Tomorrow I'll double check it, thanks.
What size main jet are u running in your pwk on 180cc?
Don't think so, it happens only after a long run at WOT.
mopedadenyou
island rider
wat happens if you dump the throttle
50cczip racer
Well-Known Member
fuel delivery problem from petrol tank either breather or vacuum tap worth checking, try a remote fuel supply they can appear fine but over a time flow can reduce to a trickle
and some inlet manifolds used can tip carb up too steep and cause starving problems
also check pump is working fine and vacuum pipes arent split or perished
and some inlet manifolds used can tip carb up too steep and cause starving problems
also check pump is working fine and vacuum pipes arent split or perished
When I pull the fuel hose off the carb, fuel flows out like crazy. Vacuum valve has been stuck in open position for the last few years, but I still have vaccum hose connected to the carb just for precaution.
Carb is mounted in upright position.
Just checked the float level, it's 23mm.
When I set it lower yesterday it was still higher or the same as 19mm and carb was overflowing. God knows does this value applies for an OKO, maybe the float weight/shape is not the same as keihin.
Anyone has an OKO laying around and is willing to check the float height, I would appreciate it very much.
Here are the instructions I followed:
http://www.carbparts.com/keihin/needles_tuning/jetting_your_carb.htm
I also checked that tunnel connecting fuel tap on the carb with a float bowl is not clogged, it's flowing amount of water at least 4 times the main jet could flow, so that's not the cause either.
When I set it lower yesterday it was still higher or the same as 19mm and carb was overflowing. God knows does this value applies for an OKO, maybe the float weight/shape is not the same as keihin.
Anyone has an OKO laying around and is willing to check the float height, I would appreciate it very much.
Here are the instructions I followed:
http://www.carbparts.com/keihin/needles_tuning/jetting_your_carb.htm
I also checked that tunnel connecting fuel tap on the carb with a float bowl is not clogged, it's flowing amount of water at least 4 times the main jet could flow, so that's not the cause either.
Last edited:
tiba_karotsu
OVERREV TUNING
that is very strange though, maybe you are consuming more fuel that the fuel delivery system can supply, as for jets you should start at 140 & work your way down leaner. i do some mods to the carb to make sure i don't experience fuel starvation at WOT on my dragscooters.
I am at 148 main, thats the biggest i have at the moment, have a feeling it runs a bit flat at WOT, like last 1/4 doesn't make any difference, maybe it even runs better at 3/4 than at WOT, hard to tell is it too rich or lean. Too bad i don't have bigger main jet to make it sure.
Tomorrow I'll try a smaller main, but i tried with 130 and it bogs on quick throttle burst, I tried with 140 and cant tell a difference between 148 and 140.
It's hard to believe that it could consume more gas than fuel system can supply, but we'll see tomorrow. It's like 5 degrees C over here and don't have a good time messing with damn thing
Btw, I have 180cc piaggio engine and open air filter.
I am at 148 main, thats the biggest i have at the moment, have a feeling it runs a bit flat at WOT, like last 1/4 doesn't make any difference, maybe it even runs better at 3/4 than at WOT, hard to tell is it too rich or lean. Too bad i don't have bigger main jet to make it sure.
Tomorrow I'll try a smaller main, but i tried with 130 and it bogs on quick throttle burst, I tried with 140 and cant tell a difference between 148 and 140.
It's hard to believe that it could consume more gas than fuel system can supply, but we'll see tomorrow. It's like 5 degrees C over here and don't have a good time messing with damn thing
Btw, I have 180cc piaggio engine and open air filter.
I've built a few 180cc engines I hope I can help for you! First of all I have to tell you, your carburettor choice isn't favorable in this case. For the standard engine (I know you have a little porting job), with a standard exhaust the 28 mm carburettor is a little bit too much, with that exhaust + configuration.
You can't exploit the 28 mm carburettor a 24-25 mm carb is even enough. Your fuel consumption increases a lot with this carburettor, but the perfomance gain is smaller than estimated your engine "waste the fuel"!
However if you would like to use it, you can adjust it without any supplying problem.
For the 28 mm PWK + tuned 180 cc engine I use 140 size main jet, and for the 200cc engine I use size 145, but all the maschines have a race type expansion chambers direct air filter, custom made combustipon chambers and not a simple exhaust!
In my opinion the 148 is too big for your engine.
In general case if your engine runs better or the same at 3/4 throttle than WOT your mainjet is too big!
You should feel if your bike runs lean! The engine soud is weaker and it suffers!
"i tried with 130 and it bogs on quick throttle burst"
I think you have wrong imagination about carburettor tuning! The mainjet is not for accelration or for throttle response. So do not adjust the part throttle with the mainjet, but with the needle position or with needle profile. Between the closed throttle and small openings the idle jet also has an effect!!
As I wrote several times in other topics, the original PWK (Keihin, Stage6, Koso) needles are not suitable for the 180cc engine, doesn't matter if we talk about race bike or standard bike, the needle profile is for 70-80 cc crisp bikes so you have a hard job if you want to use the original needle.
The 180 cc engine doesn't need so much fuel at part throttle or rather doesn't need big cross-sections because the strong vacum pulse, the vaporization is better. A quite fat type needle, with a long "straight" profile, and a steeper end profile, for higher rpm is good.
Because I couldn't find the right one at the Keihin N427-46xxx series I made a custom one, but you can give it a try to the optional N427-68 series. I don't know them but they are dual taper type so maybe they are good. There are a very cheap needle series at scooter-center you may find the right one for your bike:
http://scooter-center.com/catalog/product_info.php?cPath=4403_6331_6332&products_id=11607 9 pieces of needles only 13 euro!!
I heard and read a very good results with 28mm carb with stock pipe, with a consumption of 5-6 liters per 100 km.
I know I can't compare my bike with yours highly tuned machine. I asked for a main jet size u use just to have rough idea on what size main jet I might use. I base my jetting on engine sound and power delivery, I am not an expert but I'm giving my best.
I tried a procedure on determining correct pilot jet I found somewhere on the internet. I removed the main jet, put the JJH needle in a mid position and set the pilot jet (40) so that my mixture screw is about 1 turn out and tested the throttle response, everything was just fine, engine reved more or less crisply. Then I put the main jet in, and with 130 main it bogs when I wack the WOT at idle, with 140 it doesn't bog. I'll test it again tomorrow with a smaller main and another needle/needle position.
I haven't found original needle that bad, bike runs pretty much ok, no severe sputtering at low throttle openings (1/4) as i thought it would be, just a little bit like with stock carb, probably it could be better but I tried only with a needle in mid position without any fine tuning.
I have another BGM needle set with 3 needles, I tried the fattest and didn't feel any difference.
I still haven't solved the problem of carb running out of fuel, today I set the float height to 21mm, carb doesn't overflow and I'll test itomorrow how it runs. If it won't be ok, I'll try to set it even lower.
For pilot jet tuning: Let engine idle, turn in air screw until RPM slows down, (Too Rich). Engine will die at this point. Restart engine after adjusting air screw out 1/2 turn. This will have you at the proper air fuel ratio for the pilot jet. ... Standard deal on the pilot jet. Air screw 1/2 turn or less out and pilot is to small. 2 1/2 to 3 turns out, pilot jet is to big.
Needle jet: Go rich a clip position until the motor doesn't run good then lean it out one position.
Main jet : Go big until the engine will not take any more fuel then back off 5 to 7.5 points.
So find out if you can go richer on the needle , then find out what is to big on the main and back down.
You must have a clean fuel supply .
Needle jet: Go rich a clip position until the motor doesn't run good then lean it out one position.
Main jet : Go big until the engine will not take any more fuel then back off 5 to 7.5 points.
So find out if you can go richer on the needle , then find out what is to big on the main and back down.
You must have a clean fuel supply .
