100% throttle cut out on CR125

[Slowerthanu]

ok so i have a Kart with a 99 CR125 motor by swedetech. i was having trouble starting it a few weeks ago but it starts first turn now no problem. Reeds are good, compression is great, pumps are great, motor is strong as hell with partial throttle. the problem now is that i am not able to go full throttle. every time i do it cuts out on me. not sputtering of any kind, it just cuts out as though i let my foot off the gas completely. the Mikuni carb on it was extremely dirty because the guy who i bought it from left gas in it for about 4 years so everything needed cleaning. its like new now though. i replaced all the jets and the needle but still no luck with full throttle. 0-75% is great and very strong but every time i try to floor it it cuts out. i tried playing with the needle settings but that only seemed to make a difference with the mid range throttle. i have tried main jets ranging from 180 all the way down to 155. now i live in colorado and the weather lately hjas been around 45-50 degrees so im assuming i need more fuel but with the more dense air but i literaly felt no difference at all between the 155 to the 180 main. do i need to go even higher? the plug keeps reading rich so we kept going leaner and leaner despite my better judgement. even when we pull the plug and its wet it never fouls, starts up and runs great, unless i floor it of course. then i came across this article which tells me the plug is giving me a false reading for anything below 60 degree weather.

http://srsengines.com/technical-library/shifter-kart-engines/honda-cr125-cold-weather-jetting?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+SrsKartRacingEnginesBlog+%28SRS+Kart+Racing+Engines+Technical+Updates%29

so my question is what main jets are people running for modded cr125 during cold days? or is my problem elsewhere.

btw im running 110 race gas and redline oil 32:1 in case that has anything to do with it.

also, what spark plug gap do you guys usually run in a stock cr125 and what do guys with high compression gap the plug to? its an RS125 piston.

Edited January 18, 2011 by Slowerthanu

[Slowerthanu]

anyone? nobody has ever experienced a full throttle issue?

oh and btw im at 6,000ft elevation if that helps too.

[coley13]

your confusing me a little with main jet size with mikuni carb. Are you sure it's not a kiehin carb?

PS elevation that high requires leaner jetting, not just the main jet, needle clip position, possibly leaner straight section needle and generally more out on the air screw "determine your pilot size first"

[Slowerthanu]

yeah I'm Positive on the mikuni jet numbers. What jet numbers are you familiar with for mikuni? And I'm sure that it's a mikuni carb. Says mikuni on it and has TM up the side. My pilot is a 37.5. I don't have the needle part number off hand but it's the stock cr125 needle. I'm aware our air up here requires less fuel which is why I have been hovering around the same jet size range as people with similar motors at this altitude run. But for some reason mine is still cutting out at wot. Their range is usually 165-180 for the main and they all run the same pilot and needle. Since fullthrottle is the only one giving me trouble I'm really primarily concerned with the main jet. It does not sputter and bobble as though it was running too rich, it goes flat and completely loses all power as though ibhad just let off the gas like it was super lean although the plug being wet suggests that it is rich.... according to the article it's understandable to Need to go richer than normal with cold temperatures regardless of what the plug reads. Anyone had similar issues?

[SuzukiFan]

When you cleaned the carb did you measure the float height?

I just recently corrected a jetting issue which was linked to the float height being way off. The bike ran fine in the mid but on the top it felt really flat. It would also spooge a ton and it didn't matter how lean I went on the main jet. Turns out float height was around 19mm and it's supposed to be 16mm thus causing an increase in fuel pressure richening all the circuits.

[Slowerthanu]

When you cleaned the carb did you measure the float height?

I just recently corrected a jetting issue which was linked to the float height being way off. The bike ran fine in the mid but on the top it felt really flat. It would also spooge a ton and it didn't matter how lean I went on the main jet. Turns out float height was around 19mm and it's supposed to be 16mm thus causing an increase in fuel pressure richening all the circuits.

it is a pump around system, so it doesnt have any float anymore. it looks like this. with the two lines stickout out. one for feed and one for return.

instead of having a float to govern how much fuel is in the bowl there is a feed line and return line to the carb so when the level get to the return line inside the bowl it sucks that fuel up and back into the tank as the feed line constantly feeds it in so it keeps a constant level at all times. The pumps are working fine so i know thats not the culprit. good suggestion though.,

i think im not describing it well enough. the motor is not just flattening out like its just losing some power and nosing over a bit. when i go WOT it literally stops firing all together. when i floor it, the motor acts as though i let my foot off the gas. there is no sputtering or backfiring or firing of any kind. it just cuts out and starts to decelerate. if i pull back a tiny bit on the throttle to get back in the 75% range it picks right back up as though nothing happened and hauls ass. i can do 10 laps of partial throttle and the motor runs great and pulls hard the whole time all the way through the rpms but the second i try to go full throttle it just cuts out like i let out of the throttle. sounds like a lean main jet issue right?

[Snipetracks]

Are you sure that it is not an electrical problem? If it runs fine and then all of a sudden it just cuts out no bog or anything that sounds like an electrical problem to me. Did you check the resistance on the coil?

[Blackwoodz]

It sounds like the pumps are the culprit. Why else would it starve out for fuel?

Its that or the cdi is fubar....but that never happens to Honda cdis.

I would test it with a auxillary tank just above the carb and see if the gravity feed system will let it run wot. But you need the float bowl back inplace to check.

[Slowerthanu]

Are you sure that it is not an electrical problem? If it runs fine and then all of a sudden it just cuts out no bog or anything that sounds like an electrical problem to me. Did you check the resistance on the coil?

its not just cutting out all of the sudden at random times, its cutting out only at full throttle...at any rpm... which makes me think its an issue with carb or pumps. specifically at aspects dealing with full throttle position.

It sounds like the pumps are the culprit. Why else would it starve out for fuel?

Its that or the cdi is fubar....but that never happens to Honda cdis.

I would test it with a auxillary tank just above the carb and see if the gravity feed system will let it run wot. But you need the float bowl back inplace to check.

i was thinking it was the pumps at one point because it seemed like a starvation issue. the only thing that made be go away from that idea was that it runs well at any other throttle percentages. running a whole ten laps at as high a pace as i could (using 75% throttle or less) i figured if the pumps were not keeping up it would still starve in that situation. do you think if the pumps were bad that i would still be able to go 75% all the way to about 80mph on the back straight? i was thinking that if the pumps were bad that it would run out of fuel even at partial throttle at that pace. what do you guys think?

also i tried sitting idle for a whole to be sure that there was fuel coming through the return line, indicating that the bowl was full and tried launching the kart at half throttle...it worked great. then i waited again to be absolutely sure the bowl was full and tried launching again but this time using full throttle and it cut out immediately. so since it seemed to cut out even if the bowl was full i assumed it wasnt a pump issue, but i could be wrong still. it might be as simple as going to a much bigger main. What size mains do you guys use?

[Blackwoodz]

Does the choke ON effect it in anyway?

[Slowerthanu]

Does the choke ON effect it in anyway?

Yeah the choke works normal.

[joekarter]

It sounds like it's the vacuum signal strength to the pumps or bad diaphrams. You might want to check and make sure that you don't have an air leak at any point in the vacuum lines, and that they are safety wired at the pumps (also sometimes the housings can crack). If you don't find that, get a set of rebuild kits and install them.

On the pumper carbs you see on the KT100's and Tags you get about 3 races from a set of diagrams before they go bad so having a set that sat around for a while with fuel in them would be a prime candidate.

The reason it works at low throttle settings is that that is when you have a higher vacuum level. Think about it this way, at partial throttle you have a high vacuum on the pumps, when you put your foot in it lowers the vacuum at the pickup point and the signal strength goes low enough that the pumps quit producing a sufficent volume of fuel.

[Slowerthanu]

It sounds like it's the vacuum signal strength to the pumps or bad diaphrams. You might want to check and make sure that you don't have an air leak at any point in the vacuum lines, and that they are safety wired at the pumps (also sometimes the housings can crack). If you don't find that, get a set of rebuild kits and install them.

On the pumper carbs you see on the KT100's and Tags you get about 3 races from a set of diagrams before they go bad so having a set that sat around for a while with fuel in them would be a prime candidate.

The reason it works at low throttle settings is that that is when you have a higher vacuum level. Think about it this way, at partial throttle you have a high vacuum on the pumps, when you put your foot in it lowers the vacuum at the pickup point and the signal strength goes low enough that the pumps quit producing a sufficent volume of fuel.

I understand completely. I forgot about the whole lack of vac at wot. And I didn't realize the diaphrams go bad so quickly. I just didn't think it was the pumps because even at idle when I KNOW the bowl was full when I gae it full throttle to launch it it still imediately bogged. Would it drain the bowl really that quick? I thought i would at least allow me to floor it for a little bit. I'll give that a try since it probably needs to be done anywys. Thanks, I'll let you know if it solves the problem this weekend.

[Slowerthanu]

well i finally figured it out this weekend... it was the simplest mistake in the world. Mikuni jets are on a different scale...everyone in karting generally has a keihin carb, so people were telling me jet sizes like 165-170 depending on the weather is what works for them so thats what i should run. i would tell them that i have a mikuni carb but most people dont know much about them, so nobody told me that keihin jets are waaaay bigger than mikuni jets. a keihin 168 main jet is huuuge compared to a mikuni 168. so i eyeballed it and made a guess. i put a 360 main in it and it ran great. 100% throttle pulls like crazy now so i must be in the ballpark, now i just need to find the range so i can compensate for weather. got my times from 57's to 54's with that change which is approaching competitive times for that track. ?

thanks for the troubleshooting help guys! ill post some video from this weekend later tonight