Jetting chart for CR125 with PWM pump around

Aaron Bunch · Joined: 30 Jul 2012 Posts: 19

Does anyone have a jetting chart they would be willing to share? I have had an awful time with jetting and it seems like my instincts are always backwards or I am just plagued with bad luck.

It was in the mid 50's yesterday and I was running a 175 main and a 52 pilot...and I melted a piston. Our altitude here is about 750 feet and yesterday's barometric pressure was about 29.9

The highest EGT I registered was ~1230F. I really didn't think I was in danger...and then I had no compression....

I have an SRS motor, but I bought the kart that way so I am a little reluctant to bother John with my questions because I am not directly a customer of his.

Thanks,

Aaron

Sam Zavaglia · Posted: Mon Dec 10, 2012 6:23 pm Post subject:

I'm guessing you can't melt a piston on tune with a 175/52 if it is Stock CR125 cylinder/base gasket/head/head gasket/OEM piston/'99 ignition, assuming your fuel is 98 or higher and fresh, so in which case it must be a mod engine somewhere, hence get a engine tuner to look at it front to back if that's the way you got it and not opened it up.
Also assuming your running a suitable needle, lots of assumptions here, you know what that means.......

Otherwise there must be a problem with water flow through the engine or something wrong if it is complete stock and setup to spec Honda.
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Jimmy McNeil · Posted: Mon Dec 10, 2012 6:39 pm Post subject:

Pilots to lean. Depending on how long you are wide open on the straight, the main mite be close.

What needle you running?

I guess I should say, that would be to lean on my Swedetech stock honda.

Sam Zavaglia · Posted: Mon Dec 10, 2012 9:40 pm Post subject:

Bill Pyles

Personally, I would get rid of the pump-around, and opt for what many refer to as a "hybrid set-up" which uses a double pump, but retains the floats in the bowl.

For the ambient conditions you describe, I would start with a 55 pilot and 185 main and look at your lap times and plug and lean the main from there. You will probably end up with a 180 or 178, but leave the pilot at 55.
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Oscar Aguilera · Joined: 18 Jul 2001 Posts: 1614

call john...dont listen to us yo yos any more than you have to! lol

john is a really nice guy and he will help you no matter what,,,,if you ask.

first thing that comes to my mind is....was the radiator covered and motor running at 120-130 F? i dont leave the grid with anything less than 110, but if the rad is not covered the motor/water will cool down to much.

you may have cold siezed it....dont think you would have seized it with that jetting. other than it might not have run great on standing starts???? it should have run.

the only other thing is pump gas? you may have detonated the piston,,,it chipped/cracked and got into the head or cylinder.

chances are if it stuck in the first 5 laps it cold siezed...in that weather you would have had to do at least 5 laps to get heat in the tires to go fast enough to hold it wide open long enough for you to stick it from being too lean.

if you were up on water temp and all the above was correct....you were on a road race course? then you were too lean! you need to be 2 jets bigger than what jet you would run on a sprint race in the same conditions when it is that cold especially

just my opinion

al nunley · Joined: 13 Nov 2006 Posts: 3032

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Aaron Bunch · Joined: 30 Jul 2012 Posts: 19

Thanks for the replies and advice.

I have a theory.....

I think my fuel pump wasn't working well enough to keep the bowl full. On the tighter part of the track I am off the gas enough that the pump can keep up, but when I get on the longest straight the bowl gets sucked dry.

I have the Shark Shifter bracket that mounts the fuel pump to the motor:

http://sharkshifter.com/zc/index.php?main_page=product_info&cPath=20&products_id=219&zenid=5355254db0801f14a0667c7b028042b5

The previous owner installed the pumps upside down so that the pulse line was on top. This prevented the vacuum/pulse chamber in the pumps from self draining. So my pumps had quite a bit of oil in them. I should have seen this and I should have taken more care to clean the pumps.

My bracket design has been changed, and now has the pulse line on bottom where it can easily self-drain into the cases. Plus I rebuilt the feed pump just to be sure.

I don't think I mentioned it in the first post, but at one point I did manage to grab the choke while it was acting up..my theory was that if it was too lean then the choke would add fuel and it might run right...it didn't...but if the bowl had been sucked dry by that point then pulling the choke would not have helped anyway.

So that's my theory, it makes sense to me at least, haha. I replaced the piston, honed the cylinder and reassembled it all. I haven't been to the track again since (because of holidays plus weather).

Thanks again,

Aaron

Jimmy McNeil · Posted: Fri Dec 28, 2012 5:35 pm Post subject:

Aaron, fatten up your mid range. If you fatten it up and its to rich, you can always lean it back down. If you leave it and its to lean, you will stick it again.
BTW Ive raced at over 6000ft and the lowest I get on my mid range is a 55. No reason to ever run a 52 at you elevation.
My home track is also around 700ft when the temp is in the 50's. I would run a 180 main and a 58/60 pilot.

marc-andre hubert · Joined: 10 Apr 2012 Posts: 174

in Florida with high 60 mid 70f i run 55 main 180 and runs great top 1250 mid 900egt runs strong and clean and plug still looks a bit rich

Jason Berry · Advertiser Joined: 29 Dec 2001 Posts: 294

Are you using pump fuel?
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Jason Berry · Advertiser Joined: 29 Dec 2001 Posts: 294

If you are using pump fuel, change it to VP MS 98 with Motul Kart Grand Prix. Also, if this is a used kart, inspect the silencer for a broken core or blocked exhaust.

Once you have a good base line with fuel and the mechanics, this jet chart may help you.

http://www.swedetechracing.com/news-events-blog/swedetech-technical-help/how-to-create-a-jet-chart/
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J.R. Clasen · Posted: Fri Jan 04, 2013 12:35 pm Post subject: Tuning

You might want to consider that you 'cold seized' the engine. What temp did you get up to before going on track, and how much tape did you keep on the radiator?
JR Claesn