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Old school meets new school

29 Jun 2020 17:00
formulaz1
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Old school meets new school #41
Wow!!! Thanks for the useful pic.

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29 Jun 2020 19:33
YZBill
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Old school meets new school #42

MarkT wrote: One other thing I learned racing was jet the pilot FIRST. If you start with the main jet and get it dialed in perfectly and then significantly lean out the pilot jet (not one size... but say from a 60 to a 35), you will now be lean at full throttle. Pilot jet is not a big factor at full throttle, but it does continue to supply fuel all the way to full throttle. I learned that the hard way.


A 60 to a 35 would be a huge jump. I’ve never leaned a pilot by more than one step that I can recall. The only exception was GasGas and I owned three of them. They were horribly jetted by the factory with a 38 pilot and a needle that was way too rich. When I bought my last one in 06 I changed out the jets, needle and the slide before I ever started it up.

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30 Jun 2020 06:48
MarkT
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Old school meets new school #43
Was a "prejetted" TM38 for a Honda. Came with a 60 pilot and didn't act as horribly rich as you'd expect. Just a little "lazy". Was racing in an extremely competitive series at the time... dialed in the main jet and needle/needle jet... then it took going down to a 35 pilot to get the throttle response off idle I wanted...

If making changes to needle and needle jet I always retest the main jet... Had already spent a ton of time tuning and testing and didn't think the pilot jet would affect the main as much as it did. I was wrong. :Ugh

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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30 Jun 2020 14:11
YZBill
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Old school meets new school #44

MarkT wrote: If making changes to needle and needle jet I always retest the main jet... Had already spent a ton of time tuning and testing and didn't think the pilot jet would affect the main as much as it did. I was wrong.


Two lessons for formulaz:
1. Always retest the jetting.
2. Never make huge jumps in jet sizes.

In formulaz’s case the stock jetting isn’t far from where he needs to be, but the first time making changes to jetting can be a bit overwhelming. That’s why I suggested dropping the needle first then lowering the jet by one step if dropping the needle made it run better. His comment about all the smoke has me wondering if the primary side main seal is leaking.

I looked through some racing photos I have and found one where you can see the smoke from my exhaust. This is 50/50 110 race fuel/91 non-ethanol mixed with 927 @ 32:1.

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30 Jun 2020 23:58
MarkT
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Old school meets new school #45

YZBill wrote:

MarkT wrote: If making changes to needle and needle jet I always retest the main jet... Had already spent a ton of time tuning and testing and didn't think the pilot jet would affect the main as much as it did. I was wrong.


Two lessons for formulaz:
1. Always retest the jetting.
2. Never make huge jumps in jet sizes.

In formulaz’s case the stock jetting isn’t far from where he needs to be, but the first time making changes to jetting can be a bit overwhelming. That’s why I suggested dropping the needle first then lowering the jet by one step if dropping the needle made it run better. His comment about all the smoke has me wondering if the primary side main seal is leaking.............................................................................


The lesson I was trying to convey is to "READ THE MANUAL". People said back then and still say "pilot doesn't affect full throttle". I found out it does. And just to be clear the pilot was not changed from 60 to 35 in one step... it was done one tedious step at a time until the idle and throttle response were where I wanted them.

If I had the Mikuni Tuning Manual back then and read it carefully, I would have seen the charts showing the pilot supplies fuel from idle to full throttle and maybe it would have dawned on me to re-check the main jet... but probably not... it was late... I had a race the next day... and I'm sure the neighbors were already not happy. :S

Those lessons are often things you learn the hard way. :Ugh

What scares me about the suggestion made twice now about dropping the needle and then going down on the main and putting the needle back is that formulaz has dropped the needle all the way with some improvement and may be under the impression that the goal is to put in smaller main jets to "get the needle back in the middle" ?

From post #21

formulaz1 wrote: ......................When I drop the main to get the clip back to middle groove should I drop in increments of 5, 10, or 1/2 increments?


The factory tech sheet I have says the needle should be in the second groove... and I'm not sure if the needle (6F15) or needle jet (P-4) have been checked for wear (shank of a drill bit close to the right size inserted in the needle jet and a strong light can sometimes reveal the needle jet worn oval) and that they are the correct parts? If so, might need a P-2 or P-0 to clean up the mid-throttle range. Or a different needle.

And if the needle and needle jet are in good shape and correct... that's where I start to wonder if something else is wrong... undetected clogged exhaust or timing is off... or something... because it's quite rare to need to change the needle jet on a stock bike unless maybe operated at high altitude or something.

I think I've said my piece... I usually avoid jetting threads because there are so many methods and techniques... and opinions... and I'm more of a "follow the book" guy. Hopefully sharing some of my mistakes will help others from making them. :Buds

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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01 Jul 2020 09:55
formulaz1
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Old school meets new school #46
WOWWW, so many differing opinions on ratios, jetting, how much smoke, spooge, guess I'm going to have to review the comments and find the things most agreed upon and wing it. Thanks to all, you should all be proud of your willingness to help and share your knowledge.

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01 Jul 2020 21:05
YZBill
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Old school meets new school #47

formulaz1 wrote: WOWWW, so many differing opinions on ratios, jetting, how much smoke, spooge, guess I'm going to have to review the comments and find the things most agreed upon and wing it. Thanks to all, you should all be proud of your willingness to help and share your knowledge.


No problem. FWIW, stick with 36:1. Mark and I have different approaches based on our experiences, but you’re using the stock carburetor with the stock jetting. Make small, incremental, changes to one circuit at a time.
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