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carb flow mod with pics :)

Discussion in HPI Baja Engine and Exhaust forum, started by sclark.

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  #1  
Old 04-14-2010, 03:25 PM
sclark sclark is offline
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carb flow mod with pics :)

Hello

This credit goes to fernandez iam just posting better pic of what he is talking about

quote from fernandez


-Open the carb at the bulb, remove the cap above the High jet. The cap is a small metal push cab, i firmly pushed a screwdriver into this cap, turned it and the cap comes out. (it's just pressed)
Remove small c clip, remove small filter. Drill out the jet I am not sure on the diameter I used, 1.5 time? Leave the filter and c-clip out and place back the cap.

now as for size iam not sure if he means 1.5mm or increase the size of the hole by 1.5? maybe someone can post what he means ohh i try to added text to the picture but as you can see its small so i just wrote above the picture as you can see

picture of the cap inside the carb

cap removed picture of the c-clip and screen

screen and c-clip removed picture of the hole (note this hole is NOT DRILLED OUT YET) stock hole




Last edited by sclark; 04-14-2010 at 03:33 PM.
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  #2  
Old 04-14-2010, 03:29 PM
Shortfuse206 Shortfuse206 is offline
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interesting I might have to try this with one of my old carbs I have laying around. you should get a hold of him to clarify what he means by the 1.5.

Last edited by Shortfuse206; 04-14-2010 at 03:32 PM.
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Old 04-14-2010, 03:35 PM
sclark sclark is offline
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ya i will see if he sees this post and hopefully he will post if not i will pm him
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Old 04-14-2010, 04:59 PM
BajaBash BajaBash is offline
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Sweet!!! Now, there was mention of filing out the intake area? Any pictures of that? Do you happen to have a stock spring and a spring you cut for comparison if you did so?
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Old 04-14-2010, 05:32 PM
sclark sclark is offline
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Quote:
Originally Posted by BajaBash View Post
Sweet!!! Now, there was mention of filing out the intake area? Any pictures of that? Do you happen to have a stock spring and a spring you cut for comparison if you did so?
no not YET however there is some pics on this thread about filling out the intake:


http://www.hpibajaforum.com/forum/sh...t=84394&page=4
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  #6  
Old 04-14-2010, 07:30 PM
turboglenn turboglenn is offline
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I went today and scavenged carbs from my favorite small engien shop and picked the "gold" springs out of all the case reed engines and the large CC engines with samller carbs , I also learned a bit more and for those not up to doing this yourself, You can send me your carb and 25 bucks and i'll send you a premodded and tested unit (only have 688's right now) (and picked up a bunch og nice LARGE carbs from unusual WT seriest to HDA series carbs for 30 - 50cc engines. I'm more than willing to share my info as well and just as willing to swap you out as i have built a pop off tester and understand modding the venturi system correctly (as well as boring the 3 low sppeed and one high speed jet)
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Old 04-15-2010, 05:05 AM
fernandez fernandez is offline
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Thanks for posting, this is excactly what I did, I don'y know anymore wat diameter drill used and what is the optimum diameter, but I suggest if you have time start making yet bigger and bigger, retune and test it. You can cut the fuel flow by adjusting the high end needle anyway, it will just suck up fuel better/faster. (But will be more difficult/sensitive to tune of the needle, it can be way off stock setting, use small adjustment) I will open it measure jet diameter and post it here.

Also leave out the filter for better flow, if any small particle pass by it will unlikely block the jet as diameter is wideend.

(maybe it is also possible to slightly drill out the third jet hole of the low needele, but I would only do that very little bit, as you will adjust all 3 holes with same needle and cannot fine tune the drilled out hole seperately. Actually we need a mid range jet, 3 needle carb)


would be very happy to hear you're findings, i think the mod works best on 30mm stroked bigger bore engines, as they suck significanttly more aire than 23cc, due displacement.


@turboglenn
Would you be able to test the difference between drill out of jet, remove filter and you're light spring mod? I think in my mod the flow will have less restrictions, should give faster response? I am quite interested to know. I don't have the possibility to test with vacuum.

Maybe a combination of light spring, widend jet and lifted lever, were will be the optimal trade off?
I prefer engine perfectly tuned at midrange and a bit rich at high end, instead of perfectly tune at high end, but too lean mid range (as it seem to be the case with stock carb)

Last edited by fernandez; 04-15-2010 at 05:22 AM.
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  #8  
Old 04-15-2010, 05:57 AM
joester joester is offline
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Quote:
Originally Posted by turboglenn View Post
I went today and scavenged carbs from my favorite small engien shop and picked the "gold" springs out of all the case reed engines and the large CC engines with samller carbs , I also learned a bit more and for those not up to doing this yourself, You can send me your carb and 25 bucks and i'll send you a premodded and tested unit (only have 688's right now) (and picked up a bunch og nice LARGE carbs from unusual WT seriest to HDA series carbs for 30 - 50cc engines. I'm more than willing to share my info as well and just as willing to swap you out as i have built a pop off tester and understand modding the venturi system correctly (as well as boring the 3 low sppeed and one high speed jet)
So If I understand this correctly I will send you my carb+$25 and I'll have one of those uber carbs back? I am running a 29cc at the moment btw, what kind of carb moddage would it accept?
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  #9  
Old 04-15-2010, 09:48 AM
turboglenn turboglenn is offline
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fernandez..

THese are mods i would not recomend to anyone without an extensive knowledge or past in engine tuning with carbs. and a vast knowledge of carbs as well.. drilling the jet overly big and usingthe needle to "narrow it down" is a huge mistake made by many in the past on 1:1 and dirtbike carbs on the low speed jet, it's just not that simple and leaving the screen out is something i don't recomend either but is optional

My carb has a light spring, the 2nd and 3rd holes have been modified ACCORDING TO WHAT MY engine wanted, not all engines will have the same needs, even identical builds will differ by their fuel air needs (like the stories o the guy who gets that one engine from the factory that's stock and just "outperforms" the other stockers, all engines will perform diferently


JOester... Tell me more about your engine..porting, how it's ru WOT long passes or more "track like" driving and so forth... i'm o the opinion that any larger cc high performing engine will benefit from these mods, and the airplane guys say this is implemental in preventing lean siezing in mid-air (but proper warmup before pulling the trigger on the Tx helps with that too)

One way i've been told and have been observing to find out if you need this is to make several WOT passes and kil the engine and see if the primer bulb is still full or if it's about empty.. i it's empty you're engine is using fuel faster than it's bale to refill it's self

and yes, for 25 bucs adn yours as a core you'll get a modded and tested 668 back, if you have something other thana 668 i will mod and test that carb and send it bac untill i build a stock of other comon baja carbs (i this us something that i get much demand for to build a good stck of 771's, 813's, 257's etc..)

if your engine runs overly hot, or starves for fuel then the mod wil help for sure.. one way to know is to run and plug check.. i your plug comes out white but if you open the needles any more the engine loads up and fouls out then you need more fuel flow, the easiest way to tell is that right there
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Old 04-15-2010, 09:49 AM
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Monsterman Monsterman is offline
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Nice mod, great "How To" should come in handy for those who can handle doing a mod like this..
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Old 04-18-2010, 08:23 AM
joester joester is offline
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Quote:
Originally Posted by turboglenn View Post
fernandez..


JOester... Tell me more about your engine..porting, how it's ru WOT long passes or more "track like" driving and so forth... i'm o the opinion that any larger cc high performing engine will benefit from these mods, and the airplane guys say this is implemental in preventing lean siezing in mid-air (but proper warmup before pulling the trigger on the Tx helps with that too)

One way i've been told and have been observing to find out if you need this is to make several WOT passes and kil the engine and see if the primer bulb is still full or if it's about empty.. i it's empty you're engine is using fuel faster than it's bale to refill it's self

and yes, for 25 bucs adn yours as a core you'll get a modded and tested 668 back, if you have something other thana 668 i will mod and test that carb and send it bac untill i build a stock of other comon baja carbs (i this us something that i get much demand for to build a good stck of 771's, 813's, 257's etc..)

if your engine runs overly hot, or starves for fuel then the mod wil help for sure.. one way to know is to run and plug check.. i your plug comes out white but if you open the needles any more the engine loads up and fouls out then you need more fuel flow, the easiest way to tell is that right there
Hihi woops kinda lost this thread.
I will test my carb bulb with the WOT passes as soon as I get my gears from ddm.
Btw my driving style is just flatout basher.
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Old 04-21-2010, 08:47 AM
nico v j nico v j is offline
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hey guyz.the hole is 1mm so it's a 1.5 mm hole.either way.
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Old 04-21-2010, 01:36 PM
sclark sclark is offline
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so you mean the stock is 1mm and you drill it out to 1.5mm?
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Old 04-21-2010, 02:03 PM
adolf hamster adolf hamster is offline
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hmmm, the needles would restrics flow, but this still looks a good mod. what i'd like to see though is a larger venturi to increase air flow.
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Old 04-21-2010, 04:40 PM
strada001 strada001 is offline
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Quote:
Originally Posted by turboglenn View Post
fernandez..

THese are mods i would not recomend to anyone without an extensive knowledge or past in engine tuning with carbs. and a vast knowledge of carbs as well.. drilling the jet overly big and usingthe needle to "narrow it down" is a huge mistake made by many in the past on 1:1 and dirtbike carbs on the low speed jet, it's just not that simple and leaving the screen out is something i don't recomend either but is optional

My carb has a light spring, the 2nd and 3rd holes have been modified ACCORDING TO WHAT MY engine wanted, not all engines will have the same needs, even identical builds will differ by their fuel air needs (like the stories o the guy who gets that one engine from the factory that's stock and just "outperforms" the other stockers, all engines will perform diferently


JOester... Tell me more about your engine..porting, how it's ru WOT long passes or more "track like" driving and so forth... i'm o the opinion that any larger cc high performing engine will benefit from these mods, and the airplane guys say this is implemental in preventing lean siezing in mid-air (but proper warmup before pulling the trigger on the Tx helps with that too)

One way i've been told and have been observing to find out if you need this is to make several WOT passes and kil the engine and see if the primer bulb is still full or if it's about empty.. i it's empty you're engine is using fuel faster than it's bale to refill it's self

and yes, for 25 bucs adn yours as a core you'll get a modded and tested 668 back, if you have something other thana 668 i will mod and test that carb and send it bac untill i build a stock of other comon baja carbs (i this us something that i get much demand for to build a good stck of 771's, 813's, 257's etc..)

if your engine runs overly hot, or starves for fuel then the mod wil help for sure.. one way to know is to run and plug check.. i your plug comes out white but if you open the needles any more the engine loads up and fouls out then you need more fuel flow, the easiest way to tell is that right there

wich carb should I send you that's gonna work better on that 30.5 you did for me???
my 771 or a stock 668?? or both?? as you know.... I'm not in a rush.

by the way my buggy is in pieces just waiting for me to find some time to work on it.

send me a PM, as I have a few more questions for you.

thanks Glenn
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Old 04-21-2010, 07:35 PM
nico v j nico v j is offline
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the standard one is 1mm,and u drill it to 1.5..thats what they say.just be carefull..the material inside is soft.
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Old 04-21-2010, 07:38 PM
Martin_ Martin_ is offline
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If the midrange need to be improved, here you go, at the middle of the page:

http://webspace.webring.com/people/b...daptation.html

I have two carb with a factory half moon cut on the butterfly, I will take you some pic tomorow, they are not stamped ''walbro'' but looks identical, maybee a cloned carb.
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Old 04-21-2010, 10:51 PM
fernandez fernandez is offline
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Some summary as I see it;

- Adjusting the lever height will lead to sooner or later opening of the valve, it will offset the popoff valve opening/closing time. But will not change the steepness of the valve curve. Delta pressure from closed to fully open is the same. So fuel flow the high jets just kick's in later or sooner.


- Adjusting the spring tension, it also change the characteristics of the lever opening time. (lighter, is sooner open) But as delta presure from fully open to close is smaller, it change's the behavior and fuel flow curve to high jet is steeper curve.

- Widening the High jet of 668 and removing the filter. Engine can flow more fuel. As more air pass over the jet it will suck more fuel, lever will be vacuumed down sooner. Fast fuel flow to jet, create, more, snappier throttle response. (Needle adjustment become more tricky)

Last edited by fernandez; 04-21-2010 at 10:55 PM.
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Old 04-22-2010, 03:54 AM
big scott big scott is offline
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i just rebuilt my 668 and drilled mine out to 1/16" witch it like 1.5mm. i changed the spring to a lighter one. I'm going to put this on my drag baja and see what it does. i hope it works great and will let you guys now my findings.


i was trying to find the thread about changing the spring and changing the pop of. can anybody post a link thanks.
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Old 04-22-2010, 03:59 AM
fernandez fernandez is offline
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I hope you do not give to much fuel and have too rich running!

In my case I remove also choke, and widend intake, also run 30% methanol to pertrol.


succes!
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