carb tuning/ redrilling jets

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BoardTrackBomber

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So we have our gasbikes running pretty well after tweaking on them and adding things like expansion chambers, tuning the carbs etc etc...

It seems like on some days, our bikes run very well, they just haul and seem like the powerband never ends.. Then on other days, it seems like they randomly bog at WOT.....

Now here's my question. We're thinking about investing in some better carbs than what came in the kits..... I know the needle clip adjustment only controls the mixture up to about 3/4 throttle and the jet is what is at play during WOT......

One of the next things I'm going to try is soldering shut the jet and redrilling to a smaller hole because it seems like all of our motors run way rich, and they are all set on the top groove on the needle (the leanest setting)..

My smallest drill bit is about .040" and I have some .020" drill bits on order that should be here in a few days so I'm going to try that and see what gives there...... Also going to check my filters and go through the carb and make sure there is no trash or anything in there..

NONETHELESS, We still want to buy some better carburetors, so which one should we be looking at here???? And in Norm's thread on carb rebuilding/tuning he mentions soldering shut and redrilling the jet smaller than either .025" or .026" depending on which jet you have.... But he doesn't mention how much smaller....... Anyideas here? Norm? Anybody???

Thanks in advance.
 
crassius

crassius

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AFAIK, these jets are in hundredths of a millimeter. i.e.: 68jet is .68mm, 71jet is .71mm

get a good conversion program & maybe also a set of cheap metric micro drill bits
 
tooljunkie

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i bought a set of number drills,identified the size and went down a couple sizes.
the drill bit set i got was the size of a matchbook.
i placed the jet in my drill chuck and held the bit with a pin vise-usually found in the welding department disguised as tip drills.its the size of a pen and holds the bits nicely.

its trial and error,as each engine likes a certain mixture.

quick and dirty is placing a small strand of wire in the jet,no solder,just bend the wire over at each end of the jet.but it wont give you an accurate mixture and be careful with the amoount of strands you stuff in there.
 
2door

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Your altitude above sea level will impact your jet size. I'm at about 6000' and I'm a devoted NT carb user. I've found that a main jet, soldered and drilled to .025 to .026 is just about perfect for all round performance at any temperature. The lower your altitude, the bigger jet size. You'll know when you hit the sweet spot. Everything will come together. Experimentation will get you there.

Like tooljunkie, I use a set of orifice drills and a pin vice, soft solder and flux to fill and drill my jets. It has been my experience that all stock jets will run rich.

If your smallest drill is .040 you might want to think about moving to Death Valley, or buying some smaller drill bits. :)

Tom
 
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bill2781

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hmmm definetly got me thinking on my carb and wondering what my jets are at on my stock nt carb and what to set it all at. mine is at the second to the last slot on the lean side and mine is running rich I have been thinking about moving mine to the leanest it can go and see if it will work better
 
2door

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bill2781 said:
hmmm definetly got me thinking on my carb and wondering what my jets are at on my stock nt carb and what to set it all at. mine is at the second to the last slot on the lean side and mine is running rich I have been thinking about moving mine to the leanest it can go and see if it will work better
Click to expand...
Remember, Bill, as BTB said in the first post, the 'C' clip position on the needle will only effect fuel flow up to about 3/4 throttle. From there the jet size determines how much fuel goes to the engine. If you feel you're running rich at WOT then reducing the jet's orifice will help. Just don't go too lean.

Tom
 
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Thanks for the quick and informative replies y'all. I have bought the next hotter spark plug (NGK B5HS) and I'm going to swap that in place of the B6HS that I'm currently running. Maybe that will help a little with igniting the fuel more thoroughly, i.e. leaning out the mix a bit.

I picked up a pack of tip drills today, the smallest one is .0225" , not the .020" that I wanted to try, but maybe it's all that I need. Even if I need to go smaller, I did order some .020" drill bits that should be here by the end of the week.

Placing the jet in the chuck and using the pin vise? That is an awesome idea. I'm going to have to pickup a pin vise. Ironically, I was just looking at a pin vise and thinking that could be useful for this, and then I realized that is the deal you're talking about!!! Another trip to the parts store to buy one of those is in order. Do you just hold the pin vise by hand? How do you keep from breaking one of these tiny drill bits?? Seems like if you even moved a fraction that it could bust the drill bit easily.

All of our carbs are NT gen 2 except for one, it is a NT gen 1 carb.... I have robbed the jet outta the gen 1 carb and am planning on soldering and drilling it, does anyone see any reason why the gen 1 NT jet wouldn't work in the gen 2 NT??????


Hotter spark plug

A multi pack of tip drills
 
2door

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Here's what I use. I guess I should have put something in the photo to give it some scale but you'll get the idea.
This is a set of orifice drills. Also there is a pin vice holding a .026 drill bit.

I twist the vice between my fingers, twirling it back and forth and applying just enough pressure for the drill to bite into the solder. Never try to use power. Just your fingers. A .026 bit is pretty small and needs to be treated with care. Go slow and the bit will do its job.

Good luck.

Tom
 

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bill2781

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thank you 2door for the information . for me its mine is running to rich and fouling plugs. I am using opti 2 1.8oz oil in a gallon so I don't think its that. and I have 201 miles on it so fare.
 
silverbear

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2door said:
Here's what I use. I guess I should have put something in the photo to give it some scale but you'll get the idea.
This is a set of orifice drills. Also there is a pin vice holding a .026 drill bit.

I twist the vice between my fingers, twirling it back and forth and applying just enough pressure for the drill to bite into the solder. Never try to use power. Just your fingers. A .026 bit is pretty small and needs to be treated with care. Go slow and the bit will do its job.

Good luck.

Tom
Click to expand...
Thanks Tom. I just ordered a set of bits and the twist drill. Have also purchased a couple of second generation NT carburetors, one for my 99 Predator engine and the other as an experiment on my Villiars midget engine on a 1934 Elgin build I have never been able to run since I can't find a replacement carb or repair kit. Maybe I can get that bike running and on the road this summer. Vintage engines can be frustrating tracking down parts.

In the case of the Predator engine the swap is pretty simple, boring out the mount holes on the intake manifold a bit and making up a new throttle cable.

The Elgin may require making a custom intake manifold for it. Will see. There's a way.

Regarding the jet orifice on the NT second generation carbs... is there any sort of chart available referencing elevations? You are at a much higher elevation (over 5,000) than I am at 1,457 ft. Slowbalt is at the other end of the scale at sea level. I'd like to start at what likely be a bit lean and be able to keep drilling it out one bit at a time until Goldilocks says "this one is just right!"

But I need a rough idea of where to start. Make a guess? Or is it best to start with the stock opening, see that it is too rich, solder it, drill it out a step down... still too rich, solder it and go to the next next bit smaller? More labor intensive, but perhaps safer than damaging the engine by running too lean?

I've read about this for years now, but never done it. Like lacing wheels, once you've built one it is no longer a daunting proposition. I should know this from algebra class as a kid. If you know the formula and use it on a couple of problems it is no longer hard... it's easy! Thanks for teaching an old dog new tricks.
SB
 
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F_Rod81

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silverbear said:
Thanks Tom. I just ordered a set of bits and the twist drill. Have also purchased a couple of second generation NT carburetors, one for my 99 Predator engine and the other as an experiment on my Villiars midget engine on a 1934 Elgin build I have never been able to run since I can't find a replacement carb or repair kit. Maybe I can get that bike running and on the road this summer. Vintage engines can be frustrating tracking down parts.

In the case of the Predator engine the swap is pretty simple, boring out the mount holes on the intake manifold a bit and making up a new throttle cable.

The Elgin may require making a custom intake manifold for it. Will see. There's a way.

Regarding the jet orifice on the NT second generation carbs... is there any sort of chart available referencing elevations? You are at a much higher elevation (over 5,000) than I am at 1,457 ft. Slowbalt is at the other end of the scale at sea level. I'd like to start at what likely be a bit lean and be able to keep drilling it out one bit at a time until Goldilocks says "this one is just right!"

But I need a rough idea of where to start. Make a guess? Or is it best to start with the stock opening, see that it is too rich, solder it, drill it out a step down... still too rich, solder it and go to the next next bit smaller? More labor intensive, but perhaps safer than damaging the engine by running too lean?

I've read about this for years now, but never done it. Like lacing wheels, once you've built one it is no longer a daunting proposition. I should know this from algebra class as a kid. If you know the formula and use it on a couple of problems it is no longer hard... it's easy! Thanks for teaching an old dog new tricks.
SB
Click to expand...
There is no "one jet size" that will work for your given area. Lots of factors go into tuning/syncing the carburetor. Altitude, and weather being the biggest. Temperature, humidity will throw you a curve ball if you try tuning on the wrong day. My suggestion is buy a few spark plugs (5,6,7 heat range) and have a selection of jets to experiment with. Otherwise you'll spend most of your time soldering and drilling jets. If you don't mind then by all means. When you do this, perform it on a day where the temperature is close to what you normally ride in. Too hot and/or too cold and you will not get accurate results.

.wee.
 
2door

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Ditto:
Tuning a carburetor is not an exact science. You can come pretty darn close to 'perfect' but unlike computer controlled fuel injection that makes adjustments based on changing conditions, a carb must be tuned to be right, 'most of the time'. There will always be factors that you can't control, weather being the most prevalent. Weight load, riding style, terrain etc. can be anticipated and tuned for but any changes in them will have an impact on the way the carbureto/engine perform.

I never agonize over trying to get things 'perfect' when it comes to tuning but instead tinker and experiment until I find a condition that gives me good performance 'most of the time' because otherwise I'd spend all my time tuning and less just riding for enjoyment. I will say that the jet size is critical. I've seen very noticable difference between a jet drilled to .024 to .026. And don't forget that your fuel/oil ratio plays into the equation, if you're tuning a 2 stroke. I tune for my choice of oil that I run @ 100:1. The higher the oil content, the leaner your fuel/air mix will be. This might explain why the stock carbs always seem to be on the rich side because of the factory recommended oil ratio of 16:1 seen in many kit instructions.

As for your question about where to start; I start lean and drill until I reach about where I want to be. I never run my engines lean for extended periods so I'm not worried about damage. Just long enough, a couple of blocks maybe, to see where things stand and then go for the drill bit if it sounds/runs lean. And I don't abuse it in the lean condition.
Spark plug readings can help but in our little engines they can be deceptive. I don't rely heavily on plug color.

Tom
 
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I don't know if it's my imagination, but it seems like the hotter plug alone has helped. Still bogging at WOT though...

Tom, could you describe what you are looking for when you notice it running lean? You said based upon sound and how it runs. My bog at WOT I'm just assuming is b/c I'm running rich as most of the engines seem to do....

And I totally agree about plug reading being a potential indicator, but possible a tricky one to interpret. I've been a gearhead as long as I can remember, but I never did get too good at reading plugs and being able to judge them when you're dealing with the succinct differences between shades of color. I get that white is lean and dark is rich, but all the area in the middle is kinda a gray area LOL whoops look at that pun....
 
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2door

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A simple test: At WOT when you feel that "bogging" try putting on a little choke. If things improve with just a tad of choke then you're lean. If it gets worse, you're rich.
I suspect you are rich.

I run Opti-2 and all of my plugs are grey. Light grey when I'm lean and dark grey when rich so plug color to me is arbitrary if not nebulous.

Tom
 
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Well I tried the choke test as you suggested Tom and I can't really tell a difference either way..... Seems to run the same with the choke halfway on or completely off..... Of course, if I go much past half choke it kills the engine.

I have soldered shut and redrilled a spare jet to I believe .024" when it was originally about .026" so maybe I should just swap that jet in and see what happens then...

I know Bairdco has a thread where he detailed how he went through all the usual things trying to fix his WOT bogging (needle, jet, float, etc etc) and he eventually ended up changing the petcock for one which flows more... I might try that just to see what happens. Though as you pointed out in that thread Tom, in most cases the stock kit supplied petcock should be more than sufficient. I don't know, I guess I should slap that other jet in there first that I have worked over and see what happens.....
 
2door

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Baird and I usually agree on most everything and I don't discount his findings regarding the petcock. What do you have to loose? Try it. My view was that unless you have a highly modified engine that the kit petcock, if it flows a steady stream, will be sufficient. Nevertheless, Baird had good luck with a bigger valve. You might too.

As for the choke test, if you are really really rich or lean it might not be as conclusive but usually you'll see a difference one way or the other. One other way to tell is how the engine starts and idles. A very lean mix will usually result in a little hard starting. Meaning you have to pedal for a while and use lots of throttle. A rich mix will usually allow the engine start quicker. If it tends to die at idle speed that is another indication that you might be a little on the lean side.
.024 is a pretty small hole. Let us know how that performs for you. Do you happen to know your altitude? If not let me know what airport is closest to you. That will tell me how high above sea level you are.

Tom
 
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According to google, Guthrie OK is 981 feet above sea level.

My bike is pretty easy to start, well it's a little cold blooded from time to time but I'll just attribute that to the nature of a 2 smoker...... But yeah, pretty easy starting 90% of the time considering that.

I guess that .002" really is a pretty big difference. It has been pretty cold here lately so no gasbiking unfortunately. I'm hoping to get that jet swapped in and do a short test at least before I freeze my toes off!!
 
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