Stock carb mod

[ken murphy]

ya I like it when the hole comes out where the slide seats.My yellow bike siezed at the race my friend Tory was rid'n it,he said he had 2 peddle it back off the track,I thought it was the clutch but when I tore it down,the main bearing on the crank is like gummed up I'll pull it apart n see, I had a new motor 2 put in and it seems fine,low compreshion though even with the slant head,I took the new one apart before I ran it the cylinder looked good,I ran the hone through it the ports looked good we'll c how it likes that big carb it seeme to run fine.I'm guna try that slide modif.on the blue one.

 

[ken murphy]

Hey Egor How was your Turkey Day? I went on my longest ride yet with my two bikes that are my runners funny thing my blue bike that took third doesn't get the fuel mileage that my yellow one does with that big carb on my yellow bike the yellow bike used half the fuel that the blue one did,so in other words I'm getting better milage with that four stoke carb and it idles well I'm geues'n 200 rpm at idle real low. we took about a twenty mile ride,the silver streek (my next build) its gett'n close I checked out Nvirobike the new motor bike shop in central phx the store looks good,I cruzed by s/s scooters,they haven't had much luk w/the lil girls I let him ride mine {ole yellow) he was impressed,he had one to build c'mun up he buys the whole thing and puts it together bike and motor kit for a customer he said he has built a couple with not much luk.
that shop ss scooters is pocket bikes n scooters and some electric.
Happy trails

 

[AussieSteve]

Note that filing the inlet side of the piston also affects primary compression, therefore secondary compression is reduced. On the down-stroke, at the point where the inlet port would normally be closed and primary, (crankcase), compression would be increasing dramatically, the inlet port is now still uncovered, so primary compression has not even started yet, resulting in a reduction in final pressure forcing mixture into the transfer ports. Also, of course, it will throw the piston out of balance if it was anywhere near balanced to start with.
In the case of HT engines, the blobs of casting behind the ring location pins probably unbalances the piston anyway, in which case the filing would improve balance, but still throw primary compression timing to the wind.

Sorry, I forgot to note exactly which post I'm responding to, but that's not too relevant.

... Steve

 

[ken murphy]

I did the modifacation and it was sucessful,I was happy w/the improvement------I cut 2 small slits in the back of the slide and removed material on the the angle part of the slidedown to the bottom face,only in hope of getting more air into the engine from the carb------those carburators have miuch to be disired that is all i think I know.I'm real happy with the khine I put on.

 

[AussieSteve]

I did the modifacation and it was sucessful,I was happy w/the improvement------I cut 2 small slits in the back of the slide and removed material on the the angle part of the slidedown to the bottom face,only in hope of getting more air into the engine from the carb------those carburators have miuch to be disired that is all i think I know.I'm real happy with the khine I put on.

What's a khine? Do you mean Keihin?
... Steve

 

[Egor]

Turkey Day went well I am home from Utah, signed a bunch of papers for my our house their. I will be taking a few of my MB's up there to go for rides. I have a new kit coming, it is a 4 smoke with a Hoot box on it. They look like they have a good carb on them I will see. Fun to see the mod working out for everyone. Have fun, Dave

 

[craisin]

something to read on a lazy day
good stuff

 

[chopperjoe]

great info, i found a .040 drill bit at menards, packaged is marked #60 by vermont american paid $1.36 wasnt sure i had proper bit at first, but checked it with a caliper to be sure, also found my drill and tap reference guide, sure enough #60 drill bit is .040 if any one needs drill size reference just send message no problem looking it up. Chopper Joe ps it was the smallest bit they carried at my store

 

[chopperjoe]

hello Egor from the chicago area, just got done with the .040 carb mod also cut the small slot, a little cold today about 34 degrees, bike seems to be running slightly smoother with a little less hesitation at take off, running an NGK B6HS plug grubee skyhawk 48cc with the newer carb, close to break in. bike was running pretty well before, but cant stop messin with it. is about 32 mph about all i can expect. and do you run premium fuel??? thnx chopper joe

 

[Egor]

Hi Joe good to hear that you are giving the idea a go. It is a little cool out hear also, not that cool though. As the engine breaks in it will get better, I have found that they all need the needle at the top notch and some need the float lowered, you just bend the tabs down that touch the float. Next you need to lower the intake port if you can. It needs to go down at least 1/8 but I do all mine 1/4. If you are not comfortable with carving on the barrel you can cut the piston skirt. With the intake off look down the port with the piston at the top. If you see the skirt in the way that is what you need to remove. take a marker and draw a line on the piston remove the barrel and cut the piston on the line, I take a little more. The more you take the longer the intake stays open, up to a point it helps allot, but not more than 1/4" any more and you loose the ability to compress the charge and send it up the scavanging ports.
I worked for Drag Specialties back in the 60's building choppers, we ruined a lot of good bikes for customers, thats what they wanted. Built some wild Panheads, LOL. Have fun, Dave

PS I Need to get a 48cc and see what It will do, I just have the 66cc

 

[chopperjoe]

thnx for the advise, i will look into the carving today, out of work and snowing, nothing to do but mess around in the shed, stereo and heated, thankfully. are u using premium fuel? does it make a difference. been playing with the big 2 wheelers for some time now also. 2 sportsters 2 triumphs and my latest one pictured, 1637cc revtech about 4 yrs. build time on that one. but these little engines are fun to mess with. thnx again will update when finished. chopper joe

 

[Egor]

Ok I do use the high test gas, I think it helps but the oil kinda messes the rating up. I have a bunch of big bikes too but these are soooo much fun. I have a thing for English bikes (old ones). Let us know if you do the port timing thing, I think you will like the results. Have fun, Dave

 

[carpediemi24]

Re: Stock carb mod...

Giving a little context first. OK, just read the 18 pages here. Been reading these 2 motorbike forums and feel like my diploma should be in the mail soon

Egor, man I so would Love your opinion here. I really want to build these bikes here in CO as a hobby/job...I just so enjoy this.

Ok, my question and whats going on. I have the Starfire Gen-II-A 50cc (sticker/metal id says) and the NT carb (pic below). I live in Boulder CO which I believe is 5400ft-ish above sea level. I believe I have about 100miles or less on motor. The bottom of the main case is leaking a few drops after a 5 mile run or so. It seems to have a gray silicon like (want-a-be) gasket that is crumbling out at the bottom. So I would of course assume leaks compression a little.

Next, I keep the bike inside and temp is around 68 degrees and the outside temp here mainly now is 25-50 degrees (if this helps). I put the choke all the way up when I am about to start. Then slowly over 5-ish minutes I drop it down to idle/rev lower. It does idle I guess pretty well once warmed up...meaning it usually does not die after warmed up.
I do have to peddle a few feet before letting out clutch (85lb bike I believe with 44t front and 18t back sprocket).

Where I would LOVE and be so grateful to learn and know is, once warmed up, and going full throttle, the engine seems to bog at the top end, I let of the throttle a second or so then back to full, then bogs a few seconds later and repeat the same.

OK, I am listening, what would you experts do to the carb/engine,etc as a modification???

I am pretty handy just not sure which mod to do.

Oh, the spark plug right now is more darker chocolate than tan (richer). Stock spark plug. 91 octane fuel. 100% synthetic oil. Around 240lbs with me on bike.

I hope I listed what you need to hear to make some suggestions!?

Again, Thanks so much for your time and input.

Buck

 

[Egor]

Hi Buck - I think one of the things you are going to have to fix is the leak in the bottom of the engine. I think at this point it is actually helping at your altitude (leaking air in and leaning it out). I have never found one of these carbs that is lean and needed a larger jet. One thing a two stroke is affected a lot by altitude. The bogging sounds like you are starving for fuel. Make sure the vent in the cap is good and that all those fuel stops are not getting in the way. take the bowl off and turn the fuel on and see how fast it is flowing. The only other thing I can think of is that you are actually going fat at top and and flooding and that is takinyou out. I had to jet my old Yamaha about three times in the mountains of Utah, at one point I got it above tree line I finely had to push. Let us know what you find out. Have fun, Dave

 

[carpediemi24]

Thanks Egor for the quick replay! *I tested the fuel flow with bowl off, seems to drip fast and 3-4 drops trickling down and same time. Do see how it could be starved at WOT. (I think that means winding out top end!?) But there is a vacuum helping suck the fuel down when running too??
*I noticed that the needle clip is on the lowest of the needle, raising up the needle....would you change that?
Also, I cleaned off the bottom seam and top part of crank seam below the carb bowl where I see grease coming out of the crappy gasket. *IF I try to seal it with high heat gasket seal to test the air flow while waiting for a crank gasket, would I then need at this altitude more air coming in some other way? I have noticed others polishing the hole inside diameter that the air filter air gets sucked through to the point of same diameter of the outside of the hole!? You can see that is the 2nd pic above.
*I have a 50cc-52cc boost bottle but waiting on extra intake manifold incase I mess up this one. Not sure where this will help in what I have described!?

Pic below is just incase you need to see where idle screw is and such.
It is a snowy day here and open to working on this carb/crank issue.

I bow and thank you for your time and input!
Buck

 

[chopperjoe]

Thanks Egor for the quick replay! *I tested the fuel flow with bowl off, seems to drip fast and 3-4 drops trickling down and same time. Do see how it could be starved at WOT. (I think that means winding out top end!?) But there is a vacuum helping suck the fuel down when running too??
*I noticed that the needle clip is on the lowest of the needle, raising up the needle....would you change that?
Also, I cleaned off the bottom seam and top part of crank seam below the carb bowl where I see grease coming out of the crappy gasket. *IF I try to seal it with high heat gasket seal to test the air flow while waiting for a crank gasket, would I then need at this altitude more air coming in some other way? I have noticed others polishing the hole inside diameter that the air filter air gets sucked through to the point of same diameter of the outside of the hole!? You can see that is the 2nd pic above.
*I have a 50cc-52cc boost bottle but waiting on extra intake manifold incase I mess up this one. Not sure where this will help in what I have described!?

Pic below is just incase you need to see where idle screw is and such.
It is a snowy day here and open to working on this carb/crank issue.

I bow and thank you for your time and input!
Buck

Wide Open Throttle

 

[carpediemi24]

wide open throttle is where the problem mainly is...or the symptom...working on fixing the cause (condition that is letting this happen).
Peace

 

[carpediemi24]

...I think you were letting me know what WOT stands for Thanks!

 

[Egor]

I would not mess with the carb body. At your altitude I would put the clip on the top notch of the needle. I don't know what the jet size is they don't put a number on the jet. I have a set of jet drills some ware I will look for them. If you can find out what size yours is, I would go down at lest 5 numbers. There have been a few carbs that the float stop is no good and needs to be lapped in. To check take the fuel bowl off and hold the float up and see if it holds, if you leave the fuel on over night it would leak on the flood. I think what you are experiencing is a fat funning engine, it is cutting off with too much fuel. Have fun, Dave

PS: To find out if you are going fat on top end, turn the fuel off and make a hot pass WOT and see if it starts to run good, it will only be good for a minute but it will tell if this is the problem.

 

[mdlee1958]

Carp,
I'm up here in Ft Collins and have my needle clip all the way up to the top (last notch). I still occasionally get bogging at wot. If it is super humid here in the Poudre Valley is when it seems to be the worst. I am considering the solder and re-drill route.