HT Bicycle Engine Main Jet Size?

misteright1_99

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Mar 21, 2008
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Treasure Coast, Florida
Does anyone know what size the jet is in the 48cc Happy Time? I read Normans carb post and he lists the main jet as being .026 for stock, but he might have been talking about the larger HT engine. Is it the same size for 48cc as 68cc since they use the same carb? Doesnt seem like they would be. Any help would be appreciated..
 

spad4me

New Member
Jan 20, 2008
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Arizona Bullhead
Re: HT Main Jet Size?

I really think they use the same carb.
I modified the dog snot out of mine.
My needle was as straight as a nail.
It is radically double tapered( too much work but very nice)
It is lengthened and adjusted with extra washers on top of the slide.
My main jet was too large even after I tapered the needle.
I am too cheap to buy the complete drill bit sizes so the local dollar store bits last just long enough to drill brass.
Then solder and try again.
 

misteright1_99

New Member
Mar 21, 2008
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Treasure Coast, Florida
Re: HT Main Jet Size?

Yea the carbs all look the same, but I would think that the larger displacement of the 70cc would mean it would use more gas, therefore a larger jet. I had the set of jet drills when I was doing motorcycles, but sold them along with all my jets...I have done a bit of porting and exhaust mods and think I need to go up a size on the jet, and start the needle back on the top slot...
 

RLorange

New Member
Jun 21, 2008
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Australia
Re: HT Main Jet Size?

The Jets are the same for both the 70 and 48 engines and are 0.70mm or close.

The 70cc needs a bigger jet for sure. Luckily the jet unscrews from the pin fuel well inside the carby. Even luckier still Dellorto PHBG Main jets are a perfect fit!! and cheap at only $5.

I would try a 0.78mm to 0.80mm or for the 48cc engine and 0.80 - 0.85mm for the 70cc engine. You may have to compensate by raising the clip on the pin.

Very handy that the Chinese engineers thought to match the jets with Dellorto's

:)
 

Norman

LORD VADER Moderator
Staff member
Jan 16, 2008
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pampa texas
Re: HT Main Jet Size?

I went and checked on my metric drill sizes for the 70cc main jet mine was drilled at 0.70mm and was a little to rich I soldered it shut and drilled it for 0.65mm which is working good this was done on a Dax 70 cc my other china girls are 0.065 some that I've looked at are slopply drilled and could use a solder shut and redrill my elevation here is3200 ft above sea level and the temp is in the 90s so air density is like running at a higher elevation so a guy will need maybe need a slightly smaller jet during the summer.
Best to check with the plug nice light brown is in the ballpark.
Ok for you guys who want it in american measurements
0.70mm= 0.026"
0.65mm=0.025"
Hope this will help
 
Last edited:

RLorange

New Member
Jun 21, 2008
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Australia
Re: HT Main Jet Size?

Good info about jet sizes... You found the 0.70mm too rich?

Well I found that by using a bigger jet you get more fuel at full throttle but this only works if you raise the clip to compensate so that it is not too rich at lower revs.

My logic was that my engine was too rich at lower revs on the middle clip setting and too lean at higher revs when the jet size matters more. So by running a bigger jet and raising the clip you reverse this now having more fuel at higher revs and less at lower revs.

I promise the engine is much happier, uses less fuel and the spark plug muffler and combustion chamber are all cleaner.

I am considering running a pilot jet! I will drill a small hole in the carburettor and set a hypodermic needle into some QuickSteel epoxy resin creating a tiny second jet. This should help smooth the transitions in throttle.... Then I will have to drop back in size for the main jet
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Duvall, WA PNW
www.sickbikeparts.com
Re: HT Main Jet Size?

I will measure my spare carb jet today on the comparator. Something I've been promising to do since Norman discovered my needle didn't have a (proper) taper, way back when.

Rather than buy new jets, I may drill this one out to whatever drill comes closest to .80mm and see what happens.
 

RLorange

New Member
Jun 21, 2008
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Australia
Re: HT Main Jet Size?

Ah yes my Supplier told me a similar story about the needles some batch with the wrong taper... Hmmmm maybe I have one of those that would explain how I am running such big jets.
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Duvall, WA PNW
www.sickbikeparts.com
Re: HT Main Jet Size?

My spare carb jet is .6958mm (.7 mm) (.6958mm = .027")

This was on a 67cc engine, standard carb - that arrived in December with a fat needle and wouldn't run worth a lick until the needle was turned down.

As stated, I need to find what size drill will be closest to .8mm (.0315") (maybe there is a .8mm drill!) and drill this jet out, just for fun.
 

Saddletramp1200

Custom MB Buiilder
May 7, 2008
1,439
40
48
Houston, Texas
Re: HT Main Jet Size?

The rubber O ring that should be there does not change the gas to air mixture. The AIR needle does. The .48cc is about 23-30. Seventy cc about 30-36 at most. One to, One & One half turns. I run 140 mains in my goldwing but I'm pushin' 6 carbs.(c)
 

Mikodawg

New Member
Aug 9, 2008
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Huntington Beach, CA
Re: HT Main Jet Size?

My HT80 ran too rich on the stock jet after some 150 mile break-in. I soldered up the stock jet and began with a .4mm(way too lean) I settled on the .6mm (.023) with the needle clip is at the top. I getting a good burn on the plug and picked up about 2mph.
I am running a 32:1 mixture now using Belray MC-1 synthetic. (As a matter of interest the Powerking 80cc actually measures 69.3cc.)
 

wizzizer

New Member
Sep 8, 2008
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Seattle
Re: HT Main Jet Size?

Hi, i am new to this forum and i have recently drilled my mainjet out waaay too big. now i would like to know how to solder up the hole and redrill it. do i need brass solder? or what? thanks!
 

wizzizer

New Member
Sep 8, 2008
14
0
0
Seattle
Re: HT Main Jet Size?

this may be a dumb question but i went to the dellorto site but all the jets were listed as 5mm, how do i know what size the orafice is of those jets (.7mm,etc)?

thanks.
 

wizzizer

New Member
Sep 8, 2008
14
0
0
Seattle
Re: HT Main Jet Size?

i read in another post that some were drilling and soldering and redrilling to try to find the right size so i thought it was common, oh well. i just ordered a few from dellorto anyway.

thanks guys!
 

Norman

LORD VADER Moderator
Staff member
Jan 16, 2008
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pampa texas
Re: HT Main Jet Size?

I use a propane torch and electrical solder, I also use a resin type flux when soldering. Don't need much heat use needle nose pliers to hold jet in the fire then tough the solder to the hole and it wil flow into the hole very little solder is needed, to drill the hole I use a pin vice to with micro drill bits to drill my main jets by hand with the main jet mounted in a vice. works for me with no problems.