HOT TIP...motorcycle type carbs on 4 strokes

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silverbear

silverbear

The Boy Who Never Grew Up
Jul 9, 2009
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Thanks first to slowbalt for cluing me in to a simple modification I can make to my 99 Predator by swapping the intake and carb with one from a stock China girl 2 stroke.

http://motorbicycling.com/showthread.php?t=51619

I had no idea and didn't realize that 4 stroke industrial carbs are designed for slow throttle response, meant to be governed, etc. and really not at all what we want for our motored bike creations. We want something more like a light weight motorcycle. Vroom!

In doing a little bit of research I see that others have been making this switch for a long time, maybe not with the predator, but with other 4 strokes including Briggs engines designed for water pumps and such and used instead on racing go karts.

I was recently advised to swap out the stock carburetor on my 212cc Predator for a motorcycle type Mikuni carb. The difference is supposed to be night and day.

I find this pretty exciting! In the case of my 99 Preddy it will cost me nothing since I already have a China girl carb and manifold I can use. I'll also need to change the throttle cable end, but how big a deal is that?

I no longer have a Hua Sheng 50cc engine, but I wonder what a China girl carb would do on that? Has anyone done it? Is the Intake manifold close enough in size to use the one from the China girl? My guess is that it would wake that little engine up. It was a good motor, just had slow throttle response and was a little bit too doggy for me, always acting a little gas starved, but with a different carb... well, I wonder.

Thanks to all of you guys who know more than the rest of us and are willing to share your knowledge and skill. By the way, slowbalt, that is a fine looking bike you have there!
SB
 
professor

professor

New Member
Oct 14, 2009
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Silver- if you drill the main jet on the 212 a few thousandths (from .028 to .033) you will find the engine likes it.
The idle can go up only a very small amount, I used a welding tip cleaner as a file to open up the idle jet a tiny amount (being careful to not stab a finger with the "File").
The gokart guys go bigger on both (Bob's 4 cycle forum) but I am happy w/ the above.
All new engines are stupid lean. It is fixable.

Rejetted, with open exhaust and freer aircleaner, the 212 makes a bit over 9hp at 4400 rpm. I do not know what the hp is with the stock aircleaner - the kart guys usually do not use it, but it does not look restrictive to me.

I am still using the governor - adjusted for a bit higher top end.
The gov has one advantage- when you are crusing around- it automatically compensates for an increase in load, but still goes to wide open instantly when needed.

The response to WOT is the same for any carb unless it has a accelerator pump in it.
 
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culvercityclassic

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Sep 27, 2009
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I have never used the stock carbs on any of the four strokes I have built due to the fact of jetting issues and linkage issues. I like to be able to change jets with ease. Pictured is the hf 99 with a 22 mm carb. Bike runs great with this setup.
 

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Stormsorter

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Jun 10, 2010
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I knew you could use motorcycle carbs but had never even considered an HT intake would fit so easy until I read SlowBalts thread. Threw on an NT and now it idles low enough that I can start it up and walk away without the clutch wanting to engage when it warms up. Later I will test if the material from the stock breather will richen it up enough so I don't have to drill a jet. Shouldn't use that black swiss cheese from a stock NT anyway.
 
cannonball2

cannonball2

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Oct 28, 2010
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I have just recently got into the scooter thing GY6 based. There are zillions of speed parts for these units, my 150cc is a full house big bore is 75mph scoot. All that said the point is there are bunches of carbs for the 50/150cc GY6, CV, CVK, Flat slides, etc. They run very well and are cheap, starting at around $30. The CV/CVK is very easy to set up and jet. They generally have an auto enrichener but on some it can easily be converted to manual. I would think a 32mm CVK on a 212 preddy would rock.
 
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atombikes

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Feb 14, 2010
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OK, so when I got into this hobby of building a motorized bicycle, I went straight to the 4 stroke. I don't have a clue about 2 stroke carbs. Any pointers on what a NT is vs an RT, etc?

Also, I'm wondering about this mod on a HF engine mounted vertically...seems that if you used an intake manifold that sticks straight off the engine, won't that cause the carb to stick out real far? With the engine mounted vertically that will mean the carb will be closer to the riders leg.

Dunno, just wondering.

I like that you can rotate the carb around without messing with the intake manifold, though.
 
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atombikes

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SlowBalt said:
You can see the carb sits flush with the Pull start http://www.youtube.com/watch?v=QcEtCbhCMZ4
Click to expand...
Yeah I saw that but when you rotate the engine vertically I think the carb bowl will be interfering with the engine shroud, so it would have to stick out much further to clear. <I think> .... Anyway, I'm just throwing this out there as a possible concern for SB and others that may have vertically mounted harbor freight engines.

I have a PZ19 carb (which may be larger, not sure since I don't know squat about NT type carbs?) and with a vertical engine, you pretty much have to use a 90 degree intake to allow it to clear the engine shroud.

This pic lifted from ebay shows that it's roughly 2" from the bottom of the bowl to the centerline of the intake. Not sure if this means I could do this on my vertical engine?



If anyone has done this with a vertical HF engine, please chime in and tell me there is no interference and I will jump on this moving train as well.

edit:
I dug thru my pics and found this one (it show my PZ19 carb lined up where I want to mount it). Anyway, in the pic you can see when the engine is mounted horizontally, there is alot of room for the carb bowl; but when mounted vertically (as my engine is shown) the engine shroud is right where the bowl is.



So the next question to solve the NT carb on vertical HF engine: is there a 90 degree (or offset)intake for HT engines?
 
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F_Rod81

F_Rod81

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This mod is designed for when the engine is mounted horizontally. If your motor is mounted vertically then it might pose some problems. Generally if you mount the engine vertically then the HT manifold will almost be obsolete in stock form. You'll need to modify it further or fabricate your own manifold to give it the 90` angle to either the front or rear of the engine; so it sits flush with the recoil housing
 
F_Rod81

F_Rod81

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Jan 1, 2011
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So the next question to solve the NT carb on vertical HF engine: is there a 90 degree (or offset)intake for HT engines?[/QUOTE]
________________________

Based off this picture, if you're able to have the carb angle towards the front instead of the back you should get the ram-air effect. Plus it looks bad a$$! Similar to what another member performed nicely...

Front facing carburetor
 
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atombikes

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Feb 14, 2010
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Northern VA
F_Rod81 said:
This mod is designed for when the engine is mounted horizontally. If your motor is mounted vertically then it might pose some problems. Generally if you mount the engine vertically then the HT manifold will almost be obsolete in stock form. You'll need to modify it further or fabricate your own manifold to give it the 90` angle to either the front or rear of the engine; so it sits flush with the recoil housing
Click to expand...
OK, thanks for the clarification. I just didn't know if there was enough of a size difference between my PZ19 carb and the HT carb that would allow the HT to fit. But then the more I studied things I assumed there might be problems with the vertical mount. Oh well.

F_Rod81 said:

So the next question to solve the NT carb on vertical HF engine: is there a 90 degree (or offset)intake for HT engines?
Click to expand...
________________________

Based off this picture, if you're able to have the carb angle towards the front instead of the back you should get the ram-air effect. Plus it looks bad a$$! Similar to what another member performed nicely...



Yeah, that does look pretty bada$$! I'm thinking I might end up making my carb forward mounting, for leg room if no other reason.
 
SlowBalt

SlowBalt

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Mar 8, 2010
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This might even fit with the manic or pirate style intake. It would tuck the carb in tight to the shroud but I don't have one to test with.
 
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