5 hp flathead racing carb, tillotson, mikuni, need advice

[gran_pann]

Sooo. I sucessfully put a honda gx200 carb on my briggs 5 hp flathead that I hopped up. I made a custom intake, welded my own stainless exhaust (about .85"ID 1"OD 2X 45 deg bends in it) It runs like a dream. Direct throttle custom made by myself bolted to a pivot point on the head.

The only problem now is that i cant seem to break some RPM thresholds, and I have been told prior to putting this carb on there that it is going to be pretty restrictive. So here's my setup, and I am looking for advice on a nice carb.

NOS cast sleeve briggs engine with the large dual bearings on crank. broke in with ZDDP break in oil amsoil,
raptor piston with raptor rings
shaved eyebrows
mild port/polish
stock appearing springs
94ss cam
stock length billet rod
flywheel set to cam card, arc billet. (iirc 28-30 btdc)
High flow intake/high flow exhaust with 1"ID RLV muffler.
put on baja heat mini bike with comet 30 series torque converter, 8t jackshaft, and stock rear axle sprocket. Geared correctly. Pops wheelies on start.
Governor gone
stock appearing valves
valve lash set to cam card specs, lapped valves
.029" or .030" copper head gasket
indexed stock plug
gx200 carb with 140 e tube and #36 jet

Seems like the engine just planes out at around 3800 rpms. Makes me kinda sad. So i think ill do a little better port job focusing on upper inside corners, and then shave a little more from eyebrows. I almost want to just take it to my friend with a mill and deck the block while i'm there.

Also, the carb has got to to be too restrictive I think. Im wanting to put a mikuni 22mm in there. Is this the one to fully take advantage of the hop ups?

I am having issues though with breaking any rpm thresholds. It revs up to 4k rpm real fast, then the torque converter takes over and i get to about 38-3900 rpm before it kind of just planes out.

 

[OND]

Are the springs stock or stock appearing ? As far as the honda/clone carb we turn the clone engines 6000 to 6500 rpm's with the stock clone carb and a .615 venturi.

 

[tim mclaughlin]

Larger inside diameter exhaust pipe.

 

[David wulf]

It's not the carb doing it , it's the torque converter holding the engine back, also I find it hard to believe it only takes a 36 jet . My HS 50 with a lot less mods has a 42 in it . Try getting the yellow spring for the driven and the blue / white springs for the driver . Or you can just take off the converter and run a clutch and watch that motor wind up . I also have about the same build as you but with a mikuni 22 on it and she runs out to 7,000 no problem.

 

[gran_pann]

OND,

springs are what the cam called for (19 lb IIRC). It will allow it to rev pretty high. Eitehr way, stock springs would be good to 5500 rpm based on experience.

TIM,
I thought about this, but even the smaller diameter pipes than what I have currently let engine rev to over 5000 rpms. Also, I do not want to lose the torque from loss of exhaust velocity from a higher ID exhaust.

David,
I was thinking that as well about torque converter. I talked to the guys at checkered flag racing, and the gentleman I talked to was an experienced engine builder. He told me that the carb was too restrictive. I think i might have a double whammie with flow and parasitic drag. Also, i think jetting actually might be a tiny bit rich as it puffs a little smoke on throttle up, but burns clean after that.

Also, I am going to do gravity fed tank. Does that carb need the fuel pump or can you set it up to operate without?

Also, I am thinking about redoing the exhaust to a staged type with a better muffler. Its just too stinkin loud with the RLV muffler.

 

[OND]

Hey when you put the cam in did you set the valve lash ? And your exhaust pipe is .850 not .85 ..Right ?

 

[gran_pann]

OND, yeah, I set valve lash per cam card. Ground it down perfectly at 90 deg to bench, measured cold after lapping. Cant remember, but i think it was something like .005" and .006" It definitely has compression (just ask my arm if i try to start it on anything other than the compression stroke.

My exhaust isn't pipe (like schedule 40), rather, tubing with a certain wall diameter. It measures at around 1" outer diameter, and .85 inches inner diameter, and .85 and .850 is the same number, just with another significant figure. Its stainless tubing that I used.

 

[gran_pann]

It's not the carb doing it , it's the torque converter holding the engine back, also I find it hard to believe it only takes a 36 jet . My HS 50 with a lot less mods has a 42 in it . Try getting the yellow spring for the driven and the blue / white springs for the driver . Or you can just take off the converter and run a clutch and watch that motor wind up . I also have about the same build as you but with a mikuni 22 on it and she runs out to 7,000 no problem.

So I happen to have the cheetah clutch from noram still new in box (wow price i got it at last year was 160, now its $220!! I might just have to plop it on the mini bike to break in the clutch. Question though with gearing. Whats a good size sprocket to put on the jackshaft?

Current set up is:

Noram clutch: (comes with 13, 14 and 15t sprockets)
Jackshaft input??? What size to get?
Jackshaft output sprocket 8t
Axle sprocket: 50t
Tire size:19"

Any suggestions on gearing?

 

[joshua. c.]

Have you tried different springs in the comet 30 series torque converter? the stock springs aren't set up for high rpms. I have a similar set up briggs but I do have the mikuni vm22 and a different slightly more aggressive cam 107ss. iv got my torque converter set for 3100 rpm engagement and the stronger yellow spring in the secondary. no clue what it revs to but it pulls hard and is scary fast. also the mikuni vm22 is a lot better for power than a stock carb.

 

[ole4]

I have a number of mini's with TC's and the tav30's with their overdrive will load the engine down when in high range and limit revs. Even my salsbury which does not go into high range till engine is near 7000 during full throttle runs pulls down revs by a couple thousand RPM. I could gear lower (bigger rear sprocket) but it is too fast at top end to be safe as it is. The Salsbury 330 has different driven pulley spring holes so you can set a range where it shifts up.

 

[joshua. c.]

yes the tc can limit the rpms but the new springs allowing it to start at a higher rpm and shift slower help to let it rev much higher. I only chose to set my engagement at 3100 for drivability you can set it much higher. my point it there is improvement to be had before giving up on the setup.

 

[ole4]

It limits it only because it cant pull the high gearing, If you gear it down more it will rev into the power band but then it will really be hard to start without flipping over.

 

[PacMule]

Put the aluminum flyweights in the primary with the blue springs and it will rev up. The zinc weights are to dame heavy and they did the same thing to me,4000 rpm max. If it had the governor,it would work just as it was designed for.