Topic: Big time Stumble off idle

[snorth88]

Hello,

My newbie build thus far:
Briggs Vanguard V-Twin 18HP
Thin head gaskets
High rev kit w/ timing advance key & Isky valve springs
EC carbs ECH203 @ barrel w/ high flow intake, high rise adaptor and X-1 air filter
Precision Cheetah Cam valve lash set@0.004
2300 ft altitude

Starts fine but has a major stumble off idle...it had 0.038" main jets in it so I upsized to 0.050"...now it pukes black smoke like at wide open...and doesn't pull at all. (I'm assuming these are too large now)

I'm not great with carb tuning and not familiar with this particular carb at all
This carb has low side jets in it , but I'm not sure where they are, how they function or even if this could be part of the problem

Any help/advice would be greatly appreciated  

Thank-you!!
snorth

[Kokamo]

I'm starting to understand carburetors in-depth a lot more in recent months. I'm not familiar with your carb, but if it has the ability to change idle jets and also has an adjustable idle mixture screw, this would be very helpful in making the carb work. What this means is with an idle mixture screw you can adjust closed throttle blade fuel supply even tho the idle jet(s) are larger than what needs to be. The reason you want a larger jet here is because you should have what you call transfer ports. These ports are what transfers a correct mixture from idle to off idle (when you give it gas) it'll flow more fuel so to speak to keep it from stumbling. Now, keep in mind, if you give it 100% throttle at once, then the transfer ports won't help much because they can only flow so much fuel at once, also they don't work after there isn't a vacuum between the throttle blade and the side of the venturi. This is where the main nozzle/emulsion tube come into play. The main flow of fuel, having come through the main jet, rises up in the emulsion tube where air from the main air bleed enters through holes along the side of the emulsion tube and is discharged into the intake airstream via the main nozzle. The size of the air bleed holes and the holes in the side of the emulsion tube are most likely what your issue is. I know there are some carburetor guru's that can help with this as I'm not the guy to give advice on what size holes would correct your issue..

Good luck with it!

~Joe

[Jeff McKelroy]

Snorth~

With higher altitude, you should go to a leaner jet. So go smaller than your original jet size of .038"

[snorth88]

Thanks for your help,

I guess I should have clarified a bit....it had 0.038" jets in it before I installed the cam...I never noticed any power increase with the cam install and was quite disappointed with the next to nothing performance gain. I was advised larger cam=more air...needs larger jets to "wake it up" so I upsized to 0.050"....I guess the bottom line is either the 0.050" jets are too large or the 0.38" maybe too small, I guess I'll just keep trying jets

[outlaw#6]

I have a similar setup to you, but significant head work, a bigger cam, and a 38 cu in motor. 1500+- altitude

.050 is way too big. .038 is much closer to where you want to be. I think I'm using slightly smaller than that actually. That will need to be the first thing you fix.

To try to fix your stumble, first mess with your idle mixture screws to get the highest rpm at idle you can without moving the idle adjuster. If that doesn't get rid of the stumble, may need to go up a size on the idle jets. Figure out what size you have, then go up one size with a tip cleaning drill bit set. But....you may be too rich at idle, then it'll take a smaller idle jet. My idle jets are about .025.

No two engines are the same, it's going to take some experimenting to get it right.

[Mowerpar]

Sounds too fat on the high side

[snorth88]

Thank-you everyone!!!

I don't have any smaller jets, and the supplier thinks these jets are the same as a Nikki carb? , but pics on net show a small oring on those jets, so I don't think they are the same. I ordered a jet kit from china that should work based on caliper measurements...going to take a while.

In the mean time I soldered the jets and redrilled them back down to an estimated 0.0415" , now I'm back to the original stumble. I paid very close attention to the exhaust upon cracking the throttle this time , and as its stumbling for a second or so, it is throwing a wisp of black smoke...so like you say they must be oversized even at that. I've got a set of 0.036 in the kit, so Ill try those when I get them.

Thanks again   

[snorth88]

UPDATE!!!

Got the jets, put in the 0.036" mains. Now she pulls real well, no stumble!
Still messing around a bit with the idle mixture screws, cause idle is a bit sparatic...but all in all it runs great.

Thanks once again, You guys rock!