Box Stock

Please help! Newbie Tilly 212 struggles

Z

Zachfeen

Member
Hi there,

My name is Zach and I just built my first engine to go into my first minibike project. I am a complete newbie with small engines but am fairly mechanically inclined (own two vintage mustangs).

This is where I am at.

Engine is a Tillotson 212 e start
EC Banzai 265 cam
Arc billet rod
Stock Tilly piston
Tillotson 212 “Outlaw head” - bought complete. 29/25 valves, light port and polish and shaved .050, 26lb EC springs and retainers
Stock thickness head gasket
Head studs torqued to 200 inch pounds
NR 1.2 rockers
NR 5.340 pushrods
ARC Billet flywheel- Timing is 32 degrees
Mikuni VM22 with 17.5 pilot and 140 main jet

The issue is I can not get the engine to idle correctly. With the fuel enrichment on (plunger in up position on VM22 right?) it will start fairly easily but surges pretty badly and does not idle smoothly. When I turn the enrichment off, I can keep the engine running by revving a bit but when I let it return to idle it slowly dies. Sometimes with the choke on, if I rev, I get a puff of white smoke and backfire through the carb

Spark Plug is a Autolite 1110x and when I pulled it it is not wet, but very black which makes me think very rich BUT my logic here, is if idling decently ok with enrichment on, wouldn’t it run better with it off if it was very rich?

What I have done so far-
- Tried every combination of idle screw from .5 turns out to like 7 turns out.
- checked timing, marks on crank and cam are lined up as they should be
- checked valve lash. Set to 3 thousandths on both intake and exhaust at TDC on combustion stroke
- took off carb and reseated on adapter to the block, the o-ring is perfectly centered
- replaced intake gasket and re-tightened
-went both 1 up and down on the needle adjustment, only affected when I got out of the idle circuit as expected
- checked slide was positioned correctly and it is

I have a 12.5 and 15 pilot jet on the way but given the symptoms I’m not sure that leaning it out is going to help when it will kind of idle on choke but not with choke off. All I’m trying to do is get it to idle consistently now so I can work on tuning midrange and WOT through the needle adjustments and main jet.

In the video linked below, at the beginning it is idling with choke on and you can hear the surge, I revved a few times with choke on and then turned choke off and revved again and let return to idle, you can hear it slowly return to idle and then die.


Any suggestions or experience here will be very much appreciated!

Thank you-Zach
 
First ditch the plug put a standard plug in .
Second if it idles with choke and not with choke off .
I say its lean .
 
Thanks for the replies,

Yes, clockwise (in) to idle up. What happens when I do this, choked idle remains the same, when I turn choke off, engine speed revs up on its own to I’d say 3k and then slowly comes down until engine dies.

I thought I had a vacuum leak but replaced all gaskets and checked that everything was tight.

I just placed an order for the next size up pilot jets, 20,22.5, and 25 and already have some
NGK plugs on the way.

I do have a stock style carb I can put on to test, it’s a big bore modified carb so should match the engine ok at least for testing purposes. I may do that tomorrow

-Zach
 
Tried the big bore stock appearing carb but couldn’t even get it to light off. Spark plug wet with gas. Waiting for the NGK plugs to get here and putting the Mikuni back on and going from there. Definitely frustrating!
 
Are you running a pulse pump or direct feed? From your video it looks like you are trying to gravity feed it. Attach a pulse fuel pump on it as it will help regulate the flow of the fuel to the carb.
 
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Certainly seems like too much fuel to me too. I’m gonna try to drop the pilot jet and see what happens along with the different spark plug. Lots of people running Mikunis off gravity feed with no issue although I wouldn’t be super opposed to going with a pulse pump.
 
I’ve looked at a lot of threads, pilot jet sizes all over the place from 12.5 to 30. So I’ll just do trial and error until I get results. It seems to perform fine once the needle comes into play so I think I am just way off on pilot jet.
 
Just tossing this out there as I’m more of a Tillotson guy than a Mikuni, but does that carb really have a choke or does it have a fuel enrichment circuit? I’m guessing it’s a fuel enrichment circuit based off of the motorcycles I’ve worked on. If that’s the case, and you can get it to idle with the enrichment active, that sounds like the pilot is too small. Maybe you need a larger pilot and a smaller main. With the sooty plug, that sounds like you need a smaller main anyway.
 
It’s just a fuel enrichment circuit, so it makes complete sense that the pilot is too small given the symptoms. I bought both smaller and bigger pilots just to be sure. I think the sooty plug is likely from keeping it “choked” to run and therefore dumping tons of fuel at it. If you watch the video, when I rev it with fuel enrichment on, it blows a puff of black smoke, I think the 140 main will be close but I have 130/135 jets as well to try out once I get it idling right and out on the street to test.
 
Appreciate the help. Got all the new jets in yesterday and took a wild guess and put a 25 pilot in along with a thicker intake manifold gasket and a NGK 7ES plug and it started right up on choke and now idles beautifully off choke. Still have some fine tuning to do and still may need to up or down one pilot but I’m glad it’s nothing more serious than just a bit of carb tuning


 
Here a tip for you. When you gag the throttle from idle it should go. If it doesn’t, you need a bigger pilot. You can also reduce the pilot until it dies when you gag it, then go back up one size at a time until it goes. Then use your plug color to dial in the main jet. Keep in mind that changing the main may affect what you need to do with the pilot. I always like to get my low side circuit dialed in first, then the high side. Then go back and recheck the low side.
 
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