Need some advice on a predator build.

[Monkeyclutch]

Alright so since I'm a new member here I'll give a little back story on how I wound up here. Back when I was around 11 a friend and I co-owned a gokart meaning I supplied the engine and he had a frame, it was an ungoverened 8 HP Briggs on a 120-ish pound frame, needless to say I'm suprised we didn't hurt ourselves.
Now almost 9 years later, that same friend has talked me into getting back into gokarts so that's why I'm here.


I've got a 212 hemi predator with P&P'd head, no governor, slightly stiffer valve springs salvaged from a blown engine, intake, header, and about 3 degrees advanced timing. I just installed the header and intake, and I assumed that would lean it out enough to warrant bumping up the jet size. In an attempt to rejet by drilling I'm afraid that I might need to buy a new jet, the smallest drill bit I had was a 1/32 or 0.031, I drilled by hand only about 1/4 of the way into the jet and reinstalled to check the plug and this is where things went down hill.
After installing the drilled jet the engine would missfire on quick throttle changes, holding full throttle would bog it down but it would eventually come up to speed and no longer missfire, the weird part is that it only does this on a cold start, once it warms up it doesn't miss a beat.

So does it sound like I'm running rich and need a new jet? The engine will miss at any rpm if the throttle is opened much higher than the throttle setting for that rpm so I don't think the problem is in the low speed circuit. I checked the plug after running it and it looked a little wet but not fouled out, I forgot to check the plug after I got it to run without missfiring.

Any help or ideas on this would be great, thanks in advance.

 

[OzFab]

If the engine runs ok once it warms up, I don't see the problem...

The thing that concerns me is you mentioned removing the governor & replacing the valve springs but, I don't see any mention of a billet conrod; the stock conrod is only guaranteed for a maximum 5000rpm &, right now, your engine can & will exceed that... you know what comes next, right?

 

[chancer]

If you put the stock valve springs back you will be fine having the gov. removed. The valves will start to float at 5000rpm limiting revs and sparing the con. rod.

 

[Monkeyclutch]

Billet rod and flywheel are being shipped right now along with legitiment valve springs, the "stiffer" valve springs aren't really much stiffer. I've taken it up to valve float to get the top speed, 43 MPH with a 10 tooth clutch, 40 tooth axle, and 11 inch tall wheel/tire, so I'm hitting valve float at ~5200 RPM. I'm not hitting valve float in regular driving, I like to drive sideways instead of fast. I promise I'm responsible enough to keep from ruining the engine I've put so much work into or hurt myself. Also stock conrod is guaranteed up to 5k? Who guarantees that?
I do appreciate the warnings and regard for my safety and my engine's.

As for the problem with the hesitation, it was late last night when I got done, I ran it today and checked the plug, insulator was completely white except for the side facing the exhaust port, drilled all the way through with the 1/32 and it's still on the lean side, but the hesitation is gone. Why would a lean condition go away with a rise in temperature?

 

[chancer]

10-4 on the billet parts. The lean condition hmm. I used to race stock cars at our local track and the engine builders would walk around with a barometer checking the thickness of the air as the evening cooled. They would adjust the carb jets accordingly. But I cant remember the theory rich/lean due to thick/cool or warm thin air. I just know on fine tuned engines a warmer temperature can make a difference on air fuel mixture tuning.

 

[Monkeyclutch]

It's all about density altitude, a higher density altitude will make one setup richer, lower will lean it out. Cold temperatures, low humidity, and high barometric pressure make low density altitudes. Warmer Temps, high humidity, and lower pressure makes high density altitudes.

The whole reason I thought it was running rich was because it smoothed out at hotter engine temps. Plug readings and EGT both indicate lean, even with a .031 main jet so I'll bump it up tomorrow and see how it goes.