Carb Main Jet Question - Drill bigger?

I'm putting the final touches on my Massey Harris Mustang puller. I've got it running and plan to dyno it on Tuesday. It runs ok, but surges a lot and it appears to be starving for fuel. I've got a good supply of fuel to the carb, but the load needle has no effect on fuel/air mixture at high idle (or under load for that matter). I can unscrew it all the way and the engine still runs lean. When I tighten it down, of course the engine stalls.

The engine is a Continental F140 that's been bored to 168 cubes and compression has been increased quite a bit. Do you think it runs lean because the carb is designed to feed 140 cubes and now it needs a bigger main jet? I measured the jet hole around .040 inches and I'm considering drilling it bigger. Any thoughts on this would be appreciated.

Zack

One follow up comment - I've been through the carb twice since rebuilding it to make sure all passageways were clear and float was adjusted properly. It's clean inside and all air and fuel passages are clean and open. As far as I can tell, it should work properly.

Zack

Hi Zack,
I think you answered your own question. Needs more fuel period. More cubic inches require more fuel. I"ve always wanted my main jet a little to big as so I could have some adjustment with the main needle. When you drill out your jet, you need to use a numbered bit instead of your standard bit. Good Luck.
Charles

How much bigger should I drill it? My gut feeling says it probably doesn't take much and I'll start with a #57 bit (.043 inches).

Zack

Sounds good for a starting point Zack.
Charles

ZAck, you may have to drill the float valve to let more fuel flow into the carb, made a big difference on my 77. chuck

Zack

The Best way to set it up is on the Dyno And Keep Drilling the Jet out untill the carb is just too Rich and Looses Power !

Then Turn in your Main Jet Needle in till the Best HP and You Got It!

Take your time! A Drill Size or two at first then only 1 size till you start to Loose power with the jet open then turn it in to you gain the most power!
Make Shure you Keep Every Thing Clean!

Cliff S

if you drill it to big you can solder it up and start over ,it seems to me you have another problem though ,are you overlooking something?

Thanks to all that responded. I spent a couple hours yesterday working on it and ultimately got it running really nice. I ended up drilling the main jet out to .067 inches. I also drilled out the needle valve hole a bit to allow more fuel to flow into the carb.

With these increases I was able to get the mixture where I want it.

Also, on these tractors there is a spring in the throttle linkage that connects to the governor. Before restoration, the spring was an original that was actually broken and rebent to fit. When I initially got it running, I used a similar sized spring from the Home Depot. It turns out it was way too light and allowed the governor to surge, especially when running lean. After a bit of trial and error with other springs, I was able to get one to work properly. This not only helped get rid of surging, but also controls the full throttle rpm. I have it set to idle around 550rpm, and rev a little over 2400rpm at full throttle. Using the Twin Power clicks, it will go over 2600rpm.

I'm taking it to the dyno tomorrow to really dial in the fuel and timing. I'm really anxious to see what sort of numbers it will put out.

I also can't wait to get this thing on the track!

Zack