im running a pretty stock 88 yj with the 4.2. I nutter bypassed it and swapped the carter carb for a rebuilt Holley 2 barrel... i believe it is a 350 cfm but cant quite remember.. anyways with the carter i was getting between 13-14 and after the swap and bypass ive been getting more in the 10 mpg range. how can i make it run more efficient?
Your fuel economy is way off. With the new carb and the nutter bypass you should see at least 14. Something is wrong. Like it has been suggested the best way to get better fuel economy to to go upper end from a fuel injected 4.0L and dump the carb.
You do have something wrong with your current setup.
A rich running engine starts easy and idles smooth. That is all most folks want.
If you want good gas mileage, you must set the engine up (the entire engine, not just the fuel system) to run as close to ideal stoichiometric ratio as possible for as much of the operating range as possible.
When you stray from OEM equipment and settings, you become a field engineer, and need to do all your own research. Vehicle weight, rolling resistance of tires, normal engine speed at cruise, gear ratio, wind resistance, air filter size, crankcase pressure, manifold pressure, and a bunch of other parameters all have bearing on fuel burn, and none of these factors include carb or ignition design.
If you are just now starting out, you need to get the engine in top shape.
Do a compression check, correct as necessary
Eliminate excess slack in timing chain and gears
Do a complete tune up on wear parts (this includes overhauling the carb and setting all the internal adjustments to fact spec)
You need 20+ " of manifold vacuum at idle (correct for altitude above MSL) Correct an necessary to obtain.
Once you have baselined the engine, start tuning the distributor. There are excellent threads on distributor tuning in the CJ section (YJs are generally non tunable because most have FI, CJs all were available with carbs). Search distributor tuning in CJ section. You have to have some minimum tools if you are going to do this: A high quality timing light, a multi-meter, a vacuum gauge (also works as fuel pressure gauge), and a timing tape set. A
When you get the ignition working, then you address your carb. An exhaust gas analyzer is ideal, but with lots of work and trial and error, you can get it done by reading the plugs, and possibly by tapping into the O2 sensor with your multi-meter to read your mixture burn ratio.
Suspension lifts and oversize tires are gas burners, if you want them, you will burn more gas, but you should be able to significantly increase fuel efficiency over what you are getting. One thing, setting an engine to put out more power (without increasing compression ratio) ALWAYS burns more fuel.
All the things above are do-able, but will be a long term project. It took me over 4 months to totally go through my current Jeep and bring the eficiency back to where it was whan it left the factory, I was also getting 10 MPG when I bought my 4.2.
418 miles on 19 gallons of gas. Can't beat the BBD !
If you have the patience and skill of Old4X then go thru like he said and get the carb and timing all set up. I tried over a several month period and couldn't seem to make things work right, whether it was me, the Weber carb, or whatever I couldn't get the tune and carb set where I thought it should be. Close, but just not right. I decided to do a junkyard TBI setup and couldn't be happier. From the first time the engine cranked over it has run, perfectly. I get 17-18 around town consistently, and have no idea what road mileage is but can assume it would be better. The engine starts cold perfectly, idles fine, the smell of gas is gone and there has only been one issue, a burned vacum line because I let it rest on the engine rather than properly secure it. I would advise against the Howell (unless you have to have the sticker for California) because their setup does not control spark, only fuel. In essence what you get with the Howell kit is an electronic fuel delivery system and you still have to tune it for spark control with your distributor. IMO the real advantage of a fuel injection system is it can control both fuel delivery and ignition, setting your spark advance to whatever is optimal (or retard it when knock occurs) while controlling the amount and mix of fuel into the engine. Most people shy away because they think it is complicated or difficult, but I found the resources here and over at Binder Planet had everything I needed to install and make it run perfectly. The total cost of my install was under $400 and I used a lot of new parts.
The 4.0 fuel injection system is close to a bolt on to the 4.2 of you find a donner vehicle to pull it from cheap. Keep your eyes open for a part out on a wrangler cherokee or gc. I have heard 1 mpg on the hei setup.
Make sure your timing curve is optimized....then tune your carb for best lean running.
My 4.2 has 40K on a quality rebuild, the Team Rush mods (including recurved dizzy) and a highly-tuned MC2100. I get 16MPG in the city...and crack 20+ on highway trips w/ a small lift & 31" all terrains. Drops a bit when I throw on the larger tires for offroad duty, of course.