Jetting cars


ok, ive been gone for a long time, popping in every once in awhile. while i was gone, i was at a dirt bike website gaining knowledge that i horribly lacked about engines. i learned how to replace a top end, how to jet a carbuerator, how to split cases and put it all back together, everything. i even bought a new rm125 while i was gone…

but what i am getting at… does all of this maintainance apply to cars too? is there a huge advantage to having a car that you replace your top end on when it starts losing power? do you need to jet your car/truck if it is carbuerated when the weather changes? and if not, why not? thanks.


Itis not necessary to do a top end on a car unless it blows a head gasket or burns the valves with the latter being a rare occurence. The only car that I knew of that had to be jetted was the Mercedes Benz Gullwing, and that was only at altitudes because of its Webber carburators.


Huge advantage? No.

When an engine’s top end starts to leak, it loses a lot of combustion chamber pressure, so it’ll lose power. How much power it loses depends on how much it’s leaking. In reality, there aren’t a lot of cars running around that need their heads rebuilt.

As for jetting the carburetor, you really need to think about that. The jets determine how much fuel will be atomized in the air stream per VOLUME of air. The amount of fuel necessary depends on how much oxygen is available in the air. That doesn’t change with the weather. It does change with altitude, because air becomes thinner at higher altitudes. That means that a carburetor tuned ideally for sea level would be rich at 5000 feet because there’s less oxygen in the same volume of air at that altitude.

This doesn’t mean that you’d have to change jets as you’re climbing a hill, but it would be beneficial to change jets if you move permanently to a hill and want the best engine performance available.

Do yourself a favor and just ride your bike for a while before you take it apart to “improve” its performance. You’re not likely to improve much, but you do have a pretty good chance of hurting its performance a lot.


A top end overhaul is almost a full rebuild on an automotive engine. To renew the cylinder, piston, and rings requires that the whole engine come apart. At that time the head and valves should be serviced and the crankshaft and cam bearings might as well be inspected if not replaced.

As to the need to rejet a carburator. If you are looking for performance or economy, you need to rejet to the conditions you will be using the engine. If the engine has been modified with a new carburator, different cam, exhaust headers, and aftermarket intake manifold, the jeting will definitely need to be tuned. On advantage of a moto is that the slider needle can be adjusted up or down to get a mild enrighment or enleanment or the needle profile changed to adjust mixture at a certain throttle openning. For a car, runs on a chassis dyno is the best way to get feedback on the tuning that includes ignition timing as well as fuel air ratio. Since most performance applications are usually at full throttle, the race mechanic usually ‘reads’ the spark plugs to see if the mixure is too rich or lean. Also, symptoms of ‘bog’ or ‘flat top end’ need to be addressed. There are other fuel metering devices in an automotive carburator such as power valves, mixture needles, vacuum operated secondaries, power piston springs, acceleration pumps, emullsion tubes, etc. The one advantage of using Holly or Weber carburators is the plethora of tuning parts that they supply.

Hope the above helps you in your search for knowledge.


yes, but in a colder climate arent you getting more air into your cylinder?