Topic: Main jet clarification

[ddanz27]

Am I correct in assuming that the larger the main jets are the higher the fuel consumption will be?

Also, when you replace the main jets (to compensate for exhaust or filter changes) do you need to replace the slower jets as well?

[Shoeboxjoe]

Yes when running more air you'll need more gas which means less mpg to some extent. There's a pretty extensive Carb section in the FAQ forum that would help. Main jet is only going to help with 3/4 - WOT. You'll probably need additional carb tuning like adjusting needle position, air screw, and yes possibly changing the slow jets.

[Kong]

While intuitively it might seem like larger jets lead to higher fuel consumption like many things intuitive it may not be so.  The key to jetting is not so much to find the largest jet that will work properly,  its to find the jet that will work properly.  In fact a properly jetted engine will run more efficiently than an improperly jetted on and the more efficient the engine is running the better fuel economy it will get.  So its actually quite possible that in the process of taking a lean running (inefficient) engine to a condition where the optimal fuel/air ratio (efficient) condition exists may very well increase your fuel economy.

[Shoeboxjoe]

I agree with you Kong but in this context I believe he's talking about attempting to increase performance with new exhaust/air filters which is going to decrease fuel efficiency when properly tuned. I don't think it would be a significant decrease however and nothing to be concerned with.

[Kong]

I agree with you Kong but in this context I believe he's talking about attempting to increase performance with new exhaust/air filters which is going to decrease fuel efficiency when properly tuned. I don't think it would be a significant decrease however and nothing to be concerned with.

True that!

[crazypj]

On 1977 CB550F1, stock, minimum 23mpg, best, 38mpg
modified CB550 (591cc, cam, exhaust, electronic ignition)
20+mph faster, 18mpg better at similar speeds to stock bike (110+mph, 39~41mpg)

[TwoTired]

Am I correct in assuming that the larger the main jets are the higher the fuel consumption will be?

No.   But, you are considering only part of the equation.
Jet flow is determined by pressure differential across it, inlet to outlet.  It doesn't flow anything if the pressure at each end is the same, such as when the engine is stopped.

A larger jet will flow more volume than a smaller one, given the same pressure differential across it.  Assuming engine demand remains the same, a different air filter design may, by it's nature, reduce the carb throat pressure.  This would require a larger jet size to compensate for the pressure loss in the carb throat.  The common reaction to filter design changes is to make the fuel orifices too big, which allows the engine to run acceptably but will give horrible fuel economy.

Also, when you replace the main jets (to compensate for exhaust or filter changes) do you need to replace the slower jets as well?

That depends on the how much the carb throat pressures have changed from what the carbs were originally tuned to compensate for.  And how much the exhaust has changed the scavenging of the cylinder.

Cheers,

[motorhead55]

While intuitively it might seem like larger jets lead to higher fuel consumption like many things intuitive it may not be so.  The key to jetting is not so much to find the largest jet that will work properly,  its to find the jet that will work properly.  In fact a properly jetted engine will run more efficiently than an improperly jetted on and the more efficient the engine is running the better fuel economy it will get.  So its actually quite possible that in the process of taking a lean running (inefficient) engine to a condition where the optimal fuel/air ratio (efficient) condition exists may very well increase your fuel economy.

I agree with Kong. When you get an engine tuned and jetted right AND producing more HP. In NORMAL driving mode, you will often see better fuel economy.
Here is a PERSONAL example. I had a 1977 Ford 3/4 ton Econoline van. It had a small block 351 engine in it. The fuel mileage was about 10mpg. It felt lean in running and had low power. Mind you it was STOCK and STOCK jetted. I opened up the mains on it and my mileage jumped to 13-14 mpg. It also had better power. It also smogged well after the jet resizing.