Gas "mileage"

[bored&stroked]

Between my buddy and I, we got to thinking. He's been riding his 96 superjet with b pipe and 195psi ADA head for years. I bought my square nose last year and really started riding it this year. Its a 61x single 44mm carb with pro tec pipe and MSD. He has been making fun of me and how much gas I've been using, which is slightly more then my 650sx with a 750 and dual 40mm carbs.

He just rebuilt his round nose after transfering all the stuff into a rickter and used an all stock 61x setup from a sitdown. His gas usage has also been large, much more then his 62t/61x with dual 38's and b-pipe. My question is WHY? Everyone says the single carbs do better on gas but we have two different setups both using lots of gas. The 61x in his ski is 100% stock including jetting, my ski has 120L/150H jetting so the carb isen't that far off if it is indeed off a little. Any idea why the single carbs seem to be using more gas?

 

[Vumad]

Maybe you are riding harder? On the throttle more? That'd be a nice to hear reason.

I am personally not a fan of a large single carb over smaller duals. I can't bust out tech specs but from a logical stand point it doesn't make sense to run a single. On duals your Jets may be the same but your adjustment screws, especially the highs, are often different. A small turn of a high speed screw can make a significant difference in performance. Some motors even run larger rear jets. A single carb on the same motor is going to leave one cylinder a tad rich or a tad lean.

I also don't see how a larger single can be more fuel efficient than a a smaller carb when I can flow more air but not make more power. I can only speculate it results in a much less efficient fuel mixture. The path from a single to the cylinder is less direct than with duals and likely creates some churning. You may also run lower popoff due to lower signal and get more leaking.

I personally haven't heard any benefits to a single carb except ease of maintenance. More area to work and fewer parts to wear out. But to me, a carb job is a carb job, and an extra $75 in carb parts and an extra hour on a good rebuild every 1-2 years is money well spent. In your case, it seems like you make that back in wasted fuel.

Edit: also my 38s run 75/140 so unless you are making a lot more power, your carbs are wasting fuel.

 

[kevbo]

It is the pilot/popoff mixture that makes the difference on standups. Stock 38s are set lean at part throttle and still have good response because they are small. I have found that a single 44 with 120 pilot is as good as 38s as long as the popoff is over 30psi. This condition usually hurts response so you need to either drop the popoff or up the pilot which consumes more gas all the time. Larger carbs must have richer part throttle mixture for good response. A pair of 48s with reverse jetting will guzzle at 15mph.

 

[Vortex]

The exhaust plays a role as well in how efficiently the fuel is burned. Bpipe may have a slight edge over the protec pipe in terms of scavenging the fuel/air mixture.

I was going to run a set of novi 48s on my ported engine but I might just sell that setup and spend that money elsewhere. The stock 38s are just so easy to tune. Much more fuel consumption than the dual 38s would be crazy.

 

[Quinc]

You need to measure in Smiles per gallon.