Gas Mileage

[dbeardslee]

Ivan - I've heard other members with the mod say it had a dramatic affect on mpgs and others that say it didn't change at all - that's why I've got my fingers crossed . I will say this, however. I gassed up and went for a 20 mile spin last night. I watch my fuel gauge like a hawk, and it is definitely decending more slowly. I'm thinkin' the key to improved fuel mileage is to take advantage of the taller gearing, as in run it at lower rpms across all the gears. Anyway, that's my theory - we'll see how well it works in practice .

[Crazcnuk]

Again it comes down to riding style. Just doing a mod or two is not going to change much, if you don't change riding style.

Mpg is as much a lifestyle as it is mechanics.

And again, there has been some evidence that riding at too low an rpm can hurt mileage.

[Ivan]

Well, I think the true purpose of the 750FD is to get closer to the 160 mph mark.

Actually, if you were to drop the floats a few mm and drop the needle, you could lean the mid range cruise where the taller gears like to ride at 60 mph, and go a bit bigger on the main jet to keep WOT at a nice 8:1 for power when you need it. Might help. not sure, JAT

[dbeardslee]

Don't want to hijack, but...

Well, I think the true purpose of the 750FD is to get closer to the 160 mph mark.

Definitely helps there! It's a whole new experience on the four-lane . I just went up on the jets to accomodate the pods, and I'm really likin' the way it's running. I think I'll experiment running in different rpm ranges for a while - run a tank trying to keep them around 2k, another tank shooting for 3k, etc., just to see what happens. Also want to try an all-highway ride with a constant speed of 70mph - it's only turning over 4k there. We'll see.

[XJOK2PLAY]

Dbeardslee, am I understanding correctly that you have swapped out to a 750 FD? Do you remember what your rpm's were before the swap @ 70mph? I know for a fact that my '82 XJ1100 Maxim has not had the swap done, yet seems to be very tall in the gearing, runs exactly @ 4000 rpm @ 70mph, and gets a consistant 30mpg... just went through everything, including carbs and timing and tuning. I had heard that the average rpm for stock @70mph would be around 5000rpm, and after the swap would drop to 4000rpm. Sound right?

[dbeardslee]

XJOK - That sounds pretty close - I think I was running close to 5k at 80, now it's around 4.5k at that speed. I mentioned the 4k at 70 'cause it's a more-or-less legal speed in most states, and the motor seems to like running there. The 1100 rear end is geared 3.3/1 while the 750's is 2.9/1. With a 12% taller gear, one would expect the rpms to be about 12% lower. Of course math was never my best thing . It feels like a lot more than that though - very smooth.

[XJOK2PLAY]

Since my previous post I found your thread detailing your FD swap.
I've heard several of the ones who've made the swap make those claims, and 4000 down from 5000 rpm @ 70mph seemed to be the general concensus. Maximan's a close friend, and he and xschop, as you know, kinda created the concept.
I just can't get over how tall geared my XJ already is. 4000 @ 70mph. If I made the swap, I can't imagine being able to take off from a stop!
I've also been told that by using the 850 FD instead of the 750, it makes life a lot simpler (and more dependable) concerning the spacer. Maximan lost his first FD swap attempt after a few thousand miles due to failure, relating to the spacer. Said it locked up at speed!

[dbeardslee]

I think as long as you stay with a '79 or later FD it won't make much difference in whether it came from a 750 or 850. Mine came off a '79 750 and it was a breeze as far as the FD was concerned.

[Crazcnuk]

I think the consensus was that it drops about 700rpm in high gear.

[DGXSER]

DBeardslee has a std and therefore a 17" rear tire so his RPM at speed will vary from those of a special or a XJ which I think also has a 16" rear wheel. Obviously the smaller wheel gives lower speed for the same RPM. MY special is definitely about 5K at 70 MPH.

Do the XJs have a different gearing than the XS? I thought they were the same in drivetrain.