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terrible fuel mileage after rejetting (Spirit 750 Rejetted)

5K views 6 replies 6 participants last post by  MarkC 
#1 ·
2003 Shadow Spirit 750.

Bike was re-jetted by a dealership. After re-jetting, the fuel mleage dropped almost in half (now only 90-100 before hitting reserve). The bike has an aftermarket exhaust. I have taken apart the carbs and here are the results:

-needles were set at 3rd clip with spacer (correct)
-screws 3 turns out (correct)
-right carb main jet = 100
-left carb main jet = 102
-stock air filter

Dyno Jet recommends;

-Stock exhaust = 102 right carb, 104 left
-Aftermarket exhuast w/out baffles = 104 right carb, 106 left

3 questions:

-Why the significant drop in fuel mileage (40%) given the 100 and 102 are smaller mains?

-Wouldnt larger mains result in a further reduction in gas mileage?

-And, why would the dealership go with smaller mains and not follow Dyno Jets recommendation?

Any thoughts/suggestions are appreciated.

Paul/Chicago
 
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#2 ·
Hey Paul,

Unforunately, that is the way it goes...larger jet, more fuel into the motor. Your mileage numbers are very close to Leslie's on her 750 Spirit. Her bike has the Vance & Hines Cruzers on it.

As for why the dealership put in smaller mains than Dynojet recommended...well, it is probably because they have some experience with jetting bikes in your area...and to your local conditions. The only real way to tell if the bike is running properly on the new jets is to test the burned fuel ratio. Since this test isn't readily available, you have to rely on the "seat of your pants" test and check the plugs.

To properly check your plugs, you need to get the bike up to speed (not idling around town) for a few minutes, then pull the clutch in, hit the kill switch, and brake to the side of the road. Do not let the bike idle during this. You want a good representation of how the motor is running at speed...where your main jets are doing their work. Then, pull the plugs and check the color. If grey, it is too lean and will need a larger jet. If they are dark, then you are running too rich and need to put smaller jets in. You will need to check one plug from each cylinder to make sure both are working right.

There really are SO many factors that effect fuel mileage. Have you been idling your bike more since getting the rejet done? Do you stay in a gear longer because you like the sound of the pipes? Has the wind been blowing more than normal lately? How is your tire pressure? Is your air filter dirty?

I found that when I put pipes on my Spirit 1100 and rejetted it, I started out with a lot worse mileage (not half...but noticeable worse) because I liked to hear the pipes. I wouldn't idle it more than normal...but I would stay in a gear longer to hear them.

Out of curiosity, what kind of mileage did you get before the rejet?

Joe
 
#3 ·
Not sure if my bike was a weird one or not, but even before the pipes I got 90-100 miles out of a tank before switching to reserve. On reserve, I'd usually drive another 20 and then go fill it up and it would take just about 3 gallons.

As for the performance after the pipes, I am not sure because I am still on my first full tank of gas after switching out the pipes and re-jetting. I will have to get back to you on that. I am expecting a decline in mileage but 40% is HUGE.
 
#4 ·
Hey guys,

My wife's 750 Sprit gets about 150 miles before reserve. She's getting about 65-70 MPG. The bike is strictly stock.

I'm so happy with it that I can't see any reason to tinker with it until things start changing.

I just don't like messing with success. But that's me.

It seems like 40-50 MPG are the numbers I hear alot when people have have fiddled with carbs, you seem to be right in that area.

Check around for any leaks, restrictions, ect.


Maybe you might want to think about returning the original parts.


John
 
#5 ·
alot depends at what speed you drive at, my ace1100 96 i have 10-1 pistons and erson cams i doubled my ponies i run an 168 -170 under 70mph i get 39-40 mpg at 75 -85 31 mpg at 135mph i get 22 mph so if i keep it under 70 i pull an honest 40 mpg on a modified engine plus i forgot to say i have the dyno cdi box to get this bike to rev this high and alot of crank balancing and piston and rod rod balancing too
 
#6 ·
This is actually my sisters bike and we only track mileage on our long distance trips--fuel mileage is based on "highway miles" (70-75 mph). Prior to re-jetting, she could get 150+ miles, now down to about 100 before hitting reserve.

As a couple of you have mentioned, it probably is what it is. Considering going back to stock, but the dealership kept the stock mains/needles so I would have to round up additional parts. Was mostly interested in confirming the mileage and that the dealership did their job properly.

Just got the bike back together last night (adjusted the choke and the airbox was not seated properly when I tore it down--assume additional air into was being introduced into the airbox/system).

Once we do a test run, I will let you know if either of these made any difference.
 
#7 ·
Olivereaman said:
Hey guys,
My wife's 750 Sprit gets about 150 miles before reserve. She's getting about 65-70 MPG. The bike is strictly stock.
I'm so happy with it that I can't see any reason to tinker with it until things start changing.
I just don't like messing with success. But that's me.
It seems like 40-50 MPG are the numbers I hear alot when people have have fiddled with carbs, you seem to be right in that area.
Check around for any leaks, restrictions, ect.
Maybe you might want to think about returning the original parts.
John
Well what people have learned about these bikes is that from the factory they get GOOD gas mileage, my VT1100 would get 52mpg if you kept it under 75mph. But what I know is that a 1100cc at 52mpg is running as lean as it can in almost all the throttle ranges, and probably a lot lean in some of them.
Honda states in there OEM service manual that they use "Lean Carburetor Settings" to meet EPA standards. So if Honda tells us our bikes are lean then its pretty certain they are lean.
If that's the case then we can be certain that be lean is at a price of a loss in performance somewhere in one or more of the throttle ranges.
Yes, rejetting has its problems, getting it done right is one.
Finding some one to do it is another, if your not going to do it yourself is another.
Also, some bikes have more to gain with a rejet then do others. I've heard the VLX600's when done right has a lot to gain from a rejet.
Rejetting about any of the Shadow's 97 and up will gain a better rounded performer, they cold start better, warm up faster, have less bulky throttle response, and have better top end performance. Yes, it may cost a little in fuel mileage but I don't think if it like that. When I started I felt that my bike wasn't getting the gas mileage it should, 52mpg for a 1100 wasn't right, my thinking was that 45mpg would be more of what I expect, and it run right and not be lean. I'm near the 45mpg now, it does run a lot better than it did stock, yes its been a lot of trouble, but I've been careful to not mess anything up. I've worked at it a little at a time and I dont' have access to a dyno so that's a problem in itself. My spark plugs are a very NICE tan color. I still fill that my bike has a problem around the 2100rpm range, so that's another story.
MarkC
 
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