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I just picked up a 97 nighthawk cb750 last week, with 40k on it(well, says 40k, but the speedo/odo wasnt working when I got it, so who knows...). Anyway, the bike seems to run great, but for some reason I am not getting the MPG that I think I should be getting.

Right now, on the freeway I am getting about 37-40mpg.(at about 70mph)

Not horrible, but I've seen plenty of people claiming to get 40-50mpg. Since I got the bike, I've fixed the speedo(the wire was broken up by the front wheel hub), changed the oil and replaced the spark plugs.
I travel 200 miles per day, about 190 of it on the freeway. My speed ranges from 65-75mph, occasionally 80. I weight 150lbs. Air pressure in both tires is 33 cold.
As a test, I filled up the tank, and got back on the freeway, and kept my speed between 58-65mph, and went 70 miles, and filled up as soon as I got off the freeway. I was able to squeak out 40mpg...But once again, I've seen people say they beat on the bike and still get 40+, and I was riding gingerly getting 40 at best.

The bike does not leak gas when sitting(holds a full tank overnight) and otherwise seems to run well. When on the freeway, I keep a constant speed.

I will be replacing the air filter very soon.(could this effect the mileage that much? Its dirty, but it didnt seem too excessively dirty...)

Any ideas about why my mileage is(seemingly) bad?

Otherwise, I'm kinda stumped on where the gas is going...and not that my MPG is THAT bad, but its a 200 mile round trip to work, and I'm trying to make it home-work-home again without filling up.

I'll be ordering a 16t front sprocket to help keep the revs down a little bit, since its a 95% highway bike.

I also have a Vance & Hines exhaust, if that makes any difference.


Am I just expecting too much? I know 37-40 isnt horrible, but I feel like it could/should be better.
 

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I just picked up a 97 nighthawk cb750 last week, with 40k on it(well, says 40k, but the speedo/odo wasnt working when I got it, so who knows...). Anyway, the bike seems to run great, but for some reason I am not getting the MPG that I think I should be getting.

Right now, on the freeway I am getting about 37-40mpg.(at about 70mph)

Not horrible, but I've seen plenty of people claiming to get 40-50mpg. Since I got the bike, I've fixed the speedo(the wire was broken up by the front wheel hub), changed the oil and replaced the spark plugs.
I travel 200 miles per day, about 190 of it on the freeway. My speed ranges from 65-75mph, occasionally 80. I weight 150lbs. Air pressure in both tires is 33 cold.
As a test, I filled up the tank, and got back on the freeway, and kept my speed between 58-65mph, and went 70 miles, and filled up as soon as I got off the freeway. I was able to squeak out 40mpg...But once again, I've seen people say they beat on the bike and still get 40+, and I was riding gingerly getting 40 at best.

The bike does not leak gas when sitting(holds a full tank overnight) and otherwise seems to run well. When on the freeway, I keep a constant speed.

I will be replacing the air filter very soon.(could this effect the mileage that much? Its dirty, but it didnt seem too excessively dirty...)

Any ideas about why my mileage is(seemingly) bad?

Otherwise, I'm kinda stumped on where the gas is going...and not that my MPG is THAT bad, but its a 200 mile round trip to work, and I'm trying to make it home-work-home again without filling up.

I'll be ordering a 16t front sprocket to help keep the revs down a little bit, since its a 95% highway bike.

I also have a Vance & Hines exhaust, if that makes any difference.


Am I just expecting too much? I know 37-40 isnt horrible, but I feel like it could/should be better.

New air filter?
Have you looked at the plugs to see if she's running rich?
Brake discs aren't dragging when they're hot?
Is the gearing stock 15/38?
Is the odometer accurate?
 

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Discussion Starter #3
New air filter?
Have you looked at the plugs to see if she's running rich?
Brake discs aren't dragging when they're hot?
Is the gearing stock 15/38?
Is the odometer accurate?
No new air filter yet...should I go oem, or get the k&n one?
Plugs looked good when I swapped them for new ones
Brakes dont drag, I redid the front wheel to get the speedo working, and rear was up/no drag when putting lube on chain
Gearing is stock-I'll be putting a 16t on the front since I do 95% freeway
odo is acurate, or high by about 1-1.5%. My work is 97 miles away, and it usually reads 98-99 when I get there.
 

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No new air filter yet...should I go oem, or get the k&n one?
Plugs looked good when I swapped them for new ones
Brakes dont drag, I redid the front wheel to get the speedo working, and rear was up/no drag when putting lube on chain
Gearing is stock-I'll be putting a 16t on the front since I do 95% freeway
odo is acurate, or high by about 1-1.5%. My work is 97 miles away, and it usually reads 98-99 when I get there.
Stick with OEM air filter.
 

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Discussion Starter #5
Stick with OEM air filter.
sounds good.

I just ordered a 16t front sprocket, so hopefully that'll help.

Would a crack in the airbox effect milegae in a negative way? I haven't done it yet, but I'll be sealing those up soon. The cracks are in the edges of the airbox.
 

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Id get the air box sealed up it may contribute to a mile or two.
Think there is company makin 17t pretty sure my buddy jack had 17t? Said he did anyways lol
 

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Discussion Starter #7
Id get the air box sealed up it may contribute to a mile or two.
Think there is company makin 17t pretty sure my buddy jack had 17t? Said he did anyways lol
someone else mentioned a 17 when I was searching online, but it seemed like that was a little bit too tall for gearing...I'll try out the 16t for a while and go from there
 

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Discussion Starter #8
okay, quick update-
The mpg has gone up a few since the 16t, but I still linger around 40mpg.

However, I noticed the bike starts easier with no choke on(this is not a new issue/symptom, just one I came to realize/remember).

If I turn choke on(doesnt matter if its 100% 75% or 50%) the bike has trouble starting when cold(probably 65f outside time). But if I never turn the choke on, the bike starts right up.

This could contribute to running rich while driving around, right? Causing poor mpg?

What can I adjust to help fix this?
 

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okay, quick update-
The mpg has gone up a few since the 16t, but I still linger around 40mpg.

However, I noticed the bike starts easier with no choke on(this is not a new issue/symptom, just one I came to realize/remember).

If I turn choke on(doesnt matter if its 100% 75% or 50%) the bike has trouble starting when cold(probably 65f outside time). But if I never turn the choke on, the bike starts right up.

This could contribute to running rich while driving around, right? Causing poor mpg?

What can I adjust to help fix this?

It would depend on what is causing it.
What are the mixture screws set to?
Are the choke circuits operating?
 

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Discussion Starter #10
It would depend on what is causing it.
What are the mixture screws set to?
Are the choke circuits operating?

I dont know where/what the mixture screws are set at(how can I determine if they are set properly?). I posted up in the nighthawk forums, but is there a diagram that shows where/what each screw is on the carbs?

The choke does work, as when I turn in "on" I can see it pull the linkage near the carbs. When I turn it "off", it pushes the linkage back in.
 

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I dont know where/what the mixture screws are set at(how can I determine if they are set properly?). I posted up in the nighthawk forums, but is there a diagram that shows where/what each screw is on the carbs?

The choke does work, as when I turn in "on" I can see it pull the linkage near the carbs. When I turn it "off", it pushes the linkage back in.

Yes, it's all detailed very well in the manual.
It may be working mechanically, but the choke circuits might be blocked.
When you open the choke circuits you close the pilot circuits - which probably explains why it won't start when the choke is on.
Unless you open the throttle while trying to start, that will bypass both the pilot and choke circuits.
Has the bike been sitting for any length of time?
 

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Discussion Starter #12
Yes, it's all detailed very well in the manual.
It may be working mechanically, but the choke circuits might be blocked.
When you open the choke circuits you close the pilot circuits - which probably explains why it won't start when the choke is on.
Unless you open the throttle while trying to start, that will bypass both the pilot and choke circuits.
Has the bike been sitting for any length of time?

I have no idea if the bike ever sat for a long time, but its 17 years old so its highly likely. I also found something interesting while looking at the pilot screw adjustments/enrichment screws...carb 1 has just the screw, carb 2 still has a cap on it, carb 4 is just the screw, but carb 3? It looks like it was broken off or something...
Carb 4 looks like this(same as 1 and 2)


But, carb 3 is like this...like something was broken off/a random screw was put in there.




I know pictures arent the best, but its the best I can do until I pull the carbs.
 

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I have no idea if the bike ever sat for a long time, but its 17 years old so its highly likely. I also found something interesting while looking at the pilot screw adjustments/enrichment screws...carb 1 has just the screw, carb 2 still has a cap on it, carb 4 is just the screw, but carb 3? It looks like it was broken off or something...
Carb 4 looks like this(same as 1 and 2)


But, carb 3 is like this...like something was broken off/a random screw was put in there.




I know pictures arent the best, but its the best I can do until I pull the carbs.

Those mixture screws look like a mess.
I can only suggest pulling the carbs, strip everything out for cleaning, make sure all the orifices flow carb cleaner, and ensure everything is installed to spec.
 

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Those mixture screws look like a mess.
I can only suggest pulling the carbs, strip everything out for cleaning, make sure all the orifices flow carb cleaner, and ensure everything is installed to spec.
yeah, thats what I was think would need to happen. The pilot jets are under(well, techincally on top) of the mixture screws, right? Where can I get another mixture screw at? And should I replace the pilots too? Or get a new mixture screw(to replace the missing/broken one on carb 3) and clean everything+re install and see where I'm at?
 

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yeah, thats what I was think would need to happen. The pilot jets are under(well, techincally on top) of the mixture screws, right? Where can I get another mixture screw at? And should I replace the pilots too? Or get a new mixture screw(to replace the missing/broken one on carb 3) and clean everything+re install and see where I'm at?

CARBURETOR COMPONENTS


The pilot jets are further along the mixture screw circuit, but not necessarily underneath the screws.
You won't know what might need replacing until you strip them.
 

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I'm pretty sure what you see in the first picture is the "limiter cap" on the fuel screw. They had about an 1/8 to a 1/4 turn of adjustment. If you tried to make more of an adjustment than that the head of the screw would "break away" from the shaft of the screw. Obviously looks like that's what may have happened at one point or another. I would assume then that it's possible either someone has already been in them, or gave it a futile effort to make adjustments. You'll know even more once you get them off and see what's up. The fuel screw will not necessarily be right where the pilot jet is, however they do share common passageways in the circuit. I would just order everything you need once you get them apart and use an internet site such as partzilla or similar. Unless you have a nearby local dealer. OEM parts work really well.
 

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Discussion Starter #19
Hmmm, look like iridium plugs. I assume with your NGK DPR8EA9 boxes your going back to standard plugs? Don't look so bad to me, and don't look rich in my opinion.

I bought the "standard" plugs before removing the old ones, so I didn't know what I was gonna remove.

Would having a few cracks in the airbox (on corners/leading up to the carbs) contribute to much?(mpg, lean, rich etc) I'll be fixing the cracks this weekend, so I guess I'll find out.
 

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I bought the "standard" plugs before removing the old ones, so I didn't know what I was gonna remove.

Would having a few cracks in the airbox (on corners/leading up to the carbs) contribute to much?(mpg, lean, rich etc) I'll be fixing the cracks this weekend, so I guess I'll find out.
The iridium plugs that came out were not standard. That bike will probably enjoy the regular plug much more. And as far as the air box goes, I would imagine if it were bad enough to make the engine run crappy then it would be a problem. If the engine is running and idling well then I would say no, it's not going to make hat much difference. But seal it up nonetheless.
 
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