Exceptionally poor MPG

[Gbrown]

So I've rebuilt the carbs on my 78 GS750 and paid someone to sync them, as well as rebuilt the airbox, changed the plugs and a set of points, changed the oil, etc. etc. but the bike is getting TERRIBLE fuel economy. Of course I didn't buy a vintage bike for the sole purpose of economy, but I did have an expectation of 40+ MPG after reading these forums extensively. I've run (3) tanks of gas through it and I'm averaging somewhere in the mid-20's on MPG. This seems really low, especially considering the bike doesn't run that bad... Any ideas on what to check first? As I alluded to, the bike doesn't run as well as I think it should so I'm going to take a look at the jetting; but before I went down that rabbit-hole, I thought I'd at least poll the experts here. Any help is greatly appreciated. Thanks.

 

[Guest]

Take a look at your spark plugs, what do they say.

 

[Big Rich]

Rusted up drive chain, or bad wheel bearings?

 

[Big T]

Your expectation is correct

New condensors? Properly timed? Advancing as it should?

 

[Gbrown]

Take a look at your spark plugs, what do they say.

I will attempt a 'plug chop' and post pictures here soon.

 

[Gbrown]

Your expectation is correct

New condensors? Properly timed? Advancing as it should?

I wish I could answer any of those questions with confidence... This is the first motorcycle I've ever owned and I don't think it's run properly since I've owned it.

I rebuilt the carbs and replaced one set of points (because I didn't have spark on the #1,4 cylinder). Although, when I replaced the points I did nothing more than do my best to gap them properly, I did not check/adjust timing.

I paid a guy to sync the carbs and adjust the valves. He says he checked the valves and they didn't need adjusted (I'm looking for a second opinion on this because I find it hard to believe that the valves didn't need adjusted and I know for a fact that they haven't been adjusted for 10+ years--per the P.O.).

The bike starts easy enough but is very 'cold-natured' and I usually ride for 10+ miles before I can fully open the choke. Also, the bike cackles and pops (backfires) when I decelerate. I have noticed that the #4 header does not get hot at idle, but it does get hot when riding. Could this be a clogged pilot jet?

The bike came to me with a 4-1 exhaust but that's the only mod from stock that I'm aware of. I do not know the jet size because I didn't know to check that when I had the carbs apart and I'm dreading removing them again--it's riding season, man! There's literally ONE shop in my area that will work on an 'old' bike like mine and they're always booked solid.

I'm willing to do work myself and am fairly mechanically inclined--I've always owned old cars and done all necessary work on those. I'm just a bit out of my depth when it comes to the Japanese engineering. Any advice on what to check is greatly appreciated. Thanks all.

 

[Steve]

The bike starts easy enough but is very 'cold-natured' and I usually ride for 10+ miles before I can fully open the choke. Also, the bike cackles and pops (backfires) when I decelerate. I have noticed that the #4 header does not get hot at idle, but it does get hot when riding. Could this be a clogged pilot jet?

If it starts that easily, the valves are likely not the problem. However, riding for 10+ miles on "choke" is killing your engine and your gas mileage. With properly-tuned carbs, you chould be able to use partial "choke" to start the bike. Modulate the control just a bit to keep revs under about 2500, get your helmet and gloves on, then RIDE. By the time you get into third gear, you should be able to turn the "choke" OFF. Sounds like it's time to do some tweaking of the mixture screws to get them "right".

Your popping and backfiring can be a lean mixture from the carb or an air leak in the exhaust system.

Your cold #4 pipe could easily be due to a plugged pilot system or simply a mis-adjusted fuel screw.

.

 

[Brendan W]

Leaks are my first suspect when fuel is disappearing. Sounds like you have other issues as well. Bear in mind that some of these bikes leak gas past the tap and it can end up in the sump.

Whats happening to your oil level and is there a smell of petrol from the oil filler?

 

[Gbrown]

If it starts that easily, the valves are likely not the problem. However, riding for 10+ miles on "choke" is killing your engine and your gas mileage. With properly-tuned carbs, you chould be able to use partial "choke" to start the bike. Modulate the control just a bit to keep revs under about 2500, get your helmet and gloves on, then RIDE. By the time you get into third gear, you should be able to turn the "choke" OFF. Sounds like it's time to do some tweaking of the mixture screws to get them "right".
.

OK, that's good to know. I have to start the bike on FULL choke, and after a few seconds the idle starts to DROP so I open the choke to half and the idle goes back up a bit. I've never let the bike sit and run for more than a few minutes for fear of overheating but if I try to fully open the choke after a minute or two, the bike dies.

Your popping and backfiring can be a lean mixture from the carb or an air leak in the exhaust system.
.

That's what I was afraid of... I've heard horror stories of burning valves (or worse) from running too lean. I did have an exhaust leak (by ear) before I did the oil change and fixed the broken/missing exhaust bolts. Is there a more scientific way to check for an exhaust leak then 'by ear'?

Your cold #4 pipe could easily be due to a plugged pilot system or simply a mis-adjusted fuel screw.
.

I suppose I better pull the carbs and give em a look... Thanks.

 

[JTGS850GL]

Probably an exhaust leak at the header. Did you replace the exhaust gaskets when you fixed the broken/missing exhaust bolts? If you, then that would be a good place to start. Drop the exhaust, pull the old gaskets out and replace with new gaskets. Some add two gaskets but I've always found one to be sufficient as long as the mating surfaces are in reasonable shape.

As for the idling and gas mileage issues, I'd say it definitely time for a carb rebuild. Leaking floats can cause pretty bad gas mileage issues as well. You wont see the problem with the floats when the engine is off if your petcock is working correctly.

 

[Gbrown]

Probably an exhaust leak at the header. Did you replace the exhaust gaskets when you fixed the broken/missing exhaust bolts? If you, then that would be a good place to start. Drop the exhaust, pull the old gaskets out and replace with new gaskets. Some add two gaskets but I've always found one to be sufficient as long as the mating surfaces are in reasonable shape.

As for the idling and gas mileage issues, I'd say it definitely time for a carb rebuild. Leaking floats can cause pretty bad gas mileage issues as well. You wont see the problem with the floats when the engine is off if your petcock is working correctly.

Yes I replaced the exhaust gaskets and I JUST rebuilt the carbs. Besides listening to the header, is there a more definitive test I can perform to determine/rule out an exhaust leak?

No smell of fuel in the garage and no wet areas on the bike or floor. I believe the issue is tuning...

 

[JTGS850GL]

You wont see or smell any gas in the garage if it's a leaking float and a working vacuum petcock. It only dumps more gas into the engine and only when it's running. An excessive fuel problem on a specific cylinder will not usually be jet related as long as the carbs are clean and you have no major mechanical/electrical issues with that cylinder. That's assuming that all the jets are the same in all the carbs. What I'm say is that, if all four carbs are jetted the same, then you should not have a single cylinder going pig rich if the carbs, cylinders and ignition are working correctly.

 

[Gbrown]

You wont see or smell any gas in the garage if it's a leaking float and a working vacuum petcock. It only dumps more gas into the engine and only when it's running. An excessive fuel problem on a specific cylinder will not usually be jet related as long as the carbs are clean and you have no major mechanical/electrical issues with that cylinder. That's assuming that all the jets are the same in all the carbs. What I'm say is that, if all four carbs are jetted the same, then you should not have a single cylinder going pig rich if the carbs, cylinders and ignition are working correctly.

Very good to know, thank you. How do I determine a leaking float? What, exactly am I to look for? Thanks again.

 

[BigD_83]

OK, that's good to know. I have to start the bike on FULL choke, and after a few seconds the idle starts to DROP so I open the choke to half and the idle goes back up a bit. I've never let the bike sit and run for more than a few minutes for fear of overheating but if I try to fully open the choke after a minute or two, the bike dies.

Just to reiterate a point, the "choke" is not a choke but rather a fuel enrichener, so you are adding more fuel at FULL choke, and less as you dial it down.

When you say you try to "open" the choke, do you actually mean lever all the way right (or knob pushed all the way in)?


Assuming you have it FULL choke to start, and are applying no throttle then it sounds like that aspect of the bike is running properly. The RPMs will drop when it warms up a bit, as the bike is seeing too MUCH fuel at FULL choke. Dialing it down gets the A/F ratio back to normal, and the RPMs will rise a bit.

Really though, in the summer, you shouldn't have to ride for much more than a block or two at half-choke before turning it completely closed (no enrichening of gas).

So, to clarify, can you explain what your understanding of "OPEN" and "CLOSED" is?

 

[Gbrown]

Just to reiterate a point, the "choke" is not a choke but rather a fuel enrichener, so you are adding more fuel at FULL choke, and less as you dial it down.

When you say you try to "open" the choke, do you actually mean lever all the way right (or knob pushed all the way in)?


Assuming you have it FULL choke to start, and are applying no throttle then it sounds like that aspect of the bike is running properly. The RPMs will drop when it warms up a bit, as the bike is seeing too MUCH fuel at FULL choke. Dialing it down gets the A/F ratio back to normal, and the RPMs will rise a bit.

Really though, in the summer, you shouldn't have to ride for much more than a block or two at half-choke before turning it completely closed (no enrichening of gas).

So, to clarify, can you explain what your understanding of "OPEN" and "CLOSED" is?

My understanding is the same as the way you explained it, except I was thinking of choke 'closed' = less air and therefore more fuel, and choke 'open' = more air and therefore less fuel. My choke happens to be an up/down lever as opposed to a push knob as you implied, FWIW. So I believe we're on the same page.

Unfortunately, though, my experience has been that the bike takes far more than "a block or two" before it will run with no choke. Also, I've noted that if I try to turn the idle down below 2K, the bike is hesitant to stay running. If the bike is really warm (after a 25 mile commute on the freeway) I can turn the idle adjustment knob down and the bike will idle (roughly) at ~ 1100 RPM. For you experts out there, what's this indicative of?

 

[JTGS850GL]

It's indicative of dirty carb passages.

 

[Steve]

I agree, "dirty carbs".

You should not have to touch, let alone adjust, your idle speed knob on a day-to-day basis, let alone during a ride. When carbs are cleaned and set up properly, you might need somewhere between half- and full-"choke" to get it going first thing, Modulate your "choke" control to keep revs under about 2500 for the time it takes to get your gear on, then ride. By the time you are in third gear, turn the "choke" OFF.

What you are describing as your "normal" routine are classic symptoms of dirty, or at least mis-adjusted, carbs. Try an adjustment first, if that doesn't fix it, it's time to do a full "strip-and-dip" rebuild. If you don't feel comfortable doing the rebuild yourself, there are a few of us here that will do them for you, for a modest fee, of course.

.

 

[Rob S.]

I read these threads to try and learn.

I guess this is just a generic problem, but I'm still surprised that no one has asked for the model/year.

I guess we're not that far into the details yet.

 

[BigD_83]

I read these threads to try and learn.

I guess this is just a generic problem, but I'm still surprised that no one has asked for the model/year.

I guess we're not that far into the details yet.

Rob...You might have glanced right over it, its a 78 750 (stated in the first post)

 

[JTGS850GL]

I believe that the first line of the first post indicated that it was a 78 GS750.