Bad gas mileage.

Got my 79 750 twin road legal finally and been riding around. It has higher end uni pod filters with open exhaust headers. It has a baffle in it for back pressure because the bike will not run at all without it. I have a 2.4 gallon tank on it. Right now I have 32 miles on the bike and my tank is almost completely empty. I have 14 front and 32 rear sprockets. 60-65mph at 5,000 rpm on the highway. Won't go any faster than 80 and that's wide open.( don't need to be going 80 anyways lol ) . I do some city driving when I'm not on the highway but does open exhaust and pods really make mileage this horrible? I think my pickup truck gets better mileage :blink:

Schmeck wrote: Got my 79 750 twin road legal finally and been riding around. It has higher end uni pod filters with open exhaust headers. It has a baffle in it for back pressure because the bike will not run at all without it. I have a 2.4 gallon tank on it. Right now I have 32 miles on the bike and my tank is almost completely empty. I have 14 front and 32 rear sprockets. 60-65mph at 5,000 rpm on the highway. Won't go any faster than 80 and that's wide open.( don't need to be going 80 anyways lol ) . I do some city driving when I'm not on the highway but does open exhaust and pods really make mileage this horrible?:

YES. I am riding that exact bike with stock air box and MAC exhaust and average 38 mpg in city, highway is probably about 50 - 55 mpg ballpark. With pods and open exhaust, you are probably so far off tune that you have to ride it with the throttle half wide open all the time just to get it to run. The gas is going out the tailpipe.

Bike runs and idles perfect except for top speed. No half throttle all the time . Most of the time I give it alittle throttle unless I'm on the highway then it's 3/4 throttle at 65mph. I can push it up to 80 it just surprises me that with my gearing that it's at 5,000 rpm at 60-65mph. It doesn't run horribly out of tune , infact it drives smoothly . Just eats my gas fast. What can I adjust for better mileage? My plugs are black but have not fouled me yet. Maybe go up on a clip position? And does the idle screw effect highway speeds?

The bike has 155 compression in both cylinders cold. Went for a ride earlier but engine felt cold when I tested it hours later. It will sit and idle for however long till it runs out of gas lol . Like I said, it's just a monster on gas mileage . : /

Stock gearing and tires on a '79 750 twin will turn 5K RPM @ 70 mph. Your gearing is a shade lower than stock but that won't cut gas mileage in half.

I would wager the problem is that even if your throttle is only open a shade more than stock, it eats a lot more gas. If you look at the needle taper, lifting the slide even a bit opens the gas up a lot. When I cruise at 40 - 45 mph, my throttle is barely open which means I am running mostly off the pilot jet. If you have to open the throttle a bit and lift the slide, the main jet is kicking in and that sucks gas. Being off tune even a little shifts the operating point off and that can really eat gas.

The gas has to be going somewhere.

bountyhunter wrote: Stock gearing and tires on a '79 750 twin will turn 5K RPM @ 70 mph. Your gearing is a shade lower than stock but that won't cut gas mileage in half.

I would wager the problem is that even if your throttle is only open a shade more than stock, it eats a lot more gas. If you look at the needle taper, lifting the slide even a bit opens the gas up a lot. When I cruise at 40 - 45 mph, my throttle is barely open which means I am running mostly off the pilot jet. If you have to open the throttle a bit and lift the slide, the main jet is kicking in and that sucks gas. Being off tune even a little shifts the operating point off and that can really eat gas.

The gas has to be going somewhere.

That makes a lot of sense.

If not already done, would perform the clear-tube test on each carb to assure fuel levels are within specs.

And also sniff-test and examine the crankcase oil to check for possibility of gasoline contamination.

Good Fortune!

Carb sync wouldn't hurt either

Patton wrote: If not already done, would perform the clear-tube test on each carb to assure fuel levels are within specs.

And also sniff-test and examine the crankcase oil to check for possibility of gasoline contamination.

Good Fortune!

+1 Never ignore the obvious...... that gas may be leaking past the float valves and hitting the ground or going into the crankcase.

I had it synced 2 weeks ago when I was messing with the carbs but maybe I did it wrong? . I will double check. I have head of a dry float level check that doesn't involve clear tubes as all I have is clear brake tube which doesn't do a good job. I'm sure the clear tube test has better accuracy though? Another question is has anyone else had issues with the needle underneath the choke. Pilot jet I think but mine doesn't like to stay in. In fact I had to replace one because it backed itself out( due to the twins vicious vibrations). I haven't addressed that issue yet cuz I'm not sure what to put on it . It needs to stay in but also be adjustable.

Thanks for helping also. Things go a lot quicker with advice from you guys. I'm sure other users think the same ,

Schmeck wrote: I had it synced 2 weeks ago when I was messing with the carbs but maybe I did it wrong? . I will double check. I have head of a dry float level check that doesn't involve clear tubes as all I have is clear brake tube which doesn't do a good job. I'm sure the clear tube test has better accuracy though? Another question is has anyone else had issues with the needle underneath the choke. Pilot jet I think but mine doesn't like to stay in. In fact I had to replace one because it backed itself out( due to the twins vicious vibrations). I haven't addressed that issue yet cuz I'm not sure what to put on it . It needs to stay in but also be adjustable....

There are several types of sync tools available, such as dial gauges, mercury tubes, and others.
An eyeball manual bench sync may be hit or miss, and it's mostly miss.
A poor sync can increase vibration at lower rpms.

The dry float level check is unreliable.
The relatively quick and easy clear tube test should be properly performed on each carb.
Clear plastic tube is inexpensive and readily available in different sizes to obtain the correct size needed.

The pilot jet isn't "adjustable" and should be tightened snugly enough not to vibrate loose.

Would assure that the choke plunger is staying down in it's closed position when the choke is OFF.
And also assure that the spring on the starter plunger isn't broken or missing.

The accessible exterior "adjustable" component on each carb is the pilot mixture screw which governs air-fuel ratio in the pilot circuit.
The pilot mixture screw is important toward smooth steady idle with strong pull-away and preventing popping from the exhaust on deceleration.
The pilot mixture screw should have a spring to help keep it from vibrating out of its set position.

Good Fortune!

Assuming you have the stock BS-38 carbs:

The pilot jets are in the top of the float bowl. You need a perfect fit screwdriver for blade thickness and width to get them out or tighten them. That's why so many are torn to shreds. Get a flatblade of correct blade thickness and grind the blade width to fit.

The stock pilot jet is 45, but 50 will run better. Type is BS30-96. Available many places. Start with pilot adjust screw about 2T out from full in.

The main jets are at the bottom of the float bowl under the drain screw. Stock is 125 but with straight pipes and pods, probably need much larger.