How to get better MPG??

[mastertechy]

Hello All,

I am new to the forums and have been restoring my 79 xs11 special. I have a 4/1 exhaust with a OEM replacment K&N air filter on her and as far as I know the only "MOD" is I took off the Octy and tossed it on the self. I have just bought a set of OEM Yamaha exhaust to put back on. I have been through the carbs and rebuilt them, running all the factory jets. My issue is I am only getting around 17.5 mpg on her and from other postings I should be getting 35-40 mpg. I am not driving her like a race bike just do a little cruising. I really am at a lost if putting the factory pipes back on does not improve the mpg. I have also found a factory air filter to put on as well but not sure if I should hold off and put the pipes on and see if that does not improve anything. Would anyone know if maybe to a valve job would help, there is a very small tick? But I am starting to be at a loss and don't want to keep putting money in her to only be getting around 17.5 mpg.... Any advise would be helpful... Thanks!

[crazy steve]

Have you synced the carbs after the rebuild? Is your vacuum advance hooked up? Is your ignition system working right?

Which exhaust you have would make very little difference in mileage, whatever the problem is it's not particularly related to that. Are you sure you have '79 carbs on it?

[mastertechy]

Carbs have been sync'd, was told the carbs were of the 79 year (brass adjustments), she starts right up no problem with that and all the vacuum lines minus the ocy are hooked up correctly as far as I know.

She is very slugish in 1st to 2nd gear but once in the higher rpms she runs great... don't know if that would help..

[ViperRon]

It would help if you could also do a compression check to see what you have. If the bike has not had the valve adjustments done in a long time that may be an issue. As Steve said check the vacume advance and timing and if you do not find the issue get a guage and check compression you can borrow them free from Autozone with a security deposite. You may only need to adjust the valves or you may have some carbon build up that makes it look like a valve problem. Do the simple steps and checks first many things are dependant on other steps being good. It would help to know things like milage, how long the bike was sitting and what new parts you have on it like plugs wire caps etc.

[natemoen]

Where do you have the vac advance line hooked up? It SHOULD be hooked up to a nipple on the engine side of the #2 carb body and NOT on the carb intake boots.

[TADracer]

Yeah, it kind of sounds like an ignition thing to me. Weak spark at some point in the RPM range. Check for free movement of your centrifugal advance mechanism. Pull some vacuum on the dashpot for the vacuum advance. The arm should hold completly in at about 5" of vacuum. With the bike running and timing cover off, bring the RPMs up slowly and watch the vacuum advance mechanism. It should advance until you let off the throttle. Disconnect the vacuum advance hose and plug the carb nipple then raise the RPMs to 5200. Timing should be at 31 degrees for an SF. Make sure you have the right heat range of spark plug (I am one of those old guys that stay away from automotive plugs). Look and see if the PO put on aftermarket coils. If yes then you may need to jump the balast resistor. Check your initial timing (79SF should be at 5 degrees). In general, go through everything 'ignition' before you start chasing those carbs.

[mastertechy]

Ok let me try and check some of those things and then I will test and let you all know... Thnx for the advise, now I have somewhere else to look beside the pipes and carbs and air cleaner...