Need some ideas

[Ivan]

My 81 franken bike threw a rod, and needs some heavy repair.

So, I am about to make the trek from central KS to central TX on the 79. Problem is that, even though it runs VERY strong, it doesn't get the best mileage. Last night I had the wheel up in third gear and hit 130 no problem and was still gaining when I shut it down, so power is there, and it runs without a hiccup.

So, what can I do to increase the mileage? I bumped the timing advanced a few degrees and that helped for about 5 mpg, but I am still only at about 25. Its the early style carbs, so I could lower the slide needles a notch, but I don't want a stumble.

I am going to put a fairing on it, and that might(?) help with wind drag too, but no way to tell until I get it.

Its that or swap the engine into the 81 which is getting the Maxim tank, which should give me more than 100 miles per tank. But I'd like to keep the numbers matching. Not to mention that's going to be a lot of work to get done at nights during the week.

So, let me have some ideas...fire away.

[jmnjrpa]

What

plugs are you running? I found the E3 gave me better gas milage and HP over everything else including the iridium plugs. I also add an octane booster. What is your tire pressure? Higher pressure will be a slight benefit. Dragging brakes? Axle too tight? Mid and final drive in good shape? Clutch slippage? I really don't think any one item will fix it and I bet you already checked most of these but.....25 MPG is pretty low. Maybe quit dragging your feet?

[DGXSER]

Do you have stock air box, stock exhaust? What jetting are you running?

As mentioned, look for dragging brakes or gear boxes (final or middle).

All I got right now.

[Ivan]

Well, the PO did an immaculate job of maintaining it. Everything except the exhaust is stock, and in pretty close to showroom condition. Its a really nice bike. The clutch does slip but that is in hard accell in third and fourth.

Not sure the plugs, haven't even pulled a cap.

Tire pressure is on the high side of the range, so that should be ok.

Pretty sure everything is good as far as drag issues. When I did the fork seals, we made sure the front turned easily, when the calipers were all lined up. The rear turns as easy as you'd expect from the normal drag of the two gear boxes.

Haven't pulled the carbs yet, but I would guess they are stock jets from the condition of the rest of the scoot.

E3 plugs are NGK? I will look into them.

[tarzan]

Hey Ivan,
If the the motor with the thrown rod got better mileage , can you put the carbs off that one on the one that runs? You should get close to the same mileage.
You could also throw in a Triple final drive.JAT
Rick

[Ivan]

The thrown rod only did about 28, so not that much better. And that is with the final drive mod. Mine actually got better mileage running in fourth, so the swap wasn't worth anything other than having the lower noise level.

Mind you, most of my riding is at about 80 mph on the super slap. I know that isn't going to get fantastic mileage on anything, but when I hear about people pulling mid 40s, makes me think I should be able to get at least mid 30s.

Maybe its just a pipe dream.

[Yard Dogg]

Well, the PO did an immaculate job of maintaining it. Everything except the exhaust is stock, and in pretty close to showroom condition.

We just had a debate about exhaust on these bikes. With a gathering of knowledge we've come to the conclusion that the length of your exhaust, or should I say the distance of the first baffle from the cylinder head is the most crucial thing to insure power and effeciency. The reason being, that when the compression stroke hits, it creates two waves of energy in the pipe. The first is a sound wave which travels faster than the second wave, which is the air pressure wave. The sound wave then reverbs back through the pipe hitting the air wave (known as back pressure) and causing the spent gases to be pushed back into the cylinder head greatly robbing you of performance. When the baffle is positioned correctly, the sound wave will hit the air wave at the correct time so the spent gas is allowed to escape stroke after stroke. A cross over tube helps to equalize the back pressure between the 4 cylinders, and at the same time makes use of both mufflers for increased flow and less over all pressure in the exhaust and less bogging of the engine. Find some stock pipes, and rejett if needed, resync, and I'll bet you pick up some mileage. At the very least, make sure you're baffle is located the correct distance from teh cylinder head, this will help. If you have Spaghetti pipes, forget everything I said.

[Larrym]

Wimpy Vacuum Advance?

Hi Ivan,

Sorry to hear that the rod went Elvis on you. Just for my own curiosity's sake, if you could provide the approximate rpm this happened, I'd appreciate it.

From the symptoms you're giving, I'd guess that something might not be working up to snuff in the advance mechanism. Specifically, I'd suspect the vacuum advance. Sounds too similar to what anyone would find if the vacuum advance wasn't connected or working on any engine (bike or car). Strong running under acceleration yet when you let up off the throttle it doesn't move the timing into the range where you can coast/cruise with minimal effort and maximum MPG. That little bit can mean a lot especially when it just ain't there.

I'm not sure at what RPM the vacuum advance really stops providing any gain over the mechanical advance. I know that at the speeds I ride (under 80 mph) I do get mileage in the High 30's. My riding style does include backing off the throttle for extended periods where the vacuum advance provides the added efficiency.

If you swap the carbs and the bike still gets the same mileage then that would point to something other than the carbs as the mpg limitation.

I've got a set of fully functional carbs I use as a test bank. I synced them and did a road test: passed with flying colors on my 79 Special. Standard jetting and float settings. If you'd really like to rule out fuel systems as a possible cause then let me know. I'll pay for shipping to you and you can handle the return costs.

[GARTHXS]

floats????

I suspect float level,and check the main jetting it sounds like you are running to big a main jet, with the 4/1 it should run good with a 45 or 47.5 it will run really good with 50s but mileage will suffer.

[motoman]

sure you already know.....just reaching for straws so to say, not being present, is the 79 TCI module working as it should. Sorry to hear that on the 81, but your indications on both bikes seem to have a common denominator going on as far as mileage which yes, that is poor considering what they easily get normally. Alot of variables can cause the noticed added fuel consumption, mainly carb issues. Might pop off one of the diaphram tops and carefully inspect those vacuum diaphrams for small pin holes. Any lack of proper vacuum drawn to them won't allow the needles to set in the main jet at the normally correct location in relationship to vacuum being drawn at whatever steady given RPM. They rise with vacuum drop as in opening throttle farther and will mimmick the same with any pinholes in them. Bad enough that they become more flexable with many years of flexing causing needles to rise and set a few thousands higher than they would have when new. The early carbs having the needle clip positions helps to correct that, but as you know the tiny thin washer trick kinda will give you an in between setting of any two clip positions. Know that may be a little nit-picky with the tiny washer but if you really wanna have one of these scoots "dialed-in", sometimes that's what is needed. Just a thought as I'm sure others have many good suggestions to check and consider.

[Ivan]

I have already checked the advance mechanism, as best I can. I don't have a timing light for precise measurements, but it does fully pull by about 3500. I have my suspicions that the ethanol content in the gasohol around here may require more advance than normal. I may bump it even further and see what happens.

I will probably end up pulling the carbs and seeing what is what in there.

As far as the rod, I think it went from oil starvation, since I lost a crank seal, which was supposed to be 48mm outside diameter, but was 47.6 or some odd thing like that. So when the light came on, I killed the engine, but apparently it wasn't quick enough. Now the engine sounds fine at idle but knocks like hell on load and is getting worse. The noise moved up to the top end at idle, which makes me thing the conrod bearing or wrist pin is shot. I'll pull it after MoM and dig into it. This really surprised me after hearing about Bob's XJ that ran until it seized up with the oil pump coming unbolted, and still running afterward.

[GARTHXS]

or maybe

You could slow down! !!

[XS1100 Newbie]

Ivan. Invest in a wrist restraint, or put a speed govener on it. Just kidding, but i bet slowing down would help.

[kjohnson]

I have to tune my XJ11 up, but I think I've been getting about the same economy that you have ~29-31 mile / UK gallon. I think I have something sticking somewhere as well as it is a real beast to push. My XS750 feels like a matchbox car compared to my XJ11.

That being said, I have no advice to give, but I am gonna be taking some of the advice in the thread to track my problem down.