Tuning for Economy and Performance

[Anonymous]

I've got a 1979 GS750 L. It has the original VM carburetors, MAC 4-into-2 pipes, and a K&N filter in the original airbox. I went through a bit of tuning last august (thanks for all of the advice back then), but still am not satisfied with the results, and am looking for some fresh advice.

Fuel economy is pretty poor. M.P.G. is in the low 30's. I've thoroughly cleaned all jets and passageways in the carb bodies more than once. Here is how I've got the carburetors set up:

Fuel Screw: 7/8 turn out
Air Screw: 1.75 turns out
Pilot Jet: 15 (stock)
Main Jet: 100 (one leaner than stock)
Needle Height: clip is in the second groove from the top (stock, I think)
Float Height: 0.98"

I synchronized all carburetors, and vacuum is running around 7.5" in all four. Point gap was adjusted statically and checked with a dwell meter. Timing was set with an inductive timing light.

The plugs look good. They are gray, about like #19 on the chart at this link: http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html If anything, this looks to be on the lean side of good, but I'm burning gas like I'm too rich.

The bike seems to run well, but it is cold blooded. I have to keep the choke partially on for a very long time when the air temperature is below 60 degrees, or it will stall at intersections. I recently tried running it with the air screw at 1.25 turns out (per Clymer), and it only got 31 MPG on the last tank, but it seemed like I could take the choke off sooner.

After it warms up, it will idle OK at 1100 rpm. I've tried adjusting the air screws for the "lean best idle" speed, but turning them between 1 turn out and 1.75 turns out didn't seem to make much of a difference in engine speed.

Last August, I tried a bunch of different carburetor settings. I found that the stock main jets were too rich (fouled plugs with throttle wide open), and that having the needle clip in the middle groove was also too rich in the mid range. Lately, I'm happy with the power and drivability, but I'd really like to improve the fuel economy and the cold blooded nature.

So now, some questions:
1. Can worn or stuck rings cause excessive fuel consumption and/or cold blooded performance?
2. Can leaky intake manifold o-rings cause this problem?
3. What about the rubber gaskets on the bottom end of the choke plungers?

Any advice is greatly appreciated.

[Anonymous]

I dont know what your problem could be. # 19 shows me that you have done an excellent job of tunung your carbs. Maybe you drive in hilly country--drive in too much traffic. Mybe it just tht you think you are the burn out or whellie king.

Air in tires ninding brsks or bearingd. chain related ( I think that is a possibility some how)
Driving habbits have a lot to do wit it. i have an 1100 that gets 47 mpg when its not getting its best mileagr. My ysmmy 650 get 5 and thsn sometimes u have seen 57. Probsbly the way i fill the tank but still good gas mileage. Just remember i'm the slowpoke.

[daveo]

I'd say 30s ain't bad it's what I got on my 750 (1100 carbs and stage 3), the way I ride, especially around town, stop and go-fast style. The manifolds leaking could make it cold blooded for sure, rings possibly, but you'd notice oil mist blowing from the case vent and possibly exhaust, leaky plungers would probably make it better. Just remember you don't want to get so lean you're pinging at all, especially with the crap gas out here, could risk burning an exhaust valve or holing a piston

[earlfor]

Re: Tuning for Economy and Performance

Everything you posted seems within parameters except for this.
BIG RED FLAG!!! You dont have a functioning induction system!!!!!

Pull the carbs, take out the old "O" rings and replace them with factory "O" rings. (the factory rings are a specific composition and you cannot use auto store rubber "O" rings)) When you have the rubber induction boots off, check the sidewalls carefully for any cracks.
My 79 750 when set to a high idle of 1800 to 2000 rpm pulls 25 inches of vacuum. My gas milage is 43 to 45 mpg. (and I am running bigger jets than you are with the needle in the middle clip)

Earl


[quote="xcvii90"]
I synchronized all carburetors, and vacuum is running around 7.5" in all four.

[Anonymous]

Thanks for the advice. I ordered new o-rings from my Suzuki dealer this afternoon.

I took her for an 80 mile ride this evening, and she only got 32 mpg. I checked the tires before I went, and they were at 34 psi. I held a steady 65 mph for most of the way on a highway with no stops. There were some hills, but nothing extrordinary. When I returned, the brake rotors were cool to the touch, so the brakes aren't dragging.

Hopefully, the intake o-rings are the culprit. I've tried probing for leaks in the intake boots with WD-40 in the past, but found none. But for six bucks, the o-rings are a cheap thing to try.

Thanks again.

[Dink]

34 psi both ends!! 8O You may wish to try 34 psi front and 38 psi rear.
Dink

[Anonymous]

Follow Up

First step: I replaced the intake o-rings. The old ones were really ugly. They were hard and brittle, and the metal flanges on the intake manifolds were corroded. I cleaned the mounting surfaces and replaced the o-rings with new Suzuki parts.

Second step: I checked the valves. I hadn't done this before, since the previous owner claimed that "it had just been done", and I didn't want to spring for a $24 valve cover gasket. Big mistake. My exhaust valves were way out. I had to install thinner shims in all of them. Two cylinders required a .002" adjustment, but one required a .006" adjustment and another required a .008" adjustment. 8O

Third step: I reassembled everything and took the bike for a test ride to warm it up. When I got back, I synchronized the carburetors. They were way out of adjustment. They are all drawing about 8" now. Even though my gages (and Earl) indicate that this is too low, the mechanic at my local dealer, and the Clymer manual for my Honda CB750 both indicate that this is a reasonable value. :?

Fourth Step: I rode the bike for about 200 miles, using about half of a 16oz bottle of fuel system cleaner in each tank. When hot, I did a compression test. The compression was 107 psi to 115 psi in all cylinders. This is a significant improvement. Before my valve adjustment, I was only getting 85 psi in two of the cylinders, and around 110 in the other two. I then let the bike sit overnight and checked the valves the next day. The adjustment was still perfect.

The Results:

1. The bike is a lot less cold blooded (although it hasn't been below 50 degrees in the mornings lately). I can take the choke off after only a few blocks and she will idle just fine around 900 rpm. After she is fully warm, she idles around 1100 rpm. Idle is very smooth.

2. Gas Mileage is way up. On the last tank, I got almost 41 mpg. That is a 24% improvement over the 33 mpg I was getting before valves and intake boots. I think that it may improve a little more as the fuel system cleaner works its magic, and the valves reseat themselves somewhat.

Beyond that, I think I would need to do a ring job to get my compression back up and to improve performance further. But I don't think that is absolutely necessary, and in the mean time, I'm happy with the results. 8) Thanks to everyone for the advice.

[Anonymous]

Thanks for the excellent explanation on the subject and the update after the modification. I'm strungling with poor mileage too (between 25 to 30 mpg) and was already trying to figure out what to do about it. This post gave me a few excellent tips to check. First one will be the valves check.

[saaz]

Great result!

The compression is on the low end, but if the bike goes well and oil consumption is not a great worry..just enjoy riding!!!

It is odd, but standard my 78GS1000 used to get 50mpg (probably equal to 45 or so US mpg). Modified, heaps more power, get that around town and around 60mpg on trips!

[skip]

great job, Isn't it nice to see positive results from hard work?