Terrible Gas Mileage

[Norseman]

Okay, so this might be far out there, but I've checked everything else. My gas mileage is roughly 25 miles per gallon. So far what I've done is: 1) All valve clearences set exactly at nominal (mid-tolerance), 2) Just did the re-powering coils mod, 3) cleaned carbs three times by total disassembly and Berryman's - followed the cleaning document to a tee with all new gaskets, o-rings, etc., 2) stock pipes and jets, 3) tested and confirmed ignition and electrical systems, 4) new sparkplugs, 5) tuned the bike with Morgan Colortune and Carbtune both, 6) fresh oil, filter, and gas.

I have not done compression testing simply because I don't have a tester. However, the bike does smoke a bit (white). Not on idle, but it does smoke just a bit on acceleration. There are no oil leaks or oil in air box. The bike starts up immediately, idles and runs good, although I have never had a GS650 before so I really don't have a reference point. However, I do think the power should be much more...

My question is, and I hope it's not too stupid, could the extremely poor gas mileage and lack of power be because of bad rings/cylinders that demand more-than-usual gas to accelerate and drive? I don't know how else to phrase the question... If this could be possible, I assume I need to run out and get a compression tester.

 

[Steve]

Although the valve clearances might be correct, did you check the valve timing? :-k

It is not unheard of to find one of the cams off by one tooth. This will affect overall compression and power.

.

 

[salty_monk]

Long shot but also check if anyone has drilled the slides... heavily oversized slide holes with a stock setup made my gas mileage at least 7 or 8 short of where it should have been.

Dan

 

[Norseman]

Thanks Guys. Steve, no I haven't checked timing either - another tool I don't have. But thanks, I will have to do that next. I didn't think of it because the bike runs so smoothly, and I didn't think it would if the chain was off.

Salty, I did check that some time ago when I read about it on the forum. The slides don't appear to be drilled - the throttle slides are real nice and snug with no "slop" and they all move freely.

So, what I'm hearing here is maybe timing is off. So my theory of worn-out rings causing excessive fuel consumption is pretty much off-mark? I really can't afford a timing light right now, but I can buy a compression tester just to see what numbers I get. If I get decent numbers, I guess the next issue is timing, which honestly intimidates me - sigh...

 

[earlfor]

I believe what was meant was valve timing. For that you dont need anything more than a couple of wrenches and eyesight to see the cam timing marks. I wouldnt check compression before knowing the cam timing is correct. Ignition timing would influence how it runs, but would not alter compression readings.

Earl

Thanks Guys. Steve, no I haven't checked timing either - another tool I don't have. But thanks, I will have to do that next. I didn't think of it because the bike runs so smoothly, and I didn't think it would if the chain was off.

Salty, I did check that some time ago when I read about it on the forum. The slides don't appear to be drilled - the throttle slides are real nice and snug with no "slop" and they all move freely.

So, what I'm hearing here is maybe timing is off. So my theory of worn-out rings causing excessive fuel consumption is pretty much off-mark? I really can't afford a timing light right now, but I can buy a compression tester just to see what numbers I get. If I get decent numbers, I guess the next issue is timing, which honestly intimidates me - sigh...

 

[Steve]

I believe what was meant was valve timing. For that you dont need anything more than a couple of wrenches and eyesight to see the cam timing marks.

Correct. To check the valve (cam) timing, you need to pull the cover off the right end of the crank and the valve cover. There is a mark on the crank for 1&4 TDC. When you can see that, the exhaust cam should have a #1 lined up with the top of the head surface. There should be a #2 mark pointing straight up. You will have to consult a manual that is specific to your bike and your year, because the following number may vary. Count the number of pins in the cam chain, starting with the pin that is over the #2 mark on the exhaust cam, ending with the #3 mark that is on the intake cam. Make sure it matches what the manual calls for on your bike.

Sometimes the exhaust is not lined up with the crank, sometimes the intake is not lined up with the exhaust. Either one can affect your mileage.

.

 

[Norseman]

Many Thanks

Many Thanks

Thanks so much, Guys. I will do that first thing tomorrow. I do have the Clymer's manual for my bike, and while I would much prefer the factory service manual, I have to believe Clymer has the numbers correct.

One thing I know for sure, whoever worked on the bike before I bought it last year should not be wrenching on bikes - there were a lot of dumb issues that I had to correct when I initially bought it. I say this because it is entirely possible, considering these other issues, that the valve timing is off from incorrect assembly... We'll see tomorrow after I read the Clymer's tonight :shock: I will post my results...

 

[Steve]

I just went back and re-read your original post and saw that you have a 650. Be very careful reading the manual to see how many pins need to be between the marks. The chain-drive bikes have one number, the shaft-drive bikes have another. :shock: Make sure you are reading the rights specs. 8-[

If I was home right now, I could give you page numbers in the Clymer manual, but I won't be home until tomorrow night.

.

 

[Norseman]

Thanks for the heads-up, Steve. I have the shaft model, and will make sure I verify the number is for shafties.

 

[Norseman]

Compression Tester?

Compression Tester?

Well, checked timing and chain - all is what it should be, so the timing is not off. I guess that leads me to compression testing.

Anybody have any suggestions on compression testers? I checked out Motion Pro on Bike Bandit, and at $132 + shipping it's to expensive for me. Plus I would like to get one right away. Has anybody purchased one from AutoZone or NAPA or Murray?

Also, since this is my first compression test, here's another dumb question. I assume I run it first to warm up the engine and then without disconnecting fuel tank, just remove all four plugs and test one cylinder at a time with full open throttle? (even if this sprays gas/air into cylinders?)

 

[seuadr]

i've used a couple different cheap compression testers from local autoparts stores for cars.. they may not be pretty, but they generally get the job done. i wouldn't sweat it, if you have a murrays near you, they probably have nice ones you can rent. they are awesome like that. 'nother michigander is good to see! we might have to have a tri state area rally or something!

 

[p_s]

Make sure the compression tester has an adapter for 12 mm plugs--most seem not to.

 

[Norseman]

Thanks again, Guys. I checked out the various auto stores, and found Equus brand at Murrays for $25, Great Nech brand at AutoZone for $35, and HarborFreight has a Great Neck brand on sale for $15 (reg. $25). they all have 12mm adapters (Harbor Freight has 10mm, 12mm, and 14mm adapters).

So, since I will likely just use it once in awhile, it's off to Harbor Freight I go...

UPDATE: Don't bother buying the Harbor Freight compression tester - it broke apart on first useage...

 

[Guest]

You're bike is using twice as much gas as it should. When I had a 650G, I got roughly 45 in town, and 55 on the road. Something is seriously wrong.

 

[Norseman]

I agree, Tom, but I can't figure it out. Valve clearances set at around 0.05x, carbs cleaned and rebuilt, bike is tuned using CarbTune and Colortune, coils rewired with relay mod, new sparkplugs, compression on #1 & #4 is on the low end, #2 & #3 is around 165 psi, gas not leaking into engine...

I just don't understand what is causing this. The only thing I can think of is when I tore the carbs down last summer for a complete cleaning, I don't think I checked the actual numbers on pilot and main jets - I just assumed they were original since the bike is totally stock.

But if the PO did re-jet thinking in his ignorance it would be a good idea for more "power" or whatever, well, I guess that could cause excess fuel usage?

What else could it be? The throttle slides are fine. I just don't get it...

 

[p_s]

What else could it be? The throttle slides are fine. I just don't get it...

Brake drag?

 

[Norseman]

I don't think so - I just went out and checked. Front spins freely, and rear spins with a little resistance (I assume from shaft drive). But I definitely don't feel any drag from calipers.

 

[Guest]

With a drive shaft the rear wheel won't spin.

The magazine tests from 1981 and 1982 reported 4 seconds for 0 to 60, and a 13 second quarter mile. Sounds right to me. I once pulled a wheelie using throttle alone - no pulling back or dumping the clutch. If your 650G doesn't accellerate quite briskly, there are serious tuning problems. Of course, with that mileage, you already know that you have serious problems.

I think that testing compression is next. Sears sells testers starting under $30. Harbor Freight starts at $10. Ebay has a KD one for $10. White smoke is a bad sign.

How many miles on it?

 

[jadesystem]

While you are at the carburetion topic, and you decide to remove and inspect the carbs again, try to check the pilot and main jets for the
value vs the correct stock value in the manual. Check the seating of
the Needle Valve.

You can use a 8-10mm metric hose to the gas inlet nipple, and blow
thru the hose into the float bowl with the bottom float bowl removed.
Move the float up and down to verify it seats and cuts off. Once the
float is level it should cut-off whereas you cannot blow air thru it as
the needle valve is seated. If not, bend the tang on the float so it
cuts-off at level. Check all the varbs this way. You are simulating the
gas and verifying the needle valve is adjusted correctly. Otherwise
gas will overflow and become wasted - the metering function of the
needle valve will be defeated.

You can read your plugs too to see if they are heavily carboned. It
could be running too rich. Seat the air/fuel mixture screws CW, then
CCW 1.5-2 turns. This is a static adjustment. It is better to run slightly
rich than too lean.

When you do your compression test, take out all of the plugs so that
the engine turns over easier. Make sure your battery is fully charged.
The compression results should be within 20-25psi from each other on average. if it is way off, something is wrong with your rings or valve
seats. One way to isolate this to point to rings or valves is to pump
it up with compressed air with both intake and exhaust valves closed.
If air seaps to the bottom thru the PVC valve, it is rings. If it leaks at
the top, it is the valves. Of course, the valve cover must be off to
hear and see it (bubbles thru some excess oil).

Good luck to you :-D

 

[Norseman]

Much appreciated, Tom and Jadesystem. Tom, after reading the info on acceleration I definitely don't have that kind of power on take-off! I did a compression test and got: #1=135, #2=165, #3=160, #4=130. Not the greatest on outboard cylinders, and I will eventually re-bore/hone/new rings, but that alone shouldn't cause excessive fuel use or dramatic power loss. So I guess I remove carbs and clean again, and check the jet numbers.

Now, one more piece of information - once in awhile, during drving, suddenly the bike will abruptly really out white smoke - I mean a real cloud. Stop the bike, shut it down, wait a few seconds, and start it up again, and it will be fine.

Any ideas on this?