Poor fuel economy

kz750saskatoon wrote: Sorry to hijack this thread partially.

When I did a compression check last year I got 105 or 110 PSI across all the cylinders. That seemed low to me but it seems to run okay, even though the mileage is low.

If that is the true compression, those numbers are horribly low and below operating spec limits. It would explain the terrible mileage since you will be constantly cranking the throttle open to get any power out of the engine. The engine probably needs a valve job (at least).

2bskor wrote: It would roughly be 24 mpg or a guzzler!!

If you have checked compression and valve clearance (and they are OK) I would check for big fat sparks at each plug, check timing/timing advancer and then go after the carbs. BTW, leaky needles/needle valves will let gas leak out. Ever see any on the ground or smell it in the oil?

If the needles are sealing, it sounds like you need to work the jets and needles. The gas has to be going somewhere.

What is the compression I should expect? I checked it last year and seem to remember thinking it seemed okay. Am I looking for 120+ psi?

I should clarify this statement. I have no FSM (no luck finding one. If you have any suggestions or an electronic copy I'd be very grateful) so based on the Clymer book (I just looked) the 110psi is low. It suggests 125+ psi. I will be trying the tablespoon of motor oil into the spark plug hole and a compression test to see if that makes a difference.

What I did check last year against spec was the valve clearances. I'm new to this type of stuff. Can you expand on what all a valve job entails?

A new/rebuilt engine might see 180 PSI,put afew years on it and maybe it's down to 160 PSI. 125 will produce some power but 100 PSI or lower will have you maxing out the throttle trying to go faster(probably wide open).

The 750 twins are 155 psi nominal (new), about 115 psi service limit. I think the 750 fours are higher like 180 ballpark so service limit is probably 125 ballpark.

Jeez, I have a fairly significant problem then. Looks like some other projects are being put on hold and I have some investigating to do.

Altitude will change compression readings, higher the lower, and it is always a good idea to double check. Use a different gauge to verify, and my fav, a leak down test. Did you do the test with the engine hot, and the throttle wide open? Also add a small amount of oil to the cylinder and retest to see how much it recovers. But if it is low, then the power/ mileage would suffer.
Have you ever checked the ignition advancer? And timing?

I would love to do a leakdown test. I think I may just bite the bullet and purchase a tester. I think it will involve ordering one as I don't think any shops here will have any on hand.

Elevation here is 1580ft. I'll have to try to get my hands on a second gauge.

I do not remember if the engine was hot. I hadn't thought much about the test for over a year. Is removing the spark plugs with the engine hot not a concern? I thought I read here once that there can be a danger of wrecking the threads? I also do not remember about the throttle.

I will redo the test this weekend and repeating it with oil in the cylinder immediately after to learn some more.

I never thought the power was lacking but I wouldn't know any different about what to expect from this bike. I do know that without a windshield and into a 40km/hr head wind the bike struggles to get over 70mph which I always thought was not correct.

I believe I checked the ignition advancer and timing but again I can't remember the details of it. If it turned out bad I would have dealt with it for sure. I'll be checking that again as well.

Removing spark plugs hot should not be a problem, but let it sit for maybe 15 minutes so you won't get burned. Do use anti sieze goop on the threads and torque them correctly going back in.

:woohoo: :kiss: :blink: :sick: :ohmy:

Thanks for the hijack guys!!

bountyhunter wrote: Removing spark plugs hot should not be a problem....

May use a short length of hose that fits snugly around the porcelain part of spark plug to easily complete removal and begin re-installation while the engine is piping hot, all without burning fingers.

Especially handy for road-shoulder spark plug readings after high-speed throttle/ignition chop.

Good Fortune!