Compression testing

[bosco659]

I was just thinking. When one conducts a compression test, they would normally have the throttle in a wide open position. With our CV carbs, the butterfly may be open but is there sufficient vacuum available to lift the slides, to allow adequate airflow to get proper test numbers? Anyone know how quickly the slides lift? If not, when I put my carbs back on I may have a watch and report back.

 

[Mailman]

What I do is pull my air boxes off and I wedge fender washers , standing on edge , under the slides. Even better when you have the carbs off for cleaning.

 

[bosco659]

What I do is pull my air boxes off and I wedge fender washers , standing on edge , under the slides. Even better when you have the carbs off for cleaning.

Ok so that tells me that they would normally remain closed under cranking rpm. I’ll run a little experiment with and without slides propped up and see what the difference is in the readings. Thx.

 

[Mikey]

Is there a down and dirty way to check them when your out and looking at a new to you bike

 

[WideAWAKE]

Pull the plug, thumb over hole and crank.

Decent suction Will at least tell you if you got something to work with.

If the bike runs decent, I don’t tend to worry about compression when looking at a bike I’m buying.

If you’ve done it enough, just kicking it over with plugs in would give you a idea of what your workin with.

 

[Mikey]

Pull the plug, thumb over hole and crank.

Decent suction Will at least tell you if you got something to work with.

If the bike runs decent, I don’t tend to worry about compression when looking at a bike I’m buying.

If you’ve done it enough, just kicking it over with plugs in would give you a idea of what your workin with.

so don't bother with a compression guage when looking at a new bike

 

[WideAWAKE]

so don't bother with a compression guage when looking at a new bike

You asked for down and dirty way, not proper.


If it’s a decent runner - to me, not a big concern. Can’t hurt to test it. Just make sure you know how.

I’m more prone to search out hack wiring and things that look janky. If it’s outside the motor, just prepare for it to be in the motor as well.

 

[Jim]

I was just thinking. When one conducts a compression test, they would normally have the throttle in a wide open position. With our CV carbs, the butterfly may be open but is there sufficient vacuum available to lift the slides, to allow adequate airflow to get proper test numbers? Anyone know how quickly the slides lift? If not, when I put my carbs back on I may have a watch and report back.

There's a gap between the floor of the venturi and the bottom of the slide. I would think that it's plenty big enough for airflow that happens at the rpm provided by the starter or foot. If I had to guess, I'd say less than 2 or 3 psi difference between slides open and closed.... if any at all.
It'd be interesting for someone to run a test and see if there is any difference.... hint hint...

 

[Mikey]

Your right on the down and dirty method that would give you a good indication if you didn't have a tester
But if you had a compression tester with you I just thought you could take both plugs out and test 1 cylinder at a time while kicking it over or using the e start

 

[5twins]

I seem to get decent results with the carbs still in place, throttle just held wide open. It does make a big difference if you don't hold the throttle open. Warm motors test better than cold ones usually too.

 

[Jim]

Agree with WideAwake... there's bigger fish to fry when looking at a very used XS.
What I do is stand on the kicker and let my weight force it down through compression stroke on both cylinders. That gives you a pretty good feel for it.

 

[bosco659]

There's a gap between the floor of the venturi and the bottom of the slide. I would think that it's plenty big enough for airflow that happens at the rpm provided by the starter or foot. If I had to guess, I'd say less than 2 or 3 psi difference between slides open and closed.... if any at all.
It'd be interesting for someone to run a test and see if there is any difference.... hint hint...

I’m all over it when the carbs are back on

 

[bosco659]

Not a proper test but I didn’t have much to do on the bike today, so I did a cold compression test, with open intake manifolds. Previously 140 psi, both sides, engine warm - hot. Today, cold and dry (no oil added), 140 psi RHS and 145 psi LHS. If numbers should improve when hot, then I guess there is some restriction on the intake side to negatively affect the compression test numbers. For reference, the highest I ever measured cold and dry was 125 psi.

Hopefully later this week I’ll have the carbs back on and I’ll test and report again.

 

[Jim]

Not a proper test but I didn’t have much to do on the bike today, so I did a cold compression test, with open intake manifolds. Previously 140 psi, both sides, engine warm - hot. Today, cold and dry (no oil added), 140 psi RHS and 145 psi LHS. If numbers should improve when hot, then I guess there is some restriction on the intake side to negatively affect the compression test numbers. For reference, the highest I ever measured cold and dry was 125 psi.

Hopefully later this week I’ll have the carbs back on and I’ll test and report again.

Have you readjust the valves since the last compression check? That could change the readings.

 

[bosco659]

Have you readjust the valves since the last compression check? That could change the readings.

No I don’t think so. Maybe from the original cold test tho.

 

[hovel]

If you’ve done it enough, just kicking it over with plugs in would give you a idea of what your workin with.

Working the kick start lever by hand will give you an even better idea. I think a side-to-side balance is more important than a total number.

 

[bosco659]

Working the kick start lever by hand will give you an even better idea. I think a side-to-side balance is more important than a total number.

Yes I agree the differential between cylinders is quite important, but for me I’m interested in the actual values as well. I’ve been fooled in the past thinking a motor is healthy and “feels“ like the compression is good, only to find afterwards that it is actually low and requires rings / rebuild. Dry, then wet tests can be a good indicator of work that may or may not be needed in the near future.

 

[gggGary]

I'm in the kick it to gauge health on a buying trip.
If you already have an XS do a couple kick throughs on your bike before heading out it'll make a benchmark for feel.
I kick through throttles open. If you kick through "kinda half hard" it's easy to feel, compare both sides for evenness.
But that's buying a barn beater. Buying a $4,000 "cream puff" the gauge might be justified.
Honestly I usually can't drag out a buying inspection that long.
I did do a drawn out A-B compression test series on an XS, only significant difference was a lower number with the throttle closed. Once the butterflies are open, slide closed/opened, or carbs off gave me the same numbers.
I can get slightly higher numbers with about 5-10 kicks than with the electric starter.

 

[5twins]

I kick for my tests too, just keep kicking until the gauge stops rising. Don't recall exactly how many kicks that is but the 5 to 10 spec sounds about right.

 

[bosco659]

Here’s the results of my “un-scientific testing”. Bottom line is it didn’t make any difference in the compression numbers whether the slides were up or down (but with throttle held wide open). Today, cold vs hot readings yielded a 5 psi increase hot and pleased that readings were identical side to side at 145 psi.