Compression Test Help

[Vizzinzin]

Hi I'm a new member to the forum and have been lurking for quite a while but finally need to make my first post...

I recently purchased a 1978 xs650E (2F0-000101 type Vin) so an early model with the original engine in it and I'm having a bit of a problem with a compression test.

So the bike I have is running and I have seen it run/started it myself but it's definitely a reluctant to do so and thus I opted to do the compression test cold. I'd also like to change oil/clean gas tank and a number of other things before I start running it regularly. The battery is also dead so I figured I could kick it to build compression. I removed the spark plugs and attached the gauge and proceeded to kick with the throttle completely open and there was no compression... It would occasionally jump up in pressure but it wasn't holding. It's also worth noting since I attempted this I removed the gas tank if that has any relevance at all.

My understanding is I need to open the slides and try again. I'm EXTREMELY green to mechanics here and was wondering if someone would point me in the right direction to do this correctly. I have the oem manual and a clymer but figure someone could assist me much easier than me digging through the diagrams. I (believe?) I have to take the airbox off to access them but just want to be sure.

Any assistance would be appreciated and apologies for my lack of understanding in the matter. All I want to do is learn and understand! Unfortunately I won't be back home until tomorrow but would appreciate any good literature/comments.

Thanks!

 

[fredintoon]

Hi Vizzinzin and welcome,
on a new to you XS650 it's never wrong to take the carbs off and give them a full cleaning job.
While they are off, pull both plugs and connect the compression tester to each plug hole in turn to do the test.
Kick the bike over or jumper it from a car battery to use the starter for about 10 revolutions to get the reading.
give each plug hole about a 1/2oz of engine oil and test it again.
150 psi is great, anything above 100? Well, I'd ride it anyway. Below 100, plan a rebuild.

 

[gggGary]

Yes you can get full readings with the kick. The reading will raise with each kick, with a healthy engine say along the lines of 80 100 115 125 etc til you max out after 6-7 kicks at 150 plus. Like fred says a cold engine anything near 100 is good enough to let it run, don't worry about a high number till it's been ridden a while.

a bit of rust or carbon on a valve seat can drop compression like a rock, no worries some running usually cleans up the seats. I typically spray some rust buster in the plug hole and intake and exhaust ports while doing the wake up drill. with the plugs out I kick it over a fair number of times off and on while doing basic maintenance. then follow with some oil. This may help unstick rings and lube valve stems. It's easy to have a sticky valve on a "sitter" and just as easy to bend it with the piston on a "too soon" starting attempt.

Do a head stud retorque,even before you try to start it.

The compression gauge has a shroeder valve that holds the pressure reading until released. This can be dirty or sticky letting the reading bleed off. remove clean, and lube it.
You don't need to raise the slides. there is plenty of room for the air to pass under them.
when you kick does it FEEL like compression?
many old shed queens will have an issue somewhere in the clutch cable actuator the kick start path passes through the clutch and you may not be spinning the engine strongly.
An old hand knows this immediately but if you are not familiar with how a kick should feel........

 

[Vizzinzin]

Hey just wanted to say thank you guys for the quick responses! They're both extremely helpful. Unfortunately I didn't get very long to work tonight, only had a chance to drain the tank which I should hopefully get to cleaning soon.... I will update this asap once I get it done though.

The compression gauge is brand new so hopefully that shouldn't have any problems but I will have a look. In regards to kicking it does feel like I get compression but there's also a sound that seems as if it's not holding the compression. The engine definitely spins pretty good when I kick it, although again I don't have much of a basis to go off of here.

I have started it and seen it running so It may very well be the gauge.... I'll follow your advice and give you an update though! Thanks again.

 

[peanut]

Welcome vizzinzin
In order to get a consistant compression reading the test should ideally be done with the engine warmed up so that the rings have expanded and create the best seal on the bores .
However if that is not possible you may be able to extrapolate the readings with a cold engine.

Use the starter motor to turn the engine over . If your battery is flat attach any car battery using jump leads
'
if you have no working starter motor I would suggest that you conduct the test by running alongside the bike so that you can turn the engine over at least 8-10 times in order to build the cylinder pressure up to its maximum .
Using a kickstart for a compression test on a cold engine is hopeless to be honest.

Make sure that you remove both spark plugs and turn the ignition off.
Screw the compression tester into one of the cylinders
Select third gear and pull in the clutch.
Push the bike with the clutch in until you get up a bit of speed and then drop the clutch .
Keep pushing until you hear the engine turn over 8-10 x times and then take the reading.
Repeat for the other cylinder .

 

[XS650D]

Certain years had different compression numbers,my 77 only shows 134 psi in the book,i think the comp numbers on the early years were higher maybe 150 + then they lowered the comp i think around 76 thru 77 and then raised it again when the special came out.Not sure why ,maybe someone can comment on that.When doing a comp reading different gages give diff readings as much as 20 lbs.that i have personally witnessed.U want both sides within 10 percent and also doing a cold reading (let bike run for 10 seconds) then a hot reading,10 min ride and compare.that will tell u alot.Hold throttle wide open when kicking or cranking,if u have a cdi box(electronic ignition) u must ground the plugs while doing test or possibly ruin the cdi but i think your ok,u have the standard with no cdi.I believe only the specials got cdi boxes.

 

[peanut]

you do not need to ground the plugs if you have electronic ignition because you do not do a compression test with the ignition on... EDIT (when using the kickstart to turn the engine over) .
if you want to be doubly safe you can either unplug the electrical connector to whichever ignition module you have or remove the fuse or battery connection EDIT if you have an electronic ignition.

There is also no point in just opening the throttle wide on a mikuni BS34/38 because the throttle cables are not linked to the slides. the slides are independant and operate by vacuum.
You need to remove the filters and prop the slides open as well as open the throttle .

Lots of websites copy paste information without understanding it or checking if its correct . Eventually misinformation is all over the web to catch out the unwary.

 

[pamcopete]

1. If you are going to use the starter motor for the test, for many bikes you have to turn on the ignition to use the starter, so disable the ignition by pulling a fuse or disconnecting the coil(s) or connect each spark plug wire to a spare plug resting on the engine. Be very cautious and do not do this with one or both plugs just disconnected. You have to either ground the spark plug wire or use a spare spark plug.

2. The slides do not block much of the necessary air flow for the test, but the throttle plates certainly do, so that is the reason to open the throttles. No need to prop open the slides or remove the filters.

(The red zone is for immediate loading and unloading of passengers. There is no stopping in the white zone)

(No, the white zone is for immediate loading and unloading of passengers. There is no stopping in the red zone)

 

[XS650D]

Sorry Peanut,but your wrong,throttle must be opened (butterflys open) or your reading will be much lower.Plugs grounded to motor and in there jackets and no damage will be caused,if no cdi then not necessary!

 

[5twins]

Yes, I test these all the time with the carbs and air filters on, throttle wide open, works fine. Yes, you must use wide open throttle. Like Pete said, those butterfly plates need to be open because they do block off the air flow. I've done cold engines just by kicking too. It works but will give a lower reading than a hot motor, and you need to kick a lot.

Be aware that a compression test is not a tell-all. A bad motor can give good readings. You may have read that if you get a low reading, add a spoonful of oil to the cylinder. If the readings come up, the rings are bad. That added oil has helped seal them and given you a better reading. Well, if you have a cylinder with bad rings (or badly scored) that's passing lots of oil up past the rings, that will act just like adding that spoonful of oil. I experienced this first hand. This guy's bike gave a good reading on the compression test but when we tore it down, it was a mess inside.

 

[peanut]

Sorry Peanut,but your wrong,throttle must be opened (butterflys open) or your reading will be much lower.Plugs grounded to motor and in there jackets and no damage will be caused,if no cdi then not necessary!

I think we are talking at cross purposes here.
I have always understood that it is necessary to have the venturi open so that the engine can draw air from the whole of the venturi.it cannot do this if the slides are right down .

All the references that I have seen for conducting a compression test instruct to open the slides fully on this type of carb where the throttle does not operate the slides.
I'm going to do a compression test on my engine shortly so I'll record if there is any difference in the two methods .

 

[lakeview]

I have made a set of jumper wires that I use to ground the plugs instead of just laying on the cylinder head. Clip one end to the thread portion of the plug another end to an engine fin and for sure you are not going to damage the electrics.

 

[pamcopete]

From Airplane:

"Don't start with that red zone white zone shit"

 

[gggGary]

Pink zone warning!
So then She said.......

Look up your carb throat some time the slides on a CV carb have a huge air gap under them . Different story on VMs but the throttle cable lifts THOSE slides. CV carb gap is big enough you will never get the slightest vacuum in the intake at kicking or e-start speeds as long as you have the butterflies open.
I actually did a back to back comparison test on an engine, CV slides open and closed, 0 difference in readings. Then I did e-starter vs kicking comparisons I can get a higher number with several kicks every time.

Now THIS is a kick starter!

I think it was Pete that did the math showing you can actually kick a motor through compression faster than the e-start will do it.

 

[650Skull]

Hi I'm a new member to the forum and have been lurking for quite a while but finally need to make my first post...

I recently purchased a 1978 xs650E (2F0-000101 type Vin) so an early model with the original engine in it and I'm having a bit of a problem with a compression test.

Any assistance would be appreciated and apologies for my lack of understanding in the matter. All I want to do is learn and understand! Unfortunately I won't be back home until tomorrow but would appreciate any good literature/comments.

Thanks!

Welcome.

As a new member the question can be, "who to listen to when the answers given are contradictory"..............Sometimes a search on members public profile will help you get an idea on who knows what they are talking about

78E Points ignition model.

Does not need to have the spark plugs ground/earthed when removed from the motor and ignition on.

If the motor has an aftermarket electronic ignition, (pamco, Boyer and others) or the factory TCI ignition installed, the the plugs have to have a ground when they are removed and ignition on.

+1 with gggGary, 5Twins, Pamcopete, XS650D, fredintoon, lakeview.

 

[Vizzinzin]

Holy shit that blew up quick. Thank you all for the input it's much appreciated and frankly everything helps me further my understanding here. Definitely didn't intend to start a debate though.

So I'm going to come back to the compression test after I get her in a bit better running condition which will allow me to do the test on a warmer engine. I don't want to run her and potentially cause further issues till shes a bit cleaned up. Essentially this bike sat for ~8 years and got taken out 1-2 times a year according to the PO which may be bullshit but again she does.

I'm going to go back to the running rough/beginner threads and go through a couple more things then try this again! Wanna clean that gas tank/petcocks cleaned out (there's only the tiniest rust spot!!) check the carbs, and definitely change the oil. Got my training/endorsement tests monday/tuesday so hopefully I'll be able to get her in better order then take her out for a spin here as well!

I'll keep everyone posted and hopefully get some numbers on the engine too. Probably will start a build thread as well so everyone can watch me struggle!

 

[peanut]

I actually did a back to back comparison test on an engine, CV slides open and closed, 0 difference in readings. Then I did e-starter vs kicking comparisons I can get a higher number with several kicks every time.

Now THIS is a kick starter!

I think it was Pete that did the math showing you can actually kick a motor through compression faster than the e-start will do it.

Thanks for that Gary I stand corrected
I'd always consider owners experience over any book or uncorroborated internet source.
Any test that you guys have done is good enough for me

 

[peanut]

Sorry Peanut,but your wrong,throttle must be opened (butterflys open) or your reading will be much lower.Plugs grounded to motor and in there jackets and no damage will be caused,if no cdi then not necessary!

I owe you an apology XS650D

I have reread my original post and I can see that it was misleading.
I should have taken the time to read it through before posting but I was in a rush as usual .working a 15 hour day 7 day week for the past 7 years leaves little time for proof reading my posts and its not the first time that one of my posts has contained mistakes or ambiguity I'm afraid.

Of course the butterflys need to be wide open that is a given .
The point that I was trying to make ...badly... was that I believed the slides should also be propped up .....so that there was no restriction in the venturi .

Because the throttle cables do not operate the slides I was advocating propping them open manually so that the venturi were completely unrestricted.There are numerous tutorials advocating this on the net.

Pete and Gary have stated that they have tested compressioin with slides open and closed (butterflys open of course) and there is no appreciable difference . That is good enough for me ....as I have never personally tested this back to back on a CV type carb .

 

[XS650D]

Hey Peanut,no worries,really wasent trying to start a debate,just wanted Vizzinzin to get the rite info to start.Im no expert like some on this site but that much i was sure off.
Regards
XS650D

 

[gggGary]

We're just all having fun here!