low compression? bad test

[mcompton1973]

ok, so the fuel tank was not hooked to my bike...so I made a make shift deal to try and feed fuel to the engine and do a compression test. I bought a cheep compression tester from Harbor freight for like $10. Put that in the first spark plug hole and tried to start it. It only registered like 30. That being said when I tried to start the bike. It turns over OK...I did the little test where you take out a plug and hold it near the block and turn it over...there is spark...but it doesnt even sound like it is trying to start. no popping or anything. to me it seems like there is no fuel getting from carb into the engine...so no combustion going on at all.

I am sure the solution is pull the carbs, clean them do a bench synch and put them back on...something in the carbs is prob wrong.

Before I do that I want to know what you think about the compression test results with no combustion going on? Does the 30 worry you? or does the fact there is no combustion mean the compression would be lower etc? I almost wonder with as cheep of a tester that I bought if it could be leaking air in the hose or something instead of registering it in the gauge.

What are your thoughts?

I think that I am changing my plan for the bike. Originally I just wanted to get it on the road as quick as possible. Now I am thinking about redoing the carbs, redoing the entire wiring and charging system and really getting the whole thing tuned up good...and then riding the entire length of Hiway 6 (I live off hiway 6 in Nebraska about 1/2 way...so I could do to the east 1 summer and to the west the next or something. That is a whole other post though...lol)

 

[mike_of_bbg]

30 would be worrisome, but... Were you holding the throttle wide open? You need to do all your cylinders. And you don't need fuel or to start the bike to do a compression test - in fact you want all the plugs out. If the numbers are still low try putting 1 tsp of oil in the spark plug hole and see if it improves. Crank until the needle doesn't go up any more. Make sure your battery is well charged. You want that starter turning over as fast as it can.

Do you know if the valves have been adjusted any time in recent history? They wear tight and one could be hanging open.

 

[Nessism]

Engine needs to be hot and throttle held wide open before you can do a proper compression test. Of course, checking the compression without performing a valve adjustment and putting some miles on the engine first to clean out the cobwebs is going to be misleading anyway so keep this in mind.

 

[mcompton1973]

yeah...this keeps making me go in circles...lol.

My original thought...take out carbs, clean, new boots, new o-rings, seal airbox, etc.
Take off the wiring and totally go through and clean it.
do the valves...

Then put it all together and see what I have...if the engine is basically good...I can test the charging system...and if its good I should have a pretty reliable ride.

But if the compression is still bad...then I have to pull engine and do a rebuild or something...and that means redoing a lot of what I would have just done. Its kind of catch 22 though...i cant get good compression if I dont have good electric and starting.

ahhhhhh.....i keep taking myself in circles on this. this whole where to start thing.

 

[Nessism]

You won't have to redo anything, other than maybe pull the carbs and airbox back off. Easy stuff once you have done it before.

 

[mcompton1973]

Well maybe I need to make myself a list...start gathering parts (boots etc) and do that then.

redo wiring harness
upgrade stator and R/R
clean carbs
replace boots and o rings
seal airbox
replace petcock
do valve job
new brake pads
new brake lines
put it all together and hope that the compression is really better than Im seeing right now.

what else should I do while I have it apart?
any other "fixes"

 

[Tom R]

Well if you're worried about compression, ease your mind and do that first. But as stated earlier, take all four plugs out and have either the throttle wide open or the carbs off. It should be warmed up for an accurate reading; but if you can't start it you can't get it warm - so cold will do to find out if you even have a compression problem. Yeah 30 is low and it'd probably be worth checking into why it came up so low - was the tester sealed tight? was the throttle open? was the engine turning over at cranking speed? how many strokes? Do you have any way of verifying your cheapo tester works? (I've had success with a cheapo tester for the past 20 years or so)
Putting oil in the cylinder will tell you if it's the rings or the valves. If the reading goes up after adding the oil, it's likely the rings - if it stays the same, it's in the valves.
If you're getting spark, I wouldn't worry too much about the wiring harness. What makes you think it's bad?
Going through the carbs and cleaning them up is never a bad idea if it's been sitting for a while - but one step at a time.

 

[mcompton1973]

Harness has some issues. I can see areas where the prev owner spliced in a wire and routed it to the back and spliced it back into the harness. Areas with melted connectors or wiring etc. The harness functions kinds of...but its not reliable.

what makes me worried about the tester is just that the rubber where it connects to the threads that go into the spark plug hole can twist...so I just wondered if it can move like that....maybe it doesnt have a good seal?

"supposedly" the bike was running last summer some after having the carbs redone...but the more I look into the more I see that needs to be done. Good thing I really like this bike...and I got it cheep!