low compression across all cylinders

hello all. so im a bit confused, i did a compression test last wknd and my numbers came up all right around 70 dry and about 80 wet, 4th cyl was slightly higher at about 88ish.

ive read that bikes shouldnt even start with numbers like that, but mine fires right up. i used a cheap tester from harbor freight, and i also read that if there is too long of an adapter that could cause low numbers. does anyone think that may be the case, as it was longer than the kit with just adapters, so it was maybe 3 ft long or so.

i just dont know what i should do next? the bike seems to run ok, i dont have anything to compare it to but i am worried about running it any more for fear of really damaging something. any ideas?

 

Hey Web,

You need to find a compression tester that has the schrader valve located at the spark plug end of the hose/tube. I too started with a HF tester and got artificially low numbers across all buckets but the bike is a screamer. That didn't make sense to me. What happens is without a schrader valve near the combustion chamber where the spark plug usually sits, you are essentially adding volume (from the tester hose/tube) to the already small CCs of our engine. Sure you can continue to crank on the starter to build up the pressure, however, not a wise way to approach this particular test. I found a nice comp tester with the schrader valve in the right location as Farm & Fleet. Sears also carries the Actron series. Good luck!

 

duuuuuude, thats the answer i was hoping for but not expecting to get!

thanks man!

 

Yeah, I was also relieved when I ran numbers with the new tester. So impressed, I put the information in my signature line below. Check it out!

 

saw that, very nice!

while youre online, can you tell me which sears one would workt? im looking at them now, and there are quite a few. also a bit unsure about what exactly a schrader valve is (looking it up didnt produce many good examples of what we are talking about)

is it just something that prevents air from going up into teh tube?

 

If you think about a bike tire and the valve that holds the air in, it's the same thing. It sits in the spark plug end of the adapter. I don't have a pic for you but if you google schrader valve a picture might come up. Its also called a valve core for tires

 

Here is a link to a pic.

http://www.clubpolo.co.uk/forum/lofiver ... 45024.html

 

ahh, ok. thanks!

 

Let me know if you found a replacement and post your compression results for sharing.

 

If you have an O'reilly or AutoZone near you, you can use their 'loan' a tool program and get a professional quality tester to use for free. They will ask for a deposit but when you bring it back your depo is refunded.

Remember to have a fully charged battery, throttle held wide open and TCI unplugged.

Re - low numbers: at 80psi most IC engines can still ignite and run, but that is just about it, any lower say 60psi and it's a no go. Even so if you only had 80psi and the mill ran, it will run poorly. Mix will be affected and HP will be mostly non-existant. So if you have good HP and torque those numbers are not right.