Compression Test Question

[Jcol]

Just to summarize I picked up an 83 Aspencade last year with a blown heat gasket. I replaced the head gasket and timing belts. The bike started right up and seemed to run OK but I did not ride it as the tires were terrible. I moved on and dealt with some other issues and then got tires for it this summer. I started test riding it off and on and have put about 100 miles on it. The bike seems lazy and a little rough between 1000 and 2000 RPM. There is very little power when you start out and it is very easy to make it sound like you are in too high a gear even though you are in first. Once you get the bike up to speed it cruises along quite nicely however there isn't as much power as my 81 when you roll the throttle at 60 mph. I discovered that the bikes rear two cylinders were not firing strongly at idle because I can remove the plug wires from each cylinder one at a time and very little change in the idle occurs. I started thinking it was a coil issue however before I went any further I got a compression tester and discovered that my back two cylinders are at around 130 psi when my front two are at slightly over 150 psi. I am now thinking that that difference in compression is what is causing my issues. With such a difference in the compression will it be possible to get it to run smoother without a major engine tear down? The bike drives down the road OK but when compared to my 81 it lacks some smoothness and power. BTW I tested the compression on my 81 and all cylinders are at 160 psi - my 81 is a very smooth running engine. Any advice would be appreciated. Thanks.

 

[joedrum]

well as a recent case proved in jpwinger bike once carbs and iggy mount together as problem it don't take long for the compression to get compromised through just thilthy conditions of not firring good .....jerry left my house with atf mixed in gas with compression every bit that bad .. I cant remember what it was ... and when he got home the atf had cleanded the bad cylinders enough to raise the compression back up to 150 I think it was ... and then he battled the carb rack for a week or two and finally its screaming and running great ... I can tell you his ran just like you are describing to the t.....so I im thinking your bike is okay just severely dirty with crap gas and the crap it leaves in cylinders ... bad carbs and possible weak spark at idle ... but I don't think theres anything seriously wrong with the motor at all ... needs to be fixed though

 

[zman]

ATF works good, I use Marvel Mystery in gas and oil because I have a gallon and need to use it . I would ride it , take a freeway trip 20-30 miles minimum as many days as possible. I have heard these wings need some time and miles to free up.

 

[dan filipi]

zman » Tue Sep 03, 2013 8:18 am[/url]":jy37c8pm]
ATF works good, I use Marvel Mystery in gas and oil because I have a gallon and need to use it . I would ride it , take a freeway trip 20-30 miles minimum as many days as possible. I have heard these wings need some time and miles to free up.

Yes! They don't like to sit.
Just a couple months with same gas in the float bowls will clog the tiny idle jets.

 

[slabghost]

Agreed! Run it!

 

[Jcol]

Thanks for the advise but I have cleaned the carbs and ran ATF for the 100 miles or so of testing. I see people mentioning putting additives in the oil as well which I have not done. Is that something I should consider as well? Assuming the carbs finally clean out will the bike run OK with difference in compression between the front and back cylinders or will the compression come back as it is driven? I figured once the compression is gone it's gone unless valves and rings are replaced.

 

[slabghost]

Rings often do re seat. 100 miles isn't nearly enough some times. Get out a run a full tank through with the ATF then recheck compression with the motor warm and throttle wide open.

 

[dan filipi]

One engine I had in started out with 90 lbs compression on one cylinder, the others were like 120 to 150.
It took a good 500 miles to bring compression up to 160-165 on all 4.
At that time I didn't run anything in the 10/30 oil.

 

[joedrum]

well these motor prove to be really durable when rode a lot ... atf in gas is good atf in oli really has no effect on running character and has to be riden easy and changed ...from what you just said I thinking your compression from faulty carbs and atf probably is not going to clear them ... find the the cylinders that are giving the trouble and take off carbs and fix them ... once it starts firing again it will go the other way and start getting better and better

 

[AApple]

Compression readings between cylinders should normally be within 15% from highest to lowest. Anything more than that would indicate a problem. In your case, your lowest is 130, and the highest 150. 15% of 150 is 22.5. 150 - 22.5 = 127.5. If we go by the "accepted" 15% spread between cylinders, yours is within specs....even tho it doesn't look right. Strange that the two rear cylinders are low, but that could be a valve issue, or any number of other things. Are you checking the compression with ALL of the plugs out, throttle held wide open, and a charger on the battery?
Check compression both hot and cold, and see what the difference in the cylinders are like that. Lower compression readings when cold is fairly normal....
Just tryin to help.... :blush: :good:

 

[dan filipi]

And I'd adjust valves cold then verify adjustments a couple times before putting the covers on.
.004 intake, .005 exhaust. Should feel slight resistance pulling the feeler gauge through.

Cranking speed makes a big difference so be sure when comparing that your doing so with same good speed and the compression gauge hose and fittings aren't leaking.
Doesn't take much of a leak to drop 20 psi.

 

[Jcol]

Test was done with a warm but not hot engine. All plugs out and throttle held wide open. I did not have a charger on the battery and by the forth cylinder the crank speed was starting to slow. I also noticed that the front good cylinders got up to 150 after 3 or 4 strokes while it took 5 to 6 to get the back cylinders up to 130. I will put a few more miles on it with ATF or SeaFoam in the gas and retest. I will also do a cold compression test and see what it shows. Thanks for all the help.

 

[Ansimp]

C5 ignition :good: or other overhaul of your ignition as suggested in threads ( relay mod,coils,leads etc)