Hi,
I just checked my compression -- Front cylinder (120); Rear cylinder (110). Is this normal, low, etc...?

The reason I checked is because the bike kind of dogs it once I hit fourth gear.

Thanks,
Barry

 

Compression for a stock engine should be in the area of 180 lbs. +- a few. Your compression is rather low and some work may be in order like rings or possible a valve job. You may want to do a leak down test to see if you can tell where the problem lies before opening it up. Hopefully you opened the throttle full and grounded the ignition lead when you did the compression test or the numbers will be on the low side if the throttle was closed.

 

The numbers sound good to me. More important, the numbers are very close to the same which indicates both cylinders are in similar condition. If one were to have a very different value then that would indicate bad rings, excessive wear or a bad valve(s) in the low cylinder. I think you are in good shape.........pg

 

You guys are confusing me a little [&o] One of you says the numbers are low and the others say they're okay. I'm really a newbie at doing much of anything mechanical so I'd appreciate it if you could clarify.

Also, this is the first time I've ever checked compression. I pulled the throttle all the way open and let the motor go through five revolutions. I did not do anything about grounding the ignition lead. Is that necessary, and if so, how do I do it?

Thanks,
Barry

 

bdavis.......I certainly did not intend to confuse you. I said that I thought the numbers sound fine to me. I still say that. If someone can find, in the factory manual, where it states what the compression number should be....well, I will eat the page right here on the forum.
Your numbers are close together. This is most important because it shows both cylinders/heads are in similar condition. As I stated before, if one was a lot lower than the other it indicates trouble in that cylinder. Yours are even for all intents and purposes.
Let me ask you:
Does the bike start and idle smoothly?
Are your tailpipes free of oily deposit?
Your spark plugs do not foul with oil?
If the answer to these questions is a positive, then you are in good shape. Just ride and enjoy.......................pg

 

180 lbs for a stock Ironhead would make for a stout engine.

I have a Haynes manual. It shows minimum compression to be 120 with no more than 10 lbs psi difference between the two.

I have a '77 Ironhead. I did a top end job at 11k due to my foolish self melting a piston due to an intake leak. I got compression in the 150's out of it. I foolishly melted a piston again at 43k. I did not have to bore it, just hone an replace the pistons. I checked the compression a while back at 53k, it was 150 on one and 155 on the other.

Your's sounds a little low. However, if it ain't smoke, blowing oil, or fouling plugs I'd keep riding.

 

Amen to that Bro.

 

bdavis

I posted wrong info there, I didn't realize I was in the ironhead forum when I posted that info. Sorry about that!

Might be a good idea to post the year and model in the future so guys like me don't scramble your head on you

XXX had it right when he posted that 180 lbs would make a stout engine for sure.

minimum of 110-120 LBS would be the cutoff for me, depending on the year whether it is a 7.5 CR or higher as in the later years, I would say that your numbers are not too far off. I would like to see a bit more compression and a little closer on the spread myself.

Also not AS important to ground the ignition lead, that's mostly for electronic ignitions to prevent damage to the components, not coil and points.

Sorry for the mixup.

 

Thanks for clearing that up. I didn't mean to imply that anyone was intentionally trying to confuse me. I just wasn't sure what to do with the different sets of numbers.

I haven't noticed any problems in these areas. But once I hit fourth gear there doesn't seem to be much power left. This is a 1972 XLH. Years ago I had a 1976 Sportster and when I hit fourth there was plenty of power/acceleration left. This is what led me to check the compression in the first place.

I do have a couple major oil leaks. One is from the primary cover and I'm sure I need to replace the gasket. I haven't been able to locate the other one yet, but it is from the engine. I think possibly from several spots.

Thanks for all your help.

Barry

 

The 72 xlh was rated at 61 HP and had a 9:1 compression ratio. That should give you some decent engine compression if all is well. Have you checked your jetting and point gap along with timing yet?