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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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compression test

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  #1  
Old 3rd April 2021
Gravitas Gravitas is offline
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Default Non-Starter, Compression Test Results

Hi,

Many thanks to all those who welcomed me to the forum.

I changed the plugs and the gas. Still does not start.
The old plugs had no oil on them.
The new plugs smell of gas after trying to start.

As recommended here, I have run a compression test.

Front Rear
Initial 80 psi Initial 0
With oil 105 With Oil 25

So even though, I thought the rear cylinder was "making an effort" with starter spray and the front was doing nothing. I guess I was wrong.

So, the front has low compression and the back looks awful.
I assume that I will need to dismantle the rear cylinder head and check for bent valves or a holed piston.
I have not tried adjusting the push-rods yet, I guess that could improve the front compression, but is not responsible for the rear - is that correct?

Any suggestions for the list of things to do now would be welcome!

Regards

Paul
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  #2  
Old 3rd April 2021
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For the compression test it is important that the throttle be open while cranking, and that the engine be at full operating temperature. The front cylinder at 80 psi actually looks ok. Would make the 120 mark if fully warmed up. Perhaps not great but runnable.

You might test the rear cylinder again, unless you are experienced and confident in your technique.

Adjusting the pushrods [engine must be stone cold, I guess not a problem, eh ] may very well make a big, big difference. The valves may not be closing at all.
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Old 3rd April 2021
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If it were my bike, I would adjust the push rods first, then double check ignition timing is correct. Maybe even remove the plugs and reconnect the plug boots, ground the plugs on the engine and watch for them to spark just to prove they are working. Does your bike still have the original ignition?
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Old 3rd April 2021
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Default

Many thanks to Mick and Doodah_Man.
I'll adjust the push rods. First I need a rear stand... And Imperial spanners and sockets.
As well as a full gasket set, and additional push rod O-rings.
I assume that I will need a friend to help get the beast onto the rear stand, since it would really annoy me if I dropped it...
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Old 3rd April 2021
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There are only 2 pushrod settings that matter for starting:
1. valves close
2. valves do not close

Valves close + low reading: top end comes apart
Valves do not close: re-adjust pushrods
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Old 3rd April 2021
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Only a few standard tools are needed to adj the pushrods.
1/2” open wrench
7/16” open wrench
Straight blade screwdriver to remove cover clips
Spark plug removal tools.
Pull plugs and while in 4th gear, roll the bike to position particular valve to lowest point. Adjust tappet. Repeat for remaining 3.
Reinstall clips, place in neutral, repeat comp test.
10 min job.
With a fresh valve job, it is not uncommon for valves to get tight in initial short run time.
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Ironhead Push Rod Adjustment
http://xlforum.net/forums/showthread.php?t=44143

If you have the actual Sportster centre stand then yes, you will appreciate an assistant. There are some tricks - like rolling the bike forward, rear wheel onto a piece of 2x6, then lower the stand and kick out the wood.
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The engine must be stone cold when adjusting the valves. You should adjust them until there is free turning and a bit of up and down movement. Check the results carefully. If the pushrods are tight you will burn a valve face within a few minutes of start up. If you adjust the valves while the engine is even warm, the iron parts will shrink as they cool and the valves will be slightly open for the next time you try to start it. In those few minutes that the engine takes to heat up and expand, the valve face will be burned as combustion gases escape past the valve. If you are lucky, you might be able to hand lap the valve and save it.

As Mick says, a compression test done incorrectly is a waste of time. The engine is designed to be at operating temperature to work well. If you get a marginal Compression test don't get hysterical about it and start ripping things apart. A lot of engines have so-so compression tests and still operate fine. Have a good reason before you tear off the heads and barrels. Excessive oil consumption, blow by causing excess oil loss through
leaks and breather loss.
The work to pull the heads and barrels is easy and quick. The fix usually costs a bit since most people bore the barrels to remove taper and must have new pistons and rings. Don't get into a habit of doing things just to "Check it out" or make sure it is right.
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where are you located? there are many members here that could have that bike running quickly. or at least recognize any problem it may have.
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Old 4th April 2021
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This is the bike that was parked 8 months ago, right ?

As I said, there's usually a reason for a bike being parked like that.

If you have done at least a reasonable job with the compression test a "rule of thumb" will confirm what I am guessing has happened.

Instead of the compression tester just put a thumb over the spark plug holes and feel the compression generated. If one cylinder is really at zero it will be noticeably different.

The only times I have seen zero comp is with a holed piston. A very good reason to park a bike. It could also be a bent valve but in either case the head has to come off.

A leakdown tester will confirm if it's a piston or a valve problem.
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