Compression too low?

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byrd900sp
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Compression too low?

#1

Post by byrd900sp »

I got another high mileage bike that I want to get back on the road, but my initial compression test was disappointing.
One bank has 80lb in both cylinders, and the other bank has 70 and 90 pounds.
The oil and antifreeze look okay on first inspection, but I haven't drained them yet, and the gas tank has no rust. Electrics all work, and the body is nice, the first 75 model I've seen in white and gold with mag wheels.
The bike looks great, and I really don't want to part it out, what do you suggest? Part it out, do a full rebuild, buy a used motor, or rebuild just the heads and hope for the best?
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2008 KTM 530 EXC-R
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Re: Compression too low?

#2

Post by ericheath »

If you forgot to open the throttle wide open, you’ll get readings like that. I’ll bet you’re above 130. The other thing worth me this tons of us have had low compression in some cylinders that bounces right back after a few hundred miles. Speculation is that rings get stuck in the pistons. Heat and use frees them up.
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77 WING, 1200 engine with 77 heads, cams, gl1100 foot pegs, Magna V65 front end, 764A carbs, [-gone Suzuki M109 monoshock--, replaced with gl1100 shocks] gl 1200 swing arm, gl1500 final drive, wheel and rear brakes Valkyrie seat, Meanstreak tank, Sportster pipes, Power Arc ignition off crank.
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Re: Compression too low?

#3

Post by chewy999 »

While those figures seem low, I have read on here that the rings may free up with a few miles on the clock, and some use certain additives to help this happen. I think it is better to check for certain, so it may be worth taking the heads off just to see there is nothing else amiss such as a faulty valve or seal. Your bike looks tidy, so you could assume it has been cared for, do you know when it last ran? I would change the timing belts first of all as insurance.
All that said, it is easy to start spending more money than needed on these bikes.
Sure others will pass on there words of wisdom soon.

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Previous Rides,
1980 CB250N Good to learn on
1981 CX500 good mid range tourer, went to Austria on it!
1983 GL1100C Pride and joy, sold when I bought my 1st house, big mistake
1985 GL650 Silverwing another mistake, horrible bike
1986 CBX550 Good commuter
1989 Suzuki GS750 (1976) cheap and cheerful until a dog ran out in front of me on Xmas Eve, 1991
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Re: Compression too low?

#4

Post by low-side »

If it hasn't run in a while, rings will stick, oil drains out of the cylinders, and rust may form on the sealing ring of any open valves or their seats. I don't get terribly excited about compression readings on long dormant engines. If it's a runner and it reads like that and you did it correctly, I'd be concerned. How many miles does it have on it?
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Lucien Harpress
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Re: Compression too low?

#5

Post by Lucien Harpress »

1: Re-check with the throttle wide open. The fact they are ALL similar to each other (rather than they are low) gives me hope that's your problem.

2: Has the engine fired at all? Even with starter fluid, etc. I could be wrong, but if compression numbers were too low to run, they'd be too low to fire at all. I'd say it's worth a try to (at the VERY least) get in running, even badly. Then you can go from there.
1997 Valkyrie- Light Cutomization, but Too Busy Riding
1980 KZ1300- Bike's Haunted
1976 GL1000 (Yellow)- It Runs (Poorly) and Doesn't Leak (Mostly)
1974 Velosolex 3800- Better Than Walking
1972 CB750- Broke the Chain And Ate the Motor
1969 CT90- The Most Fun You Can Have on 90ccs.
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byrd900sp
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Re: Compression too low?

#6

Post by byrd900sp »

The PO said its only been sitting a few months, and the tires are brand new and dated 2017. He said that after sitting out in the rain for the past two months, it would not start, and gas leaked down the side of the engine when he tried to start it.
Its got 26,000 on the odometer, and the PO said its turned over once, so 126,000 miles on it.
I haven't tried to start it yet. I will start with a carb inspection and cleaning next.
Thank you for the responses!
Oh, and I did have the throttle all the way open and the choke on, but I will take another reading once the carbs are off.
My current bikes
2008 KTM 530 EXC-R
1998 Honda GL1500C
1997 Ducati 900SS SP
1996 Honda XR650L
1993 KTM 250SX
1984 Honda XR500R
1980 Honda GL1100 Standard
1975 Honda GL1000 Scrambler
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Re: Compression too low?

#7

Post by Rat »

The gas leak could be a stuck float ....

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byrd900sp
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Re: Compression too low?

#8

Post by byrd900sp »

Update, I drained the carbs, changed the oil, and made sure the antifreeze was only green, and then gave it a squirt of starting fluid and it started right up! I drove it around the block and it ran great! Lots of smoke on start up, but it cleared up. It has a very quiet exhaust, so the motor seems extra loud. There's definitely some piston slap or clutch chatter or something clangy in the engine at idle... but I do get paranoid about these things. I only paid 400$, so I didn't really expect it to be a runner.
My current bikes
2008 KTM 530 EXC-R
1998 Honda GL1500C
1997 Ducati 900SS SP
1996 Honda XR650L
1993 KTM 250SX
1984 Honda XR500R
1980 Honda GL1100 Standard
1975 Honda GL1000 Scrambler
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Re: Compression too low?

#9

Post by Track T 2411 »

byrd900sp wrote:The PO said its only been sitting a few months, and the tires are brand new and dated 2017. He said that after sitting out in the rain for the past two months, it would not start, and gas leaked down the side of the engine when he tried to start it.
Its got 26,000 on the odometer, and the PO said its turned over once, so 126,000 miles on it.
I haven't tried to start it yet. I will start with a carb inspection and cleaning next.
Thank you for the responses!
Oh, and I did have the throttle all the way open and the choke on, but I will take another reading once the carbs are off.
"...and the choke on..." would be a cause for low compression numbers...
"I have no special talent. I am only passionately curious." - Albert Einstein

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Lucien Harpress
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Re: Compression too low?

#10

Post by Lucien Harpress »

A bit clattery at idle could be due to a carburetor sync that's just a bit off, or (if the bike idles down and holds a steady 1.1-1.5k rpm) just something the motor does. I've heard people talk about motors that sound like old diesels, but they go and stop just fine.

But the choke was definitely the cause of your low compression numbers. You closed those butterflies in the throat, literally "choking" it.

....hey, I wonder if that's where they got the name from? crossy.gif :lol:

(Also, I'm in before the inevitable "Change your belts before you get too crazy!". Because you really should. :-D )
Last edited by Lucien Harpress on Fri Jan 03, 2020 8:34 am, edited 1 time in total.
1997 Valkyrie- Light Cutomization, but Too Busy Riding
1980 KZ1300- Bike's Haunted
1976 GL1000 (Yellow)- It Runs (Poorly) and Doesn't Leak (Mostly)
1974 Velosolex 3800- Better Than Walking
1972 CB750- Broke the Chain And Ate the Motor
1969 CT90- The Most Fun You Can Have on 90ccs.
1965 CA77 Dream- Needs a Full Teardown, but Complete

All advice I give is only valid until an expert corrects me.
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Re: Compression too low?

#11

Post by Gowing »

Nice shape, really good price, welcome
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Re: Compression too low?

#12

Post by Sidecar Bob »

^^^^^What they said^^^^
Where is the noise coming from? Early 'Wings often have a rattling sound from the rear of the engine at idle that goes away when you increase the RPM, caused by the primary chain becoming loose from normal wear (later models have a tensioner to prevent it); It isn't a big deal if you know what it is and as Lucien mentioned it is more noticeable when the carbs aren't perfectly synched.

BTW: Those are NOT "mag" wheels. They are Comstars
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Re: Compression too low?

#13

Post by SnoBrdr »

Sidecar Bob wrote:^^^^^What they said^^^^
Where is the noise coming from? Early 'Wings often have a rattling sound from the rear of the engine at idle that goes away when you increase the RPM, caused by the primary chain becoming loose from normal wear (later models have a tensioner to prevent it); It isn't a big deal if you know what it is and as Lucien mentioned it is more noticeable when the carbs aren't perfectly synched.

BTW: Those are NOT "mag" wheels. They are Comstars
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Any idea why Honda mentions Comstars are tubeless on the CX/CBX but not on the GLs?
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Sidecar Bob
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Re: Compression too low?

#14

Post by Sidecar Bob »

I think it had something to do with when tubeless motorcycle tires were introduced. Maybe. (It has been discussed here before)

Some people have reported successfully using tubeless tires on early 'Wing wheels but from what I've read the shape of the inside of the rim needs to be different for tubeless tires.

Others will argue the fine points but I happily use tube tires (with tubes) on both of my bikes so I'm not inclined to do much research about it.

BTW: I forgot to mention that those ComStars are not original to the '75. I think they were introduced in '78 or '79.
Mr. Honda ('83 GL1100/Dnepr) summer How a motorcycle evolves thread
The Famous Eccles ('84 CX650EI/VeloUral) winter Never Ending Build (CX500forum)
Click: Colour schematics for all GL1000 & GL1100 and GL1200 standard models plus instructions on how to download the full size version
"A guy with two sidecars can't be all bad." - Cookie
Another guy with two sidecars..... Hmmmm... must be something to that....
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Re: Compression too low?

#15

Post by Track T 2411 »

As the 'Red Page' says, the ComStar wheels were introduced for the '78 model year. As for tubes and tubeless, I'm wondering if the tubeless tires of the time didn't have the appropriate weight ratings for the GoldWing...
"I have no special talent. I am only passionately curious." - Albert Einstein

"He that is good with a hammer tends to think everything is a nail" - Abraham Maslow

"If you can't take the time to do it right the first time, how are you ever going to find the time to do it over?" -Unknown

Current Rides:
'Grumpy' - '81 Standard, now fully dressed.
'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
'Scarlett' '76 'Survivor' nekkid as a j-bird!

Under Construction:
The 'Jalopy' '78-'79 Mash-up
'Quikie' '81 gl1100I back on the lift, project with the step-son!

In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
'82 gl1100I, My first 'Wing, and an expensive lesson!
New2U Bike? Read Me.
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