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Old 07-01-2008, 10:21 PM
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Would Every Cylinder's Rings Be Bad???

Hi guys, I've been working on a 1980 Honda CB 900 Custom for a while now. I bought the bike in boxes and put it together, only to have it bogging real bad when I started it. I did a compression and spark test, spark is good. Compression in each cylinder read around 105 psi. I have a running 1981 Honda CB 900 Custom that I checked and the cylinders run around 140 psi.

I squirted a bit of oil in a cylinder in the non running bike and i got about 160 psi and it held. So, could each cylinder have bad rings, I would think it would occur on only one set but on all 4??

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Old 07-02-2008, 12:13 AM
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All four cylinders have the same amount of wear, expect the rings to wear equally as well.
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Old 07-02-2008, 07:28 AM
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Quote:
Originally Posted by 68NovaSS
All four cylinders have the same amount of wear, expect the rings to wear equally as well.

Strange because it only had 10k miles on it. Now, can I go with a stock rebuild or do I need to bore it over due to the wear.
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Old 07-02-2008, 07:38 AM
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Any thing will fire off at least 80 psi. My lawn mower is 70psi. .
Are you taking the compression reading at Wide open throttle or with the throttle closed?
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Old 07-02-2008, 08:10 AM
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Quote:
Originally Posted by n-gin
Any thing will fire off at least 80 psi. My lawn mower is 70psi. .
Are you taking the compression reading at Wide open throttle or with the throttle closed?

Wide open throttle. She'll fire, but bogs horribly until you reach about 5000 rpm. I took coils, ignition modules, carbs, battery off of same style running bike and get the same results.
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Old 07-02-2008, 08:36 AM
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Have you done a leak down on the motor yet ?
If not, it will tell if a ring or valve.
What about blow-by or burning oil ?
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Old 07-02-2008, 12:24 PM
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I have some experience in working on these older bikes and can tell you one thing, the wires go bad, and battery power is everything. Make sure the battery is good if any dought replace it. Now check to see if you are getting more than 11.9 volts to the coil withthe key on. The wires going to the coils can go bad. I had a similar problem with low rpm on my 1977 Kaw, yes its a KZ, as soon as you hit higher rpm it was OK at best, but lower rpms it was bouging. Finally test to see if you are getting 12.9 or higher at idle. At 4000 rpm you should be over 14.3v at the battery.
I actually ran a separate positive wire to my coils to a relay off the ignition and man did it get a kick in the pants Whow!
Shortly after I made my own ignition from two GM ignition moduals, It looks crude but a hell of a lot better then the old ignition. Now I just have to find out how to make my own coils I'm not joking
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Old 07-02-2008, 12:25 PM
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Battery is brand new. I never thought of it being the wires to the coil, but very interesting. I'll check that tonight and report it back. Thanks

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Old 07-02-2008, 01:54 PM
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Make sure the positive wire is on the correct side of the "+"coil, as it will cause a miss at high rpm, as well as incorrect spark plug wires.
Point type is solid core wires.
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Old 07-05-2008, 09:24 AM
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Quote:
Originally Posted by n-gin
I have some experience in working on these older bikes and can tell you one thing, the wires go bad, and battery power is everything. Make sure the battery is good if any dought replace it. Now check to see if you are getting more than 11.9 volts to the coil withthe key on. The wires going to the coils can go bad. I had a similar problem with low rpm on my 1977 Kaw, yes its a KZ, as soon as you hit higher rpm it was OK at best, but lower rpms it was bouging. Finally test to see if you are getting 12.9 or higher at idle. At 4000 rpm you should be over 14.3v at the battery.
I actually ran a separate positive wire to my coils to a relay off the ignition and man did it get a kick in the pants Whow!
Shortly after I made my own ignition from two GM ignition moduals, It looks crude but a hell of a lot better then the old ignition. Now I just have to find out how to make my own coils I'm not joking

Checked the voltage at the coils, they are reading about 11.7.
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Old 07-05-2008, 10:19 AM
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Are you sure the cam timing is correct, that could affect both power and compression. I would have another look at it I think.

Chet
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Old 07-05-2008, 10:40 AM
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One thing about rings and compression tests.
Remember the aluminum piston, when cold, has additional clearance to the cylinder wall. This allows the piston to rock, unseating the rings. Oil squirted in helps seal the minor leaks so compression goes up.
Commonly done on airplane engines.

This is the reason that the newer car pistons are of an alloy that expands little and the piston clearances are tight at 2.5 thou total. Less ring blow by for less smog. Older forged pistons set up around 6 thou. Think what happens when a piston and cylinder gets worn. It is not just piston slap.
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Old 07-05-2008, 12:08 PM
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I was a Honda Tech for many years and was working there when those bikes were released. Because of the SOHC 750 that proceeded it, they weren't maintained as they should have been. The 750 had screw/nut adjusters on the valves and the DOHC models are shimmed. They take more time and are considerably more costly to have the valves adjusted. I would check valve adjustment, they tended to burn the exhausts if not maintained correctly. The other,MOST common,problem is carbs getting plugged. They have 3 circuits and although the idle and high speed circuits usually get cleaned, the mid range isn't typically. The fact that it hasn't run much(10,000 is about what I put on a bike in a year) would account for the low compression too. I would still aim at the valves, the initial maintenance check may never have been done. As the valves tighten, and they will, it screws up everything...low compression, weird running/accelerating. By the way, if you have too tight valves and they aren't corrected...it is a pretty pricey to repair. At the time I think the valves were about $12 each, multiply that by 8 exhaust valves,add labor and gaskets....well it gets pricey.Have the carbs synched as well, that is also a long delayed maintenance item. The low compression would point me to the valves.
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Old 07-06-2008, 07:11 AM
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11.7 I imagine that is key off, Thats all right to have as your front headlight is on. Mine has a off switch for the light. Any way, What is it at running?
Get it running again and uncrew the oil fill cap to see how much psi is in the crankcase, you will probobly need to cap off the breathers on the engine since it is an 80. That will give you some idea of how much blow by you have.
Do adjust the valves it is posible for you to be bleeding off cylinder psi if they are out of adjustment.
Are the cams stock cams? I know on the old Kaws you can put 750 and even Gpz 750 cams in the 650. These cams have longer duration and can effect compression.
As for the carbs you dont have to vacume sync them right know to get it running good. Just take a drill bit about the smallest one you have about this * big and slide it under the slide of the carb, do not force it at all. You want it to have a slight drag, like if you were checking with a feeler gauge. Adjust the slides that way. IF you have CV type carbs then you need to look through the carb and see how much sun light is showing through the buterfly and bore. Remember the smaller the adjustment the closer you will be.. This goes for both style carbs. Be sides have the carbs vacume synced is over ratted. I have had my carbs synced by a profetional and my bike had less power and didnt run as smooth. So I use the drill bit method.
As for the circuits on the carb you have a Idle wich is your Pilot (easily clogged and most bikes can go up one size due to the leaning of carburation for polutants) Idle air bleed for richening or or leaning the idle circuit, (turn in for more fuel turn out for more air. Some my have a starter jet wich is a jet to measure fuel with the chock/enrichening system on. Most carbs on bikes have a richening system, not a choke. Acelerator pump found in some carbs this injects fuel into the back of the air horn twords the engine. Main metering (center jet for some throttle response and mostly for WOT) Clip position for main metering wich moves the fuel curve. Make sure the jets are not clogged or have imperfections were a screw driver has scrathced them, especially on the head of the jet. This WILL efect mixture more then you think. I would just replace them and be carefull on installing them. Do not over tighten .

These bikes have a mechanical type of advance unit for the ignition make sure that it is lubed and working with no binding. Also check the timing.
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