S65 carb woes

6

69ST

Well-Known Member
Sams65 said:
Ok, ordered a proper compression tester (about time I got one anyway) and will post results here as soon as I get my hands on it. The continued input and suggestions are very much appreciated (also happy to be troubleshooting something other than just carbs for a change)
Click to expand...
Two things to note...
First, open the throttle, if the carb isn't already off the the engine, to allow complete cylinder filling. Second, the most accurate PSI reading will be had by placing the gauge directly in the sparkplug hole; that's usually impossible. Thus, it's possible...and likely...that the gauge number will be lower than what's actually in the cylinder. With a typical-length hose connection, 140psi+ is a good reading.

With small displacement, hose volume can and usually does affect peak gauge readings. I won't go into a mathematics lecture. Suffice it to say that 65cc displacement with 9:1 CR means 7.2cc open volume at TDC. 7cc is a tiny volume. Just adding 3cc hose volume drops CR down to 6.5:1, hence the lower-than-actual psi numbers. Most compression testers are made for use with automotive engines. When there's 500c+ cylinder displacement, hose volume becomes insignificant.
 
red69

red69

Well-Known Member
To add to what Bob said, with an 18" hose on my compression tester, my two K0's read 135 psi and 137 psi. on original engines with about 2500 miles on the odometer. My S90 with 4700 miles and a previous head rebuild at 4100 miles, read 135 psi. All bikes pull my 190 pound ass very nicely. The K0's will do about 36 mph and the S90 gets to 53 mph on the flat.
 
Gary

Gary

Well-Known Member
Here are some official Honda spec's that may help-

honda scan.jpg
 
S

Sams65

New Member
Ok, got some compression numbers... Although I wasn't able to warm the engine up completely to operating temperature (since it's pretty much unrideable in its current state) I'm hoping these numbers might still provide some clues...

With a warm(ish) engine: 125 psi
With a cold engine: 115 psi

*Both tests done with throttle wide open.

Based on racerx's recommendation of 140+ psi I'm guessing these are looking a little abysmal, but not sure how much of that is due to a lukewarm engine.
 
L

lukelaw1

Active Member
I'd test cold with throttle wide open and record readings. Then Add a teaspoon or two of oil down the spark plug hole and retest again with throttle wide open. Compare the numbers.
 
S

Sams65

New Member
lukelaw1 said:
I'd test cold with throttle wide open and record readings. Then Add a teaspoon or two of oil down the spark plug hole and retest again with throttle wide open. Compare the numbers.
Click to expand...

Good tip. Here are the numbers for that:
Cold, no oil: 115 psi
Cold, w/ oil: 135 psi
 
6

69ST

Well-Known Member
Those numbers aren't necessarily accurate. Highest reading I've seen, firsthand, using my own compression gauge...on a 49cc motor...was 132psi. With 72cc, typical numbers are in the 140s. Highest reading I've seen was 155psi, with a stock 110 Nice motor, closed throttle; with 9.5:1 static CR, the real number was probably closer to 195psi. No matter...this just means that using the same gauge + hose assembly gives you the same amount of error and that can be used with a correction factor. I'm not being coy here, either. Short of getting the gauge head threaded-into the sparkplug hole with a very short metal fitting, there's not much way of getting an accurate reading. Even the internal volume of the gauge itself impacts the psi reading. Thus, what I'm saying is to use the same tester, that will give you consistent results...IOW you'll get the same amount of error with any given displacement. With time, you'll become familiar enough with what you're seeing to mentally "correct" the low readings. Exact, it ain't. With practice and experience it can be very useful.

Consider those who've posted that their engines ran with 90psig cranking compression. IMO, 90psi actual, isn't enough to make an engine run. Obviously, the true cylinder pressure is higher. We cannot know the exact amount of error, it doesn't necessarily matter. If one of these owners suddenly sees 60psi and the engine won't fire, it's obvious that compression is way down. Or, seeing a big increase after pouring-in oil is still a usable result. Despite being inaccurate numbers, the amount of error is consistent enough to be used as a reference standard of its own...using that tester with a given displacement.

Between gauge numbers, top speed numbers, and using the "shadetree mechanic's compression tester", i.e. cranking the kickstarter by hand, it is possible to get a reasonably accurate indication of top end condition. Full compression will always manifest as sharp resistance being felt through the kickstarter, on the compression stroke. With a properly-executed fresh rebuild (bore, hone, full valve job) or a known healthy engine, you have something to use for establishing your baseline gauge readings...with your tester.

I really didn't want to get this longwinded, it's almost unavoidable, though. There's no simple & concise answer to this.
 
L

lukelaw1

Active Member
Last resort, maybe time to pull the head, rotate the flywheel and check/measure how much the piston protrudes out the cylinder. Should be able to straight edge the top of piston and measure back to machined face of the top of the cylinder. Then straight edge the machined face of the head and measure down to the combustion chamber. See what you have for numbers. We are trying to get an idea of how much clearance there is between piston and head. Dont forget to add the thickness of the head gasket.

If feeling up to it remove the cylinder and check/measure old piston to new. Look at the rings to for proper placement and orientation.
 
S

Sams65

New Member
Yup, agreed. I’ll crack things open and will start measuring and comparing components.

Compiling a nice check list here with all the input. More to come...
 
S

Sams65

New Member
Finally got around to spinning some wrenches on this again but unfortunately no eureka moments. I took the old parts off and tried to take some measurements but lacking accurate tools, I had to make do with a digital caliper. Readings were inconsistent to say the least...

I even threw the old parts back on the bike (this included original head w/valves/guides/springs, original cylinder, piston and rings) and still got the exact same results with massive rich condition. Was hoping it would at least lead to low compression and oil burning instead. I'd take anything else... But it's like nothing's changed.

**Note: one thing I did notice when trying to fit the old piston with new rings into the old bore, I couldn't compress the rings enough with my fingers. And I don't have a ring compressor small enough for these teacup pistons (tried a hose clamp but no luck) So just ended up using the old rings. With the new parts I was able to do it easily with my hands, using the bevel on the underside of the cylinder to drive everything in. I measured the end gaps of the new rings and everything's within spec. So this is potentially something I might investigate again.

Attached pics of old and new parts in case any of you more eagle eyed might notice anything way off.

New combustion chamber:
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1227.jpg
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1212.jpg

New on left, old on right:
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1229.jpg

Cylinders - new on left, old on right:
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1233.jpg
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1235.jpg
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1234.jpg

Pistons - new on left, old on right
https://i1257.photobucket.com/albums/ii510/sam84i/IMG_1221.jpg


To recap so far: Have now tried 3 different carbs, 2 different heads, 2 different cylinders, 2 different pistons and rings, 2 different air filters (and no air filter) and all result in the same pig-rich running condition.

At this point I might see if I can get my hands on some bore gauges/micrometers and do proper measurements. Any other thoughts or opinions?

Question - could the exhaust system factor in to this in some way?
 
S

Sams65

New Member
lukelaw1 said:
Last resort, maybe time to pull the head, rotate the flywheel and check/measure how much the piston protrudes out the cylinder. Should be able to straight edge the top of piston and measure back to machined face of the top of the cylinder. Then straight edge the machined face of the head and measure down to the combustion chamber. See what you have for numbers. We are trying to get an idea of how much clearance there is between piston and head. Dont forget to add the thickness of the head gasket.

If feeling up to it remove the cylinder and check/measure old piston to new. Look at the rings to for proper placement and orientation.
Click to expand...

Before I took everything apart I removed just the head and measured from the piston crown down to the top of the cylinder. I did it with cheapo digital caliper so not entirely confident in the accuracy, but here are the numbers for that:

Without gasket: 5.71mm
With gasket: 4.65mm

I currently have the old piston and cylinder on the bike so I'll do the same with those parts.
 
S

Sams65

New Member
lukelaw1 said:
When you changed carbs, did everything get changed slide, needle, cap?
Click to expand...

Yeah, three different carbs with three different sets of internals each. Even purchased new jets/needles and floats, gaskets etc.

The slides and caps stayed original but I interchanged them between different carbs.
 
S

Sams65

New Member
What are the collective opinions on lapping brand new valves on a brand new head? Yay or nay?

I had the head off and filled it with solvent for a couple hours but nothing leaked through.

I got a few small bubbles when i forced compressed air through the intake side but not sure if this could have been due to the air forcing the valve to open slightly... No bubbles appeared around the exhaust valve though. (So not sure how accurate/appropriate this test is)

(As a recap I did also perform an actual leakdown test with the engine assembled, which showed nothing)

Still have some nagging doubts about the compression numbers I listed above.

In other news I also measured piston to bore clearance and all related specs with a bore gauge and mic. All are within the specs listed in the service manual.

So really chasing ghosts at this point.
 
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