Compression question, yet another...

Sorry guys, I have searched and read countless threads with the hopes of understanding the health of my engine, but still need some help. Here are the basics: 1979 Honda CT70 with 2,035 miles and unknown ownership prior to my purchase several months ago.

When I bought it, it seemed to idle okay but bogged down terribly at anything more than 1/2 throttle. I pulled the carb, cleaned it, and readjusted after install. Now it starts typically 1st or 2nd kick, but doesn't seem to like idling very well after approx. 1 hour of easy trail riding. In fact, it kept dying at one point.

Fast forward a little bit, my spark plug has black soot on it, my exhaust always seems to smell heavy (rich fuel - not oil I believe), and if at any point I open the choke, it dies instantly regardless of cold or hot. My thought was too rich at this point. Additionally, at idle, unless I adjust the idle speed screw to a fast idle, there is clicking noise (sorry - don't know how else to describe it) which I though was the valves. They have since been adjusted but were very close to specs and the noise remains. I've tried to play with the pilot (air mixture) screw but anything besides the starting point of approx. 1.5 turns out seems to cause the idle to fall even lower and the engine typically dies. Currently needle is in the furthest down (richest) position possible.

So... Having said that, I thought I would test the compression. Engine was warm, choke open, and full throttle for the test. Call it 127 PSI after about 7 kicks and then, with about 1 tsp of oil added through spark plug hole, measured 140 PSI after the same 7 kicks. Compression tester is a loaner from Auto Zone but looked to be solid and in good shape without much use.

What am I missing at this point? It seems to run well enough to ride causally and it'll never be a daily commuter, but something just doesn't set right with me. I feel like I honestly don't know the subject matter well enough to effectively diagnose what, if anything, is going on. Any thoughts or suggestions would be greatly appreciated.

I should mention that I live in the San Francisco bay area and only in the last 2 days got this beast street legal. As such, I have not had a chance to test top speed, but my guess is roughly 30 mph. Don't let this be the killer thought though, it has a 4-speed transmission from a 1983 Honda ATC, but previous owner didn't change the front & rear sprocket so I'm currently rockin' 15T and 35T which I assume need to be updated (best starting guess is 14T and 45T but that's another conversation).

Sorry for the long post!



1983 Honda ATC transmission (listing for my future reference):
1st = 11/36 = 3.27
2nd = 16/31 = 1.93
3rd = 20/27 = 1.35
4th = 23/24 =1.04

sorry again - one last thought: I was told to remove the oil dipstick while the bike was running and see if there was any air movement / coming out. Not sure how scientific this is, but the answer is "yes". While idling, if I remove the oil dipstick, there is most definitely air coming out of the hole. Does this mean piston / rings / cylinder issues?

Removing the dipstick when the engine is running is like someone asking you to pull their finger. It's not going to be good. Not a valid test and makes a big mess. It sounds like you should move the clip up one groove and retest, then pull the plug to see if the plug looks better. My guess is it wil,l as it sounds too rich..

OLD CT said:

Removing the dipstick when the engine is running is like someone asking you to pull their finger. It's not going to be good.

Click to expand...

A sober truth with a side of humor...nearly a coffee-on-keyboard moment.

Any time the choke is needed to keep the engine running, there's a carburetor problem. Gotta figure out what is restricting fuel flow. My guess(es)...emulsion tube & pilot jet. There may also be a vacuum leak. I'd start with OC's very logical advice. Things only get more complicated beyond this point.

Compression is getting weak. Makes me think the intake valve is leaky. If the intake port is clean, this is ruled out, a black intake port confirms it. Those psi numbers are consistent with an oversized tester; gauge & hose volume matter with small displacement.

That 4-speed tranny is a sweet upgrade. A little math and optimal sprockets can be determined. Top end should be in the low 40s...with a healthy-running motor.

While reading the OP I was thinking rich and this carb needs a good cleaning. Air filter is also important for a good tune so address it as well. Are the gaskets and isolator for the manifold good leak free and in the correct locations?

15/35 sprockets are standard gearing for your bike. 14/45 would make it slower on top end.
With the added 4th gear, you should be able to approach 45-50 mph, no problem.

Definitely sounds too rich from your description but just carb settings shouldn't slow it down that much. I may be wrong.
Check for any restrictions in the air box intake.
Fuel flow...
Float level...
Is it a stock carb?

Does the motor sound like it's reaching full rpm's at top speed?
If so, maybe the clutch is slipping.

Adjust the cam chain and see if the rattling goes away.

Take out the battery holder and look down in there to see if the fuel lines are squeezing up against the air inlet snorkel. If they are, thats acting like a secondary choke.

wanrep said:

15/35 sprockets are standard gearing for your bike. 14/45 would make it slower on top end.
With the added 4th gear, you should be able to approach 45-50 mph, no problem.

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No disrespect...but...you've gotta be precise when it comes to gearing these little machines. They don't make much torque thus, if you're more than ~3% off, you'll feel the difference, beyond about 5% the motor will either fall on its face or you'll be giving away the store in needless additional revs per mile. With a non-stock transmission and who-knows-what primary gear pair, I'd want the gear teeth count for the 4th gear pair & primary drive pair before making any recommendations on sprocket combo. A 72cc tune will pull 4.77mph/1000rpm optimally and not much more (5.00mph would be over-the-top) unless there are exceptional circumstances.

As for top speed, fastest stock 72cc-powered stocker I've heard of belongs to our own CJPayne...50mph, in a full tuck. That's really hauling arse for a stock CT70, even with a fairly light rider aboard. Basic physics say that top speed is limited, ultimately, by horsepower...and that usable power starts running out at 80% of that number. With the available torque, most CTs have good, usable, speed into the high 30s and that's about it. Gearing will never compensate for lack of horsepower but, incorrect gearing can cost you in acceleration and mph.

racerx said:

No disrespect...but...

Click to expand...

Certainly, none taken. I was imagining Kansas Corey headed down one of those steep San Francisco streets on a top end run.

To your point...I understand what you're saying and completely agree.
Just because you gear one of these to theoretically go 80 mph, doesn't mean the motor has enough steam to pull it off.

Primary gear ratio is the same between the ATC70 and CT70 (pretty sure).
Only difference would be the trans. gear ratio. 3rd gear for the CT is 1.19. 4th gear for the ATC is 1.04.

Once Kansas Corey gets the carb sorted out, he can let us know how it pulls 4th gear.

Nice to see another Bay Area on here.

OLD CT said:

Removing the dipstick when the engine is running is like someone asking you to pull their finger. It's not going to be good. Not a valid test and makes a big mess. It sounds like you should move the clip up one groove and retest, then pull the plug to see if the plug looks better. My guess is it wil,l as it sounds too rich..

Click to expand...

OLD CT - will test it this weekend if I can't get to it tonight. Thanks!

racerx said:

Any time the choke is needed to keep the engine running, there's a carburetor problem.

Click to expand...

racerx - thank you commenting. I think possibly you misread my original post? Anytime I open the choke (turn it on), the engine instantly dies. Basically, I leave the choke off and kick start it first or second try. Probably running way to rich, but we'll find out soon. Will try adjusting the needle and resetting the idle + air pilot screws. If I'm feeling adventurous, I might tear the carb apart again this weekend and double check the emulsion tube & pilot jet. I did use a small piece of wire brush to clean them last time, but now understanding the overall importance of perfect operation in this area, will go over them in much greater detail.

Deoodles said:

Air filter is also important for a good tune so address it as well. Are the gaskets and isolator for the manifold good leak free and in the correct locations?

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Deoodles - air filter is brand new. As far as gaskets & isolator - everything looks complete from what I can see. I did take both carb cleaner then starting spray (ran out of one and switched to the other) and soaked the entire area with the bike running- there was no change in idle speed so I was assuming no air leaks?

racerx said:

With a non-stock transmission and who-knows-what primary gear pair, I'd want the gear teeth count for the 4th gear pair & primary drive pair before making any recommendations on sprocket combo.

Click to expand...

racerx - seriously always appreciate your insight on these beasts. Does this help? Has 15T up front and 35T on the rear.

1983 Honda ATC Transmission:
..1st = 11/36 = 3.27
..2nd = 16/31 = 1.93
..3rd = 20/27 = 1.35
..4th = 23/24 =1.04

Of course, as wanrep posted directly below, happy to tackle this once I get the carb and/or compression sorted out. Thanks again!

wanrep said:

Primary gear ratio is the same between the ATC70 and CT70 (pretty sure). Only difference would be the trans. gear ratio. 3rd gear for the CT is 1.19. 4th gear for the ATC is 1.04. Once Kansas Corey gets the carb sorted out, he can let us know how it pulls 4th gear.

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wanrep - seems like a fair path forward. Appreciate the input.

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Thank you everyone for the input - sounds like I'm much more concerned over the 127 PSI compression and the 13 PSI gain when done wet than anyone else. Heck, I was convinced that this was my primary problem! I'll go through the carb and adjustments again this weekend and see if I can get some improvement (idle, throttle response, and overall drivability) as well as try to get a top speed run in just for fun!!

Another good idea is the take the intake off to see if the port is black like racerx suggests. Black means the valve itself is leaking and losing compression.It is a really good test to ''gauge'' the engines condition. If you do not need to use the choke cold you have a rich situation. Sometimes it is good as you know the bike is safely jetted and will run cooler. If the plug is black and sooty on the other hand, the clip is the first thing to adjust and do not cost anything. Sounds like tripod is kinda close, always good to make a friend here. This forum has a lot of good people and we actually like to meet in person at events or if one is stumped with a running condition and such for help, when time permits. Good luck.

wanrep said:

Certainly, none taken. I was imagining Kansas Corey headed down one of those steep San Francisco streets on a top end run.

To your point...I understand what you're saying and completely agree.
Just because you gear one of these to theoretically go 80 mph, doesn't mean the motor has enough steam to pull it off.

Primary gear ratio is the same between the ATC70 and CT70 (pretty sure).
Only difference would be the trans. gear ratio. 3rd gear for the CT is 1.19. 4th gear for the ATC is 1.04.

Once Kansas Corey gets the carb sorted out, he can let us know how it pulls 4th gear.

Click to expand...

Okay, now that I'm no longer using wood-burning interweb access...

Stock sprocket combos:

• 15/38 = 2.533
• 15/35 = 2.333
• 14/44 = 3.14
• 15/45 - 3.00

Primary gearing:

• 18/67 = 3.72
• 17/69 = 4.05

Top gear ratios:

• 1.19
• 1.04

Cutting to the chase, stock gearing (crank-to-wheel) was 11.21 for the K0, 11.24 for K1-later...a difference of 0.3%. The 4-speed motors make the same torque, thus need the same crank-to-wheel gearing, within ~3%. The difference between 1.04 & 1.19 is ~14% and that's just for the top gear alone. A little basic math, once you know what your primary ratio is, will tell the story, with precision. I just don't see a 72cc tune being able to pull ~9.6:1 gearing effectively, under typical riding conditions.

Morning everyone. Okay - quick update: I move the needle clip from the highest point (richest setting) down 2 spots to the middle setting. Afterwards reset the carb to roughly 1.5 turns out on both pilot air screw and idle screw. Bike was slight bit harder to start and had to use choke when cold, but started and idled on roughly the 4th - 5th kick. Took it out for a quick run after it was warmed up. Top speed was 31 mph per the speedo (my GPS on iPhone wasn't working for some reason). Pulled really well the bottom half of the throttle but once about 50% of max RPM's, ran smooth but with no umph. I assume this is 1 of 2 issues possibly: either carb isn't perfectly tuned so my air fuel ratio on top 1/2 of the range isn't quite right - or - possibly gearing as touched on above.

OLD CT - I looked at both intake and exhaust valves and neither one looks black / sooty. Later today, I'll swap in a new spark plug, go for a quick ride, and check the condition when I'm done.

racerx said:

Okay, now that I'm no longer using wood-burning interweb access...

Click to expand...

racerx - Love This!! Just wish I loved your gear ratio input as much... Not that I don't appreciate it, as I do tremendously, but I'm fairly good with math & numbers, the whole gear ratio stuff confuses the heck out of me honestly. Probably because I understand the basic concepts, but have never ever done anything with it. Later today I'll try to do a little studying and see if I can work through the numbers. Thank you for the help!!

Will update again later today once I get through my "honey do" list

** forgot to mention above: yesterday I had been doing some maintenance work on my Suburban. After my carb adjustments and my quick ride, since I was already dirty and had tools out, I quickly changed the oil on the Honda CT70. I was really disappointed to see the oil that came out was dark gray in color and had a lot of "star dust" look to it. Obviously metal of some sort and not a good sign I assume?

The intake port & visible portion of the valve should both be clean metal. Any discoloration would be the result of a leaking intake valve. If the port is clean, gray, aluminum...throughout...the valve is sealing properly.

Ever ride a bicycle with more than one gear ratio? Remember have much farther you traveled, per crank revolution, in top gear...and how much more effort was required? On steep grades, you probably had to choose between downshifting and walking the bike uphill. That's usually the easiest analogy I can provide. When a motorcycle is overgeared (a.k.a. gearing that's "too tall") the engine can't pull it and feels weak on the "seat-of-the-pants dyno". I suspect that that your setup is way overgeared...a K1 sprocket combo paired with a 4-speed...and that's why it has "no umph" above 30mph. There's simply not enough torque available. In oversimplified terms, I'd try a 45t wheel sprocket to go with the 15t cog on the countershaft.

As for finding stardust in the oil..."it depends". With a new engine, or fresh rebuild, it's to be expected, especially in the early stages of break-in. The first few changes will be very sparkly. With a fully broken-in engine (anything with more than 1000-1500 miles on the clock) stardust is not a good sign. That said, it may mean nothing more than than an oil spinner that's long overdue for a cleaning. IOW, it's filled to the point that it can no longer remove the abrasive gar-bahge from the oil.

FWIW...a 14/37 sprocket combo would come close to the stock crank-wheel ratio. 11.13

You mentioned a clicking noise at idle in the first post. If that clicking noise is the cam chain, slapping against the tunnel in the head, it's wearing a groove into the aluminum head casting with every click.
That aluminum would all go into your oil. Have you tried adjusting the MANUAL cam chain tensioner yet?

Also...a magnet will tell you if that stardust is steel, or aluminum.

wanrep said:

FWIW...a 14/37 sprocket combo would come close to the stock crank-wheel ratio. 11.13

Click to expand...

Something just didn't smell quite right. So, I checked the parts manual. H-models came fitted with 18/67 primaries, 24/23 4th gear combo...which is .958, not 1.04. Thus, 14/44 would be the closest sprocket combo, at 3.14.

(3.72 x .958) x 3.14 = 11.19

FWIW, 14/45 cogs would give a final drive ratio of 10.68, ~4.5% taller. 14/37 would give 9.40...16% taller than stock.

FYI...this all depends upon the primary gears being 18/67 and the 4th gear pair being 24/23. If any of those numbers are different, all bets are off.

kirrbby said:

You mentioned a clicking noise at idle in the first post. If that clicking noise is the cam chain, slapping against the tunnel in the head, it's wearing a groove into the aluminum head casting with every click.
That aluminum would all go into your oil. Have you tried adjusting the MANUAL cam chain tensioner yet?

Also...a magnet will tell you if that stardust is steel, or aluminum.

Click to expand...

Good catch, Mr. B. That one got right past me...nothing but contrails...

racerx said:

Something just didn't smell quite right. So, I checked the parts manual. H-models came fitted with 18/67 primaries, 24/23 4th gear combo...which is .958, not 1.04. Thus, 14/44 would be the closest sprocket combo, at 3.14.

(3.72 x .958) x 3.14 = 11.19

Click to expand...

Right on...but...just to make sure we're on the same page...
He's got a '79 3-speed motor with a '83 ATC70, 4-speed tranny,

Primary gears are the same between the two...17/69 for a 4.059 ratio or 4.06 for argument sake.
4th gear in the ATC trans. is 23/24 or 1.04
If I'm figuring this right (calculators do lie), 4.06x1.04x2.64 (14/37 sprocket) = 11.14

I do think, now, that this gearing question has a lot to do with his lack of top speed.

Are you sure that it's 23/24 and not 24/23? Anything is possible, I suspoze...especially at OEM level. But, consider Honda's consistency, year after year. IMHO, it seems a little odd that they'd reverse the M/S & C/S gears. Even the Wave 100 & Nice 110, new-gen (circa 2000) use the 24/23 (.958 ratio) pairing and that's an all-new, much beefier, transmission. All of which having been said, I'm not as familiar with the ATC tranny as I am with the other 4-speed flavors from the 1970s. IOW, if you know the teeth counts to be as you stated them, then go with that...in that case, I'd be wrong. It's just a machine, actual parameters matter...egos, not so much (a.k.a. nada).