Honda Passport USA C70 Low Compression help

[Cub650GS]

Hi, I am new to the forum I just joined because I seen some Honda Cub topics and thought maybe I could get some help here.

I have a 1982 CDI Honda C70\Passport USA (I'm Canadian) and it runs but has low compression. I did a valve adjustment and still low compression. I then took off the head and gave it a good cleaning, it was carrbed up real good. I cleaned the valves noting that the exhaust valve was really dirty (it actually didn't come fully clean). I then tried to lap the valves with a drill and a lapping tool. Still, bad compression...

I sounds like it is seeping from a valve... I can hear it squeezing out when I hand turn the motor. I took the head off again and tried to fenangle the valves a bit to see if they would seat but nothing changed.

So, I am looking at heads on Amazon and ebay and wondering if I should just pop on a whole head or try and rebuild the one I have.
Is there a hot head option? Or head and cam setup that might be better than stock?
I don't really want to get into big bore kits or motor swaps because of cost and I don't want to mess about trying to find the right jets for what some report as little gain.
Thanks or the help; cheers!
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[uberkron]

don't use a drill to lap valves. hand tool with a suction cup, back and forth 1/2 turns clockwise and then counter clockwise.

every couple of minutes, lift the valve and give it a quarter turn.

then, sharpie the valve seat, then apply the valve with no grinding paste and give it a spin. if your ink dissapears evenly, the valve will seal/seat.

simply spinning the valve will make any leakage worse.
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i ride a zip 50 2smoke. It overtakes a hundred times a day.

[stinkwheel]

From memory, the C70 head IS the "hot head" for these engines, to the point C90 owners will fit one to their bike as a tuning aid.

Clean it properly, thoroughly inspect the valve and seat for damage like cracks and nibbles and seat it properly with a spit-stick and grinding paste. If it's pitted and nibbled on the seat/valve, you might need the valve to be reground or the seat to be recut and then lap it.

A quick and dirty test is to fit the plug and with the valves in place (pushed in, not necessarily with the springs fitted) sit it upside down over a bit of cardboard and put some petrol in the head. See if it leaks out. If it does, you aren't finished.

While you have the head off, you may as well check the bore too. Visual inspection for damage/wear. Look for signs of blowpast on the piston and carefully remove a piston ring and check the end-gap in the bore at a couple of points on the working area.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jeffyjeff]

[Robby]

How poor is the compression? I don't think it was ever that high on these engines, but maybe the 70cc was a bit higher.

I say this because even doing an imperfect valve job, you should have a pretty good seal on the valves. You can get reasonable compression with quite poor valve sealing, and using a suction stick rather than a drill is only going to yield minor improvement.

You haven't mention the piston rings or barrel. Worth measuring both. Also make sure your valves are straight, and the timing is right.

TL;DR: These bikes are the sort of thing that people get to learn how to take an engine apart. It's quite possible that someone has done that, and put it back together wrong. It also takes quite a lot for a single cylinder Honda engine of this era to actually run poorly - I've had a big split in the intake boot for my 250 single, and hardly any change in performance. So check the big things before chasing the small things.

[stinkwheel]

[Cub650GS]

Lots of good response; even the snarky ones

"How poor is compression?" 90psi, since I last took it out and tinkered with it, it won't start but I don't have a tester to see if the compression dropped. I think I got the timing off.

"don't use a drill to lap valves. hand tool with a suction cup, back and forth 1/2 turns clockwise and then counter clockwise." I never lapped a valve before and the mechanic friend who told me how just said to spin it with a drill. I actually bought a drill suction cup one that didn't suck to the valve, It was as hard as a rock even after applying heat. I was gonna buy one that had a mechanical gearing to step up the drill speed but the suction cups were too big for my little valves. I'm taking it these aren't the right tools?

"If indeed compression is leaking past a valve, you should be able to ascertain which valve it is by listening carefully to the intake and exhaust ports on the cylinder head as the piston comes up on compression stroke." A local mechanic told me it was low compression off one of the valves and that it needed a head job and it does sound like it is coming from the exhaust valve. He was going to rebuild the head for me, gave me a quote and everything, but didn't get back to me when I tried to book it (multiple times). I must assume he is either too busy or didn't want to. The other mechanic just wanted to throw amazon parts at it and I was less enthusiastic about that.

I don't want to get down into the piston and cylinder; I don't have the tools to deal with it, or hone it, or the personal constitution. If it needs all that, it will have to go to a shop.


"From memory, the C70 head IS the "hot head" for these engines, to the point C90 owners will fit one to their bike as a tuning aid." That's what I figured, I thought there might be a better head for it that was worth upgrading to.

Aside from a really dirty exhaust valve it looks good. The casting is a bit rough but there are no gouges, cracks, or anything that looks out of the norm. I did look at the bore and it looks smooth, my fingernail didn't catch anything and I don't want to get into it. I know there are people who feel strongly about going all the way all the time, I just don't have the confidence to.

"I was suspecting half a job." Kinda like your half a comment? Haha! You know I never done this before; it's not like I went "Huh, must be good enough", it was more like, "Hummm, if I keep using the sand paper on this am I going to ruin the valve? Should I use a wire brush? My honing stone?...".

After all this I am thinking maybe it's best to send the head to a head shop? There is one local to me and I am working with probably not the right tools.
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[jeffyjeff]

[stinkwheel]

The valves are hard steel, you can use a wire brush/wire wool to remove the carbon, especially on the face, just keep off the valve stem which is a ground diameter. Your spit-stick won't stick to carbon deposits, needs a clean, shiny metal surface. The valve seats are hardened too but the alloy of the head is soft so you can be fairly rough but in a more targetted manner with them.

[slowasyoulike]

[Cub650GS]

I spoke to the head guy before coming back to check responses and you guys all seem to generally agree. He thinks I should try to clean and the valve better and making note not to touch the stem (which I did the first time :O ), and to check for play in the valve seat and seal (which I also did and there wasn't any). When I touched the stem it was with a little cleaner and some 1200 grit. I didn't use a lot of pressure or force....

He also said that if I get the valve to seat and it bumps compression it might cause the rings to leak. Especially if blow by is what caused the initial build up. That said, there should be no harm in doing the valves and seeing what happens. Worse case new gaskets. I am assuming the mechanic I had it at did a leak down test and that is why he was confident that it was indeed something leaking in the head. I only vaguely recall the conversation and the emails suggest that was done. I was going to have him do it but I he never got back to me all winter.

This weekend I will hit the valves with some more cleaner and scotch bright and steel wool etc... basically just pick through the comments and get some cleaners and we will see what happens.

If I cannot get it with valve cleaning can someone point me to a good piston ring set and a valve set? I may as well put new valves, springs, keepers, piston rings, etc... in. I noticed some piston ring kits have 3 - 5 rings and I am not too sure what I'm going to need. I suppose I will know when I get it out and look.
New piston too?
Will I need to hone the bore or is that only if there is damage?
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[that_impulse_guy]

doing the vales with a stick is like 5 minutes max. very quick. I'd suspect using a drill isnt that great an idea.
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Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[stinkwheel]

Get OEM parts if you can unless they are stupidly expensive. They lasted this long... There are loads of aftermarket parts out there for these motors if price is an issue but in my experience, honda OEM stuff is top quality.

Normally when asessing a bore, you check for a lip (which you have done). Look for any damage, scores and smears with a good light. Have a look at the piston, see if gas has been blowing past the rings.

The proper way to check the bore is with a bore guage but they are fiddly and tricky to use, there's a knack to it.

The good enough way is to get a new standard size piston ring and push it down into the working part of the bore with the piston so it goes in totally square, then measure the end-gap on the ring with a feeler guage. There will be a specified tolerance for this in your workshop manual. If the gap is too wide, you may need a rebore. Or it may be cheaper to buy a complete barrel and piston kit for these bikes.

Here's a picture of me checking the ring end-gap in a different engine. This is a 2-stroke so it has ports cut in the barrel that you don't have.
https://lh3.googleusercontent.com/pw/AP1GczPRA7uH6Bfsss1xwYHrj_z9-839Dgfel35v6I-MLeTBnQ4uCdevQP9a9xi4Igo2_3QI-wE5Jhw-nBhUENwmmDLie51Z5orKPuyYPJqVnnCqpa5jtvhbjCqz_oXszXNaeW6fVPi6GnXxGTI3381dcRhW=w1547-h870-s-no

If it was me, I'd start by carefully (they can snap) popping one of the existing rings off and checking the end gap. If it's within spec, and there's no significant blowpast, I'd reassemble. If it's within spec and there is some blowpast, I'd hone the bore and reassemble.

If the gap is outside spec but there's no visible wear on the bore (that step you checked for) I'd order a new set of standard rings and check the end gap on those. If it's within spec, hone the bore and assemble with new rings. If it's outside spec, you just wasted the cost of new rings (that's just the nature of the job) and either need to have it rebored for the next size up piston or (may be cheaper for one of these) buy a barrel and piston kit and just swap in a new barrel.

If you're getting it rebored, you can usually buy a "piston kit" which has the piston, rings, gudgeon pin and circlips in it. You'll also need a top-end gasket set which should have head gasket, rocker cover gasket, valve stem oil seals, carb flange seal, exhaust gasket and base gasket.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Cub650GS]

Good news! It has much better compression now. I cleaned the exhaust valve and port with a scotchbright pad and then did the lapping as described and it certainly has compression now.
BUT...

Now it revs super high when I put on the choke. I removed the air filter box to see if it was plugged but that made no difference. I reset the mix screw to 2.5 turns out...
To my knowledge this is due to a lean condition so I need more fuel? I noticed some air in the fuel lines could that be the issue?
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Bork Bork

[A100man]

[stinkwheel]

Throttle valve could be sticking, most commonly due to an old/badly routed cable. You should be able to see it in operation if you take the air filter off.

Also a fast/variable/hanging idle can commonly be due to an air leak round the inlet manifold (causing air to get in and a lea running condition) so check the rubbers for cracking, make sure any gaskets or o-rings are in good condition.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Cub650GS]

"check the idle adjustemnst and only use choke when needed. 'Feather' the choke to achieve a resonable warm up idle." I'm not following; I can get it to start with a hit of throttle and it will idle but as soon as I touch the choke to open it, it dies. If I crank the idle up enough to get it to stay running when the choke is open it will idle like mad when the choke is closed. If I take off the air box and cover the intake it will die, but if I put the airbox on and cover the 4 intake holes it just runs like normal :~. It has stabilized fuel in it, could that be an issue?

"Throttle valve could be sticking, most commonly due to an old/badly routed cable. You should be able to see it in operation if you take the air filter off.

Also a fast/variable/hanging idle can commonly be due to an air leak round the inlet manifold (causing air to get in and a lea running condition) so check the rubbers for cracking, make sure any gaskets or o-rings are in good condition." I can stall it by covering the intake, I don't see any leaks or cracks in the seals. I put a new intake gasket on when I put it back together.

The throttle seems to operate fine, so I don't know. Before the compression issue I had it tuned in, it was just smoky due to the incomplete burn. Now it is running clean but it won't idle and I have to give it a hit of gas to start.
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Bork Bork

[A100man]

Your terminology is not helpful..

'Choke Open' does that mean choke on and feeding extra fuel/less air or does it mean carb mouth fully open and full air flow?
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[Cub650GS]

Open means full air flow
Closed mean restricted air flow i.e. choked

Think of it like first aid...

Open means the airway is open and I am not choking.
Closed means the airway is closed and I am choking.
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Bork Bork

[Cub650GS]

The new head gasket is no good.
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[jeffyjeff]

[Cub650GS]

Well, it isn't sealing; on start up there is a *pop* and at first I thought it was a shorting plug so I replaced the plug but it is actually the head gasket not sealing right near where the plug enters the head. My uncle said the gasket was no good...
I have to take it back off and see if there is something I can do about it.

He also noted that the head could be warped; I can get back to it this evening.
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