Modern Vespa

I recently bought a winter project - a 2017 Honda CB300F with about 5000 miles. It's not running. I'm told the bike ran perfectly, right up until it was parked 2+ years ago. Everything I've seen on this bike checks out so far (i.e. very clean) and PO showed me paperwork of mechanic review.

Anyways, I've gone through the expected issues and made sure they're good: fuel pump and injector working. Bike has good spark.

The problem: I checked compression yesterday and it's really low at 60 psi. Throttle was wide open, but obviously engine was cold.

I've read that this can happen on a sitting engine from stuck rings or valve. Does this seem plausible? Has anyone here encountered and solved this before? I'm hoping I can get it going with penetrating fluid in the cylinder (or similar) rather than opening the top end!

Sticking valves are absolutely plausible causes for low compression on a motor that has been unoperated for a while.

The word "penetrant" can have several meanings. I wouldn't say "penetrating oil" like Kroil or ATF/acetone would be helpful in this situation, as it is bad for gaskets and rubber. I would suggest adding one ounce - no more - of a light oil called Marvel Mystery Oil into a full fuel tank of fresh gas. There is a lot of speculation as to what is in MMO, as it smells like wintergreen, but it's probably just a modest concoction of light machine oil and a bit of methyl salicylate, a mild penetrant.

Then crank the engine over a few times, remove the plug, and shoot a teaspoonful Marvel Mystery Oil into the cylinder with a plastic syringe. Let it sit for 15-20 minutes, and repeat starting with the cranking to see if she starts.

The idea here with the syringe oil is to lubricate the valves, and possibly the rings, so that they move and seal enough to allow repeated combustion to take place and the engine will start, though it might stumble. The idea with waiting before cranking is to allow the oil to dribble past the valves into the sump so there isn't too much compression during cranking. The idea with the oil in the fuel is to allow a bit of extra lubrication for a few hours of running so the valve corrosion can work its way off. Once the engine gets running you don't need to keep adding MMO to future fuel.

(This is a fairly standard procedure for small engines used seasonally like lawn mowers, pressure washers, and snowmobiles.)

I had a CB300F---First get it running as is---or maybe first some Marvel Mystery down the plug hole----run some, change oil (Walmart house brand 10-40 has worked for me for the last 2 wheel miles 500,000 miles) Ride a couple of hundred miles, then check compression---if still low, maybe pull it apart. Also check valve clearance. My CB300 did over 30,000 miles...including James Bay Road....always ran well.

Agree with Juan - add some oil and check again with a known good compression gauge. If the readings jump, you have a ring problem. If not, it's a valve problem.

Marvel Mystery Oil... there's one I hadn't heard of in a loooong time...

Legendary stuff that!

A few drops of oil into the cylinder and a second compression test can give you an idea of what the issue is. If the compression jumps after a bit of oil is added the issue is likely stuck rings as the small amount of oil will settle around the piston and improve the seal marginally during the test, if it makes no difference its likely a valve thats not seating.

I don't know this engine - but perhaps it has a reduced compression while cranking, just like Piaggio engines?

Thank you (all) for the ideas!

I didn't have MMO available, so I tried with about a teaspoon of engine oil into the spark plug hole. I repeated the compression test. It was still too low, but had increased quite a bit to 100psi. Just to see, I then reinstalled the tank (annoyingly, this bike has the spark plug under the gas tank and down in a narrow ~4" deep hole) and tried to start it. No luck.

But the increased pressure suggests there's a ring problem? I guess I'll repeat this process a couple of times and see if the rings loosen up?

This is a good idea, but I don't think it's the case. I'm following the Honda Service Manual, and they specify the normal compression with this method (on a hot engine, mind you) should be 188psi.

It sure looks like a fun bike... if I could just get it running!

It's rare for someone to just park a running bike for two plus years, but not rare to hear that when they go to sell it. Sort of like the ads that say it just needs some $40 part to make it run, but the owner doesn't have time...
It was probably running very well two plus years ago, until it broke. Then it got parked.
You will do better trying to figure out what broke when it stop running than what would cause a well running bike to lose compression sitting in a garage for two years. My guess is they ran it out of oil.
When it comes to used vehicles, buyers are liars and sellers are worse.

5000 miles is about right for blowing up at "never had its first service or oil checked". Speaking from experience here unfortunately.

Entry level model for every squid who wants a CBR1000RR. Ridden accordingly.

Yes, this is possible and always my first thought when buying a non runner used vehicle. I may end up being wrong, but a few things leave me optimistic that's not the situation here.

I met the seller at his house. I saw paperwork where he had a mobile mechanic come by a year ago to try and get it out of "hibernation". The bike wasn't that expensive. And I just generally got a good feeling from meeting with him a few times.

Of course I may end up disappointed. But I did want a project

So it really only sat for a year. The plot thickens.

You guys are more cynical than me, which is saying a lot!

Seller is a 40ish banker from France. Has a 3 year old child, which lines up (I'm betting no coincidence) with the bike being parked.

About a year ago he says he thought to pick it up again and had a mechanic over. It had already been sitting a while at this point. Mechanic changed battery, checked for spark and a few other things. Suggested he bring it to shop to drain tank and have throttle body and injector cleaned. This didn't happen and bike continued to sit.

I will open the engine if it comes to it. I was just surprised the compression was low and curious if it's possible to have been caused by sitting. (Otherwise I'd have to agree the only possible explanation is I was misled.) I was also curious if anyone had experience/success un-sticking in place.

Thanks for the additional information. It's helps!

The three year old stuffed a fluffy toy in the exhaust.
You're welcome.

Did you check the air filter? And the exhaust? It's not crazy to find that mice have built a nest in the air filter or that something is plugging up the exhaust.

People do terrible things to motorcycles like not service them so anything is possible.

Make sure, make very sure you are using a good compression gauge. You can get some really wonky readings with the cheap ones.

I'll tell you a crazier way to check to and you already own one - your thumb. Have a helper open the throttle fully, hit the starter button while you place your thumb firmly over the spark plug whole. If your thumb gets moved off, you have enough compression. If not, it's on to the next test. Remember to ground out the spark plug, some of the electrical systems are touchy.

It is not cynicism, it is prudence and then it is also healthy to insinuate a legitimate doubt. But isn't a vintage motorcycle in your thoughts?

Ran fine... right up until Jr. Squiddy missed a shift when going for the next gear at redline and bent a valve over revving it.

Or, ran fine until that accidental double downshift while chasing his buddy into a tight curve.. over revving it and bending a valve.

To check for the above, Set the crank at the correct angle for checking the valves per the marking on the rotor... makes sure all valves have correct clearance.

Actually... it could have run fine until he tried adjusting his own valves...

Your thumb is a good ruleout of VERY bad compression, but it starts to slip off at about 70psi. That feels like a lot of compression but is probably not enough to start the engine. It can't really tell the difference between 70 and 100 and 130.

It is also hard to finagle a thumb over the plug hole on a lot of Piaggio engines.

Easy though on "flat four" engines like old tractors, forklifts, and many cars where 70psi will probably be enough to start them.

... 5 bar may be few ... it also depends on other parameters.
Of course in a side valve engine they are enough ... maybe.

These bike have a rev limiter, so it's hard to over rev one.
They also have a slipper clutch, so again, hard to over rev.
But, yeah, you can adjust the valves wrong and bend them, I'm sure

First valve check on this bike is 16,000 miles ( Honda ), so I don't think they will have been adjusted yet.

Well, a bit of "progress".

I sourced some Marvel Mystery Oil and tried a daily teaspoon for a couple of days. Compression improved slightly, but maxed at ~90 psi.

So I started disconnecting the engine in preparation to remove it and have a closer look.

I was able to get a squinty look at the valves once I removed the throttle body and exhaust. Intake valves look clean, but I seem some rust on the exhaust side. I'm guessing this is my problem.

I know I have a USB borescope here somewhere, I just can't find it now that I need it

I'm tempted to try and clean this up from here and maybe MMO from the outside. Only because I feel a lot of this crud would clean up on its own once the engine were running again.

To got a little more information, I'm going to try and see if the valves move when I rotate the crank, and also measure the clearances.

Realistically I'm going to have to open the engine and clean things up properly. It's just taking forever to get parts, it seems. So I suspect the engine will be taking up shop space for weeks. At least it's winter, so not a huge issue.

My advice before tearing the engine apart would be to spend a bit of time cycling the valves to see if you can knock off enough corrosion to get them move them smoothly enough to start the engine. The hope is that if you can move them manually a few dozen times the engine will catch, and run rough for a while, but then after thousands of cycles with that fuel with extra MMO lubricant in it the engine gets back to normal.

I don't know if this will work on a Vespa but on other engines with impaired valves, the recipe I've used has been to goose the starter with the plug out, which will leave a sticking valve in the OPEN position, spritz a little MMO over it, tap the valve down with a tool like an Allen wrench (this can be a lot like arthoscopic surgery where you can't see what you're operating on), and repeat this cycle a few times. Then install the plug and see if the valves are limber enough to allow the engine to start.

EDIT: the engine won't catch with 90psi of compression? I am a little surprised by that.

This sounds like good advice - thanks! I'll continue working on it and see where I get.

I would says the 90psi was fleeting, at best. I got that reading once, but I think it was because oil happened to be helping seal things up. When I tried again immediately after, the reading returned to ~50psi. So I'm thinking stuck valve. My understanding is this is an interference engine, so I'm hoping it's not bent!

Or...maybe maybe maybe...the valve cycled once and got stuck again. Do keep at it, my fingers are crossed for you.

I tried to clean up the exhaust valves with Scotch pad, and sprayed some MMO on them directly. Then I manually rotated the crank a few times over the course of a day. No material improvement in compression

I was able to see that intake and exhaust valves were all cycling (i.e. nothing was obviously stuck, as far as I could tell).

Then I dug a little deeper and removed the valve cover. Things looked pretty good to me. Nothing burned or scored. Timing was spot on. Timing chain had good tension.

I measured the valve clearances and found both intake valves to be in spec. But both exhaust valves were too tight (clearance 0.20 mm vs min 0.24mm / preferred 0.27mm).

I know that valves out of spec are a common cause of hard / no start, so maybe that's it. I'll buy new shims and test this out.

That said, I've heard of bikes with basically no clearance that still start, so I'd be a little surprised if 0.04mm too tight is causing no start. Is this plausible? (And I'm a little surprised they're out of spec at all, given the service manual says first check at 16,000miles.)

Well I attempted a (sort of) leakdown test where I supplied compressed air and listened for leaks. The intake and exhaust were good, so valves probably OK. No bubbles in the radiator, so head gasket probably OK. But a solid "breeze" coming out of the oil filler hole, so something's wrong with the piston side of things... out comes the engine.

Bad news inside the engine. I found a chewed up piston, cylinder, and wrist pin. In my opinion, far too much damage for just trying to start it after a long sit. But who knows? Cynicism wins, I guess.

Anyways, it was a cheap project bike. Should still work out alright if I could replace the cylinder, piston, rings, gaskets, etc (looks to be about $250 USD) and move on.

But... I'm a little concerned about damage to the outer edge of the connecting rod bearing surface. I think it's mostly wrist pin material that has deposited onto the connecting rod. I haven't tried cleaning anything up yet because I don't want to make it worse. There may be some scoring on the connecting rod too, but it's hard to tell without cleaning things up.

Any suggestions on how to proceed would be welcome! I'm thinking maybe a small honing tool, if they make them that small (the diameter is about 17mm). But most of the bearing surface looks undamaged and I don't want to make a problem there.

BROKE DOWN AND These French bankers, they cannot be trusted, n'est-ce que pas?

They make hones that small. I have a 10mm (cleaning up a stuck autolube pump) and a 12mm one (cleaning up a conrod end on an Elite 80). They come in multiple grits too, so choose carefully. Check the Brush Research catalog for options.

That said, the rod looks pretty ugly. You might check the parts catalogs for a rod kit and source a machinist to press a new rod/bearing/pin in.

Somebody might have run it out of oil at one time, hence the toasted piston. I bought a 1991 Nighthawk 250 several years ago with a bad carburetor in pieces, rear brake cam way out of its useful range rendering the rear brakes useless, a burned out starter motor and the directional signals didn't work due to a burned out relay and damaged handlebar switch gear and a gutted exhaust sytem.

This was the result of my efforts. Later I toned down the exhaust.

Honda builds a very tough bike. Somebody did a number on yours

If you ever own a motorcycle shop, you will become just as cynical in about a week.
When they take it off a truck and say it runs, you learn in a hurry to have them start it in front of you. Then it usually "was running yesterday."
Yes, until it broke.
Parked for years almost always means it stopped running and none of the easy fixes got it running again. There are plenty of bikes with blown motors and brand new spark plugs, air filters and motor oil in garages.
If you are concerned about the bottom end, it will probably be cheaper to find a used motor out of a wrecked bike than to tear it down and rebuild the engine.

Boy, that piston got toasted. No oil and or overheated to the max. It may not be worth putting any more time and money into that engine unless you can determine if the bottom end is still in good shape. Has the bike been tracked?

I don't think this bike has seen a track, but it looks like I didn't get the real history on this bike, so I can't say 100%.

I don't see any evidence of heat damage or scoring from what I can see of the bottom end. Everything moves smoothly. The connecting rod doesn't have any slop / vertical free play on the camshaft.

Unless it's certain to fail (?), I'm inclined to clean up the connecting rod small end and give it a go.

This is good info - thanks!

Fair points!

The thing is, I am normally very cynical about these things. From everything I checked, I really didn't think this one was a typical blown engine story. I like to think I'm also a good judge of character, which is the real reason I'm disappointed here! I've bought many used bikes and cars, and haven't felt misled before this one.

Oh well. The bike was cheap and I wanted a winter project, so I took a gamble. Obviously, this one came up short! You win some, you lose some The bike was cheap enough that I could buy a new engine and still break even... it's just that I wasn't planning on that much of a project, especially in terms of time investment/hassle!

Can I salvage the connecting rod and get away with rebuilding the top end? Or am I guaranteed another near-term engine failure going that way?