Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board 1969 T120R magically loses compression

Got my oil tank back from a welder on Friday - really nice job and hardly any damage to the paint.

Put the tank back on this morning, filled with oil, and rolled the Bonnie out into the yard to take my first ride in just over three weeks. Tickled the carbs and kicked her over. Kind of fired once and then died out when I gave some throttle. I started pumping the kickstarter to get to the next compression stroke but it just kept kicking through.

There's hardly any compression - a little more than if the plugs were out, but not much. I checked the valve clearances - ok, so all 4 were a bit on the wide side and I readjusted them.

Still no compression.

The bike was running nearly perfectly when I parked it just over three weeks ago because one of the oil tank brackets broke..

WTF?

Perhaps confirm with a compression test and get a psi number for reference. Then follow with a leakdown test.
Check clutch isn't slipping badly.

With the front tire up against an immovable object and the gearbox in 4th gear I can stand on the kickstart and it doesn't budge, so I'm thinking the clutch isn't slipping.

Compression test shows around 70lbs in both cylinders. I put some oil down the drive side cylinder and it made no difference.

Two things to remember here - 1) the bike was running like a champ when I shut it off three weeks ago 2) when I tried to start it earlier today I was definitely up against a compression bump for the first kick. After that, no (low) compression.

Hi Excalipur, Kind of sounds like fuel wash down on rings.

Squirt some motor oil in both cyl & kick through a few times. About 2 table spoons or so. Don't short the oil. Any excess with blow out put hole & into exhaust as you kick through. Reinstall plugs. Tickle carbs as normal & see what happens. After start up smoke will stop when oil burns off. Might take 15 min. or just seconds.

I've seen this many times at work when fuel distributor would stick & squirt gas in uncontrolled. The only way to start was remove plugs & add oil in plug holes.
Don

Hi Don, I'll give your suggestion a try in the morning. I have my fingers crossed because otherwise this could be the end the line for me and the Bonnie.

Don’t say that ! I pulled my jugs off three time before I found the problem with my bike.
I’ll tell you one thing I can get the top end off and on real quick now !

The clutch is not between the kickstart and the rear wheel.

TT's right, your clutch slip test method is flawed Bruce... you'll figure out why ;-)

Looking at it logically, there aren't many things that can cause a loss of compression.
Since it happened suddenly before you even got the engine started, I'd rule out the usual suspects, seizure or other bore/pistons/rings related issues.
A sticking valve might happen, but unlikely on both cylinders at the same time. Still, it' s something I'd look in to. Usually accompanied by loud clicking noises.
Loss of valve clearance can happen too, I've seen it many times, but it doesn't happen in an instant, like in this case.

Since both cylinders seems to be affected, your head gasket may have let go and permits leakage between the cylinders. This is often caused by the rocker box gaskets compressing and being squeezed out, losing tension on the inner 4 head bolts. This would obviously affect both cylinders.
Nothing to worry about, armed with a set of new gaskets you're looking at a few hours work. Make sure to use copper rocker box gaskets, or even better, the CovSeal variety. Franz and Grubb have something similar, probably good too. They won't ever squeeze out.

My bike does this sometimes but runs fine usually. It is just from flooding the engine too much as Don says.

On yours it can't be anything else as nothing has been touched except the tank! If you open the throttle to full it should start up (let the throttle go quickly!). My clutch does slip a bit but this feels different. It is also different from kicking with the plugs out. Once it has started and is stopped the compression should be back.

Try your compression test when hot with the throttle full open to get a true reading. The rings are probably a bit worn on mine. When hot my compression is about 120psi- not too bad.

Always suspect the simplest thing first.


Dave

Don - you're the man!

I did exactly what you said, rolled the bike outside and tried to start it. Dead. Completely dead. Back into the shop. Pull the plugs - enough oil on them to close the beach. Put new plugs in, jumped on the kickstart lever, and she fired right up like always.

Whiiiiile I wait for the smoke to clear out of the shop...

Actually I wheeled the Bonnie outside and started her again. After two minutes of warm-up, compression had returned. This episode probably explains why in the past I have observed that compression seems to occasionally drop off, only to return to normal shortly later.

TT, Roger Stein - yeah, I get it - I was on the wrong side of the clutch. Morri - you're right, I shouldn't allow myself to get so discouraged. Dave - you're right, not like there was NO compression, just very reduced. I should do a hot compression check today.

I ride therefore I am. I ride a Triumph therefore I wrench. Under duress.

I can't figure out these machines with one right in front of me - you guys diagnose problems from 3,000 miles away! Logic and knowledge make a formidable combination!

Sometimes I wonder if the benefits of the Internet really outweigh all the downsides. Hard to say, but one thing I know - if it weren't for the Internet and the dedicated knowledgeable people on the Britbike and TriumphRat discussion boards I would have been at the end of my Triumph road a long time ago.

Thanks so much to all of you. Wish I could invite you all over for a barbeque and awards ceremony!

Right now I'm going for a ride. All day! The woodpile and garden can wait today.

Well done Bruce, and well advised Don, happy it worked out without a tear-down! Strange thing to happen though, but I'll keep it in mind for future reference.

I had a flat head Ford with trips on it {3carbs}. I flooded it and washed the oil off the cylinders basically. Had to drain the oil and put some oil in the cylinders. Basically the same thing you did.

I'd be wondering why bores got washed down in the 1st instance. Perhaps gas taps aren't shutting off 100%??

It's the cylinder pressure that expands the rings. The oil just must give it enough seal for the engine to start. Something to keep in mind if you buy a bike and the compression is low, it may not need torn down. My trident wouldn't read over 40 psi cold when I first got it. Put some oil in, got it running and was getting 145 psi hot. Another thing that keeps compression down on an engine that's been sitting is carbon holding a valve open.

+ 1 on carbon holding the valve open. Valve lash is correct, but the valve is not closed.

Hi Hermit, Very welcome. Just learn things along the way. I'm glad it worked out for you.

I think the internet is great. Lots of info. Back in the early 80s most the Triumph dealers went under. Very hard to get parts. Only mail order from catalogs. Never had many swap meets in my area. Still don't.


Regarding carbon chips holding valves open that is the real deal & can make you chase your tail. I actually don't remove plugs before a valve adjustment to help insure carbon doesn't drop onto valve seat. Even though it makes engine much harder to turn over. When I was a Porsche tech on 911 motors if you removed plugs before valve adjustment almost 100% of time would get carbon on valve seat.

Anytime you do compression or leak down test I most strongly recommend back plugs off a few turns. Start motor & gun it a few times to blow out any loose carbon that may (will) have dislodged from plug threads. Then remove plugs & proceed with testing.

Cold compression test is a crap shoot for accuracy. I warm motor, test one side, put plug back in. Start motor briefly to oil the rings then check the other side.
Don

Glad you got it sorted Bruce! I know how you feel, I've not really been able to ride my Bonnie for 2 years due to ongoing Columbo moments - "now just one more thing" ..........

We are lucky to have Don on our pages!!!