2 Stoke no power under load

Ill admit the 2-stroke world is new to me so im stumbling through this. Recently purchased a '69 Benelli 125 (piston port engine with magneto ignition) in great condition. It had been sitting for decades in a New Mexico shed and was in great condition.

I gave the bike a once over; cleaned the fuel filters, changed the gear oil, put a new gaped plug in, etc. With fresh gas in it cranked right up and idled perfectly. Flip the throttle and it revs through the range well. The problem comes when you go to ride it and put it under load. It bogs and just has no power. You can keep it running and get it up to speed only by feathering the clutch.

Heres what ive done to narrow the problem so far:
-Taken carb apart and cleaned thoroughly (was clean to begin with, no varnish or trash). Reassembled with idle air screw in same position.
-Cleaned air screen (and tried without screen at all as well)
-Removed exhaust and decarbonized using both the caustic soda and wire whip methods. (it now weighs ~0.2lb lighter )
-Installed new exhaust gasket
-Tightened all case bolts
-Removed head to inspect piston and cylinder for damage and reinstalled with new gasket
-Replaced seals in both petcocks and installed new fuel lines.

After all that it runs exactly the same. Before I go too far I wanted to get some input on best plan of action. I can not get a good enough run to do a plug chop, but it appears dark upon removal. This all leads me to think its running rich at mid throttle and I should adjust the jet needle. Thoughts?

The carb is a Dellorto ME if that helps.

And I know this is my first post, but ive been a member since 2011 and have read through tons of threads including the great Carb 101 thread so any real insight will be appreciated.

Check compression

Sounds like its running rich. I didn't see you mention changing the original spark plugs, change the plugs and try it again, if it gets dark again, its definitely running rich.

Good luck

Forgot to mention, I did check compression. Results were around 80psi before and after all work. This seems slightly low to me but I suspect may be pretty close for a mass market 2T of the era.

Plug was changed from the get go with an exact replacement gaped to the specs in the bike's manual.

Additional info: Running fresh premium pump gas with 50:1 Maxima 927 oil.

Pretty sure 80 is really low. Do you have a shop manual?

I do, but it does not give specifications for compression. It only states a compression ratio of 8:1 which is low to begin with. Ive previously read that 2T tend to have lower readings compared to 4T so I dont know how to translate this to an expected compression reading. (seems there is much theory on compression ratio correction)

I would also suspect my el cheapo car compression test kit is reading slightly lower than actual pressure.

The cylinder, piston, and rings all look great with no scoring, ill try to get a picture tonight.

do a leak down test , it will tell you more than a compression test on two stroke.

You might have leaky crank seals ( and the black plug could be a little trans side oil being sucked in )

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get a good compression gauge . . you need at least around 115 for it to move . . you really need 150 for it to be normal and up to around 170 for optimum perf on pump gas.

if you did not clean the tank first, you might have had varnish in it which will liquify and destroy the engine.

a leak down test as suggested is a good idea . . if a crank seal leaks on the flywheels idle, it will idle high . . if it leaks on the crankcase side, it will suck oil in and turn your plug dark.

i would run the maxima at 40:1 . . its safer

Ran a couple more compression tests. Got the same 80psi dry, add a little oil and it only goes up about 2-3psi. Even if the scale of my gauge is off this should indicate a serviceable piston and rings in my experience. Thoughts? A case leak is a possibility, but I would think that couldn't have an effect on compression readings right.

Looking at the plug again I've convinced myself it's running lean. The needle looks to be the absolute smallest offered and clip is on third clip from bottom. I will move down and try to ride when the weather clears.

The head dome is gigantic as well, and as you can see the piston is relatively flat. No squish zone like I'm used to seeing. That coupled with the baffle muffler that came on these and they had to be terribly inefficient even new.

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ck.mecha said:

Ran a couple more compression tests. Got the same 80psi dry, add a little oil and it only goes up about 2-3psi. Even if the scale of my gauge is off this should indicate a serviceable piston and rings in my experience.

Click to expand...

well your experience is wrong . . i gave you the CORRECT answer . . would you like me to give you one that makes you happy instead?

ck.mecha said:

Looking at the plug again I've convinced myself it's running lean. The needle looks to be the absolute smallest offered and clip is on third clip from bottom. I will move down and try to ride when the weather clears.

Click to expand...

moving the needle will not fix your extremely low compression prob, nor will it have much affect on jetting at all . . it is merely a fine tuning adjustment.
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A leakdown test has been on the agenda, once I find a decent way to block the intake.

However, can you please explain how a case leak would cause compression loss, I just don't see it. The cylinder is essentially at atmospheric pressure until it passes the exhaust port, that's where compression would start. From there the only areas I could imagine that would effect compression is either the rings, head gasket, or loose plug. Am I missing something. Got any resources

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your crank seals are not leaking enough to cause you to only have 80 psi of cranking compression . . you have bad rings and/or a bad bore or the wrong cylinder head or a horribly inaccurate gauge, its that simple.

Your plug looks good in that photo. I would figure out the low compression and not worry about the jetting. It will change once you increase the compression.

Just to be sure, did you check compression with the throttle open or closed? Should be with throttle open. No way to fill with air otherwise.

Yes throttle open but even with the throttle closed you will eventually get close to max psi with enough kicking.

Is there some reason you would think 80 psi is enough? It isnt. You either need rings, a piston, or both along with a rebore or hone. Is there some reason you think you can visually judge the sealing properties of a used piston, rings or bore? Unless you have laser eyes and a computer in your head it isnt gonna tell you shit. You need to measure things or take it to somebody that will.

8:1 static is not a low advertised cr for a 2 stroke. Regardless, advertised cr is next to useless.

Eventually, sure. But if you think you're supposed to do it in a dozen kicks you're going to deduce there is an issue. At 10 - 12 kicks, a compression reading of 60 - 80 is about right for a closed throttle. I've seen engines never get over that reading no matter how many times you kick it over.

Two strokes do not run lower compression than 4 strokes of the same era but modern two strokes tend to quote compression from the point at which the exhaust port closes and that is equally meaningless. Just as an extreme example, a TZ250 used to have 15.5:1 CR measured from the bottom and 7:1 measured from exhaust port opening IIRC.

warning: Overly simplified explanation coming up..
On a two stroke gas/air mixture in the bottom end is compressed by the piston as it goes down and that mix is forced out under pressure as the transfer ports open. So the gas is potentially at higher than atmospheric as the pistons starts to rise - depending on how much goes straight out the exhaust and how much is pushed back by the exhaust.

Back to your motor. The open head design is known as a quiescent chamber and they were common on two strokes up to the mid seventies. They burn efficiently but cannot stand very high compression as a rule. If you are using a harbor freight tester, there's a high probability that it is reading low. Borrow a good one. Do the compression test without the carb or with it wide open and kick until it stops rising. It will probably get closer to 100 psi if it is working well. It will never be 150 or above. It is a really old design and it was not designed to go fast and it was designed to work on very low octane fuel that was available post WW2 which is where teh design dates back to. That said, 80 sounds lower than it should be

If the sparks are good if look at a choked exhaust next. Those old exhausts were reasonably quiet but not very efficient from a power perspective. Your plug is fine for now. Put the jetting back to stock.

My turn.

No explanation needed... It's old and needs a fresh top end.

Measure the piston (bottom skirt) if stock get a stock set of rings and sctchbright on a rag on a hone in and out like a 20 year old to deglaze the jug, put it together and see how it runs, if decent ride it until you get the new piston and rings to bore it the next size over.

Or just coat the dang thing... you won't be riding it to work.