Robin 295 losing power as it warms up.

[JRL]

Sounds good. Post what you find.

[Nubes4876]

So, this is what I tried and noticed.

I got the engine running and then when it started to die down, I clamped the fuel pump pulse line and let it continue running to see what would happen. I could see that fuel was no longer flowing through the filter. I may not have let it go long enough but I didn’t notice any improvement is its running. Next I rigged up an external gravity flow tank. This did not make any difference either. I then pulled off the air filter and intake line. I found that the inside of the carb and intake hose was wet with fuel. I ran the engine with the hose removed and watched the intake. As it was running I began to see a mist of fuel blowing out of the intake. I’m assuming that as it runs, it begins to flood itself. I’m not quite sure why it would blow back out since it seems like suction would want to pull it into the engine. Going to keep playing around with it but any thoughts are appreciated.

[JRL]

Leaking carby, although if run long enough with the fuel supply blocked it should clean out just as it runs out of fuel if the carb is the problem. Fill the bowl, clamp all fuel supply lines and give it a minute or two. If the carb's leaking, as it runs out of fuel, it'll clean up for a few seconds
AND, an improperly, or worn out intake valve will cause what you describe. It will adjust correctly, but if the valve face is worn, it won't seal properly. Blows backwards out the intake on compression stroke.
I'm a motorcycle mechanic and not Robin literate. Unsure how common valve face failures are on 295's. Someone will advise.

[JRL]

If when you adjusted the valves, and when initially checking, the intakes appeared tight, it's very likely an intake valve(s) is/are failing. Or, they are incorrectly adjusted and too tight. Compression test might not catch this, although a leakdown test will reveal improper valve seating.
Leakdown test with top dead center and air leaking from the intake is positive for failure. Does your engine have a cast iron exhaust manifold?

[Nubes4876]

I do believe it is a cast iron exhaust manafold.

I don’t have a leak down tester yes but I’m going to make one. I did notice when I first removed the air filter to run it, it seemed oddly loud so that may lead some credence to compression coming back through the intake valve. Since this seemed to come on quite quickly during a drive one day, what do you think about a timing issue?

[JRL]

I read that the cast iron (or removeable) exhaust manifold engines had less than the best valves from the factory. If the valves, probably intake only, are sinking, do both cylinders and resurface the seats. Check valve guides too. The exhaust valves are harder and probably ok. Running without an air cleaner, or with a nasty air cleaner will screw over intake valves and rings....and eventually pistons and cylinders.
RE timing, doubtful that the cam timing would suddenly change if belt's ok. Ignition timing won't change unless something comes loose...flywheel or stator/pickup. If all'set tight, probably not timing related.
I suppose that a failed compression release could cause issues, it wouldn't relate to intake valves.
Lock the crank at TDC on one cylinder at a time and pressurize the combustion chamber. You'll get some crankcase leakage, some is normal, but no return thru the carb or intake. Make sure both valves on the test cylinder are closed, and jam a pull handle and socket onto the crank flywheel. If the crank can, it'll turn and toss the wrench. You can make an adaptor for pressurizing the cylinder from an old spark plug. It won't be a leak-down tester, it'll be a "leak" tester good enough. You can borrow or rent leak down testers from auto parts suppliers.
There should be no air leaking thru the intake valves when closed and the rocker arm is free.

[Nubes4876]

I know its been a while but I finally got to doing some testing. I pressurized each cylinder with air and for both, I am getting air coming out of the dipstick and the smell of gas vapors. I will take this as I have damaged/worn rings and or cylinders. Before I move on to the repowering process, anything I could be missing?

[Cartmaster]

JRL is spot on here i reckon. Dont bother with any more leak testing. Just pop the valve cover off and look at the adjusters for the valves. If the two inner adjusters (inlet valves) are showing a few threads more above the locknut than the two outer ones (exhaust valves) then its almost a certainty that the inlet valves need replacing. This is a very common failure on those earlier robin twins. There should be no need to face the seats, just a valve lapping should suffice when replacing them. Find the engine maintenance book in the tech section and follow the simple instructions for replacing the valves.

basically what happens, is that contamination gets past poorly fitting air filter and then wears down the edges of the valves. You will most likely find that the inlet valve heads have sunk into the seats and are paper thin at the edges (careful as they are sharper than a box knife when like this). It will be obvious when you pop the head off for a look.

[Nubes4876]

I already had the valve cover off during the testing. I did so to make sure that I had the piston in the right place and there was no force on the valves (observed cam lob location and rocker arm play). At first I thought the tool was bad because I heard so much air leaking. I could not tell where it was coming from. It wasn’t until i tried putting the valve cover back on did I notice how much air was flowing. When I held the cover in place and removed the dipstick, there was a ton of air coming out. I had the inlet pressure around 80 or 90 psi, and quite a bit was coming out. Bad valves wouldn’t let air into the crank case right? It would have to be getting past the rings. I’m pretty mechanically inclined but the expense of my options has me second guessing myself.

[JRL]

Generally if rings are worn, and they may well be, the engine will present with low compression and it'll smoke blue smoke when when warmed up.
Bad intake valves won't smoke. However, a badly worn engine could have all of the above failing due to wear and tear.
Do a compression test and post results on both cylinders. When checking compression, have the carb throttle wide open, both plugs out, and good battery.
If the unit was operated by kids, or without an air filter, or didn't get regular maintenence, it's likely tired.
If the engine has known correct and fresh engine oil, in correct amount, and smokes blue smoke...might want to grab an engine repair manual and have at it. You'll be intimate with valves during the operation and I'd recommend new intake valves.
If the engine doesn't rattle at idle, you might save the pistons. Just measure it all up. Don't try to fit oversized anything like rings unless you do the machine work that goes with oversized parts. Or recoat the cylinders or whatever Robin does. Someone will advise, but if chrome bore, a light dingleberry is OK, but chrome cannot be roughed up. If iron sleeves, it can be bored. If bored or rechromed, supply pistons with the block for final fit.
On tiny 4 strokes, details count. Do the evaluation and assembly per the manual. Good torque wrench is required. Ring end gaps and location are important. Cleanliness is vital go good outcomes.
BTW, the reason the air-leak noise was worse with the valve cover removed is that the return oil galleys are allowing air leaking thru the rings to come out thru the head oil return drains. The valve train area and crankcase communicate.
Post some pics. Interesting project!