Two stroke power loss.

[Bob Purton]

Who can help me with this problem, a two stroke engine which pulls fine for its first couple of miles but then loses power once its warmed up, it doesnt missfire, just slows down untill I have to change down a gear, the engine has all new seals and a rebore, new piston, no obvious air leaks, has good fuel flow. Any ideas anyone please?

[AndrewG]

First (admittedly rather pessimistic) thought: it's trying to do a hot seize.  Is the piston size definitely right for the rebore?  One way to check would be to pull the barrel and check the bore to see if the honing marks are still there - or any other sign of piston scraping the bore.

[Bob Purton]

Yep, I removed the barrel and there were no obvious signs of scraping, I must admitt I did wonded if it could be the opposite and that the bore was too loose thus when hot thinning the oil and losing some of the pumping action. Someone mentioned that sometimes a coil can have this effect when it hots up, I have never experienced that before though.

[Big Al]

So heat related problem. Are there any bolt together joints to the induction/crankcase that could suffer from alignment problems on assembly and torque down?

I give an example of the Goggo twin barrels. It is not wise to tweak these down and then put the inlet manifold and exhaust on last. They should go on first to set the barrels interface flat and the spacing/orientation of the barrels correct before clamping down the heads, thus the barrels. Clearly a misaligned inlet can / will leak air into the charge weakening it. Heat can increase the effect. The engine therefore tends to run weak or to rich on tick lower revs if tuned for higher revs.

Now all the Goggo owners are back in the room.

Similar issue but simple gasket issue?

Little end a tad to tight? (you can get treatment on the NHS)

Main bearings wrong grade? You can buy too good a quality. Most mains are C3. Go for expensive high quality they have not got the play for the task and are made for some sort of precision installation like a machine tool or something. Mick did this on a Schmitt as the main on the Sachs is a general type bearing in gearboxes etc. The top grade will not run in a Sachs. It cost a rebuild but the bearings, eer, came free from chummy's work so not as expensive as it could have been.

Vaporisation of the fuel leading to a weakened charge, plug should show evidence of that?

A steep hill has appeared down the road when you were not looking?

[steven mandell]

I did wonder if it could be the opposite and that the bore was too loose thus when hot thinning the oil and losing some of the pumping action.

You could try doing a compression check whilst the engine is warm and running well and compare it to the results when it is hot and not providing enough power to help rule out or support that thesis.

[Bob Purton]

All good suggestions chaps and I will work through them all in a process of ellimination. I had never thought of the bearing issue that Al mentioned, the crank only has one roller bearing which I replaced with a new one which wasnt very expensive so I'm hoping its not a finer tolenance than a C3. The other side of the crank is in a bronze bush with a rotary hole type valve which I also replaced. The engine doesnt feel tight when the power falls away so I dont think its this but cannot be sure. I took the plug out last night and it was black! I'm wondering now if the jetting is wrong, too rich which would make it go well when cold but choke it up once warmed up. Will check that today. Thanks again for the suggestions.

[AndrewG]

A steep hill has appeared down the road when you were not looking?

That is clearly a silly suggestion.... and anyway any spare steep hills have all been installed 'oop north round here where I go (pedal) cycling.

[marcus]

A steep hill has appeared down the road when you were not looking?

That is clearly a silly suggestion.... and anyway any spare steep hills have all been installed 'oop north round here where I go (pedal) cycling.

They also have a massive Hill Storage Facility down in Kent's Mudway Towns

[AndrewG]

Drifting well off topic, I am reminded of a table of data from a car magazine in my youth.  In the column of 0-60 times for the Fiat 500 was 138*.  Down the bottom of the page was a footnote:

* average time to find steep enough hill.

[Barry]

Still off topic (and microcars) here are the crazy Swedes on their mopeds.


http://www.youtube.com/watch?v=ChM6k9qaL_I&feature=share

[Big Al]

Still off topic (and microcars) here are the crazy Swedes on their mopeds.


http://www.youtube.com/watch?v=ChM6k9qaL_I&feature=share

Aren't Swedes a type of brassica, Bob? Falling off into a field of blond female type ones might be far more of an adventure than we will ever hear about!

[Jim Janecek]

Who can help me with this problem, a two stroke engine which pulls fine for its first couple of miles but then loses power once its warmed up, it doesnt missfire, just slows down untill I have to change down a gear, the engine has all new seals and a rebore, new piston, no obvious air leaks, has good fuel flow. Any ideas anyone please?

Getting back on topic, despite the appearance of proper fuel flow, could you still be running lean?
Lean via old fuel, too much oil/vs fuel, or an obstruction in the main jet some place?
I had exactly the same problem recently and was pleasantly surprised to find I was unable to shift to third on a flat and had to remain in 2nd if I wanted to maintain any sort of speed at all.
After burning a hole in the piston, the answer presented itself to me on a nice little aluminum platter will small flakes and bits that I must now scour the engine to remove.

[Bob Purton]

Sorry to hear that Jim. Mine appears to be getting plenty of fuel, the plug is a good colour etc. I'm now suspicious of the barrel being rebored incorrectly, I am having another barrel rebored and new piston fitted be someone more competent. I will try this one on the engine and see if it eliminates the problem. Will keep you posted.

[Bob Purton]

Still off topic (and microcars) here are the crazy Swedes on their mopeds.


http://www.youtube.com/watch?v=ChM6k9qaL_I&feature=share

Aren't Swedes a type of brassica, Bob? Falling off into a field of blond female type ones might be far more of an adventure than we will ever hear about!

Err Yes, landing in a field full of young Brit Eklands would be a far more exciting experience. Has it ever happend to you Al ?? and I dont mean whilst on medication!

[Big Al]

Who can help me with this problem, a two stroke engine which pulls fine for its first couple of miles but then loses power once its warmed up, it doesnt missfire, just slows down untill I have to change down a gear, the engine has all new seals and a rebore, new piston, no obvious air leaks, has good fuel flow. Any ideas anyone please?

Getting back on topic, despite the appearance of proper fuel flow, could you still be running lean?
Lean via old fuel, too much oil/vs fuel, or an obstruction in the main jet some place?
I had exactly the same problem recently and was pleasantly surprised to find I was unable to shift to third on a flat and had to remain in 2nd if I wanted to maintain any sort of speed at all.
After burning a hole in the piston, the answer presented itself to me on a nice little aluminum platter will small flakes and bits that I must now scour the engine to remove.

To much oil in the mix is a classic own goal the uninitiated often make thinking they are urring on the side of caution. It can take some time to get them to understand why if they are not mechanically minded. Old fuel seems ever more a problem. Our sort of two strokes should be able to run on paraffin but dead petrol seems to have about the same effect. It is known that running paraffin makes the engine hotter. The timing is normally altered to allow for the differing combustion dynamics. For volume it has more energy in it as diesel has again. The reason that diesels can give superior efficiency.

Was not a Goggo that burned out was it. They are known for doing that if not tuned up for the fuel. I had one cylinder go on my T300 at Story. Of course the Germans do not readily use 300cc engines so I could not get parts and ended up bolting a pair of 250cc top ends on the 300cc unit. It worked surprisingly well. In the end the problem was found to be a bent, yes a bent, crank where someone had attempted to remove the dynostart with a lever, we think. The points were thus sparking twice unless the gap was on max despite being set correctly. This eventually killed the piston. The problem was only identified after I had sold the thing, rebuilt as a 300cc of course, and it caused a great deal of embarrassment and hassle. The recipient still does not believe it was a genuine situation I think. So easy to gain detractors despite anything you do to mediate the situation.