[Al Posts: 875 Status: Offline WWW]

Really it can only be three things, Fuel, ignition or compression...once you have all three it should work. Timing as well but its fixed. So I will just go back and double check everything one thing at a time.

So what was it Pat?

[Pat Tobin Posts: 107 Status: Offline]

Totally agree. A discussion might spark something in someones mind that eventually leads to the fix.

[Nick Flint Posts: 4853 Status: Offline]

By bantering stuff back and forth, we all learn, even the "so called" teachers/ advisers.
Its good to discuss these intermittent hard diagnoses.
Its the energy of this forum IMHO. 

[Pat Tobin Posts: 107 Status: Offline]

Really it can only be three things, Fuel, ignition or compression...once you have all three it should work. Timing as well but its fixed. So I will just go back and double check everything one thing at a time.


Heading away on business so it will be a week before I get the chance.


Thanks for all the input


Pat

[Warby Posts: 1041 Status: Offline]

Gaz,
We've all been there, listening to advice!! I actually built a HC about 40 years ago using 2 stroke engines.
I was with the HCGB on a beach near Barmouth when the thrust engine, an Aeriel Arrow 250cc, packed up. There were many theories as to what had happened including a failed crank seal between the two cylinders. I actually stripped the engine to find the seal was ok, the fault was ignition related!!!!aaarrrrgh

[Gaz Posts: 2344 Status: Offline Cruising around Cornwall WWW]

Warby;
I think you may have misunderstood me. I was not belittling the advice given to Pat. I was reporting on my personal experience. I was listening to others reporting an engine fault with the clubs loan craft that I was expected to fix.

The full story;
It was reported to me that a coil had failed so without going to see the craft and investigating the fault personally I purchased said coils (I got 2 to be sure) I fitted said coils to an already stripped motor and ran it up. I was then greeted with that well known "well that didn't work" comment. An hour later the carb had been stripped and cleaned, no change... Engine ran fine on choke but not without and there was no power. I then set about replacing the fuel system bit by bit before finding the split in the vacuum pipe and case solved.

The advice given by those in the know is never to be scoffed at. I was close to asking for pointers myself, it was sheer chance that as I was fitting the fuel pump I spotted the split.

[Warby Posts: 1041 Status: Offline]

Too many senior moments!!
How do you check compression when an automatic de-compressor is fitted to an engine?
I believe they only operate below about 300rpm so shouldn't affect idle, but that would mean to get an accurate compression reading the engine would have to be turning over above the operating speed of the de-compressor!

[Warby Posts: 1041 Status: Offline]

Gaz
Re remote diagnosis, all the comments so far put forward here by members  have been suggestions of possible faults which can be checked with a little time no money needed!!
Pat has done all the first basic checks, and I would be surprised if it isnt a simple thing like an air leak or something in the carb
Another worthwhile check would be a fuel delivery test.


When an engine plays up, is not performing as should etc the first checks should be mechanical ie Compression check, valve clearance, A compression check is one of the most important. I've watched people (who wouldn't take advice) replace just about everything only later to find compression down on a cylinder due to a burnt valve!
With Pats engine, being 15 years old I would check everything that can perish, harden or split with age like fuel pump diaphragms fuel pipes, vacuum pipes etc
I once had a car play up time and again,(with a twin choke carb) I was convinced it was a fuel supply/carb fault I stripped the carb three times. Finally finding a hard piece of muck in a fuel passage (not in a jet) it was moving back and to in the passage to the main jets intermittently blocking the fuel flow.

[Gaz Posts: 2344 Status: Offline Cruising around Cornwall WWW]

The Clubs Scout was playing up recently, it wouldn't rev and finally would only run if the choke was out. I finally tracked the problem down to the small rubber tube that connects to the fuel pump, the one that provides the vacuum pulse to drive the pump. It had a tiny split in it that was hidden from view which helped make diagnosis difficult.  I wrapped the pipe in tape and the engine sprang in to life. but only after I spent about £100 on parts others told me were faulty.  The replacement pipe was a tenner!
Lesson learnt was for me to go and look at the fault myself and not listen to remote diagnosis.

Hijack alert; The 1387 Squadron surveyor build will be finished and ready for use early next year so HoverClubs' loan craft, The Scout, is now looking for a new home. If anyone has a youth organisation near them that could make use of it it let me know by PM. The craft is available immediately  (apologies for the hijack Pat)

[Warby Posts: 1041 Status: Offline]

If valves and ignition ok may still be a carb problem.
To check for intake air leaks, with engine running try spraying (SPARINGLY) WD40 or similar with a straw attached around gaskets on intake manifold, if theres a leak engine speed will change, but be careful with a hot engine (fire risk)!!!

On the amazon photo there appear to be two electrically operated "anti diesel" or fuel cut off valves, are these working ok?
In a diagram I found on net it showed two idle mixture adjustment screws (the ones with a small spring)
Has your carb got two? If so have these been taken out and cleaned?

[Al Posts: 875 Status: Offline WWW]

I had a similar issue 40 years ago with a grass track bike.  After driving me crazy for weeks I found the issue one evening in the dark.  The spark would short out now & again to the engine.  It couldn't be seen normally, but in the dark the spark would now & again run down the HT lead and short out on the engine. 


Another thing to check is that you have the correct coil orientation.  Don't ask but on briggs its easy to put the left on the right and vice versa because when fitted correctly the HT leads look like the coils are on the wrong side.  The coils are sided so wont run correctly if on the wrong side.
 I found that out the hard way on my first surveyor

[Ronnie L Posts: 507 Status: Offline]

Do you have a strobe light with an induction pickup, so that you can watch the ignition system with the engine running.

[rossfloyd Posts: 2025 Status: Offline]

Not something simple like the HT lead tracking on the cowling ?


A problem I had - I doubt this is what is happening here as its only one cyl , but caused me a complete engine change - was an air leak on the input side of the fuel pump.  When warm or the engine moved on the mounts at full power it let in a tiny amount of air and after a few minutes began to lean out and eventually die or only run on the primer.  Drove me nuts and in desperation I fitted a new engine.  All down to a bit of  ( b expensive ) hard fuel pipe.  Much cussing when found !

[Pat Tobin Posts: 107 Status: Offline]

Thought that myself so disconnected the diode and no difference.
The coil seems like the obvious problem but changing it made no difference. 
Going down the valve possibility...I am going to run the engine until the problem occurs then measure the valve clearances when fully closed. If they are  way out out I will know I have a valve problem

[Ian Brooks Posts: 3550 Status: Offline]

Another thought - What about the little diode in the kill line?

Ian