HKS 700e engine problem
 

I'm helping a friend struggling to cure a severe rough running problem with his HKS 700e. I consider myself more than fairly competent with engines and the aircraft I fly uses the same style of Bing carburettor as those on the HKS.


Right from the outset, let me assure you that the problem is definitely not related to the ignition or spark plugs. The plugs are new and the first step was to swap them to opposite cylinders, however the rough running remained, with the left cylinder EGT showing only 150C while the right side showed 500C. All plugs had previously been removed from their holes and tested on their leads to show healthy sparks when cranking the engine.


The second step was to completely swap plug leads and their coils between cylinders, effectively swapping ignition systems, which is do-able because the ignition is a "wasted spark" principle in which a spark is produced every time the pistons approach the top of their strokes. No change was seen in symptoms.


After each swapping step the engine was run and the ignition circuits switched off in turn, but the fault remained the same with the engine running violently rough and refusing to accelerate when throttle was advanced. Always left cylinder warm, right cylinder hot.


Cylinder compressions have been tested and found very healthy.


I have been involved with Bing carburettors for decades due to them being fitted to BMW motorcycles and I'll happily strip them to the smallest part. This is what I did to my friend's carburettors and can't find any problem. Slide top springs, diaphragms (not split), needles and their clips, main jets, emulsifiers, floats and their needles and seats, choke valves, butterflies, throttle and choke cables working perfectly in unison, return springs good, rubber flanges, inlet manifolds all check out A-okay. Internal fuel and air passages blown through; clean and clear!


Fuel supply to both float bowls more than adequate and the floats float well with the float needles stopping fuel flow exactly as advertised.


Just in case I've missed something in a carburettor, the next step will be to swap carburettors between cylinders (not easy due to mirror imaging) and see if the fault swaps sides.


Intriguingly the right cylinder, which gets hot very quickly, also makes its plugs black and sooty, while the plugs in the left cylinder look normal. Something doesn't add up!


What am I missing? Help please, because I can't find a clue using google.

Ignition timing?

Ignition system is dependant on battery / charging circuit so I would also be looking at the regulator.

As previously indicated, ignition timing problem very unlikely due to healthy sparks being produced when testing each plug on its lead.


Furthermore the battery cranks the engine robustly, so it also is in excellent nick.


My semi-ignorance of things electrical hasn't prevented me from considering the regulator, but reading elsewhere online tells me that even if the regulator was on the blink / failed, the battery alone would still power the ignition system to produce sparks, provided that the voltage was above 9v.


Thanks for the idea, though.

Are both throttle cables and carb butterflies opening exactly together? Balanced.

Ok. Just re-read that you have checked carb throttle and carb balance. Looks like all that is left is to swap the carbs over.

I agree with swapping the carbs. But before going into that elaborate job, have you compared pressure in the inlet manifolds? Either with twin manometers or with a long plastic tube half filled with water?

The difference in colour on the spark plugs seems to point to differences in mixture, though it could alse be due to some issue with valve timing.

Which might be harder yet to diagnose than swapping the carburettors, I'm afraid.

Check for air leaks between the carb and the engine.
Perished or damaged rubber, O rings, Gasgets, vacuum takeoff point etc.....

I once found that a vaccum takeoff point was missing its stopper screw.

Yes. That's the kind of issue that could cause differences in manifold underpressure, and is precisely why I suggested checking and comparing those.

You may be getting sparks but are they occurring at the correct moment?

A nice fat spark is of little use when the piston is on the down stroke with the exhaust valve open.

All of the other suggestions have been covered by not only me, but three others checking for air leaks, etc.


"You may be getting sparks but are they occurring at the correct moment?"


Yes they are and if there was a problem with either ignition system, the symptoms would swap cylinders when we swapped the coils together with the plugs leads. In effect we reversed the ignition systems sending spark to each cylinder. This was further confirmed by switching off each ignition system in turn.


My next suspect is the diaphragm mechanical fuel pump, which I opened and checked that the diaphragm wasn't split. This potentially could have let fuel into the air suction line which connects to the right side cylinder inlet manifold and caused the rich mixture, evidenced by sooty plugs on that side.


I've connected the electric facet booster pump directly to the pipe supplying the carburettors and blocked the air suction line. When it stops raining tomorrow, I'll do another test run keeping fingers crossed. However I don't have great hope, because the pump diaphragm appears undamaged.

I don't know - I have a few hours testing an HKS engined X'Air, and thought it was a horrible underpowered little lump, but never really got to know it very well.

I do know that if I had a problem, I'd probably call P&M Aviation at Manton, as they have a few people there still who were involved in certifying the HKS engined AX2000 and they're probably a knowledgeable as anybody in the UK on the thing.

G

Genghis,


Thanks for the suggestion. I spoke to a helpful person at P & M this morning who told me that there's a bit of history with HKS 700e engines and valve seats unseating, if I understood him correctly. Because the cylinder heads are oil-cooled, it's possible for oil to enter the combustion chamber through a faulty cylinder head.


Sure enough, after a couple more very rough ground runs today during which the left cylinder remained cool and the right got hot, several drops of oil fell from the left cylinder air cleaner. It was very definitely engine oil diluted a bit with petrol and the only possible way it could have reached the cleaner was through the inlet tract and carburettor.


This would account for the oily plug insulators in the left cylinder, looking quite unburnt. Shining a torch through the plug holes revealed an oily piston crown.


So it looks like a "dockyard job"; one or both cylinder heads off and sent to an engineering workshop to replace seat/s and maybe valve guide/s.


Thanks to all for the quick and considered advice.