KH500 Lean Running Problems

[Les Carter]

Has anyone any idea what the two holes (marked) at each side of the KH head are for? They go from the mating face through a couple of fins and stop. There are no corresponding holes in the block and are in addition to the stud and pushrod holes. I'm looking for an air leak. Thinking of plugging these up. The carb flange is OK and it doesn't seem to be in the inlet tract, I can blow in it and no leak obvious. The head gasket has been leaking oil so a new one is to be fitted. Mid throttle is lean with 376 monobloc even with the needle on bottom slot and with a size higher 107 needle jet, 220 main jet and std 31/2 slide. Bought a new 276 carb the correct spec for this bike but had similar lean running problems even when significantly over jetted.
Any views will be welcome.

[Roger Gwynn]

Those holes are not normally there and I can see no reason for them, however I doubt very much that they are causing your problem as they are smaller than the push rod holes. If the head gasket is leaking oil it is likely to be sucking in air as well and causing weakness. I would check the gasket surfaces for flatness and have them skimmed as necessary.

[brianbarbary]

Hi Les.
I have a 1949 KH i have checked the photos i took during my rebuild my head has the same 2 holes they are not used i can only guess that they were used as locating dowl holes during machining to align the head for line boring the rocker spindles and machining the valve seats etc during manufacture at Ariel
Have you tried filling the head combustion chamber with liquid to see if it leaks past the valve seats.
My KH runs a bit richer on one of the cylinders it is fitted with a 376 Monoblock i tried fitting the 276 pre Monoblock but could not get it to run properly so went back to the 376.
There is not anywhere air could leak into the head via the inlet ducts, if the carb gasket is ok and the carb flange is not bent it should be ok you could have a pin hole in the casting but i would not think so just a thought.
Hope this helps you.
regards Brian Barbary

[Les Carter]

Thanks for you replies. I guess those holes were for some manufacturing reason and I am now sure they are not the cause of lean running. I taken the head off and vacuum tested the inlet tracts and proved no leaks here. I am to fit a new club Otto type head gasket which will hopefully prevent the oil leaks. The head and barrel faces are flat when checked with a steel rule. Compression tests before I stripped it down were good 10 bar each cyl so don't think enough air could be sucked in through the oil leak places to affect the mixture.

Should I use a sealant on the Otto head gasket? I am aware of the need to re-tighten the head bolts

Brian - could you let me know what jets you have in your 376 carb.
Thanks
Les

[Paul Slootheer]

You might start to fit a thicker heat insulator between the the carb and the head and also check the complete fuel line, including the petrol tap. There might be some obstruction?
I would stick to the original copper/asbestos(?) gasket. The seal from the head onto the barrel is a bit of a flaw in the design i.m.h.o. and there’s not a great access to nuts which doesn’t help either. I owned two iron headed KH’s myself… For the head gasket I would stick to the original gasket in combination with a light smear of grease. Once fitted the gasket warm up the engine properly untill it reaches it’s temperature. Let it cool down and re tight the head. I repeated this procedure at least three times. It enables the gasket to settle down. good luck!

[jj.palmer]

I had a similar problem with a cylinder head oil leak on my KH500 some years ago, after checking all possible causes I finally came to the conclusion that I was not able to apply sufficient torque to the nuts with an open ended spanner.

I therefore trimmed a small amount of material from the cylinder head so that I could fit ring spanners (home made) to the nuts and apply sufficient torque, this cured the problem, see pics.. I also did similar to the cylinder base nuts.

When trimming the material away I used a sectional thickness calipers to constantly check the wall section thickness.

[Paul Slootheer]

It looks that those two holes are quite common. Reason still unknown… I saw these two KH cilinder heads on the website of ebay.uk on offer and bearing the same holes…

[robjameson]

There is no specific instruction to use a sealant for the Otto gasket, but it certainly wouldn’t hurt to use some high temperature RTV around the pushrod tunnels to assist in preventing leaks.

[brianbarbary]

Hi Les
My 376 carb has,
pilot jet 30,
carb slide 3.5
main jet 210,
needle 106
needle position 3
I would have a chat with the guy from Surrey Cycles he has been helpfull to me in the past his phone number is 01342 716120 ( email )
info@surreycycles.com
one other thought could you have a problem with fuel supply to the carb ie float level low or float in upside down, restriction on the fuel getting to the carb from what you are saying it seems that the different settings you have tried have made no difference to the weak running.
I would not put anything on the head gasket as if the surfaces are flat and the head tightened down even then you should not have a problem, i did not put anything on my head gasket and it has been OK.
Hope this helps.
Brian

[paul.jameson]

Brian's point on fuel flow is a good one. I remember an AOMCC Chairman, who held the role for many, many years, having his KH unable to keep up with my RH350 one day due to poor fuel flow, caused in that case by polish clogging the vent in his petrol filler cap !