Jump to content

Plug fowling


Recommended Posts

Hi

I have tolerated this problem for years and years: If I start the engine using even the minimum amount of choke No.2 plug fowls up and needs to be pulled and cleaned as it will not clear on its own. In the summer I tend to start not using any choke and no fowling and runs smoothly after a minute of warming up. Not a carb problem as fitted new carbs this summer and problem existed with old ones as well. I am guessing a worn valve guide. Sorry to sound ignorant but does the engine (like modern engines) have changeable valve guide seals or is it just the fit of the valve in the guide that stops oil seepage into the bore.

 

Another question: I changed the PCV diaphragm yesterday. The workshop manual description/diagram shows the internals being a diaphragm and a spring with no mention of a 1 inch diameter plate with a small round solid tube attached that on mine went between the diaphragm and the spring. Is this plate a later mod or what?

 

Cheers

 

Simon

 

 

Link to post
Share on other sites

Hi Simon,

the valve stem simply runs up and down in the valve guide.

There is no seal (normally) to stop oil running down.

 

There must be a very slight gap otherwise the valve stem may seize in the guide.

With the suction from the manifold oil will be drawn down the guide.

I am investigating valve guide oil seals at the moment. If I find any good info I'll pass it on.

 

Sounds like your guides may need replacing.

 

Roger

Edited by RogerH
Link to post
Share on other sites

Simon, I had never noticed but you are right, the WM diagram of the PCV valve assembly(page 1.314 in mine) simply shows the diaphragm and spring. That in the Moss catalogue isn't sufficiently exploded, simply showing the diaphragm, I don't know if the plate with tube attached that you mention was a later addition or on all such valves but it is present on my 4A built on 17.10.66 and acquired in 1970.

 

No valve stem seals were fitted to this engine as built.They can of course be retro-fitted and some do this. I was advised, however, that this might starve the valve stems of that little bit of lubrication needed in the guides. No doubt more knowledgeable buffs will be along shortly.

 

Tim

Link to post
Share on other sites

"Sounds like your guides may need replacing"

 

Is this a job for a machine shop or would I (the sort who likes to give it a go) be able to do this job? No sarcastic comments about aerosol spray cans please.

 

Simon

Link to post
Share on other sites

Simon,

In summer you dont use choke and it doesn't foul. To me that rules out oil and guides and indicates fouling with fuel.

Try disconnecting the choke on the front carb and see if it starts without the plug fouling.

Also check the HT supply to #2 plug- plug connector, HT lead, cap etc

Peter

Edited by Peter Cobbold
Link to post
Share on other sites

Can't resist it, sorry. Stop putting chickens in your petrol tank.

 

:P

 

Seriously though, if you have such a leaky valve stem as to cause fouling with oil then you'll probably notice other problems. Such as a puff of smoke if you accelerate after a period on a trailing throttle and if really bad you'll have pinking no matter how much your retard the ignition. Have a sniff of the plug, if it's fuel you'll tell. If you can't smell it then it's more likely to be oil. You can fit oil seals to the valve stems but it you want it done properly you'll need an expert as the best seals (arguably) need the valve guides machining with a groove. I think they originate with Mitsubishi and these are what Pete Burgess fitted to mine.

Edited by peejay4A
Link to post
Share on other sites

Hi Pete (Cobbold)

 

Thanks for you input. I was never very sure of my self diagnosis of worn valve guide/stem. Thinking about your suggestion of disconnecting the front choke, again, It did not seem to be the answer because starting without any choke was achieving this anyway. The fouled plug always seemed to be a mixture of fuel and oil and I always tended to think towards a fuelling problem. Anyway, I did look at the choke mechanism and noticed that the screw on the front carb that should mate with a cam on the choke linkage was not touching so when the choke was pulled out the throttle was not opening that fraction it was supposed to. A couple of turns of the screw did the trick. Full choke start = all 4 singing in harmony. Deep joy It is a warm day so the real proof will be a freezing cold start.

I should have checked this earlier but t was only your input that got me thinking about the problem correctly. That's what is so good about this forum.

 

Time to go out for blast and find a nice pub for lunch.

 

Cheers

Link to post
Share on other sites

OK so it was a case of too much choke for the throttle position on the front carb.

I get similar - No. 4 plug will foul if I leave the choke on for too long. My throttles are all set up correctly, but still - it's always No 4 !

Also a colourtune check (I have now assembled 4 of them ! ) shows No. 4 is much richer on tickover then No. 3.

 

Still, never mind eh ! It still runs OK

 

Bob.

Link to post
Share on other sites

Don't be upset Roger, you just beat me to the punch and I hadn't seen your post. Of course I definitely consider you to be one of the more knowledgeable buffs on here!!

 

Tim

Link to post
Share on other sites

Bob

I guess your analysis is correct. Too much choke for the throttle position on the front carb. Why no.1 did not foul is a mystery. Amazing how a turn of a screw driver can so dramatically alter the running!

On your problem perhaps you have an air leak into no 3 intake making that lean on tick over that was compensated by enriching the idle mixture causing no. 4 to be too rich??

 

Simon

Link to post
Share on other sites

If the engine slopes down at the rear then liquid fuel will tend to flow more towards 2 and 4. if that makes the mixture to rich to burn then the plug can foul.

Peter

Link to post
Share on other sites

Simon.

I checked for air leaks around No. 3 manifold by spraying carb cleaner around there - it made no difference at all, so not that.

 

Peter.

I agree with you, but why No. 2 not showing richer than No. 1 ?

 

Bob.

 

Posted 14 August 2017 - 01:58 PM

Every so often I find that No. 4 plug gets carbon'd up, & causes a misfire.

Today I decided to check out my mixture settings.

Previously I have used the "lift the piston a little, & see what the revs do" method, followed by road testing, & fine tuning for distance travelled before choke can be pushed in without engine hesitating on acceleration.

 

coloutune (I have 4 !) results here:

 

 

post-12009-0-73488700-1502715317_thumb.j post-12009-0-47072400-1502715324_thumb.j Slow idle

 

post-12009-0-12387000-1502715331_thumb.j post-12009-0-64545900-1502715335_thumb.j Fast idle

 

Note No. 4 is very rich, while No. 3 is spot on. 1 & 2 were much the same as being slightly rich.

 

I then did compression check:

 

No.1 145 No.2 168 No.3 127 No. 4 160 PSI

 

So No. 3 is somewhat down, would that cause a weak burn, which I have compensated for in the carb, leading to No. 4 being too rich ??

 

Bob.

Edited by Lebro
Link to post
Share on other sites

 

Simon.

I checked for air leaks around No. 3 manifold by spraying carb cleaner around there - it made no difference at all, so not that.

 

Peter.

I agree with you, but why No. 2 not showing richer than No. 1 ?

 

Bob.

 

 

 

Bob.

 

Bob, When the butterfly is part-open the air turbulence past its edge is extreme. At less than 30deg opening the air flow is always sonic ( thast how the buttelrfly limits the flow). So the fuel will be atomised and the droplets follow the air flows equally between cylinders. If the butterfly is fully shut liquide fuel may flow in the gap without atomising, biasing the wall-flow down-slope.

Peter

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.