1776 - No1 cylinder, plug carbon fouling

[Zcat7]

I still haven't fully run my 1776 engine in since building it some months back. It does pull well when up and going but to be honest it hasn't been running as smooth as I'd hoped at lower revs and I'm getting a bit cheesed off trying to set it up. The crap weather has played its part in that but the latest problem has occurred twice now, the second time after about a 30 mile run. No1 plug carbon fouls to the point that the spark fails and it runs on 3 cylinders. No2 plug works fine and if I'm reading it correct, has a good colour indicating indicating that it is firing fine and the mixture is good.

I'm running twin solex 40s rejetted/venturi'd for 1776 by KaddieShack, have replaced all plugs, leads and the dizzy cap. The dizzy was changed from a 009 to an 034 SVDA with Autospark points to try to cure a mild vibration at around 1600rpm (and it got better) but I'm still a bit confused about what timing I should set. Currently it is set to 7.5 BTDC (vac off) but that gives me about 32 degrees max advance (vac connected). It doesn't rev up smoothly to accelerate from standing, if you blip the throttle it hesitates before spinning up so I'm thinking I might need a touch more advance. But that would make my max advance about 34 degrees, which I'm guessing is too much?

Compression check shows all is good and within a couple of PSI around 122, which I think is good for a sub 200 mile engine. I have also reset the tappets twice, although all was OK anyway.

Anyone got any suggestions what could cause a poor burn in 1 piston only and what I should do about the timing?

 

[jonboylaw]

Hesitation on acceleration ? Check your accelerator pump diaphragm and the jets etc.

How does it run on wide open. Throttle ? What about tick over? Is it smooth or lumpy and missing?

What plug as are you running and when were they changed ? What about the leads ?


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[Zcat7]

Accelerator diaphragms are good but I guess I could crank them up a touch.

WOT seems OK too but it's hard to tell exactly unless I drive another 1776 on the same setup. Pulls much stronger than my old 1664 setup.

Starts well and idles OK, although 1 and 3 do suffer from the "robbed vacuum" effect of a shared venturi.

I'm running NGK BP6S and changed the plugs and leads after the first carbon fouling episode.

Why the carbon fouling on one plug only though? Is the BP6 too cold a plug and combined with the poor air/fuel mixture at idle for 1 and 3, is only showing up on 1 for some reason? Is the timing wrong?

 

[Bluesnailman]

I'm no expert, but adjusting the timing seems like the first thing to try. Home strobes are not 100%. Try moving it by no more than 2degrees.

Also, if the petrol in the tank is old - could be that, especially if from supermarket

On other cars in the past having suffered months of this sort of confusion I ran the tank down then popped in super unleaded, retunned it and it was perfect. Ran it on super from then on, and for the performance I was happy to pay the premium

Carbs, manifolds and atomisation are fickle, that's for sure......


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[jonboylaw]

Definitely something a little iffy with #1.
I would try a set of Bosch plugs as a first step. I have heard others having problems with non-Bosch plugs for some reason. Also, are you able to swap the dizzy with someone to eliminate that? You may have a sticky base plate.

On the timing, max static advance is around 30 BTDC, on overrun the vacuum will pull this higher, so make sure your advance measurements are on WOT. What you should see is the dizzy advance jumping noticeably once you come off closed throttle as the vacuum comes on and gives it the advance it needs.

I run a Megajolt and have around 40 degrees on high vacuum overrun conditions.

Do 1&2 share the same Venturi and fuelling in the carb? If so then they should be getting the same mixture which does suggest electrics to me.



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[Zcat7]

Bluesnailman - fuel was put in just before the 30 mile run and the tank was showing pretty much empty. I filled it with BP regular unleaded.

Jonboylaw - good advice on the timing., will recheck and be diligent about WOT and overrun. Yes 1&2 do share the same venturi, solex 40s are twin single chokes. I still have the 009 with Pertronix that I could fit so might try that again.

What do you think about plug temps? I was wondering if the engine is running too cool due to current weather. I don't have the thermostat and flaps fitted at the mo but have sourced a set and plan to install them once the monsoon season subsides.

The only thing I can think is that as #1 and #3 naturally fire poorly on idle this gives a tendency for the plugs to start fouling. But then, as piston #3 is known to run hottest out of the 4, maybe it runs just hot enough to self clean the plug and #1 doesn't quite make the cut (if you'll pardon the pun!) and the fouling increases. What do you think?

Thanks for the replies, I have a couple of things to try now.

 

[Trikky2]

I had exactly the same problem with a 2 liter I built.

How's your oil consumption?

The old engine never needed topping up but the new one used about a liter of oil every 1000 miles. Initially I put it down to running in and larger than stock capacity etc.

I was wrong.

After the second time the plug fouled with carbon deposits I took the engine out and dismantled it. It turned out there was a manufacturing fault on one of the oil control rings.

The spacer piece that supports the two thin oil control rings was slightly too narrow and had allowed the rings to slip between the spacer and the piston groove which meant they were no longer being pushed against the cylinder wall.

All the compressions were perfect and there was never any blue smoke from the exhaust but it let enough oil by to carbon foul one plug.

After replacing all the rings the engine used zero oil between 5000 mile changes (ext oil filter fitted).

Hate to be the bearer of bad news but your symptoms match the problem I had.

Ps. Wrong mixture or ignition timing will not cause carbon build up the way you describe it. Excess fuel will leave black fluffy deposits, not hard carbon that stops a plug working and ignition timing, apart from not causing carbon deposits, would of course affect all cylinders in the same way.

 

[Trikky2]

Double post due to Internet hiccup.

 

[Zcat7]

Hmmm, not sure about oil consumption as it's such low mileage but I think it is doing some... Sounds like it could be the same as your problem Trikky.
I have to have the engine back out to fit the thermostat and flaps so popping the head off and pulling a piston isn't too much more work I suppose. I can't quite picture what you're saying about the spacer and the oil control rings to be honest, even though I've read your post a few times so I'm not exactly sure what I'm looking for. Will it be more obvious once it's in my hand?

 

[67panel]

had terrible trouble with the race beetle when we first ran it with twin 36s kept fouling plugs,turns out it was a weak spark so check your coil and ign feed to coil as well as it comes all the way from the fuse box(low volts and poor coil was our prob)since then I think its good to fit a relay to coil feed so it only has to come from the battery .

 

[Trikky2]

Hmmm, not sure about oil consumption as it's such low mileage but I think it is doing some... Sounds like it could be the same as your problem Trikky.
I have to have the engine back out to fit the thermostat and flaps so popping the head off and pulling a piston isn't too much more work I suppose. I can't quite picture what you're saying about the spacer and the oil control rings to be honest, even though I've read your post a few times so I'm not exactly sure what I'm looking for. Will it be more obvious once it's in my hand?

The oil control rings of the type I mean are two very thin rings that are in the top and bottom of the lowest groove on the piston. They are held in place by a springy metal spacer piece which keeps them apart and supports them from underneath the ring, to push them out against the cylinder walls.

If they are not properly supported by the spacer they don't scrape the excess oil off the cylinder wall and hence a little more oil gets into the combustion chamber. This gradually builds up a hard carbon deposit on the plug till it stops working.

This type of deposit is hard and dry and has to be scraped off the plug, often in chips.

It's a wholly different type of deposit compared to a worn engine, where the deposits will often be wet - or in an over fuelled engine where they will be much more soot like.

Hope this helps explain it a bit better.

Ps. There's also another common fault that causes the same problem. The "springy metal" spacer between and under the oil control rings is often made of a wavy metal strip. When fitted to the piston the colour coded ends should be aligned as per the instructions but it is quite easy to overlap the ends by accident, which means it is a smaller overall diameter in the groove and hence does not push the control rings against the cylinder wall.

 

[Zcat7]

This makes more sense now and is very interesting Trikky. No1 piston is the only one where I had the need to remove and replace the ring set during the rebuild and that surely cannot be a coincidence. I recall that the wavy metal spacer you mention was easy to overlap but I can't in all honesty say that I checked how the thin scraper rings sat ON the spacer, only against it. It could be that you have hit the nail on the head.

The carbon buildup I have on my plugs does come off with a wire brush as opposed to a scraper but it is very dry and not at all wet or oily.

I never got any instructions on how to fit the rings as they were already assembled on the piston kit but I of course exercised diligence in removal, orientation and reassembly. Guess I'd better get searching for some instructions!

67panel - thanks for that. I'm pretty certain I have a good spark as it nearly stopped my heart the other day when I pulled the plug lead off and I got a shock through the hand holding the lead and my left hand near the bumper! I also suspect that the other plugs would exhibit the same problem unless it was one lead or a cracked/tracking dizzy cap, which isn't the case.

So, the fun begins again but I'm less irritated than I might be as

a) I have to drop the engine and partial strip it anyway to fit my thermostat and flaps
b) I have some idea of what I'm looking for now!


Many thanks all, now praying for some clement weather...

 

[Zcat7]

Well its only taken me since the end of January to get around to pulling the engine but I have now looked at this problem and reinstated the thermostat flaps! As Trikky suspected there was an oil control ring problem. I found a large glug of oil in number 1 piston when I pulled the head off. Stripping the piston showed that the wavy oil scraper spacer had indeed overlapped when I reinstalled it (twat...).

So, I popped it all back together again and she fired up within a couple of turns. The idle is altogether different, smooth and even again. I think I need to set the carbs up properly/again though as things don't seem quite right but I'm in that hyper-critical phase at the moment where I'm listening to every noise and tick and maybe scrutinising the performance too much. I still fancy that there is a touch of hesitation on acceleration but I'm pretty certain that all the vibration has now gone at around 1600 rpm. There is some popping and farting on hard throttle when cold but this goes once she's warmed up.

So far as I can tell the flaps and stat are working OK too, which hopefully will contribute to a better all round setup!

 

[Trikky2]

Glad to hear you got it sorted in the end. :mrgreen:

Your not a twat, its very easy to accidentally overlap and mis fit those three piece oil control rings. Horrible things. They are not the best design IMO and have caught out many a professional mechanic over the years.

The popping when cold probably means either the choke needs adjusting or the tickover mixture is a bit weak. Try undoing the small mixture screw a quarter turn and see if it fixes it.

 

[Zcat7]

You're very kind Trikky! I have to say that when I reassembled that ring set this time round I found it to be a royal pain to stop it overlapping. I ended up checking the alignment and gap spacing about 20 times before I finally slid the barrel on!

I thought that running weak might be causing the popping. I'm running twin Solex 40s so no choke. I guess I need to compromise and richen the mixture a touch?