Carb air leak with lots of popping & banging

[peteyg]

OK, what am I doing wrong? 

Heard an air leak on the front carb (nearest bulkhead) which I suspect is the cause of my slow dropping revs and lots of popping on overrun. Sounds a bit like when the choke is on. Have since replaced the spacer O rings twice and following Tim Engel's very detailed process of 'soft' mounting the carbs using Hylomar AF on the O rings to create an extra seal but the leak is still there. The only way I managed to remove the leak was to crank up the mounting bolts as tight as they would go therefor crushing the O rings and creating a rigid mount. I did this before following the soft mount procedure and the slow dropping revs persisted, especially when the engine was warm after being sat for a while.  When soft mounted the front carb spits and bangs on the inner barrel and when revved, spits so hard and eventually blows out one of the O rings on the spacer. What could be causing this? Spray carb cleaner over the spacer and the engine picks up happily. All spacers replaced with new from SJ and O rings replaced from 2 different sources in case they were the problem. If I pull hard on the carb the seal improves and the engine runs better but the exhaust is still banging away though this could be down to the carbs not being tuned/balanced. Mounting faces of carb and manifold appear flat with no obvious scoring.

Not sure what I'm missing. Some spitting from the rear carb but nowhere near as aggressive as the front.

Help!

Pete

[JimmytheTurbo]

I had to tighten mine more than they recomend to make them seal.

All parts were new but I tightened until there was still a gap between the cupwasher but less than what Des Hammils book recomends.

At the end of the day they need to seal and all need to be done up the same amount.

I used no hylomar.

[drdoom]

Spitting from carbs indicates lean condition as does popping on overrun, your air leaks central to the situation. Question is, what specifically is out of sorts? Anything recently replaced should be considered suspect firstly. It seems that some parts offered nowadays are out of spec, so be wary. On the other hand be open to the possibility it's something other than spacers/rings, perhaps throttle shaft seals? Might have bothers in 2 or more spots concurrently.

[jonwat]

12 hours ago, peteyg said:

Mounting faces of carb and manifold appear flat with no obvious scoring.

There are drillings in the mounting faces which have been plugged with lead plugs, seen here.

When the mountings have been overtightened the lead plugs can be forced back into the drillings so facilitating an air leak so check for this.

Sounds as if something has been distorted through overtightening.

[peteyg]

@jonwatI think you might be right. I spotted that as I was about to throw my toys out of the pram in frustration and thought surely the holes will be hindering a proper seal so decided best to sleep on it. There's no lead but a hardish brown substance that has sunk about 1-2mm on both barrels. Any idea what I could fill the holes with? Would rather avoid anything that will need filing.

Pete

 

[jonwat]

2 minutes ago, peteyg said:

Any idea what I could fill the holes with? Would rather avoid anything that will need filing.

I used ordinary body filler on mine, just a thin smear with a plastic scraper or old credit card did the trick. 

[peteyg]

Really thought I'd found the root of the problem. Filling the holes seems to have cured the air leak but still banging and blowing out the O ring. Always the same barrel though all of them spit and pop but less violently. Any other ideas?

[910Esprit]

Think you should also rule out any mechanical issues, eg incorrect clearance/burnt valve on the inlet valves (compression test).   I've not seen such violent 'backfiring' even if the mixture & balance is all over the place.

FWIW I also see that your arrangement on the carb mounting studs are different to mine, with what appears to be a cone shaped spacer? (I have 2 identical cupped washers for each rubber washer), although that wont have any bearing on your current issue.    

[skiing]

yes the washer arrangement etc looks different to mine as well 

[jonwat]

Yes, I agree with Steve. You should have two cup washers sandwiching a rubber spacer.

Also, it does sound like your timing is too advanced or maybe leaking valves to create enough pressure in the manifold to blow out the O rings.

[JimmytheTurbo]

21 minutes ago, jonwat said:

Yes, I agree with Steve. You should have two cup washers sandwiching a rubber spacer.

Yes should have a cup each side

 

Edited June 29, 2023 by JimmytheTurbo

[peteyg]

It had 2 cup washers previously but I was following a recent article by Tim Engel who seriously knows thes engines and his instructions for soft mounting recommends the setup you see in the photo.

Pete 

[JimmytheTurbo]

1 hour ago, peteyg said:

It had 2 cup washers previously but I was following a recent article by Tim Engel who seriously knows thes engines and his instructions for soft mounting recommends the setup you see in the photo.

Looks like it works well... 😆

Edited June 29, 2023 by JimmytheTurbo

[Sparky]

Been following this with interest.  Wow, never seen an O ring blown out.  I suspect that's cylinder pressure.  Compression test?

[cweeden]

Popping can also be caused by an exhaust leak or leaky exhaust valves, a compression test will help eliminate the valves at least. 

[peteyg]

Well, put both cup washers back on and cranked the nuts tighter than spec and all seems to be well. Carbs balanced and running smoothly. Only had a quick test drive but none of the spitting and banging just a bit of exhaust pop on the overrun. Will have more time to test tomorrow.

Pete

[drdoom]

Ahh, now this is sounding as i would have expected. Though respectful of TE's wisdom I concur with those here in differing on the cushions installation. I found it necessary to crush the rubber cushions between two cups when running my Elan with DCOE's, and there was just a little gap between the cup edges by the time it held vacuum sufficiently. So you've made meaningful progress Pete, should go over carb and spark timing settings to resolve what remains of any popping and spitting.

I am perturbed at what appears to be an o-ring extruding out of shape in the lower reach of that one barrel.

[peteyg]

Yup. They clearly were not done up tight enough using TEs method which is why air was getting in and  the O ring kept blowing out. 

[Sparky]

Blimey, were they really that loose?  You can go pretty tight and still isolate from vibration.

[TAR]

I'd imagine that the mating surfaces aren't completely flat and true  

[drdoom]

8 hours ago, TAR said:

I'd imagine that the mating surfaces aren't completely flat and true  

That would be worth considering if it were not for the o-ring seals which will accomodate more variance than could possibly arise in either carb or manifold.

[peteyg]

9 hours ago, TAR said:

I'd imagine that the mating surfaces aren't completely flat and true  

I did check that and they are fine.