Losing compression

[den912]

I’ve had my 2.2 engine rebuilt with Ecams put in, it has a new set of PMO carbs put on with a sports Scart exhaust fitted, it also has a 123 dizzy fitted. The cars only done 2400 miles had its first service but has a reoccurring problem where it loses compression in cylinders 4&5, I've had the car on a rolling road and dynoed several times, it gets set up perfectly and runs great for a couple of hundred miles then gets really sluggish with no get up and go, the plugs soot up on these cylinders and causes the loss of compression has anyone any idea what could be causing this, it’s been suggested that going with hotter plugs could help with the problem, what’s the pros and cons of going that route, could there be a problem maybe with the new PMO carbs, I’m really not that mechanically minded so I’m not able to answer any technical question about the engine, the guy who built the engine is a very respected engine builder so it’s not down to trying to skimp, any help would be great fully excepted
Brian

[RobFrost]

When you stop, is there a smell of petrol in the engine bay? Sometimes the carbs get hot.

If they work the same as Webers you can adjust the mix per cylinder. Maybe it's just rich on those cylinders.

Does it backfire at all, pop in the carbs or in the exhaust?

Are the ignition leads on the plugs and distributor properly?

What makes you think it's losing compression? Have you done a compression test?



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[den912]

No smell of petrol and the carbs have been set up correctly, when it’s been on the Dyno they can see it’s not got full compression, the engine builder has also told me that’s why it’s so sluggish when the problem flares up, you can also hear it from the engine making that “Tuff tuff tuff “ noise every time I think it’s running correctly it comes back ☹️

[RobFrost]

If it really is losing compression then make sure spark plugs are fully tightened, which is the quickest and easiest fix.

A leak down test will tell you whether it's valves or piston rings leaking. On a recently rebuilt engine you may need to retorque bolts.

Valve clearances should be checked and adjusted.

The problem arising only when warm suggests to me valve clearances may not have been set when the engine was warm, so now when things expand it puts them out. Be sure to set valves when engine is hot or according to manufacturer's instructions.

There are others who know more than me. My advice is general engine advice, not 911 specific.

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[RobFrost]

You seem to say sooted up cylinders causes a loss of compression but sooted up cylinders can't cause this. It can cause those cylinders to lose power or to not fire. But if that happened I would expect the unburnt fuel to backfire through the exhaust.

Sooted up cylinders is caused by unburnt fuel, which either means the mixture is too rich or the spark is not igniting the mixture.

Classic cause of mixture too rich is carb adjustment. If the mixture can't be fixed by adjustment this is usually caused (on webers) by a damaged jet letting too much fuel through, which makes adjustment ineffective. You get new jets on those two cylinders or get them soldered and redrilled. Other causes are faulty float or needle valve in the carb.

Were new pistons, rings and cylinders used in the rebuild, and if old ones reused, were they within spec?



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[Walt Fricke]

You say it is losing compression in 4 and 5. Can one assume this is based on doing a compression check? So what were the numbers for these cylinders, and where generally were the other four?
Sooted up plugs have nothing to do with compression. Compression is lost when one or both valves leak when closed, rings are worn out or haven't yet seated, or all of these. Diagnosis involves a leak down test.

I'm guessing you don't mean compression - you mean you lose power, and you assume 4 and 5 are the culprits because of how those plugs look compared with the others?

This would point to an issue with the carburetor settings for these cylinders (e.g., see post 5), or a spark issue, or perhaps both. Carbs would seem more likely, as spark issues don't tend to be episodic - but then carb issues don't seem to come and go either. Did you use a tool which measures relative air flow to balance the six carb barrels?

As to valve lash - on our engines the valve lash grows as the engine warms up. Always good to check lash, but not a very likely cause of this behavior.

You are saying the engine behaves erratically on the dyno - sometimes giving expected numbers, and sometimes not? And when down on power you are hearing a tuff tuff tuff noise? This does sound like something not related to spark or carbs, but rather a leak of some sort? A valve isn't seating? Is the noise RPM related? If it isn't, you should be able, with a stethoscope or a rubber tube as a substitute, to figure out where it is coming from. Doing a leak down test most of us also listen - in the carb or other intake, in a tail pipe, and in the oil tank (because the breather exits there and you can hear piston ring leakage there).

Carbon does sound like there is ignition (otherwise no carbon), but as mentioned, too rich. A problem with combustion chamber seal would suggest you wouldn't draw as much air and fuel into the combustion chamber, not too much. Easy to see why you are perplexed.

[Richey]

Why doesn't your " very respected " engine builder diagnose the problem ? Most anyone can assemble and tighten the nuts and bolts , the hard part is making an engine run properly .

[faapgar]

I love these posts.Do a leakdown when hot and you will get your answer.

[safe]

Originally Posted by den912

I the guy who built the engine is a very respected engine builder so it’s not down to trying to skimp, any help would be great fully excepted
Brian

If he built it, let him fix it. It's his problem not yours.

If its making “Tuff tuff tuff “ noises it may be leaking between the cylinder and head (look for soot on those cylinfers), but again the builders head ache.

[stownsen914]

As suggested, I'd take it back to your builder and ask him to make it run correctly. Any competent mechanic should be able to diagnose the source of the issue. Sounds like a compression or leakdown test is in order if not already done. Might want to have the cylinder head nuts torque checked. They can loosen and you might get lucky. Also, pulled studs are a common problem on the early motors due to the magnesium cases.

[Walt Fricke]

Right - we are not told if the head studs in mag cases issue was addressed in this or a prior rebuild. I'd say it is something always done by shops which work on these engines. If one didn't do it, the most likely reason is the customer didn't want the extra expense.

[den912]

The main problem is after a time the plugs on cylinders 4&5 are sooting up and no firing so the cylinder isn’t doing its job, on the dyno once the plugs are cleaned the car runs perfectly but after 200 miles odd they begin to soot up and not fire, by adding hotter plugs would this damage the engine at all

[Jonny042]

I had issues with plug fouling when I was trying to use Beru plug wires. If this sounds like you throw them as far away as you can and get decent new ones. Clewett best of all.

[Trackrash]

I had a similar problem. Check your float levels. Even a millimeter too high and you will foul your plugs.

[safe]

So you arent loosing compression??

Do a compression test and leak down an all cylinders so you can rule out mechanical issues. If all is ok then you recheck the carbs.

What spark plugs are you using? Its easy to go a step hotter and check if it helps.

[RobFrost]

Hotter plugs without diagnosing the cause is unwise in my view. It'd be great if you answered these questions:
-Have you checked ignition leads are all on correctly?
-What is the CO or O2 sensor reading on each bank of the exhaust?
-Are the spark plugs fully tightened?

At the moment there are two differential diagnoses:
Running rich on those cylinders
Inadequate spark on those two cylinders.

Plugs not hot enough would not explain the difference between those two cylinders and the other 4.

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[icarp]

Originally Posted by Jonny042

I had issues with plug fouling when I was trying to use Beru plug wires. If this sounds like you throw them as far away as you can and get decent new ones. Clewett best of all.

Clewett best of all.

do you know what every single jet size is , air, fuel, idle, choke, aux vent, for both carbs .
Do a leak down , you must or you will never solve this , much more to follow .


Ian

[icarp]

Ask me how many times I have opened up carbs only to find that 4 to 6 of the jets are different sizes , just what they had in the jet box , it will be ok, It's not a race car !
Too many
Check them all
Ian