Rebuilt engine - low compression on cylinder no.2

[Charles73]

I've recently rebuild the engine on my S4 primarily to convert from strom to webers.
I purchased a overhauled and fully assembled head from a reputable TC man here in the UK, valves seated, clearances set etc.
The block has been skimmed and rebored to accommodate a new set of forged pistons and rings.
Running on a refurbed pair of 31 DCOE 40's, electric fuel pump & pressure regulator.
Brand new electronic ignition ('up to 150bhp' TTR).

The first issue I noticed was no.3 and 4 were running way too rich, no.2 didn't seem to be firing at all and no.1 seemingly fine.
I pulled the leads off with engine running and this confirmed 1, 3 and 4 were firing and no.2 was not.

Adjusting the ignition slightly gave no.2 some life briefly, but it was running way too lean - no matter how much I adjusted the carb.

I pulled the plugs out. No.1 was slightly sooted, no.2 was clean, No.3 & 4 were completely coated in soot. I am getting a spark from each when out of the engine.

I then compression tested each cylinder and 1, 3 and 4 all gave the same reading but no.2 was considerably lower (can't recall exact readings, something like 14,14,9,14).

It seems to me that I have three separate issues going on; induction, ignition and maybe no.2 valve not closing fully?!

What would be the best way to proceed and in what sequence? Any suggestions or experiences would be most welcomed.


Charles.

[nmauduit]

first thing I would try if you have not already, is an other compression test with some thick oil in the offending cylinder (to see if that restores the compression to the level of the other cylinders) - if this restores the compression you may wan to look into a leak-down test as a complementary diagnostic.
I understand the engine has never run well since being rebuilt (and that everything else is within specs, including valve gaps). You may want to try next wiith a new set of sparks with good gaps for each (lots of bad sparks these days) and not too cold, a poor spark can quickly turn your engine startup sequence into a nightmare.
One thing keeps poping in my mind is a slightly bent valve during reassembly (possibly when timing the camshafts): are you confident this can be ruled out (yet the low compression is not that low...) ? if not, taking the head off may be soon on your todo list, just to make sure (though if the engine has sat a long time there is always the remote possibily of a small deposit preventing a valve from closing properly, but that is quite rare and if it does not go away after a few minutes running one will need to give it a closer look anyway). Before considering deeper investigations, I would spend a day keeping trying, getting the best carbs settings I can first (using a synchronizer if not already done) even if a final carbs adjustment will require a properly running engine.

good luck !

[rgh0]

I agree you need to spend some time working on what is the problem with no2 using a leak down test. My bet would be a bent inlet valve during assembly.

The other problems look like fuel or ignition and should be easily fixable but hard to work on solving with one cylinder down so much on compression

cheers
Rohan

[promotor]

What are your individual valve clearances? I would check each clearance a few times to check the average, just in case there is a difference each time you check, which will indicate there is something there to investigate further.

However, if one plug is showing signs of being clean I would want to check that there is a healthy spark at each plug - it could be a faulty plug as there are fakes/copies out there.

[2cams70]

A long shot but how did you go about setting up the cams?

[USA64]

This was just out Tue. You set the head down on the bench was my first thought (and very pleased to find Rohan suggesting it). Not very pleasing to you of course!
https://www.hagerty.com/media/opinion/t ... 80e7908bfd

[Charles73]

Ok thanks for your thoughts so far.

I realised I hadn't had the throttle fully open whilst testing compression. So I held the throttle open and retested. This gave a reading of 19 bar (or 275psi) on all cylinders! I did the test a few times to be sure it was consistent and got the same results.

I also did it with the throttle closed again, just out of curiosity. And no.2 still gave a lower reading, but 1,3 and 4 didn't give as a consistent reading as the initial test, but it was still higher than no.2.

I will also add that the engine was hot on the initial test, but cold on the recent tests.

So hopefully my compression problem is now not a problem after all!?

I will now try a new set of plugs, check the distributor and leads (all of which are brand new), and go from there.

Also, I haven't yet done the ignition timing as I haven't got all cylinders firing yet.

What is quite marvellous is the fact that, even though I've only had 3 cylinders, when giving it some revs it sounds fantastic...so I can only imagine how great it will be with all four!

Many thanks, any more thoughts welcome.

Charles.

[alan.barker]

Sorry but,
Bent Valve, how did you set Cams.
Before fitting Head you should set Crank at TDC. Before putting Head in position set the Two cams at the TDC position. Then lower Head onto Block. Don't turn Crankshaft or Cams with no Timing chain fully fitted.
Please remove head before you do more damage. Bent valves can make a very nasty expensive mess.
Alan

[Charles73]

Sorry but,
Bent Valve, how did you set Cams.
Before fitting Head you should set Crank at TDC. Before putting Head in position set the Two cams at the TDC position. Then lower Head onto Block. Don't turn Crankshaft or Cams with no Timing chain fully fitted.
Please remove head before you do more damage. Bent valves can make a very nasty expensive mess.
Alan

Hi Alan,

I did all of the above when fitting the head.

So even though I now get a consistent compression reading on ALL cylinders with open throttle, you still think a have a bent valve?

[Charles73]

A long shot but how did you go about setting up the cams?

Using a timing wheel on crank pully, DTI on cam follower on no.4 set to:

Inlet fully open: 100 deg ATDC
Ex fully open: 106 deg BTDC

Cams are Q420. Sprint size valves.

I have a feeling ill be double checking the accuracy of my measurements...

[alan.barker]

If you have same compression on all Cylinder your Valves are ok.
Good luck
Alan

[Craven]

Good compression on all 4 on wide open throttle but dissimilar when closed indicates to me that there is quite a difference in position of the throttle plates between 1 & 2. I see you have recondition carbs but check the relative position of throttle plates via the progression holes, plenty on here tells you how.

[Donels]

You have got the plug leads in the right order on the distributor cap?

[Charles73]

You have got the plug leads in the right order on the distributor cap?

I'm just checking the distributor now.

For initial setup I point the rotor arm directly at no.1 cap pickup at TDC. Then plug leads in 1-3-4-2, going anticlockwise.

Correct?

[Charles73]

Good compression on all 4 on wide open throttle but dissimilar when closed indicates to me that there is quite a difference in position of the throttle plates between 1 & 2. I see you have recondition carbs but check the relative position of throttle plates via the progression holes, plenty on here tells you how.

Good idea, will do.