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MG Midget and Sprite Technical - Compression worries - follow up thread

This is a follow-up to an earlier thread headed 'Compression - should I worry'. As that earlier thread had reached 70 responses I thought it more effective to start a new follow up.

To recap, engine suffered a HG failure but still runs very badly and is blowing through the carb. All the obvious things have been checked and the cause of the poor running has, so far, remained undiagnosed.

I have just been checking the cam timing. I marked up a sticky label in degrees ATDS and stuck that on the crankshaft pulley. I then put a dial gauge on No1 inlet valve. While this is a bit rough and ready (it depends on the accuracy of my label) the results suggest that the cam is retarded by around 4 degrees. The required setting is 108 degrees ATDC but the average of 4 measurements was around 112 degrees. Roughly the same result (more or less) was obtained on each of the 4 measurements so is probably not too far out.

If my measurements are correct is 4 degrees of retardation enough to cause the symtoms I'm experiencing (or should it still run but not be optimal)?

If I need to reset the cam timing is it possible to do so in situ, i.e. is it possible to raise the engine sufficiently to get the timing chain cover off in order to gain access to the vernier gear - or is it an engine out job?

Chris
Chris H (1970 Midget 1275)

4 degrees is negligible.

Timing cover can be removed relatively easily with the engine in situ, should you feel the need to alter it.
D O'Neill

Chris I did a job like that with the engine still in the car, not optimal but it works..
Alexander Sorby Wigstrom

Retarding the cam makes more torque at the expence of top end power.I retarded my cam by 3 degrees - on purpose and it all runs perfecty fine, so this is not the problem. When checking the timing always do so with the engine in the direction of rotation - a couple of degrees can be lost if you go backwards.
F Pollock

It sounds like the cam timing is not out by much, if at all. As Fergus says, the problem must be elsewhere. In some respects I wish that cam timing was the problem as at least I would know how to sort it. At present I'm still floundering about looking for the cause. Can't cure it without knowing thwe cause!
Chris H (1970 Midget 1275)

Where abouts are you Chris?
F Pollock

retarding the cam loses bottom end power advancing the cam increases bottom end power. EG cam meant to be 108 degrees. 112 is retarded 4 , 104 is advanced 4 degrees. retarding seldom works better than standard whereas advance can improve performance, especially if you fit high lift rockers. 4 degrees retard is bad.

Peter
P Burgess

Hi Fergus - I'm in Wolverhampton.
Chris H (1970 Midget 1275)

Hate to suggest this, but you seem to have excluded all other possibilities, without taking the head off again.

Prior to that though, I would once again set the ignition static timing, and re-check the firing order.

Impossible to say as I'm not there. But reading through all you have posted on this and the first thread, my gut tells me the solution is a simple one, probably staring you in the face. A fresh pair of eyes might well see it. Often the way, and then you kick yourself afterwards.

In some ways you probably hope it will be a complex fault, to justify all the effort and time you spent on it so far. I know that's how I would feel. On the other hand, easy is cheap.

But whatever, don't lose heart, you'll fix it, and then you'll know even more than when you started.

Lawrence Slater

Hey Peter,

You beat me to it, i was going to give that same response, but it was your info i was stealing... I took your advise and did exactly as you discribed...its torqy for sure

1 qustion, by advancing the cam... Would that increase the compression psi ... Not the compression ratio but the but the compression pressure its self

Agian thanks for that advice

Counter clock wise is advance ... Clockwise is retard

One last not...never move the chain backwads...always forwards...if you move it back even just a little... It will give false readings by several degrees I learned that one the hard way
Prop

Hi Prop

You are quite right the compression pressure will read higher with advancing the cam, traps the mix in earlier.

Have you spent all your lottery winnings yet? I keep getting tax refund offers and all sorts of crap!

Peter
P Burgess

Sorry peter

I cant seem to spend it fast enough and so it just keeps piling up in the corner....lol

I have got to get back to work... Or i will never get this job done.

Prop
Prop

Swopped just about everything now (dizzy, coil, leads etc.) and now just fitted the original twin SUs to replace the HIF44 (in case that was the problem). Nothing works and the misfiring and blowing through the carb remains. Nothing for it but to take the head off and give it a thorough examination.

Chris - I need my head examined - Hasluck
Chris H (1970 Midget 1275)

Have you done a compression test?
Dave O'Neill2

Yippee - I've finally diagnosed the problem. First, many thanks to Dave O'Neil for calling round to have a look. He didn't come up with the answer but he reassured me that I was not doing anything too stupid.

Here is what it was NOT. The following were OK:
* petrol pump
* carb
* distributor (no condensor either)
* ignition leads
* ignition timing
* plugs (thanks Dave)
* cam timing
* cylinder head (good compression, no leaking/burnt valves, broken springs)

So, anyone venture to guess what the problem was? Gold star for anyone who gets this!!

Chris
Chris H (1970 Midget 1275)

Congrats. :)

Coil? Don't see how but it's not on your list.
Firing order? But I think you already eliminated this.
Inlet manifold gasket? Not likely.
Valve gap? inlet valve open on compression stroke?

I have no idea, please releave us from the misery of guessing. lol.
Lawrence Slater

Lawrence
Sorry - I forgot to mention that the following were OK as well:
* coil
* inlet/exhaust manifolds
* firing order

I set the valve gaps carefully on completion of HG replacement but you are close! Hint - I have pressed steel rockers.

Chris
Chris H (1970 Midget 1275)

You had a partially seized or broken rocker? Stripped thread in the adjusting screw/lock nut?
Lawrence Slater

Broken rocker on an exhaust valve?

If so, I suppose that could have contributed to the initial HGF.

I would have thought that it would have shown up on compression test though.
Dave O'Neill2

Badly worn rockershaft
Onno Könemann

You havn't got tube spacers between the rockers have you? These can bind up when they get hot (as can bronze valve guides) and cause misfires. Give in.
F Pollock

Gold Star to Lawrence! The adjuster screw on #5 rocker had popped up out of the rocker. I have some Cooper S competition valves fitted and they seem to have too much resistance and partially stripped the thread inside the rocker arm. I think 1/2 inch clearance is too much - I normally go for 12 thou!

I think the sequence of events was as follws. The rocker failed, the exhaust valve remained partiall/fully closed, the exhaust stroke became a compression stroke and the pressure was relieved when the inlet valve opened and it blew out through the carb. In the meantime the pressure was also blowing out the HG in the process.

I did not notice this when dismantling the head and when reassembling all of the rockers adjusted and locked as normal. I had a vague sense that the engine ran normally for a second or two when first started (but thought it was my imagination) but it was obviously the time taken for the screw to pop out again. I've had the rocker box off several times since but failed to spot this (I was always looking at #1 cylinder and #1 inlet valve to check timing).

Anyway, it seems I've found the problem but I' going to strip everything again as (1) I don't trust those rockers anymore and will replace with forged ones and (2) I would like to be sure the new HG has not been damaged when running in this way, even for a short time. I may even put some weaker springs on (my current one may be way too strong).

Thanks to all who provided suggestions and especially Dave O'Neil. Dave, you did actually suggest valve gaps at one point and I said "no I had set them OK" (which I had). It was checking them one final time that led me to spot the problem.

This is the second time that someone off this BB has actually called around to offer help with a problem. That's great - I hope I can return the favour some time for somebody else.

Chris



Chris H (1970 Midget 1275)

Good to hear Chris

Had a simulair incedent after setting te valves on my old mini when i forgot to tighten the nut...

You might just want to try fitting a new set (why not go for high ratio rockers)
I believe you said compression was fine so HG must be ok.
If it is gone you won't ruin anything with it
Onno Könemann

Onno

"Why not go for high ratio rockers"

Peter May, who built the engine told me he did not recommend high lift rockers with the cam that he fitted so I left them off for this engine build. I had some HLR fitted to my old engine (same head) but the valve springs (those currently fitted) were getting coil bound and so that convinced me to follow PM's advice. As I would have to change the springs to fit HLR I was going to fitting 1.3 replacement rockers. If I change the springs I could go down that route - but this is getting expensive!

Chris
Chris H (1970 Midget 1275)

Often the way Chris.

Previously I posted,

"my gut tells me the solution is a simple one, probably staring you in the face. A fresh pair of eyes might well see it. Often the way, and then you kick yourself afterwards."

Obscure but simple, and look as the problems is causes. You were very unlucky in that the thread didn't strip completely, you'd have spotted that right away.

Good thing is, now we all know another way to be tricked by an engine, and might spot it future. :)
Lawrence Slater

Chris
Why do you think the springs are to hard?

Valve springs are matched to the cam as to lose springs can damage a cam severely.
The only thing 1.5 rockers add it more lift so the springs must be able to cope with this.

To strong valvesprings only add resistance and cost power but do not cause nearly the damage lose springs do

If PM build the engine and adviced not to use 1.5 rockers he must also have given advice about what valvesprings to use....

So get springs with the appropriat rating and fit the rockers.

these will most likely be appropriate
http://www.minispares.com/Product.aspx?ty=ai&pid=33655&sku=0&tc=1#submenu
And are not expensive
Onno Könemann

Hi Onno

The problem is I just transplanted my cylinder head from the old engine to the new one. The springs came from a full-race Cooper S but I have no other information about their rating/spec.

I thought about buying exactly those springs from Minispares but when I phoned Minispares yesterday they told me those springs were out of stock and would not be available until mid-january 2012!!!!

Anyone suggest a similar good set of springs?
Chris H (1970 Midget 1275)

Call Peter
or use the other usual suspects

http://www.med-engineering.co.uk/catalogue/cylinder-head/springs-caps/med-road-valve-springs

http://www.minisport.com/mini-spare-parts/info_ADLH-LIFT.html
Onno Könemann

Good result.

Intersting difference when the exhaust valve fails to open on a big valve MGB is that the total load from the gas pressure on the bigger inlet valve is greater than the buckling strength of the pushrods. So instead of acting as an exhaust valve the valve stays shut and the pushrod bends.
Paul Walbran

Gulp! I hope that's not the case here. I'll check the pushrod for straightness.
Chris H (1970 Midget 1275)

I've used std single A+ springs in many heads I've built over the last few years with very good results.. Don't make them too strong...

Mark.
M T Boldry

Probably OK.

A series big valve is less than 2/3 the area of B series big valve, so less than 2/3 force working against the pushrod. And the A series pushrod is much shorter, albeit thinner too.

On top of that, it depends how far foot was in at the time of the happening. Full power = lots of pressure on the inlet valve if the exhaust doesn't open. Idle/part throttle results in much less pressure.

The reason it happened with me is that I was experimenting with carbon fibre pushrods and big rocker ratios at the time. The resin bonding the carbon fibre went soggy with the heat and collapsed. They were operating the exhaust valves, I wrongly thought not much would happen if they didn;t work out. What puzzled me initially at the time was why the inlet pushrod had bent as well, at only 6000 RPM when it had exceeded that substantially countelss times before. The I did the maths and the penny dropped!
Paul Walbran

Just checked against a short straight-edge and pushrod is still straight. Thank goodness.

Rather than just throwing everything back together with my replacement rockers, I'm going to assess every aspect of my cylinder head for reliability while stil looking for performance. Regarding springs, there seems a wide range available. I have not come across Minisport (as suggested by Onno) before. They seem very cheap but are they any good? Also, which is best for a road and track day car - 200lbs or 240 lbs (is the latter too much)? I also looked at Swiftune's springs which were rated at 180lbs.

Chris
Chris H (1970 Midget 1275)

This thread was discussed between 09/11/2011 and 20/11/2011

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