Help!!! Am I a tooth out?

[Shawn]

I've just fitted a 948cc engine, that I believe is in good condition. As part of the process I fitted a 1098 camshaft. I cannot get the engine to run with any force. I'm using the same ignition and carb setup as the engine that I removed, and although I messed with both the mixture and timing, I think I've got them close enough to eliminate them as a source of the problem.

I searched and used the method below to check the cam timing and found the gaps on #1 cylinder to be very different. On removing the front cover... well see the pics below.

So - rocker cover off - spark plugs out - turn the engine so #4 cylinder is firing - both valves will be closed -and set it exactly at Top Dead Centre. Looking now at #1 cylinder - the two valves will be slightly open (this is the overlap at TDC) , if the timing is right they will each be open by exactly the same amount. If you had a dial gauge - you would use it to measure how far each is open. Since no dial gauge - you need to slacken off the valve gap adjusters on both valves until there is the slightest of gap at each valve - check with say 5 thou feeler. Now crank the engine over one turn - and measure the valve gaps now on that #1 cylinder - they should both be the same at ~ 40 thou or so. The value doesn't matter - but they should be the same! If not the same (within 5 thou) - then the camshaft is not timed in correctly. Simples !
Be sure to reset these valve gaps before running the engine !

Both of the pics were taken without turning the engine (ie at the same enging position, I didn't move it between shots), one from outside the grille.

Am I aligned or not?

Help!!! I'm starting to have sanity issues.

[bmcecosse]

That looks fine. What else was done to the engine - 948 head or 1098 head?

[Shawn]

It's the same 1098 head (shaved 50thou) that was fitted to the previous engine, which also had a 1098 cam. It's backfiring on the over-run and has no power. The head was swapped with the engine swap, with a new gasket set.

Would the valve gaps on #1 suggest that the cam timing is out?

[bmcecosse]

If you have carefully run my 'test' - what gaps do you measure on overlap at TDC? It's always possible the sprocket(s) are not correctly dimpled - or keyed.... Or a faulty cam - let's see the measurements please.

[Shawn]

Thanks, I'll run the test again in the morning, but from memory, the exhaust was more than 50thou (guessing about 80) the inlet was much less, 25-30ish.

[bmcecosse]

Hmm - well as long as you are SURE it was dead on TDC - that's not good. Try moving the cam back one tooth (on the cam sprocket)- and then run the test again

[philthehill]

Looking at the photo you have a very slack chain that has allowed the relationship between the cam sprocket and the crank sprocket to be nearly 1/2 tooth out. I put a ruler across the photo and if you do the same you will get the same outcome. Whilst it may have nothing to do with your problem I would fit a new chain and possibly sprockets before doing anything else.

[IslipMinor]

Roy,

I have seen this cam timing check suggested a number of times, but surely it is only correct when the cam timing of Inlet Open and Exhaust Close are the same number of degrees before and after TDC? None of the BMC cams have symmetrical timing around TDC, and only a few of the A-Series aftermarket ones do, e.g. Kent 256.

For example the 948 Minor cam has a timing of In Open 5° BTDC and Ex Close 10°ATDC (15° overlap, and the exhaust valve closes 5° more after TCD than the inlet opens before TDC) and the 1098 cam has In Open 5° BTDC and Ex Close 21° ATDC (26° overlap, and the exhaust valve closes 16° more after TCD than the inlet opens before TDC). This would suggest that at TDC the 948 cam exhaust valve will be open more than inlet, and with the 1098 cam quite a bit more. To have the same valve openings the crank with a 948 cam would need to be set to 2.5° ATDC and the crank with a 1098 cam 8° ATDC - or am I missing something?

Shawn has found that the exhaust has a larger gap than the inlet, which is correct as the adjuster will have had to be undone more, if the above explanation is correct!

Shawn, the timing looks fine, apart from the slack chain that Phil pointed out.

[katy]

Looking at Fig AA.16 in the BMC manual it shows the holes in the camshaft sprocket displaced nearly 90º ftom the timing mark. in Shawn's pics they appear to be in a different location. I don't know if all the camshaft sprockets have the holes in the same locatiion or not. If they are all in the same relative position then that tells me that there's something goofy w/the camshaft sprocket, possibly the timing mark is in the wrong location?

[philthehill]

Looking at and referring to the illustration AA16 in the wksp manual the important points to look out for are the relationships between the woodruff key slots and the indentations in the crankshaft sprocket and the camshaft sprocket. In the case of the crankshaft gear that relationship is correct. In the case of the cam gear because the nut and lock washer are still in place it is difficult to determine that it has the correct relationship but I suspect that all is well in that area. I have never come across one that did not have the correct relationship and from the photographs above the cam gear looks to be original equipment. The large holes in the cam gear can be anywhere around the gear with two holes providing the balance; some gears do not even have the holes. Shawn can you please post a photo of the cam gear with the nut and lock washer removed?

[Shawn]

I'm thinking that the cam timing is ok. Using the test, I found gaps at 43thou inlet, 48 exhaust, so I guess we can call that close enough.

Here's the pic with the nut removed and the woodruff keys visible.



This chucks me right back where I started. Bugger.

So, on a known good carburettor+inlet and a known good Hitachi dizzy, where do I start looking for an appalling lack of power and backfiring on the overrun?

[bmcecosse]

I can assure you all - the 'equal lift on overlap' test works if the cam is correctly installed. And indeed Shawn has now found that to be so ( a few thou is neither here nor there ). So - if you believe the dizzy is correct - you need to look to the carb...is the piston free to rise and fall easily?? Could you run a compression check on the engine - and post up the results?

[MarkyB]

appalling lack of power and backfiring on the overrun

Manifold gasket?
What colour are the plugs?

[bmcecosse]

Backfiring is unburned fuel - lighting up in the exhaust. So this suggests an ignition problem. Sure the plug leads are on right order - 1342 anticlockwise round the dizzy cap.

[philthehill]

Shawn
Thank you for your photo of the timing gears with the nut and washer removed.
If the two indentation timing marks are still in line - the cam is about 1/2 turn out. I have just checked a known cam timing gear which was used on my engine and the woodruff key slot is just off (approx. 185 degrees (clockwise)) opposite to the indentation as per Wksp Man illustration AA16.

[bmcecosse]

The sprocket only fits the cam one way...the check shows it's correct, all chains are a 'bit slack' after a few miles running. The problem is not camshaft timing........

[philthehill]

Yes the cam sprocket only fits the cam one way round BUT we still do not know if the timing indentation mark on the cam sprocket is in the correct relationship to the cam sprocket woodruff key! I was hoping that Shawn would provide a better photograph showing the whole of the cam sprocket with the timing indentation clearly visible . Whilst it is highly unlikely that the woodruff key in the cam and cam sprocket is incorrect I personally would need to know that the basic relationships between cam/cam sprocket and crank/crank sprocket are correct. Get the basics right before looking elsewhere for a problem.

[bmcecosse]

Indeed - and there have been instances of the 'dot' being wrong on cheapo replacements. But these look like tried and trusted components..so I don't think they are wonky. And the check appears to show pretty much equal lift on overlap - so it's not a mile away. I do believe the problem lies elsewhere.

[IslipMinor]

Roy,

I have just checked ours, which is fitted with a Piper 270 cam, and does have symmetrical timing around TDC of 31°, i.e. 62° overlap. It also has the same lift for both inlet and exhaust, as, according to Vizard, do all BMC standard and ST cams, except the 998 Metro.

Using the timing marks for TDC and a dial gauge, the valve openings at TDC are 0.086” exhaust and 0.076” inlet. To get the inlet to the same lift as the exhaust requires 3° more crankshaft advance, so at 1-2° ATDC both are the same.

I didn’t use the timing marks when fitting the cam, but started from the basics and used a dial gauge on the piston crown for setting TDC and a dial gauge and degree dial to set the cam to the correct advance. Next time I have the head off I will check the accuracy of the pulley timing mark! If not, could be the result of a bit of timing chain wear, so will reset the timing on the vernier camshaft sprocket.

This method has been an excellent cross-check, so I have copied it into Word for future reference – thanks.

No matter what the cam timings are, the process is still very valid, and is also very quick and easy to do, but I believe you need to adjust the base setting for half the amount of ‘asymmetricity’ for BMC, and some aftermarket cams, such as Kent.

Which leaves the original question – I would like to understand how you can have equal lift on the overlap on a cam with asymmetric overlap timing around TDC - please explain it to me?

[bmcecosse]

That's pretty close Richard - chain stretch/inaccuracy in machining of keyways etc. Remember - this is measuring the lift at the point where the exhaust is closing and the inlet opening at TDC on 'the overlap' and will always be true on any normal camshaft. There may be specially developed exceptions - but I've never found them. In theory this is how the cam should be installed - BUT - dyno testing may give better top end power if installed slightly advanced, and possibly better low end power if installed slightly retarded. The Mini lads spend ages discussing the merits of installing camshafts within a degree (offset keys/adjustable sprockets etc..) ...how they ever detect any difference on the road is beyond me...
Edit -just noticed you have vernier - if I had that I would install slightly advanced to allow for future chain stretch. For a road engine I just advocate installing dot to dot, and then checking to make sure nothing seriously out of kilter. Way back in my rally Minor days I bought a reground camshaft and was always disappointed in the performance (didn't know about lift checking) -I tried it one tooth forward and it was even worse, then tried it one tooth back and it was very much better, so left it there. I just assumed it was poorly ground.