For ages I have had an issue with this engine not running correctly.It runs really rough.
History was when I bought the tractor many years ago the oil pressure was way too low running at 5psi at running temp. So an engine rebuild was done using parts from davidbrownparts in the UK who were fantastic to deal with.
Since this was the first tractor engine rebuild I took my time disassembling the engine and took particular interest in this part of the engine, valve timing. I noticed at the time before undoing anything here that with the timing mark on the flywheel in the centre of the inspection hole, piston at TDC, both valves fully closed and the distributor facing #1 position that the dots on the cam did not line up with the gear on the crankshaft. Now at this stage the engine was running well but lacked oil pressure and compression was down a bit so I made my own new marks by making it have double dots on the crankshaft and cam gear. However if you look closely at almost the 1 o'clock position you will see some original dots there.
From memory when I discovered this I think I posted it up here and again from memory I was advised that this gear was possibly from a diesel engine and is why those dots are where they are.
So after trying many things over time I have now dug deep into here to see what really is the problem. As you can see the double dots are lined up as I found them when I stripped it originally and now they are still in the same position. I have been told that my next step is to put the engine on TDC and check the position of the valve at #1 and #4. At TDC #1 both valves should be fully closed and #4 should both be rocking, meaning the valves should be half open and half shut.

Does this sound correct?
My intention next is to remove the cam gear and rotate the cam until I get both valves closed at #1 and both on the rock or rocking on #4. Any thoughts are appreciated.
Cheers Cobba

If in doubt start at the beginning

Find and prove TDC
Dial the cam and use what ever special keyways and keys you need

Then you know it is correct

I always try to eliminate things, doesn't matter if I think it is correct I need to prove it is correct otherwise Im just chasing my tale

A good tradesman taught me this as a apprentice

Paul

For ages I have had an issue with this engine not running correctly.It runs really rough.

What problem are you trying to solve? The running rough?

Yes, recheck the valve timing, but importantly, check spark timing and spark timing stability across revs. Compression and carby, valve clearances and general backlash movement in things. check for HV breakdown.

If it starts and runs, I'd be betting the valve timing relative to crank is ok.

btw, what is the brass bush for? There is a large gap.

Thank you for the replies, they all help to point me in the right direction to solving this ongoing problem I have had for years.
Past history for this was it was totally rebuilt many years ago but as it does very little work you just tollerated the what sounded like a misfire. It always started well but never ran smoothly and was a problem on a road run.
SO I am now officially the worlds fastest at removing, stripping, inpsecting and replacing a David Brown SP30-V carby. IT has a new needle and seat as the old one would leak a bit and flood also new seals on the mainshaft. I have tested the ignition coil even bought a brand new one and that made no difference. All new spark plugs and brand new wire cored HT leads with new brass ends fitted to a known good distributor cap and tested with the multimeter for resistance and passed with flying colours. The distributor has been totally stripped to be cleaned out but inside top to bottom was spotless and the bob weights were also clean and free so I put it back together with a brand new and tested condensor plus the points are in perfect condition, cleaned and gapped. I can't tell you how many times I have been through the electrics and find no issues with all the components. I should mention that I didn't just rely on my multimeter for testing the coil and condensor. I have a bench tester for this and it can thoroughly test both parts, pre heat them to operating temps and test the spark gap inside a glass window. SO you test everything from cold then heat then re test and see what the readings are. In this case there was no drop in th readings so they also passed the test.
The head was fully reconditioned when the rebuild was done so good valves and guides plus the springs were all good and the tappet settings have been done and never had to re-adjust any. I have done a compression test across all four cylinders a little while ago and all were around 75-78psi which is excellent on a low compression engine.
I have good clean new fuel so that is not an issue and after so long and asking many people and the last resort is to look here at cam timing as it is the only thing left to consider.
So just now I had a reply from a retired David Brown field serviceman and he gave me the tip on setting or checking the valve timing. Knowing full well about the 2 sets of marks on the gear he said to ignore them and simply set both crank and cam gears with the keyways in the vertical position. This cannot be seen in my pictures but I know the crank gear is in that position so tomorrow morning after work I will take the nut and washer off the cam gear to have a look and see what I can see.
So hopefully it is 1 tooth out then I know the answer as to why it ran like a pig.

The valve timing is worth checking if for no other reason than to just have piece of mind that it is correct

I dont know if any of you have had a timing chain jump, the motor still runs and runs smooth at idle but is gutlless and wont make any power

Not saying that it is even closely related but I would check and as I said I would dial it in

I remember reading in the book "The Nullarbor kid" when either a crank twisted or the cam twisted

Or maybe they snapped and grabbed, what ever it was, things happen that are unusual

With any luck you will find everything is perfect and lined up and timed as it should be

But you will now know its right

Good luck and keep us posted


Paul

To add to that, another symptom if it's one tooth retarded can be cranking for a while, then really fast before it catches.
After doing the timing chain on my Dad's ZH 302 years ago, it started instantly. He always had to crank the guts out of it previously.

Jarrod.

Cobba,

You've made no mention of backlash in the distributor as it picks up the cam/crank gear or in the advance plate. Is there any? It will affect timing and dwell if there is and it's moving about.

Where is it (RPM) running rough? Carby isssues are broken into 3 engine speeds; idle speed (idle jets and vacuum leaks), as it transitions from idle to high speed (vacuum and emulsion tubes), high speed (main jets).

I forgot to answer a question further back about the bronze bush so this is what fits there then it is connected to the governor gear.
Well today I went back to the manual to confirm the info about the position of the key ways and yes both are at 6 o'clock as stated in the manual so it was assembled correctly at the rebuild. Now to clean up all the surfaces ready to reassemble the timing case. I went back through the valve clearances and all were perfect but one was a little loose so I did that up. Now they are all to spec again. This is another section that can be cleaned up and put back together.
Spark timing. There was mentioned on the DBTC forum about a "M" on the flywheel for timing the spark. I have seen this and have since found out that this is 8' BTDC for the points to be opening. This is new to me, I always times this engine to crack the points at TDC so this may be part of the issue. Having said that the engine even now starts with the least amount of starter motor, it has almost always kicked off immediately with the only exception being when the engine used to flood due to a bad needle and seat.
The distributor does have some back last and also some weights under the plate. The bob weights are free and are able to move when a centrifical force is applied but there is no moveable plate to alter the advance and retard that I remember. I have from time to time checked the rotor button position and is always hard against a tooth on the camshaft. As to how much back lash I do not know for certain as I have never measured it.
If backlash is the problem how do you adjust that?

I will go diving back into the manual to view the distributor to confirm this advance and retard.
As for when does it run rough this starts from above idle and the more revs the more pronounced. Sounds like a miss fire but I have done everything on the electrics that can be done that I can think of. Only thing that has not been replaced is the cap and there is no signs of arcing inside or out but it is worth looking at again. The Solex FV-30 updraft carby is so very simple with only a main jet and an air bleed jet. Both are spotless inside and out with no tiny little holes blocked. I have installed new seals on the mainshaft to stop any vacuum getting in here and have sprayed either at the shaft both sides with no changes in the revs so it is not sucking in through there or anywhere with the air filter hose connected.

Backlash isnt a problem ,in fact skew gears often have a lot of backlash,but you need to allow for it setting the timing......so dont go turning the engine backward when setting the timing .......the backlash is all taken up when the engine turns in the direction it runs.....Sometimes distributors wont return to rest position just on the advance springs,and need a bit of help.Turn the spindle in the opposite direction to running to take up the slack.

OK, so the crankshaft drives the governor idler gear which drives the governor gear; what does the governor control or interfere with?