Ariel missfire

[craignation]

my Ariel 500 all iron @ 1949 starts to missfire when it gets hot  , or about two miles from home , this problem has me scraching my head . things i have done , repaced mag twice , re laped valves , fuel cap breather is clear , reground head to barrel , changed carb and cheaked fuel surply . chq compresion . changed plugs at least 10 times , anyone see what i have missed here or had a simalar problem , any ideas welcome , Ive never had a problem like this that seams to go against all i know ( not enough ) and im sure be something simple but what , If it makes and odds im running a Renolds cam and concentric carb

[JFerg]

Try changing the HT lead and the spark plug cap.
JFerg

[craignation]

Try changing the HT lead and the spark plug cap.
JFerg

changed the HT twice and have no cap on now as i thought this could be problem , thankyou any other ideas

[Goldy]

Something that can cause missfire when hot is heat transfer from the cylinder head to the carb causing the carb body to get warm.It is eliminated by fitting a thick fibre spacer between the carb and head. Used to be made from tufnol but now possibly some sort of insulation or fibre board. All the best. Goldy

[craignation]

tufnol spacer about half inch fitted , i was told about heat transfer from iron heads , thankyou any other ideas welcome

[33d6]

This has a horribly familiar ring about it.
In my case it was a sticky exhaust valve that would not quite shut after 10 minutes or so running. When the engine stopped it would cool a fraction whereupon the valve would shut so everything was fine again. It took weeks of slowly eliminating all other possibilities before I found it.
Your case is a little different. You say you have reground the valves since the problem started so the chance of a sticky valve stem is small BUT if you have replaced the valves and valve stems the  newstem/guide clearance may just be a little too tight and sluggish in action when the engine warms up.
Also you don't mention the rockers. They can work well when cold but be be just a little gummy when hot. All it takes is just a little bit of hesitation when the engine is hot so the valve doesn't seat completely and power is out the window.
Finally there is the pressure regulator. Not only is this used to adjust the pressure shown on the oil gauge it also varies the amount of oil supplied to the rocker system. Are your problems caused by too little oil getting upstairs? In my case it was too much. From your description it doesn't appear to be too much in your case but is it getting enough?
Hope the above has given you some clues. Let us know how you get on.
Cheers,

[craignation]

you may have something here as all valves and guides are new , i will try some spare rocker boxes i have also , this bike is a bitsa and is made from Ariel ridgid VB rolling chassis with RH engine fitted , there is no regulator fitted , oil to top end is via a branch off the return from pump after spin on filter , it goes up to inlet rocker then over to exhurst where there is another branch which goes down to a pipe which drops oil straight on top of the cam ( there used to be another T here which went round and fead oil into the rear of the cylinder which i did away with during the rebuild ) , this might be asking to much of this surply , but the engine has some local race history where it performed very well prior to me rebuilding it , the tolarances now may be just to tight , i have also put a standard piston back in as it looks like it was running on Methonol or dope , thankyou again for your help and it has given me some new ideas any more are welcome ( apart from give up ) ,

[33d6]

Hi again,
You possibly have two problems.
1  Slightly too little valve stem/guide clearance
2  The upstairs lubrication pipe layout.
An easy way to check the valve stem/guide clearance issue is to buy yourself an aerosol of CRC silicone spray. Take the exhaust rocker box off and spray silicone down the valve guide and stem. Reassemble and see if things improve. I suspect the exhaust valve as you say it just runs out of power. If it was the inlet you tend to get some spitback through the carb before the engine stops.
The upstairs lube pipe layout you describe is a pretty standard Ariel race layout. It is fine for racing which is in short bursts but not so good for everyday road use. The standard layout goes to the exhaust side first for good reason. It needs more oil than the inlet. The incoming fuel cools the inlet and provides a tiny bit of lubrication. The exhaust always runs much hotter and needs more. I suspect your inlet is getting the lions share of oil and the exhaust side is struggling. Change back to the standard factory layout.
We'll talk about how you regulate the supply to upstairs another time.
Cheers,

[craignation]

Change back to the standard factory layout ......... the timming chest has no oil union ( must be off VB ) so standard is out for now  , but i will feed the exhurst rocker first as i think ( hope ) you may be on to somethink there , thankyou for your help , any other surgestions welcome

[33d6]

I've been giving your OHV lubrication setup some further thought.
Like water, oil always flows the easiest way it can. Because your oil line has been modified and continues past the rocker gear to go down to the cam gear I think most of the oil flows straight past the rocker gear and continues on to the cam.  The oil has to either push hard to get through the rocker gear and out to the valves or it can take the easy path and just flow straight on to the cam. I think most is flowing straight on.
Until you get the matter resolved and get a timing cover with the proper oil line connection I would block off the extra feed to the cam.
I like the extra feed to the cam, it certainly adds oil to where it is needed. My preference would be to revert to the original factory standard OHV lube set up for your engine but also keep the current return side oil line arrangement and run it direct to the cam. That way you get the best of both setups.
In their final few years of production Ariel changed the OHV lube set up from being a positive feed from the timing cover to a return line set up so you could have a late model timing cover. By that time very few new Ariel Singles were coming out to Australia so I have no experience of them.

[craignation]

I've been giving your OHV lubrication setup some further thought" thankyou , so have i and what you say makes good sence and i will be altering the setup taking your advice . Also i now have a remote filtter , car spin on type and the T off for the rockers is After the oil has passes trough it on its way to the oil tank , do you think moving the T to before the oil gets to the filter might help deliver more oil to the rockers , Im thinking there might be more resitance before the filter and this may force more up to the rocker ? all help is welcome thankyou PS when i work out how to upload pics i will post some of the bike , its not standard but i like it .

[33d6]

I confess I'm not a fan of aftermarket oil filters. I believe nowadays most old singles don't do the mileage to warrant them and regular oil changes are more than adequate.
I believe far more in air filters and think they are far more important. In the old days air filters (if fitted) were the first thing to go so as to 'improve' breathing. No wonder our engines wore out comparitively quickly and got full of muck. A modern K&N airfilter mightn't look in period but they do a terrific job.
As for your T piece question, I don't really think it matters too much. There is barely any oil pressure to speak of in any roller bearing engine, so the difference in pressure before and after the filter is pretty small.
It's been a very interesting discussion so far, I look forward to a few pictures.
Cheers, 

[craignation]

There is barely any oil pressure to speak of in any roller bearing engine,,,,,,,,,,,,I agree but this plunger pump will deliver very high presure dependant on what its working against , my thinking is by offering a resistance to the return i may get more oil up top , i think this will be a trial afair as to much will cause another problem and one is enough for now . thanks again for your help