TR6 General Inquiries.....Camshaft and valve clearances

[Erich72TR6]

Hello all,

I've had an expensive few months rectifying issues that are the continuing consequences of an unprofessional engine build by the former outfit PRI/MotoCancelli. The lynchpin was a decked block and shaved head resulting in high conpression and wrecked valve train components.

I have purchased an early head as a replacement, and am having the valves installed and it cleaned up. I decided not to have any material removed as it will have been made for a higher CR than the late model heads anyway. I am going for reliablility, flexability more than anything else from here on out.

Something I've not been able to find out is what camshaft exactly is in my motor. The 'mechanic' is no help as in unable to contact him, and there aren't any markings on the front face; I removed the timing cover and sprocket to check. It is not a stock cam as there is a slight lope, and I was instructed that the the valve clearances should be set at .014 and .016, intake and exhaust. However, I don't think it's out of line to doubt the accuracy of this information, given the other **** I've had to undo. I don't want to get the new head on, install the rocker assemblies and have them wear unduly because the clearances aren't quite right. Does anyone have any advice regarding the 'unknown cam?' Maybe a method of setting the proper lash without knowing the specs exactly? At this point, the head is off and I COULD remove the cam...bit the wife is starting to catch on and watch my spending. I don't want to buy another cam if I don have to.

 

[TR3driver]

I think the next step would be to plot out the cam profile (measure lift every few degrees) and try to match that to one of the commonly sold profiles. There aren't very many of them. Best summary I know of is at
https://t2000.kvaleberg.org/t_tune.html#camshaft
Or just map out the ramp area and make sure the lash you choose falls on the ramp. See https://www.tildentechnologies.com/Cams/CamDesign.html for more info on ramps and lash.

Although there are alternatives, my suggestion would be to buy a dial indicator with magnetic base, and make or buy a degree wheel. Lots of web sites have info on how to degree a cam, here's one https://www.lunatipower.com/Tech/Cams/HowToDegreeACam.aspx

OTOH, there is some argument that you are over-thinking this. A slight lope at stock idle isn't bad at all, and you can turn the idle up to reduce the lope. I drove a 3/4 race cam on the street for a few years and it wasn't too bad. It wouldn't even run below about 1200 rpm, so I set the idle around 1500 and just drove it. Idling is no fun anyway, no matter how smooth it is

The main thing is to check and double-check that you have enough travel in the springs to accommodate the cam lift. Coil bind is likely the cause of your wrecked valve train components. Also don't get carried away with seat pressure if you want the engine to live a long time.

Also, any time you mess with engine breathing, you're also going to affect mixture. The stock "emissions" carbs don't give you many options for changing the mixture profile, but there are alternatives (including using different carbs).

 

[Erich72TR6]

Thanks, Randall, I think those pages you've linked will be good resources.

I may indeed be overthinking some of the aspects of this, but I've got my first kid on the way and want to close up this lump and have regular maintenance on the horizon for the next 50k miles if I can!

The valve springs that were installed as part of the build which I am transferring to the new head are double 'updated' springs, marked with green paint. I was told that the cam profile made them necessary, as the stock springs produced binding as you caution against.

I spoke at length with Richard Good, who was very helpful in coming to some important conclusions. I had originally suspected his GP2 cam was the one installed in my motor, but his sales records said otherwise. His determination is that the head being shaved, plus more Spring tension, plus the decked block, plus the cam profile would almost certainly have necessitated shorter pushrods...which they were not. The result was the adjustment screws on the rockers almost being bottomed out and wear evident on the pads, and both ends of pushrods after only 1,500 miles or so.

So, I suppose that I'll use my dial gauge to measure the lobes on the intake and exhaust and before I have the head on and compare them to some other mid range bump sticks out there. I was told by a local enthusiast that I can't really go wrong setting the lash between .014-018, unless it's a very aggressive race cam, which I doubt.

The lope at idle is pleasant and the motor stayed smooth all the way down to 800rpm, as well as pulled away smooth from a stop. I just couldn't ever eliminate predetonating and overheating.

 

[AltaKnight]

I put in a TH5 cam sourced from BPNW about 10 years ago along with a 0.100 head shave and shorter pushrods, the spec sheet for that cam called for the same .014 inlet and .018 exhaust you are referencing. My idle is lumpy/lopey at 800 rpm so I have the idle set up to 1100rpm.