[Trunnion]

Hello folks, I was wondering if anyone on here can help with a problem I have with my 39 5T.

I have just finished a rebuild of what we in the UK call a "basket case", i.e. a bike that arrived in bits (including the engine). I took it out for a first run and it stopped 3 miles later with a valve stuck open. The valve has seized in the guide and I believe the valve to be very slightly bent.

I have racked my brains for a cause. There is good oil flow to the top end. I put a degree disk on it to look at valve timing. I am pretty sure both cams are E1485 as per the 39 parts book but on closer examination of timings I believe I have a mixture of cams. The inlet timings are as per 39 and the exhaust timings are as per 46. According to Roy Bacons book the 39 cams are: Inlet = E1485 Exhaust = E1485 and the 46 cams are 46 cams: Inlet = E2302 Exhaust = E1485. So both 39 and 46 use E1485 as the exhaust part number but the timings seem different, see below.

Taken from Woolriches book the timings for 39 are:

IVO 21 degrees BTDC
IVC 75 degrees ABDC
EVO 75 degrees BBDC
EVC 21 degrees ATDC

and for 46

VO 26.5 degrees BTDC
IVC 69.5 degrees ABDC
EVO 61.5 degrees BBDC
EVC 35.5 degrees ATDC

So could the mismatch have caused a clash of valves? Or are the E1485 cams interchangable? I guess I need a pair of 39 cams or a pair of 46 cams but I am looking for second opinions as I cant see anything else wrong that could have caused the breakdown.

Thanks.

 

[xsteinauge]

If the valves hit on overlap close inspection should reveal a little mark on the margin of the valve.More likely it was a tight new guide and the delightful quality of modern "gasoline" that caused the problem.Allow about .002" stem to guide clearance on the exhaust valves and use 1 ounce per gallon" marvel mystery oil" in the gas.That should cure your problem.

 

[Duncan of Kent]

New guides should really be reamed to the tolerance xsteinauge says. I had the same thing last year on the Wife's T100. It got a bit warm on the IOM, while on a good fast lap of the circuit, and a valve stuck open. This caused the push-rod to jump out of the cam follower etc. After a few mins, when the engine cooled again, BOING, the valve shot back in its seat.

I'm rebuilding it anyway, but I'll ream the valve guides once in. Strangley, it's the first time I've actually had it happen, rather than just hearing about it.

 

[Mr.Pete]

They only changed the cam timing because they worked out it was lousy cam timing in the first place.Personally,I reckon they should have advanced the intake timing another 10 degrees more than they did.There's not much chance of intake and exhaust valves making contact with those timings.
If you have iron guides,you might get away with 0.001" intake clearance,0.0015" exhaust guide-to-stem clearance.You'll need more if the guides are bronze,about 0.002"-0.0025".

 

[fouraceswes]

My money is on a bad valve job, i.e. improper tolerance valve and guide. However, anytime one messes with cam timing or puts different cams in a motor, one should degree the cams then clay the pistons. If a valve hits a piston it can stick it in the guide. Although there can be "options" when timing cams, there is usually only one clear choice as to the optimum timing.

 

[Trunnion]

Thanks for the input guys.

The valve job was done for me by a local engineer who has a good reputation. However I will be replacing all the guides myself as a precaution and I do have a hand reamer of the correct size so I will make sure they are done properly.

I spent some time early today with a dial gauge and degree disk and the inlet is as stock but the exhaust seems to be closer to the 46 spec but still about 10 degrees out at either end of its cycle, i.e. its open 20 degrees longer than it should be. I did replace the timing gears because the ones that came with it had been messed with and had multiple timing marks so it could be that someone has re profiled the cam at some point in the past. I have ordered replacement 1485 cam so I will fit that when it arrives.

I will post an update when its back together and put some pictures up of the build from box of bits to complete bike.

Thanks once again.