Timing a V Twin

I’m hoping someone here can assist me before I go completely mad!

I have built the fairly well known V Twin hoglet, but I’m struggling to set up the timing after trawling google. Some people have said to set the piston at TDC, turn to 38 degrees and set the inlet valve to open, others have said to rotate 90 degrees and set the inlet valve to open.

I have tried all of these methods but I can’t seem to get it started, it will cough and splutter slightly but not to the point where it will run, I have also noted that you can feel air being pushed back out of the carb, which tells me something isn’t right! (It isn’t a great force but enough to be concerning) I’ve used a DTI to set TDC and a degree wheel on the crankshaft for accuracy.

I’m getting plenty of fuel as the plugs are wet and getting a decent spark, it sparks both plugs so it isn’t an issue of mixing up the wrong magnet on the hall sensor. Can anyone shed any light because it’s keeping me awake and distracting me at work.

When you say "set the valve to open" are you setting the intake valve to the fully open position, ie right on the peak of the cam? And are you positioning the crank with the front (righthand) cylinder to TDC then rotating it 90 degrees clockwise?

Also, spark timing is separately timed, usually to spark right at TDC on the compression stroke on the front cylinder to start with. Then advanced a bit by trial and error as it runs.

If all else fails, do what we do on the full size Hog motors and set a degree wheel on the crankshaft and a dial indicator on each cam follower and make a chart of where each valve is opening and closing. This should give you a good picture of what is going on.

Set it as Ozzie46 suggests in this thread and it should work like this one

Thanks for the replies

Yes I had been setting it so that it would be fully open.

When people say on the front cyclinder which one does that refer to? As I’d tried it on the left and the right hand one individually.

There is no "One size fits all"

Often the Inlet Valve timing is about 5 - 10 degrees btdc to start opening, and closed by about 5 - 20 degrees abdc.. Exhaust Valve timing is going to be about 5 - 10 degrees bbdc opening and closing about 5 degrees atdc.

Optimum valve timings will depend upon the Induction and Exhaust systems, and the speed at which the engine is required to run.

Ignition timing is likely to be about 5 - 10 degrees btdc, depending upon compression ratio, and speed required of engine. The timing at which it starts may not be the optimum for high speed running, (and the latter will depend on the applied load, because of ignitition delay within the cylinder, and the in cylinder residual temperature after combustion. For a given speed, combustion temperatures will be higher on full load than on light load.)

The component design and manufacture should be such that having set one cylinder "correctly" the other will function in the same manner. Being a doom monger, with identical settings, this may not mean that both cylinders will sound/perform the same, because the Induction and exhaust systems may cause differences in the mixture distribution.

(Once, I came across a V8 which was down on power and sounded rough. The silencer, common to both banks, had lost a baffle for one bank, so that the back pressures and pulses were different between the banks. Fitting an undamaged silencer solved the problem)

Hope that all this rambling is some help.

Howard

Edited to remove double negative!

Edited By Howard Lewis on 23/01/2018 22:29:44

The right hand cylinder is generally referred to as the "front", a la Harley Davidson orientation. Likewise, check your crank rotation is clockwise when viewed from the cam gear side of the engine.

Sounds like best thing to do is fit a degree wheel to the crank and chart what degrees each valve is beginning to open at, and finally closing at. Should give you a pretty good idea of how close you are. Howard's figures above are a good ballpark starting point.

Once you have the valve timing set somewhere in the ballpark, it may be a matter of then adjusting spark timing. I would start with it at TDC on the compression stroke and keep advancing it a couple degrees at a time if no firing at that setting.

Wow, many thanks for all the information! Certainly learning a lot.

I do believe however I have found the culprit. My dad took the camshaft to work to measure the degrees on the cam lobes and some how 2 of them are out by 10 degrees I have no idea how that has happened as it was cut on a rotary table on the milling machine. So I’m pretty positive this has thrown the timing out massively and is causing all the hassle. I will be making a new one over the next few days and I shall report with the results.

... I have no idea how that has happened as it was cut on a rotary table on the milling machine. ..

The nut holding the handle on the rotary table?

Sounds like you are on track to getting her fired up. Be sure and post a video for us when she fires.

I've been reading up on making the camshaft again, as it's not something I find easy. Originally we did it with an offset cutter holding the workpiece vertically, which produced nice enough cams but obviously somehow got the degree angles wrong.......still remains a mystery so instead i'm looking at doing it horizontally which I think would be a far better option give the equipment I have.

The only downside is you need to use camcalc which doesn't work on any of my web browsers, currently on windows 10 and it won't allow me to install an older version of the web browser. Does anyone have an updated version of this program? Otherwise i'll be tempted to use keyways and position them that way.

No idea about camcalc but if you make the lobes individually you could probably just Loctite them onto the shaft in the appropriate positions without needing keyways.

So I managed to get camcalc to work and I’ve remade the camshaft. Tried to get it running and it’s definitely firing on one cylinder as it gets quite warm after a while but the other doesn’t get as hot so I’m not sure what else could be wrong.

One of the lobes is about 2.3 degrees out, is that really enough to make it not run? I don’t have any experience with IC engines so not sure how accurate it needs to be.

Edited By Neil Bottle on 29/01/2018 18:05:11