a (sort of) basic timing question

[interferencefit]

I've been building a 1973 75/5 for the last year and it won't start. I have spark (good blue spark). I have fuel (carbs are adjusted as recommended by BMW for brand-new installation). I have electricity to the starter (it turns over quite well and the battery is new and topped up). It will turn over and after 5-10 seconds it will backfire quite loudly. I figured it is probably timing. I know that the cylinders are at the top of their stroke when OT is centered in the window (I used a piston stop and it is dead-on). It has new heads and rocker arms, as well as camshaft and crankshaft sprockets (the marks were lined up correctly after the chain was under tension), and timing chain. So here is my question:

When setting the valves, I was not entirely sure which cylinder was on its compression (as opposed to exhaust) stroke, as everything was new and loose. I loosened up all of the valve adjuster bolts (backed them all of the way out until they were stopped by the rocker arm) and turned the engine to OT. Then I looked at both heads. Valve clearance on one side was huge, the other side was considerably smaller. My assumption was that the loose side was the one that was on the compression stroke, because the pushrods were all the way in. I assumed this because when the pushrods come out it forces the rocker arms to rotate and the valves to open, so two closed valves would have the pushrods pushed into the crankcase, and would correspond to a lot more valve clearance (rocker arm rotation). Have I done this correctly? I adjusted valve clearance on the loose side to .006" intake and .01" exhaust (new valves as well). Then I rotated the crankshaft 360 degrees and did the same thing to the other side when OT was back in the window. It just feels like the timing is really far off and I wanted to make sure this was right.

As a side note, it has dual plugged heads with green dyna coils and Boyer microdigital ignition (a kit from Rocky Point Cycle). I have power going through the coils (hence the spark) and all of my other electrics work correctly. Confirmation of my valve adjustment decisions would be appreciated. If that was done properly then I'll look at the ignition.

[Deleted User 62]

The way I do it is to turn the engine with the kick starter or alternator bolt and watch the valves on the side you want to adjust. When the intake valve opens, and then closes, that's when it's coming up to OT on the compression stroke.

[Airbear]

G'day Interf.
You are on the right track re the valve timing. I'd set the intakes to 0.004" and the exhausts to 0.006". Since the motor is just going together you know you'll be back in there soon to re-torque the heads and check the clearances.

The backfire is most likely indicating an ignition timing problem. When you get to setting the static timing, I suggest you check both sides - that is, once you get the points plate correctly positioned so your timing light goes out when the S is in the window, rotate the engine through 360 degrees and check again. A slightly bent cam nose can have a big effect on timing synchronisation.

Good luck, and welcome to the forum.

[interferencefit]

The way I do it is to turn the engine with the kick starter or alternator bolt and watch the valves on the side you want to adjust. When the intake valve opens, and then closes, that's when it's coming up to OT on the compression stroke.

Thanks Tim,

That's precisely my problem, I think. The valves don't move unless the camshaft pushes the pushrods to rotate the rocker arms to move the valves. All of these parts (rods, arms, valves) are new and just I put them on the bike. I need to be certain of where my camshaft lobes are on a particular side for the purpose of setting valve clearance. So my question still is, was my method of locating closed valves (pushrods all the way in and maximum rocker arm to valve clearance) the correct one? Cheers.

[Kurt in S.A.]

So my question still is, was my method of locating closed valves (pushrods all the way in and maximum rocker arm to valve clearance) the correct one? Cheers.

Sort of. If the pistons are all the way out at TDC, you have closed valves...sort of. As was stated earlier, you need to watch the valve action. When you see the intake valve open, then close, that side is approaching TDC on the compression stroke...turn until you see the OT mark in the window. You can also remove the plugs and put your finger in the plug hole and turn until you feel pressure on your finger...you're approaching compression at TDC. The other side might look like it has closed valves, but in reality, they will be at TDC on the exhaust stroke and they will be tight and/or partially open.

If you have your rocker adjusters turned all the way out, you won't see the valve action. They need to be set in the ball park so you can watch things move in/out.

Kurt in S.A.

[ME 109]

It's times like these when Me 109's home remedies can may be of help.

Bike on centre stand, rear wheel off the ground.
Transmission in 5th gear.
Spark plugs out.

Stick yer left thumb in the left hand spark plug hole.
Grab the back of yer rear tyre at the 2 o'clock position with yer right hand, and 'knock' the wheel in the forward direction.
'Knocking' the wheel in the forward direction will slowly turn the motor over.
With your thumb stuck in the left hand spark plug hole and knocking the motor forward, you will know when the piston is coming up on the compression stroke because air will be wanting to escape past your thumb.
As soon as you feel/hear air wanting to pass your thumb, keep knocking the wheel around, looking through the timing hole for the OT mark.
When you see the OT mark, you are at TDC on the compression stroke, left hand side.
Basically.

Exhaust .15mm
Inlet .1mm

[ME 109]

I gotta learn to type quicker Kurt.

[Ken in Oklahoma]

Finding TDC by watching the OT mark on the flywheel can sometimes be a bit of trouble. The OT mark flies by pretty quickly when "bumping" the flywheel via the rear wheel with the transmission in gear, or when turning the allen wrench stuck into the alternator rotor bolt. (Of course it helps a lot to have the OT area on the flywheel painted white for visibility.) But sometimes, for whatever reason, I like to feel the top of the piston directly. On those occasions I will go to my box marked "Sticks" (clever eh?) and extract a bamboo BBQ Shish Ka Bob stick. Said stick is stuck into the spark plug hole and I now have a fine TDC locator. (With a bamboo BBQ stick, even if you ham fistedly crack the stick, the bamboo slivers will keep the pieces stuck together.)

I suppose a drinking straw or swizzle stick would work pretty well too.


Ken

[ME 109]

I suppose a drinking straw or swizzle stick would work pretty well too.


Ken

There is a very handy swizzle stick screwed into the engine block Ken. The dip stick.
In the hands of a professional, the dipstick can get you to TDC within .1mm/8 thou.

The dipstick is perfectly counterbalanced for this purpose and will not damage anything.

There is an art to reading the dipstick in this application.

[interferencefit]

Thank you all for the posts. I have no concerns whatsoever about knowing when it is at TDC - I have made sure that the OT mark is exactly correct (checked it with a degree wheel), the only concern was with TDC exhaust or compression when OT rolls around and all the valves are loose.

Kurt - I'll go check the valve movement now that the valves are tightened up somewhere "in the ballpark". I can't believe I didn't do that earlier - too much time thinking about rotation and lobes, I suppose. Many thanks.