B33 Timing Specs

[stevec27]

I have a 1951 B33 500cc plunger bike. The engine (500cc) is a long rod model with 7:1 piston. I believe it is listed at 6.8:1.
I'm looking for the timing specs. Should it be 3/8" or 7/16" btdc? I have found in different publications where they list both specs.

Also the cams I have have the following p/n on the cam gears.
EX--65-2436
IN--65-2438
Are these the correct cams for this model as the parts info I have found list something different.
Thanks in advance for the help.

Steve

[neil1964]

The cams you have are Goldstar from an early 50s ZB34.
In a ZB with alloy top end the clearances are 1 thou.  In your engine I would stick with 3 thou as per B33 spec.
Is yours a loose or tight wire advance/retard lever?
Ignition timing with modern fuels is more of a guide and the real answer is does it start and run properly.  The difference between 7/16" and 3/8" is
I would set it at 3/8" btdc at full advance with 12 thou points gap and 18 thou plug gap.
It should run quite nicely like that even with those cams (I have a pair of NOS ones of those in my spares box ready for my ZB.

[stevec27]

Neil,
Thanks for the reply.
For the advance I have to pull the lever so I guess it is a tight wire.
I use the 0.003" for valve clearance.
The bike has started easy (usually first kick) and has run great.
The problem I'm having is that I have scuffed the piston twice and seized the exhaust valve in the guide once. The first scuff was after 40+ miles, the second was after 21 miles. The seized valve was about 2 miles.
I had the timing at 3/8" btdc. I have had a terrible time trying to set the timing. It will always move when tightening the mag gear nut.
DO you know how much variation you can get away with?
Kind of at a dead end trying to figure this out.
I was thinking of trying 7/16" to see what happenings.
I think there is something going on inside the mag that is changing the timing.
Thanks for your help.

[idie]

If you are having problems with the timing changing as you tighten the nut check that the gear pinion is not cracked. I have had many over the years. The taper may be bottoming out either due to wear on the mag taper or in the gear. A good trick to get you out of trouble, use a spring washer under the nut.You should be able to tap the gear onto the taper. Not if it is cracked though.

[stevec27]

idie,
Thanks for the reply.
I have checked the gear pinion and it is not cracked. I have even tried a new gear but still had the same issue. Also have lapped both gears to the mag shaft and it is a very tight fit. I ordered the spring washer from the parts book just to make sure I had the correct one. I have checked to make sure the gear is not bottomed out on the mag shaft. Even when tapping the gear still have the issue.

Thanks again.

[Pete C]

I feel for you! I've had the same problem for ever and tried all the suggested remedies. It seems to be a case of luck whether it slips during tightening or not and how much. Sometimes it has worked frst time and other times I have been at it for an hour or 2.
I know this doesn't help with theproblem but you are definitely not alone.
Good luck!

[Dan_B33]

Hello,
I have been stripping my mygdyno recently.
After putting it bak together I was no table tap in gear. It was always slipping.

Turns out I have put my bearing in magneto too far from slip ring 0.5mm can make difference. That was my case. The symptoms are the same.

Your magneto shaft might not be in the timing chest far enough.

How to check?

Just loose the magdyno metal strap lightly to see if magneto moves when you press in the shaft.

Push the gear on to the shaft and if magneto stays in the place means there is no mention  and it won't be tapable.


You need to figure out why it is not in the timing  chest deep enough.


That was my experiences of problem solved just yesterday.

dan

[Dan_B33]

In addition check the nut wit corral. The one pushing the star towards clutch gear inside magneto.

If too far out you have no chance to get the grip on pinion

[stevec27]

Thanks for the replies

[Bigsingle]

I had to increase the hole size in my rocker feed bolt to 1.7mm , I also inserted a 1.25mm copper wire into the oil tank discharge return to slightly restrict and increase flow to rockers and valve . Cheers