Topic: Spark Advancer Mystery

[starf]

Hello all,

I have a mystery I'm hoping you can help with. I have a CB750 K7 which had a "333" spark advancer fitted. Why would a "333" be on a CB750? Would they (could they) have been on from the factory?

I did a little reading and found that this is probably not the correct unit for the K7 and bought a (remanufactured) "300". I just compared the TDC marks on both units and they don't match. They are a few degrees off at the "T" mark and a lot of degrees off at the "F". (F on the 333 is halfway advanced on the 300)

Photos of TDC attached. I found TDC using the "300" and then swapped to the "333" without touching anything else.

I have to assume that the "333" advancer was fitted when the top-end was rebuilt - does this mean that the timing chain is potentially off by a link? I was hoping not to have to pull the engine, but I will if I have to... Is there a way to verify which is the "correct" TDC mark for the engine - i.e. find exact TDC without using the advancer mark?

Thanks in advance ( )!

[Don R]

 Fortunately, it has nothing to do with the cam chain. Your marks are directly on the crankshaft. That's a significant difference isn't it? 
 I might be inclined to find top dead center with a piston stop.
 Here is some information,
http://forums.sohc4.net/index.php?topic=113338.0

[jaytee-nz]

I believe "333" is the Honda model code for a CB350 four. That would explain the difference.

[starf]

Don R; thanks for the link. Certainly a complicated topic. My concern is that the cam was set using the wrong TDC mark when the chain was installed. I’ll see if I can check with a piston stop before pulling the engine

Jaytee-nz: it’s a mystery how it ended up on the bike. Maybe a PO had it laying around..

[pekingduck]

My recollection from '70s racer boy tuning was that using the 333 advance (from the 350 and 400 fours) would slightly delay the advance curve due to slightly stiffer advance springs than 300 or 323 advances.  Also, the 333 points were used because they had slighly stiffer springs to prevent point bounce.   

[bryanj]

333 points are nodifferent to 300 points ad thedifferent advancer is probably all a po could get, when i checked all the tdc and f marks are the same but advance may differ

[Oddjob]

It's possible the PO timed it with a 300 advance unit then swapped to the 333 advance unit for whatever reason.

[starf]

when i checked all the tdc and f marks are the same but advance may differ

The T and F marks are different in the photos attached to the original post, but your comment and this thread: http://forums.sohc4.net/index.php?topic=169207.0 make me wonder if they are supposed to be the same.

[bryanj]

If you compare the T mark to the "peg" on the points cam they apear to be the same, the F on the 333 is earlier so earlier spark , possibly due to top revs and capacity

[starf]

If you compare the T mark to the "peg" on the points cam they apear to be the same, the F on the 333 is earlier so earlier spark , possibly due to top revs and capacity

Thanks. I'll have a closer inspection later, but the pictures were taken by lining up the T mark on the 300, then swapping to the 333 (with the crank in the same place). Even allowing for a little slop between the peg and hole, the T mark is in a different place. Surely that means that the peg vs mark position is not the same?

[rotortiller]

Surely that means that the peg vs mark position is not the same?

I'd say from what you have measured/pictured and stated that seems factual. Get to TDC through the plug hole with a stick or DIAL test indicator and see if the specified advancer shows the correct indexing as someone suggested. You could use a degree wheel to see that the cam is following correctly by removing valve lash and observation of  valve movement if you like. I thought the peg position was the same on all SOHC4s for TDC but you have proved different. Good that you caught a potential issue.  I would not be pulling a pig engine until I was sure it was snafu'd.

[scottly]

Your "manufactured" unit is bogus! Compare the spacing of the T and F marks with my original K7.

[rotortiller]

Your "manufactured" unit is bogus! Compare the spacing of the T and F marks with my original K7.

Still has to be sorted out unfortunately. Good to see so much kind assistance.

[starf]

Your "manufactured" unit is bogus! Compare the spacing of the T and F marks with my original K7.

That is great info, thank you. I'm hoping that is the case and that an original 300 "T" will line up with the 333 "T" I have. I've ordered a used K7 300 unit from eBay - once it arrives I'll hopefully know for sure.

[scottly]

Where did you buy the bogus advancer from?

[starf]

Small update: I bought a used TEC 300 advancer unit. The "T" lines up exactly with the "T" from the 333 unit I have, and the repro unit is off by a few degrees as the photos in the original post show.

I fitted the used 300 and re-set the static timing. Not only was the 1-4 timing off (as expected), but also the 2-3 timing in relation - meaning that the repro unit had the angle between 1-4 and 2-3 different than the used unit.

Lesson learned. A slightly crusty used part may be more accurate than a shiny new part!

scottly: The repro unit was from vintagecb750.com.

[BenelliSEI]

Fascinating….. Ihad a similar problem when I was doing the static timing on my K6 project. I got frustrated and tried an old unit and problem solved! Thanks for the explanation.

[bryanj]

So my agd brain was correct and all will fit just different advance curves

[HondaMan]

The repro unit was from vintagecb750.com.

You might want to let VintageCB750 know about it: they are usually faithful folks who will fix their inventory, at the least. They might refund your purchase, too. I know PartsNmore is good about this stuff and glad when you help them find the bogus parts.

[scottly]

The timing marks are very close to the same between the 333 and 300 advancers. Also, the pin lines up with the T mark.