Topic: 1977 hondamatic almost back on road!

[Ratmatic77]

New to the forum, and new to working on a cb750a. I have been building a cb750a for a few weeks now. I almost have it all back together anddddd..... No spark. I have 12 at the kill switch. 12 to the b/w wire that feeds the coils. Continuity of primary ignition circuit. No continuity of secondary. I can't see both my coils being bad tho... Any suggestions of where to look?

[raymond10078]

Do you have power at the points?

Otherwise - how are you measuring - I'd remove the spark plug caps from the wires (they just screw on) and measure from there.  You have to measure from cylinder/wire 1 to 4 for one coil and cylinder/wire 2 to 3 for the second coil.

Spark plug caps will degrade over time.  They should measure about 5k ohms through the cap.

Spark plug wire ends degrade also.  Most will cut off about 1/4 to 3/8" (to get to clean copper) and re-install the caps.

[Ratmatic77]

I have a DVM

I took the caps off and tested between 1 an4 and 2 and 3. Nada.

How do I test to see if there is power at the points?

[raymond10078]

Well - if you have "nada" through the coils - then they're dead.  BUT - is your DVM a multi-range or an auto-range?  The secondary resistance of the coils should be thousands/tens of thousands of ohms - so if you are on the wrong scale, it will just shown on open circuit.  Also - just to be sure - for example - are you connecting the test leads to wire 1 and wire 4 - measuring through the coil (i.e., these are not, for example, wire 1 to ground tests).

As far as the points - take off the right cover on the engine.  When the points are open, should measure 12 V (to ground) at the connected wire/post.  When closed, the voltage should be zero (to ground).

Don't leave the key on and the kill switch on for too long - it may burn up the points.

[jagans]

The Honda kill switches are notorious for developing copper oxide that shunts to ground. Im pretty sure there is a black with white stripe wire near the left coil that attaches to ground. I think this is from the kill switch. (Its been a while, so forgive me if its black with yellow stripe) try disconnecting this and see if you get spark. If you do, you have corrosion in your kill switch shorting out your ignition.

[raymond10078]

The Honda kill switches are notorious for developing copper oxide that shunts to ground. Im pretty sure there is a black with white stripe wire near the left coil that attaches to ground. I think this is from the kill switch. (Its been a while, so forgive me if its black with yellow stripe) try disconnecting this and see if you get spark. If you do, you have corrosion in your kill switch shorting out your ignition.

The black/white wire at the coil is the power source for the coil.  He's already measured (and found) the expected 12v on this wire - so the kill switch is OK. 

[Ratmatic77]

I got it figured out. Got it to kick on. Once I got the wiring right for stock I threw away the points an got electric ignition. Idr what kind right now. The one they sell on cb750supply.

Everything works perfect now. No shorts or power losses due to bad grounds or connections, But! One of the brass fuel adjustment screw's tip broke off. I want to get rebuild kids for all my carbs (not hard) I just can't find the screw thing. Everywhere is sold out.

Any advice?

[raymond10078]

If you are talking about the idle mixture screw, then generally you'd have to get one from a used carb (e.g., ebay).

The part is common to the following bikes:

1976-1978 CB750A
1977 CB550K

I see there is one on ebay right now (not mine):

http://www.ebay.com/itm/1978-78-CB750A-HONDAMATIC-CARB-CARBURETOR-AIR-FUEL-NEEDLE-JET-SCREW-SPRING-SET-A-/330946969079?pt=Motorcycles_Parts_Accessories&hash=item4d0df9fdf7&vxp=mtr