'79 CB750K backfiring or knocking when revving

[gbwithacb]

Hello again everyone - so just a quick refresher: Got a stock CB750K, it runs but very rough. It had sat for a long time but looked like it had been prepped for long term storage. Backfires all the time, not rideable at all. There is no compression in the third cylinder. Low compression in the other cylindres.

In the final hopes of preventing a total engine rebuild, I measured the valves and they vary quite a lot, even between same cylinders.

1
Exhaust 0.08mm | 0.1mm
Intake 0.04mm | 0.08mm

2
Exhaust 0.04mm | 0.04mm
Intake 0.1mm | 0.08mm

3
Exhaust >0.04mm | 0.04mm
Intake 0.1mm | 0.08mm

4
Exhaust 0.08mm | 0.08mm
Intake 0.08mm | 0.08mm

This is the first time I've ever done anything with valves so I'm not sure how far out they can be. If shims might fix the problem I'll buy them, but if not, I'm ready to let the whole project go at a loss - the DOHC is too complicated for me.

 

[Chris in Dorset England]

calulator

Hello again everyone - so just a quick refresher: Got a stock CB750K, it runs but very rough. It had sat for a long time but looked like it had been prepped for long term storage. Backfires all the time, not rideable at all. There is no compression in the third cylinder. Low compression in the other cylindres.

In the final hopes of preventing a total engine rebuild, I measured the valves and they vary quite a lot, even between same cylinders.

1
Exhaust 0.08mm | 0.1mm
Intake 0.04mm | 0.08mm

2
Exhaust 0.04mm | 0.04mm
Intake 0.1mm | 0.08mm

3
Exhaust >0.04mm | 0.04mm
Intake 0.1mm | 0.08mm

4
Exhaust 0.08mm | 0.08mm
Intake 0.08mm | 0.08mm

This is the first time I've ever done anything with valves so I'm not sure how far out they can be. If shims might fix the problem I'll buy them, but if not, I'm ready to let the whole project go at a loss - the DOHC is too complicated for me.

Hi, All your gaps are too tight. 5 thou is the minimum gap, that is 0.127mm. There are several vids on UTube on how to check and calculate the correct shim thickness, quite clearly you will need to make some adjustments. Take heart, the engine is not complicated, just different, all 4 stroke engines have valves just differing ways of operating them.
Don't buy a whole set of shims! Use this to find which one you will need https://www.bing.com/videos/search?q=calulating+valve+shims+on+dohc+cb750+Honda&&FORM=VDVVXX The one with the hand written formula.

 

[gbwithacb]

Oh, I'm scared to get stuck into it, I just don't want to face the disappointment of doing the shims and still having problems. This bike has fought me every step of the way, it hasn't been pleasant working on her so I'm at my wits end. Still, ONE MORE TASK! Let's do it! Thanks for your reply