Cold cylinders.

[loxley007]

Hi all, I’m having difficulties getting my 1979 CB750 firing on all cylinders - cold pipes on 1,2 and 3 and hot on 4.

I have been working on diagnosing the issues and I feel like I’m getting nowhere fast. Any insight, ideas or wisdom out there??

Context and problems:

- I’ve done a full top and bottom end rebuild
- It’s running stock carbs and airbox (new boots on the carbs)
- I have rebuilt the carbs including removing and cleaning the pressed in pilot jets (they were clogged)
- carb one doesn’t appear to be getting much fuel - when I do the float level test with the clear tube method, carbs 2, 3 and 4 are fine but carb one has no fuel coming out (the overflow tube is clear as I can blow through it) - clearly some sort of issue here.
- I’ve put new HT lines on, spark boots (all tested fine on resistance) and new coils - I get spark in all plugs.
- compression test shows 150, 100, 150, 160 PSI. (some sort of an issue in cylinder 2 - would it be carb related?)
- valves have been checked, new shims fitted and rechecked and all are now within tolerance

Any help/insights/advice? What would be the list of issues to test/work through.

Thanks,

Andrew

 

[kirky]

you might have fine dirt back in your carbs from the petrol tank , 1and 2 getting fuel first , it only takes very small bits to upset the carbs, i know getting the carbs off and on is a pain but maybe try compressed air in the bowl drain holes first.

 

[loxley007]

you might have fine dirt back in your carbs from the petrol tank , 1and 2 getting fuel first , it only takes very small bits to upset the carbs, i know getting the carbs off and on is a pain but maybe try compressed air in the bowl drain holes first.

I’ll give that a shot.

 

[PWBII]

A lot of questions. How are you determining "hot" What temp with an infra-red thermometer maybe is each head pipe? Just hot to touch I wouldn't recommend doing that. What is spark condition & is it good on all 4 cylinders? Timing? If 1 & 4 are cold, it could be coil. Remember 1 coil fires 1 & 4, the other 2 & 3. If top end is rebuilt, I'm thinking it's not broke in yet, that's why compression is all over the place. How does it start? Any smoke out the pipes?

 

[loxley007]

Hi, good questions to my many questions:

- temp taken using infrared thermometer
- spark condition looks ok (each plug has spark)
- timing - good question. I couldn’t figure out how to check the timing on the ‘79.
- when it starts it is fine - no smoke from the pipes

 

[dirtdigger]

see if it fires with some carb clean or squirt a bit of fuel in the carb.

The low compression is an issue. If it was rebuilt correct you should have max compression from the start. With correct machine work there really is no break in. More than likely you have a valve not sealing unless a ring was broke during install. Did you just hone cylinders or bored with new rings? How was the head rebuilt? New guides and valve seats cut? New valves? Honda valve clearance is too tight from factory, need to open up to .005" minimum or you will burn valves.