Topic: exhaust 2-3 remains cold engine runs bad

[JP1]

Hello, CB 750 F2 stored 15 years. I cleared cabs, sparks work, when running the engine with start pilot, engine runs not very good: exhaust 2-3 remain colder compared with 1-4. I changed bobine 1-4 with 2-3, same result, so bobine is ok. Timing of 1-4 is exact good, 2-3 is a little bit too late, say about 2-3 mm compared with the "F"  I guess this is not a problem, I guess the cabs are also OK, somebody has a suggestion how to proceed or what could be the problem?

Regards Johan

[Scott S]

 What do you mean by "bobine"?

 You swapped the coil wires? 1&4 for 2&3? Did the problem follow the wires?

 If you open the drain bowl on the carbs, do they have fuel in them?

 What do the plugs look like on 2&3?  Pull the 2 or 3 plug  and ground it on the head and see if it's sparking.

[JP1]

Ahh, sorry, bobine = coil, and yes I swapped the coil wires, all drain bowl have fuel, the sparks, or plugs are black and wet

[Scott S]

 Clean the fouled plugs. Lay a plug on the head and try it....do you get spark now?
 What happened when you swapped coil wires? Did it follow the wires or 2/3 still not firing?

[JP1]

I just change the wires to check coil 2-3. I also exchange sparks 1-4 with 2-3. But they all fire!, but still 2-3 remain cold.

[mystic_1]

The magic formula is gas plus compression plus spark, so if you have spark on all four and gas in all four, then check the valve clearances next I would think.

Also, just because you have gas in all four cylinders, doesn't mean that the carbs are adjusted quite right.  Bench sync them if you can, then try again.

Now, having said that, a problem with 2+3 tends to indicate spark, because those cylinders share a coil.  You said that 2+3 fire a bit late, 2mm-3mm past the mark, are you unable to adjust the 2+3 subplate to bring this into spec?

mystic_1

[evanphi]

Are you getting adequate fuel to 2/3? You may be getting SOME but not enough.

Compression results?

[JP1]

Break contact 2-3 are on the end of adjusting, so I have to re-ajust the whole plate 1-4 and 2-3 together, after this I think it should be possible to tune 2-3 on time, but do you think this is so critical?

@ mystic_1 That's also a good question how can I check thus?

[JP1]

I mean @ evanphi How can I check if carbs 2and 3 are ok? I still cleaned them all.......

[evanphi]

I mean @ evanphi How can I check if carbs 2and 3 are ok? I still cleaned them all.......

Have you done a clear tube float check? Your 2/3 could be overflowing causing overly rich wet plugs.

[Phinn]

One bad condenser? A weak spark combined with borderline timing or mixture issue could contribute to missing/absent combustion.

[mystic_1]

Break contact 2-3 are on the end of adjusting, so I have to re-ajust the whole plate 1-4 and 2-3 together, after this I think it should be possible to tune 2-3 on time, but do you think this is so critical?

Maybe, maybe not, but it could certainly be contributing, especially if combined with other factors.  That's why I asked if you thought you'd be able to adjust it. 

Running out of room on the adjustment slots could possible indicate a problem with the point (worn or dirty or just a poor quality part) but it can also be related to the point plate being slightly off-center in the cranckcase bosses, if you search around here some you will find threads where people discuss using shims to center the plate during timing.

If you're only a couple of degrees off then the bike should run reasonably well though, so no i don't necessarily think this is the only issue.

mystic_1

[JP1]

Thanks for advice! I will start with the clear tube float check!

Best regards, Johan

[JP1]

Hello again, does anybody know the coarse setting of the 4 setscrews at the airinlet of the carbs. in the attached files?
Also carb. no 3 remains overflow at the pipes shown in picture1190962. I think all floats are adjusted, but the level in the transparent tube remains high, but when I remove the floatchamber 3, the level of gasoline is less compared with the other three??

Any ideas/suggestions?

Thanks in advance