Topic: One carb is out of sync, and can´t be adjusted

[Martin Thomassen]

Hi

This is my first post, but I have been reading here for some time now 
A couple of months ago I bought an 1978 CB 750 F2, where the engine has been replaced by a F1. The engine rattles somewhat, even after I have done the basic tune up (adjusted valves, camshaft chain and ignition, so I loaned and original Honda sync gauge in hope that this would cure the distinct rattling noise. I drove the bike for 15 minutes, to insure that the engine had reached its working temperature, checked the idle was around 900-1100 rpm, and then connected the sync gauge to the carburators. Carburator 2-3-4 were all ~22 cm/Hg, but cylinder 1 was way off at about 35 cm/Hg. I then started fiddling with the mixture screw, but the gauge reading did not change when I made adjustments the carburator 1´s mixture screw, not even when the screw was fully closed  (I tried to switch the hoses on the sync gauge, just to rule out that one was bad)
So I guess the carburator suffers from some sort of air leak, but I´m rather puzzled regarding the source, as the insulators has been replaced recently.

I´m sure there are loads of people in here, that has a lot more experience in handling these carburators than me, and might be able to point me in the right direction 

[strynboen]

syncro adjusting  are not gas/air mix
,its the posision of the 4 trottel pistons/ ..to give the same airflow to all cilenders..so to adjust it turn the schrevs to adjust every trottel pistons lift,, to same airflow
sorry for my englisk..

[lucky]

The gauges should read 8-10 millibars of vacuum. NOT 22

You borrowed the gauges not loaned them.

With the vacuum that high, something is off.
The fact that one of the carbs had no reaction to the mixture
screw adjustment means the idle jet on that carb could be clogged up.


BTW....strynboen is correct. Syncing the carbs is adjusting the height of the slides to get the idle smooth. What it is doing is making sure that all four slides are in sync to the vacuum pressure.


Do not adjust the mixture screws when syncing. Leave them alone.
Just make sure they are all the same.

[strynboen]

hvere in danmark are you..i can help..have done this on all my bikes..
lives on langeland/sydfyn...

[Martin Thomassen]

The gauges should read 8-10 millibars of vacuum. NOT 22

You borrowed the gauges not loaned them.

22 cm Hg equals 0,3 bar - I think that is somewhere near what it should be.

And yes I borrowed it 

[Martin Thomassen]

syncro adjusting  are not gas/air mix
,its the posision of the 4 trottel pistons/ ..to give the same airflow to all cilenders..so to adjust it turn the schrevs to adjust every trottel pistons lift,, to same airflow
sorry for my englisk..

Arhh, I guess I got something mixed up 
These are my first carburators since I gave up my last moped 15 years ago 
Will try and see if I can get the sync right tomorrow evening then, but am still puzzles regarding the lack of reaction when fumbling with the mixture screw on carburator 1.

Regarding location I'm from Østjylland, so Fyn is a little far from me, but thanks for the offer 

[mrbreeze]

The adjustment is made by turning the screw on top of the carb. Loosen the lock nut and turn the screw. Don't worry about the number of vac inches. What you are trying to achieve is making all four numbers match. When you get it where you want it.... Tighten the lock nut.

[TwoTired]

The carb vacuum sync adjustment is under the top cap of each carb except #2.   All carbs are adjusted so their vacuum reading matches #2.
 (Assuming the bike has PD style carbs as we have here in the Americas.)

Highly recommended to have the Honda shop manual when doing work on the bike.

[Martin Thomassen]

The carb vacuum sync adjustment is under the top cap of each carb except #2.   All carbs are adjusted so their vacuum reading matches #2.
 (Assuming the bike has PD style carbs as we have here in the Americas.)

Highly recommended to have the Honda shop manual when doing work on the bike.

I actually have the shop manual, but must have misinterpreted the description. How close should the readings be before accepting it as good enough?

[TwoTired]

The carb vacuum sync adjustment is under the top cap of each carb except #2.   All carbs are adjusted so their vacuum reading matches #2.
 (Assuming the bike has PD style carbs as we have here in the Americas.)

Highly recommended to have the Honda shop manual when doing work on the bike.

I actually have the shop manual, but must have misinterpreted the description. How close should the readings be before accepting it as good enough?

The closer to equal, the better.  The shop manual states within 3.0 cmHg.

[strynboen]

you must have a fan/blower to cool the engine..it can only run for few minuts vitaut airflow in the cienders..it  can take some time"s,, to adjust all kaburettors korrekt

[lucky]

you must have a fan/blower to cool the engine..it can only run for few minuts vitaut airflow in the cienders..it  can take some time"s,, to adjust all kaburettors korrekt

Good point STRYNBOEN!
Got to keep those air temps consistant.