Tuning problem on 1980 CB750F brat racer

[norzilla]

Hi All

I have a freshly built 1980 CB750f Brat racer with CV carbs, pod filters and a stainless 4-2 exhaust and reverse megaphone cans.

I have rejetted the carbs, sync the carbs and am now experiencing some odd behaviour.

I am getting strong power from cyl 1, reasonable power from cyl2 poor power from 3 & 4 at idle the cyl 1& 2 exhaust temp near the head is about 200deg C and on 3 it is about 60 deg C and 4 is about 80 deg C.

Now the carbs are perfectly snycd until a mercury style nanometer with a stable reading within 1inHg.

Adjusting the idle screws really only has a marked effect on cyl1 and a lesser effect on cyl 2 and no effect of 3&4

Where should I start looking for the problem, it is a carb issue or a coils/ignition issue?

[seestheday]

First off.  Your bike looks bad ass.  Good job man.  Wow.

Now your question - have you run a compression test?  Any issues with cylinder 3 & 4?  I'd start there before looking into timing etc.

[robs750]

[QUOTE]I am getting strong power from cyl 1, reasonable power from cyl2 poor power from 3 & 4 at idle the cyl 1& 2 exhaust temp near the head is about 200deg C and on 3 it is about 60 deg C and 4 is about 80 deg C. [QUOTE]

Sounds like no spark at #3,4 or no fuel.
The easiest thing to do first is to verify spark at the plugs.If there is no spark,I would think the plugs would be wet from fuel.

If there is spark,I would check the compression.If the compression is good,then I would be looking at the carbs.
If the compression is low/uneven,check the valve clearance.

[TOOLS1]

It sounds like an ignition problem, but a compression test will give more diagnostic info.
TOOLS

[alehman]

I'm sorry I can't offer any good advice for your problem, but where did you get your exhaust?

[rrgunslinger]

Also check the vacmue around and between the carbs. 

On Thu, Jun 28, 2012 at 6:34 PM, alehman [via Honda CB750'S] <[hidden email]> wrote:

I'm sorry I can't offer any good advice for your problem, but where did you get your exhaust?

1979 CB750 10th Anniversary Edition

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[norzilla]

First,

Thanks to all the replies to my post :-)

2nd as a culmination of the ideas in the post and some other stuff here is what I know now.

- I pulled the carbs down again to reclean and found that the idle jet hole in 3 was blocked and in 4 for partially blocked. the blockage in 3 was from a perished O ring around the idle mixture screw, so replaced the O ring in all idle screws and clean or idle jet holes.  Now I am getting fuel to the engine at idle.

- I tested the coils, leads and plugs , all tested ok, but just in case bought a new coil lead and plug set an swapped out the old one, no difference in performance

- float levels are ok and of the non adjustable plastic float type.

- I drowned the exteriors of the carbs in WD40 and found no change in the RPM so assume no leaks?

- I have not done a compression test yet

- I fitted a colour tune kit and looked and the flame while re jetting and have found some curious results. starting with prim jets  at #1 :70  #2 :75  #3 :75 #4 :77.5  

The bike is now idling pretty well, but still a little lean on 3 & 4 , I have ordered some more jet to go up one more size on the 3 & 4 , hopefully will try this today.

As a side note , I have actually just built 2 of these Honda brats the other has a 4-4 stainless system with 4 reverse megaphones and pod filters and this bike also exhibits the same idle charateristic/problems, these to have been largely solved with similar idle jetting sizes

NB I made the exhaust system inhouse out of stainless and had it polished, it is the same configuration as the OEM system.  The other bike I also made a stainless 4-4 then wrapped with black pipe wrap  photos to come