hi i have recently rebuilt my old manual choke hitachi carb with new gaskets and accelerator pump because my new auto choke carb was just rubbish!

so know i tried tuning it i can get a smooth idle at 750rpm but its running too rich the plugs go black is seconds the mixture screw is 5 and half turns out anything leaner and the engine start to sputter so i cant lean it out.

also when the revs are held say at 1500rpm you can hear the revs go up and down like high then low but less noticeable at full throttle! if i lean the mixture out it does this at idle as well then will die!

its running so rich that my choke wont function either in the mornings!

stuff i have checked!

1.air leaks
2.distributor points dwell position and timing

also when i put my hand over the carb to see if their is any air leaks the suction is really strong like it sucks your hand in then dies immediately is this much vacuum normal?

sorry for the long post!

Sykine can probably tune it for you using his dyno
if you are not too far?

what plug gap you use? try 0.8mm. if misfire happens, reduce it by 0.1mm until there's no misfire.. also, what heat range? using plug colder than it should be tends to foul the plug itself as it is too cold for the application.


i recommend NGK BP5ES with 0.8mm plug gap.

hi,

yes sykine has offered to tune it for me but he lives quite far from me!

i use standard plugs for a12 engine over the counter so i think heat range is fine i will try gapping it smaller!

also the carby has a spacer in between the manifold i noticed when the spacer sits on the manifold in the middle were (between the two circle ports) the spacer doesn't sit flat so there's a gap!

could this cause this problem? maybe i will try without a spacer just to see?


thanks!

There should be no gaps between spacer and manifold or spacer and carb. Removing it will not help as the linkage will foul on the carb. Did you put the paper gaskets in-between the spacers and the surfaces? Might just mean you need to file the spacers down flush. Then retune them

yes i used gaskets i will try to file it down and see if it helps!

Thanks!

the bakalite 'spacer' you mentioned should never be removed. it acts as the insulator, to prevent heating the carb. try reface it, and readjust. see what happens. maybe you need to find another unit if filing doesnt help.

Had a similar problem a while back. The problem with those little Hitachi carbs is that they are getting old . Being old they have been on an off a few times and stripped downn etc.

What happens is the main body warps as does the air horn.That causes an internal vacuum leak between the body and the air horn. When that happens the power valve remains open at all times and produces the rich running you are experiencing.
Run a straight edge over the two mating surfaces and look for a gap. Make sure you place the straight edge on the ears where the screws are inserted. They are particularly prone to warpage.
What you need to do is strip the body down to the bare body and run the top surface over a surface plate with some 180 wet and dry and some kero . That will bring it back to a flat surface.

Similarly with the air horn you can to the same thing. However this is a bit trickier as you have the lugs for the float and the spring loaded plunger for the power valve in the way . What I did was to make a surface plate out of a piece of 100mm by 8mm plate about 150mm long and made some slots in it so that I could move the air horn back and forth about an inch. I then simply place a piece of similar wet and dry over the plate, punched holes in the wet and dry and then sat the air horn on it, aligning the dangly bits with the holes and started rubbing until it was flat. That should sort it out.

hi thanks fredeuce i just tried tuning the engine and its running rich that the manual choke wont even work just dies and and when you lean the mixture the engine dies because not enough fuel!

is this the same problem you had?? did fixing the warpage help?

thanks!

I am not sure exactly what you are saying in relation to the operation of the choke.

Have you check to see what the fuel level is in the bowl?

Did you remove the jets ? If so , are they in the correct positions ?

In answer to your question about the rich condition being cured by re-surfacing the mating surface, the answer is yes.

I also query if you have the little ball valve in the accelarator pump and the corresponding pump discharge needle both in place? If that check needle is missing then fuel can be drawn at all time through the discharge nozzle.

Have a look at these things and make sure they are all there and doing their job.