Subject: More @#% ZS carbs
> Well this saga continues to drive me up the wall, and now I'm more
confused than ever. Someone please help. I'm not a carb expert, and
everything I've done has been by the book. And at this point I'm at a loss
as to what's going on.
>
> Here's the scenario....I'm using the Haynes carb manual...
> I've got a TR250. When I got it it had matched 3385 ZS carbs with B1AF
adjustable needles. I verified that both bowl jets are fine, throttle shafts
are fine (no leaks there), air valves, dampers and needles are OK. However,
it was running really lean. Adjustment on the needles to a full rich
position didn't seem to do much good based on the valve lift test. It always
started to slow and stall. So, I rechecked for vacuum leaks somewhere and
found nothing except the PCV (EGR?) line that connects to the air cleaner.
When it is plugged...engine speed drops down to a comfortable 1800 RPM.
>
> So, I changed out the needles for a pair of B2Y needles in non-adjustable
air valves and found the following symptoms:
> Once its warmed up, with the slow idle screw backed out of contact with
the throttle cam it idles at about 1800 to 2000 RPM. This with the choke cam
on its closed stop.
>
> In this state, quick goose of the throttle drops RPM to about 1000
immediately but recovers quickly to previous.
> Lift test results in RPM drop dramatically and usually stalls out.
>
> Also, if the choke is pulled when warmed up, the engine balks and dies.
Now I'm really confused.
> What should I look for next?
>
> Anyone ever run into this before? What should I do?
> Thanks everyone for the help!
> Mark
Mark,
With the idle screws backed out, the throttle plates should be closed and
little or no fuel should be reaching the intake maifold and the engine
should stall. Your note suggests you already know that. Two things you
might consider: one or both the bypass valves are defective allowing fuel to
pass or the throttle plates aren't closing tight. To see if it is the
bypass valves, remove the bypass valves one at a time (the three slotted
head screws), and then cover the two holes in the carb body with duct
tape/masking tape, etc and see if the idle speed drops. Second choice is
that something is keeping the throttle plates from closing. You'll have to
pull the carbs to visually inspect the shafts and plates. I believe that if
the plates are installed improperly, they will no go fully closed.
I also have trouble getting a good mixture adjustment by lifting the air
valve. I use it for a first cut only and then check out the color of the
plugs. There is a three part article on rebuilding these carbs on our club
website, link below. Part III gives some suggestions on tuning the carbs.
I'll be updating that part later this evening with photos on how the plugs
should look if the carbs are adjusted properly.
www.nextek.net/BuckeyeTriumphs
There is no use trying to adjust the mixture until you solve the problem
with the idle speed. That problem likely upsets the mixture, exspecially if
the bypass valves are defective.
Nelson Riedel
Granville, OH
76 TR6 (just painted, now repairing damage from fender bender)
68 TR250 (next project - new paint)
70 TR6 (all the parts in a pile ready to assemble)
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