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Old 15 Mar 2006, 05:53 am
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Default Does your carburated Honda run rough? Lack acceleration? Smog/NOx trouble? READ THIS FIX

Does your 86-89 carburated honda hesitate, falter from a stop, sputter
on acceleration, and/or surge at cruise? I bet 99% of 86-89 Honda carb
owners have this undocumented problem. Likely other years too. Here
is the simple solution to an apparently widely unknown problem.

My quest started with a failed california smog test (NOx too high;
"gross polluter"). Over time, I tried many things to diagnose and fix
the problem. I replaced the catalytic converter, but that made little
difference. I replaced various old vacuum hoses, rebuilt the
carburator, replaced the carb gasket, replaced the water pump, and
flushed the cooling system, among other things--but altogether that
made little difference. I didn't find the problem until I
systematically started checking all the vacuum diaphragms. The leaking
diaphragm I found wouldn't seem like a big deal, but it was in fact a
major problem.

Do you know what a bowl vent valve is? I bet you don't, and I bet your
mechanic doesn't either. (I paid $400 for a carb rebuild job that
didn't touch this valve). It's not a real useful part, but its failure
causes crucial problems with the car. I bet that yours has failed, and
you don't even know it. If you fix it, your car will run MUCH better.
If you're like me, your car lost 40% of its power (!) and you didn't
even notice because it happened slowly.

The carburator bowl vent valve sits on the right front of the carb
(near the top), as you face it from the bumper. Its function is to
remain open when the engine is running and slam closed when you turn it
off. It is normally closed, meaning that with no vacuum, a spring
holds the valve closed. The purpose of the valve is let the vapors be
sucked off the top of the bowl when you shut the car off, and to
prevent venting to atmosphere from the bowl when the car is off. The
problem is that every night some vapor leaks toward the diaphragm, plus
splashes when you drive, and while the engine is running, the gooey
diaphragm is constantly being sucked on to hold the valve open. So
over the years, this diaphragm is guaranteed to fail, with big

Here's why the failure is so impactful.
1. The leaking diaphagm works to starve the engine of fuel! The vent
to atmosphere on the bowl is very small, so the vacuum leaking in from
line 8 causes negative pressure in the bowl. This reduces fuel flow
under all conditions (!), because the metering at the jets is
calibrated to the differential between venturi pressure and expected
atmospheric pressure (as felt at the bowl). With vacuum on the bowl,
the differential and thus the fuel flow is much less.
2. Secondary carb barrel stuck closed! The hole reduces vacuum on line
8, so manifold pressure is increased on those lines that precede line
8. Line 8 also runs to the vacuum-operated secondary. As long as line
8 leaks, your secondary will open slightly and rarely, if ever. Major
loss of power.
3. Retarded timing. Line 8 is T'd off of line 2 (inside the smog box),
which goes to the distributor vacuum advance. The reduced vac on line
8 retards timing, forcing you to over-advance the timing in order to
correct base timing. This over-advanced timing creates more NOx.
4. Leaned mixture due to vacuum leak. (More NOx)
5. It sucks fuel vapor (plus splashes) up into line 8 and beyond, aging
other hoses up the line, provoking more leaks and failures.

These are crucial problems. You lose some 40% of your performance.
But the fix is easy.

So if some mechanic has told you to rebore your jets, or to adjust the
mixure screw (requires removing the carb), or you notice that your
timing has drifted, or several other unexplained problems... You know
now what the real problem was.

To check your vent bowl diaphragm, just remove line 8 at the valve,
replace it with your own hose and suck on it. If it doesn't hold
vacuum, you're in luck--the fix is easy and the result will be

My carb bowl vent valve diaphragm was a gooey mess with a hole about 5
mm wide. To check yours, just move the air filter assembly out of the
way, and take out the 3 screws holding the bowl vent valve cover on. I
think my diaphragm failed about 5-10 years ago, and the performance
took a nose dive, but it happened slowly (as the hole grew) so no one
noticed the change.

I wanted to replace the diaphragm, but all the valves I saw at the junk
yard had the same problem. None of the carb rebuild kits I
found--including from the dealer--contain this diaphram. I tried to
fix the diaphragm with gasket maker, but it failed within a couple days
by turning to goo again. But try forcing the valve open AND plugging
the vac coming in (line 8), and you'll have a brand new car. DRAMATIC
improvement. You do not need to take the carb off to fix this--it's
fast and easy.

You'll see a MAJOR improvement! Try it!

This information took an incredible amount of trial & error and an
embarrassing amount of time to compile. I've found almost no
documentation on the valve, and not one word on the huge impact of its
almost-guaranteed failure. If you're as thrilled as I am by the
improvement in your car's performance after fixing this huge little
problem, please consider sending me $5. It makes so much improvement,
you may even feel compelled to send me 10! Or just email me if the fix
works for you. ;-)


PS: For more information on smog problems, read my post here:

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