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 consequences.
Here's why the failure is so impactful.
- 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.
- Secondary carb barrel stuck closed! The hole reduces vacuum on line
- Line 8 also runs to the vacuum-operated secondary. As long as line
- 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
- Leaned mixture due to vacuum leak. (More NOx)
- 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 thrilling.
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. ;-)
Greg
PS: For more information on smog problems, read my post here: