42 replies to this topic

[TexanIdiot25]

Yes- the idle speed should decrease when you unhook and plug the vacuum hose. If idle speed increases, then your advance diaphragm was bad and you plugged a vacuum leak (just leave the hose plugged).

Idle speed doesn't change audibly, but the throttle shafts in the carb do leak a little too so the idle is a bit unsteady as is (Just have the idle speed knocked up to 1000. Fast, I know, but it's smooth). Will hook a timing light and test it again tomorrow. This is with the "lower" of the two vacuum lines unplugged, you can feel the line pulling vacuum.

I have to agree with Scott on this. plus Euphoric, is dead on, there is a model (83-85) accord, SOHC that has a similar cap to the civic/crx, and yes, it is off by about 45 degrees difference. Easy way to tell, get the cap off, get the cam to TDC (align those marks on the cam to the head) while you are at it, check to see if the block is there too.

Cam timing is spot on, we checked it and the timing belt. Belt was in great shape, not stretched, loose, cracked or worn looking, so we left it be and then did the valve adjustments. Plug wires from AZ didn't fit, so will have to try another store's...

What plugs did you use? Did you change to colder plugs? I see this a bunch with modular fords. Don't use the Bosch platinum +2, +3, and +4s. For some reason when they are used I see the same symptoms. Also when tuning my numbers are all over the board.

NGK replacements, from RockAuto, gapped to the factory spec on the hood. But this was an issue even before the new plugs, so I'm ruling them out.

I should say this is a $200 beater, so budget is more important than perfection. Just need to get rid of this missfire since a lot of my driving is freeway, and entering and staying at freeways speeds is always an adventure, but not a fun one.. And, it's really annoying in the back roads too

Edited by TexanIdiot25, 28 May 2010 - 09:23 PM.

[Rampage]

I would check that cap for any cracks or heavy deposits inside.

Also, sometimes a clogged fuel filter can feel like a miss, and it is especially noticeable at higher speeds.

[TexanIdiot25]

Eh, it can rev up the RPM range at part throttle fine. Replaced the after-pump filter, but couldn't get at the after-tank one. My CST10 eats filters every few months, literally feels like it's running out of gas at high RPM, so I'm accustomed to that sensation.

Plus it back fired out of the carb today when I got on it too hard, seems ignitiony to me. I'll mess with it tomorrow morning or sunday. Thanks for the suggestions everyone, I'll sit down with the car this weekend and get at it. Busy week and I'm exhausted.

[EuphoricBlue]

It sounds like mostly ignition to me. You might have a slight vacuum leak elsewhere but mostly it sounds like the main culprit is the leaking vacuum advance.

Here's why I think that:

Vacuum advance bumps up advance at idle to run better on a lean mixture. Press the gas and vacuum drops, retarding the timing, and creates a rich mixture. Stay in it and a bit higher up the RPM band the centrifugal advance takes over.

Someone sets advance without checking the pot. The pot is leaky and thus the vacuum advance does not work AND the mixture is too lean due to the vacuum leak. Bump up either the idle or the fuel mix to compensate. And then the timing is set at the recommended 20 something advance at idle. Re-set idle speed at "proper" timing.

The issue comes in when you step on the gas. The vacuum advance is supposed to get disengaged, retarding the timing, for the fast burning rich mixture. But since it's not working you remain at 20 degrees advance and the fast burning flame front hits the piston way too soon and you get knock. Later on in the RPM range these engines deal well with advance, so the knock goes away.

AND, the knock you had earlier would have "knocked" some carbon buildup off the valves/pistons/head and lowered compression, thus reducing the knock. Plus, this kind of knock is sensitive to temperature due to richer/leaner mixture.


Also, when my pot was bad and bypassed I too had knock at the lower/mid range under load since I had the distributor cranked for better idle timing.

Disclaimer, this is not guaranteed to be your problem... but it is a known issue and the symptoms fit... I'd address the vacuum advance and timing before moving on to something else.

(Was that long winded enough for you? >_<)

[TexanIdiot25]

Long-winded is OK,

But, if I understand right you're saying the miss should (in theory) start to cut out at higher RPM as the mech-advance takes over? The miss sometimes lessens up, but is still there at higher RPM at WOT. I can't say for sure how consistent or how much it misses with any certainty, as I don't drive it up there at WOT with 250,000 miles and the violent (for lack of a better term) misfiring in the mid range.

It can be brought to redline, but only with a given amount of part throttle, balancing to keep it from missing. I usually have to shift at 4,000 rpm anyways to keep moving at a decent pace to enter freeways lol

edit: whaoh, the price of vacuum advances... I'll be damned to find these in my junkyards too

Also, which plug wire set will for sure reach to the side-mounted dizzi terminals?
http://www.rockauto....9,parttype,7228

Edited by TexanIdiot25, 29 May 2010 - 01:48 PM.

[TexanIdiot25]

Unplugging the lower vacuum line (and plugging it with a small bolt), and gave it a good 15 minute drive to running temps, NO MISSFIRE! There seems to be an occasional miss, or it could just be me. But, no mid range shut down and it pulls quite fine in all gears. I can even put it in 5th at ~25mph with the motor putting along at 1,000 rpm, just about as much load as you can put on it, and it's smooth! Hell, the RPMs can even dive below 1,000rpm when clutching out with out the car shaking violently

Idle hasn't changed much, but with the throttle shafts leaking vacuum, I don't expect it to improve much. Good thing this was it's last year of emissions test. I'll keep you guys updated later this weekend when I get it out on the freeway.

[EuphoricBlue]

Sounds like you really have to go through everything. It really does seem more and more like someone messed around with all kinds of settings trying to compensate for improper advance operation and vacuum leaks.


As for the vacuum advance bit, I don't know how inventive you are. I was able to spend some time at a scrap yard and assemble parts that work and ended up with working vacuum advance again. No more misfire.

My misfire was mild though, and I didn't intend to imply that it should go away at higher RPM. Only that it does on mine.
It might have been that my centrifugal advance doesn't work and doesn't further advance timing as the RPM goes up, I haven't tested it on this distributor.


Is your HF a feedback carb system? As in, is there an O2 sensor attached to the exhaust manifold?

[TexanIdiot25]

I haven't looked for an o2 sensor, but it's not on the top of the manifold (what's obvious around the plugs).

Well, it's semi-fixed. Drove out to a classic car show a few miles away. First half of the trip it was quite all right, and then the miss fire returned, with varying severity. The drive home at night (colder outside temps) and no fits of missfire, maybe one random miss here and there.

Bummer, I was hoping it had worked!

Edited by TexanIdiot25, 29 May 2010 - 10:38 PM.

[Bubba]

Hmm, I have an 87 HF and it should have a 5 wire oxygen sensor sticking straight up out of the center of the exhaust manifold. I wonder if someone has put a 1.3 manifold on it (no oxygen sensor hole on that engine). The 87 HF is a feedback carb system.

My 87 HF also has an EGR valve position sensor on top of that valve and I noticed you don't have that. I also noticed that your exhaust emissions box is missing. Someone has definitely been messing with the vacuum components...

[TexanIdiot25]

I noticed it didn't have the emissions box on the side, but saw in a few other CRXs that they didn't have it either. I don't see routing for the 2nd fender mounted box on the vacuum diagram either, so I didn't think anything about it.



Also I found no exhaust O2 sensor mounted on the exhaust manifold, at least not in plain site w/out jacking the car up.



Also I should note this car was originally purchased in California, according to the owner's writing in the owners manual.

[Bubba]

Very interesting- yeah the vacuum diagram does say that it is California vehicle.

I agree that there is no oxygen sensor hole in that heat shield. The aluminum air cleaner and plastic valve cover are definitely for an HF.

Mine is an 87 high altitude HF.


Hmm- I just looked on www.fueleconomy.gov and sure enough there is an 87 HF listed that is not FFS (feedback fuel control system). That must be your car.

As far as the msifire- maybe a cracked vacuum line that opens up when it gets hot, or a slightly cracked rubber base plate under the carb.

[TexanIdiot25]

VIN says it's an HF as well.

Only vacuum leak left that we've been able to trace is the throttle shafts, but it wouldn't cause this much of an issue. Base plate gaskets were replaced.

Didn't drive it much today, but the missfire was inconsistent, it was the upper 80s during the evening and it seemed to either misfire like normal, or after dinner it wanted to miss with just about any throttle, it was hard to get smooth. And, of course, outside air temps drop after dark and it drives great all the way home. I'll replug the vac advance and see what happens. I also have the hot air intake tube unhooked, I was attempting to see if maybe the vacuum control that opens that valve in the air cleaner was maybe causing it to pull hot air all the time, but when I check the air cleaner it's still set to draw from the rest of the intake.

[EuphoricBlue]

Place your hand over the carb (with the filter can removed), start to block air flow... What does the engine start to do first? Does the idle go up first or does it drop?

[TexanIdiot25]

Place your hand over the carb (with the filter can removed), start to block air flow... What does the engine start to do first? Does the idle go up first or does it drop?

Idle jumps to 2,000 rpm. Yeah, lean. I hooked the vac advance back up today, during the heat of the day (near 100 i wanna say) it would just constantly missfire with out the vac-advance hooked up. Of course, this is with only the one vacuum line off and plugged, the one that pulls vac at idle.

It's strange that it's so dependent on outside temps, especially if lean. If I'm thinking right, while Lean it should run worse in the cold considering there is more air, basically...

Still averaging 37mpg as well. Can't bitch that it runs like hell and averages better than most modern cars.

Edited by TexanIdiot25, 31 May 2010 - 06:01 PM.

[EuphoricBlue]

The hotter the temp, the less dense the fuel is too. Maybe that's why it works opposite of what you'd expect. That's just a guess though.

The RPM is supposed to go up when you cover the throat, but it does sound like the fuel mix is too lean.

I'm gonna say your first priority should be to try to richen the mix a bit on the CVCC port and maybe the primary port. I don't know off hand how to do that on the CVCC tough, no experience with those carbs.

You are sure you have to other vacuum leaks? Did you spray carb cleaner at the vacuum lines/base plate of the carb? Have you tried reversing the brake booster check-valve to make sure the booster has no vac leaks?