heavy stumble at low RPMs - Catalytic Converter?

[Nunyas]

Hey folks,

I did some work on my car this past weekend, and took her out for a spin (15 - 20 miles). Everything seemed to be running ok some minor adjustments (to get rid of a stumble under acceleration from a stop) would help, but ok overall I thought. When I finished the drive, I let the car idle and popped the hood checked the fuel filter; it appeared over half full initially which is better than normal after a drive that distance. Then as I was looking at the filter I noticed the fuel level in it started dropping very rapidly it emptied in under a minute with the engine idling after the drive. When the fuel level in the filter started dropping rapidly there was a notable change in the engine idle... it behaved really lean: unsteady idle speed, random sounding misfire, and "splashy" exhaust note. The only thing I can think of to cause this is maybe a lack of air flow evacuating the exhaust heat from the engine compartment, and the fact that I have only half the original heat shield pieces in place (I got the car with NO heatshields). I'm considering getting some aftermarket moldable heatshield material to attempt to block off the remaining exposed areas of the exhaust manifold from the carburetor area.

I had a question to ask at the beginning of all this, but I seemed to have strayed a bit. More to the point I want to know about is no matter how fuel rich I make the mixture in my carb I can't seem to be able to get rid of a stumble from idle. Lately, it seems to be getting worse, and it definitely seems to be worse when the engine is fully warmed (e.g. driving in city traffic is becoming less and less "enjoyable" due to the stumble from a dead stop) than when just started. If I rev the engine before attempting to move the car I can usually get a smooth start from a stop light, but the longer I drive the car the higher I have to rev the engine to keep it from stumbling. So, by the time I get to the last stop light from my house I have to rev the engine to 2000 - 2500 RPMS (and sometimes even higher) to keep from stumbling on take-off. Steady state driving with the engine RPMs over 2K seems to be ok with little to no hesitation. I'm beginning to suspect that I may have a partially clogged catalytic converter, but I'm uncertian on how to check without cutting up my exhaust. When the engine was fully cooled I tapped on the cat (after market non-stock cat) and it sounded "solid" (as in not hollow), and I tapped on the muffler it sounded hollowish but without any reverberation (like a metal container with heavy sound deadening). I don't think this is anyway to determine if a cat is bad, but short of removing it I'm not sure how else to tell. Any of you know? Also, can a partially/mostly clogged exhaust cause all of the behaviors I'm getting (e.g. heat build up in the engine compartment, heavy stumbling at low RPMs, etc.)??

Thanks for any information you guys can shed on this topic. /ubbthreads/images/graemlins/smile.gif

 

[DrEntropy]

Re: heavy stumble at low RPMs - Catalytic Converte

My experience with clogged CAT's is the car will run for short periods then shut down completely. Sluggish acceleration ALL the time leading up to the "short running time/shutdown" phase. The "thump" test works okay with cantelope, not so well on catalytic converters, IIRC /ubbthreads/images/graemlins/wink.gif

Are you SURE your fuel pump has both delivery pressure and volume? *Sometimes* an electric/electronic device fails gradually as it's temperature goes up, like CPU's. IYSWIM. Try an' check volume/pressure after a good run, when everything's up to temp... and use due caution, huh?

 

[Nunyas]

Re: heavy stumble at low RPMs - Catalytic Converte

Well, the acceleration problem seems to be getting worse... It seems it only accelerates well on cold mornings before the engine reaches normal operating temp. Heading out for lunch is almost to the point where I'd rather not drive. And hesitation under low RPM acceleration is like saying a Yugo is a little slow compared to a Lotus Elise... IYSWIM...

I'm sure the pump has good flow. I did check that the day I saw my fuel filter drain like a toilet flushing. Flow rate seems good and trying to stop the flow with my thumb over the end of the line gave me the same effect as trying to stop a garden hose with my thumb... /ubbthreads/images/graemlins/eek.gif Of course that's no accurate measure of preasure.

Thinking back, it seems I've been setting the carb progressively richer and richer over the past year. Each time was to deal with a stumble under acceleration that slowly developed since the previous adjustment. The last major change I made prior to my latest drama was changing the rocker assy. When I changed the rocker assy, I also changed the valve cover gasket to a silicon type. Since then, I've noticed my carb is sucking large amounts of air through the line connected to the crank breather/oil sepparater pipe, and I've noticed a new oil drip from the manifold side of the engine (exact source is still unknown). Plugging the line or plugging the port on the carb allows me to get into an adjustable range for mixture. I have gaskets for the side covers, but haven't installed them yet.

Could it be that my mixture really is just too lean because of a leak? I'm trying to think outside the scope of "doctoring" the symptom and trying to figure out the root cause so I can fix it right and get out of the cycle of continually adjusting this or that to doctor up the same symptom over and over... ya know?

 

[tony barnhill]

Re: heavy stumble at low RPMs - Catalytic Converte

what carb setup? Are all the emissions there or have you removed part of them?

 

[Nunyas]

Re: heavy stumble at low RPMs - Catalytic Converte

I'm on a ZS carb. All emissions parts are there. Gulp valve has been blocked off due to leaking (it's stuck slightly open and never shuts off). EGR valve appears to work properly (my last emissions test agrees). I discovered not too long ago that my smog pump ... well isn't pumping. It spins freely but pumps nothing. Any air entering the exhaust it being pulled in. I feel suction on the smog pump intake when it's connected to the air rail. When I disconnect the pump it does nothing but look pretty and spin. The valve cover to charcoal canister line is on its last legs. I'm using a fuel line that I stretched out one end to fit on the oil separator breather pipe (the other end is clamped to the carb). The air rail check valve is new and definitely working.

I'm having a hard time finding a source for hoses that are suitable for use with the emissions equipment. The gulp valve is being a pain too. I think I'm stuck with having to order that part from Moss or the likes. All the "major" chains look at me stupid when I walk in looking for one.

errr... looks like I went off on a minor tangent. The emissions parts are all there... but in various stages of working.

 

[Nunyas]

Re: heavy stumble at low RPMs - Catalytic Converte

I just had a thought. Could this "new" oil drip that I'm getting on the manifold side of the engine and the over abundant amount of air being pulled into the carb through the oil sepparater be related?