Rochester carbs really getting under my skin...

[scottydont]

.... (grumble grumble).... stupid ratchesters.....

I am nearly at my wits end with these carbs. Need help and I'm really hoping that there's something obvious and particular to these carbs that I've missed.

When I got my transvair, the carbs were off the engine and had been for years uncounted. Before trying anything, I went ahead and rebuilt them. This isn't my first rodeo and I have a pretty high level of confidence that I didn't do anything blatantly stupid during the rebuild. After reinstalling them (and an electric fuel pump) I was able to get the engine to fire and run, but it was in need of some fine tuning when I pulled the thing into the garage for suspension and brake work...

Well, here we are and I'm trying to get the engine running the way it should... I am caught in this repeating cycle that I can't get out of. I can get the engine running and stable at about 1200 rpm, but if I start inching the idle speed down, it'll deteriorate suddenly and die once it gets below about 1000. Then won't restart and idle, so I'll bump it back up and it will idle high so I start over again and the whole cycle repeats. It is also very hard starting (like need starting fluid almost every time hard)

I've double checked the timing and dwell, voltage to the coil, and gone through the whole process of synchronizing the carbs and linkage to factory specs...

It's acting like the idle and acc pump circuits aren't working on either carb, but if the passages were all clogged I would have caught it, and I'm confident that everything went together exactly like it came apart, so I'm stumped.

Any ideas? Anything unusual that I might have overlooked? Please? I'll bake cookies for whoever can solve the problem....

Thanks guys!

[Scott V]

maybe you have a vacuum leak somewhere & when you try to get the idle low you are closeing the throttle plates too much.

how tight are the throttle shafts in the carb? loose shafts = vacuum leaks.

-Scott V.

[scottydont]

throttle shafts didn't seem excessively worn when I did the rebuild. I've definitely seen much worse.

In my opinion I have to keep the idle speed screws much farther in than they should be for the rpm that it will hold. They're pretty close to bottomed out and that'll hold between 1200-1400 rpm. I don't think they're closing too far, but if I had to guess what's happening is that it's dropping down onto the idle circuit, which isn't functioning properly... I just don't know why. Is there possibly an external linkage or connection that's easy to miss or get installed wrong?

[Augie]

I had a similar issue and Mine was fixed by going through and re-balancing the carbs again. Not saying you did it wrong but that is what I did and all is better now.

Also do you have a PG? if so the Idle needs to be set with PG in D.

Not saying you would have missed that, but you don't give a ton of info to go on, other than they aren't working

[scottydont]

Manual trans, so no issue there.
Tonight I was able to get it started and idling around 850-900 almost immediately. It was stable there long enough to warm up thoroughly, double check the timing, check the mixture with a propane bleed, check for vacuum leaks by spraying starting fluid around all the likely suspects (no leaks found) do some random head scratching about what to check next, and double check Idle mixture by adjusting the idle mixture screws and watching rpm. They seemed about right, but at fastest idle (maybe 1 turn out-ccw from where they were set, which showed an rpm drop when checked with propane), it had a noticeable off idle flat spot. As I was adjusting back to the baseline I started at, the idle got ragged again and it eventually died. Wouldn't restart easily after that and wouldn't idle.... same old problem and it was as I was moving BACK TOWARDS the setting it had been performing the best at!

The whole PCV hookup seems kind of funky to me. I know it's just as it should be (new pcv valve too, and I've double checked to make sure the orientation is correct), but the best idle I've had so far was with the crankcase vent to the air cleaner plugged. To me that says its too lean at idle since it's under slight vacuum from the pcv.... but the other tests I did dont back that up.... I have to be getting something wrong... Just don't know what at this point.

[scottydont]

Hmmph... I really don't like it when cars faith heal themselves. It just leaves me with nagging questions about what caused the problem and whether it will come back.

After my last post, I went back out to the garage, started the engine again, and low and behold, it idled steady with no changes from when it was acting up. I adjusted the idle down to about 900 and left it there while I went through and checked everything again. The whole process took about 40 min while it was just idling along with the occasional blip from me. The off idle flat spot has even gone away and I haven't done anything to address that anywhere along the line. I even spent more time looking for vacuum leaks and wiggling fittings trying to duplicate the sudden changes in idle characteristics that I've been seeing.... Couldn't find anything amiss at all...

[flat6_musik]

I know it's just as it should be (new pcv valve too, and I've double checked to make sure the orientation is correct), but the best idle I've had so far was with the crankcase vent to the air cleaner plugged. To me that says its too lean at idle since it's under slight vacuum from the pcv.... but the other tests I did dont back that up.... I have to be getting something wrong... Just don't know what at this point.

When you say "new PCV valve too"......you know these cars don't actually have a PCV valve, it's just a pinhole orifice in that metal breather tube. You don't actually have a PCV valve on this car (like from a regular engine) right? If it were my car, I would also start by plugging every single vacuum port (even the crossover) and go from there.

[davemotohead]

Sounds like dirt in the idle circuit to me,and the PCV valve? what year is this car? some early's had an actual valve,but late's don't, I was going to tell you to remove the idle mixture screws and blow some air thru the hole to blow any dirt out,as far as balancing, I have all the tools but don't use them much any more, I seem to be able to get them balanced by ear and feel Helen Keller style and they run great,there is really not much to these things. my carbs always balance!

[scottydont]

The engine is a '65 vintage 110. What came out was definitely not just an orifice, and the local oreilly looked up the PCV valve by year and hp and came up with an identical part to what came off of it....

dirt in one of the carbs was my guess as well. The carbs got dipped prior to rebuild and passages blown out with copious amounts of solvent, but I can't think of anything else that fits the symptoms. They have new brass filters, and a big inline filter below the tank so the incoming fuel should be clean. At this point I'm going to cross my fingers and hope it was just a one time deal and not a recurring issue.

[davemotohead]

Your 110 should have no valve,it should only have a piece that looks like this, can you post a pic of your PCV set up?

[Augie]

Sounds like dirt in the idle circuit to me,and the PCV valve? what year is this car? some early's had an actual valve,but late's don't, I was going to tell you to remove the idle mixture screws and blow some air thru the hole to blow any dirt out,as far as balancing, I have all the tools but don't use them much any more, I seem to be able to get them balanced by ear and feel Helen Keller style and they run great,there is really not much to these things. my carbs always balance!

I don't use a tool to balance them either, I set them initially from a base line with the throttle linkage disconnected from both carbs and the crossover disconnected from the rod coming out of the firewall, go to baseline, then it is all about uniform adjustments from there and it seems to work.

[scottydont]

I'm at the office so cant take a photo right now, but except for the air cleaner setup (mine are stock) the breather/pcv assembly looks identical to this one right down to the location of the valve in the rubber line.http://www.google.com/imgres?imgurl=htt ... g&dur=1055 The PCV valve is a few inches from the end of the metal tube (right at the top edge of the photo).

[scottydont]

On an unrelated note? What does that roughly 3" vent from the fan shroud to the firewall (right below the pcv in the photo) connect to in a stock corvair? Mine is in a VW bus and is not plugged. I'm going to cap it but wasn't sure what its original purpose was.

[bbodie52]

The attached EM and LM shop manual sections and Corvair Assembly Manual section all describe and illustrate the forced air heater system used in the Corvair.

[scottydont]

Still fighting with these things. I've gone through every step of the old video that unsafe posted on youtube... twice. Checked for vacuum leaks... the fuel pump and filter are brand new... new plugs and wires... etc.

What's happening is that if I fiddle around enough I can get a semi stable idle (usually around 1000 rpm or a tad under) and it'll stay there almost indefinitely, but as soon as I start trying to make changes to fine tune things, the idle will fall apart and the engine will die. Undoing the change won't return it to where it was and I have to fiddle around with a bunch of things to get it to idle again. Then as soon as I make a change it falls apart again and the whole cycle starts over.

I've tuned dual carb engines before. I have a good idea what I'm doing and it shouldn't be this difficult. I'm absolutely stumped as to where to go from here....

..... although in past few weeks I've seen two corvair engines for sale that part of the ad was that the seller converted to a singe carb because they couldn't get the multi carb setup running....

The only thing I haven't done is run the valves, but that's because I haven't taken the time to look up the procedure for setting them without the engine running. I'm beginning to suspect that there may be one or several valves holding open (both through process of elimination and the fact that I get a LOT of fluctuation on a vacuum gauge when I hook one up)... Do the valves on these engines tighten or loosen over time? Ive seen it go both ways depending on the make of engine.

[bbodie52]

Reading your description I wonder if one of the two carburetors has a faulty or blocked idle fuel circuit? The car may be idling on one carb only, with the other cylinder bank essentially doing nothing at idle. If the remaining fuel passages in both carbs are OK, the engine may rev OK or seem to run OK at speed, but it simply won't idle well.

If such a condition exists, you may be able to figure out which side is "dead" at idle by disconnecting spark plug wires. Disconnecting a plug wire on the "dead" side would have little effect at idle, while disconnecting a wire on the other side (that is carrying the whole engine at idle) would have a profound effect. You may also be able to notice a difference in exhaust note between the two sides on a dual exhaust engine, but a single muffler exhaust would mask the sound difference from a bank of dead cylinders at idle by combining the sound from both sides.