Been having trouble with my '69 350 HO. It has a Rochester 4 bbl 4MV. The car starts and runs fine through the warm-up. When I get out onto the road it sometimes continues to drive well. However, other times when I pull up at traffic lights, (still in drive) it seems to lose RPM and feels/sounds like it starts to run rich. It gets that murky off beat kind of sound. If I leave it long enough, it stalls. It always starts again, but can take a bit of encouragement. If I give it a bit of gas before it stalls, it picks up again (albeit with a bit of effort) and when the lights change, off it goes again, driving nicely until the next lights. If I'm driving down hill with my foot off the gas it can feel like it's going to stall as well. If I just leave it in park after initial warm up it doesn't miss a beat. It's quite intermittent, though it does seem to be getting more consistently a problem.
I checked my carby. It had a new kit put through it about two years ago, and has only driven about 2000 miles since. The petrol is clean, 98 Octane and I do run a valve softener (but always have). The mixture seems ok, though I discovered that the the screws don't seem very sensitive for the amount of vacuum. Even so, with everything connected, it's pulling about 17HgVac (manifold) at about 750RPM running temperature idle.
I've read a whole bunch of stuff about it, but nothing's making sense, so I'm turning to the experts for help. Any help would be appreciated thanks.
Intermittent problems are always the toughest to solve. when you say the engine sounds rich when the problem occurs that may very well be but by what you are describing about the mixture screws not having much effect on idle quality that is a lean condition in the Carburetors idle circuit or a vacuum leak. This may or may not have anything to do with your intermittent problem and if it doesn't still needs to be addressed. Have you meticulously checked for vacuum leaks? Another thing to look at is the idle stop solenoid if you are using the factory setup. If dropping out intermittently it could be the cause. One last thing to consider is spark. If you have a cylinder dropping out or partially dropping out due to crossfire intermittently. Check over your system carefully, things like overlapping secondary wires, wires touching each other and wires touching metal. the easiest way to find light crossfires or groundings is in the dark. Also don't forget to look at the plugs, and especially look closely at the cap for any carbon tracking between posts. all that said with the limited amount of info about this problem My guess would be carburetion and checking for Vacuum leaks. Just check all the basics and things I talked about before disassembly of your Carburetor. By the way here is a read on deciphering vacuum gauge readings https://www.tpocr.com/Maniflod_Vacuum.html
Ya, it's not in the service manual but in the diagnosis manual. I don't have access to one but Here is a link to Harold B's site for a 68 one https://www.firebird400ho.com/mdocuments-library/ Not sure why you need it unless you are unsure that things are hooked up right?
Thanks for the link. So there’s a diagnosis manual as well as the Fbody manual and service manual ? Wow. I thought a couple of 1000 pages was enough.
But to answer your question if I’m looking for a a vacuum leak, wouldn’t I want to check all vacuum fittings? For example, if the hose that supplied vacuum to the brake booster was split it would suck air back to the carby Wouldn’t I check the whole system?
Sorry, sometimes it's hard to tell someone's skill level through the internet. No you don't need a vacuum schematic to check the system they are very basic. All you have to remember is that anything between the base plate of the carb. and the manifold needs to be checked. This includes the manifold itself on the small chance of a hairline crack, all fittings that screw into it (except on the front most coolant runner), all around it's gasket surface, all hoses coming out of it and the carb. all along there length and where they terminate. you also need to check things like the modulator valve if auto, the vacuum advance dashpot on the distributor and the power brake booster if you have one. The best way to check these is with a hand held vacuum pump like a mity vac as they are inexpensive. As a mechanic doing it for a living years ago I always found oxygen if you have a set of torches the easiest and cleanest to sniff out the leaks in the hoses and such while idling as it will cause the rpms to raise up when there is a leak. If none a home plumbers torch with propane works just as good and some people use carb. cleaner but be careful if you have a pretty engine as it can discolor some paints, also be careful when spraying near a hot manifold. For these reasons I like the above two methods better. Also as I said don't forget to check all around the baseplate of the carb. but be careful if you have an open element air cleaner as it can suck it through the filter and fool you. This whole process won't take you more then 20 minutes to a half hour at most.