Ok as the header says. It runs up fine is you don't hammer it from Idle. It is worse when under load. Hei is hooked to ported va*** on the carb. Initial timing is 10-12 Deg Full(total) timing is 22 deg I don't think the Va*** adv is working (New vac mod) Idle is set at about 600-650 RPM ( Guessing!! no tach) Any thought Oh yeh it will fire through the carb. Somtimes when you stomp the pedal in park and want to die.
I vote for more timing,but I always vote for more timing... what is it in? sounds like distributor, but I need more info. what is it idling like lean or fat? vac*** leaks? and if all else fails, check the firing order again.1-8-4-3-6-5-7-2.
I vote for a worn accelerator pump in the carb. It's easy to check. Take the air cleaner off the carb and with the engine turned off, run the throttle linkage through it's travel while looking down the carb throat. Try it fast and slow and watch the stream of fuel. Is it a good stream like you'd piss after drinking 3 PBR's? If so, check if the secondary air flaps are opening too fast. They should open slowly, as the engine load and RPMs increase. YOu need to have the engine running, car in neutral for this test. Let the engine idle and quickly open the throttle WFO. The secondary flaps should open slowly or not at all in neutral. This is cool. If they flop open as soon as you hit the throttle, you may have a broken tension spring. Replace it.
What he said. Annddd....although this might not be specifically related to the stumble, I gotta say 22deg. advance all in sounds a little weak. With engine off and cap removed check the vac advance and make sure the diaphram isn't broken by ****ing on it with a piece of hose if the advance moves stick your tounge in the hose and see if it stays put.If it doesn't replace it. Then check the mechanical advance to be sure it's free, the rotor should turn clockwise a few deg. and spring back into place freely. If not lube mechanical advance and or replace springs as necessary. Good luck and Merry Christmas,T.OUT
Worse under load and it boots up through the carburetor? It could be timing related, but since it's a Chevy small block, make sure it isn't suffering from a case of the flat cam blues. It wouldn't be the first time I saw someone try to tune a lack of valve lift out of their car.
It is a brand new Vac advance no leak The ported va*** on the carb doesn't seem to be working Could that cause not enough total timing? And would the improper timing ramp cause the pop through the carb? The carbs a a liitle rich, Trying to get timing real lose before tuning carb (I have spent way too much time trying to tune a carb when timing is the issue. It is in a non traditional Chevy Blazer
Do a search in the tech archives for "timing" there is a post there by c9 that says it ALL !! Do what he says and you can eliminate timing from your possible problem (AND make the motor run a HELL OF A LOT better too) ..... Next would be carb ( my vote --- is it a holley ?).... Firing order is CORRECT--right ? Post what the timing fix does ...might do it ..if not ...carb is next .....and I've seen "flat cam lobe syndrome" cause diagnosis havoc before too ...... jersey Skip
I my best guess would be accelerator pump not working, or way to lean on top end. but it could also be a intake leak! Any way I would not drive it like this or you might end up with a hole in one of your pistons!
I haven't messed with a SBC in 5 years, but it seems like the advance is supposed to be on a constant source. I had a similar problem with a SBC when the advance weights in the distributor was sticking... it would only advance a little bit, then stick. Made it have ****ty acceleration, and spit back through the carb.
As I suggested above: try the vacuum advance hose connected to full, all the time, manifold vacuum. As referenced in another post and another thread, SBC use full time vacuum. That is why Chevy manuals say: "disconnect and plug the vacuum hose before doing static timing at idle." If they used ported vacuum one would not need to pull the hose and plug it to set the timing. I made this mistake on my first car that came with a Holley carb. I ran it for some time with it set up as the previous had it: ported vacuum. It ran just like the setup you are trying to fix: poor acceleration and popping thru the carb when nailed. I ran across an article that pointed out that SBC used full vacuum. I switched the hose to the correct fitting and the problem went away.
Damn I hate it when someone thinks of something I forgot.LOLThat's a didtinct possibility too. T.OUT Oh and yes 12 initial and 22 mech. and vac. for the 34 total.
Everything everyone else has said is good to check, but I have one more possibility: I had a '73 Cutl*** that was doing pretty much the same thing, and it was the spark plug wires. They were touching the bottom of the air cleaner, and apparently they were only arcing under acceleration under load. It had got to the point where getting on the highway int he morning was a little scary. I replaced the wires and never had the problem again. Just a thought. (And by the way, the wires on the car were only a couple years old.)
...great advice from every bodyso heres two more things check and make sure allthe plug wires are hooked good at both ends.. dist.cap check while your there lastly look under the hood at night you can see wires shortin out better at night I know thats 3 things ..sue me
they are not called a Quadrabog for nothing. It is hard to get that off idle bog out of a old quadrajet.
There's the problem! ;^p Check the dwell. SBCs are touchy about the dwell. ~ Paul aka "Tha Driver" You talkin' dirty or hep?
Driver- Its an HEI there is no dwell....but I am sure you were joking. Here is just a shot in the dark. Has the engine had any major work? 11 to 1 pistons, decked block, major head work, new cam? And is this the original carb from before when it was a mild 180hp smog motor? On rochester carbs there are metering rods that regulate fuel flow. On your high HP motors and your larger CID motors the metering rods are very aggressive. Very thin and have steep angles on it. If this is a smog motor carb and the engine has had alot of work. You could simply be starving it for fuel. You can get metering rods fairly easily. Hit any junk yard and look for rochesters. On everything from station wagons, to buses, cube vans, trucks. The rochester was used on just about everything. And the BBC had some nice metering rods in it . So check your old Suburbon, lol. Vacuum advance should be ported. If your total advance is only 22° then that would be the first place to start. I would check both your advances. Get a small hand held vacuum pump with a guage and hook it to the distributor. With the cap and rotor removed, apply vacuum and see if the plate moves and holds position. I have seen bad diaphrams before. Also try and move the weights under the rotor by hand. See if they move smoothly. Also if a shadetree has been in there before, and he has "rebuilt" the distributor. Quite possible it has the wrong weights and springs. If all else fails, advance the timing at idle and check it at WOT. Manually adjust the timing until you are between 38° to 42° at WOT. This will make it run poorly at idle but if it solves your WOT problem, then you know its timing and not something else. Also do a quick check of the plug wires. Squirt them down with soapy water and start the engine. Lower the hood so it is dark and look for arcing and leaky wires. A common place is #5 and #7, since they are next to each other on the engine and on the firing order. Do these timing checks first, since they are easier and cheaper to check. Next do the carb checks that were mentioned. The accelerator pump, the secondary flaps. Also check for play in the throttle shaft, it would be small but it is unmonitored air if it is bad. Common on older carbs. If the problem is still there after all of this. Check for a flat lobe on the cam. Pull the valve covers and disable the ignition. If you think the valve lash has never been adjusted. Then turn the engine over by hand until both vavles are closed and check the lash. It will be excessive on the flat lobe. If it has been adjusted, and/or you dont see excessive lash. Then find or borrow a dial indicator and check each rocker for lift. I am a huge fan of the Rochester carbs. They are cheap, they are everywhere and even in the smog years they can flow over 850 cfm. With a cheap rebuild kit and an ***ortment of metering rods. Vacuum operated secondaries, it works perfect for a street/strip carb. The reason people don't like them is they can be complicated if you don't understand them. Hope that helped.......Snafu
I would pull the top off the carb and check to see that the secondary accelerator tubes are still installed. It's a common problem with Q-jets that these tubes fall down into the wells.
That'd be my guess. No one ever wants to admit that they put the plug wires on incorrectly, but I had it happen to me. Did the same **** as it's doing to you, didn't sound bad. Did everything in my power to figure it out, a friend of mine checked the firing order and it was off. I always ***umed that the wires were in the right order, the truck in question was owned by a supposed mechanic, for crying out loud.
Thank you all for the advise I will be doing all of these today or at least checking them all today I really think it is a combo of two small problems that are making a big stink. Always able to come to the hamb and get more questions than i can come up with.
Actually no. I may be a SBC dumb***, but I *do* recall (although it's been awhile) working on one that had the *same* symptoms & I couldn't get it to run right, & someone said to check the dwell, & I'm like "but there's no points", but that's what the problem was. Ran great after setting the dwell. Don't recall if it was an HEI or some other type of electronic distributor... ~ Paul aka "Tha Driver" Easy on the Giggle Cream!
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