Fresh build. 78 chevy truck. 72 350 olds 30 over pistons , new rv cam and roller timing, new distributor hei. 7a heads. Timing 12 degrees at 800 rpm. Idols perfect, slow accelerate okay. Backfires out carb (600 holley) on quick throttle, stalls out, no power. Cold starts right up. Acts the same cold or warm doesnt change symptoms.

 

Secondaries opening too fast?
What controls the rate of opening on this carb?
On a QJet it is the secondary air valve spring wrap and the link to the choke pulloff which also serves as a motion damper for the secondary air valve.

If you open the back half suddenly, the fuel flow can't keep up with the flow of much less massive air, and you get excessive leanness.

 

Vacum leak , would be my first place to look or real lean carb.

 

It does act like its real lean. Im still working on idol screws but I dont have a vacume gauge. Im not sure about vacume leaks, not seeing where it could be, at least obvious. Doesnt smell rich at exhaust.

 

When no vacuum gauge is available what I do is lean it out on one side until it almost dies and back it out an 1\8 th of a turn past the point where is starts to run smooth. You do both sides or if you have 4 corner idle you do all four. Make sure your timing is advancing properly . I have had a carb back fire due to the timing advance not moving because it was stuck. I have run an edlebrock 600 out the box on an otherwise stock 350 with out having to even touch the idle mixture . Usually just the fast idle.

 

Are you getting even streams of fuel when you mash the accelerator I had that problem and it seems I had a clog I fixed it and it ran fine after that

 

X2, first place and easiest place to look.

 

Sounds a little more sensatice than I was thinking lol. Which was is lean then? I was thinking clockwise to tighten it made it less lean. So turn it untill it starts stumbeling, then go other direction 1/8th turn.

 

You gotta make sure clockwise leans it out . Some carbs are oppoisite but for the most part clockwise leans it out. Yes you turn it until it stumbles then you back it up until it just runs smooth then just an 1/8 th out. That would give you a lean best misture but its a good point to start.

 

Okay. Thanks all. Ill tweak on it some more.

 

Try adding about 8 degrees base, bet it is way retarded, timing wise. My low compression Olds love a lot of timing. My 8.5 to 1 Olds 350 would not run with less than 10 base on the sbc swirl port curve in my 94 Z71. I am currently running 19 base on that truck. Factory sbc setting is 0.

 

Ill try that too. It seems fine with timing retarted more. Just cant seem to get the tweak out of the carb issue. Idols perfect sitting still, thats just not going to get me places tho lol. After I have this done, I have a 403 on standby I want to put these 7a heads on at some point. I hear thats a pretty good mix putting the 403s intake valves into the 7a heads helps too. Might be pointless because Ill never go to a track. I do want to smoke the tires and hear that killer compression sound comming out the exhaust tho.

 

My current engine likes 20 degrees initial . My previous mild engines liked between 10 and 14 . Anywho I just remembered I had a similar issue once with a bad accelerator pump. It would idle fine but as soon as you revved it ., it would cough and run really rough. it would back fire underload .

 

Before I took the motor apart, when driving it would hesitate and take a few seconds to "catch up" when accelerating. Would that be something also caused by acceleration pump? I want quick snappy responce. Give gas, goes...

 

That would be a bog . It is either leaning out under acceeration or pigging out and loading up with fuel. You need to look at your plugs and see what they read . If they are white , ashy or gray its lean if they are black its rich . Depending on what it looks like you can decide where your carb needs to tune wise . Then tuning under power to transition between idle and power you will need to tune the acelerator pump . Its goes from there . You need to familiarize your self with the cuircutry of the holley carb once you understand how it works tuning is that much easier. What holley carb do you have ?

 

With the good idle and only trouble when you give it fuel I would check accelerator pump first. Causes extreme lean condition when throttle plates are opened quickly.
Your pump nozzles could have clogged partially or completely when it sat. Sometimes the crap gas today leaves deposits in the fuel bowls that clog them. Please let us know what you find.

 

Easiest way to check accelerator pump is to let it idle, then use a small screwdriver to push the pump rod down without opening the throttle. A good shot of fuel from the accelerator pump should kill the engine, no muss or fuss, with no need to peer into the carb with a chance of backfire.

Of course, if there are other problems (excessive lean due to vacuum leaks, say) the extra shot of fuel might just make it idle smoother for a couple seconds.

 

Now we are talkin. Im not sure how to id it. Trying to attach a pic.

 

Look in it without it running. Slowly and continuously push throttle. 2 good solid streams should be observed through the whole motion. Let it return, push fast and again solid streams should be seen. Of course there has to be fuel in the carb. Let us know what you see and then we can go from there.

 

I do not think timing comes into play in an off idle stumble. When throttle plates open quick there is no vacuum or enough RPM yet to advance timing until the engine begins to rev. I could be wrong, but all I have read and seen says 90 % of stumble immediately when accelerating is lack of fuel.
(There are actually carbs out there where loss of vacuum operates the accelerator and no mechanical connection is made)

 

Fuel came thru both sides. Not so much a stream, it was more splashing. Same thing on second quick throtle. Was hard to see. But the sound both times sounded like a squirt or stream. The fuel I saw seemed to be equal on both sides however.

 

I do not beleive any vacume leaks are playing a roll. If there was a way for me to post pics I can do so, so you can see my setup.

 

I seem to have a little more power. Still hesitation or bog. Still backfire out of carb on heavy acceleration (just drove around the block ((city block)). Idol screws set to two half turns from bottomed out. Wish you guys lived closer. I dont have any friends that live close that do anything. The only ones I had stopped comming over because they said I was an @$$ hole. I just got tired of them comming over and drinking my beer without ever brining any or doing anything but standing around with their hands in their pockets (drinking my beer) talking about how difficult it looked like I was doing was. I almost droppd my engine block one time trying to turn it over on my stand bcause I lost balance. This guy was standing there saying "damn that looks hard" (again with my beer in his hand) and just stood there with hand in pocket. Then Im the @$$ hole because I said I was busy when he wanted to invite himself over later. Im sorry, I think I just vented, my bad.

 

You really should get a vacuum gauge to see what it does. A decent gauge can be used to diagnose (or at least rule out) a wide range of problems. A cheap one will often have too much dampening to be super useful, but with a good gauge even how the needle twitches can be enough to steer you in the right direction.

 

The A/F mixture screws are only in use at idle. I'd bump the timing up to 18*. Is you vacuum advance hooked to manifold or ported vacuum?

 

Vacume advance hooked to port on carb. Ill try and get a vacume vauge asap. Already wanted to get one but money for it got soaked up by spouse for Christmas stuff. I half indended on getting a dash gauge for checking it. They are cheaper. 20-25 bucks. Gauges are another prob. Dont want to veir away from carb issue, but I got my dash mostly working. Had to rewire it all. Dad had it riged to work off headlight switch before. Turn on headlights and dash would shut off. I got that all fixed now. But oil preasure gauge pins. I think sending unit might be for dummy light. Regardless...if i get a dash vacume gauge, would that work good for diagnosing? And should it be hooked to manifold or carb port to be more acurate?

 

You want manifold vacuum for a gauge. Most dash gauges are heavily dampened, but can give at least a general picture.

 

I would say the oil pressure is probably the sending unit. Make sure your advance is working properly. Vacuum gauges are relatively inexpensive. You can also use a tach to adjust your carb. Make sure the a/f mixture screws are adjusted about the same. Turn in until the rpm drops, then back it out for the highest rpm, then turn in 1/4 turn.

 

You bumped up the timing? If you are running cast replacement pistons, you need the extra timing. You will be lucky to have 8 to 1 compression with 24CC dish pistons. I would replace the accelerator pump and try a stiffer secondary spring. You could need richer jetting but start there. I had a 78 1/2 with an Olds 307 then an Olds 350. With the 2.76 gears even the stock 81 307 killed both the sbc 350's power wise. You may have 2 something gearing, even more demanding with heavy truck weight.

 

Yeah I was thinking that too. Sending unit was in it, and also same one in the 403 i picked up. They look lime senders for dummy lights, which would make gauges pin? Gauge senders should be cans correct?
It will be a day or so before I can get back to tweaking the carb. Ill change timing to 18 too, that would be at 800 rpm too? Its real easy to see. I spent alot of ,ime detailing. I used the plastic marker off the 403 cleaning and pai ted it black, detailed numbers white.

 

Ooh, and needs to be warmed up whil adjusting things each time correct?

 

18 at 800 rpm is fine. Just run it long enough for the fast idle to come off.

 

By george I think I got it. I couldnt wait. Will know know after some test runs. I used my old school tach. Its an ancient analog big red MAC tool monstrosity ,hat does dwll volt and tack.

 

For now I think it will work. Still a few things to button up and maybe keep adjusting playing with rpms, timing and carb adjusts. Guess I need a vac gauge still to get a better more acurate adjustmnts usin all of the above in conjunction for better responce times and whatnots. But I think I can actually drive it comfortably for now. Thanks guys, your expertise got me this far. Backfires are gone and power restored. Now I can sleep well to tonight. You all do the same and memrry christmas.

 

Glad it's coming together, Happy Holidays to you and yours.

 

While I seem to have that in order for now, maybe I can get some input for sending units. The watte has always "seemed" to be running hot. Oil hasnt functioned in past untill now, in which case it pins. The major delema I have had is to get right stuff from parts stores on the gm 350 they seem to think its all universal, then I get into it with them about "unless Im here to get a disttibutor cap, stop selling me chevy parts for my motor." Everything has been mission, timing gears, groments, freeze plugs. This one is tricky tho. I need a water temp sending unit to read accurate from my olds 350 to my 78 OEM water temp gauge. As well as an oil preasure sending unit for my olds 350 to give me an accurate reading to my 78 chevy 0-60 OEM oil preasure gauge. Thoughts?

 

You may have to adapt the chevy senders for the truck gauges to work properly. Another thought is to just go out and buy a set of aftermarket gauges and be done with it. I use a Sunpro set like this under dash. Very simple hookup.

 

Get the senders for a Chevy 1/2 ton, just say 350 to appease the parts guy. The sbc temp sender just moves to a spot in the intake. The oil gauge sender off a sbc 350 fits, I used a short of pipe and adapter from the plumbing section, for more clearance. Worked for both my Olds engines. I did find the Standard oil senders plugged often, would read 1//2 what they should. Both wires may need lengthened but is easy to do. Sounds like more timing helped.

 

You stated it was a 600cfm Holley. If you had a backfire you more than likely destroyed the power valve unless it's a newer Holley with the check ball installed. If it's the old style you are probably running rich from the blown power valve. Try replacing the power valve and see.

 

rdhigh74 ,I read one of your previous posts stating your friends just hung out... downed your beers without helping you.Then call you an a-hole, Wow !! I can't tell you how much thses guys that are helping you out with your engine, helped me out with mine as well !! No questions go unanswered here with great guys on CO. Put-r-ther !!!! These guys are awesome !!! Good to hear you have your Olds running better now. Mine continues to run awesome !!!
Cheers
Eric