Cam/Vacuum/Carb/Stall Issues

[jmel]

I've brought this up before...have tinkered a bit, and think its time to bring it up again.

I had a a 305 with a stock cam, quadrajet carb, & stock torque converter. I picked up a thumper cam from comp cams and while it absolutely sounded kick *** - it would stall whenever i put it into gear. Had to put it into neutral when coming to a stop. Am afraid that its a bit rough on the tranny constantly throwing it into neutral and back into gear...on top of the fact that its a pain in the ***.

First thought was that i needed a higher torque converter. Called comp cams to get their recommendation and they stated that the stock torque converter should work just fine. They thought it was the rochester carb that i'm running. Stated that it requires more vacuum than that cam can produce.

Don't have money to dump into new carb (although if a good deal on tri power came along, i'd probably wh0re myself out to get the money). I'm about to put old cam back in but kills me to lose that sound.

Any other options that i'm overlooking? I heard mention of a vacuum reserve tank? But know nothing about them.

Any suggestions or thoughts?

 

[Sniper]

Hook up a vacuum gauge and see what your actually getting at idle. You need something to base your next move on. Maybe it's something as simple as a vacuum leak in the system. A fat cam, and a small CID engine can be enough to cause a major vacuum drop, add a leak and your into a bog/stall situation.

One goofy problem that crops up from time to time is the missing plug/bolt in the front of the small block Chev engine. Not sure if the 305 is affected or not but it's easy to check. Years ago the small block had a 'U' shaped motor mount bolted to the front of the engine with 4 bolts. It was located under, and up both sides of timing cover. Even on the later engines the boss's are there and tapped for bolts, though not used. The kicker is, the one bolt hole goes completely through to the area of the mechanical fuel pump pushrod. Leave that bolt/plug out and you have a major vac leak. (A lot of pushrods and pumps were ruined (bent) by bolts that were to long.) Since the hole lines up with the pump push rod it's not to hard to figure out which one it is.

 

[RPM]

Hook up a vacuum gauge and see what your actually getting at idle. You need something to base your next move on. Maybe it's something as simple as a vacuum leak in the system. A fat cam, and a small CID engine can be enough to cause a major vacuum drop, add a leak and your into a bog/stall situation.

One goofy problem that crops up from time to time is the missing plug/bolt in the front of the small block Chev engine. Not sure if the 305 is affected or not but it's easy to check. Years ago the small block had a 'U' shaped motor mount bolted to the front of the engine with 4 bolts. It was located under, and up both sides of timing cover. Even on the later engines the boss's are there and tapped for bolts, though not used. The kicker is, the one bolt hole goes completely through to the area of the mechanical fuel pump pushrod. Leave that bolt/plug out and you have a major vac leak. (A lot of pushrods and pumps were ruined (bent) by bolts that were to long.) Since the hole lines up with the pump push rod it's not to hard to figure out which one it is.

If that bolt you are talking about is left out you get an oil leak, not a vac leak.

 

[rainman]

Like Sniper said hook up a vacuum guage and see what you've got. Try different spots with you're dist. (timing) to see how much it changes. Set it where you have the most vacuum, (within limits on timing). See if this helps. Also make sure you're carb is adjusted properly this could have some effect on it too. One other thing, I know the thing you're most likely to do is turn the idle screw up, but don't go to high with the idle this will only make it worse. Hope some of this helps!!

 

[old lady's mad]

i would look for a vacume leak. can you hear a whistle sound from the intake area ? you can care fully squirt some carb cleaner around on the intakes gaskets and vacume ports. if it picks up rpms then you found your problem.

 

[adam]

What's your base timing? You probably need 15-20 degrees initial timing to clean up the idle with a cam like that, and then would also have to lower the idle speed, because it will rise with adding timing. Also make sure your total timing isn't way too high if you do this, also. A vacuum can has nothing to do with making enough vacuum to help a carb that need more vacuum-it only stores vacuum. Two different things.

 

[Sniper]

If that bolt you are talking about is left out you get an oil leak, not a vac leak.

If your running any kind of a PVC system you definitely will have a vac leak, just the same as a bad end gasket/seal on an intake or valve cover gasket leaking.

 

[Tripper]

Any other options that i'm overlooking? I heard mention of a vacuum reserve tank? But know nothing about them.

I've run big cams like that & lost vacuum but it usually only affected my power brakes. I tried a elec vac unit & it was horrible (IMO). Noisy & didn't work that well. I finally went to a Hydroboost system for braking.

BoB

 

[jmel]

Thanks for all the replies. There are no vacuum leaks (at least that we can find). From my understanding, the rochester requires more manifold pressure than that cam (on my relatively small displacement engine) can provide. We tried going to a holley 750, but that proved to be too much cam for that engine. Also, the fact that the previous owner kept it sitting on his shelf for years with gas in it causing it to be a complete PITA to clean probably didn't help either.

Went to a car-show last night and talked to a buddy who has an Edelbrock 500 that he'll loan me to see if that works for me. Said i can use it for a while until i find a replacement for myself.

The Holley fixed the problem with the stalling, but again, just dumping way too much fuel, and running way too rich - couldn't lean it out enough.

In my very limited understanding, there is a modification that can be made to the Rochester, something about lightening the spring and maybe adding weight to the power piston - doing so will let carb operate with less manifold vacuum.

I'm keeping my fingers crossed that the Edelbrock will work for me...and hold me tight until i can convince the wife to let me put a few $$$'s into three 2's. [dr

 

[Toad]

The Holley fixed the problem with the stalling, but again, just dumping way too much fuel, and running way too rich - couldn't lean it out enough. [dr

I'm surprised a Holley 500 would be dumping too much fuel, those are usually what you go to when you're trying to save gas. How did you try to lean it out?

Toad

 

[King Herald]

......Even on the later engines the boss's are there and tapped for bolts, though not used. The kicker is, the one bolt hole goes completely through to the area of the mechanical fuel pump pushrod. Leave that bolt/plug out and you have a major vac leak. ......

I'm trying hard, but I can in no way see how that bolt hole can possibly be connected to the air intake vacuum

It is in the crankcase and if anything will leak oil out, not air in.

I'm actually using those four bolt holes for an engine plate on my pick-up.

 

[old lady's mad]

that hole would be an oil leak. the only way that would be a vacume leak is if some one had a totaly sealed crankcase with no pcv valve and a straight hose going for the vac port to the valve cover or something. then you might have a vacume leak on that hole if left out. that hole is actualy ment to hold the fuel pump push rod in with a long bolt. so it dont fall down while changing pumps. just remember dont leave the long bolt in it .

 

[jmel]

I'm surprised a Holley 500 would be dumping too much fuel, those are usually what you go to when you're trying to save gas. How did you try to lean it out?

Toad

The Holley was a 750 that was dumping too much gas. I went with a Edelbrock 500...and am pleased to announce that it worked like a charm.

The ol' T never ran so strong. Sounds great, and performs stronger than it did with the Quadra. Happy as could be.

Still might be running a bit rich, as the fumes are still quite strong, although that might have something to do with lakestyle pipes blowing fumes in my face.

 

[sgtpontiac]

low vacuum letting secondary metering rods open

Thanks for all the replies. There are no vacuum leaks (at least that we can find). From my understanding, the rochester requires more manifold pressure than that cam (on my relatively small displacement engine) can provide. We tried going to a holley 750, but that proved to be too much cam for that engine. Also, the fact that the previous owner kept it sitting on his shelf for years with gas in it causing it to be a complete PITA to clean probably didn't help either.

Went to a car-show last night and talked to a buddy who has an Edelbrock 500 that he'll loan me to see if that works for me. Said i can use it for a while until i find a replacement for myself.

The Holley fixed the problem with the stalling, but again, just dumping way too much fuel, and running way too rich - couldn't lean it out enough.

In my very limited understanding, there is a modification that can be made to the Rochester, something about lightening the spring and maybe adding weight to the power piston - doing so will let carb operate with less manifold vacuum.

I'm keeping my fingers crossed that the Edelbrock will work for me...and hold me tight until i can convince the wife to let me put a few $$$'s into three 2's. [dr

power circuit starts to work under low vacuum..... quadrajet metering rods are opened by spring pressure after vacuum drops(rods rise out of the metering jets).... due to low vacuum you are probably having to have the idle speeds too high (open throttle blades) and the idle circuit is already out of the picture.... edelbrocks still require a vacuum signal to pull down the metering rods at both corners to keep it leaned out at idle.... as was said, need a base timing setting at 12-15 degrees, check vacuum signal and make sure your quadrajet to holley adapter plate is not leaking (if it's not a holley that was made for quadrajet manifolds) if the vacuum is 14 or 15" at idle then it should be enough and carb tuning / replacement is required and or timing needs to be sorted out.... in any event, the carb throttle blades must be closed at idle or the power circuit (rich condition) will result...... sounds like you are at lower than the 14 or 15"'s of vacuum and will cause a flock of problems.... vacuum cannisters won't help....

 

[rainman]

Thanks for all the replies. There are no vacuum leaks (at least that we can find). From my understanding, the rochester requires more manifold pressure than that cam (on my relatively small displacement engine) can provide. We tried going to a holley 750, but that proved to be too much cam for that engine. Also, the fact that the previous owner kept it sitting on his shelf for years with gas in it causing it to be a complete PITA to clean probably didn't help either.

Went to a car-show last night and talked to a buddy who has an Edelbrock 500 that he'll loan me to see if that works for me. Said i can use it for a while until i find a replacement for myself.

The Holley fixed the problem with the stalling, but again, just dumping way too much fuel, and running way too rich - couldn't lean it out enough.

In my very limited understanding, there is a modification that can be made to the Rochester, something about lightening the spring and maybe adding weight to the power piston - doing so will let carb operate with less manifold vacuum.

I'm keeping my fingers crossed that the Edelbrock will work for me...and hold me tight until i can convince the wife to let me put a few $$$'s into three 2's. [dr

One other thing, are you running an electric fuel pump? It's possible if you're not using a pressure regulator, that you could be giving the carb too much pressure. That could make it simulate a too rich condition! Just a thought.