|11-07-2007 09:47 PM|
I think, in your particular case ... with leaking intake gaskets ... that it has to do with engine load.
If the engine is not under any load, (not in gear) it would maintain enough port velocity to deliver a lean fuel mixture ... but still enough to make each cylinder "carry it's weight", at least partially. The momentum developed by the "good" cylinders would also help to carry the ones not making a full contribution.
Once you drop it in gear ... the load increases, the port velocity decreases, and the momentum effect is largely absorbed by the torque convertor. Those cylinders that are not carrying their own weight are much more of a liability now ... and idle quality suffers.
Similarly, if you have a header gasket leak ... you don't notice it nearly as much until after you drop it into gear and put a little load on.
|11-07-2007 03:16 PM|
|1drwgn||main reason vacuum drops is because rpm drops. the engine is not pumping as much air at a lower rpm and therefore is not pulling as much vacuum against the butterflies of the carb. just like when you are cruising on the highway, higher load at a lower rpm vacuum will be higher, something like 20 to 25 or even higher depending on the engine size and whatnot, because of rpm and lack of opening in throttle blades. hope this blurb is correct and helps.|
|11-07-2007 04:37 AM|
Because the carb might be too big thusly when the rpm drops the airspeed cannot keep it's velocity due to the carb size??? Just a very random idea. I really couldn't tell ya.
But now that I'm on that subject, I don't recall what carb you said you had but could it be possible your secondaries arent closing all the way?
It's probably best you ignore my random train of thought
|11-06-2007 10:14 PM|
Sorry about the confusing post. Im wondering why the vacuum is so greatly affected by shifting into gear. At park the engine idles with 17 Hg" of vacuum. The moment I shift into reverse or first gear, the vacuum drops to 10 Hg". Im just wondering what is happening inside the engine that causes this significant amount of drop in Vacuum?
|11-06-2007 08:10 PM|
The magnetic stand just lets you put the dial indicator anywhere, except on an aluminum engine.
What gears are we talking about changing. If the timing gears or off the cam can be retarded or advanced from where it should be, this would affect engine vacuum. Changing rear end gears would have no effect on idle vacuum but will change cruise vacuum because the amount of throttle open is changed for the mechanical advantage of the gears.
|11-06-2007 06:38 PM|
what does the magnetic stand do for the dial indicator or measurement? would I be able to measure what I need to with just a dial indicator ?
If it is a manifold leak (and im almost positive it must be) then how would changine the gears decrease the vacuum as significantly as it does. I understand how a vacuum leak would cause the symptoms the engine has, but why does the gear change double the trouble?
|11-06-2007 04:49 AM|
|11-05-2007 05:47 PM|
305 sbc good idle, Dies when in gear
for STOCK to MILD engines i used a safe bet rule(rules meant to be broken though) double the CI equals carb size...305 x 2 = around 600
|11-05-2007 03:27 PM|
Thank goodness I have you guys and this site. I havent taken the heads off of an engine so I was a little excited to rip em' off. But I'm really glad that I dont have too. Its definately oil, and I wasnt positive that it was coming from the heads. I think its probably the intake manifold gasket that is leaking.
I recently took the manifold off and put it back on without replacing the gaskets. Now I can do it right (shouldve done it the first time ). Im on a time crunch to get the car out of the unit and on the street within the next two weeks. I just want her running so I dont need to pay for such a huge storage unit. I'll get a smaller one and store the accessories, carpet kit, chrome etc. in that. I also have a few ideas for turning a smaller unit into a "workshop" that I can run a small ebay side business from.
Fbird88, the car has always been really rough and boggy off the line. Its never been this bad and I havent noticed that much oil from the manifold gasket as well as had this bad of an idle since I put the manifold and carb. back on. This time I'm obviously replacing the gaskets and Im going to run a bead of sealant along the front a rear of the manifold instead of using the rubber gaskets that have been on their (which is where the leak SEEMS to be comming from)
Thanks for all of the advice and words of encouragement. It helps more than y'all know (no we dont usually say y'all in Southern California but it seemed appropriate ) Have a wonderful week everybody!
|11-04-2007 06:23 PM|
|TANGO||I think your oil leak is a valve cover or the rear of the intake . Check it better before pulling the heads . Also try adjusting the valves on that engine . It may idle real good when your done . That engine sounds mild like a stock cam . Also the next time you pull the plugs do a compression test . You may have a low reading on one of the cylinders . I had a 305 that ran like that it had a bad valve spring . A compression test will tell you that . Dont let that engine get you down . You will fix it .|
|11-04-2007 06:20 PM|
i see all of this talk about a 750cfm QJet being too big for a 305. wait, what 4bbl carb did GM install on all their 305s from the factory ? Yes, a 750cfm QJet. also, with the way the QJet was produced, with secondaries open, it only netted a little less than 600cfm total.
dont forget, the QJet is an air dam carb, it will only flow what is needed, nothing more nothing less.
lets take my engine for example:
305 bored .040" over making 312ci
dish top pistons
Edelbrock Performer intake with EGR
port matched 601 heads
Lunati Bracket Master II 00016LK cam (.458/.458 lift, 218/218 duration)
Dynomax blackjack headers with 1 5/8" primaries to 3" collectors and dual exhaust with turbo mufflers
2.29 rear gear
ok, now im getting almost 16MPG in the city and 25MPG on the highway with my "big" carb. granted, the rear gear helps the mileage, but still, isnt that impressive for an SBC that is nearly 30 years old ? oh, and yeah, its still putting out 210hp to the wheels.
1970Chevy, ive given you this advice already but apparently it falls on deaf ears.
|11-04-2007 06:14 PM|
What parts are you taking off???
A head gasket doesnt leak oil.It leaks water or glycol fluids.
A valve cover gasket leaks oil.only 4 bolts.
Is it leaking oil out of rear of intake manifold and seeping down ?
|11-04-2007 05:51 PM|
Its just straight headers right now. I have delta 40 series under the car but they arednt bolted to the headers yet. I cant do it yet
So today I went down there bright and early. After running the engine for about ten minutes she ran out of fuel. Then I noticed a significant oil leak toward the rear of the engine. I did some poking around with a flashlight and mirror and found out where the leak has been coming from. Guess........the drivers side head. I think that this explains the symptoms of a vacuum leak!
I began to drain the radiator fluid AGAIN, and slowly taking parts off. It was a crazy morning to say the least and too much to handle on a sunday. I figured if God takes the day, I might as well too. I have a lot of work ahead of me. Im hoping its just a head gasket. I'll also replace the manifold gaskets.
I dont really need to get this car out of that storage unit. I cant afford new gaskets, my bills, and $220 for rent at the unit. Hey, Ive been doing this same thing for the past few months, whats another week or two. Ive gotten it done this far, no stopping me now ha ha ha
any thoughts? am I crazy or would this actually be the cause of the elky's symptoms?
|11-04-2007 03:01 PM|
|stroker_SS||Just curiosity but, what kind of exhaust do you have on that thing. Pretty freakin loud.|
|11-04-2007 08:50 AM|
You have good point td
One thing for sure,
If I owned a business in California I'd hunt that kid up and hire him 'cause he ain't afraid to 'dig in'.
Only sometime your diggin' in wrong spot....haha
yer doin' a fine job bud(Matthew) have a good day
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