low engine vacuum at idle help

[sassy383]

sorry for the delay i have had alot going on and no time to mess with my car. for the torque converter it is a 2500 stall tci. i have come to the conclusion that i will problem try to get another carb possible speed demon or holley and see if it runs any better. it seems to have its moments. i have tried just about everything with this carb and nothing seems to work. i have checked all the base engine and all looks great. the engine is a fresh build with about 100miles on it. compression, leak down, equal lift, it all checks great. i feel as if the carb is just working for me.
i will keep you posted
thanks
pat

[Charles]

I have never ran that cam,  but I really dont see it being that low on vac because of it.  What ever the problem is,   dont let it get you down, when you hit the fix your car will change instantly

I agree. Keep looking. Whatever the problem is, it's not because the cam is too big, as others have already pointed out. I had the same cam in a (re-ringed, basically stock) '69 383 and it idled with barely any lope, with 11-12" vac.

I know you said you've doublechecked the initial timing. I may have missed it, but did you verify that your TDC reference mark is actually TDC? If it isn't, your 16 degrees isn't right either.

-Charles

[Pro Street dart]

P-WOMAC1,

Don't switch to an Edelbrock Carb. It looks like I have the same problem as you do. I have a brand new Eddy 800 AVS carb.

I have to start checking things everybody mentioned in the thread.

[speedymopars]

High drop in idle speed when going in and out of gear is caused by too lean condition.

[signet]

the problem more likely is with the carburetor.

you had a problem earlier and this as it turned was a float level that was way too low.
if you were to go out and get a new/properly rebuilt carb the problem would probably go away and you could assume Carters/Edelbrocks are junk.

but the problem was more than likely someone with good intentions messed up on the set-up or there is a bit of dirt were it should not be.

you managed to set the float levels properly so you should be able to totaly disassemble the carb and reassemble it.

get a book on the Carter AFB and get a gas torch orifice cleaner/tip cleaner and the proper kit for the carb, blow out the passages and clear all those small ports ect.

a pro could do it in less than 2 hours unless it needs a hail-mary(shafts worn out or severe corrosion).

[sassy383]

you are correct the carb was the problem. i got a carb from a buddies dad that he had sitting on the shelf for several months after taking it off a car he used it on for 3 years (it really needed a good cleaning on the outside). bolted it on and hooked everything up and away i went. no stumble or hesitation at all, no backfire through the intake at all. it ran like a bat out of hell. it was also a 750 edelbrock just like the brand new one i started with. it was bone stock jets, metering rods, etc. i guess i must have got a bad apple. i called edelbrock and got an authorization to send it back, called summit and they are going to replace it. what luck, hopefully the next one will work better than the first.
i appreciate everones help.
and by the way i have a steady 8hg of vacuum in gear. idles and runs great with the old junky carb.
thanks
pat

[signet]

P-WOMAC1

that was a brand new carb? and the floats were that far off.

[sassy383]

yes the floats were way off, and i seem to think that the carb has some sort of casting flaw some where. it does not seem to work very good in any aspect.
pat

[thecarfarmer]

X2 on the throttle being open too far:

Somethings just not righht here....

Moving the metering rods up/down in/out should have absolutely no affect on idle speed or mixture because thats not the idle circiut, it's the main circuit and power enrichment circuit. The only effect, if any, would be a small vacuum leak if you were to remove the metering rod pistons completely while it's idling.

Here's how you check if the primary throttle blades are open too far:
Looking at the front of the carb there are two small vacuum ports on the front of that carb. One sets low near the base and the other sits a little higher on the other side. The one low near the base is manifold vacuum. The one a little higher on the other side is venturi vacuum.

When at idle there should be no vacuum signal at the port that is raised a little higher, the venturi port. If there is any kind of vacuum signal on the venturi port during idle then the primary throttle blade is open too far and you are drawing fuel out from the main circuit and it will be nearly impossible to tune correctly.

If you're able to change idle mix by pushing on the metering rods, you're pulling fuel thru the main circuit.  That's not right.  Either the motor has a mechanical problem (cam in wrong, dead holes, vacuum leak, etc), the ignition timing is off, or the fuel mix is off.  That's really about it.  Oh yeah, it could be a mix... but you'll catch that if you systematically check every little thing.



I'd check the fuel mix and the damper for being right.  If you really have 14 deg of initial advance, the car should idle strong with a modest cam like that. 

Break out the propane tester, and check how lean it is at idle.  To me, nothing beats having a #.  I'd set a performance car up for maybe a 50 rpm boost (will only gain 50 rpm when feeding propane into the air horn).  If you don't have a setup to do it, PM me and I'll give you instructions how to make your own tester for about $10 or so.

If you have a good mixture and still can't get it to idle right, try bumping up the timing several degrees.  See if that does it.  If that solves the problems and the car doesn't ping a bunch... betcha' your damper is off.  If that doesn't do it, check for leakdown, cam timing, other stuff...

-bill

[Pro Street dart]

Hey Guys,

Looks like I am having the same problems as P-WOMAC1. This a brand new Edelbrock RPM Performer & BRAND NEW Eddy 800 AVS carb. My old MP M1 Intake & Eddy 750 did the same thing. I'll start checking things in this thread. No vacuums leaks to be found. Could the floats be off? Shouldn't be.

I don't run a PCV valve. Port is capped. If I hooked one up could this help? 

 I could never get the 750 to come off idle right. Always stumbled. Messed with it til I was blue in the face. See what "Sam" from Century Performance says about the 750:

http://www.centuryperformance.com/forum/topic.asp?TOPIC_ID=63

"...Now, in regards to the 750 version, just SAY NO! The carb is a slug. It has the worst signal of anything we have ever tested/dynoed, and is just not an efficient or powerful carb. In plain words it is a .
The 500-600cfm versions are great, the 750cfm is unworthy of being on an engine. 
Best Regards."

[70cuda]

I agree with clonevertible, I had a very similar problem with my 440. After years of sitting, the engine ran just like yours with about 6'' of vac. I figured the carb must be the problem and after 3 rebuilds on a fairly new holley no luck, still low vac. I changed the power valve to compensate for the vac but it just covered up the problem, not solving it. I tried the spray around the engine to find the vac leak trick but nothing changed. The simple problem was loose manifold bolts. This was on an engine with a fresh, mostly stock rebuild, lots of new parts. The simple solution was to tighten the intake manifold bolts on the edelbrock manifold! Hope this helps as I couldn't believe it when it happened to me. Good luck.

[dusterman]

You certainly have received a lot of advice on solving your idle problem. I would like to suggest you start by taking a compression test with the engine warm and throttle open fully and give us your readings. This will help determine if any valves are sticking in the guides, not rare in new heads. Also with 236 degrees @ .050 usually requires at least 9:1 compression to limit exhaust dillution with the incoming intake charge. If you have less than 9:1 a cam change to around 230 degrees max at .050 will really help your situation. Keep us informed of your findings .

[NYrr496]

I read further back that you have paper gaskets on both sides of your tin intake gasket. I did that once and immediately turned my car from a perfect running 440 into a low vacuum, poor idling mess. The port alignment was off and I'm sure it was leaking at the bottom of the ports. Remove all the gaskets and add sealer to the tin. I bet the problem goes away.

[sassy383]

at this point i will try anything, what kind of sealer do you recommend
pat

[sassy383]

dusterman, the cam i have is advertised duration 238 int and 240 exh. the pistons that i have were advertised 8.94:1 with 88cc heads and my heads are performed rpm with 84cc. i think that puts me really close to 9.1. could this still possibly be the problem
thanks
pat

[signet]

that cam should give you 20" of vacuum easy.

[Pro Street dart]

at this point i will try anything, what kind of sealer do you recommend
pat

P,
 I ran a search @ Moparts last week about sealing the valley pan.  80% of reply's were "Dry" both sides 20% were Permetex head side only.This is what I did just to be safe.  Paper gaskets too: no. (unless the heads and or block has been milled or just one.

Correct me If I'm wrong but,

Permextex's products are not Gasoline resistant, although they should? not be subject to raw fuel.

I dunno.

[Pro Street dart]

that cam should give you 20" of vacuum easy.

I am getting 10" of vacuum @ 1000RPM in Neutral w/ a purple stripe 484/284AD.

Car still runs like a bat out of hell.

[NYrr496]

at this point i will try anything, what kind of sealer do you recommend
pat

I don't remember exactly what I used, but it was a liquid product by Permatex that gets brushed on. It sets up very thin. I believe in my heart this will solve your problem. Even try dry. I know in a pinch, I've re installed my gasket without applying new sealer.

[dusterman]

Pat, even though the pistons were advertised at 8:94 comp. ratio, this is assuming that the deck height is at some specified number. If your piston is further down in the hole, not uncommon in chrysler blocks, your actual comp. ratio could end up as low as 8:1. That is why a compression test at this point is important. Hughesengines.com, a mopar only engine builder, recommends a minimum of 170 psi. cranking pressure with a cam having 236 degrees @ .050. When we know what your cranking pressure is, and if all cylinders are close to the same, we can determine if the cam is  right for your application. So get us some numbers when you can.

[sassy383]

ok i checked the compression, they are all very low between 90-95 psi. does this mean i have low compression ratio or do i have a base engine problem. all cyilnders were about the same just low
thanks for the help
pat

[dusterman]

Pat, those are not good compression numbers.I assume your compression gage is accurate and you did have the carb butterflies fully open. If not, recheck a few of the easier accessible cylinders to see if there is any change in pressure,since they were all very  close in your original test. I would then reccommend you perform a leak-down test on some or all of the cylinders to determine if there are any serious leakage problems with the valves sealing, or piston ring sealing. The leak-down gage will give you a percentage of leakage in the cylinder and will identify what is leaking by listening where the air is escaping. This will determine if the engine is sound or what will need to be fixed before assuming the cam is too big in duration. Leakage should be less than 10%.A well built engine will have less than 3% leakage. Good luck and let us know what the results are so we can advise you further on correcting your problem.

[sassy383]

i did not do an acutally leak down percentage but i did pressurize the cylinders with air and listen for leakage in intake, exhaust, and crankcase. i did not hear any leaks. could the low compression be due to the cam duration being too high. i will try to find a leak down gauge in the mean time
thanks
pat

[dusterman]

Pat, very low or no leakage is a good sign,but the low cranking pressures are not. With your present cam of 236 degrees at .050, I have to assume that your deck height must be somewhere around .080 or your cam might be installed in a retarded condition and/or your timing chain may be loose. If this is not the case, you can advance your cam 6 to 8 degrees to raise the cranking pressure or install a cam with about 220 degrees at .050. The best, but the most labor intensive and possibly expensive fix, would be to pull one of the heads and measure the deck height to verify what it actually is, and if it is more than .040, a change of pistons to ones with a compression height to bring the piston as close to 0 deck as possible would certainly raise your cranking pressure to what it needs to be with your present cam. This will really wake your engine up. Keep us informed of what you find.   

[NYrr496]

AND get rid of those paper intake gaskets.

[sassy383]

my cam has holes to advance or retard 4 degrees. if i advance my cam 4 degrees, you say it will raise the compression a little. will this also give me more vacuum. if i can get more vacuum to help my idle the car will run much better. the power on accell is pretty good. just has bad idle
thanks
pat

[dusterman]

Pat, advancing the cam will raise the cranking pressure but not the compression ratio. But doing this will help raise your cranking pressure above the 90 to 95 psi. you now have. This will happen because you will be closing the intake valve sooner and you will have more degrees of rotation before tdc. How much cranking pressure you will gain I dont know but it will certainly help your situation. As your cranking rises so will your low speed performance. Keep us informed of your progress.

[sassy383]

i think i will try advancing my cam and see how it idles and what the vacuum does. that seems like th easiest thing to do at the moment. thanks for all the help and i will keep you informed.

[signet]

"you are correct the carb was the problem. i got a carb from a buddies dad that he had sitting on the shelf for several months after taking it off a car he used it on for 3 years (it really needed a good cleaning on the outside). bolted it on and hooked everything up and away i went. no stumble or hesitation at all, no backfire through the intake at all. it ran like a bat out of hell. it was also a 750 edelbrock just like the brand new one i started with. it was bone stock jets, metering rods, etc. i guess i must have got a bad apple. i called edelbrock and got an authorization to send it back, called summit and they are going to replace it. what luck, hopefully the next one will work better than the first.
i appreciate everones help.
and by the way i have a steady 8hg of vacuum in gear. idles and runs great with the old junky carb.
thanks
pat"

are you working on a new or different engine now?

[sassy383]

it is the same engine that i had the carb problem with, i solved the backfire through the intake and stumbe or hesitation with a different carb but still have the poor idle. i have found out the idle is because of low vacuum not holding the metering rods down in the carb and the engine is loading up. i now have to pinpoint the reason for the low vacuum or low compression. my guess is the compression ratio of my motor is way too low. i feal really confident that all is put together correct and adjusted correct.i am a master ford technician so i have pretty good automotive experience (9years). i just know nothing about carbs, but lots about new cars and fuel injection. all you guys are superior about these motors.i think a little reading up about 383 engines and there poor compression ratio calculations and low piston deck heights would have probably help me from the beginning. i can tell you that when i checked piston to valve clearance when i built the engine i remembering being shocked at how much clearance it was. i could not tell you what it was becuase i have slept since then but this is the reason i feel it is low compression ratio. i guess the excitement of putting the motor together then did not spark  suspicion at the time. you live and you learn.
thanks for all the help, i will let you know what i find
pat