Vacuum

I have a new motor that I purchased built and I am trying to figure out some vacuum and carb problems...

Here is what I have:

76 block std bore 10 to 1 stock 70 pistons stock rods
70 heads with Brand new stage 1 valves & TA valve springs
Using the 70 rocker arms (alum) New AC spark plugs
New TA 413 cam & lifters New TA fully grooved cam bearings
Std/std crank all new bearings and rings
Felpro gaskets set used
Double roller chain
Booster plate oil mods with 5/8 pickup with a 550 rpm drill 58lbs oil pressure
Heads and manifolds were surfaced and I now have the Performer intake
70 455 carb rekited and cleaned


I have been messing with trying to get the carb working better and I am only getting about 7 to 8 inches of vacuum. I have the motor idling around 800 RPM's at this reading. I am sure there is a slight leak in the carb. If I cover the carb with my hand slightly, my idle speed increases to about 1000 RPM's and vacuum jumps to around 15 inches.

I have sprayed carb cleaner at the base and notice the idle speed increasing moderately, but only around the rear/passenger corner area and 1/2 way in each direction.

I do have my brake booster vacuum connected and the top driver side port to my distributor for timing advancing, and to the PCV valve, with the rest blocked off.

I definately have a problem with my brakes at lower speeds.

Does anyone else have a similar combo that I have that can tell me where on the vacuum scale I should be? And ideas of what in the carb could be causing the leak in the base area?

I did put a new base gasket in and also ensured all screws were tight for the carb main body to its base mount.

Thanks for ANY help...

Mike

 

Couple of suggestions:

1) Is the manifold surface perfectly flat where the carb attaches? Take the carb off and check it with a straight edge. If it isn't perfectly flat, you can use a thick gasket for something minor or have the manifold machined flat.

2) Is the carb base warped? if the carb isn't snugged down evenly by tightening the four bolts/nuts a little each then moving to the next one until they are all tightened enough, if they aren't at an even torque value or if they are over tightened, you can push the base to the point where it is no longer flat and in extreme cases, break a piece off!! Same deal with the straight edge.

3) What kind of gasket are you using? Thin paper, steel shim? In some cases, the wrong gasket can allow a vacuum leak even though it "appears" to be correct. I had installed a rebuilt Quadrajet on a stock 68 Riv with the wrong gasket and had a big time leak. Take the carb off, drop the gasket on the manifold and make sure it seals completely. If it is even close to not fitting properly, go find the right one. Flip the carb over for the same test.

4) Dumb question: It IS a NEW gasket, right? Most everytime I have tried to reuse a carb base gasket, it bites me and I have to purchase a new one anyway.

I'm not sure what kind of vacuum you should see with your combo. Things like cam timing (advanced, straight up, retarded), ignition timing make a difference. Perhaps one of the guys running a similar combo with chime in for the numbers.

By the way, which carb are you using?

 

The carb, I believe is a 1970 Q-Jet. Which is slightly smaller CFM than 71 and up, I believe.

There was origonally a spacer gasket in there to help my air cleaner clear the HEI that was put in for a short while. I have removed it and put the points dist in and purchased a Mr Gasket Q-Jet gasket for it. With both gaskets, I had a leak in the same area, with exact symptoms.

I will try the straight edge trick this weekend hopefully. It looks like I will send it off to get rebuilt and set up. I have heard a lot about John Osborne being the best. So I am looking to go that route.

I hope someone with a similar combo as I have will chime in and let me know what numbers they are running.

Thanks for any help...

Mike

 

Mike,
Sure sounds like a vacuumn leak. GS464 gave you good advice. I am putting in my new motor this week with almost the exact combination (have an SP-1 manifold.) Will let you know what I get for vacuumn as soon as I get the reading. Currently waiting for my carb back from Bowtie Overdrives, they are modifying the linkage to use with 200r4. You should be getting 10 to 15 inches of vacuumn with a 413 cam.

 

Thanks Doug,

I will definately look to hear what numbers you get. I spoke with John Osborne last night and he agreed that the numbers should be higher. I am going to send the carb off to him to build and calibrate for me. I was thinking a good 12- 13 inches of vacuum at least. I still have a few more items to iron out as well while the carb is off, what is another 2 weeks? Heck I had the motor sitting on a stand waiting to be put in for just over a year. But the big problem is that I have driven it, even though it is not right, I got the big block fever back again !!!

Mike

 

Michael
Just fired mine up and am also only getting about 8 inches of vacuumn. Motor does sound wicked though. Need to get some time on it and work on the tune. I think that I am a little lean on the carb so will work on that as soon as I get the transmission up. Let me know how your carb works out.

 

I sure will. John has my carb and dist right now to do up for me. I should get it back just around labor day weekend, but will not put them on until after the weekend, due to previous plans. I am definately anxious. Keep me posted as well. Mine also sounded very WICKED... What a word, huh? Definately a crisp response for sure.

It was funny, I just moved down here to GA. My neighbor was telling me about the guy across the street - a talk they had. I have not met him yet. He was telling my neighbor that I had just brought a car in, AND IT SOUNDED PRETTY HEALTHY !!!

That made me PROUD !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

At least down in GA now, I have a lot more of the year to work, tinker and play with my combination.

Mike

 

I think the problem is a leak at the base plate gasket. I sprayed WD40 at the base and the vacuumn jumped to 10" and smoothed out the idle. Going to put on a new gasket and a light coat of sealer and check that out. Will let you know Thursday how it works out. Usually its something simple.

 

Finally got the car back from the tranny shop and got to drive it a few miles. I am still only pulling 10 inches of vacuumn and don't know where the timing is because my nephew borrowed my timing light. I do know that the carb is acting weird. I put on new base gaskets and did get rid of the leaks but I only gained a few inches from 8 to 10. When I go to adjust the idle screws I can screw them all the way in and have no effect. When driving the car it acts very lean.

This is the same carb that was perfect on my old motor. Don't know how it is jetted because I did not have to before. Going to take the car over to Joe Taubitz next week and see if we can get it tuned in.

Any ideas out there let me know.

 

I received my carb and distributor back from John Osborne Thursday. I am looking to install today and/or tomorrow. I will post how it is doing with numbers. I sure hope it is all good. John told me that the carb had been through the wringer to say the least. So I hope it was definately the problem.

Mike

 

Mike,
What's the latest. I have been pulling more vacuumn as the motor breaks in. Have not had a guage on it lately but it was at 10" last time a checked and feels better the more I drive it. I am running 18* initial advance also. With my distributor setup it is at 34* total. I still think I need some carb mods but waiting to here how yours worked out.

 

I got the carb back last week and put it on. John Osborne did the re-work. He told me to drive it for a while before messing with it and let the motor break in more. I did put my gauge on it, but it is not at 0 anymore. It sits at like almost 2 lbs of pressure not connected. But I was slightly above 11 now with that gauge, so I am probable around 15 to 17 inches of vacuum at idle. A definate difference in the brakes. I do find myself a little out of vacuum every now and then, but nothing like it was before. I still need to do my exaust up and I have a miss now. I replaced my plugs, wires along with having the distributor reworked by John as well. I believe I have a bad plug in there, and have to diagnose it when I get back from Cali. I am out here for school for work. Can you say "A paid vacation", LOL...

I did talk to John today and he re-stated to definately drive the car a while before making any adjustments, as the motor definately needs to break in.

I will let you know more when I get back on it.

 

Doug,

I got back yesterday and took the car out today. It still has a slight miss and I have not figured that out yet. I need to find out a good plug to run, I just put AC R44TS's in and I am not sure of thier heat range. I know that is seems like the back 3 plugs on the passenger side are doing nothing, at least no difference at idle when pulled.

I hooked the vacuum gauge up and tweeked a little on the screws. My gauge needle is off when disconnected, so I know it is not correct when connected, but I would say I am around 12 inches of vacuum at idle and raise well up into the green when revved up.

I am thinking my vacuum is not as good as it will be with the miss in there.

How are your tests coming along?

I definately have much better vacuum now, and the brakes show for it.

Mike

 

Mike,
I am using the r44ts also. This is a pretty standard plug for street use. Would probably drop to a 42 for racin. I do not have any miss but I am running lean and out of gas at the top end (I guess at that point I have a big miss.) Was trying to buy an Edlebrock 850 on Ebay today but I figure anything used should go for half of new and the bid got out of hand. I know I need a new fuel pump and carb mods. What kind of miss?

Try this, take your timing light and hook it to the cylinders that seem week. See if you get a good signal off of each wire with a nice strobe from the timing light. If not you have and ignition problem.

 

Mike,
We have not discussed your ignition system. Are you running an HEI system? If you are you should check your voltage at the coil. If you cannot maintain 12.5 volts at idle I think you have a problem with your alternator and you will have poor performance at idle.

 

Doug,

I am not running a GM HEI. I do have a Crane XRI module in my points distributor. I checked by removing wires and found cylinders 4, 6, and 8 not having a difference when disconnecting them. I was concerned, and did a compression check. All cylinders are 155 to 170, mostly 160. I had my plug gap at .045 and was told that may be a little too much for my system, so I closed them some to around .038 and the miss is not there anymore. I am running a MSD Blaster II coil, which is the size of a stock coil. I thought it would have handled the gap, but I guess not. It is funny that the miss was only at the lower RPM's. When I got on the throttle, it did not seem to be there. It definately had power to me.

Needless to say, I believe my problem is solved. But John Osborne recommended a Motorcraft plug (non-resister), that seems to be unavailable (AF42C). Not even at the Ford dealership, they said they have never carried it. I am thinking about going with a NGK plug, if I can figure out which one would be best. I am not concerned about radio interferrance, as I drive my car for the pleasure of driving it, not for the tunes I can hear, if you understand what I am saying.

I am going to post another question of suggested plugs for my application, and what the advantages are between resister and non-resister plugs.

I appreciate any info you have to help, thanks !!!

:TU:

Mike

 

Doug,

By the way, I have been told by a couple of different motor buffs here to not run vacuum timing advance. To disable it and run the 34 to 35 degrees at 1500 to 2000 RPM's with the mechanical advance kicked in. Put the lightest springs on the weights for quickest advance.

So I have no vacuum advance at all. There is a drastic difference of mechanical advance between an HEI and a points distributor, some FYI to think about also. I only have about 6 degrees advance with my points system.

 

I am running the reccommended 35 degrees timing, and even had 87 octane gas and was not pinging a bit. Which would be better to boost me up, the UR4 or UR5?

It sounds like you are saying the UR5 will be like advancing some. I believe I can afford it, to say. I have always liked NGK plugs, and never heard a bad word about them.

Thanks, Mike

 

Mike,
I think you have a weak ignition system. I was very surprised at the performance boost I got when I did a complete rebuild and upgrade on my system. Here is a good link on spark plugs, ignition systems etc www.centuryperformance.com go into their tech section.