So I just recently got the engine back in the car after rebuild and paint. Its been nearly a year since the car was driven last. Engine work was mostly top end (new cam, lifters, rockers, springs, pushrods, timing set). I'm using the same carb, distributor, and ignition as before rebuild and it all ran fine previously. Spec's below.

I primed the oil system and ran a good break in cycle, didn't notice any problems. On the first drive, I got a lot of hesitation on take off and misfire. Even when running a constant 40 mph on the road, I get a random misfire. Happens more on acceleration. Also seems to get worse as the car heats up.

I started at 10 deg initial timing, and have since played around betwen 6-12 deg and timing doesn't seem to have much influence.
I'm also noticing at idle, it won't keep a constant RPM. It jumps around 30-40 RPM at any given set idle speed. I was trying to adjust the mixture screws on the carb and can't get an accurate feeling of an RPM swing because its jumping around so much.

I checked the plugs after the first drive and they seemed a bit black and had a little oil residue. Not sure if its related and I'm hoping the slight oil on them was just from break in before seals and rings found their home. All 8 seemed very uniform. Also not sure if this is related but I have very low (~5 psi) oil pressure at idle but it jumps up well on accelleration.

After searching the archives here, it sounds like I may have a vacuum problem..causing RPM to fluctuate? Would this be related to the misfire? I don't see any leaks or gaps around the intake and the only vacuum connection I have at the carb is for the tranny (no vacuum advance distributor or power brakes). To check vacuum, do I have to check at a port in the intake or can I check one of the unused plugged ports on the carb?

Also I was going to check compression across the cylinders but haven't gotten that far yet.

Any other suggestions??????

Engine specs-
1967 400, 670 heads with dished pistons to lower c/r to ~8.6
Summit 2801 cam
performer intake
MSD ignition & distributor
Edelbrock 1910 quadrajet

Based on your description of the plugs, it sounds like you have a rich running problem?? You can check engine vacuum at either an intake port or at the carb, but make sure the carb port is in the base plate. Looking for 20 inches or so of vacuum at idle. Based on the drivability issue it sounds like a lean misfire which doesn't account for the plug description. How old is that quadrajet? The primary throttle shafts will case a vacuum when they get some miles on them. Maybe time for a rebuild? They don't stay very happy either when they just sit around for a year or so. I'd point a finger at the fuel system first.

Let us know!

What fuel you running? 8.6 is very low and you could be running into alot of unburnt fuel with that big cam for that 400. IMO, too low.That is a 214/224 grind. You are running 2 barrel compression with a decent 4 barrel carb and camshaft. Not good at all. Engine is under performing and falling flat on its face.

The closed chamber 670's are not helping any either. Those are the best flowing low lift performance heads being hindered to the hilt.

Engine not pulling vacuum because it can't. Signal to carb muddled and mixed and no matter what you do, you will never get it right unless you change out the pistons for 9.5 nominal.

1) Not sure on the age of the carb. I know they don't sell this model anymore so its at least several years old. I didn't give it much attention in the past but it would probably be good to rebuild it anyway

2) Banshee, I guess I was thinking 8-9 C/R was OK for a pump gas street driver..? I usually run mid-high octane pump gas. The 8.6 number is an estimate. I didn't take volume measurements when I had the heads off because I thought I was in the ballpark. 670 heads but the block was bored .030 over and it had the 4 scallop top pistons. The cam I took out was a 216/228 lift that ran fine so I thought the new one wouldn't have much change. I didn't know any of this before I bought the hardware and opened up the engine.
The carb is a 850 cfm designed for the performer RPM intake which I don't have. Sounds like its too much carb anyway.

I suspect a carb problem also.
But you need a base line for the tune up.
Do a cranking compression 1st.

FYI: I also have an Edelbrock 1901 Q-jet replacement carb (there is no 1910). You will find that Edlebrock still carries the instruction manual on their website as a PDF file.

Mine dates back to just before I bought the car in 1997.

OK its been a while but I've been working on this for weeks. Figuring my compression ratio again, I think the post above about mine being too low makes sense. However I didn't do anything in the rebuild to change c/r, and the car ran OK before. The only real change was the cam.

Old cam was a Crane 216/228 @ .050, 272/284 duration, 112 deg sep, .454/.480 lift. New cam is 214/224 @ .050, 272/282 duration, 112 deg sep, .442/.465 lift.
Is it conceivable this slight change would hinder performance that much? Would it change my dynamic C/R?

Compression checks good on all cylinders.
The best vacuum I can pull is 15-16", and thats when the ignition timing is advanced way beyond what it should be.
I tried a new carb with metering rods supposedly leaned out for my elevation and had worse performance and flooding. I'm guessing the old carb that came on the car has been jetted way lean just to get it to run.

If the 2801 cam is too tall for the 670 heads, should I be running something more like the 067 or 068 with closer to a .400 lift? High octane isn't available and I'm at 4600 ft elevation.
And/or start over with new pistons aiming for 9.0-9.5 c/r and keep the cam and carb I have?

Thanks

Did you degree the camshaft to make sure it was installed at the correct intake centerline? Tolerances can make a camshaft vary if you simply try to line up the marks and assume it's where it should be.
Did you check to make sure the balance properly marks TDC?

What is "advanced way beyond it should be?" Did you record what the timing was set to back with the old camshaft?

Yes to degreeing the cam, and also tried it at 4 deg advanced with similar results.
If I start at 10 deg ignition timing, I only get 10-12" vacuum with a lot of misfire and hesitation. If I wind it up to 40-50 degrees, vacuum will climb to 16" at most at idle. It will run better but still with the same symptoms.
I thought at first the balancer slipped, but it checks OK at TDC.
Never thought to check timing on the old cam before I took it out.

I know this is going to sound stupid, but are you sure you're checking #1? I usually time from #6 since it is on the same side of the motor as the timing marks and there's less chance of tangling the timing light wire in the fan -again. (Oops)!

A healthy engine will produce about 20 inches of vacuum at sea level, which you are not, so those numbers you're getting are not that far out of the ballpark. You need to get with someone familiar with setting the carburetor up for altitude, plus it looks like it sat for a year or so before you put it back into service. Not good. I'll bet a large lunch at a drive up window a carb rebuild/modification and rebush the primary shafts will solve most of your problems.

Dito with Jeffs thoughts on altitude and Vacuum. I'm at 5400 feet and pull about 15" with a healthy motor. Also your motor would probably benefit from a leaner running carb cause of the altitude. Approximately two jet sizes smaller (with rods ~.030 smaller) would be a good place to start if your carb has the original jets. However, if indeed you're experiencing lean misfire, you will need to solve that problem first. But black plugs would tend to tell otherwise. 15" of vacuum is enough of a signal for the carb to function properly once it's jetted correctly.

Thanks, good to know. Any thoughts on why I would have to advance ignition timing to 40+ degrees at idle to get to 15-16" and better running though? Balancer and distributor are lined up right on #1 TDC.
The new carb that kept flooding had just rods changed out per Edelbrock recommendation for my elevation, not jets.

Sorry, My fault.....I thought we were talking about a Rochester Quadrajet. I'm not very familiar with the Edelbrock quads. My thought by dropping two jet sizes and using matching rods (.030 less)is you would be leaning both the wide open and cruise mixtures, which would be a good thing at higher elevation. As far as your vacuum readings, what is the needle doing?? Is it steady, or does it jump around or drift back and forth?
My thoughts on the 40+ ignition setting at idle (if in fact it truly is 40)is the cylinder mixture must be burning really slowly, which is why you have to fire it off so soon....Just a thought....

850 is a large carb for that engine. i think it should be more in a 650 cfm range.

yes 850 seems very large. Not to be insulting but a did you re check the firing order? I have seen this same issue several times, very easy to switch 2 plug wires.

How do your plugs look?

Yea, I've checked and double checked twice over the plug wires as I've made the same mistake before.
Plugs are black as coal, rich and flooded. I took the 'new' carb apart and think it may have been sticking. Had a slight amount of some sort of white crust or mold in the float bowls.
Cleaned it out and I'll test it out tomorrow night. This carb is an 800 edelbrock thunder, yes also a bit large but with adjustable secondaries throttled back per advice of edelbrock. Car still won't run right with the 'old' carb, but I guess its possible both were messed up.

Any time a carb sits especially with fuel in it you risk something getting clogged. I try to run some stabil through them then drain well. Be sure you verify the floats are good. Not sure what kind you have, but they can either be weighed if composite material or a gentle squeeze under water watching for air bubles if they are hollow. A bad or leaking float will case this condition. Float level is also very important. Clean the carb well blow carb cleaner through every passage (wear safety glasses)! blow air afterward. If a pressure or carb tank is available be sure to take advantage of it. That crap gums up everything. Hogwash brand is my favorite solvent.
Tony

Almost forgot smell the oil! if it smells like gas change it
T

Mabey the gas is bad. Try running a line from a gas can as a gravity feed and see if it runs better. If the gas in the tank is a year old it could have gone bad. Also check your plug wires. If they are old it may be arching across them and making it backfire.

Cam is not to big. It fact, its a baby.
Carb is not to big. Just needs tuning.
67-74 Q-jets where about 760-780 cfm.
75 or 76-79, they used 800. Even on 350 ci.
Either the jets are to big, AND/or its out of adjustment.
Try'n find my old 67-400(w/744 cam) timing specks tonight.
From memory the closed chamber heads need more lead time.
Or other way around? Been since 82.

Again.
Pull all the plugs and do a cranking compression test.

And after the cranking compression test, hook up that vacuum gauge and let us know what the needle is doing, say with timing around 12-15 degrees and idle around 900.....is the needle steady, moving, drifting, jumping etc...

Compression checked OK as far as I understand. #2 cylinder checked 135 PSI and all the others checked 145-150. The book says at least 100 PSI and all 8 within 30% ?

Both carbs now perform very similar after cleaning out the new carb. Both have had fresh gas from the start.

At 12-15 deg timing at idle, I get a steady ~ 10" vacuum at most, and rough idle. Turning the distributor up to 40 degrees, vacuum rises to steady 15", and after re-setting idle speed, idle sounds better. Any more timing than that, I get no more vacuum increase.

In all cases, if I quickly give it throttle, vacuum drops quick, engine sputters and sucks air, then vacuum spikes to 20" and the engine catches up. In the case where I'm at 40 degrees timing, it doesn't spike down as low which I guess it why it runs better there.

Plugs still black. I guess I'm still too rich even with the new carb and rods, flooding the plugs at acceleration since it can't get enough air? I'll order the next two leaner jets & rods but thats as lean as they go.
Still confused as to how the car ran great previously and now the same old carb and new carb run so similarly, still makes me think its not carb related but I'll try anything.

Thanks again all-

Just to be clear, you didn't touch the bottom end, or replace the valves or seals, correct? Basically a cam swap?

The valves looked pretty good so I just cleaned and lapped them a little by hand. The play in the valve guides checked ok with a dial indicator per the tolerance in the repair manual. I did put on new stem seals, intake only. Nothing to the pistons or bottom end

A low steady reading on the vacuum guage would indicate either a slight vacuum leak, retarded valve timing, or ignition timing set to low. A big cam would bring in a low reading but the engine would still run OK. I would think that 12-15 degrees ignition timing would be in the ball park. Obviously 40 is not correct. Incorrect valve timing could explain the black plugs. And a vacuum leak could explain the low vacuum reading and rough running engine. Since the needle is steady there doesn't appear to be anything wrong with the valves themselves......Just my 2 cents.

Here's a good link for those that have become interested in this thread. It's about how to diagnose engine problems with a vacuum gage. Good basic info.

http://www.secondchancegarage.com/public/186.cfm

That's a pretty useful site there. I will add it to my collection.

Thanks.

Scenario 4 from that website sounds like what I have-

quote-
"Worn rings or diluted oil. Normal operation shows about 15-17 inches. Rapid acceleration causes needle to drop to zero. Rapid deceleration causes the needle to rise only to about 20-23 inches (normal would be 26-30) Reason? Lower vacuum due to gases bypassing the rings."

But my compression test checked OK....
?

Don't forget those numbers are base on sea level. Subtract 1 inch per 1k feet of altitude up.

From what you described, with the way advanced timing issue I think you have a late cam timing issue.

When you swapped back to your old carb, did you keep the fouled plugs in? Hows the spark getting to the plug look like? Is it a good strong spark?

To be honest, your overall compression numbers aren't great (not horrible, but not great). Don't know how a 100psi rating came to be in a service manual, but from experience I would expect it to be in the 150 to 200psi range depending on camshaft and compression ratio. At least they are all balanced within 10% of each other.

When you replaced the head gasket, did you go with a much thicker one?

Compression readings at higher altitude will be effected just like vacuum readings. At his elevation of 4600' the compression readings will be about 12% lower than at sea level. So if compression at sea level was 170lbs times a correction factor of .88 for the higher elevation, the readings would come in at 149 lbs. I'd say his compression is fine. Looking back at the threads the only things that chaged in the engine is the cam (which wasn't enough of a change to make any difference), and the dished pistons (but his compression readings are more than adequate for the motor to run. I'd still leaning toward something being installed wrong during the rebuild, like cam timing or vacuum leak.

And I almost forgot. You mentioned way back in the beginning of this thread that you "didn't see any vacuum leaks around the intake manifold". Did you check for vacuum leaks around the manifold and the carb by spraying carb cleaner or something like that where the mating surfaces of the manifold and carb come together? Sometimes those leaks can be pretty hard to find. Careful you don't start a fire though..

Oops, forgot about the altitude.

Plugs - I've been changing them like once a week due to the heavy blackening, and wire brushing them in between.

Head gasket -not sure what the old one was but new one is .040. I don't remember the old one being noticeably thinner - or thicker

Spark - haven't checked but it should be blue color?

vacuum - yea I checked with carb cleaner around the carb base and intake base. No noticeable response. But I'll check again.

I think we need to go back to the beginning. The only parts you changed out were camshaft/valvetrain related. You said the motor ran good before. Now it doesn't. I'm still going to point my finger at late cam timing. Unfortunately, pulling all the crap off to check and adjust is no fun. I could be wrong, but I think we're chasing our tail at this point. Backtrack all your work.

Late meaning cam too far advanced or retarded? I have it back to zero now (both dots at 12:00 on firing stroke). I previously tried advancing cam 4 degrees with timing gear and didn't notice any difference in running.

Let me ask, did you adjust your valves after initial break-in?

I don't think we covered that in this thread yet, and don't think anyone has mentioned it.

If not, do it.

Good idea Dr.
Maybe thier not properly adjusted.