Rough Idle
 

I know this question has been asked before but here goes.
My 74 has a 650 Holly DP Carb, new distributor(no points) aftermarket cam and Hooker Headers.
Also, has roller rockers and as far as I know (no suspected vac leaks.)

I have adjusted the carb until my hands got tired and had to idle it up to 900 RPM's to smooth out the idle a bit.
Car still has a rough ide, could this just be the cam.

As I mentioned before I have no idea what cam it is.

The only thing I think might be causing the proble is my Headers with 3" collectors reducing down to 2" stock exhaust causing plenty of back pressure.

Thx,
Schoon

Back to the basics
 

Lets start with the basics, like spark plugs, plug wires, timing and so on....

How old are the wires? Headers have a funny way of burning them up. The plugs may be fouled, incorrectly gapped or just the wrong application.... Is the firing order correct? Spark plug wires that run close to each other could be cross firing too. Has the timing been set or checked? How much vacuum are you getting?

Lot's of questions - I know.....

Anyway - if the carb ain't responding then it probably ain't the carb.....

My .02 worth, I hope it helps!

Plugs and wires have less then 5k on them.
Timing was set when new distributor was installed 500miles ago.
Vac must be okor my power brakes would not be working good.
I will check anyway.
Ben think about putting insullated sleves on wires by headers, have them on my camaro and they work well.

You don't think it could be the back pressue or just a nasty cam?

Did the idle get bad recently without any other changes or was something worked on and then it went bad?

Another thing to check would be stuck vacuum or mechanical advance.

The idle has been very rough since I got the car 3 years ago.
It was really bad then until I rebuilt the carb and that helped some.
Then I replaced the distributor and that helped a little.
Yesterday I sprayed starting fluid looking for vac leaks, no luck.
When I get on it it runs very strong in first and second gear.
Cruises very good @ 60+

I am begining to think it's just the cam doing this?

2" exhaust should be OK. Do you have catalytic converters? Sometimes they collapse internally and block the exhaust flow. A restricted exhaust would limit top end speed and you would hear the pressure building up in the exhaust and leaking out someplace ( a hissing sound).

It still sounds like a vacuum leak to me.... or an ignition miss...

How is the oil level? Low oil pressure will not pump hydraulic lifters enough to give good valve opening/response and will result in a rough idle too...

Keep us posted!

No converters, oil pressure is very good.
The stock 2" exhaust is connected to 3" collectors on the headers.
I bought new plugs today and will set the gap wider then before, 40 instead of 35


Stay tuned!

Watch your engine vacuum @ idle. Sounds to me you have a lot of valve overlap (big duration cam) tall lift, big duration means rough idle because the intake and exhaust valves are slightly open at the same time during idle.

When you say watch engine vac at idle, what am I looking for?

Pulled plugs today, 2 were wet with gas.
Put news one in and car has a lot more power then before, idle still on rough side.
Now the question is what made the plugs wet?

Schoon

A fouled plug will get wet from not firing properly. You can get a bad plug from the parts store on a regular basis, seems not to be much quality control f/ any of them. If you got alot more power after installing new ones, that'd probably be your answer. Now that the engine is running pretty good, check the color of the plugs after driving it afew days to see if they are all the same, meaning all the cylinders are firing equally. If one is different from the others, you can run the checks on that cylinder. If you trace wet or dry plugs to one manifold runner, then one side of the carb isn't working correctly; maybe a stopped up port, etc.
As mentioned in another reply, a rough idle can be the cam you have. A vacuum gauge will tell you alot of things. If you have one, use it and read the booklet that comes w/ it.
If you don't have all the crap smog devices that came on the early/mid 70s to early '80s cars, you're better off, as having a strictly mechanical and smog device free engine makes it easier to figure out when something goes wrong. It's straight simple mechanical logic.
If you have more problems with the car not running right, I'd look at the carb. Read the plugs first.

Jim,
My carb is shot!
Havn't re-checked the new plugs yet.
Tried to reset the foats and one of them fell apart.
My idle was set at 6oorpms and every time I stopped it would be at a different rpm.
Ordered a new 700cfm Holley dp from Sumitt.

Schoon

Forget the plugs. W/ a bad carb, it won't matter. When you get the new carb, you can adjust it, install some new plugs and then keep a close eye on the them, they'll tell you everything.
Glad you found your problem.

Bad day today for me.
Put the new Holley 700cfm, dp on the vet.
Started her up and there was a very loud noise coming from engine sounded like a supercharger.
Checked all the vac connections and snugged the nuts holding the carb down.
Still same noise plus the car would only run for a few seconds and die out.
Now I am at a total loss so I call Holley tech and speak with them.
They thought I was nuts so I let them hear the noise over the phone.
They said never heard anything like that and I should call Sumitt and get a new carb.
Makes me think they have had this problem before.
So now I call a mechanic friend of mine who said put the old carb back on and see if you hear the noise, why didn't I think of that.
So I put the old one back on and when I was tightening one of the nuts I cracked one of the bosses on the manifold containing the threads. Maybe this was the problem, a slight crack causing a nasty vac leak.
Now I need to find some type of epoxy like JB W eld and see If I can fix the new problem.

Not my day!
Schoon

Oh ****!! Total drag. Yes, the noise could be a vac leak caused by the carb not seating, but did you actually have that cracked thread situation the first go-round? And w/ a leak like that, the idle should have gone WAY up. Did it?-I mean during the first go-round, the first time you did the changing of the carb-old to new. Even w/ three sides snugged though, w/ a good gasket, there shouldn't have been THAT big of a leak. If indeed I'm understanding what the noise really sounded like.

There is some kind of apoxy that'll fix that thing. I have used JB weld on a manifold years ago, my 223ci 6 cyl. Ford pick up engine, where there was a vacuum fitting (a capped-off source f/ something that my truck didn't come w/) that I can't remember how I did it (19 years ago), but I cracked the manifold and filled the whole thing in, threaded hole, crack and all, w/ JB Weld. Today, it's still the same way and runs like a champ. I fixed that manifold as I had bought, three years prior, the only other manifold like it, and there were no others, locally, to be had.
Please post and let me know what's happening w/ your situation.

Jim,
You are right the first time I started it up after installing the new carb the idle went up to approx 2000rpm's.

I used Loctite weld to fix the manifold now waiting for the replacment carb from Holley.
Stay tuned.

Schoon

Schoon,
Did the carb base have some kind of warp to it? WOW, what are the chances. Yes please, keep us posted...

OK, finally got her all back together.
Had a tough time keeping the accelerator bracket from moving back and forth on one of the studs holding the carb in place.
Had to weld a piece to keep it in place.
Anyway first couple of times I took her for a ride and nailed it i wasn't really impressed.
The new carb made a lot of nosie but the Vet just didn't fly.
Went home made some adjustments and took her back out.
This timethe Vet scared the crap out of me, holy **** rubber in 3 gears.
No that's what Im talking about.
Don't know what happened but I sure am pleased with the results.


Schoon

You gotta love it! Love the smell of burning rubber in the morning.
How was the service when you swapped the carb out?
Any fix that took that long will require a lot of seat time to prove it out. :)

Burning rubber smells like victory. ;)

The service on the carb was really quick.
I called Holley and told them their carb was f****d up so they said call Summit and send it back.
Summit was great they sent me a new one before they had the old one picked up.
Anyway there is a very noticable difference in performance from going up to 700cfm from 650cfm and now my Vet actually idles a lot smoother at 800RPMs

The only problem I now have is my accelerator cable is hanging up somewhere and somehow.
I put the pedal down in 4th the other day and it stayed down, scary moment or two,
wound up turning the key off and took it easy going home.
I think the cause might be because I put a Holley 1/2" spacer under the carb, it is the kind with 4 holes in it.
The secondary butterflies might be rubbing.

Stay tuned,
Schoon

Was the new Holley jetted the same, same secondary spring, and have the same power valves as the bad one?

Just wondering since the 50 extra CFM shouldn't make much difference unless the engine is heavily modified.

I guess it wouln't make that much of a difference but remember the old never worked the way it should have.

Schoon

The throttle plates could be hanging up on the spacer or they could be hanging up on the carb base plate. Before you remove the carb to check this, remove the air cleaner and have somone push the pedal to the floor. Look at the cable for bends that are too tight. Then have them release the pedal. If it sticks, gently touch the cable to see if there is some play at the connection point between the cable and the carb. If there is play (you can wiggle the cable without the carb releasing from full throttle) then it is not the cable. It is either the spacer or the base plate.