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Chandler63
11-08-2014, 10:19 PM
I apologize in advance for the long post. I am having trouble with cylinders 4&7 not delivering power. Each cylinder is getting spark and compression is good on all cylinders (160-170). Engine was rebuilt recently. Vacuum is steady at 18-19 inches with very little movement. I am running a pertronix ignitor 1. Carb has been rebuilt. Distributor was bench tested. Wires, plugs and cap are new. Heads were checked. I am thinking there is a problem with the intake runner which is common to 4&7. Maybe a crack? When the engine was out I didn't pay much mind to the intake manifold. I am considering pulling it and having it checked at the machine shop. Has anyone had a problem like this? The steady vacuum is what puzzles me. It idles ok it is just missing enough to irritate me.

jopizz
11-08-2014, 11:13 PM
Have you driven the car at high speed. I've had intakes that had carbon buildup that was causing it to miss. A good run at 55+ mph usually takes care of it.

John

simplyconnected
11-09-2014, 01:44 AM
John, you remind me of the old days when valves used to load up with lead... remember? A good long trip at high speed usually straightened it all out. Now, we don't have lead.
NOTE: John is a great master mechanic. He's the guy I would bring my car to.

Chandler, welcome to Squarebirds.org. We're honored to help. If you have a recent build, I would go through the basics again.

Switch plug wires and see if the problem changes with the wires.

Change the #4 plug to different cylinder and see if the problem moves with the plug, then do #7. We can sense high voltage at the wires but we cannot see a fouled plug short circuit to ground inside the chamber. BTW, what do the #4 & #7 plugs look like? Are they wet? Describe.

Pull each rocker covers off, one at a time, and watch the exhaust valves. A bent exhaust pushrod will allow high compression and high vacuum, but produce no HP. A worn cam lobe will produce the same, so watch the valves. You want to see at least 1/2" to 3/4" stroke.

If you suspect a vacuum leak on a runner, take your car outside and let it idle. Then, use an UN-lit propane torch, turn on the gas slowly waving it around your suspected areas. If your idle jumps up, you found the problem. You said your vacuum is steady at ~18" Hg, so I do not suspect a crack.

Check for any vacuum leaks. Plug the entire vacuum line, first. If the problem goes away, start looking downstream for leaks.

Did you change your cam for one with a long duration?
Do you still have a flat tappet cam?
What oil are you using?

As you can see, we know only what you wrote about your engine. So for us, it turns into a 'what if' situation. Don't apologize but instead, give us as much info as you can about your engine. We can solve this mystery. - Dave

Chandler63
11-09-2014, 02:19 PM
First of all thanks for the suggestions I should also add that I am a new member. I neglected to mention that in my initial post. I tried a couple of those suggestions. I inspected the plugs. All were about the same. They have a greyish tint, not wet or oily but I would say that the engine is running cool and rich. I don't drive the car much. Basically Fridays to work. It will get a short run on the freeway for about 2 miles. Beyond that I run it in the garage trying to troubleshoot this issue. I swapped the plugs but that had no effect. I closed off the pcv, oil filler cap and manifold fittings. I checked for vacuum at the oil dipstick tube looking for possible crankcase pressure and found nothing. I have checked the valves with the covers removed they all move the way I think they should. No bent pushrods. The car seems to run slightly better at higher rpm. I notice the miss when the idle kicks down. I here it skipping out of the exhaust. This was an ongoing issue before my rebuild. I had a mechanic do a balance test and 4&7 failed to deliver. I am hoping someone may have come across a similar issue.

simplyconnected
11-09-2014, 08:01 PM
Chandler, you only need three things for fire; air, fuel and spark.

If you are confident that your valves work as they should and that Pertronix is delivering the way it should, that only leaves one possibility... your carburetor.

Carbs are not built equal. They depend on air velocity to suck gasoline into the ventures with a concentration of 14.7:1 air to fuel. Some carbs only dribble gas in.

New cars atomize gasoline under fifty psi, easily mixing gas with air. Carbs have a difficult time doing just that. Check your carb, since the problem was there before your rebuild. - Dave

jopizz
11-09-2014, 08:10 PM
I agree with Dave. The right side of the carburetor feeds cylinders 1,4,6,7. If the idle passage on the right side is partially blocked you will notice it more at low speed. One thing you can try is to remove the idle mixture screw on the right side and spray Gumout or any carburetor cleaner into the hole using the long plastic tip. I had constant problems with the idle passages on my Autolite 4100 carburetor getting clogged. After I changed to an Edelbrock carburetor the problem went away. I assume you adjusted the idle mixture screws on your rebuilt carb after you got it back.

John

Chandler63
11-09-2014, 09:18 PM
I rebuilt the carb (4100) about a month ago. I usually rebuild it once a year as a matter of maintenance. I soaked it for a week in Berrymans and paid particular attention to blowing out all of the passages. Having said that I have never sent the carb to a professional rebuilder. That is something maybe I should consider. The throttle shafts and bushings could probably use some attention. I won't have my new car budget until March next year so I will have to wait for that. In the meantime the valvetrain looked like it was all moving when I had the valve covers off. I am wondering if I might have a lifter issue. It passed the eyeball test but I should at least pull the lifters for 4&7 and check them out. I'll let you know if I find anything. Thanks for all of the input. There have been some thoughtful suggestions.

jopizz
11-09-2014, 09:33 PM
I wasn't aware you had done it yourself. You obviously know what you are doing. I've always rebuilt my own carburetors and never had a problem until my last one. The only thing I can chalk it up to is the Ethanol in the gas. Nothing else made sense to me. Hopefully you'll find a simple solution.

John

simplyconnected
11-09-2014, 10:35 PM
Have you got a buddy that will lend you his four barrel for a few days? You will find out very fast.

Lifters translate to rocker arms. I don't see where it's to your advantage to pull any lifters. Maybe a video would work with the camera fixed on one bank of rocker arms while you crank the starter. Don't let it start.

Then, you can look at the video as many times as you need to. After one side, do the other. - Dave

Yadkin
11-10-2014, 08:45 AM
I rebuilt the carb (4100) about a month ago. I usually rebuild it once a year as a matter of maintenance. I soaked it for a week in Berrymans and paid particular attention to blowing out all of the passages. Having said that I have never sent the carb to a professional rebuilder. That is something maybe I should consider. ...

Don't, unless the guy is really an expert, you already know more than most young mechanics.

The first thing I would do is examine the tips of your idle mixture screws carefully to see if they may have been overtightened in the past. A tell-tale on this is aluminum from the body burnished onto the steel tip. Compare the right to the left and see if they are the same. If possible sight down the screw bore and see if the seats are in good condition. Re-install by lightly seating and back out 1.5 turns.

Next I'd check lifter preloads, making sure you do it right with each cylinder in-turn at TDC on the compression stroke.

Then I'd measure the valve lift at each cylinder, noting intake and exhaust and making sure they are all the same. If not check the lifters for leak-down.

Chandler63
11-10-2014, 05:30 PM
I am seriously hoping it's not a cam lobe. Checking lobe lift is a PIA. I think I will pull the lifters for 4&7 first, clean them and see if things improve. I'll post the results. Thank you all.

Chandler63
11-13-2014, 11:32 AM
So I cleaned the lifters for 4&7 and still no change. I am going over the fuel-spark-compression triangle and I think I need to focus on the carburetor like what was suggested before. I think I will try to find some new idle mixture screws since the ones on there currently may be worn. The other remote possibility is the intake manifold fuel passage which is common to 4&7. I will keep at it. I will probably start a new thread at some point when I get some new info. Cheers!

fcar
11-13-2014, 11:57 AM
Good luck!

Yadkin
11-13-2014, 03:36 PM
Mike's premium kit includes new idle mixture screws: http://www.carburetor-parts.com/Autolite-4100-Premium-Carburetor-Kit_p_2522.html

simplyconnected
11-13-2014, 09:31 PM
I'm all about 'clean lifters', but how did you go about cleaning them?

The only way I know is to loosen the rocker shafts, pull the pushrods, pull the intake manifold off and start extracting lifters. Sometimes gunk at the bottoms will impede the check valves making them collapse and become noisy.

If that's what you did, it might have been a good opportunity to inspect the intake manifold and head ports, in case little squatters took up residency in the Manifold Motel. Just sayn'. - Dave

Chandler63
11-18-2014, 10:39 AM
Power has been restored! The suggestion pointing me toward the carb, specifically the idle mixture screws was the ticket. It turns out that I had the wrong mixture screws installed. The new ones I purchased from Quadrajet Parts were about a 1/4 inch longer and made a huge difference. I am now running on all eight cylinders. Thanks again for all your help. Cheers!

jopizz
11-18-2014, 10:44 AM
Congrats. The simplest solutions are always the best.

John

fcar
11-18-2014, 04:35 PM
Ditto on your success!

simplyconnected
11-18-2014, 05:21 PM
Here's the REAL winner; John you nailed it. (I knew you would.) Good show, ol' boy.
...The right side of the carburetor feeds cylinders 1,4,6,7. If the idle passage on the right side is partially blocked you will notice it more at low speed. One thing you can try is to remove the idle mixture screw on the right side and spray Gumout or any carburetor cleaner into the hole using the long plastic tip. I had constant problems with the idle passages on my Autolite 4100 carburetor getting clogged. After I changed to an Edelbrock carburetor the problem went away. I assume you adjusted the idle mixture screws on your rebuilt carb after you got it back.

John