new carb, lost a cylinder...

markyb

markyb

Senior Member
'31 Model A, was running decent, although would stall when coming to a stop, and couldn't adjust the idle mixture screw properly. I decided to have the carb rebuilt, turned out to have an incorrect idle mix screw, a few other issues...

the new carb has been flow checked, full rebuild, and upon starting for the first time I am losing a cylinder 90% of the time. Can a carb cause this? I hesitate to start replacing other parts since this is the only item that I've fiddled with. It actually sounds best at low idle with the spark lever full up! If I take the idle speed up she shakes like never before...
 
Have you shorted each plug wire to test cylinder? I doubt one cylinder is dropping out. If one cylinder is not dropping out, go back to the basics check timing, points setting, condenser and check for intake leaks (with wd40) etc. then let us know how you are doing.
 
Don't understand your "loosing a cylinder".... did you mean like a "misfiring" engine ?

Did you do a compression check ? All cylinders should be near equal PSI. Check the distributor point gap and ignition timing. Check the distributor cap surfaces for hairline cracks anywhere. Clean and gap all plugs then short them out one at a time and see if there is no noticeable difference in engine speed on any one of the cylinders. Check the intake manifold nuts for tightness.
 
all good advice, but since you only worked on the carb, check for vacuum leak between it and intake manifold, one cylinder not firong makes me think maybe a plug
 
Why is it that nobody here ever uses a vacuum gauge to test an engine? This and a compression gauge are the best tools in the world for diagnosing problems.
 
I'm certain that it is only one cylinder, It ran fairly smooth with the old carb, although none of the bucking that I have now. I'll short out the plugs one by one today and isolate the bad one. Thanks for all of the ideas, I've been wrenching on cars for the last 40 years, although the A is new to me, only had it for about a month from my Dad.

The weird part to me is that after the rebuilt carb was put on, it seems to run smooth with no advance and at a very low idle speed. At higher RPM there is a definite pattern of three cylinders running and a fairly good shake to the engine.
I cleaned the plugs off and re-gapped, there was a lot of black soot, expected since the old carb was set very rich to compensate for its' problems...I'll move on to point gap and timing when I'm comfortable on how to do it correctly, I better move slowly on that so I don't mess up something else and have a "compound" problem.

I'll run compression check as well, it'll be good to have a reference number for each cylinder since the engine only has about 3500 miles on it since a full rebuild.
 
If it runs good without advancing the spark--dose this mean it runs worse bringing the advance lever to the nine o'clock position?
 
James Rogers said:
Why is it that nobody here ever uses a vacuum gauge to test an engine? This and a compression gauge are the best tools in the world for diagnosing problems.
Click to expand...

James, I see that you're in Asheville, you might know this car, my Dad (Jerry) lived in Hendersonville up until about 4 years ago, he's in Vegas now, and the car is back in the Detroit area with me. He ran with the gang there in local events.

I can hear the misfiring cylinder kick in on occasion, but very rarely, I'll get new plugs, and inspect the cap/rotor, etc...I see a brass plug on the intake, I can put my vacuum meter there, although not sure what a good reading should be, any thoughts?
 
Phil said:
If it runs good without advancing the spark--dose this mean it runs worse bringing the advance lever to the nine o'clock position?
Click to expand...

Yes, only now after the rebuilt carb was put on, that's what has me baffled.

It operated as expected with the old carb, a bit rough with no advance, and smoothed out as I moved the lever towards the 9 o'clock position...
 
markyb said:
James, I see that you're in Asheville, you might know this car, my Dad (Jerry) lived in Hendersonville up until about 4 years ago, he's in Vegas now, and the car is back in the Detroit area with me. He ran with the gang there in local events.

I can hear the misfiring cylinder kick in on occasion, but very rarely, I'll get new plugs, and inspect the cap/rotor, etc...I see a brass plug on the intake, I can put my vacuum meter there, although not sure what a good reading should be, any thoughts?
Click to expand...
Try these links.
http://www.earlycuda.org/tech/vacuum2.htm
http://www.gregsengine.com/using-a-vacuum-gauge.html
http://www.international-auto.com/f...uges/tips-on-reading-gauges-vacuum-gauges.cfm
 
Ok, hopefully getting closer to resolving this problem, A vacuum gauge at idle shows that it's trying to stay at 19, and when misfiring (about 50%-70% of the time) it's dropping back to 14. I would think that it's not a valve stuck with the intermittent nature of the problem. I checked the points, were at .008, so I reset them to .018" per the book. The contacts inside the distributor cap look nasty, almost like at one point in the past the rotor was grinding away at the bottom half of the contact point. Odd that the damage is low on the contacts, and now the rotor is passing at the middle of the contact with a .015" gap.

I'm ordering up a new rotor, cap points, plugs (W19Y) and condenser. I saw a little conversation on the forum about crappy modern day condensers, so I'll do a little reading before I order to make sure I get a decent product.

One other issue...my trouble started happening after installing the rebuilt carb, when I try to adjust the idle mixture screw I can stall the engine by tightening it down, but I can turn it CCW to the point that it would come all the way out and it still doesn't affect the idle. It will stall at about 1/2 to 3/4 turns open and then no effect all the way open from there, any thoughts?

Thanks very much for all the help and suggestions...
 
To adjust the idle screw correctly, all the other components have to be correct. I think when you install one of the other components you have ordered you will find the problem if you do it one at a time and check the run in between. I think the distributor cap or condenser.
 
that's on my list of things to try before I change anything else. The carb on it now is my old one that's been rebuilt. I can't believe that it would run so erratic from a carb issue, but since that's what started this problem it's the logical thing to blame. There's a few local guys with A's and T's nearby, friends of a friend, we'll see if I can ask a favor and borrow one, or even invest in a backup one for my own use.
 
I'm going BACKWARDS!....new plugs made no difference, installed new points and condenser, tested for continuity between the point arm and bottom plate, no continuity as desired, turned the key on and found 6V at the points...good...started the car and it runs HORRIBLE...I'm going to put the original condenser in and see what happens. I have a modern points system, by the way.

I was going to check the timing, and was looking at. the settings on my timing light to verify 6v settings, then looked at the battery and see that it is hooked up with negative as the ground! I don't know the entire history of this car, although it is fully restored and has an alternator , can the negative ground be correct?
 
Most 6V alternators sold expressly for Model A use are usually positive ground, but there's no reason one couldn't have been set up with negative ground. For many years I ran a 6V negative ground Delco generator with external voltage regulator.
 
"I have a modern points system, by the way".

Aha! Put the original lower and upper plates and points in and see what happens. Those modern lower plates are notorious for bad behavior.
 
if you have the modern lower plate with the brass contact that takes the place of the wire
get rid of it. if its not your problem now it will be in the future
 
maybe I'm not too far off now, the car would not even start with the NEW condenser, I put the old one on and it fired right up. Although I have not messed with the timing yet, I have a Nu Rex timing kit, and reading their specs I should slow idle at full retard...If I do that the car idles beautiful. It hates any advance at slow idle. I just ran up the throttle and it sounds pretty good with advance now, so I'll take it for a test ride.

Nu Rex says 5-10 degrees ATDC full spark retard at slow idle, and 30-35 degrees BTDC for high speed. I need to borrow a hand crank from a neighbor tomorrow to run the full set of timing tests and see where it ends up. I also noticed a fair amount of wiggle to my distributor, so I snugged the lock screw per the manual, I read that this could've given me erratic running too...

Off for a quick test drive, thanks to all for ideas with the troubleshooting. Also, as I read here on the forum and didn't believe it, I had a new in the box condenser that wouldn't let the car run.
 
Mitch//pa said:
if you have the modern lower plate with the brass contact that takes the place of the wire
get rid of it. if its not your problem now it will be in the future
Click to expand...

I have not removed the upper plate yet to see what's under there, although if you're referring to the wire that comes up to the points, that's what I have now. I'll look for an original style plate setup for future use.
 
You must log in or register to reply here.
Back
Top