Persistent Rough Idle

[jcatnite]

Well, the “1500 Rebuild” post has gone off in such a direction that I felt I might need to start a new one. Let me start by saying that I know how difficult it is to try and troubleshoot something that someone is describing to a bulletin board…lol. Anyway, my 1500 that is in the 76 Midget is still idling pretty rough. I have a lot of information about it just having trouble putting it all together and hoped that someone here might see something that I am missing. First off the car has almost 80k on it so compression is 135 psi on 1-3 and 125 psi on #4. The PO removed the OPUS distributor and put a 25D from what I think is a 74ish 1275. The coil is new. Cap, rotor, condenser, points, and wires probably have around 250 miles on them. The dizzy seems pretty tight in that there isn’t much play in it and when I rev the engine the timing mark is stable. There is some evidence of arcing on the points and in the cap but not too serious. Plugs have about 50 miles on them and they are nice and brown. Carb seems to load up a bit towards the rich side when the car idles for an extended period (probably not related but fuel and ignition play off of each other so much I thought I would include it). OK now for the weird stuff. The car’s vacuum is only around 15 to 16 inches. To get that much I have to run the timing around 18 degrees before. Anything less than that and vacuum drops to an unmanageable 13”. I have the idle speed set at around 900rpm can get it to idle at 800 but it is a bit smoother at 900. I have sprayed carb cleaner in all the usual places looking for vacuum leaks and don’t really seem to have any. While the engine is running, if I pull the plug wires (from the cap…lol) from #1 or #2 cylinder, the rpm’s drop by 150 to 200 but when I pull 3 or 4 wires I only loose 75 to 100 rpm with 3 taking more away than 4. I’ve swapped wires around to see if it could be in the wires with no discernable change. So I guess my question is how smooth should I be able to get this engine to idle? It shakes the car so much right now that you couldn’t use the mirrors if you wanted to. Also any ideas why the timing would need to be so high? I know the valves probably need adjusting as I can hear them just a little. I’m waiting on a gasket set from VB before I tackle that one. Guess I better stop as this one is getting a little long and will probably loose everyone’s attention if I put anymore in…lol Oh, one thing I forgot to add...the dwell has slipped a bit from 58 to around 50 but that is still too close to make me want to mess with adjusting the points right now.
Thanks in advance,
JC

 

[Dave Russell]

Hi JC,
If the dwell has changed from 58 to 50, it means that the point gap has opened up a bit. This would tend to advance the timing. The fact that the "indicated" timing has to be so far advanced would lead me to check the distributor advance, both mechanical & vacuum. Also, you should verify the timing marks. If the engine doesn't kick back on starting & doesn't ping under load, a "work around" would be to leave it advanced. If you check timing with a strobe, you should be able to see it advance to something like 35 degrees as rpm is increased, Vacuum disconnected.

The fact that you have converted from Opus to points would lead me to question whether you have the correct coil & wiring. The Opus used a 6 volt, 1.5 ohm primary resistance coil with a resistance wire of about 1.5 ohms built into the wiring. If the Opus was originally correctly installed in "your" car, it would have some sort of dropping resistor with a 1.5 ohm coil.

The 25D distributor you now have, for use with points, would use a 3.1 ohm coil with straight 12 volts & no resistor. If you have the 3.1 ohm coil with straight 12 volts, & no resistor, you should be ok. If you have the 3.1 ohm coil plus additional resistance such as another resistor or resistance wire in the harness, you are only getting partial plug voltage.

OTOH, if you have a 1.5 ohm coil with no external resistance, the points & or coil will cook in a very short time. Sorry for the long answer.
D

 

[jcatnite]

Thanks Dave! It does have the coil for a 76 in it with the ballast resistor mounted on the fender well. As far as voltages and such I will have to check it this evening. I never even thought of the coil not being strong enough. In my limited experience they either work or they don't...lol. Thanks for the long answer. Logic and reasoning along with advice provides invaluable training for those of us who are new to LBC's..
As far as the timing, I have pretty much set it by ear and manifold vacuum for years so I may just use your "workaround" for a while. Was kinda toying with swapping the dizzy out for a new Mallory dual point but the old one isn't wore out and I hate to do that.
Thanks again,
JC

 

[sparkydave]

JC:

Are you sure that's the ballast resistor? The ballast resistor for a '76 is a resistance wire built into the wiring as Dave said. My '77 is the same way, and the only resistor visible (mounted on the pedal box cover) is a separate resistor used for the Opus module. My car still has it even though it's not connected since I replaced the Opus guts with a Pertronix.

-Dave

 

[jcatnite]

I might be out to lunch on that then. I haven't dug into it too deeply. It is a ceramic looking square with the low tension wires for the coil connected to it. I will look at it a bit more closely this evening when I get off work. The PO told me that it was a ballast resistor but we'll see this evening.
JC

 

[sparkydave]

Ah, if it's ceramic and goes to the coil primaries, maybe the PO put in a different ballast resistor. The other resistor I was referring to is a metal one, and doesn't connect with the coil primaries. Hopefully the PO also knew about the resistance wire, and bypassed it if he added this other resistor. If it is the stock coil, then when ignition is on and the points are closed, then you should be able to measure around 6-7 volts across the coil primary (engine not running). If the resistance wire is still in the circuit, then potentially the extra ballast resistor might be lowering the coil voltage too much.

BTW, I read that you live in Aiken? I used to live near Hitchcock woods, a few blocks off Whiskey Road. Thanks for sending the link to John Twist's Zenith tech tips too, that is a useful thing to have!

-Dave

 

[jcatnite]

That's insane...I would have never guessed that anyone from Ohio knows where Hitchcock Woods is...lol I am going to take some readings this afternoon and will let you know what I have going on with the coil. Looks like I have some resistance readings to take as well...

 

[703MGB]

You might to want to check a real basic thing. My car had serious idle problem and I did check everything except the sparkplugs. Then I plugged my timing gun on cylinder 1 to find that the light was not coming on. I actually though my gun was defective until I decided to plug it against number 4 and the light started flashing.

My problem was simply a bad plug. 5$.

It is just a thought

 

[jcatnite]

Thanks Eric, I just replaced the plugs about a 100 miles ago. Plus I pull them after every ride to look at the color as a back-up check on the carb tuning that I am learning to do. I am using the bosch platinums that are recommended for the 1500.
OK quick update. The coil has a resistance of 1.6 ohms and the ceramic thingy that we have identified as a resistor has a resistance of about 2.5 ohms. Voltage to the resistor about 11.5 with car off and to the coil was about 5.5. Oh and btw, I found out that the resistor heats up to a phenomenal temperature. Lets just say it didn't take me long to inspect it...lol. We won't be making a trip to the burn center but JC is now wearing a band-aid... So with that being said what do you think I should do? I am feeling like that coil may not be giving me enough juice for the points type distributor. I'm also wondering if the platinum plugs that are recommended for that car aren't putting too much load on it too? There is another wire that looks to be white with green stripe out of the harness near the dizzy but the PO has cut it off at the harness and couldn't get a voltage reading on it. Can't play with the car much this afternoon. The kid is off to a sleepover and the wife is acting jealous already. Maybe I can use it to my advantage :}
JC

 

[Dave Russell]

The numbers quoted are not out of line for your setup. Eric has a good point. I have known of new plugs being bad from the start. The white/green wire was likely the original resistance wire, not needed now. Think I might concentrate on the timing part of the equation next.

It is better to point to some parts rather than to touch. Electricals come to mind.
D

 

[703MGB]

Also check valve clerance if too tight some cylinders may not produce power.

If it would be me...I would replace those points immediately with an electronic system from Petronix or Crane.

Also a small amount of end play in you distributor shaft will make all your point gap adjustments strait to the garbage. May fire on 2 cylinders and do nothing on others or not enough spark.

My dad always said you need 3 things: compression, Mixture and fire to get it running....

 

[jcatnite]

So if I jump in and go ahead with the pertronix, can I use the coil that I have now or will I need a 12v setup anyway? I think I remember reading that on either this board or the mg board.
One question about the timing, would loose valves cause the timing to need to be advanced or to appear late as far as manifold vacuum is concerned? This could be a gross conceptual error on my part but it would seem that it would. I don't think the valves are too tight, if anything they are too loose based on the po's statements and the sound coming from the valve cover.
JC

 

[703MGB]

I have 2 british cars with different set-up's.

My 66 Sunbeam has a crane system with a crane supercoil and a resistance.

My 73 MGB has the same crane system with a Lucas sport coil and NO resistance.

If I remember correctly the type of coil makes the difference. If you check into the Moss catalogue you will see annotations to that effect.

 

[sparkydave]

I used the stock coil and ballast resistor with the Pertronix. I did need to run the red lead of the Pertronix to 12 volts instead of the positive side of the coil since the voltage drop was high enough across the ballast resistor, but the instructions talk about how to install it with a ballasted coil.

-Dave

 

[aeronca65t]

All good info above.
But also don't forget to check for vacuum leaks, especially around manifold gasket. The manifold "hangs" on these engines and can get a bit loose over time. This will cause rough idle.
To check for vacuum leaks, let engine idle and spray WD-40 around manifold gasket and other possible leaks sites. If leak exists, it will suck in WD-40 and slow down a bit.

Also, look for high voltage "leaks". Best way: start your car in a dark area and "mist" the engine with water from a plant sprayer. If it lights up like a Xmas tree, you may have poor insulation or bad connections on some wires.

 

[jcatnite]

Cool man thanks. I've been trying to decide whether to go with the pertronix or get a new mallory dual point distributor. The Pertronix is a lot cheaper so that may be the route to go. Thanks for the advice.

 

[703MGB]

Stay clear of the mallory distributor unless you are a weekend racer.

Mallory distributors do not have vacuum advance. And the points are very expensive when in need of replacement.

 

[jcatnite]

Ok I will follow that advice...it seems pretty sound. If there is a little slop in the distributor that I can't detect will the crane system make up for that? I'm looking harder at the crane system as it seems to be fender mounted instead of trying to pack everything inside that tiny distributor.

 

[703MGB]

If the shift in your 25D4 wobbles it shots your point gap.

But the crane system is a system that use light through a slotted disk inserted under the rotor. Both crane and petronix are the way to correct bad distributor shafts.

Still you must also make all the other checks included in this tread including the vacuum leaks.

 

[Radford]

[ QUOTE ]
Also look for high voltage "leaks". Best way: start your car in a dark area and "mist" the engine with water from a plant sprayer. If it lights up like a Xmas tree, you may have poor insulation or bad connections on some wires.

[/ QUOTE ]


That sound so cool I'd almost just do it for kicks and giggles.