Diagnostics Part II


When I wrote the other article about Diagnosis, I was experiencing a problem with my Olds. It wouldn’t idle. It would run fine off about 1500 rpm up but would die below that. At first I thought the problem was dirt in the fuel again. Turns out it wasn’t this at all, and I was a bit miffed. I spent a couple of hours rebuilding both 1850 Holley 600’s for nothing. During that process I became suspicious of my original diagnosis because I didn’t find much debris in the carb float bowls, or in the needle and seat assemblies. 

So the next logical step in the situation was to bolt the carbs back on after rebuilding them to see if I had exorcised the demon or not. Well I was not surprised to find that the demon was still there. The car ran exactly the same. It was just as if the idle circuit was completely turned off.  So I decided that maybe the 1850’s had circuits in them smaller and more prone to plugging up that did say any of my 4150’s. I had a spare 4780-3 4150 kicking around my shop that came off the race car. I also had a single four barrel top for the Offy Tunnel ram on the car now. I also wanted to see how the car ran with that top on it. After remaking both float bowl hoses, fabricating a new throttle cable bracket and a return spring bracket I put some fuel to it. I always pressure test the carb before I light it up don’t like fuel squirting all over the place with the engine running. This, by the way, is another reason to run an electric pump over a mechanical one, you get to say when the carb gets fuel and not the other way around. It’s also a great theft deterrent too. They might get your ride started but if they don’t know where the pump switch is they ain’t getting too far. In the case of my Olds they couldn’t even warm the damn thing up before it was out of gas! LAMO!  

Alrighty so no leaks. I lit the motor and it still did not want to idle. I didn’t think this combination would fix the problem I just wanted to swap the dual 4’s out for a single four. Now I’m thinking this carb was just rebuilt too so I know that all the gaskets and stuff inside the carb is all on the same page? Maybe. I’m getting ahead of myself here. I decided that the ignition system was worth a look. Mainly due to the fact that the distributor cap had been on there for over a year, as had the rotor. I’m running an MSD Pro Billet distributor on it with the small cap and rotor. I pulled the cap off and the center carbon electrode in the cap was toast. The contact strip on the rotor had a semi-shitty looking black spot on it where the cap had been shooting lightning bolts down to it. I ordered a new cap and rotor from Summit and got it a couple of days later. I love these guys.  Anyway, I installed the cap and rotor and cranked it back up. It was running a little better. It kinda, sort of wanted to idle now but still wasn’t smiling at me much. Ok so I decided to pull a spark plug. These were older than the cap and rotor and they looked it.  These were NGK BKR6E’s. I went to a heat range colder, BKR7E. Remember NGK heat ranges go colder with a larger heat range number. This is a V-groove plug and they have worked great in my race car and this engine as well.  I gapped them out to .041” too.  MSD won’t give plug gap recommendations per se. They have some suggestions however. I am running the HVC II Blue coil on the system so I have plenty of spark energy; I decided to run a wider gap. The more plug gap you have the more load it puts on the entire ignition system. It also helps get the mixture burning sooner and in most cases easier too, kind of a tradeoff. This is something you are gonna have to decide with your particular ignition set up. Most MSD systems in good health should smile at you on a .040” gap. 

This made quite a bit of difference. The motor fired up a lot easier and it was running about 50% better than before so I felt like I was getting closer. Sometimes when I get to a cross roads with a problem  I’ll just say pass and not mess with it for a while. My mind runs scenarios in the back ground while I’m trying to ignore the problem. So it dawned on me one morning drinking a cup of coffee.  First let me say that I pulled the valve covers off of it and checked all the valve springs to see if any had broken. They all looked good. I checked the timing and it was exactly where I had set it initially. I did a check for carb and manifold leaks. Found none anywhere. I rolled the motor over by hand with the plugs out and watched the rocker arms and valve springs, all of which looked fine. I did a compression test on it and had two cylinders that were low by 10 lbs. but nothing to worry about. One was 5 lbs. above the rest. These inconsistencies can be caused by cranking speed, due to a battery charge state, valve lash adjustment etc., so I didn’t jump to any conclusions. I wasn’t about to tear the heads off of the engine because of a 10 lb cranking difference in compression. Usually if the compression is bad in a cylinder the motor will run rough or even drop the low cylinder out of the firing sequence. My engine wasn’t doing this. 

So by running a compression test that told me a lot about the engine. It says that the valves and rings are good, and the timing was on and it was rock steady too. You have to start covering bases when you are chasing a problem like this. Then all you have to do is consider the symptoms and start ruling things out. I decided that I probably didn’t have any internal problems with rings, valves, broken valve springs, so I got to cross those potential problems off my list. One thing kept hitting me over the head and crashing into my face head on and that was that the engine was behaving like there was no idle fuel feed to it. 

Today I went back outside and pulled the carb off. I took the base off of the carb. If you are running a 4150 double pumper of any sort pay attention to this: Holley’s have about 4 different styles of base gaskets and all of the damn things will fit the bottom of the carb. That doesn’t make them right however!  4150’s like the early versions I have use a rudimentary secondary idle system in them. It’s not as sophisticated as the new four corner idle carbs on the market now days. However there is a transfer passage that runs from the front end on the primaries around back to the secondary idle orifices. Some of the gaskets have a hole that allows both holes to show thru the gasket and others only allow one of the two holes to show. These holes are on the very front of the primary side of the throttle base. They are located on the corners of the throttle bore radius on each side. If you install a base gasket that covers up the outside hole you have shut off idle circuit fuel and the carb will act like it has no idle system. Your idle mixture screws won’t do Jack Shit. I found this out the hard way on the race car one day. I rebuilt the carb and bolted it back on the motor. The car wouldn’t idle when I started it back up. Removing the base plate I found the problem. Changing over to the other gasket solved the problem. You’d think I’d have remembered it wouldn’t ya? Well I did, but I overlooked this when I was putting the carb back together because I had my mind on something else, something I should not be doing. What the hell I’m human. Well I found the exact same problem with the base gasket that was on the carb I had just rebuilt. Clumsy Me.   I swapped the gasket back out and installed the carb on the car. I fired it up and it still didn’t want to idle real well, but it was a hell of a lot closer now than I’d had it since this problem started. I ran one of the idle mixture screws in until it bottomed and it didn’t change the idle speed much. I tried the same thing with the other side and the idle started decreasing. So I put it back where it was and opened up the other side. Holley says a baseline setting is 1 ½ turns out from lightly seated so I opened up the shut down side one and half turns and the idle picked up above what it had been! Good sign! So then I bottomed the other side and ran it back out one and half turns. Now the motor was going about 1600 rpm. I closed the curb idle screw down in small increments watching the tach. When I got it down to 1100 rpm it was back to its cackling, gravel throated, bluesy, Olds voice. Love the way this motor sounds. Throttle response up from idle was excellent. It’s still pulling fuel thru the transfer slots but that’s because this carb was on a car with a big roller cam in it that had 4 inches of manifold vacuum and had no idle needle adjustment. One fix for it is to drill a small hole 1/8” or smaller on the leading edge of each throttle butterfly. This allows more airflow which reestablishes air flow thru the idle circuit. It gives back some idle needle adjustment. However on this engine that isn’t needed or at least the holes are a bit too large for this engine. I’m gonna install some undrilled butterfly’s in it and it should get the idle needle adjustment spot on. This will allow the throttle blades to be lowered back over the transfer slots so they are out of the picture. They aren’t supposed to run curb idle in the first place. 

So now the motor is happy again. And when she’s happy I’m happy. Funny thing about that ya know?

Dave

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