Stubborn Engine Problem
 

Hello all you helpful gear heads.

I have a lingering problem (or multiple problems). I did a motor swap in my '68 this past winter and have been chasing a problem since then that I cannot seem to figure out.

The first part of this is located here if any of you feel like reading.

http://67-72chevytrucks.com/vboard/s...d.php?t=786199

For those that don't, the short of it is that I seem to be having some kind of misfire that is occuring mostly during off idle transition. I'm not entirely sure it's a misfire, but it sounds like pulses through the exhaust at the wrong time. If you look at the youtube videos in the other thread, you can hear it. At the end of that thread, I announced it as solved with a plug wire swap... well that turns out to be not entirely true. I was finding (and I still am finding) that it was idling high without having turned the idle set screw off its minimum setting.

I pulled the intake manifold off and put a new one on and made sure I properly sealed around all the runners. I put new plug wires on since the ones I used to fix the problem were old plug wires. Basically, after that swap, it was back to square one, and in fact it was worse.

Since I don't have enough experience with noises like this, meaning I don't know what problem is associated with the noise I'm hearing, I'm looking to this great forum to list the things I should be checking.

Here's what I've done so far.

1. Timing is set at 12 deg initial with 36 total. DUI distributor has 24 deg mechanical advance. I've also tried 14 deg initial with no change.
2. New plugs. Gapped at 50 per the DUI distributor instructions and also at 35 just to see if it made a difference.
3. I re-tightened the exhaust manifold bolts. They are the spread bolt type with a set screw in the middle to make sure they don't back out. They were pretty tight but even tightening them further made no difference.
4. I unplugged all vacuum accessories. No difference.
5. Fuel pressure is at 6.
6. Tried re-routing wires. No difference.
7. Tried the old distributor cap. First drive seemed better (but not perfect) but second drive was back to crap.
8. Fuel bowls are set correctly at half way.
9. Vacuum gauge currently shows a somewhat steady 15". I think for my cam, that's about what I should expect.
10. I thought maybe I was running rich and what I was hearing was unburnt fuel in the exhaust ingniting. I leaned out the idle air bleeds. After that the plugs looked very white like it was too lean. I changed them back to the setting that produced a good looking plug.

Here's what I plan on still checking. Please add anything else I should check and hopefully I will find out what the hell is going on.

1. I read the DUI needs to have a new wire without a resistor. Originally this truck didn't have DUI but it did when I bought it. Not sure if they used the original wire but it's worth running a clean wire direct from the battery and testing it.
2. I have a spare carb I will try. It's undersized but should be able to tell me if something in the carb is off.
3. Replace the bottom half of the distributor. If the vacuum advance is malfunctioning, this will tell me.
4. Try running it without Vacuum advance.
5. Valve adjustment.

Thanks everyone for helping. These forums are why I felt I could take on projects like this. The collective knowledge really makes a difference.
Drew

Re: Stubborn Engine Problem
 

I don't like to read. Where did you acquire the engine you did a swap with? Used?

Re: Stubborn Engine Problem
 

Try driving it with the vacuum advance disconnected. Sometimes when you get a high amount of total advance it will cause that.
George

Re: Stubborn Engine Problem
 

This is from the first page of the other thread.

I have a fresh 383 stroker from west coast engines. Alum heads, roller cam and rockers. Here are the specs.

Horsepower: 450-475
Torque: 450 lbs
Gas: 91 Octane (Premium Unleaded Gas)
Max RPM: Idle to 5500 RPM
Stall Recommended: 2300-2500
Crankshaft: Scat (Steel)
Cylinder Heads: R.P.C. / ProMaxx Aluminum (64cc/195cc)
Cylinder Head Gasket: Fel-Pro
Cylinder Head Bolts: Elgin
Valve Springs: Pioneer
Lifters: COMP Cams
Valve Size: 2.02/1.60
Camshaft: COMP Cams / Trick-Flow
Camshaft Lift: .540”
Camshaft Duration: 230° Int / 234° Exh. @ .050” Lift
Camshaft Lobe Separation: 110
Vacuum at Idle: 16″
Push Rods: COMP Cams
Rockers: Elgin SSR (1.5 Roller Rocker)
Rocker Arm Stud: ARP
Intake Manifold: Edelbrock Performer RPM (Aluminum)
Pistons: Federal Mogul (Hypereutectic)
Piston Rings: Federal Mogul (Moly)
Connecting Rods: GM Forged Powder Metal
Timing Set: Melling (HD Roller)
Bearings: KING HP Series (Tri-Metal Performance)

Re: Stubborn Engine Problem
 

OK.. I haven't had time to look into this yet but I am adding to the list of things to check in the original post based on comments here and the other thread. Hopefully I'll get to do some of it this weekend.

Re: Stubborn Engine Problem
 

What you said about the power wire for the coil..... that needs to be a fat wire like a 10 or 12 guage hooked up to "ignition hot" . Mine is connected to a terminal in the fuse block labeled as such. The stock wires are not beefy enough to properly feed an electronic ignition. Edit: I just checked mine, I'm using a 10 guage wire.

Re: Stubborn Engine Problem
 

Update on the current condition:

Worse than ever!

I had taken a break from truck work to redo a deck at my house. Turns out I'm much better at that than engine troubleshooting.

Well I got back into it this weekend after finishing the deck and nothing I've tried has improved it at all. I first tried swapping an old carb I had which isn't even that old, but at 550 or 600 cfm, it's ultimately too small. It made no difference.

I tried running it without vacuum advance. No difference.

Next a did a valve adjustment. No change whatsoever. The valve train looked well oiled and nothing looked bad.

Next I tried running a wire directly from the battery to the HEI. This made no difference.

The only thing left on my list is to swap out the distributor shaft for my old one. I'm not sure what that could even improve, but I will try it.

After that, I'm fresh out of ideas. It seems to have gotten worse over the course of the summer. It's now idling poorly and stalls at stoplights unless I two foot it. But I know I have a mismatched torque converter, so there are other issues. I also continue to have problems with a timing light reading the pulse. I've tried it on 3 different sets of plug wires and 2 different lights. I bought a cheap harbor freight one and got to work sporadically enough to verify I was at 12 degrees initial.

I may have to take it someone. Anyone have any recommendations for someone in the Portland, OR area who knows old school small blocks?

Re: Stubborn Engine Problem
 

Since you’re getting sporadic timing read outs then maybe you should time it by ear.
Advance it until it pings under load, then bump it back a touch to where it doesn’t ping or start hard.
Then see if your light works.
Then adjust the mechanical timing in the dizzy.

That much cam needs likely closer to 20 initial.

Re: Stubborn Engine Problem
 

From your description it sounds like some kind of surging at the off idle transition. The other carb didn't change it at all?

Re: Stubborn Engine Problem
 

It's not surging at all. If anything, it's more like a hesitation. The carb didn't change it at all. This feels like an ignition issue to me, as every time I've played with the carb settings, it's made no difference.

Just a thought, what if my timing mark on the balancer is not accurate. It was done by the engine builder but I've never verified TDC is actually where the mark is.

I still don't understand why a timing light wont read the spark. I know it's sparking. All the plugs are looking good (at least on the 750 carb).

Re: Stubborn Engine Problem
 

Hey Drew,

I want to bring my truck over and you drive it. If it does the same thing we will be onto something.

I am local. And harmless haha

Re: Stubborn Engine Problem
 

Sorry, I'm not following. Are you having the same issues, or are you saying it's just me?

Re: Stubborn Engine Problem
 

So, is it an actual hesitation, misfire, or stumble in the engine, or just an off idle noise that in annoying? Could it just be an exhaust manifold gasket leak that you are hearing at these conditions before other noises rise up and drown it out?

Re: Stubborn Engine Problem
 

I don't have enough experience to know for sure. It sounds like a misfire, has maybe a slight hesitation at part throttle, but not bad. I don't think it's a stumble. Mash on the throttle and it goes away... although lately that's become less true as it's gotten worse. It got worse after changing intake manifolds for sure, which is, of course, when I also reinstalled the distributor. I'm going to try and verify TDC, see if the balancer is indeed accurate, and then set it by ear if I can't get my timing light to work.

If that doesn't work I will try and take some video so you guys can hear it.

Re: Stubborn Engine Problem
 

I don't see where you tried changing the ignition module or coil. Weak spark can cause similar issues. Plus the fact that you can't get consistent readings with your timing light makes me think of those 2 things.

Re: Stubborn Engine Problem
 

I'm going to be diagnosing a very similar problem when I get some time. My 78 Caddy has a surge/stumble just off idle. It has a funny sound like it's flooding. It runs fine everywhere else except idle. So far I'm blaming the carb until I find a vacuum leak. I think a weak ignition would show itself in other area besides idle.

Re: Stubborn Engine Problem
 

I didn't read all of both threads. What were the results of the compression test with engine warm and WOT?

Re: Stubborn Engine Problem
 

Timing! Timing! Timing!

After putting a bit more timing into it by ear, my light finally started to work. I set it a bit above 12 degrees. It's finally running like its supposed to. I'm so relieved. I hear a little tiny bit of that exhaust popping, so it may want a few degrees more. I gotta be careful because with 24 deg in my HEI anything over 16 deg will push me past 40 deg total. DUI doesn't like you messing with their weights, but has anyone done just that to take some total timing out of it?

Thanks for all the help.

Re: Stubborn Engine Problem
 

Without accurately confirming tdc of #1 piston to check the timing tab/balancer how will ever know how much initial or all in timing you have.

Re: Stubborn Engine Problem
 

I forgot to say I did that first. The mark is spot on for TDC. They have a second mark that is at 12 deg.

Re: Stubborn Engine Problem
 

What did you use for a piston stop?

Re: Stubborn Engine Problem
 

A plastic straw from the coffee shop, plus sticking a small screwdriver in there to confirm. It's in no way scientific but from what I could tell, it looked to be right there where the mark was.

Re: Stubborn Engine Problem
 

Do some research on how to "curve" the timing on your engine/distributor. You will be amazed at how your engine wakes up. It's kind of like tuning your distributor to the truck itself. I only use timing lights to get the engine running and broken in and really only helps in stock situations which will be rarely seen any more. I curve all my ignitions now. It takes into account the trans gearing, rear end gearing, the weight of the vehicle and the gas that is in your tank. And of course the engine. Like finding the sweet spot. I used to do it for customers and they really liked it. Stock vehicles can even benefit from this procedure. Here come the nay sayers but as For me I believe.

Re: Stubborn Engine Problem
 

I believe too!
I grew up living and hotrodding at higher elevation. Optimum timing was always the first priority even on stock just off the car hauler vehicles.
Of course setting up timing involved accuracy right from the start. No sense even starting without checking the timing tab for accuracy. Using the proper tools. Always used a piston stop if the heads were on or a dial indicator if the heads were off. There’s no other way to do it accurately. Straws, pencils, screwdrivers etc don’t work.

Re: Stubborn Engine Problem
 

Would a stretched timing chain be an issue like this