The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

drew8mc · 08-04-2019, 01:31 AM

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

08-04-2019, 01:31 AM	#1
drew8mc Registered User Join Date: Mar 2017 Location: Portland Posts: 63	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 Last edited by drew8mc; 08-07-2019 at 12:36 PM.