hesitation under load help

[rd100]

I think I adjusted something too much.

When I test drove the truck just now, it feels like it's got less power than previously and there seems to be a slight hesitation when I'm doing a steady 55mph.

It idles great.

I do have an exhaust leak that I haven't gotten to yet.

Today I adjusted the timing and the carb mixture.

I've got a 1970 302.

I set the timing to 6B, Dwell is at 26 degrees.

Do I need to re-adjust the carb mixture? Or am I being too picky here? I drove it 10 miles and it got me back home fine.

Before I adjusted the timing it was having this "bub bub" sound on certain cylinders, the timing then was around 20B, it would make a dull pop noise when coming off of revs.

[rd100]

Or do I need to adjust the timing to something other than the charts specified due to wear? The engine has 82,000 miles on it, I have no knowledge about the timing chain.

When I adjusted the timing down to 6B from where it was the engine idle went way down and I re-adjusted the idle, but like I said the idle smoothed out.

[Banjo]

the best $20 I ever spent was on a Harbor Freight vacuum gauge. You hook it to a port on your intake and adjust your carb mixture to attain the highest level of vaccuum while it's sitting there running in park. If the vacuum needle is bouncing, you have a vaccum leak that must be found/fixed. I wish I'd bought one 20 years ago to help me adjust carburators....

[Banjo]

sounds like the other pop sound you mention was it leaning out. I suspect if you feel a hesitation under load, it's leaning out a tad, just not enough to backfire or pop.

[fomocoguy]

You can never be too picky. I would adjust on it until it's right. Your idle mixture won't affect the performance at 55 mph, so I wouldn't go there just yet. When you adjusted your timing, did you pull the vacuum hose off the distributor and cap it? If you didn't you will get a false reading and it will show more advance than it actually has. I ask because it sounds too me like you may have pulled out too much advance when you adjusted it....

Keep in mind that a lot of times these old engines will perform quite well with a few degrees of advance, like maybe around 8 to 10 btdc for your app.

[rd100]

Yes, I pulled the vacuum advance off of the distributor and plugged it when I set the timing to 6B. I'll try advancing it towards 8 to 10 and see if that's better, I'm suspecting it will be.

I have a vacuum gauge that I put in the system when I was checking it out however I didn't use it for the carb mixture, I used a tach for that, is it better to use a vacuum gauge?

[rd100]

I put the timing at about 10B instead of 6B and that seemed to help. It's not solid smooth perfect but it has more responsive power now at 55mph and I don't feel the hint of shudder I felt before. I'm not sure if it's now just in my head or if there's still an occasional hesitation feeling at 55mph.

Using the vacuum gauges to adjust mixture seems better than using the tach. It was at 19" when I started and at 20" when I was done adjusting the mixture, the tach was reading pretty much the same thing before and after so the vacuum gauge method seems more sensitive to finding the precise point.

[knightfire83]

Another item to check would be the vacuum advance itself. Sometimes the diaphragm inside starts to leak and it doesn't advance the timing as it should.

[rd100]

Is there an easy way to test the vacuum advance?

I did aim the timing light at the marks and revved the engine with the vacuum back on the distributor and the timing did change there, though there's no way to tell what it advanced to.

[knightfire83]

Easiest way is with a hand vacuum pump to see if it holds vacuum. If your daring you can do it by mouth also with a length of hose (engine off). If it has any kind of a leak, it is bad and the diaphragm inside has cracked.

Or you can do as you did with the engine running and see how far the advance goes when you rev the motor via timing marks.
Rev it once with the hose disconnected then again when it is connected. Without the hose connected it will still have some advance with the centrifugal weights inside the distributor. But when the hose is connected it will show allot more advance while revving the engine. If the advance is bad, there won't be a change after hooking up the hose.

[67_p_car]

same problem here - only it is on a 240 - 68' (the engine AFAIK is from a 66)

starts up good
nice idle
hesitation under load
then a pretty good (not good) backfire when I turn the key off.

I set the ignition timing @6 with the vacuum plugged

1 other note - the carb has a slight leak - damn rebuilt - the body of the card is aslightly wet with gas after a good drive across town

I'm guessing you all would suggest a new carb first.

boy, do I love this truck though - especially because the radio is disconnected!