WHYDTYTT: What Have You Done To Your Truck Today?

[Gary Lewis]

The dyno testing was probably all done at WOT and with essentially no vacuum to the dizzy. So while the mix is probably right at WOT, it might be lean at cruise.And, while the dizzy has a modified advance, I doubt that they corrected the vacuum advance, which is where the extra timing comes from during EGR. And too much advance can cause roughness - especially with a lean mix.The test is simple - pull and plug the vacuum line to the dizzy. If the vibration goes away you've found at least part of the problem.

Gary,Yes, yes, and yes! The dyno pulls started at 2800 RPM and went to 5500 RPM if I recall correctly, so any fuel corrections made were in reference to those RPM's under load.I have the Crane adjustable vacuum advance kit installed, and I have it set for maximum advance (weakest springs). Truth be told, I meant to go back and play with this some more to check and/or adjust it if necessary. I did play with it initially after installing the engine, but never did go back to fine tune it. I will check on this for sure.One potential issue I had with the centrifugal advance was finding out exactly when it was advancing. I was having a tough time checking this stuff since I couldn't rev the engine and check the timing at the same time...or not very well at least. The centrifugal timing was supposed to be all in by 2800 RPM (I think), but it did not seem like it increased gradually like it was supposed to. It seemed like all of the advance was above maybe 2200 RPM. Again...going by fuzzy memory here, but it is all good because now I have a very good excuse to check all of this stuff over to get it dialed in.

Before you delve into it too deeply, just pull and plug the vacuum line and see if the vibration/roughness goes away.

 

And, I'm glad you have the Crane adjustable vacuum advance. Those things are great. And the instructions are shown below.

 

But, if you had separate spring those are probably for the centrifugal advance. They control how quickly and at what RPM the weights start swinging out. And the weights advance the plate the pickup sets on, which advances the timing.

 

With the lightest springs they say the advance starts at 800 RPM and is all in at 2600. That should work with your tranny, but might be a bit fast for an auto.

 

[Rembrant]

The dyno testing was probably all done at WOT and with essentially no vacuum to the dizzy. So while the mix is probably right at WOT, it might be lean at cruise.

Gary,

Forgive me for dumb questions, but I have had almost NO experience with carbs.

I was just thinking...if the carb was jetted for WOT dyno pulls, I forget what AFR now, but I think it was 12.5 or there abouts, wouldn't it by default end up running a little rich at cruise RPM's?

I know the plugs tell a different story, but I'm just trying to understand how the jetting works.

I remember having the discussion with the engine builder that I might have to re-install the leaner #66 jets when I got the engine re-installed. The rationale at the time was that with the shorty headers and full exhaust(s) with mufflers might decrease the flow enough to make the AFR more rich...

And I seem to remember reading a lot of internet chatter that Holley carbs usually run too rich out of the box. Oddly, my experience has been the opposite. Maybe my engine is flowing more air than usual.

 

[Gary Lewis]

The dyno testing was probably all done at WOT and with essentially no vacuum to the dizzy. So while the mix is probably right at WOT, it might be lean at cruise.

Gary,

Forgive me for dumb questions, but I have had almost NO experience with carbs.

I was just thinking...if the carb was jetted for WOT dyno pulls, I forget what AFR now, but I think it was 12.5 or there abouts, wouldn't it by default end up running a little rich at cruise RPM's?

I know the plugs tell a different story, but I'm just trying to understand how the jetting works.

I remember having the discussion with the engine builder that I might have to re-install the leaner #66 jets when I got the engine re-installed. The rationale at the time was that with the shorty headers and full exhaust(s) with mufflers might decrease the flow enough to make the AFR more rich...

And I seem to remember reading a lot of internet chatter that Holley carbs usually run too rich out of the box. Oddly, my experience has been the opposite. Maybe my engine is flowing more air than usual.

Yes, my experience is that Holley's run rich out of the box.

But the power valve is what makes the difference between power and cruise AFR. The power valve is a diaphragm with vacuum on one side and a spring on the other. The spring has been adjusted so the valve opens below a specified vacuum setting, with PV's available every 1" of vacuum from something like 2.5" to 8.5" or higher.

When your manifold vacuum is above the rating of your power valve it'll be closed so you are running on the main jets only. That's your "cruise" AFR. But when you push the throttle down and the manifold vacuum gets down to the rating of the PV it'll open and enrichen the mix, and now you are into the "power" AFR. And if the PV doesn't open in time the mix will go lean and the engine will not want to accelerate - or maybe even die away. And right on the border line it may run rough.

So it could be too lean of an AFR. Or it could be too much vacuum advance. Or it could be combo of the two. Or something else altogether. So, one step at a time......

[Rembrant]

So it could be too lean of an AFR. Or it could be too much vacuum advance. Or it could be combo of the two. Or something else altogether. So, one step at a time......

Thanks for the info Gary!

I have a list of things to check now, and that's all I needed to get started.

I'll be back with results later!

 

[Rembrant]

So it could be too lean of an AFR. Or it could be too much vacuum advance. Or it could be combo of the two. Or something else altogether. So, one step at a time......

Gary,

Good call! I disconnected the vacuum advance, and the engine runs much better without it. I think I knew this, and forgot...lol. I did play around with it last year and I knew it wasn't right, but then the truck sat basically for a year, so I forgot that I needed to deal with the vac advance. I will work on this, but glad to know that is at least part of the problem.

A friend came over tonight to help me with a compression test and timing checks. Base timing is still good and solid at 12 degrees BTDC. The centrifugal advance is still 20 degrees and it's all in by about 2800 RPM. The increase is not as smooth and gradual as I'd like it to be, so I must read up on this a bit and see what can be done with the springs. It's a bit of a flat curve that ramps up sharply at the end, and I'd rather it be more like a straight line if that's even possible.

Compression looks pretty good I think. This was with the engine hot, all plugs removed, and throttle tied open. I never did do a base line test on dyno day, so I didn't know where it was.

#4: 177 PSI #8: 170 PSI

#3: 170 PSI #7: 170 PSI

#2: 177 PSI #6: 170 PSI

#1: 178 PSI #5: 168 PSI

The biggest spread is between cylinders #1 and #5, but still reasonable. I don't know what is the allowable deviation. I must go read up on that. I know my 302, in it's original form was only about 150 lbs compression, but it was more even across all cylinders.

Oh, and I found #4 spark plug to be loose...not sure if that was a problem yet or not. Haven't test driven it yet;).

Oh, and I checked exhaust temps at the headers next to the heads, at idle, and they were mostly in the 265F range. They kind of varied a bit, but mostly in the 250-285F range. Cylinder #5 was the coolest of them all coming in at only 225F, but it also turned out to be the lowest in compression (I believe cylinder #7 was the hottest).

I think I need to finally bust open the piggy bank and get myself an AFR gauge. I think as you suggested I've had too much advance and too little fuel.

 

[Gary Lewis]

So it could be too lean of an AFR. Or it could be too much vacuum advance. Or it could be combo of the two. Or something else altogether. So, one step at a time......

Gary,

Good call! I disconnected the vacuum advance, and the engine runs much better without it. I think I knew this, and forgot...lol. I did play around with it last year and I knew it wasn't right, but then the truck sat basically for a year, so I forgot that I needed to deal with the vac advance. I will work on this, but glad to know that is at least part of the problem.

A friend came over tonight to help me with a compression test and timing checks. Base timing is still good and solid at 12 degrees BTDC. The centrifugal advance is still 20 degrees and it's all in by about 2800 RPM. The increase is not as smooth and gradual as I'd like it to be, so I must read up on this a bit and see what can be done with the springs. It's a bit of a flat curve that ramps up sharply at the end, and I'd rather it be more like a straight line if that's even possible.

Compression looks pretty good I think. This was with the engine hot, all plugs removed, and throttle tied open. I never did do a base line test on dyno day, so I didn't know where it was.

#4: 177 PSI #8: 170 PSI

#3: 170 PSI #7: 170 PSI

#2: 177 PSI #6: 170 PSI

#1: 178 PSI #5: 168 PSI

The biggest spread is between cylinders #1 and #5, but still reasonable. I don't know what is the allowable deviation. I must go read up on that. I know my 302, in it's original form was only about 150 lbs compression, but it was more even across all cylinders.

Oh, and I found #4 spark plug to be loose...not sure if that was a problem yet or not. Haven't test driven it yet;).

Oh, and I checked exhaust temps at the headers next to the heads, at idle, and they were mostly in the 265F range. They kind of varied a bit, but mostly in the 250-285F range. Cylinder #5 was the coolest of them all coming in at only 225F, but it also turned out to be the lowest in compression (I believe cylinder #7 was the hottest).

I think I need to finally bust open the piggy bank and get myself an AFR gauge. I think as you suggested I've had too much advance and too little fuel.

Cool! Glad you are making progress.

And by the way, those are excellent compression readings.

I don't think I'd worry about the temp differences as engines don't usually have perfect balance at idle. Or even at cruise for that matter, but it should be closer than at idle.

And with #4 loose, you may have just figured out all your problems.

[fords4life]

Cool! Glad you are making progress.

And by the way, those are excellent compression readings.

I don't think I'd worry about the temp differences as engines don't usually have perfect balance at idle. Or even at cruise for that matter, but it should be closer than at idle.

And with #4 loose, you may have just figured out all your problems.

No work on the truck today, but I did get the long block ordered. Need to make a decision on whether or not to rebuild the trans, get the exhaust ordered, and driveshaft rebuilt and I'll be good to go.

[Rembrant]

No work on the truck today, but I did get the long block ordered. Need to make a decision on whether or not to rebuild the trans, get the exhaust ordered, and driveshaft rebuilt and I'll be good to go.

I haven't done anything to my truck today, but the mailman just dropped this off!

I also picked up a new carb spacer and new gaskets this morning, so that will be this evenings project. I have a feeling that I have a vacuum leak under the carb, and I will explain more later after I take it apart.

Anyway, some good reading material for the long weekend! (Canadian Thanksgiving this weekend!).

I already have a couple Holley carb books, but they're more on general information and rebuild basics. This one gets into tuning and I need all the help I can get.

 

[Gary Lewis]

I haven't done anything to my truck today, but the mailman just dropped this off!

I also picked up a new carb spacer and new gaskets this morning, so that will be this evenings project. I have a feeling that I have a vacuum leak under the carb, and I will explain more later after I take it apart.

Anyway, some good reading material for the long weekend! (Canadian Thanksgiving this weekend!).

I already have a couple Holley carb books, but they're more on general information and rebuild basics. This one gets into tuning and I need all the help I can get.

I have several of David Vizard's books, and find them to be some of the best-written and informative ones I've found. I think you are on the right track.

[85lebaront2]

I haven't done anything to my truck today, but the mailman just dropped this off!

I also picked up a new carb spacer and new gaskets this morning, so that will be this evenings project. I have a feeling that I have a vacuum leak under the carb, and I will explain more later after I take it apart.

Anyway, some good reading material for the long weekend! (Canadian Thanksgiving this weekend!).

I already have a couple Holley carb books, but they're more on general information and rebuild basics. This one gets into tuning and I need all the help I can get.

Not today, but yesterday I grabbed my spare instrument cluster pieces to see if I can get an oil pressure gauge that will read some semblance of pressure (1992-96/7 clusters use the "gauge" as an oil pressure indicator) There is a resistor on the printed circuit that when the switch on the engine closes, makes the "gauge" read roughly 1/2 scale. Jumpering this resistor and installing an older oil pressure sender converts it to a gauge. The problem comes if the pointer sits beyond the 0 position (red line) when the key is on, engine off, then it may not really do anything.

What I did was take the loose gauges and connect the ignition and ground which is the initial condition before starting the engine. I then carefully removed the pointer and reinstalled it on the red 0 line. Grounding the signal post gives full scale to the high end red line. I will have to swap this one into my cluster possibly tomorrow as I have to go to Rejuvinix in Virginia Beach today for the 4th of 5 scheduled injections.