Distributor curve

[barnett468]

I am running a Holley 770 Street Avenger

I'm trying two sizes up from 70's to 72's

That is carb a fairly big carb for fairly big hp engines.

I would buy a spring kit and try different springs . You can often pick up a little power by installing a lighter one . The lightest is the spring with white paint on top and this is also the most commonly used but on some engines, it can cause the secondaries t open a little to soon which can cause a little bog.

Since your primaries are a little lean, your secondaries might be also, therefore, if you have some bog/hesitation with a lighter spring, it may be due to lean secondary main jets instead of the light spring so you could try two sizes larger on the secondaries also if it bogs, however, if it has 74's in the secondaries, i doubt a bog would be cased by those when they open . Also, the secondaries must be at least as big as the primaries [front] jets . if the front is screamin lean, the secondaries will also be if they are jetted smaller than the primaries . in other words, if you find an improvement with the 72's, you need at least 72's in the secondaries . Some people run around 6 sizes bigger in the secondaries, but in my experience, this is rarely necessary and just 2 sizes seems to be a good rule of thumb.

springs.

http://www.summitracing.com/parts/hly-20-13?seid=srese1&gclid=CMv37vzDm8wCFY9hfgodODoNiQ

I would also adjust the gas level in the carb so it is just below the inspection holes with the engine idling or immediately after you turn it off . put a bunch of paper towels next to the carb when you remove the screws because it will dribble a bit of gas onto your engine until they are lowered.

 

[jpaz]

I am running a Holley 770 Street Avenger

I'm trying two sizes up from 70's to 72's

That is carb a fairly big carb for fairly big hp engines.

I would buy a spring kit and try different springs . You can often pick up a little power by installing a lighter one . The lightest is the spring with white paint on top and this is also the most commonly used but on some engines, it can cause the secondaries t open a little to soon which can cause a little bog.

Since your primaries are a little lean, your secondaries might be also, therefore, if you have some bog/hesitation with a lighter spring, it may be due to lean secondary main jets instead of the light spring so you could try two sizes larger on the secondaries also if it bogs, however, if it has 74's in the secondaries, i doubt a bog would be cased by those when they open . Also, the secondaries must be at least as big as the primaries [front] jets . if the front is screamin lean, the secondaries will also be if they are jetted smaller than the primaries . in other words, if you find an improvement with the 72's, you need at least 72's in the secondaries . Some people run around 6 sizes bigger in the secondaries, but in my experience, this is rarely necessary and just 2 sizes seems to be a good rule of thumb.

springs.

http://www.summitracing.com/parts/hly-20-13?seid=srese1&gclid=CMv37vzDm8wCFY9hfgodODoNiQ

I would also adjust the gas level in the carb so it is just below the inspection holes with the engine idling or immediately after you turn it off . put a bunch of paper towels next to the carb when you remove the screws because it will dribble a bit of gas onto your engine until they are lowered.

I do have the gas level just below half of the sight glass.

I also have a spring kit.

The reason I went with the 770 & not the 670 is because it was stock with a 715cfm carb, so I figured I could go a bit bigger.

 

[barnett468]

The reason I went with the 770 & not the 670 is because it was stock with a 715cfm carb, so I figured I could go a bit bigger.

You have a 1971 BOSS 351?

 

[jpaz]

The reason I went with the 770 & not the 670 is because it was stock with a 715cfm carb, so I figured I could go a bit bigger.

You have a 1971 BOSS 351?

No, I have a 72 Q Code


I forgot to mention when I was changing jets I found some sediment in bottom of bowl. Was kinda surprised cause I just put that carb on last summer w/new filter. Even put a new filter on this year. Gas tank is new from restoration just under 2000 miles since it was done.

I usually use 93 octane gas.

 

[Don65Stang]

Dude, chill. I see some of the sketchy advice you pass around and someone is going to end up screwing up their car because of it. Thank you for the negative reputation point Barnett, take care

Hey moderator, remove the negative rep please. Not deserved in my humble opinion.

Paul of Mo

I agree!

WARNING!!! We've had enough intervening of posts where members take pop shots at other members. If you wake up and see that you have been permanently banned, just remember that I publicly warned everyone. If the shoe fits...if not then you have nothing to worry about.

 


DO NOT PM me or anyone on the staff about this.

 

[Don65Stang]

Back to the topic at hand... there is a really good (old) post on here somewhere with a lot of great advice about distributor curves.

Found it: http://www.7173mustangs.com/thread-timing-advance

 

[jpaz]

Back to the topic at hand... there is a really good (old) post on here somewhere with a lot of great advice about distributor curves. It is worth searching for.

Thank's Don, I did do a lot of reading and searching about this stuff b4 I started this thread. Read some that I understood & some that I didn't. This is my 1st 351 Cleveland, 1st Ford muscle car for that matter.

Anyway, from all that info & all the great advice on this thread it is really helping me to understand timing & the dist curve. Sure hope it can help others too, Thank's to all!

 

[barnett468]

This is my 1st 351 Cleveland, 1st Ford muscle car for that matter.

Anyway, from all that info & all the great advice on this thread it is really helping me to understand timing & the dist curve. Sure hope it can help others too, Thank's to all!

Yes, unfortunately it is an area that is too frequently overlooked which is why I frequently mention it to people, especially since it is a fairly easy thing to check . I have commonly seen gains of 10 hp on engines and even up to 30 on some others.

After you have finalized everything, I will tell you how to determine how much advance your vacuum advance can ads if you want to use it . The non adjustable ones typically provide too much advance if one sets the advance curve using the example I posted for you . If your vacuum can is not adjustable, you can buy an adjustable one . They are around $25.00 . SMP p/n VC31 or BWD p/n V311 . There is a BWD V311 that has a letter after that part number and you do not want that one.

One or the other of these can be gotten through any auto parts store . Rock Auto and Summit Racing have some of the lowest prices on them . These are the only part numbers I am aware of that are the adjustable can . If a store tries to sell you one that "cross references" to a different number than either of these, I would not buy it because it likely will not be adjustable.

.

 

[jpaz]

I put the 72's in the mains & went for a drive. It does seem like its a bit better, starts real easy too. Checked plug & it didn't have as much black on the end of threads. I still need to check what jet size is in the secondary & check for more crap in the bowl.

I changed to a lighter spring too. Mine had a silver one in it & I changed to a yellow one. It came w/3 springs, yellow being the lightest one. I'll have to wait till tomorrow to drive it.

I do have an adjustable, single advance on my distributor. I checked it when I had it out to change intake manifold. I think it was set 3 turns out. Thank's again for all the help.

 

[barnett468]

I put the 72's in the mains & went for a drive. It does seem like its a bit better, starts real easy too. Checked plug & it didn't have as much black on the end of threads. I still need to check what jet size is in the secondary & check for more crap in the bowl.

I changed to a lighter spring too. Mine had a silver one in it & I changed to a yellow one. It came w/3 springs, yellow being the lightest one. I'll have to wait till tomorrow to drive it.

I do have an adjustable, single advance on my distributor. I checked it when I had it out to change intake manifold. I think it was set 3 turns out. Thank's again for all the help.

No prob, you're welcome . Just glad it has made an improvement, unfortunately, none of this type of ignition tuning is in any book that I am aware of . I learned it around 40 years ago mostly through experimentation but it was a commonly used method for a zillion years before that.

The bigger jets will not make the end of the plug threads cleaner, and if anything, they would make them darker, however, if your timing was less than optimal and you increased it to a more optimal level, this can clean the plugs up, however, it typically takes several miles.

Also, if the jetting is close, it typically takes around 20 minutes of driving to get enough color on a plug to make a reasonable assessment of the overall jetting.

You can do a "plug chop" as Jeff73 mentioned and this will show up right away, however, it should be done on a new plug and the color formed starts at the base of the porcelain so you need good eyes or need to cut the threads off the end of the plug to see it unless it is too rich at which point it will be easier to see.

Here's an example of richer on the left going to leaner on the right done on the same vehicle with only main jet changes.

 

[jpaz]

I didn't drive it enough, I will try to today.

The pics of the plugs are great! And like you said, reading them is different than I remember because of the ethanol in the gas.

 

[73pony]

Do you have any stations that sell the gas without the ethanol added? It took some digging and investigating but I found 4 places in town here that sell either 91 or 93 and clearly marked "NO ALCOHOL"

 

[jpaz]

Do you have any stations that sell the gas without the ethanol added? It took some digging and investigating but I found 4 places in town here that sell either 91 or 93 and clearly marked "NO ALCOHOL"

Yes 73pony, I checked last summer. No stations like that close to me. There are some by the lakes, but that's a good 25 miles away. Too far to deal with & way to much traffic.

 

[Jeff73Mach1]

With your vacuum being only about 12, I think it might be wise to look at the power valves in the carb.

Especially with a lumpy cam, they can tend to "bounce" open enriching the idle mixture erratically.

In general most people set them up at 1/2 the measured idle vacuum ie., if you have 12 inches of vacuum then you should have a power valve that is numbered 5.5. Now to be candid-power valves can be tricky as idle vacuum is not really the best measure for when you need fuel enrichment under load. A vacuum gauge hooked up while driving will help you find the highest part throttle vacuum your engine is producing.

While you are tuning your carb, this is really a pretty essential step.

and then there are the accelerator pumps, their cams and squirters that may or may not need a little fine tuning . . .

 

[jpaz]

After changing to the lighter secondary spring, I took car out for a drive & seems much better. It even breaks the rear tire loose pretty good. I was stopped at a light & just mashed the gas in drive, & it spun the rear tire. Never did that b4 without a little break torque.

I pulled the rear bowl off carb to check the secondary mains & those are 75's. I had 70's in the front & changed those to 72's. No hesitation when I punch it.

I think we're getting this dialed in pretty good now.

With your vacuum being only about 12, I think it might be wise to look at the power valves in the carb.

Especially with a lumpy cam, they can tend to "bounce" open enriching the idle mixture erratically.

In general most people set them up at 1/2 the measured idle vacuum ie., if you have 12 inches of vacuum then you should have a power valve that is numbered 5.5. Now to be candid-power valves can be tricky as idle vacuum is not really the best measure for when you need fuel enrichment under load. A vacuum gauge hooked up while driving will help you find the highest part throttle vacuum your engine is producing.

While you are tuning your carb, this is really a pretty essential step.

and then there are the accelerator pumps, their cams and squirters that may or may not need a little fine tuning . . .

Jeff, how do I check to see which power valves I have in carb? Are there numbers on them?

 

[Jeff73Mach1]

yes, there are numbers stamped into them, though they are sometimes a bit cryptic.

There are also high flow and standard flow valves. the high flow have 4 openings and the standard have two. I'd start with standard flow valves and a 4.5 or 5.5 in the front. I'd leave the rear alone initially as you don't want to lean out on the top end and harm your engine

 

[barnett468]

After changing to the lighter secondary spring, I took car out for a drive & seems much better. It even breaks the rear tire loose pretty good. I was stopped at a light & just mashed the gas in drive, & it spun the rear tire. Never did that b4 without a little break torque.

I pulled the rear bowl off carb to check the secondary mains & those are 75's. I had 70's in the front & changed those to 72's. No hesitation when I punch it.

I think we're getting this dialed in pretty good now.

xlnt . glad you got noticeable results . i wouldn't touch anything including the power valve unless you just want to see if you can tell a difference . a lower number will open a tiny bit later which may cause a slight hesitation . i use 6.5 valves 99% of the time and many of the engines i build have around 12 hg of vacuum.

 

[jpaz]

So is a 6.5 power valve usually what comes in 4150 style Holley's? I'm not sure what they come with.

It's running so good I was planning on driving it like this for awhile. I haven't taken it on the freeway yet.

 

[c9zx]

This http://documents.holley.com/techlibrary_carb_numerical_listing.pdf is a link to the Holley Numerical listing for most of the carbs. It lists power valves, jets, pump cams, etc.. You may find it helpful.

Chuck

 

[barnett468]

So is a 6.5 power valve usually what comes in 4150 style Holley's? I'm not sure what they come with.

It's running so good I was planning on driving it like this for awhile. I haven't taken it on the freeway yet.

most carbs come with a 6.5 and that is the most common power valve used.

holleys power valve suggestion guide used to be pretty much useless because in fact, they used to tell you three completely different ways to determine what the best power valve was and they all contradicted themselves to some degree . i found this out around 10 years ago when someone else mentioned it to me because i had never read any of their info . i have been building and tuning cars since 1970 but learned some on my own and some from others, so i never needed to read it.

i just looked online and saw the link below which is different than what i had seen before and it is far more correct, however, it is only a guide and if you still have probs using their method, you should try something else because tuning is a bit of an art and what works for one app, doesn't always work quite the same for another.

http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning

you spent maybe $10.00 for two jets and maybe a total of one hour tuning your engine based on suggestions on your thread and it now roasts the tires from a dead stop and will spin them when it hits second gear and it has absolutely zero hesitation . in my experience, once a car does this and still idles decent etc, there is little, to nothing else that will make it any better, however, it never hurts to try.