Tuning new 383
07-23-2016, 05:34 PM
#43
Melting Slicks
Thread Starter
Since Ill be out of town for the next week and unable to tinker, help me think through and process how to begin tackling the tuning issue.
What I know:
We know its running rich, plugs are black.
15" Vacuum is good.
Will idle as low as 500.
Doesn't run hot.
Power Valve is a 6.5 and isn't blown.
Carb is a 3310 Vac secondary 750 76 Primary jets.
At low rpm, accelerating, it will "miss" not quite a pop, or buck, maybe a "trailer hitch"..that clears up at higher rpm. Over 3k it spins smooth and strong.
Cruises at 2200-2500 on the freeway, smooth no issues.
In 5th gear in the freeway putting my foot down, I had no secondary surge or sudden acceleration as I would expect.
So, Idle circuit...Lean? or Rich? Symptoms of either..?
Centrifugal advance...sooner or later?
Went to a slightly stiffer spring, not much difference.
Idle mixture screws are about 1.75 turns out. Turning them in would be leaner, correct?
In the process of elimination, lets take one thing at a time and try to rule it out......
Thanks for the help....lets do this
What I know:
We know its running rich, plugs are black.
15" Vacuum is good.
Will idle as low as 500.
Doesn't run hot.
Power Valve is a 6.5 and isn't blown.
Carb is a 3310 Vac secondary 750 76 Primary jets.
At low rpm, accelerating, it will "miss" not quite a pop, or buck, maybe a "trailer hitch"..that clears up at higher rpm. Over 3k it spins smooth and strong.
Cruises at 2200-2500 on the freeway, smooth no issues.
In 5th gear in the freeway putting my foot down, I had no secondary surge or sudden acceleration as I would expect.
So, Idle circuit...Lean? or Rich? Symptoms of either..?
Centrifugal advance...sooner or later?
Went to a slightly stiffer spring, not much difference.
Idle mixture screws are about 1.75 turns out. Turning them in would be leaner, correct?
In the process of elimination, lets take one thing at a time and try to rule it out......
Thanks for the help....lets do this
Last edited by DucatiDon; 07-23-2016 at 07:13 PM.
07-23-2016, 06:59 PM
#44
Le Mans Master
Don, one thing I would avoid till you have 500 miles on this engine is 5th gear acceleration. Low RPM with a lot of throttle puts more load on your pistons than you want in a new engine. All these parts are still breaking in. RPM for short periods is no issue. Guys drag race or dyno engines right after building them but they don't load them at low RPM. Low RPM loading is not recommended until you break it in or it can scuff piston skirts. Skirt coating helps but it can't protect against everything.
No doubt you are rich at idle.
I know you have done most of this yourself and that is great but have you considered taking it to a Dyno tuner. They install sniffers in your exhaust that give real time fuel mix readings and would know exactly what to do with the carb while running on a Chassis Dyno with no guess work.
Don at Motor Machine on Fair Oaks Blvd is good and I am sure there are others in town.
I have watched him tune Motors on the Dyno, it is fun to watch and see the Horse Power jump as the tune is perfected. You could spend 4-500 pretty quick if they need to change jets and timing springs while tuning but it would be nice to see your fuel curve perfected from idle to max RPM.
No doubt you are rich at idle.
I know you have done most of this yourself and that is great but have you considered taking it to a Dyno tuner. They install sniffers in your exhaust that give real time fuel mix readings and would know exactly what to do with the carb while running on a Chassis Dyno with no guess work.
Don at Motor Machine on Fair Oaks Blvd is good and I am sure there are others in town.
I have watched him tune Motors on the Dyno, it is fun to watch and see the Horse Power jump as the tune is perfected. You could spend 4-500 pretty quick if they need to change jets and timing springs while tuning but it would be nice to see your fuel curve perfected from idle to max RPM.
07-23-2016, 07:00 PM
#45
Race Director
07-23-2016, 07:12 PM
#46
Melting Slicks
Thread Starter
yes, vac gauge was used. You're right, of course...In for leaner.. Ive edited my post.
07-23-2016, 07:15 PM
#47
Melting Slicks
Thread Starter
And when I say low RPM on the freeway 5th gear, we are talking 3k and up..... no low rpm, or lugging until its well seated for sure.
(AND Im sure he will charge me EXTRA for having to deal with the fussy hard fuel lines...)
Last edited by DucatiDon; 07-23-2016 at 07:16 PM.
07-24-2016, 10:19 AM
#48
Safety Car
I don't know where you got that 3310 from, but pri mainjet size should be 72 max. With your mild cam and corked exhaust, I'd start with a 70 pri jet.
FYI: The carbs used for the 1970-72 LT1 engines (750/780 cfm 4555 with m/t) used 72 pri mains.
I have a 750 CFM DP race QuickFuel (no choke horn) on my 327 which has a bigger cam than you have (9 in-hg @ 900 RPM: compared to engine displacement, although on its face not a whole lot bigger than yours, although the lift is substantially higher) and it came equipped with 74 pri/84 secondary mains. I changed to 72 pri mains/82 secondary and it is perfect.
The FIRST thing that I would do, if I were you, would be to install #70 mains on the pri side, remove and clean the plugs, then take it for a drive and see if your plugs stay clean. The only way to do this properly is to drive on the highway (or, "Freeway" in CA) steady state cruise at max engine vacuum, then disengage the clutch, shut it off, pull over to a safe spot or coast to a rest area, pull all 8 plugs and inspect (use your phone camera to photograph all 8). Post up results. Fine tuning of WOT A/F ratio changes v RPM is a function of main air bleed size as well as secondary mainjet size. It always starts fat on the low RPM side and steadily goes leaner as revs climb.
I don't know if your carb is new, but it sounds to me that there is a problem with it other than the too-large pri mains. Once at op temp, idle speed should be stable as you know. There might be a fuel or vacuum leak due to expansion as it warms up.
You shouldn't need a fancy PCV valve, although if my friend Dave suggested it then I know that it will work for you. I use a standard PCV from a 1967 SBC (AC CV726C, discontinued and superceded by AC CV679) Corvette plumbed into the standard 1965 setup without the .090" orifice. Since your idle vacuum closely matches that of an L79 engine, then this should work well for you. However you proceed, know that the proper way to check a PCV is to idle the engine, remove the hose from the DOWNSTREAM end to expose the pintle (conical thingie that is spring loaded and meters the suction based on manifold vacuum) and put your finger over the opening.....................the RPM should drop by 50-100 RPM. No more, no less. Anything more means that it's pulling too much air into the intake manifold (too big a vacuum "leak"). Anything less means that the crankcase is not being sufficiently ventilated and you risk pressurization at WOT.
And...........NO you won't feel a surge of power when you nail it at .67:1 gear ratio. I think you have a 3.70 axle ratio like I do. You SHOULD NOT feel any hesitation and just a moderate, steady pull. That's what happens with my little mouse with 750 carb and mechanical secondaries. When I mash it at slow cruise, say, 60 it pulls steadily and pulls harder and harder as the revs climb. As mentioned before, not only are you lugging the engine, but if you have a vacuum gauge, you'll see that this puts you DEEP into the power valve for a long time which will foul plugs. Keep your 6.5 PV. Mine is a 4.5 PV, which I monitor with the vac gauge. I try like hell to stay out of it and only very rarely do I go into it rather than downshift.
If you put your car on a dyno now as someone suggested, since it is not even close to being dialed in, you'd be wasting a ton of money. I didn't use a dyno at all. It was perfectly dialed in before I brought it for test pulls. Your idea of getting closer first is a solid one.
BTW: Once you get your new air cleaner installed, the vent stacks should be at least 0.500" below the lid underside otherwise your a/f ratio will go leaner than it should at high RPMs. Cut down the stacks if necessary.
FYI: The carbs used for the 1970-72 LT1 engines (750/780 cfm 4555 with m/t) used 72 pri mains.
I have a 750 CFM DP race QuickFuel (no choke horn) on my 327 which has a bigger cam than you have (9 in-hg @ 900 RPM: compared to engine displacement, although on its face not a whole lot bigger than yours, although the lift is substantially higher) and it came equipped with 74 pri/84 secondary mains. I changed to 72 pri mains/82 secondary and it is perfect.
The FIRST thing that I would do, if I were you, would be to install #70 mains on the pri side, remove and clean the plugs, then take it for a drive and see if your plugs stay clean. The only way to do this properly is to drive on the highway (or, "Freeway" in CA) steady state cruise at max engine vacuum, then disengage the clutch, shut it off, pull over to a safe spot or coast to a rest area, pull all 8 plugs and inspect (use your phone camera to photograph all 8). Post up results. Fine tuning of WOT A/F ratio changes v RPM is a function of main air bleed size as well as secondary mainjet size. It always starts fat on the low RPM side and steadily goes leaner as revs climb.
I don't know if your carb is new, but it sounds to me that there is a problem with it other than the too-large pri mains. Once at op temp, idle speed should be stable as you know. There might be a fuel or vacuum leak due to expansion as it warms up.
You shouldn't need a fancy PCV valve, although if my friend Dave suggested it then I know that it will work for you. I use a standard PCV from a 1967 SBC (AC CV726C, discontinued and superceded by AC CV679) Corvette plumbed into the standard 1965 setup without the .090" orifice. Since your idle vacuum closely matches that of an L79 engine, then this should work well for you. However you proceed, know that the proper way to check a PCV is to idle the engine, remove the hose from the DOWNSTREAM end to expose the pintle (conical thingie that is spring loaded and meters the suction based on manifold vacuum) and put your finger over the opening.....................the RPM should drop by 50-100 RPM. No more, no less. Anything more means that it's pulling too much air into the intake manifold (too big a vacuum "leak"). Anything less means that the crankcase is not being sufficiently ventilated and you risk pressurization at WOT.
And...........NO you won't feel a surge of power when you nail it at .67:1 gear ratio. I think you have a 3.70 axle ratio like I do. You SHOULD NOT feel any hesitation and just a moderate, steady pull. That's what happens with my little mouse with 750 carb and mechanical secondaries. When I mash it at slow cruise, say, 60 it pulls steadily and pulls harder and harder as the revs climb. As mentioned before, not only are you lugging the engine, but if you have a vacuum gauge, you'll see that this puts you DEEP into the power valve for a long time which will foul plugs. Keep your 6.5 PV. Mine is a 4.5 PV, which I monitor with the vac gauge. I try like hell to stay out of it and only very rarely do I go into it rather than downshift.
If you put your car on a dyno now as someone suggested, since it is not even close to being dialed in, you'd be wasting a ton of money. I didn't use a dyno at all. It was perfectly dialed in before I brought it for test pulls. Your idea of getting closer first is a solid one.
BTW: Once you get your new air cleaner installed, the vent stacks should be at least 0.500" below the lid underside otherwise your a/f ratio will go leaner than it should at high RPMs. Cut down the stacks if necessary.
Last edited by 65tripleblack; 07-24-2016 at 04:06 PM.
08-01-2016, 03:21 PM
#49
Team Owner
Member Since: Nov 2005
Location: Beach & High Desert Southern California
Posts: 25,501
Received 2,346 Likes
on
891 Posts
Very nice Ed and I have to ask, what would it take to get that analysis on my 406?
Don, this discussion just jogged my memory about a very similar problem I had but have since forgotten and it too involved the PCV system. I determined that the valve I was using was allowing too much air flow at idle and giving me a lean stumble as well. I believe I detailed it in a separate thread that I'll see if I can find. One I realized actual specs ares unavailable, I ended up buying a handfull of valves and testing each one till I found one with the desired characteristics.
Found my earlier thread....
https://www.corvetteforum.com/forums...ir-flow-2.html
It runs a little long as there were several back and forth posts clearing up differing semantics on PCV systems but most of the relevant info is on page 2.
Don, this discussion just jogged my memory about a very similar problem I had but have since forgotten and it too involved the PCV system. I determined that the valve I was using was allowing too much air flow at idle and giving me a lean stumble as well. I believe I detailed it in a separate thread that I'll see if I can find. One I realized actual specs ares unavailable, I ended up buying a handfull of valves and testing each one till I found one with the desired characteristics.
Found my earlier thread....
https://www.corvetteforum.com/forums...ir-flow-2.html
It runs a little long as there were several back and forth posts clearing up differing semantics on PCV systems but most of the relevant info is on page 2.
I can try a EA-3.2 model if you post the engine specs (I failed on finding the link).
Don, is the performance as you expected, and are the little bugs worked out?
08-04-2016, 09:25 PM
#50
Melting Slicks
Thread Starter
I honestly haven't driven it enough to say how it performs yet...... Seems plenty strong so far...
I have a ridiculous amount of hours now into the damn oil pan gasket. When a whole mess of The Right Stuff didn't fix it, I decided to swap on my other oil pan, with a new gasket. That pan had just been replaced on the 327 and sealed fine with just the rtv in the corners.
So, I get the new gasket and oil pan on and guess what? Same F^&*ing drip. Pas side front corner...it seeps out between the gasket and the block, pools and then drips and runs down the front seal onto the ground. I immediately pulled it back off and slapped some Right Stuff in the corner.....so we will see tomorrow. Ive thought it might be dripping from somewhere else, but i can't find a wet spot of oil anywhere on the motor except that corner..... Grrrrr!!!!!!
Oh, and Dumbass error #2 (or is it 20 now???) My Vac gauge was broken...it read 7" high. I noticed too late, yesterday that it never dropped to 0. So...new Vac gauge, and the Vacuum is actually 8.5" not 15" after all. So I need a B28 Vac can.... the one I have doesn't start to pull until 10"-12" so I'm not getting any vac advance at idle at all. I think that may be the hesitation I feel, as the vac advance surges on when the vac comes up....
I also rejetted the mains down to 72....
Test drive tomorrow. If it continues to leak Im gonna just ignore it and go to Reno anyway.... Doing my part for ruining the environment.......
Will update again after the next drive.
On the + side, Im getting pretty good at getting the oil pan on/off quickly.....
I have a ridiculous amount of hours now into the damn oil pan gasket. When a whole mess of The Right Stuff didn't fix it, I decided to swap on my other oil pan, with a new gasket. That pan had just been replaced on the 327 and sealed fine with just the rtv in the corners.
So, I get the new gasket and oil pan on and guess what? Same F^&*ing drip. Pas side front corner...it seeps out between the gasket and the block, pools and then drips and runs down the front seal onto the ground. I immediately pulled it back off and slapped some Right Stuff in the corner.....so we will see tomorrow. Ive thought it might be dripping from somewhere else, but i can't find a wet spot of oil anywhere on the motor except that corner..... Grrrrr!!!!!!
Oh, and Dumbass error #2 (or is it 20 now???) My Vac gauge was broken...it read 7" high. I noticed too late, yesterday that it never dropped to 0. So...new Vac gauge, and the Vacuum is actually 8.5" not 15" after all. So I need a B28 Vac can.... the one I have doesn't start to pull until 10"-12" so I'm not getting any vac advance at idle at all. I think that may be the hesitation I feel, as the vac advance surges on when the vac comes up....
I also rejetted the mains down to 72....
Test drive tomorrow. If it continues to leak Im gonna just ignore it and go to Reno anyway.... Doing my part for ruining the environment.......
Will update again after the next drive.
On the + side, Im getting pretty good at getting the oil pan on/off quickly.....
Last edited by DucatiDon; 08-04-2016 at 09:26 PM.
08-04-2016, 11:05 PM
#51
Team Owner
Member Since: Nov 2005
Location: Beach & High Desert Southern California
Posts: 25,501
Received 2,346 Likes
on
891 Posts
I honestly haven't driven it enough to say how it performs yet...... Seems plenty strong so far...
I have a ridiculous amount of hours now into the damn oil pan gasket. When a whole mess of The Right Stuff didn't fix it, I decided to swap on my other oil pan, with a new gasket. That pan had just been replaced on the 327 and sealed fine with just the rtv in the corners.
So, I get the new gasket and oil pan on and guess what? Same F^&*ing drip. Pas side front corner...it seeps out between the gasket and the block, pools and then drips and runs down the front seal onto the ground. I immediately pulled it back off and slapped some Right Stuff in the corner.....so we will see tomorrow. Ive thought it might be dripping from somewhere else, but i can't find a wet spot of oil anywhere on the motor except that corner..... Grrrrr!!!!!!
Oh, and Dumbass error #2 (or is it 20 now???) My Vac gauge was broken...it read 7" high. I noticed too late, yesterday that it never dropped to 0. So...new Vac gauge, and the Vacuum is actually 8.5" not 15" after all. So I need a B28 Vac can.... the one I have doesn't start to pull until 10"-12" so I'm not getting any vac advance at idle at all. I think that may be the hesitation I feel, as the vac advance surges on when the vac comes up....
I also rejetted the mains down to 72....
Test drive tomorrow. If it continues to leak Im gonna just ignore it and go to Reno anyway.... Doing my part for ruining the environment.......
Will update again after the next drive.
On the + side, Im getting pretty good at getting the oil pan on/off quickly.....
I have a ridiculous amount of hours now into the damn oil pan gasket. When a whole mess of The Right Stuff didn't fix it, I decided to swap on my other oil pan, with a new gasket. That pan had just been replaced on the 327 and sealed fine with just the rtv in the corners.
So, I get the new gasket and oil pan on and guess what? Same F^&*ing drip. Pas side front corner...it seeps out between the gasket and the block, pools and then drips and runs down the front seal onto the ground. I immediately pulled it back off and slapped some Right Stuff in the corner.....so we will see tomorrow. Ive thought it might be dripping from somewhere else, but i can't find a wet spot of oil anywhere on the motor except that corner..... Grrrrr!!!!!!
Oh, and Dumbass error #2 (or is it 20 now???) My Vac gauge was broken...it read 7" high. I noticed too late, yesterday that it never dropped to 0. So...new Vac gauge, and the Vacuum is actually 8.5" not 15" after all. So I need a B28 Vac can.... the one I have doesn't start to pull until 10"-12" so I'm not getting any vac advance at idle at all. I think that may be the hesitation I feel, as the vac advance surges on when the vac comes up....
I also rejetted the mains down to 72....
Test drive tomorrow. If it continues to leak Im gonna just ignore it and go to Reno anyway.... Doing my part for ruining the environment.......
Will update again after the next drive.
On the + side, Im getting pretty good at getting the oil pan on/off quickly.....
08-05-2016, 11:52 AM
#52
Melting Slicks
Thread Starter
Drove it to work today. Runs FANTASTIC! No off-idle miss, buck etc. Perfectly smooth acceleration and cruise. Pulls very nicely. Cruises nice cool and quiet. (BTW I lined the console with heat barrier and what a world of difference in both heat and noise..and exhaust smell.)
Ill swap a clean plug tonight and see how the color looks, but just based on smell and driveability Id say the jetting is close.
Still have a F%^&*%ing drip of oil......... gonna just have to live with it for now...I am out of ideas/options. Thankfully its minor, and gonna make a mess, just have to wipe the bottom down after every trip. Ugh.
The last thing I really have to do at this point is replace the Vac can, and readjust the timing. Currently idles at 750rpm with 8" vacuum.
Next project.... deciding what to do with the .040 over 327........
Ill swap a clean plug tonight and see how the color looks, but just based on smell and driveability Id say the jetting is close.
Still have a F%^&*%ing drip of oil......... gonna just have to live with it for now...I am out of ideas/options. Thankfully its minor, and gonna make a mess, just have to wipe the bottom down after every trip. Ugh.
The last thing I really have to do at this point is replace the Vac can, and readjust the timing. Currently idles at 750rpm with 8" vacuum.
Next project.... deciding what to do with the .040 over 327........
08-05-2016, 03:26 PM
#53
Race Director
Good to hear Don, sorry about the oil leak. I can't stand a car that leaks oil, and I've torn apart and eventually fixed all my oil leakers over the last 50+ years(pity the poor guy that has to take them apart after I depart!). I'm sure you'll get it sorted.
Glad it runs good and that's the main thing.
Are you going to make it up to HAN? It is just packed this year. I was downtown on Wednesday and being a little late, I had to park on the bridge over the Truckee. That makes a solid half mile of cars going north on Virginia St. to where the HAN parking ends, and all the side streets leading in are full too. Normally the parking ends a couple of blocks north of the river. Tons of really nice cars. I'll post some pics in another thread.
Glad it runs good and that's the main thing.
Are you going to make it up to HAN? It is just packed this year. I was downtown on Wednesday and being a little late, I had to park on the bridge over the Truckee. That makes a solid half mile of cars going north on Virginia St. to where the HAN parking ends, and all the side streets leading in are full too. Normally the parking ends a couple of blocks north of the river. Tons of really nice cars. I'll post some pics in another thread.
08-05-2016, 03:29 PM
#54
Melting Slicks
Thread Starter
We are seriously thinking about coming for the day Saturday....not registered though.
If we do, id love to meet up!
D
If we do, id love to meet up!
D
08-05-2016, 04:45 PM
#55
Race Director
Dave
08-05-2016, 11:38 PM
#56
Melting Slicks
Thread Starter
Yeah....DEFINTELY runs strong.. Tires start to break loose in 2nd gear right around 3k.......heheheh..........Pretty damn good for full stock exhaust......
Get the oil leak fixed and Ill be completely happy and content. As it is, Im still pretty happy.
I DID THIS!
Get the oil leak fixed and Ill be completely happy and content. As it is, Im still pretty happy.
I DID THIS!
08-06-2016, 12:24 AM
#57
I also kinda sorta remember you had two original style oil pans, but they had different dimensions at the front seal area.
08-06-2016, 12:40 AM
#58
Melting Slicks
Thread Starter
One piece gasket. And I was incorrect both pans take thick seal..... Not sure why I thought either one was thin.
The oil appears on the top of the pan gasket in the front corner...then runs down the front seal....while driving it gets flung all over....including the bottom of the radiator hose and water pump...... I'm wondering if it's not coming out of the pan seal, but collecting there from somewhere else..... I thought the front timing cover seal at the balancer, but the inside of the balancer is dry....I suppose it could be dripping down from there....maybe....
I have been hunting with a paper towel searching for dampness with the engine running and haven't found anything promising......
The oil appears on the top of the pan gasket in the front corner...then runs down the front seal....while driving it gets flung all over....including the bottom of the radiator hose and water pump...... I'm wondering if it's not coming out of the pan seal, but collecting there from somewhere else..... I thought the front timing cover seal at the balancer, but the inside of the balancer is dry....I suppose it could be dripping down from there....maybe....
I have been hunting with a paper towel searching for dampness with the engine running and haven't found anything promising......
08-06-2016, 12:58 AM
#59
I had no luck trying to use the 1 piece gasket on my 1985 block with a brand new pan. As you can see in the picture below, the pan is resting nicely on the front "hump" (and also on the rear), but is nowhere near the side rails. I tried snugging it down but quit before I bent the pan. I gave up and bought a 4 piece and it fit nicely.
.
.
08-06-2016, 10:01 AM
#60
Melting Slicks
Thread Starter
It's the same setup I had on the 327 and it was leak free.....