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  #16 (permalink)  
Old 05-06-2019, 11:17 AM
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Same engine combo here with a Holley

Should be close. Maybe a step lower to 62's on the primary's to lean it up. I believe that is where I went. Has a 268 HE -218/218 cam or pretty close to yours. The jump to 74's I think is a little wide and why you maybe getting the hesitation. Out of the box they are 72's. If you narrow that spread that should make it a lot better. Less fuel is always better than being fat. In stock out of the box they will try to sell a good safe start point....

From Holley's website..
Model HR-Series
Primary Main Jet 68
Primary Power Valve 65
Primary Pump Nozzle Size 28
Product Type Carburetor
Secondaries Mechanical
Secondary Main Jet 74

Also since it is a Vacuum secondary carb get the Vacuum secondary Spring kit. You can then tailor the tip in of the secondaries. Heavy truck it may need to be heavier so it does not snap open to early causing a bog.

Tune the carb with a vacuum gauge and seek the highest number at idle. That way you will for sure have the lean best idle. Mine goes about 14lbs.

Timing with Vortec's you do not need to so far in total. Bring it down slowly.

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Last edited by arnoldsel; 05-06-2019 at 11:25 AM.
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Old 05-07-2019, 07:11 PM
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Quote:
Originally Posted by arnoldsel View Post
Should be close. Maybe a step lower to 62's on the primary's to lean it up. I believe that is where I went. Has a 268 HE -218/218 cam or pretty close to yours. The jump to 74's I think is a little wide and why you maybe getting the hesitation. Out of the box they are 72's. If you narrow that spread that should make it a lot better. Less fuel is always better than being fat. In stock out of the box they will try to sell a good safe start point....

From Holley's website..
Model HR-Series
Primary Main Jet 68
Primary Power Valve 65
Primary Pump Nozzle Size 28
Product Type Carburetor
Secondaries Mechanical
Secondary Main Jet 74

Also since it is a Vacuum secondary carb get the Vacuum secondary Spring kit. You can then tailor the tip in of the secondaries. Heavy truck it may need to be heavier so it does not snap open to early causing a bog.

Tune the carb with a vacuum gauge and seek the highest number at idle. That way you will for sure have the lean best idle. Mine goes about 14lbs.

Timing with Vortec's you do not need to so far in total. Bring it down slowly.
Thanks for the info. I only have 10" vacuum at idle and my secondary's have a adj. screw. I have driven it a bit and there is no bog really anywhere it just stubbles a bit at super light throttle right off of idle, just barely touching the gas. If you stomp it it's not there.

Anyway I have been collecting parts , I have a Quick fuel 5.0 power valve, pump cam kit , jet kit and some ifr's , so I have stuff to play with. I am also thinking I may Dremel out my vacuum advance can a bit so I can get a true additional 10 degree's advance at idle and cruise , right now its only giving 6 6 degree's. This with opening the secondary blade's a bit should help my transition slot's not be over exposed I hope.

I have the carb. off but have been busy with work so it might be a day or two before I try again.
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Old 05-08-2019, 04:50 AM
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Try reducing the IAB .003-.005 and see if that helps.
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Old 05-08-2019, 07:39 AM
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Quote:
Originally Posted by TommyK View Post
Try reducing the IAB .003-.005 and see if that helps.
That is on my list as well , I forgot to pick some of those up at summit. The air bleeds are for fine tuning , correct ?
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  #20 (permalink)  
Old 05-08-2019, 08:12 AM
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Quote:
Originally Posted by hirschdalechevy View Post
That is on my list as well , I forgot to pick some of those up at summit. The air bleeds are for fine tuning , correct ?
The IABs are a quick easy way to adjust the off-idle mix, as long as the transition slot position is good - pretty much same as changing IFRs. However, the hi-speed air bleeds are not a good way to try to richen or lean main cruise or power circuits (even though they are easy to change). They change the slope of the AFR curve (bigger hi-speed ABs lean out the mix more at high RPM than at mid-rpm - and vice versa). I just went from 0.028" to 0.030" hi-speed ABs all around in my own carb to correct a slightly dropping AFR ratio at higher RPM under full power. Then I had to jet ever so slightly bigger on PVRs and secondary mains to compensate for the extra air. Cruise mix happened to still be satisfactory based on my previous choices for IFRs and primary jets, but could have required changes there as well.

Last edited by 2001Blazer4x4; 05-08-2019 at 08:23 AM.
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Old 05-09-2019, 04:02 AM
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I have a 350 crate with Blueprint aluminum heads, 213/217 @ .050 cam, and about 9.4 compression in my '75 K25. I have a TH350 and 4.10 gears, and stock height tires. My engine seems to run much better and have more mid range power with base timing of 15-16 degrees, and about 20 degrees from a fairly quick mechanical advance curve. My vacuum advance adds about 15 degrees more at cruise.

Based on my limited experience, I would increase bas timing to about 16 degrees, but also dial back the mechanical a little so you don't exceed about 36 degrees total.

Bruce
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Old 05-09-2019, 06:29 AM
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Quote:
Originally Posted by 75gmck25 View Post
I have a 350 crate with Blueprint aluminum heads, 213/217 @ .050 cam, and about 9.4 compression in my '75 K25. I have a TH350 and 4.10 gears, and stock height tires. My engine seems to run much better and have more mid range power with base timing of 15-16 degrees, and about 20 degrees from a fairly quick mechanical advance curve. My vacuum advance adds about 15 degrees more at cruise.

Based on my limited experience, I would increase bas timing to about 16 degrees, but also dial back the mechanical a little so you don't exceed about 36 degrees total.

Bruce
Yep, this is pretty much the gold standard performance timing curve for a small block Chevy (2)
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Old 05-09-2019, 07:19 AM
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You think that with vortec heads I still need more than 32 to 34 total? I am at 11 initial , 30 at 2,000 rpm , 32 at 3,000rpm , and at 3,500rpm and up it goes to 35 and seems to stop advancing.
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Old 05-09-2019, 07:29 AM
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Quote:
Originally Posted by hirschdalechevy View Post
You think that with vortec heads I still need more than 32 to 34 total? I am at 11 initial , 30 at 2,000 rpm , 32 at 3,000rpm , and at 3,500rpm and up it goes to 35 and seems to stop advancing.
I think you are already at 35 total! You need less mechanical advance (say 21 instead of 24), so you can set your base timing higher (maybe 14) and still end up at 35. All in at 3500 is a little late but generally acceptable IMHO. What you have is pretty conservative, but if it runs well for you - then I would use it. But wait! You have an off-idle stumble you are trying to fix.

I would be more concerned why your vacuum advance is only giving you 6 degrees of advance. Vac advance unit might be completely wrong for your cam. How much advance does it give at say 2500 rpm when engine is making more vacuum or just put a hand vacuum pump on it if you have one? Extra timing, especially down low can be the cure for off idle stumbles. How much timing does it idle at with the vacuum advance plugged in?

Last edited by 2001Blazer4x4; 05-09-2019 at 07:46 AM.
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Old 05-09-2019, 08:06 AM
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Actually I found that in your initial posts - 17. I think timing is probably most if not all of your problem. A mild cam like yours should make more than 10" vacuum at idle and if timing is hugely retarded, you'll never get that carb set up because you'll be way too far into the transition slots in order to get it to idle.

Try setting your initial timing to 30, leave vac advance unplugged, and reset idle speed. Now note the vacuum at idle. Check and see if the off-idle stumble is gone, but keep engine under 2000 rpm and don't drive on the street this way. If this cures your stumble issues, we can help you get your distributor set up correctly.

Last edited by 2001Blazer4x4; 05-09-2019 at 08:11 AM.
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Old 05-10-2019, 07:28 AM
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Quote:
Originally Posted by 2001Blazer4x4 View Post
Actually I found that in your initial posts - 17. I think timing is probably most if not all of your problem. A mild cam like yours should make more than 10" vacuum at idle and if timing is hugely retarded, you'll never get that carb set up because you'll be way too far into the transition slots in order to get it to idle.

Try setting your initial timing to 30, leave vac advance unplugged, and reset idle speed. Now note the vacuum at idle. Check and see if the off-idle stumble is gone, but keep engine under 2000 rpm and don't drive on the street this way. If this cures your stumble issues, we can help you get your distributor set up correctly.
Thanks, I am going to play with it tomorrow ,(have a day off), I have a vacuum can that will give like 20 degrees at idle and cruise , I could put that in as well.

I am at 5,500 feet elevation as well so that may be why I only have 10" vacuum at idle

Last edited by hirschdalechevy; 05-10-2019 at 07:33 AM.
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Old 05-10-2019, 04:11 PM
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I have read somewhere that GMPP recommends app 32 degrees of total timing, and that's my experience as well.
I have modified Vortec heads and cam on a '93 350 small block.
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Old 05-12-2019, 08:51 AM
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Messed around with some parts yesterday , set the t-slots at square and marked the idle screw, put the carb back on. Had to open the idle screw one full turn past my mark to keep it running so I got my allen key out and opened the secondary blade's up about another 1/3 turn or so , then I could back off my idle screw a bit to try and close up the t-slot some. The idle screw is still 3/4 of a turn past my .020 mark. Then I hooked up my vacuum gauge to the carb. and hooked up my mighty vac. to the vacuum advance can , set the mix screws to highest vacuum, I got 11" at 820 rpm, that is with the vacuum advance pulled in all the way which give's another 6 to 7 degree's which makes it 17 degree's initial.

Idle was pretty good, then I would release the mighty vac. so I was back to 11 initial and the rpm dropped so I had to crank the idle screw up a bit to bring the rpm back up, (it seemed to run good there as well) but I think I am past the .040 max t-slot setting

Anyway , reading the vacuum gauge.
11" at 820 rpm , (wont idle to well much lower than that)
17" at 2,000 rpm
17" to 18" at 3,000 rpm

Pulled the vacuum adv. back on with the mighty vac., pretty much all the same readings.

When you snap the carb. it revs up right now with no stubble.

AFR readings,
12.4 at idle
13.0 in gear

Then I put it in gear a power brake it ,(moving the gas petal super slow) it is smooth to 1,260 rpm then it starts to stubble and the afr goes to 16.5 to 17.5 until I get to 1,650 rpm and it smooths right out and reads 13.5 on the afr meter.
I dropped the ifr's to 28 so you think I need to drop the idle air bleeds as well?

Then I realized that I didn't install my fuel pressure gauge , so I put that on and it was at 9psi at first then it settled down at 7.5 to 8 psi so I stopped there and I am going to run to summit and get a regulator and maybe some air bleeds as well before I go on.

I am also going to do the 30 degree timing thing as well.

Thanks for all your help guys , on my way to summit, (I love that place).
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Old 07-06-2019, 09:32 AM
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Had to go out of town for a bit , so no playing with toys. Got back and took the blazer for a 30 mile run with the 28 ifr's and timing at 11 initial with the vac can on ported and it didn't do so well at all. At highway cruise it was cutting out so bad I had to either be on it or off it and got way worse dropping in elevation. Got to my buddys house and swapped out the 28 ifr's to 30's and it was better, (good enough to drive it home) but still there a bit, (no pinging that I could hear). So now I am messing with it in the drive way, I figured I would go back to the timing first and try to get it right, so I set it at 14, 16, 18, 20 initial with no vacuum can hooked up and it seemed to like 20 initial at idle the best and my vacuum went up to 12 to 13.I power braked it in the drive way at all the different initial's, (super slowly) and it still cuts out at 1,260 to1,300 rpm just the same as before. I also went and got a pcv valve for lower vacuum as well, no change.

So I think I will stay with 20 initial and try the screw thing to limit the hei to give me a total of 30 to 32 , run the vacuum can after the fact to see how it does and then go back to the carb.

What do you guy's think?
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Old 07-06-2019, 09:48 PM
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So I limited the hei, (ground down allen head with nut on bottom)so at 20 degree's initial I am at 31 total at 1,300 up to 3,200 and then it backs off to 29 degree's above 3,200 and stays there ,(no vac. can in the mix yet) and it still has the stubble at 1,280 rpm with a super lean afr. (which was like before), so hei is limited at 29 to 31 with a super early curve , ( one stock spring and one soft spring).

So I pulled the carb to check the transfer slots and they are close to square,(maybe just a tick over) and the idle screws work well, (drivers side at 1/2 turn and passenger side at 3/4 turn out). Secondary blades are 3/4 to 1 turn up from bottom, (no t-slot showing).

So thinking it was lean I just wanted to do one thing at a time so I went from the pink pump cam to an orange one,(1st hole) and the stubble is 98% gone while power braking in the drive way , (super slow). I didn't know a pump cam would work like that on super slow throttle, but I think I am close now.

Had to many beers to drive it so that will have to happen tomorrow , I hope the timing is not to hot and the curve is not to fast, we will see.

Had the stock springs in at first and it would get to 30 degree's fast but as you held it at 3,000 rpm it would back off to 28 degree's for some reason, so I put back one light spring.

Thanks for all the knowledge from you guys on here.
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