sb tuning problems

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i'll open it up and take a look at the jets as soon as i can. will be easier to change the jets for now rather than the carb.
 
back in the 80s....friend had a 73 340 duster, had a mopar 557 cam....eddy victor...10.5 compression...dont remember the heads but they were cast iron...

8 inch converter 4.56 gears...9 x 28 sllcks

Had a holley 1050 dom....ran 11.50s in Vegas...I was thought it was too big...one TNT night..we put a 750 dp on it...it slowed down 3 tenth......that is 11.50 in Vegas ...which would be 11.20 at sea level track..
 
ap6street said:
sorry to sounds rude before, i dont doubt you. when the dyno place told me 1050 i asked at least 20 different people, friends and people from my facebook who constantly are racing and they all said no way. 1050 is way too big.
both carbs were brand new.

im not sure of the jet sizes. id have to take it apart.

the idle speed screw is one turn away from the engine stumbling to a stall as in turned out ? dono if that makes a difference.
idle mixtures are 1/2 turn out from fully wound in .

jets id have to open up and take a look.

850 diamond series quickfuel q
1050 race q quickfuel
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So here is how to base tune a carburetor. Run idle mix screws all the way in, back them out 1 1/2 turns. Fire up the car and set idle then floats. On the floats - fuel should just wet the threads when idling. I then adjust the idle mix screws 1/2 turn either way and set them to what gives the cleanest idle. Then I set the accelerator pump usually a blue or an orange cam with a 30-35 squirter. I run a 4.5 pv in my 406 - should have an 8-10 jet spread if you are running a powervalve. If you plug it run the same jets in both the primary and secondary plates. Something in the 72-78 range primary and 82-88 secondary based on what you have posted so far should get you in the balpark with a 6.5 powervalve.

As for the idle, I usually set them high on the bench, run the screw in enough to visibly see the butterflies opened up once fired up I back them down to desired idle 800-1000 rpm.

I run 14-16 degrees initial 36 degrees total timing at 3000 rpm. Advance is all done by 3000 use one medium and one light spring.

Also check for vacuum leaks around the intake and carburetor when its cold and again when its warmed up. Spray carb cleaner around the base of the carb / intake if the engine rpm goes up you have a vacuum leak. Make sure all you auxillary ports on the carb are capped off.
 
Good god. Have fun with this one.

Jetting had little or nothing to do with the black plugs at 1500-2000 rpm. Hotter plugs won't help either.

BS on the carb being too big. I WOULD NOT touch anything on the carb at this point. Is it a good street carb, maybe not the best solution. There are plenty of 1050 carbs on street cars that run fine. It will need fine tuning of circuits OTHER than the jetting. Thumperdart is pretty sharp on carbs.

I've fixed a lot of cars just like this and most of them had CRAPPY timing profiles as a starting point. Fix that FIRST, and you may be surprised how the rest of the settings come around. Anybody that says it should run with less than 20* isn't going to have this thing run worth a ****, GUARANTEED! Patient says their chest hurts, so you put a cast on their ankle. Let the ENGINE tell you what wants. You can do that without ever driving the car.

I'll let you know if I get some bushings.

Did I mention...


TIMING FIRST!!!

Good luck
 
I fail to see a need for 20 degrees initial timing, imagine that would make it difficult to restart when hot... I've run racecars at 36 degrees locked out, but typically the ignition isn't switched on until the engine is already rolling over... Problems here are a combination of timing and carb issues, 20 degrees initial is not a cure all
 
sorry bud, but i'm a 110% with crackedback!:burnout:
 
tnts72 said:
I fail to see a need for 20 degrees initial timing, imagine that would make it difficult to restart when hot... I've run racecars at 36 degrees locked out, but typically the ignition isn't switched on until the engine is already rolling over... Problems here are a combination of timing and carb issues, 20 degrees initial is not a cure all
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I think everyone agrees its the combination. The reason to start with timing is, at least for most of us, based on our own experiences that the tuners who taught us that were right. But I think your question is fair. Why something around 20 intial?
In my mind, the simple answer is Reversion.
And that's why, as the Demon selection chart indicates, the bigger the carb, the more initial.

Of course that's a pretty simplified answer. To really get into what is happening in the combustion chamber, take a look at some of the following linked threads with posts by Bruce Robertson (aka Shrinker) who had the smarts as well as the experience and equipment (such as a 5-gas) to really get much further into this than most of us could ever dream.

Relationships of fuel, cam and timing Go down to Shrinker's reply 9/7/07 at 4:11 AM.
Sorry about the archive format, but its worth reading! Cut and paste to word processer if you need.
The following is a snip of the some of the most relevant to your question:
How this all relates to idling is that the LSA is the un-throttled connection of the inlet to the exhaust. If the exhaust gas is going gang busters and doing all super scavenging stuff and being nice and not blowing up the intake then you can get high manifold vacuum. High manifold vacuum helps you to turn the fuel into a gas before it gets compressed.
Theres not much gas being compressed inside a cylinder when the engine is idling so you dont get much compression energy. An engine wont run at all unless the fuel is turned into gas and its the amount of gas at ignition time that determines the quality of the burn and the quality of the burn is what causes the exhaust to behave properly or be a pain in the butt.

Initial Timing thread snip at MSV Posts 2 and a few following are the most relevant.

Timing & Jetting at MSV

Initial Timing Method at RacingFuelSystems forum
 
I have looked at home and now at work this morning and the smallest bush i have is the 21 degree one. Your welcome to have it. I have lots of springs too
 
ap6street said:
So the car ran fine before when first in i hadnt driven it much 34 total with the 850cfm quickfuel.
Took to a chassis dyno shop to get it tuned. Pretty reputable shop (lol) they said at WOT i i have 2.5" of bacuum so which i guess makes sense needs a bigger carb . They said go with a 1050cfm i realise at this stage this carb is massive for the cubes/cam setup . Everyone advised against but i gave the boys a chance to prove me wrong and put on a 1050.
The printout showed 330hp now up to 350hp 20 extra all the way up the curve.

Took it home and drove it . Sounded a lot crisper . Especially coming off the throttle while cruising . But a bit gutless. Checked the throttle and was only going half way. Fixed it up now and it went hard . I called the place and they said they were 99% that it was fine . (Very strange)


interesting, the 850 made 330 hp, and the 1050 with half throttle (approximately 525) made 350 hp
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Well its possible half throttle . They claimed to have checked it at the shop.
Not long after this i took it to a more local dyno place the guy was a dickhead . Didnt strap the car down and said making too much torque and wheels are spinning . Lol so he couldnt get a reading.

So i gave up after that
 
The truth is that AT IDLE,(and to some degree;slow-speed),the engine does not care one tiny bit what primary jets are in there, or even if there are jets in there at all.Fueling is handled 100% by; the manifold vacuum,the fuel level & bowl vent, the Idle circuit(including bleeds), and the T-ports.If fuel gets in there any other way, the carb is in trouble.
Of these, the fuel level and manifold vacuum are easy to adjust. The T-port sync-up is only a bit more complicated. These three are in an intimate relationship, and Until you accept that, nobody can help you.
The order of adjustment is 1) fuel level, 2) manifold vacuum(initial timing), 3) T-port sync,4) idle speed, using idle-air bypass as may be necessary, and 5) mixture screws.

Now,Go do it!

I have a dash-mounted dial-back timing device wired into my timing system, It has an operating range of 15 degrees. How I set it up is like this: I put the knob in the center of its range, and then set the initial timing, to what I think the engine might like. Then I can load test it and make timing adjustments +/-7*,from the drivers seat. I keep a log book. I have all my curves previously mapped out. By looking at the maps,and adding any dial-back changes, I can, in mere moments, tell, exactly how much timing the engine is seeing under ANY circumstance. How it responds to the changes tells me where and how much of a hard correction is required.I make the change(s), and remap the curve. I Repeat as often as may be required.
One thing I can tell you,Almost every single timing change that affects the low speed operation,likes to have an A/F correction. What this means is; pick an initial timing number, correct the power timing to work with it, And then work the carb for a while. Do not be tempted to keep making timing changes unless you just can't seem to get the low-speed circuit bugged out. Make changes to the T-port sync, against the bypass air, trying more or less transfer fuel, and changes to the idle mixture screws. Keep trying.Big cams like a lotta bypass air; but if all that air ends up in just 4 of the cylinders, you will never be able to get a nice idle.If the exhaust starts to burn your eyes out, there is something wrong.
 
ap6street said:
360/408 sbm
360-1indy heads unported
10.28 comp
Solid roller 245/245 612/612 108 102.75
34 total timing i think around 14 initial from memory
Was around 10" vacuum at idle 850 -900rpm
1050 quickfuel raceq
Using bcpr6ey-11 bgk plugs
Msd ignition .

Although i like to open up the throttle a bit. The car does see a lot of cruising rpm 1500-3000 and idle time.

Problem being after driving for 20-30minutes the engine wont idle rpm slowly drops down to 500rpm and then will stall at lights.
By this stage the plugs are fairly black.

Questions ? Comments ??
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To start, that plug is WAY too cold if it`s an NGK............
 
6 is sort of cold, comparable to a 9-10 champion plug.
 
Just a note for the OP: A big carb versus cubes is going to be a lot touchier to get the mixture right across the idle and transition phases of the carb operation than a smaller one. In fact, with too big a carb, it may never be fully correct all across the low end RPM operations at light throttle and idle; the fineness of adjustment is just not there, and the transition operation in particular assumes a certain % of maximum flow versus RPM and throttle opening; you are undoubtedly operating below that % flow with the large 1050.

That is the compromise that you are making with the 1050 carb to gain that 20 HP on the dyno. One question you may want to ponder is if that 20 HP is of that much value in your use of the car and engine; sometimes, people get too caught up in every last HP on the dyno and lose sight of actual operation. Spending a lot of time idling and at lower RPM's points to a smaller carb.
 
nm9stheham said:
Just a note for the OP: A big carb versus cubes is going to be a lot touchier to get the mixture right across the idle and transition phases of the carb operation than a smaller one. In fact, with too big a carb, it may never be fully correct all across the low end RPM operations at light throttle and idle; the fineness of adjustment is just not there, and the transition operation in particular assumes a certain % of maximum flow versus RPM and throttle opening; you are undoubtedly operating below that % flow with the large 1050.

That is the compromise that you are making with the 1050 carb to gain that 20 HP on the dyno. One question you may want to ponder is if that 20 HP is of that much value in your use of the car and engine; sometimes, people get too caught up in every last HP on the dyno and lose sight of actual operation. Spending a lot of time idling and at lower RPM's points to a smaller carb.
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If all you do is jets and idle mixture screws, as this statement implies, then you are WAY off and missing a huge opportunity to get a carb right on an engine.

Unfortunately, most people think jetting and idle mix screws are all you can do. The universe to tune a carb for an application is out there. Same as the total timing crew with ignitions... they miss out on so much performance and driveability.
 
crackedback said:
If all you do is jets and idle mixture screws, as this statement implies, then you are WAY off and missing a huge opportunity to get a carb right on an engine.
Click to expand...
Well actually, I did not think or imply anything like that.... but that never stops people from inferring what they want. The reality is that unlike EFI with an O2 sensor and feedback, any simpler carb's transition operation is factory calibrated along some sort of flow and vacuum curves; you can vary around that curve but not change the curve in the simpler carbs that get dealt with commonly here. IF you are dealing with more tunable carbs like Weber DCOE's, THEN you can actually monkey around with the transition curves by changing the emulsion tubes as well as the idle air jets and idle fuel jets. Those variable parts just don't exist in these simpler 4 BBL carbs. (If you have never done DCOE tunes, then it may be hard to appreciate this point.)

My point is that with too big a carb, then at idle and transition, getting the engine to operate right along the factory fuel calibration transition curve is going to happen at one narrower range but not along the whole range. You'll find it easier to operate along a wider part of the curve with a smaller carb. This goes along with a good comment that someone posted here recently, that if you are not into tuning carbs, then a smaller carb will likely be easier for you to manage.

I just want the OP to be aware that there are compromises in the range of smaller throttle openings that come with such a large carb.
 
Yep, "Factory Calibration", that reinforced my statement.
 
I love my 850 for driving around. I have to agree, if you want more driveability, drop the carb size.
 
crackedback said:
Yep, "Factory Calibration", that reinforced my statement.
Click to expand...
Would you prefer factory design' for the transition characteristic? In a carb like a Holley, it is fixed in the metering block. That is one reason to have different metering blocks for different size carbs in the same series. Not sure what the problem is here.
 
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