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Engine Tuning Issues

Mr. Fixit

Member
Hi All,

I need to seek the advice of some of you pro engine builder/tuners. I am having issues with properly tuning and have a stumble/flat spot during mild accelleration. The following are the components of the engine.

SB 350 bored .040 over
Flat top forged pistons
6" forged rods
Comp Cams Extreme Energy advertised duration 294/300
2 - Holley 4160 450 CFM carbs.

I have set all of the floats correctly and adjusted the idle mixture screws using a vacuum gauge. It seems that the best fairly steady vacuum that
I can acheive is -5hg with the idle mix screws set at about 1.5 turns out.

Is this -5hg somewhat normal for what I have described? The idle circuit seems fine and it runs very strong when throttled in park, but as I drive off, it generally stumbles slightly but rarely stalls, and does the same while accelerating while crusing at any speed. I havent checked the power valves to determine what rating they are yet either. Any thoughts, suggestions or next steps would be greatly appreciated
 
We're gonna need some more info to get you dialed in OK......We're gonna need to know your Vacuum at idle, at about 1500, at about 2000, and your gonna have to give us your timing curve #'s as your motor comes up on Rpms, also your Powervalve #'s, and jet #'s. You say she's running strong, just a stumble off idle? Or is it as you press on the accelerator slowly as the RPM builds?

Could be the big Vacuum drop, if your doing it slowly and its flat.....Could be your timing. Are you running a progressive throttle on this setup? Is it like a Offy lowrise dual plane 2x4 setup or is it a TR?

More Info needed! Also, what CR are you running? And are you running straight pump gas?
 
We're gonna need some more info to get you dialed in OK......We're gonna need to know your Vacuum at idle, at about 1500, at about 2000, and your gonna have to give us your timing curve #'s as your motor comes up on Rpms, also your Powervalve #'s, and jet #'s. You say she's running strong, just a stumble off idle? Or is it as you press on the accelerator slowly as the RPM builds?

Could be the big Vacuum drop, if your doing it slowly and its flat.....Could be your timing. Are you running a progressive throttle on this setup? Is it like a Offy lowrise dual plane 2x4 setup or is it a TR?

More Info needed! Also, what CR are you running? And are you running straight pump gas?


Thanks for the response.

Here is what I know from where I am sitting.

I have a Weiland hi rise manifold
MSD 6AL-2 Ignition
Blaster Coil
Mallory Unilite Optical Dist (no vacuum advance)
TCI Streetfighter Converter (3000 - 3400) Stall

When I set the timing, I only set it running at about 2500 rpm and set to 34 degrees. I will need to check the timing at idle and the other rpm levels you have suggested.

The vacuum at idle in drive is right at a fairly steady -5hg and doesn't seem to change much with idle mixture adjustments.

It is a small stumble off idle, but it also does it at any speed as I slowely accellerate. I havent taken her up above 65 either.

I am running pump gas and no it isnt a progressive set up.

Thanks again..
 
I did just check the timing curves at various rpms

At 1000 rpm 10 degrees
At 1500 rpm 14 degrees
at 2000 rpm 24 degrees
at 3000 rpm 34 degrees

At 17000 rpm my garage roof blew off :hey:

Thanks again...
 
OK, your not running a Vacuum adv, timing sounds about right. BUT......BUT It still could be your timing is not where it needs to be with your setup. if you put your cam in straight up, and running a Dual quad setup, with your timing set the way it is, it also sounds like it might be in the carbs, possibly powervalves leaning things out. Start out with bottoming out your idle mix screws, then backing them out 2 full turns.
Do you have your carbs sync'ed or balanced? You know, the tool over the venturi thing and adjusting things to equalize......If not, you need to.
If it were me,get your motor warmed up. Not feeling your setup or reading your plugs, start at a idle speed of about 750 to 800 rpm, if your cam is causing a light lope, idle it up to about 1000rpm, set your idle mix screws to get the best smoothest idle you can.
I guess your running the big oval aircleaner? Take the top off each one or off the one, hold your throttle to achieve about 1200 to 1400 rpm while you bump you dist. up til the motor just ever so lightly begins to labor. (You want to do this with the aircleaner elements still on the bases, and just the tops removed cause if you don't and your running a engine driven fan, the air from the fan blowing across the top of your bases will throw things off....your motor will miss! You'll have to put your hand across the front of the carb to block the airflow......) Snug your dist. down, hit your throttle really hard, your listening for a really hard, crisp rev. You need all your centf. advance in by 1800 or 2000. If its crisp, but not HARD, your timing is off.....if it revs hard but its not crisp....its in the carbs.....
Hop in it, run down the road and see how she acts.
If its still stumbling, move on to your carbs. Your setup sounds really close though.....

Just for S's & G's, adjust your linkage between your carbs to where you have a good bit of slack in it, then take it for a spin and see if its better, and let us know......
 
Just one other thing to check. As you get towards serious camshafts the idle vacuum drops off and you need to crack the butterfiles in the primaries open a bit more to give it the air it needs to idle.(not much sucky motor). You don't have to open the Holley butterflies very far to bring the transfer circuits into play and when this is in effect a bit more throttle opening just chucks in more air, and you get a flat spot. Your idle mixture screw settings seem quite lean, and you say the mixture screws don't change much, which suggests the transfer circuit is contributing fuel to the idle condition.To get the butterflies shut and take the transfer circuits out of the game you can drill an air bleed hole in the primary butterflies, very old trick. Start with a 1/8" hole in each and see if that gets you enough air in that you can shut the butterflies almost to the stop, 3/16" holes seem to be what most guys end up with. Once you have it idling on the idle circuits alone, you can get a vacuum reading and size the power valves so they do their job of transiting the engine from idle to the real thing properly.
If you run a fast advance as Screamin' says, thats a big help, just be sure the power valves richen it up down low before the accelerator pump shots hit, so it doesn't detonate when you hang your leg in it.
 
OK, your not running a Vacuum adv, timing sounds about right. BUT......BUT It still could be your timing is not where it needs to be with your setup. if you put your cam in straight up, and running a Dual quad setup, with your timing set the way it is, it also sounds like it might be in the carbs, possibly powervalves leaning things out. Start out with bottoming out your idle mix screws, then backing them out 2 full turns.
Do you have your carbs sync'ed or balanced? You know, the tool over the venturi thing and adjusting things to equalize......If not, you need to.
If it were me,get your motor warmed up. Not feeling your setup or reading your plugs, start at a idle speed of about 750 to 800 rpm, if your cam is causing a light lope, idle it up to about 1000rpm, set your idle mix screws to get the best smoothest idle you can.
I guess your running the big oval aircleaner? Take the top off each one or off the one, hold your throttle to achieve about 1200 to 1400 rpm while you bump you dist. up til the motor just ever so lightly begins to labor. (You want to do this with the aircleaner elements still on the bases, and just the tops removed cause if you don't and your running a engine driven fan, the air from the fan blowing across the top of your bases will throw things off....your motor will miss! You'll have to put your hand across the front of the carb to block the airflow......) Snug your dist. down, hit your throttle really hard, your listening for a really hard, crisp rev. You need all your centf. advance in by 1800 or 2000. If its crisp, but not HARD, your timing is off.....if it revs hard but its not crisp....its in the carbs.....
Hop in it, run down the road and see how she acts.
If its still stumbling, move on to your carbs. Your setup sounds really close though.....

Just for S's & G's, adjust your linkage between your carbs to where you have a good bit of slack in it, then take it for a spin and see if its better, and let us know......


I need Advil and a few of these :pint:

My brain, as small as it is, is on overload. On another site, I am being told that I need to lock out the distributor advance and set the timing at 34 - 36 degrees at idle and it will remain there without changing. Ever heard of this? Does that make any sense?

I am beginning to feel like I have a tremendous amount of things to explore, but no place to start. I am cranky. :cry:
 
Just one other thing to check. As you get towards serious camshafts the idle vacuum drops off and you need to crack the butterfiles in the primaries open a bit more to give it the air it needs to idle.(not much sucky motor). You don't have to open the Holley butterflies very far to bring the transfer circuits into play and when this is in effect a bit more throttle opening just chucks in more air, and you get a flat spot. Your idle mixture screw settings seem quite lean, and you say the mixture screws don't change much, which suggests the transfer circuit is contributing fuel to the idle condition.To get the butterflies shut and take the transfer circuits out of the game you can drill an air bleed hole in the primary butterflies, very old trick. Start with a 1/8" hole in each and see if that gets you enough air in that you can shut the butterflies almost to the stop, 3/16" holes seem to be what most guys end up with. Once you have it idling on the idle circuits alone, you can get a vacuum reading and size the power valves so they do their job of transiting the engine from idle to the real thing properly.
If you run a fast advance as Screamin' says, thats a big help, just be sure the power valves richen it up down low before the accelerator pump shots hit, so it doesn't detonate when you hang your leg in it.

You may be on to something. I can run all four of the idle mixture screws in to about 3/4 turn out and it is still driveable and will idle. It seems that if I turn the screws out to 2 turns they are a bit wobbly in the holes. Not being too familiar, is this normal?

Thanks,
 
I need Advil and a few of these :pint:

My brain, as small as it is, is on overload. On another site, I am being told that I need to lock out the distributor advance and set the timing at 34 - 36 degrees at idle and it will remain there without changing. Ever heard of this? Does that make any sense?

I am beginning to feel like I have a tremendous amount of things to explore, but no place to start. I am cranky. :cry:


Screamin, I forgot to mention that I dont know what the gizmo is that you mentioned to set or balance the two carbs. Where do I get this? How does it work.

Thanks again..
 
Without the proper adapters{$$] Those bal. deals are useless,besides, it's not something you really need IMHO. Your setup sounds fairly close to mine so I'll pass along what I've leaned over the past 8 yrs. You're initial timing seems pretty low for that cam. You don't want much more than 34-35 degrees total, so you're gonna have to do a little dist. work. get an adv. kit [springs &DIRECTIONS from mallory[cheap]. your cam would probably like 14 - 16 degrees initial 34 total all in by 2K. you'll have to limit mech adv. to 20 degrees, [it's all in the directions] . That'd be a good place to start , as an aside, I really don't understand why your idle vac. is so low , mine is 13 @ 900 rpm . Go this far & let us know what happens . REMEMBER-ONE CHANGE AT A TIME > yea ,I'm shouting

dave
 
Ok, @ 2 turns out, they should not wobble. Are the carbs used???? Did you buy them used????? The carb syncro tool you can get at any auto Parts place, they'll have to order it. Its something they don't have in stock. I have 2 or 3 of them. No, its totally not needed, I do mine freelance, but I'm mostly working with dominators with the 2.3" bores on motors with 2.3 intakes with raised cam locations, with cams with journal size of 2.0 to 3.12, over .800 lift and rocker ratio of 2.5, with cr's at about 14.1. Maybe a little less.......
Alot of folks don't use dwell meters on their points dist., but to do it right, its needed. For ease of use, if your not a proficient tuner, the syncro tool helps. Goodson has them....if your gonna get one, get a good one.
6x1's& 3x2's can be a bitch to set, 2x4's isn't as problematic, 1-4 or 2x2's isn't bad.....

Mango is right.....the more cam your running, the more 'loadup' you'll have at idle. When you add a cam, and all your goodies, essentially your making your engine want to run efficiently at a higher rpm. Ever heard the old WWII fighters idling out to the runway? They barely idle without loading up. They're tuned to run at peak horses and torque up high in the powerband and at high altitudes. They probably only idle about 10 minutes out of every 100 hours of flying time. Their cam(s) hold the valves open to get a big gulp of that thin air and compress it , use it, and get it out effeciently

Alot of the motors I build have the airbleeds in the butterflys so they'll idle. Its also done with a small triangular file, just put a small notch in it. But do this only at your own risk! You can easily trash a carb if you don't know what your doing. Only use this if you have alot of cam, put them in the same location, and do the same size. Use calipers to make sure they're the same size......
I usually blue them up, lay them out very carefully, and measure, measure, measure......
 
Also, the motors I work with idle at 1500-2000, if you want to call that idling.....
The SBChevys had performance motors with 3x2's and the 2x4's.....Mostly in the Z's and the vettes, but could be bought as options for the street/strip., these were without the airbleeds, so, your setup is doable. Just take your time.....
The BBChevys had alot of those options in the 409, 427 and the 454, mostly in the Impala's, Chevelles, Vettes. One of the finest running motors was the 427 vette option with the 2x4's....rated at a very conservative 450 horses.
 
Also, the motors I work with idle at 1500-2000, if you want to call that idling.....
The SBChevys had performance motors with 3x2's and the 2x4's.....Mostly in the Z's and the vettes, but could be bought as options for the street/strip., these were without the airbleeds, so, your setup is doable. Just take your time.....
The BBChevys had alot of those options in the 409, 427 and the 454, mostly in the Impala's, Chevelles, Vettes. One of the finest running motors was the 427 vette option with the 2x4's....rated at a very conservative 450 horses.
Hey, Screamin',
He's got plenty cam, 300deg of it, which makes me think bleed air, especially with the Holleys. Drill or file, doesn't matter. I drill 'em and start off with a small hole say 1/8" and if it needs more I open it up with a taper reamer little bit at a time. Mr Fixit will prevail, one of the fun things about roddin' is nutting your own shit out for yourself. As the saying goes, "faint heart never won fat lady", or was it................hmmmm...................................
 
Without the proper adapters{$$] Those bal. deals are useless,besides, it's not something you really need IMHO. Your setup sounds fairly close to mine so I'll pass along what I've leaned over the past 8 yrs. You're initial timing seems pretty low for that cam. You don't want much more than 34-35 degrees total, so you're gonna have to do a little dist. work. get an adv. kit [springs &DIRECTIONS from mallory[cheap]. your cam would probably like 14 - 16 degrees initial 34 total all in by 2K. you'll have to limit mech adv. to 20 degrees, [it's all in the directions] . That'd be a good place to start , as an aside, I really don't understand why your idle vac. is so low , mine is 13 @ 900 rpm . Go this far & let us know what happens . REMEMBER-ONE CHANGE AT A TIME > yea ,I'm shouting

dave

Dave,

I think that the principal is becoming much more clear to me and appreciate your input.

Thanks,
 
Ok, @ 2 turns out, they should not wobble. Are the carbs used???? Did you buy them used????? The carb syncro tool you can get at any auto Parts place, they'll have to order it. Its something they don't have in stock. I have 2 or 3 of them. No, its totally not needed, I do mine freelance, but I'm mostly working with dominators with the 2.3" bores on motors with 2.3 intakes with raised cam locations, with cams with journal size of 2.0 to 3.12, over .800 lift and rocker ratio of 2.5, with cr's at about 14.1. Maybe a little less.......
Alot of folks don't use dwell meters on their points dist., but to do it right, its needed. For ease of use, if your not a proficient tuner, the syncro tool helps. Goodson has them....if your gonna get one, get a good one.
6x1's& 3x2's can be a bitch to set, 2x4's isn't as problematic, 1-4 or 2x2's isn't bad.....

Mango is right.....the more cam your running, the more 'loadup' you'll have at idle. When you add a cam, and all your goodies, essentially your making your engine want to run efficiently at a higher rpm. Ever heard the old WWII fighters idling out to the runway? They barely idle without loading up. They're tuned to run at peak horses and torque up high in the powerband and at high altitudes. They probably only idle about 10 minutes out of every 100 hours of flying time. Their cam(s) hold the valves open to get a big gulp of that thin air and compress it , use it, and get it out effeciently

Alot of the motors I build have the airbleeds in the butterflys so they'll idle. Its also done with a small triangular file, just put a small notch in it. But do this only at your own risk! You can easily trash a carb if you don't know what your doing. Only use this if you have alot of cam, put them in the same location, and do the same size. Use calipers to make sure they're the same size......
I usually blue them up, lay them out very carefully, and measure, measure, measure......

The carbs came with the car when I originally bought it years ago, so I am not aware of their condition overall, or if a shadetree mechanic did a number on them.

Being much more of a machinist, metal worker, fiberglass specialist and electrics guy, the whole engine tuning deal is more of a mystery to me, BUT, you guys are bringing clarity to it. Based upon all of the good advice regarding timing, I will start there by increasing the initial setting after learning how to modify the distributor's mechanical advance as you, Mango and Dave have suggested.

I will be away for about a week, leaving tomorrow, so I thing I will try to orger the mod pieces for the Mallory so that I can get to it when I return.

Thanks again to all of you.. More to come...
 
Here's what I do if I haven't got my test gear to check I'm getting all the advance. Check the TDC notch on the balancer is in fact the TDC notch - use a piston stop. Measure the circumfrence of the balancer, divide the measurement by10. Measure against the rotation 1/10 the circumfrence, make a mark. This is 36deg advance. Warm it up. Take the springs out of the advance weights. Put the rotor and cap back on, disable vacuum advance, connect light, start it up (it may buck a little) run it at a smooth rpm (say 2500) and set the timing to the 36deg mark. Now you know for sure the distributor goes to 36deg. Put the springs back in, check that she hits the mark again at 2500rpm, if not, get a spring kit and lighten the springs up till it is good and firm at 36deg by 2500rpm - its OK to use different springs on either side. The distributor probably has 20-22deg advance in it, so your idle timing should be somewhere about 12deg, or 1/3 of the way from the TDC notch to the 36deg mark you put on. If its more, go up a spring size one side at a time until it comes back to that 12deg.
Now you have eliminated one possible problem, onto the next, one at a time and evaluate the changes each step makes.
Happy tinkering!!!!!
 
34 degrees total timing seems way to low for my likes. Try bumping that timing up to 38-40. I run 38 in my blower motor. It was a real dog at 34. Won't cost you anything to move the timing!
 
34 degrees total timing seems way to low for my likes. Try bumping that timing up to 38-40. I run 38 in my blower motor. It was a real dog at 34. Won't cost you anything to move the timing!
Unless it detonates [and you can't hear it over the pipes] then "just some pistons or bearings] !!
Blown engines are a whole different animal , IMHO
dave
 
He is running a pretty low compression ratio with flat top pistons, and they will have valve releifs in them also. A stock Chevy motor with a vacuum advance on it will run timing way up in the 50 degree range. I don't think he will hurt that motor at 38-40.
 

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