Engine Tuning Issues

[Screaming Metal]

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...................................

The repairmans reamer is a really good idea, you can open it up a little at a time. Use some calipers that way you can get them the same size.....

 

[Screaming Metal]

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!!!!!

The Centrif. Advance kit as Mango said IS one of the cheap 1st things that needs to be done to any perf. motor. Mr. Gasket, Accel and others sells them cheap and its a must have. Also make sure your advance works smoothly and freely!

 

[Screaming Metal]

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...

Hey, just take your time, and as Dave suggested, do only one thing at a time. You have a possibility of 3 things that could have you there, just take your time, do it one step at a time.
Tuning a motor is like turning a shaft on a lathe that has a worn spot in the ways. You know that theres a problem and to get around that taper you have to do certain things to make it livable.
All motors are setup close to the same carbs, jets, cams, etc., when racing. What sets you apart from the pack is in the tuning. The more you twist that screwdriver on multiple carbs, the better you'll get.
I learned mine a Chevy Mouse Motor with a TRY-1 and 2-4's that I had for years.....but I actually LEARNED on a old wore out Tripower setup. My main tool in the pits is a little flathead screwdriver with a pocketclip on it. It goes everywhere I go. Its my weapon of choice when coming up on the staging lanes......

 

[2old2fast]

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.

With 5K wrapped up in an engine I'd much rather error on the side of caution, just me !?!?

dave

 

[Mr. Fixit]

Even though I am on the road for the next few days, I thought I would check in. Thanks for the added input. I did order the distributor spring and cam kit to adjust the advance and will get on it when I return.

Thanks again...

 

[Mr. Fixit]

The repairmans reamer is a really good idea, you can open it up a little at a time. Use some calipers that way you can get them the same size.....

Oh, I forgot to ask a dumb question, if I get to the point of needing to add the air bleeds in the butterflies (and I suppose I will), does this get done on the primary side of each carb, or secondary side also?

 

[2old2fast]

Before you turn those carbs into junk, IF you need more air ,you can simply adjust the sec. flaps open a little more & see where that takes you , but , attend to the timing issue first [ if I sound like I'm scolding, sorry, I am] Sorry......... I learned the hard way

dave

 

[Mr. Fixit]

Before you turn those carbs into junk, IF you need more air ,you can simply adjust the sec. flaps open a little more & see where that takes you , but , attend to the timing issue first [ if I sound like I'm scolding, sorry, I am] Sorry......... I learned the hard way

dave

I understand and that is my plan.

After all, I always say that "Experience" is a word used for having already made mistakes. When one learns from other peoples experience , they should be able to avoid those costly mistakes. Thanks Teach...

 

[Mike]

Before you turn those carbs into junk, IF you need more air ,you can simply adjust the sec. flaps open a little more & see where that takes you , but , attend to the timing issue first [ if I sound like I'm scolding, sorry, I am] Sorry......... I learned the hard way

This is the best direction to head, because he will be able to see if more air helps or not. And he's not sticking a fork in his throttle blades, until he knows for sure. How many of us have seen people putting pop rivets in throttle blades, trying to crutch a crutch?

 

[Screaming Metal]

Definitely a good suggestion. I've been beating, bending, grinding on a pr. of bent needlenose pliers for 20+ years that fits that secondary tang perfectly for bending to check for this.
You can get a penny and put it against between that stop and the setscrew on the secondarys to hold it open enough to see if thats what the problem is........

 

[Mr. Fixit]

Definitely a good suggestion. I've been beating, bending, grinding on a pr. of bent needlenose pliers for 20+ years that fits that secondary tang perfectly for bending to check for this.
You can get a penny and put it against between that stop and the setscrew on the secondarys to hold it open enough to see if thats what the problem is........

I hope to be able to get on the timing as prescribed in the next couple of days when I return.

Thanks...

 

[Mr. Fixit]

Ok, all resolved! I locked the distributor and set the timing to 36 which clearly is the sweet spot for this engine/set up. The vacuum is now at a solid 8.5, up from the original reading of 5. The carbs are set with the idle mixture screws out 1 3/4 turns and she idles beautifully at 900. Starts perfectly when all warmed up and drives tire shreading strong.

Thank you all for your input and a very special thanks to Dave who has assisted me greatly in sorting this all out over the past few days.

 

[Mr. Fixit]

Today I put on about 20 miles (taking things slow) and she seemed to run fine. When I returned, I pulled a couple of plugs (on Dave's suggestion) to keep a close check on things. The plugs that were once black, sooty and full of carbon are now turning off white on the porcelain area and the electrode is silvery tan in color, so I guess that's good.

Just for S & Gs I put the vacuum gauge back on it and it was up over 9 at 900 rpm idle, which was a small improvement. What is puzzling me now is when I wack the throttle quickly, the reading goes over 20 and then back to 9. I thought that the vacuum was supposed to drop to zero or close to it in this scenario. I am I mistaken???

Thanks,

 

[2old2fast]

when the throttle is opening , until you let off , it MAY go to zero or bounce from near 0 to 20, then as soon as the throttle is closed it should go as high as 20-21, then stabilize [at 9 in your case] ...caveat: if you have a slow reacting [read cheap/old] gauge , it's hard to say what it will do.

dave

 

[Mr. Fixit]

when the throttle is opening , until you let off , it MAY go to zero or bounce from near 0 to 20, then as soon as the throttle is closed it should go as high as 20-21, then stabilize [at 9 in your case] ...caveat: if you have a slow reacting [read cheap/old] gauge , it's hard to say what it will do.

dave

HI Dave,

Actually i tried it a couple of different ways. If I accellerate slowly and smoothly, it starts at 9 and continues to climb over 20. If I wack it and let it go, it doesn't go to zero either, but rather the other way. It is a fairly new Holley gauge connected directlto the base plate of the carb just above the manifold.

Thanks...

 

[Mr. Fixit]

A little birdie (Dave) suggested a while back that I was giving up to soon on the tuning and I now believe that this was true. I have continued to make adjustments, the major of which was to go to a progressive linkage on the carbs. This was a vast improvement off idle and so far seems to be doing well for me. After trying a variety of methods, I got the best results from having both carburator's mixture screws in play, even though the back carb is only being used for power on heavy accelleration.

Although this was all good, yesterday I played some more with the timing and here is where I would up. 28 degrees initial and 10 mechanical advance all in at 2000. This makes a huge improvement off the line, but to get a great response, I now have 8 HG of vacuum and had to crack open the primary idle speed on the back carb. In doing so, It now idles in neutral at 1500 and in drive at 1000.

With this set up, it definately has MUCH more pull from a stop, however, I am feeling that the TC may be too tight now at 3000 - 3400 stall. With all that said, I would like to hear from the engine pros out there with any thoughts to this.

Thanks as always..

 

[RPM]

"28 degrees initial and 10 mechanical" Are you sure about those numbers. 28 sounds WAY too high?

 

[Mr. Fixit]

"28 degrees initial and 10 mechanical" Are you sure about those numbers. 28 sounds WAY too high?

Yes, quite sure.

 

[PotvinGuy]

5" of vacuum is the big clue, that is way low. My crate SBC idles at 20", with a stock cam. Your cam (do you have the part number?) is a bad ass street/strip item. Big cams kill vacuum, make the engine run rough, etc. Screwing with the carbs won't fix it.

Timing is not your problem either, but FYI I run 20 initial, 15 vacuum and 15 speed. A street machine really needs a vacuum advance distributor; it runs cooler and smoother, gets better mileage, has more power and better throttle response. It's a no brainer. Check the MSD 8394. It has electronic speed advance (no springs, bushings, etc) and vacuum advance, all adjustable with a screwdriver.

 

[2old2fast]

Ya' might want to ask Lee 'bout the "quality" materials MSD uses. I personally don't feel it's mandatory to run vac. adv. , but , to each his own.....

dave