Hi
Some of you may have seen my last post re my 'lacklustre' 435HP, rated by me at maybe 250-300. Per before the car is in superb factory condition throughout, the engine was rebuilt around 5K ago and fitted with an L88 cam. Its has correct rebuilt tri carbs (by Holley service MI), transistor ign with blanked vac advance.
I did check a few things - there is about 8deg backlash in the cam chain.
I adjusted the static timing from where is was set at 12deg BTDC to around 20, with an immediate uplift in idle speed, it was still picking up past 25 but I set it at 20. Like that I recorded mechanical advance up to just over 40deg, maybe at 3000 rpm (couldn't see the tach at the same time).
This did seem to pick things up a bit, but not 'night and day'.
I then forced the front and rear carbs to open as the main throttle did (in effect locking the sliding linkage), now we are talking. Below 3000 rpm at full throttle the engine would bog, as I read it would, above this the whole character changed and it turned into the creature I was expecting!

But I can't run like this....I know that the vac diaphragms in the f/r carbs take vacuum from a port which is also open to the carb above the butterfly, so only a partial vac is achieved depending on throttle - from my analysis I think the carbs are not being pulled in when they should be.

Your thoughts, especially if you have had to fix this, greatly appreciated.
Re ign timing, could I run at 25 BTDC - when does it get risky?

Thanks!!

 

Timing 40 deg all in would be my first concern, (40 deg all in sounds like too much to me JMPO), does the engine ping running that much advance? Probably hard to tell if it pings when its bogging first.
One thing at a time would be my approach, put the carb linkage back to the way is was and check that you aren't getting any pinging (detonation) from the engine when you give it a lot of gas under load. If you only need more timing advance at idle why not run manifold vacuum advance? You say it has been blanked off on the distributor, do you know why?
Once the timing is sorted then I would be looking to see why the front and rear carbs aren't opening the way you think they should. As I understand it carbs one and three are demand carbs in the same way as the secondaries are in a four barrel vac secondary carb. If they are correctly built and setup as you say they are they will only open as far as the vac signal from the engine will tell them, if it runs better when you force them further open that indicates to me there is an issue in the setup. JMPO.

 

You can purchase two Holley Spring Kits and go to a lighter spring (same color in each carb.), the orange spring is a medium rate spring. The kit has a chart, I also have a graph showing more spring rates (less common colors). Let me know if you want a copy of graph.

 

thats the prolblem with tri power carb setups, if you put a 850 DP mechanical carb
on there with an air gap intake that car would rip

 

When the factory ran multi carbs it was mostly because they didn't have one good carb that would be big enough, and when not getting on it a tri power is pretty street friendly running off the center 2 barrel then as things progressed you see GM stepping to the Holley for the really mean engines L-88 etc, the single carbs always outdo the tri powers in unbiased testing.

Don't get me wrong a tri power is just about as old school cool as one can get but one has to trade off a little, I would run one if they were not so ridiculous expensive,

 

Hi
Thanks for the input on this. What I don't get is that the installation was fully developed by GM, as such I don't want to get into experimentation if I can avoid it, surely with the orange spring as the factory fit the car should run properly....should I start playing with springs I'd have no idea how far to go or why.
But I'd be really interested to take a look at the chart, sure - if you can send it my email address is paul.pb1@virgin.net.

Many thanks!
Paul

 

HI
Thanks for getting back to me. I was thinking that I could carry on experimenting with the tri set (which is what I'd be doing) or jump ship to a 4 barrel and keep the tri set for originality...
What do you mean by an air gap intake? Ive had a look and there seem to be many aluminum 427 intakes, do you have specific experience / can recommend one?
Thanks, Paul

 

Hi Paul, I used an air gap by edelbrock in my 69 427, it basicly has
a gap under the carb so the carb stays a little cooler, gas that is cooler
=more HP. I am in the prosess of building a 68 427 vert and will have the
same setup, I always use a L-88 hood, not sure if a stock BB hood will clear
that setup, good luck

 

Stock BB hood will not work with the Air-Gap. L88 hood will be needed.

 

OP, is your throttle linkage mechanical or vacuum controlled. If mechanical, have you put someone in the car (with the engine off, of course), and had them push the pedal to the floor, to make sure that your linkage is indeed opening the end carbs, and opening all three, all the way?

 

There is a point where you don't even need to run an advance. Since you have a pretty big cam I would just lock out the distributor, and run it at 34-38 degrees all the time. Most engines like to run in this range. Almost all of my street builds get 496-540 CI engines with a mini starter, and locked out ignition timing.
As someone else pointed out 40 degrees is probably too much. You could hook the dist up to manifold vac instead of ported, and then just set the total timing where you want it. It will idle great with all the timing all in, and makes the carbs easier to tune.

 

This ^^^^^ is what I've done with my LS-5 454 with a FAST fuel injection tb system. Right now, it is very responsive on the throttle. I'm not finished tuning the fuel injection yet, but it can't be that far off with the way it runs and responds now.

 

Your carbs could be the cause of your problem, make sure you check the basics before you spend a fortune and lots of time. Check your manifold vacuum at idle. Should be 19-20 inches with a stock cam. Spark plug gap should be .035" Check the balancer and make sure it hasn't spun, then all your timing adjustments will be a waste of time. I could go on, but you get the picture. Much too often people waste all their time, money and effort when they never checked the basics.

 

He's running the L88 cam, so vacuum will be much lower with that cam.

 

Is that really a good street cam?

 

Bats, it's legendary because of the racing history behind it and it did make big power in its day. Most of the power is from about 4000 RPMs on up to about 7000+ RPMs. I've never personally driven one but an old buddy built one for his '69 Vette. It was properly built with very high compression, which you need due to the gobs of duration the cam has.

Because of the duration it's cobby coming off idle and isn't very happy at lower RPMs. So, I wouldn't call it a really good street cam although the cool factor is strong. The idle is serious business- nasty! I'd look at a number of other and better options if I wanted to build a strong big block for the street.