In my continuing saga to straighten out my new 69 L71, I noticed that the secondary carbs aren't working. The linkages on these outer carbs are bolted through a slot in the center carb's arm but the slot is long so that the outer carbs never are activated even at full rotation of the arm (i.e., full throttle). Does anyone have a close-up photo or diagram of the L71 tripower linkage? Thanks alot.

 

are you thinking that the mechanical linkage will open up the end carbs?

I may be wrong on the '69's but I know that the '67 L71's don't operate that way. The end carbs operate and open up from Venturi vacuum based off the center primary carb.

To test if that's operating correctly you do need to have the motor under load so revving it up in the garage won't open up the end carbs, you need to be driving the car for them to operate but if they are you should be able to feel the difference when they open up.

The reason for the mechanical linkage is to make sure to CLOSE the secondary carbs when the center carb closes to prevent them from hanging open

 

barryk has it right, all tripowers are that way 67-69.

 

BarryK has it right. That is how the Chevy system was designed. If you must keep it original, you'll have to learn how to set up that system to get it working correctly. I always preferred the 'solid' linkage systems better than the vacuum activated method. That allows you to stay on the center carb only until you decide to go WOT;using center carb only in normal traffic was good for 'economy' [if you can really use that word with "tri-power" in the same breath]. The outer carbs would be tied together to start opening at about 50-60% throttle and be fully opened at WOT.....it worked great.

 

Progressive (mechanical) linkage was on my tripowered 65 GTO so I assumed all GM systems worked the same. These L71 secondary carbs are definitely not operating under load. Is there any way to check the vacuum on these carbs? To make matters more interesting, the center carb must not be working properly now since the engine stumbles and runs badly.

 

you may have to take the pods off and check the diaphragms...with age the rubber deteriorates. Maybe you have the black spring, heaviest, that prevents the secondaries from opening....in any case, you should take the covers off....

 

If you know where the secondary vacuum units are to "plumb into" the vacuum source, you can tie in a vacuum gauge to that source {disconnect and plug off the line to the vacuum units} and run the gauge hose into the interior so you can take it out and try it. You run the throttle and have your passenger right down what the vacuum is at various rpm increments as you call them out. It may take a pass or two to get that data. You shouldn't need to run it past 4500-5000 rpm.
If you are not seeing a good increase in vacuum as speed increases, there is some problem with the vacuum source. Also, you can hook a hand-held vacuum pump up to the vacuum-feed line to the secondary units and see how much vacuum it takes to get them fully opened. Having those two pieces of information should point you to your problem(s).

 

Not a close-up but I hope this helps. This is also on a 69 L-68

 

Thanks for all your input. I've tested the vacuum pods and both seem to be working correctly. The secondary linkages move smoothly when a vacuum is applied. I disassmbled the fuel lines and found that the (brass) filters seem to be clogged so this may be part of the problem. Also, I noticed that I don't have a vacuum advance distributer. I thought that all L71's had these. Is this correct?

 

You can use your mouth to suck on the hose connecting to the vacuum pod. If you apply vacuum it should open the throttle blades.

 

I don't know the answer to that question, but your A.I.M. should detail whether your engine option uses a vacuum advance distributor or not.

 

I'm 99% sure that your L71 came with a vacuum advance. From the factory it would have been hooked up to a ported vacuum source.
You should replace your current distributor with a correct one that has the vacuum advance cannister but when you go to hook it up do not connect it to the ported vacuum port on the carb. Instead, cap off that port and "T" into the full manifold vacuum line that is being used for the choke.
Replacing the vac adv can in your system and connecting it to full manifold vacuum will give you a smoother idle, better throttle response, better fuel economy (yeah, yeah, yeah, I know - on a BB motor and a tri-powered one at that it's a relative thing but what the h*ll), and lower operating temps.

 

My car's choke is thermostatically controlled choke and is not controlled by vacuum. The only vacuum hoses on my carbs are the ones attached to the secondary pods. I know the engine was rebuilt (with a high lift roller cam, Pertronix ignition, etc) so it looks like I have some work ahead of me to figure things out. Besides originality, what are the advantages of a vacuum advance (vs cetrifugal)? My timing light shows that the car starts with about 18 degrees of advance and goes up from there smoothly with increasing revs. Thanks again.

 

Your engine already has both vacuum advance and centrifugal advance. The vacuum advance can is strictly for smoothing out the idle and low throttle use; there are flyweights/springs inside the distributor which regulate the rate of 'mechanical' advance as engine RPM increases.

 

Two excellent articles you really should read regarding the vacuum advance system are these posted on my website's tech articles page:

1.
http://lbfun.com/Corvette/Tech/vette...101Article.pdf
This is by John Hinckley
scroll down to the section called Vacuum Advance on page 3

2.
http://lbfun.com/Corvette/Tech/vette...Adv%20Spec.pdf
This is by Lars Grimsrud

Both articles do a great job of explaining the reasons and advantages of the vacuum advance system but as a quick summary, the centrifugal advance in your motor works solely on engine RPM, the faster the motor rpm the more timing advance you get.
The vacuum system on the other hand works solely based on engine LOAD. The more load on the motor the less advance the vacuum advance adds and the less load on the motor the system adds more advance.
Both the centrifugal and the vacuum advance systems are independent of each other but work together to achieve proper and optimum spark advance at any given time during varied driving conditions which your car being driven on the street is seeing.

The overall advantages of a properly functioning vacuum advance system will be better idle characteristics, better throttle response, better fuel economy, and lower operating temps.

 

Thanks for these references. The Lars article is particularly helpful but it doesn't list an application for a 69 HEI distributor. Should I start with a B1? As I mentioned, the engine has a Comp Cams 280 degree roller cam and is set for about 18 degrees of initial advance.

 

At the risk of over simpifying this, here's how the secondaries work: As you increase the load on the engine (hard acceleration), ported vacuum is applied to the vacuum pots through the outside vacuum lines on the passenger side of the carbs. At the same time, vacuum increases in the venturi's and equalizes vacuum to the pot thru a bleed port in the venturi, thus allowing the pot to work. It make for very smooth operation, compared to manual setups like the GTO. Those Rochester progressive linkages really let you know when you hit the secondary carbs, but would bog badly if they were setup wrong.

If you apply vacuum (wth a hand pump or whatever) to the pot with the engine off, it will not operate because of the bleed port. It has to be under load! Best way is a dyno, but the suggestion of a gauge mounted where you can see it will work as well. Just tape it to the windshield in front of you and road test it. I use third gear because it makes for a long smooth pull.

You may have a problem based on what you said about the timing and cam specs. 18 initial is a bunch and may indicate low vacuum, probably due to the cam profile. If it's pretty wild, you may never get the ported vacuum you need to get the carbs to work right. Do a test and let us know what kind of vacuum readings your seeing at different speeds and loads I may be able to help further. I'm a sucker for multiple carb setups, and these can work real well with some basic tuning. Good luck!

Hans

 

Hans - thanks for your suggestions. I plan to rebuild the carbs first since, at a minimum, the brass filters are clogged. Do you know where the distributor vacuum advance lines connect the the carb(s)? The only ports that I see on my carbs are the ones for the secondary pods. I'm not sure if I'll connect the vacuum advance to the carbs (or to the manifold) but at least it would be helpful to know how to do it. Thanks again.

 

I just went out to my L71 with brand new Holleys, unhooked the linkage, undid the hose coming from the center carb and applied vacuum via my mouth. The secondaries smoothly opened. This test proves the vacuum pods are in good condition and will open if the correct amount of vacuum is applied.

 

were there HEI distributors in '69? I thought in '69 there would only be points units or the TI units and HEI's came out a few years later in the early 70's because the HEI came about with the requirements of higher spark voltages with all the emissions equipment and greatly retarded spark advance, but I may be mistaken.
Regardless, if you have an HEI unit on your car you can select the proper vac adv can for your motor based on what vacuum level your motor is producing at idle and the amount of advance that you need to add. I wouldn't go so much from that cars are listed as the application so much as the specs on the units themselves to fit to your motor.