69 Tri Power idle issue

[Grahame M.]

Hi All,

I've just installed my new refurbished carbs (done in Sydney Australia) and have an issue where the idle speed is around 1000rpm (with the accelerator cable and outer carb rods disconnected) unless I rev the engine quickly and hold the throttle linkage hard in the closed position. I can't adjust the idle speed using the adjusting screw, below 750rpm.

I feel its something binding but i'm not sure and haven't removed the carbs to see. I have included some pics below and have adjusted the idle (as best i can), then the mixture screws when i could get the idle below 900-1000 rpm. Once i got it to around 750 idle turning the mixture screws in would make the engine stumble on the drivers side and stop when turning in on the passengers side (still a left hand drive car).

I thought I would reach out to the experts to see if something in the pics was obviously wrong.

- The engine doesn't make any noticeable change if i cup my hand over the centre carb
- I have adjusted the bowl vent rod as it was binding on the actuating rod at the step
- I have checked and adjusted the fuel bowls, with only the front one being too high
- Timing is set in accordance with Lars's paper. 36 deg at around 2850 with correct VAC advance unit and VAC stopper at 12 deg
Pic 3.jpg
Any help would be greatly appreciated. I did have a popping from the side exhaust at idle but that seems to have gone since making the adjustments above. One other thing is, it smells like its running rich (no smoke), but not sure if this symptom points to an obvious reason for the high idle. Its really difficult getting tri Power help here in Oz but the company who did them are a Holley agent, but don't see many tri Power set ups.

Many thanks again and i look forward to your responses.

Grahame

[David D.]

Where are your primary carb idle mixture screws adjusted? They should be approximately 1.5 turns out from lightly seated for the best idle quality. If they need to be adjusted out more than that, it would indicate you have a vacuum leak somewhere.

A couple of observations - your rear secondary carb has the wrong fuel bowl. The float level sight plug should be lower (see your front secondary carb - it has the correct float bowl configuration.) This probably does not have a significant impact on your idle issue. Also, I see your secondary close-down linkage is disconnected. Perhaps you have left it disconnected while you are setting your idle speed, which is fine. Just wanted to make you aware of it.

[Grahame M.]

Thanks for the quick response David, yes i had left the secondary rods and accelerator cable disconnected to make sure it wasn't affecting the issue i have. I hadn't noticed the bowl, i remember getting a new one (from Chicago Corvettes) for the repairer as the original one was cracked. I'll check it against the other set i have (not date correct) but are a good reference point before i sell them.

The fast idle cam is a much brighter green than the one on my other set, would having the incorrect one cause any of the issues i'm having?

Cheers
Grahame

[Stewart L.]

Keep in mind the secondary’s are vacuum operated, the secondary throttle rod limits their opening to when the throttle is depressed. Don’t run the car with the rods disconnected or you could have an unwanted WOT situation.
are your secondary butterfly’s closing with a good seal? Put your hand over the secondary’s to block airflow in and see if it dies while at idle. If they don’t have a good seal the vacuum will suck gas and the idle won’t adjust down.
do you have proper fuel pressure?

[Duke W.]

Is it a 400 or 435 HP engine, and does it have an OE equivalent camshaft?

What is the target idle speed you are shooting for.

What is the ID data stamped on the VAC mounting bracket (I assume it's an OE type Delco distributor), and is it still the OE ported type or has it been converted to full time.

Have you talked to the carb rebuilder about the problem and if so what do they say?

When you set the timing at 36 at 2850 was the VAC connected or disconnected?

Did you rev the engine higher than 2850 to see if it advanced beyond 36?

Duke

[Grahame M.]

Thanks Stewart,

I've only had the car in the garage running, not on the road without the rods connected. Unfortunately i found out the hard way when i forgot to connect them before taking it for a quick spin around the block. Luckily no damage.

I have tried blocking off venturis with my hand and no noticeable difference and from what i can see all carb butterfly's are sealing. This is only a visual inspection but they look ok. Is it possible i have wrong gaskets? Would that cause higher RPM, butterfly's sticking?

My next job was to take them off and inspect them thoroughly.

[Grahame M.]

Hi Duke,

Thanks for the questions, please see below in order

- 400HP L68
- Around 750, its a 4 speed car
- Its a B26 with a limiter fitted to 12 deg. Full time attached to the nipple at the rear of the manifold. last time i checked i was getting around 20Hg of vacuum with the gauge fluttering ever so slightly
- Haven't talked to the rebuilder yet. One of the next steps was to take car to the rebuilder to put on his dyno. Unfortunately duke i'm not that confident they know a great deal about this carb set up but there's no one else here in Sydney.
- Vac disconnected and i did go up to about 31 deg but timing mark didn't move. I have an adjustable timing light so i checked both ways as its very hard to see balancer with A/C and P/S in the car

Thank you again.

Cheers
Grahame

[David D.]

Thanks for the quick response David, yes i had left the secondary rods and accelerator cable disconnected to make sure it wasn't affecting the issue i have. I hadn't noticed the bowl, i remember getting a new one (from Chicago Corvettes) for the repairer as the original one was cracked. I'll check it against the other set i have (not date correct) but are a good reference point before i sell them.

The fast idle cam is a much brighter green than the one on my other set, would having the incorrect one cause any of the issues i'm having?

Cheers
Grahame

..
The fast idle cam should not come into play when the choke is fully open. The hot idle adjustment should be only based upon the idle speed screw on the driver's side. The fast idle cam shouldn't be holding the idle high with the choke fully open unless the fast idle lever is improperly adjusted.

Did you determine how many turns the idle mixture screws are currently set at? Anything more than 1.5 turns indicates a possible vacuum leak.

[Grahame M.]

Sorry Duke,

From what i know and records i have the Camshaft is still OE. The engine is original from dates and stampings.

Cheers
Grahame

[Grahame M.]

Thanks David,

Yes, sorry i wasn't very clear on that. I did both adjustments to 1 1/2 turns from soft seat, it ran a little off (not too bad) but smoother with another 1/4 turn.

Cheers
Grahame

[Grahame M.]

Sorry Stewart,

I missed answering you question on fuel pressure. Its the only thing I've never checked, i'll have to get myself a pressure tester to do this, but I haven't checked only because i have full fuel bowls.

Interestingly, when i took it for its first run a couple of weeks ago the engine spluttered and stalled going around a roundabout so i thought low floats but checked that on the weekend and only the front carb was very high, 1 3/4 turns. The other two carbs were right at the lip and fuel came out if i gently shook the car.

Do you or anyone else have a pick of a fuel pressure tester setup? I'm assuming its connected to the fuel line at the first block from the pump, near the carbs? Is it tested by cranking or starting the engine? I've seen some with permanently installed pressure gauges but with the issues these tri Powers have with fuel leaks i'm a bit reticent to install one. Having had leaks in the past i have my fuel lines very well sealed at the moment and hopefully they stay that way.

Cheers
Grahame

[Duke W.]

An OE equivalent camshaft should idle at about 600 @ 15", with just a hint of lobe. Upping it to 700/750 will increase manifold vacuum to over 16" and the idle should be very smooth. My recommendation is to set idle speed as low as possible commensurate with acceptable idle, quality, and that's for the owner to decide.

The B26 VAC should be okay for whatever idle speed you choose. The max advance is nominally 16 @12", but since you've limited yours to 12 degrees its probably about 12 @ 10" which is okay.

You did right by disconnecting (and plugging I hope) the VAC signal line to set timing using the simulated WOT method. Have someone rev the engine while you adjust the light to keep the balancer notch at zero on the tab, assuming you can see it. As you should know if you have a CSM or have downloaded the information kit for your car from the GM heritage Web site , max OE centrifugal is 30 @ 5000. So unless it's been modified you would have to rev the engine to over 5000 to use the simulated WOT method.

So at least find the point of max centrifugal. If it's still OE install lighter springs to bring it in by 3500, then you can set total WOT advance at a little over 3500. If you have a single point ignition make sure the points are in good condition and set dwell at 30 before you do the timing.

Once total WOT timing is set connect the VAC and check total idle advance which will be initial plus full vacuum plus maybe a few degrees more if the centrifugal starts below normal idle speed. If you want as a point of reference you can then rev the engine to above the point of max centrifugal to see what theoretical max cruise timing is if you so desire. Record this data for future reference.

Since I've never owned a car with a Holley (but I've worked on several), the first thing I look at is the CSM and COM for that model year car. If you don't have a CSM and COM you need to procure them They should explain how to set the secondary throttles. They are slightly open even when "closed" and there is always a small amount of fuel being added. This is to keep the fuel in the secondary bowls fresh.

So check out the manuals and once you've got the secondary throttles dialed in redo the timing, then the last step is to go through the idle speed-mixture adjustment procedure.

Do the above before worrying about fuel pressure. Excess fuel pressure will not allow the needle valve to seat and excess fuel will spill into the venturis via the main nozzles. This will cause rough running and black smoke out the exhaust at but doesn't have much effect on idle speed other than maybe causing the engine to stall due to excess fuel. Too little fuel pressure will cause the bowls to run dry at WOT, medium to high revs and the engine will just "shut down" like turning off the ignition switch.

Duke

[Grahame M.]

Thanks Duke,

I'd be happy to run at 600 as it still seems a little high at 700-750 but i can't get it any lower with the idle screw not even touching the actuator at this speed, hence my problem. Thanks for mentioning but I did remember to plug the vacuum tube to the dissy when i checked timing. My ignition is TI and has the lighter springs fitted.

Cheers
Grahame

[Grahame M.]

Hi everyone,

Thank you again for your guidance.

I had some time on the weekend to look at the carb issue and found the accelerator pump was adjusted far too tight (set to 5 thou, some say 15 others say 10 and then there's mention of just making it touch the actuating arm with the screw head), and also the pump cam (red) using the number 2 screw position which i moved to position 1 which apparently is used for engines that rev lower than 1000rpm.

There didn't seem to be much difference when i set the pump cam up on position one as i could still see the second hole in the red cam, as in pic attached. have i installed it correctly?

I also found some fuel coming from around several of the bowl screws so i tightened them a little more, they were quite lose.

Would these adjustments help with my high idle issue? I couldn't start the car on the weekend but will later this week.

Cheers
Grahame

[Duke W.]

The fundamental cause of high idle speed is too much air entering the engine, and the two primary causes are a vacuum leak or secondary throttle stop(s) out of adjustment.

Did you check the COM for the procedure to set the secondary carb throttle stops? You will probably have to remove the front and rear carbs to adjust.

Duke