69 L71 just dies after a warm idle

[Guest]

Re: 69 L71 just dies after a warm idle

Something going on with the centre carburator you may have a warp metering plate or metering gasket leaking vacuum and sucking fuel in from the main body somewhere, disconnect both rear linkages when setting up Idle and mixture, you should be able to idle as low as 650 - 700 rpm unless you have some crazy Camshalf

[Chris H.]

[Duke W.]

Re: 69 L71 just dies after a warm idle

You are saying that total centrifugal is 32.4 and initial is 10.3. That means total WOT advance above 3270 is 42.7, which is beyond the 36-40 recommended by Chevrolet for maximum performance.

I'm thinking that you don't understand the difference between centrifugal advance and total WOT advance.

Total WOT advance (VAC disconnected and plugged) = centrifugal + initial advance.

Set total WOT advance a few hundred revs above where centrifugal is all in as high in the 36-40 as the engine will tolerate without detonation.

If you want to then lower engine speed to below the centrifugal start point to check initial, that's fine, and the difference should be about 30 degrees for an OE L-71 distributor.

So do the above, and if you want to try lighter springs, okay, but don't bring it all in much below 2500 and the ultimate limitation is detonation.

Idle vacuum is a good way to verify an OE cam. The L-71 with full time vac. advance for mid to high twenties total idle advance should be in the ballpark of 14" @ 900. You can probably get it to idle lower, but that means less manifold vacuum. A 900 idle speed should yield a fairly smooth idle with just a little lobe, and it will be easy to get going from a dead stop with minimal clutch slippage.

Duke

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

You are saying that total centrifugal is 32.4 and initial is 10.3. That means total WOT advance above 3270 is 42.7, which is beyond the 36-40 recommended by Chevrolet for maximum performance.

I'm thinking that you don't understand the difference between centrifugal advance and total WOT advance.

Total WOT advance (VAC disconnected and plugged) = centrifugal + initial advance.

Set total WOT advance a few hundred revs above where centrifugal is all in as high in the 36-40 as the engine will tolerate without detonation.

If you want to then lower engine speed to below the centrifugal start point to check initial, that's fine, and the difference should be about 30 degrees for an OE L-71 distributor.

So do the above, and if you want to try lighter springs, okay, but don't bring it all in much below 2500 and the ultimate limitation is detonation.

Idle vacuum is a good way to verify an OE cam. The L-71 with full time vac. advance for mid to high twenties total idle advance should be in the ballpark of 14" @ 900. You can probably get it to idle lower, but that means less manifold vacuum. A 900 idle speed should yield a fairly smooth idle with just a little lobe, and it will be easy to get going from a dead stop with minimal clutch slippage.

Duke

Duke thanks for the explanation:
My digital timing light reads with the VAC plugged, 33.4* at 3270 RPM. So I am assuming that is WOT (initial + centrifugal).
When I lower the engine speed to around 900 RPM my initial is around 10*. So less than the 30* per the CSM and your recommendation. Perhaps I still need lighter springs? I have not experienced detonation to this point.
I will check idle vacuum next. I did not have a vacuum gauge with me this weekend.

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

Ok I will give this a shot.

[Duke W.]

Re: 69 L71 just dies after a warm idle

Duke thanks for the explanation:
My digital timing light reads with the VAC plugged, 33.4* at 3270 RPM. So I am assuming that is WOT (initial + centrifugal).
When I lower the engine speed to around 900 RPM my initial is around 10*. So less than the 30* per the CSM and your recommendation. Perhaps I still need lighter springs? I have not experienced detonation to this point.
I will check idle vacuum next. I did not have a vacuum gauge with me this weekend.

Yes, you are reading total WOT advance, which should be 36-40, as high in that range the engine will tolerate without detonation, but either you didn't rev it high enough or the distributor doesn't have the OE 30 degrees centrifugal.

Try installing the lightest springs you have. Then rev the engine until the timing clearly stops advancing, and document that RPM. Then rev it a few hundred above this number and set it at 39-40. If it doesn't detonate you're done. If it does detonate try some stiffer springs and/or back down lower in the 36-40 range.

To determine initial timing you need to determine the centrifugal start point. You can usually do this by lowering idle speed to 700 or less then bring it up slowly and record the point that the timing starts advancing. If it starts advancing immediately from 700 then the centrifugal might start lower and be difficult to determine the start point. You need to know this in order to set initial timing, other wise you might be setting it at a point where a few degrees centrifugal has been added, so total WOT advance will be less than optimum. That's why I prefer the total WOT method to adjust the distributor especially when you install light springs that brings the centrifugal all in by 3500 or less.

Duke

[Mark G.]

Re: 69 L71 just dies after a warm idle

Is the center carb bowl vent working?

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

Is the center carb bowl vent working?

Mark,
Not sure? Do you mean is it clogged?

[Mark G.]

Re: 69 L71 just dies after a warm idle

Does it open when at idle?

[Mark G.]

Re: 69 L71 just dies after a warm idle

There is a hole in the float bowl covered by a rubber plug. There is a spring loaded lever that seals off the bowl opening anytime the throttle is off idle. At idle the carb linkage pushes on the lever raising the rubber plug off of the hole venting any pressure in the bowl.

[Doug M.]

Re: 69 L71 just dies after a warm idle

Ok I will give this a shot.

Lawrence, from the GM service manual

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

There is a hole in the float bowl covered by a rubber plug. There is a spring loaded lever that seals off the bowl opening anytime the throttle is off idle. At idle the carb linkage pushes on the lever raising the rubber plug off of the hole venting any pressure in the bowl.

Ok I know what you mean. I think it is working, but will have to confirm.

Thanks

[Chris H.]

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

Thanks guys. I will work on that and report back. Fun stuff...first tri-power experience.

Lawrence

[Lawrence S.]

Re: 69 L71 just dies after a warm idle

I adjusted the carb correctly with the secondaries disconnected. When I set the linkage per the above the car idled back up incorrectly. With the carb idling at 750 - 800 I hooked up the linkage to the secondary carbs. Idles nice, however, with the car off and I pull the accelerator to full throttle the secondary carbs throttle blades are not all the way open. I assume they should be?
The main linkage connection on the center carb has been bubbaed. It is not correct. There is a jive ass bolt with a nylon nut which I assume is causing issues.
Look forward to your comments