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Hello,recently purchased a 68 L-71 that seems to lose power accompanied by a rattle around 5000 rpm.Could this be pre-ignition?
Thanks
Thanks
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Re: 5000 rpm rattle
I could be detonation. I don't have the timing and centrifugal or vacuum advance specs for you engine. What are they?
You could try retarding the initial timing, depending on the spec. (That's why I want you to post all the advance map data).
Another way to isolate it is buy five gallons of race gas or 100LL avgas and dump it in when the tank is near empty. If the problem goes away it was probably detonation.
The next step is to determine the blend of high octane gas you need to mix with pump premium to keep it out of detonation or modify the advance map so it will run detonation free on pump premium.
Duke -
What is your oil pressure reading when the
rattle starts?Comment
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Re: 5000 rpm rattle
Thanks for responding Duke,here are the factory settings;
centrifugal 0 @ 900,30 @ 3800.
vacuum 15 @ 15.5
I know from reading many of your posts that you recommend NAPA vc1810,and I have gone to my local NAPA dealer and asked for one but have been told it is unavailable(I don't know if this is because it is a Canadian NAPA dealer).Anyways I will follow your advice and let you know how I make out.
Much appreciated,
NickComment
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Re: 5000 rpm rattle
Like L-72, I believe your L-71 has a ported vacuum advance, so at idle (assuming less than 900, which is where the vacuum centrifugal starts) all you have is the initial timing of about 10 degrees. This considerably increases EGT and heats up the engine and engine compartment surfaces even if the coolant temperature is within bounds, and this will considerably heat up the inlet manifold and carbs.
You can check for ported vacuum advance by just removing the vacuum signal line and plugging it. Does the idle speed/quality change? Can you feel vacuum on the signal line? If not it is ported vacuum advance. You could also check idle timing with the signal line connected and with the signal line removed and blocked. The difference is vacuum advance at idle, if any.
For a few dollars you can buy a 1/8" tee and a couple of feet of 1/8" vacuum tubing. Assuming it has ported vacuum advance rig up a full time vacuum advance by fabing a new choke vacuum break hose with the tee and connect the free end of the tee to the vacuum can with another length of the tubing.
Then reset the idle speed to about 1000 so you (hopefully) get 15", which will keep the OE vacuum can at of near the stop. Also check total idle timing with the vacuum can connected. Assuming the initial is 10 degrees the total idle timing with full vacuum advance should be about 25 or a couple of degrees more since the centrifugal starts at about 900.
The above modification will lower EGT several hundred degrees and will keep surface temperatures cooler during normal driving and reduce the engine's tendency to detonate or percolate the fuel in the carb bowls. If you have access to an IR gun check exhaust manifold temps before and after the mod. Engie should be fully warmed up and then idling for several minutes when you check.
I believe with about 25 degrees of total idle timing the engine will pull about 14" of vacuum at 800-900. In this case you can use a vacuum can that provides full vacuum advance at 12" rather than the 8" of the VC1810.
The NAPA/Echlin can that would work in this case is VC1765, 0@6, 15@12". Interestingly, these vacuum advance specs were used on L-72, and if my understanding is correct that they will pull 14" at idle then there is no need to install the VC1810 on a L-72 - just convert to full vacuum advance.
As you may know, my recommendation is to have a vacuum can that delivers full vacuum advance at 2" less than typical manifold vacuum. GM did not follow this rule for your engine, but in the case of an engine with ported vacuum advance the rule is meaningless.
DukeComment
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