Author Topic: Timing recommendations  (Read 739 times)

Offline leadlobber7

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Timing recommendations
« on: January 10, 2018, 11:38:21 PM »
'86 Chevrolet Silverado K30 454 th400 90k miles
Freshly rebuilt Quadrajet 800 cfm carburetor. Carburetor had gas leaks and cracked vacuum plugs. Was a stinking pig of a carburetor but the truck ran great.

Tuned carburetor by intake vacuum. 19 hg @ 550 rpm and
20 hg @ 700 rpm.

Truck runs good but not great. Hesitates a little bit off a stop. I'm thinking my next step is timing but I have heard many different opinions on the best timing set point for a Mark IV 454 BBC.

Any opinions?

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« Last Edit: January 10, 2018, 11:43:48 PM by leadlobber7 »

Offline roundhouse

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Re: Timing recommendations
« Reply #1 on: January 11, 2018, 05:59:57 AM »
I always adjust by ear

Find a hill with not much traffic , stop at the bottom and run up it full throttle
Advance the timing two degrees and do it  again

Keep making trips up the hill until you can hear it start to ping /knock.  Then retard timing two degrees

I have one truck with 27 degrees advance , and it runs great
Obviously the pointer is off or the pulley is marked wrong , no engine should run great with 27 degrees but timing By ear gets the best performance

If you have an automatic you can do it sitting still by chocking the wheels standing on the brakes with one foot and the gas with the other

and getting a helper to wear some thick gloves and slowly advance the timing while you have it in drive and are mashing the gas
Advancing till it pings and back off just a little 

Offline Henry

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Re: Timing recommendations
« Reply #2 on: January 11, 2018, 12:28:02 PM »
Hi Lobber7:
Well, I would suggest that you set the timing per the factory specs assuming your engine is basically stock. Should be on a decal on your air cleaner or a VECI decal somewhere in the engine bay...if all gone, then you should be able to find the specs on the technical section of this forum. If you still get hesitation, you may want to go back and tune your Q-jet using the "lean-drop" method as recommended in the Chevy service manuals. Have you checked the specs and linkage lengths for the vacuum dashpots on the carb?
Regards,
Henry

Offline blazer74

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Re: Timing recommendations
« Reply #3 on: January 11, 2018, 07:03:10 PM »
Check and make sure you have an immediate accelerator pump shot with the slightest throttle movement, no delay.
The accelerator pump shaft and lever should have positive contact at all times.
You may need to move the linkage to the green lever to the outer hole  for quicker/more pump shot if nothing else helps.

Offline bd

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Re: Timing recommendations
« Reply #4 on: January 11, 2018, 09:20:20 PM »
The outer hole of the pump arm retards the pump shot (it's further from the fulcrum).

Leadlobber, will the vacuum advance hold applied vacuum?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline blazer74

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Re: Timing recommendations
« Reply #5 on: January 11, 2018, 10:06:49 PM »
Thanks for the correction.

Offline leadlobber7

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Re: Timing recommendations
« Reply #6 on: January 11, 2018, 10:31:59 PM »
The outer hole of the pump arm retards the pump shot (it's further from the fulcrum).

Leadlobber, will the vacuum advance hold applied vacuum?
bd I'm learning more everyday and increasing my mechanical know how but as of right now I do not entirely understand what you mean by vacuum advance holding applied vacuum. I know that there is a vacuum pull off looking port on the side of the distributor but...

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Offline bd

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Re: Timing recommendations
« Reply #7 on: January 11, 2018, 11:49:29 PM »
Attach an 18" or so length of 5/32" vacuum hose to the distributor vacuum advance and suck on the hose, then plug the end of the hose with your tongue.  Does the vacuum can maintain the vacuum or does it bleed off?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline leadlobber7

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Re: Timing recommendations
« Reply #8 on: January 13, 2018, 03:50:17 PM »
Attach an 18" or so length of 5/32" vacuum hose to the distributor vacuum advance and suck on the hose, then plug the end of the hose with your tongue.  Does the vacuum can maintain the vacuum or does it bleed off?
It turns out it was set at 0 on timing and once we got it up to 10 it runs better than it ever has. I'm guessing that the last time they timed it the carb was in bad shape with a whole bunch of vacuum and fuel leaks and now that it has no vacuum leaks with a freshly rebuilt carburetor, setting at 10 on timing is perfect? The truck runs like you dropped a bigger higher performance engine in it! I have no idea that timing would mean that much.

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Offline FlatBlack77

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Re: Timing recommendations
« Reply #9 on: January 13, 2018, 06:29:30 PM »
Timing can make all the difference in the world. If you ask me there is quite the learning curve with SBCs and timing. Read ALOT. bookmark whatever you have to for later reference
"When you are a hammer, everything looks like a nail"
'77 C/10 - 350/350 mild street motor

Offline leadlobber7

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Re: Timing recommendations
« Reply #10 on: January 15, 2018, 08:42:33 PM »
Timing can make all the difference in the world. If you ask me there is quite the learning curve with SBCs and timing. Read ALOT. bookmark whatever you have to for later reference
Hey thanks man. It's very satisfying when you find something that makes this big of a difference. This truck is 91000 original miles but has sat a lot! I've had to make my share of repairs. When I bought the truck I thought it ran great, but I had never driven a big block Chevy paired up with a turbo 400 transmission. So everything that I've done step by step has given this truck much more life.

Belts - found worn/slipping belts, all new high performance belts

Replaced battery and alternator with heavy duty setup

Electric choke - adjusted choke (No secondaries)

Rebuilt carb - carburetor had multiple Fuel and vacuum leaks (cracked plugs)

Tuned carb - tuned carb using intake vacuum at 19 in/hg @550 rpm

Air cleaner - replace old stock with high performance hi flow air cleaner system

Timing - initial timing was set at 0 is now set at 10

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