Timing and Jetting ?'s
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Timing and Jetting ?'s
Morning everyone
Here's the set-up
1991 roller 350 block junkyard special, magged and bored
Scat 9000 3.75 stroke crank and rods
KB-100 pistons
Comp Cams .560 lift 280 duration roller cam
Brodix IK200 heads
Summit aluminum full roller rockers
Hooker headers
Edelbrock RPM Air-gap(fits under the stock hood with about 1/4 clearance)
Holley 750 carb vacuum secondary
MSD ignition
BTO 200 R-4 w/ 2200 stall converter
3:73 posi rear
Dewitts radiator
I'm in the middle of tuning my new 383, right now using #73 jets in the carb, and 8 degrees advance timing. Engine doesn't seem to like larger jets(surges at freeway speeds), plus I did the Lars-style timing(using the blue springs to achieve 36 degrees @ 2500rpm) with an end result of 8 degrees BTDC with the vacuum advance plugged.
Does this sound OK, or is there anything I should do differently?
Here's the set-up
1991 roller 350 block junkyard special, magged and bored
Scat 9000 3.75 stroke crank and rods
KB-100 pistons
Comp Cams .560 lift 280 duration roller cam
Brodix IK200 heads
Summit aluminum full roller rockers
Hooker headers
Edelbrock RPM Air-gap(fits under the stock hood with about 1/4 clearance)
Holley 750 carb vacuum secondary
MSD ignition
BTO 200 R-4 w/ 2200 stall converter
3:73 posi rear
Dewitts radiator
I'm in the middle of tuning my new 383, right now using #73 jets in the carb, and 8 degrees advance timing. Engine doesn't seem to like larger jets(surges at freeway speeds), plus I did the Lars-style timing(using the blue springs to achieve 36 degrees @ 2500rpm) with an end result of 8 degrees BTDC with the vacuum advance plugged.
Does this sound OK, or is there anything I should do differently?
Burning Brakes
Joined: Nov 2003
Posts: 990
Likes: 0
From: North Texas
Looks like you may have too much mechanical advance. If you max out at 36 and that leaves you with 8 - the delta is 28º. I believe you should be closer to 20º.
Which distributor are you running? It sounds like you may be missing the stop bushing on the mechnical advance.
Which distributor are you running? It sounds like you may be missing the stop bushing on the mechnical advance.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
recourve the dist to run about 16 intial and about 18 degrees mech.
You only want about 32-34 degrees max on modern heads. Some of the fastburns are even down to 28 degrees max.
Are you running flat top pistons? If you are, California 91 octane with your high compression you want to set your all in closer to 3000-3200 rpm and limit additional vacuum advance to less than 8 degrees with an adjustable vacuum advance can.
Just off hand i would say that 73 is way rich for a 750 carb
You only want about 32-34 degrees max on modern heads. Some of the fastburns are even down to 28 degrees max.
Are you running flat top pistons? If you are, California 91 octane with your high compression you want to set your all in closer to 3000-3200 rpm and limit additional vacuum advance to less than 8 degrees with an adjustable vacuum advance can.
Just off hand i would say that 73 is way rich for a 750 carb
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
I'm using the MSD tach drive distributer
Should the total timing be with the vacuum advance hooked up?
I'm a little confused by the terms initial and mechanical in the same sentence, I thought it was initial(mechanical) and total(vacuum advance added)
Lots more to timing than I thought!
#73 jets were in the carb when I got it(used), I'll try #71's see what that does
Engine runs at 11" vacuum @ 700 rpm idle/park, the power valve in the carb has a 6 and a 5 stamped on it, I take it to mean it opens at 6.5", is this correct?
Thanks for your help
Should the total timing be with the vacuum advance hooked up?
I'm a little confused by the terms initial and mechanical in the same sentence, I thought it was initial(mechanical) and total(vacuum advance added)
Lots more to timing than I thought!
#73 jets were in the carb when I got it(used), I'll try #71's see what that does
Engine runs at 11" vacuum @ 700 rpm idle/park, the power valve in the carb has a 6 and a 5 stamped on it, I take it to mean it opens at 6.5", is this correct?
Thanks for your help
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
That is 6.5 which should be fine.
measure the "total" advance with the vacuum disconnected. Dial back timing light work great. this number should be 32-34. Vacuum at 3000 rpm might jump it to over 40.
Automatics like high initial.
measure the "total" advance with the vacuum disconnected. Dial back timing light work great. this number should be 32-34. Vacuum at 3000 rpm might jump it to over 40.
Automatics like high initial.
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Originally Posted by DZRick
Damn, the more timing posts I read the more confused I get!!
Ain't that the truth!
Think I have enough to get me going.
Thanx to everyone that responded
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Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Make just one change/adjustment at a time.
Just my 2 cents here. Troubleshoot and dial the ignition first then nail the carb.
Or tune the carb first then the ign.
Most important here is make only 1 adj at a time - and look for the results to mark your progress. Even just 2 adj at same time can throw things out of wack and ruin all progress - needing to start over.
Hope this helps.
cardo0
BTW did you get to look at the bottom end with pan off?
Or tune the carb first then the ign.Most important here is make only 1 adj at a time - and look for the results to mark your progress. Even just 2 adj at same time can throw things out of wack and ruin all progress - needing to start over.
Hope this helps.
cardo0BTW did you get to look at the bottom end with pan off?
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Haven't taken the pan off yet. I've only taken the car out 3-4 times trying to tune it. Now I'm having some electrical problems(quality control issues with the new turn signal switch among others) I need to fix before I can go back to the motor....
The fun never ends, does it
The fun never ends, does it