Engine tuning / is this right?
Thread Starter
Instructor
Joined: Jan 2003
Posts: 244
Likes: 0
From: Thomson Illinois
Engine tuning / is this right?
Greetings all.
I have the 383 together and she runs! I have not drove it much at all yet, the engine doesnt seem "right" (this is where a lack of knowlege shows itself!).
The engine is really "lumpy" and hesitant. But yet smooths right out at a higher (1500 or so) RPM.
I have read, and re-read posts here and on Corvettefaq and so far, I am not able to make it smooth out (within reason) or "come alive".
Here are my specs"
4.030" bore
3.75" stroke
5.7" rods
.150" dome pistons
timing gear drive set at 0
cam - .480 int. - .480 exh. / 230*dur @.050 hyd.
76cc heads w/1.94 int / 1.5 exh match ported
Crane single valve springs.
1.5 roller rockers
std. length push rods
Flow Tech headers
7530 Team G intake
Holley 750 Dbl pumper mechanical sec.
New distributor cap, rotor, coil, wires and plugs (AC - R45S).
93 octane pump gas.
All components are new/never used (exc. block, heads and crank)
The distributor is the original stock dist (which I do plan to replace).
At 850 RPM's I am setting at 10*btdc and 11 on the vacuum gauge.
there seems to be no hesitation above 2000 RPMs. These numbers are not driving coditions, they are driveway/garage conditions.
Should I be able to smooth this out a little and take the hesitation out of it?
Thanks!!
KC
I have the 383 together and she runs! I have not drove it much at all yet, the engine doesnt seem "right" (this is where a lack of knowlege shows itself!).
The engine is really "lumpy" and hesitant. But yet smooths right out at a higher (1500 or so) RPM.
I have read, and re-read posts here and on Corvettefaq and so far, I am not able to make it smooth out (within reason) or "come alive".
Here are my specs"
4.030" bore
3.75" stroke
5.7" rods
.150" dome pistons
timing gear drive set at 0
cam - .480 int. - .480 exh. / 230*dur @.050 hyd.
76cc heads w/1.94 int / 1.5 exh match ported
Crane single valve springs.
1.5 roller rockers
std. length push rods
Flow Tech headers
7530 Team G intake
Holley 750 Dbl pumper mechanical sec.
New distributor cap, rotor, coil, wires and plugs (AC - R45S).
93 octane pump gas.
All components are new/never used (exc. block, heads and crank)
The distributor is the original stock dist (which I do plan to replace).
At 850 RPM's I am setting at 10*btdc and 11 on the vacuum gauge.
there seems to be no hesitation above 2000 RPMs. These numbers are not driving coditions, they are driveway/garage conditions.
Should I be able to smooth this out a little and take the hesitation out of it?
Thanks!!
KC
Drifting
Joined: May 2001
Posts: 1,329
Likes: 12
I'd have to hear it but my guess is your timing isn't right. Do you have the vaccum advance canister hooked up?
if the problem is that it "stalls" when you open it up, that would be the accelerator pump shot/cam.
if the problem is that it "stalls" when you open it up, that would be the accelerator pump shot/cam.
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
timing is probably not right.
Start by reading Lars's paper, it is easier than typing all of the details:
http://www.corvettefaq.com/redir.asp?site=26
Is the orginal distributor points or HEI?
Thanks,
Joe
Start by reading Lars's paper, it is easier than typing all of the details:
http://www.corvettefaq.com/redir.asp?site=26
Is the orginal distributor points or HEI?
Thanks,
Joe
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Check for vacuum leaks.
Add 6 more degrees initial advance to the distributer.
How are the idle mixture screws set? Is the idle rich or lean? Are the idle air bleeds clean? Is the float set properly?
BigBlockk
Later.....
Add 6 more degrees initial advance to the distributer.
How are the idle mixture screws set? Is the idle rich or lean? Are the idle air bleeds clean? Is the float set properly?
BigBlockk
Later.....
Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,376
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by kdcollins
At 850 RPM's I am setting at 10*btdc and 11 on the vacuum gauge.
Initial timing is not really relevant. You need to set the timing up for 36 degrees total timing, and this should come all in by 2500-2800 rpm. With 36 total, you should have about 18-or-so initial. Anything less than 16 initial will not produce good idle or throttle response with your setup. You also need to run the vacuum advance - you need a unit pulling in 16 degrees, and hook it up to manifold vacuum so you get your initial timing (18) plus the vacuum (16) all in at idle for an idle timing spec of 34 degrees.
Last edited by lars; Dec 18, 2005 at 05:10 PM.
Racer
Joined: May 2005
Posts: 470
Likes: 1
From: Amelia Ohio
Sounds like the timing may be the issue, also make sure your weights and everything under the dist. cap is free and not rusted. Looking at your compression and cam you should be lumpy at idle and the power won't come in until 200rs. Just speculating.
Chris
Chris
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Melting Slicks
Joined: Oct 2001
Posts: 3,025
Likes: 12
From: Pleasant Grove AL
Originally Posted by whizzerr
When the initial timing is set at 16 to 18, do you run out of reference on the fixed saw tooth indicator? My timing mark is approx .25 inch to the left of the saw tooth. A 1972, standard 350.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 244
Likes: 0
From: Thomson Illinois
Thanks for all of the replys , they are appreciated, but I found another issue that is probably the culprit. My cam is defective I suppose. 6 of the lobes are nearly gone already and I am under 200 miles on the new engine. They are happy to replace but I am going ot go with a different maunfacturer this time.
I have already ordered and recieved a Comp cam from Jegs today (I ordered it last night!!), it specs at: .507/int. .510/exh, .240/.246 dur @ .050 and 110* lobe seperation. Maybe it will get a little more mileage out of this one and breath a little life into the beast.
KC
I have already ordered and recieved a Comp cam from Jegs today (I ordered it last night!!), it specs at: .507/int. .510/exh, .240/.246 dur @ .050 and 110* lobe seperation. Maybe it will get a little more mileage out of this one and breath a little life into the beast.
KC
Instructor
Joined: Feb 2005
Posts: 188
Likes: 0
From: Kearney Nebraska
The hesitation could be a lean condition. Mine was going lean and would hesitate if I'd goose it or accelerate. I'm pulling 11 to 12 at idle. I changed the power valve to a 10.5. It did have a 4.5pv in it and it was opening way to late.
