Jets are gettin big!

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CurtDawg510

CurtDawg510

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So i got a 70 duster with a ratty 360 in it
Got about 245deg of duration and 107 deg lobe seperation, 10;1, 3.55 gears, 25" tire (street car), 26" 3row raiditor, 8 blade water pump, 160 high flow thermostat, dual 12" spal fans (it sucks a pice of paper to the grill easy).... Timing is set 10deg at idle and 32deg max (im guessing around 2500rpm)
So i got a 4150 holly 750 dbl pumper, ive got it idleing on idle circuit, powervalve is dailed in, im stumped on the main jets (primarys) .... I started at stock 70, temp went to 200-210 quick, and about 14.5" of vaccum at 40mph...took her back home and swaped in sum 74s.... Temp droped around 190-200 and vaccum picked up to about 15.5".... Swapped in sum 78s and temp stayed about the same and picked up another .5" of vaccum
How much more gas is this small block gunna need??
I would like it to run 160-170 on 70deg days, so it can survive in the 90deg traffic days!!
Or is that asking for too much??
 
245 degrees of duration? Advertised or at .050?

If it's .050, you don't have near enough initial timing. I'll guess stacking issues inthe carb due to a poor idle tune up.
 
AGREED TIMING WAY TO LOW.18-20 initial 36 total.Probably to big a carb.Try a 650 dp
 
Ill try movein the timing up sum... How much is too much total timing? 35-36 deg?
Wouldnt advancing timing make it run even warmer?
Thanx 4the imput guys
 
You will probably end up around 20* initial.
 
Sweet, ill have to turn up the timing n see what happens... Its got a stock dizzy rite now, ill prob end up gettin msd or something aftermarket thats easy to recurve
 
Cylinder heads?

If it's a quench engine, i could see 32ish. If it's a stock iron head, that's pretty low in most cases. We do have dogpiss for gasoline out here. :)

245 at .050 it will likely want 22-24ish for initial timing. 20 is a good starting point.

Simple test, start car, turn distributor a little CCW. If it picks up RPM, it wants the timing. Reset idle speed, do it again. Keep twisting it a little at a time. Find the point it doesn't pick up. Turn it off when warmed up and see if it restarts easily. If it kicks back or rolls over slowly, back out a little at a time until it starts at a flick of the key. Put light on it see what you have for initial timing.

If it's not pinging under load, start turning timing into it and find the point it starts to rattle. Back it off a couple degrees and get a timing reading. Rev it up until it stops advancing. DO NOT pick a number and think that's it. You may have some advance above your picked RPM number. Use a light and verify. My friends killed and engine doing the 2500 number...

Good luck with it.
 
crackedback said:
Cylinder heads?

If it's a quench engine, i could see 32ish. If it's a stock iron head, that's pretty low in most cases. We do have dogpiss for gasoline out here. :)

245 at .050 it will likely want 22-24ish for initial timing. 20 is a good starting point.

Simple test, start car, turn distributor a little CCW. If it picks up RPM, it wants the timing. Reset idle speed, do it again. Keep twisting it a little at a time. Find the point it doesn't pick up. Turn it off when warmed up and see if it restarts easily. If it kicks back or rolls over slowly, back out a little at a time until it starts at a flick of the key. Put light on it see what you have for initial timing.

If it's not pinging under load, start turning timing into it and find the point it starts to rattle. Back it off a couple degrees and get a timing reading. Rev it up until it stops advancing. DO NOT pick a number and think that's it. You may have some advance above your picked RPM number. Use a light and verify. My friends killed and engine doing the 2500 number...

Good luck with it.
Click to expand...

Thank u, i heard 34-36 deg is about max, and thats where i org had it set, then it wud go down to 10 at idle... I put 74 jets back in (i was tryin 70-74-78-82, 74s were runnin best at that point) and cranked idle timing up to 18, it max out around 39.... I took it for a spin and its got more power, crispy throttle, had 2" more vaccum, and ran at 170 deg!!!!! :)

It kept revvin up as i was spinin dizzy at idle, im sure it can use more...shud i keep spinin it till it boggs out a lil like how u said above... Shud i b worryed about that 39 deg jumpin in the 40s? Is it time for a recurve of the dizzy? Im planin on gettin a msd dizzy soon
Thanx
 
And stock 360 heads with 1.88/1.60 valves (for now).... Ill b lookin for sum good x or j heads with 2.02s later, open them up and slap them on... It started as a "budget build"
 
Give it as much initial as it will take with out kicking the starter back. You will need to recurve the dizzy for total timing. As far as jets I think you are pretty close with 74 primary. When does the temp come up? Idle or cruise?
 
Idle it runs 160 all day rite on the number... When i cruz it climbs up to 180-200, ive seen it as high as 220... I jus turned up the timing to 18idle/39max and took it for a lil spin and it ran 170...so i think im on the rite track with timing, but how far is too far for max timing?? Or is their no set number, jus back it out when it starts pinging??.... Im runnin kb-107 pistons with stock 360 (1.88/1.60) heads,67cc chambers (i think) machine shop said compression is rite around 10:1...air gap intake with holly 4150 750
 
And all driveing is 50mph or slower, never took her on the highway yet...gotta dail this in 1st
 
Ok, Im willing to learn.
I have never heard of tuning a fuel curve on an engine of this caliber using cruise temperature.The reason I doubt its accuracy is because at any legal cruise speed the carb is not even maxing out the transfer fuel supply yet, never mind the mains. If you doubt that, put some 60s in there and I guarantee you can cruise at 60 mph with them. Getting to that speed is a bit of a slow process, tho.
I have had a combo like yours but with Eddies on it. I even ran a plug in the PV port.And all kinds of different MJs. None gave me temperature troubles. But then my timing was already pretty close.And I was running a mechanical fan.(Before this is over I think you will be too).
So, help me out. Where did you get this tuning idea?
 
Insufficient timing will cause exhaust temps to increase. Lots of water passage area around the exhaust ports.

Keep advancing the timing. It could take 40+ degrees of total without any vacuum advance.
 
I figuered i had timing rite when i started, (10idle/32max)... Thats what the machine shop said the engine wud prob like.... So i thought it was gettin hot from runnin lean...i put the jets back to 74s and cranked up the dizzy to about 18idle/39max and the temp droped down to 170ish on the next drive.... Now i think i need a dizzy with less of an advance sweep, or even lock it out
 
As suggested, find the point where the engine kicks back on the starter when hot and use that as your initial timing point. 18 is a bunch closer than where you began.

You also need to find the total timing number the engine likes. Simple math from there.

An MSD is super easy to dial in with a bigger bushing than msd supplies.
 
If you get your timing set and jetted and temp still climbs you may not be getting enough air flow through the radiator at speed. Get everything else dialed first though and go from there.
 
I do not have NEAR the camshaft you do......But I have pretty much everything else similiar to your setup.

4777-2 650 double pumper., with Proform 750 chokeless center section
89' Roller Cam 360, gasket matched heads/intake, otherwise STOCK
True Dual Exhaust, with 340 manifolds
Weiand Single Plane X-celerator intake, no spacer....(you made no mention of which intake your DP is sitting on)
Champion 26" Aluminum Radiator......180* thermostat

My DP is jetted as follows:

72- Primary
6.5 PV
84- Secondary

My engine idles at 750/800 rpm....never gets over 195*....even in 100* heat.

My timing is set at 11 degrees intial, with 34 degress total.

My setup flat out screams....no complaints here....

Your not too far off....a Wideband o2 sensor, and some A/F tuning might be of some benefit to you.....do You have a Chassis Dyno shop anywhere close?
 
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