Hello everybody been awhile since I’ve posted. I’ve got a problem with a rich running 750dp on a new engine and can’t seem to sort it out. The motor is a 357 sbc MSD pro billet dizzy brodix IK180 heads JE Pro forged pistons(10.5:1 compression) forged gm crank, forged eagle h beam rods, comp xe268h cam, 1.6 ratio rockers 1 5/8 headers 3 inch exhaust with straight through hooker mufflers and an edelbrock performer rpm intake. I didn’t break the motor in a buddy of my dads did on an engine stand so timing I’m not sure of, but the balancer is marked 28 from the engine shop and that’s what he set it at. The carb is a Holley 750DP 04779c and I’m the stock configuration with a 28primary discharge nozzle(70jet) 31Secondary(80) jet and the stock pink cams it would foul plugs like crazy vacuum gauge shows 17 at idle and stays consistent no matter the idle mixture adjustment(they do kill the engine if turned in all the way). The car weighs about 3800 with me in it and has a built Th400 from Vickers in Ohio and a built quick performance 9 inch with 3.55s. I had an air fuel gauge welded in the passenger collector so I could try and get a handle on it without going through 35 dollar ngk plugs every couple weeks. I had a bog off idle when I matted it so I changed discharge nozzles and that went away but didn’t solve my plug issue after the afr gauge install i could see the car was cruising around 10.3-11 and if I stuck it it was flashing lean then bottoming out the gauge. I went down to 67 on the primary and 78 secondary with little to no change plugs were completely black so I’ve been stepping it down on the secondary to now it’s at 73 and the plug from number 8 was white after 3 miles and one hard pull with afr showing 11~also I’ve noticed if I get into the secondaries at all it will start popping from the exhaust like gun fire. I’m lost on what I need to change and don’t have anyone local to help. Ive got a video to show the pull from today with the 67-73 jetting 28 primary dn and 31sec with pink cams in both just don’t know how to upload it any help would be awesome
You need to find out where the timing really is and what it is doing at what rpms. All mechanical or vacuum advance? Without correct timing you will never gat air fuel correct. The plug you show doesn't look too bad. You will probably find the factory jetting will be close once you get the timing set right. You need around 38-42 total when advance is all in.
That’s what a guy at the muffler shop said when he put the bung in the collector, but it was right before the bend on the strap on the plug. Here’s a pic of an old plug from last week I was also told not to use aluminum anti seize and to switch to copper which I did today the balancer doesn’t have a timing tape the block does have just a pointer should I put the motor on tdc and add a timing tape to the balancer?
You covered the bog with fuel I would have given it more timing, 33-35 aluminum heads and 35-38 iron heads, total timing all in by 2200 rpm.
Great I’ll get a light on order and a timing tape for the balancer along with some springs for the mechanical advance in the distributor
If you get a light with a dial, just set the dial to 35 and crank the distributor until the timing mark shows -0- you don't need the tape
Okay great thanks for the tip also I checked the distributor box and it has all the springs and weights. The pack was open so I think the engine builder changed out whatever was stock in it.
There are a few threads here on how to start your carb setting especially the rear. Since it’s a DP you will need to start correct. This can be part of your rich problem as you are idling on all 4 holes…
I checked my transfer slots the rear isn’t exposed at all and the front is as far wide as it is tall that’s what I read on Holley website I’m not sure if that’s what you’re referring to. Still trying to learn about these things it’s pretty much new to me. Most of my older cars had the carb stuff worked out the only one I’ve ever had to fool with was a quadrajet and it ran great after I rebuilt it
If it has 4 corner idle screws there is also a screw to slightly open the seconsary blades at idle, it will lean out the the secondary idle circuit "similar to how the front primary circuit works" The bad news is you have to remove the carb from the intake manifold to adjust it from the bottom.
It does have 4 corner idle and I’ve had it off like 4 times for jet changes so I’m getting good at it lol
Sometimes you can be on the wrong side of the fuel curve and be lean. When you think your rich. Lippy
I can't remember but can't you take the setscrew out of the base for the secondaries and screw it in from the top or something so you don't have to take the carb off everytime to adjust the secondaries? Lippy
If I come to a stop and punch it off idle to wot and it flashes lean and then goes rich on the afr gauge should I increase the primary or secondary discharge nozzle? The afr gauge shows 12.6(idle)-flashes 15.6-falls to 10.6 you can hear it start clear up as it gets moving then it shows 11.5 on the 2/3 shift and 12.2 in 3rd before I lift
When an engine is modified, each cam, compression ,etc requires a new timing curve. It's not just initial timing and total advance, it also has to have the correct advance as the rpm increase.if the engine has a multi spark ignition, the plugs should not be fouled if multi spark is working properly.
Here’s a plug from before any changes to the carb everything was the same on the car besides having the stock 8.2 10 bolt with 3.08 gear and open diff. The cars stored in my carport which is attached to my house so only one side is open to the elements I’ve been wondering if the distributor cap could’ve gotten moisture in it but it’s not acting like that
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