So, I haven't isolated this as the problem yet, but I may be running at least somewhat lean, and it maaaay be because of the jets I have in right now, so I figured I'd get this thread going, as I pop off and apart my carb.

I have a Holley 600, vac secondaries, electric choke. It's on a 408, with flat top pistons and a stock cam set straight up, for a static compression ratio of 9.2, and a DCR of 7.97. Stock open heads. Slightly heavy valve springs. Headers into duals. Weiand dual plane intake. Highway is 2600 RPM, back roads are 2200, idle in gear is 700. The only time I ever take it up over 3000 is horsing around, but I would like to have some power up there....

What size jets should I be running on this? I know it is totally down to trial and error on this, but I'm curious to see what numbers my jets are currently, and if I'm even within a reasonable range.

Thanks, AleX

 

That's almost an impossible question to answer.

The best way to figure out what an engine needs is on a dyno stand. An alternative is on a rolling dyno and hooked up to an emissions analyzer that can spit up air/fuel ratios by RPM. Other than that, your guess is as good as mine.

 

Stock should be sufficient to get around... should be around 68.

I had 62's in my dual Holleys.

How many miles on the engine? Have you checked the lift on all the rockers to make sure you haven't wiped a cam lobe? It happens all the time these days.

One moment the engine is spectacular, go in for gas or a bite to eat and come out and fire it up and the engine runs like butt.

Josh

 

What causes the lobe to flatten? is it because of the lack of zinc?

 

Zinc is part of it, but also bad batches of lifters and bad cam cores.

Also modern cam lobe ramps with aggressive spring rates is probably the biggest reason.

Running a diesel grade oil whether 10w30 or 15w40 is considered sufficient for the most part after breaking-in. I am biased towards Rotella, but Delo and Delvac are great as well.

Josh

 

oh ok. Ive been hearing horror stories of this happening a lot lately also, and everyone seems to blame the zinc. I wasnt aware of the spring rates/ cam profile relationship having anything to deal with it.

 

Well mine had 65's..... I know this carb was set up for a plow truck 351M by the PO, and he knew his stuff, so it would seem that he was probably going for economy on a somewhat smaller, rarely used motor.

I'm gunna go to the store and see if I can get some 70's or so, pop em in, and just see what that does for me.

Simple simple test at this point.

I haven't checked for wiped lobes, but this was an old stock cam, and brand new lifters, and I broke them in carefully. The engine is running fine also, just pinging. I don't get the sense that I have something like a whole valve not opening.

I'll letchu guys know though, thanks for the help so far.

 

The secondary circuit on my carb uses a metering plate, not jets. Will swapping in bigger jets in front but keeping the rear the same even change anything?

Also, how does idle mixture screw adjustment play into all of this? Is there no fuel flowing through the main jets at idle? Do the mixture screws only adjust the amount of air getting in? Could I richen up my mixture noticeably just by turning the idle mixture screws?

 

I have a 600 Holley on the 400(stock) in my Bronco. I think I changed the jets to 72(I can't remember for sure). The jets it came with were to lean(I think 68) plugs were white and it poped out the carb when you got on it. All problems I had were fixed when I changed jets, runs great now(plugs nice tan color). Secondary plate I didn't have to change it was perfect out of the box.

 

Well that's comforting....

2 parts stores beat me to closing today so I didn't get to grab any new jets, which is frustrating, but if 68's were too lean that makes me feel better.

What do you get for gas mileage?

 

Hmmmm, 72's are .079", and a 5/64 drill bit is .078125"...... I'd hate to ever want these 65's again but......

 

I may not be the most educated mechanic, and I didn't stay at a Holiday Inn last night, but I don't think I am too far fetched in saying:

THERE IS NO SUCH THING AS DETONATION DURING DE-ACCELERATION

The throttle plates are closed, so hardly any fuel is getting drawn thru the boosters, but rather thru the idle bleed circuit.

If it doesn't ping under load and doesn't ping on an aggressive acceleration in top gear, then the engine isn't pinging from detonation.

Take a video and slap it up on Youtube.

Josh

 

The 72's are just what I had laying around from another carb so I put them in. Seemed to be just right though(going by plug color and how it performs now compared to the other ones).

I have never checked the mileage so I don't know, 10 maybe.

 

I can try to get a video when it's running again. But if the idle circuit is all that's feeding the engine during deceleration, then I should just be able to crank out my idle mixture screws a little, right? Maybe it's the idle circuit that's very lean?

 

If the idle circuit was too lean the engine wouldn't even idle.

Josh