Dist/carb tuning question

Would have to check my old Holley book on carby sizing and V/E. Most stock engines only run about 85% VE. But considering a 351 cu in motor handles a 600cfm carby you would think that a 454 cu in which is approx a 30% increase in volume would require a 30% larger carby i.e. 780cfm carby.

I forgot it is GM so obviously wouldn't be as efficient as the blue oval hardware.

What model was your 750 Holley. Was it a square bore or did it have extremely large primaries like q-jet.

I think you should have been able to dial it in OK may have required some changing of jets & PV's. I think at one stage you said the vac secs were opening early. This may have been your problem. Possibly a stronger spring may have solved problem.

Maybe to late now if you have ordered a 650cfm anyway. The smaller carb should give better economy anyway.

lewy2001 wrote: Would have to check my old Holley book on carby sizing and V/E. Most stock engines only run about 85% VE. But considering a 351 cu in motor handles a 600cfm carby you would think that a 454 cu in which is approx a 30% increase in volume would require a 30% larger carby i.e. 780cfm carby. I forgot it is GM so obviously wouldn't be as efficient as the blue oval hardware. What model was your 750 Holley. Was it a square bore or did it have extremely large primaries like q-jet. I think you should have been able to dial it in OK may have required some changing of jets & PV's. I think at one stage you said the vac secs were opening early. This may have been your problem. Possibly a stronger spring may have solved problem. Maybe to late now if you have ordered a 650cfm anyway. The smaller carb should give better economy anyway.

I agree the 750 is more appropiate

but I dissagree regarding VE, the VE on the BBC will be vastly better than the SB ford.
The BBC has canted valves and the valves and ports are more smartly formed and sized for top end.

secondly, to send the " sh|tcan the small carb" idea home...

An undesized carb will not automatically give better economy, he will be into the secondaries sooner in the throttle. this also gives less precise throttle control due to the vacuum actuation.

thridly, the undersize carb can be harmful for the engine. say if the engine were to produce 2.5" of vacuum at WOT, this could shut off the PV and make it run 25% lean at max rpm and load, this could frag a piston.

like has been suggested, one can't fine tune a carb till the timing is better addressed, and that would be difficult to do properly and safely with an undesized carb.

Thanks all for the input. I probably don't word things as well as can be, but I think it's 24 degrees of advance at 3000...so at 14 initial I have 38 total at 3000?

Tom, thanks for helping to look after me; I read what you have to say closely! You and Lewy make me think I have the right carb.

Symptoms as follows...
The 750 had the motor starting immediately where the 600 wants it to turn a few times before it hits. The 750 ran rich(sooty plugs) and stumbled at 36-3700 rpm's with a 7.5 PV and stumbled at 38-3900 with a 4.5 PV.

The 600 runs up to 4100 but it feels like there should be more. It also stumbled out of the hole with 6 people on board, but behaves with 2 on board. I'm not the pro like yall, but my gut feeling there was that it was thirsty so we parked it after 1 run. The gas mileage has gone down drastically like Tom said. I haven't been running it much since the 600 has been on it...not much at all really due to break in concerns and what I've read here about running lean.

All that said, I put the black(strongest) spring in the vacuum pot and put in a 2.5 PV. From here I'll do 1 thing at a time and see what happens.

It may be a while before I report back(busy,busy), but thanks again for all the help!!!

I would but the 750 back on get a real timing light and found out exactly what the timing is at idle, 1000 rpm, 2000 rpm and 3000 rpm and quit guessing what it is or what it is supose to be know exactly what it is by verify it,, engine tuning 101 kids that chase tails.

the stumble at higher rmp is because of the carb and it's settings/jetting/metering of the fuel/adjustments use smaller primary jets and the 2.5 PV in it so the secondaries don't kick in and flood it out or riching it up to early. To bad your not closer to Indy I'd have you dial in an a hour or two depend if you had the right jets or had to go get them.

you still need a miniumum of 18 inches of vacuum at idle, did you compare between the 750 and 600?
your playing the carb game, there is no way you can dial an engine in with 13 to 14 inches of vacuum, you need to get the engine to 18 and no less, whether you have a severe vacuum leak or carb size.
its the same as you trying to dial in an engine with 50 lbs of compression, vacuum also pulls fuel into the 2 front cylinders and i would guess those 2 fronts are running a little lean or not as rich
remember......what controls secondaries on your boat a a a compression? cable? solenoid? your wife sits by the carb and you tell her to open them up?
first things first my friend

Im hammering you, lol
what is the first thing the doctor does when you go there? and what are his 2 most important tools? a stethascope and a blood pressure guage.   

when he checks your blood pressure and its off the charts does he go on to the next step....no, if your heart is beating at 200 beats a minute, to the next step? no

a compression guage and vacuum guage are nearly equal to these tools the doctor uses

if your compression is bad are plugs going to cure the problem? no

if your vacuum is low are plugs going to cure the problem? no

Vacuum is as important as compression, with out either one the engine WILL WILL not run as it should, so take care of first things first

lewy2001 wrote: In saying that it starts much easier on the larger carby suggests that the main jets may be a little large.

FAIL. If he's starting the boat on the mains, he's got bigger problems. Thats just not happening.

Other than that your advise is spot-on

OK then, it's all lack of vacuum. Good call Eric. I sat in the back and opened the secondaries manually at WOT, and it ran as it should.

Where oh where did my vacuum go?!!?

exactly as Doc is saying, the intake stroke creates vacuum, the compression strokes creates....guess what? compression.
look at it this way, if you were a tooth off which i doubt you are, the piston will come down and start its journey up, if the valve is still open she will fall flat. also if you took your shop vac and put your hand over the end and completely closed it off the vaccuum increases, as you open your fingers the vacuum will decrease.....its the restriction that creates vacuum,
there is nothing you can do electronically or fuel wise to create vacuum, basically it is controlled mechanically. as earlier stated i wouldnt try anything until that guage reads 18 to 20 inches of vacuum at idle whether it be cam timing, rings, carb or so on
an engine will miss all day long with the kind of vacuum your seeing and thats why i said to you earlier, what controls secondaries......vacuum, low vacuum early open

pop that 600 back on and check the vacuum, easiest things first, im not saying that is the problem but.....if you dont try it you will never know.
I go through this stuff everyday, I have a guy who comes around my shop all the time and he costs me nothing but money, he insisted the other day that a fuel pump was bad on a truck because he said the same thing happened to him, so i told him if i drop the tank removing all 30 gallons of gas, swap out the assembly and it doesnt work will you pay for the 400 dollar assembly?
I logically troubleshot the thing and couldnt pick anything up on my scanner, the fuel psi guage was doing funny things, it took me 4 hours to figure out it was the psi regulator under the plenum
point being, anybody can swap parts and eventually you will hit the nail on the head but in the meantime if i would of put a fuel pump in this thing i wouldve had to pay for it and the labor
approach it like a see-saw, if that side goes down what will the other side do?

Not much else to check...guess it'll go in the garage soon while I can still get it out...

Whoa where the heck have I been, I figured you were off living the Nautique life whilst I had been on shore, sorry to hear thats not the case. Before you get absolutely crazy are you 100% sure your plug wiring is correct.

if you think its cammed wrong you'll be able to pick that up on a compression test, the low vacuum is a sure sign that something is not quite right.
Why do you think you may have the wrong cam

If it was only running on 5 cylinders you would have very rough idle & you would not be getting anywhere near the 4000rpm you are getting at WOT.

You have verified the timing curve manually with timing light? I assume so but have not seen you confirm that in the post.

You mentioned earlier that the secs were opening too early now it seems you are saying they are not opening at all on just vacuum.

Have you verified it is low vacuum with a gauge?

One possibility is that your rings may not have bedded in correctly yet so compression & vacuum are both low.

If so have you tried lighter springs in vac secs?

An old trick that I have seen done that may help you troubleshoot this problem. Is a small nut & bolt in the slotted cam on the secondaries butterfly shaft so that at large throttle percentage it manually helps the sec's open. The primary shaft has a linkage that connects to the secondary shaft with the slotted cam. The small nut & bolt uses this linkage as a form of mechanical secondaries. Similar to your string over the shoulder idea.

Hope this is of some help I have the feeling you are becoming frustrated. Maybe 79 will pay a surprise visit and sort it all out.

Wish I was closer so I could lend you a hand often two heads working together helps.

Bit like a dual core processor.

I re-verified dist rotation and firing order. The compression #'s mean the cam is right?

I did go down to the next to lightest spring in the vacuum pot the other day. I have the lightest yet to try, but I wish I understood the vacuum loss.

87(Jim right?) does your set up have a wedge plate with the pcv plumbed off of it? I think I'll take mine off a check the #'s...I'm sure it'll cause float probs, but that's par for this course.

That leaves the intake gaskets and rings that may not be seated...right?