Hey guys, i'm still having a weirdness with my nova. When i advance the timing to 10degrees, it wants to break up a bit at 1/4-1/2 throttle. But i'm haring it more than feeling it. Maybe i'm running too lean???? its pops out the exhaust when i'm slowing down too sometimes. But runs smooth when on it. I just had pipes put on and dumped at the axle and they are ashy inside. Not black. I'm going to be pulling the plugs tomorrow to take a look. If i have to jet it up, what do you think i should try? I dont know much about carbs and dont even know what it came with jet wise... But i cant run the car with good advance, so i'm not getting my full HP potential.
Thanks
Ed
By the way the carb is a 750 vac sec. WIth elec choke...maybe 2 years old. The choke is stuck open. I'm not sure of part number...thanks again
Ed

I would definitly richen up. Go up 2 jet sizes and see if that helps. It may need more. At WOT your getting mixture enrichment from the power valve so WOT may be getting enough fuel. The jets set the cruise and high vacuum mixture. The Power Valve Restriction Channels (in the metering block and not really user adjustable) control the mixture at WOT when the PV opens.

You might try running a vacuum guage into the cockpit and see if the PV is opening at part throttle, causing a rich condition. The carb probably has a 65 PV (pretty much standard on most Holleys). If, at 1/4-1/2 throttle, the vacuum drops below 6.5", the PV is open and you should install a lower value PV. I suspect, though, that it is a lean condition and jetting up will solve the problem.

Hey Hatboro
How big is the cam and what is the manifold??

My cam is a Blue Racer 575ish lift...300ish duration. Not 100% sure but thats what the builder told me. The manifold is a Team G.
I'm going out tomorrow to go get hose for the vacume guage and see what that scores me. I will reply tomorrow with what i have and let you know for sure. what jets do you think i should get!??!?! should i get some for the secondaries too? or are they ok because its getting enough at high rpm? I have an 850 dp....but i think that will be too much for the motor. DO you!??!

The 850 will work,it might not be any faster but it will work and it will run richer on the low speed circut which is controlled by your transition slots and idle fuel bleeds.That cam is pretty stout it might only make 5" of vacuum or so at idle.Like Pat stated you need a Power valve lower the that.If you want a hand my email is igrinditshrp@netscape.net

ED,it does sound like it's a lean cond so check into the jets . That's a large duration cam so also check into the power valve too as mentioned above with the cam your running.

Also,with 383 cubes,14:1 comp (did i read that right?),large cam with a lot of duration/lift ,and all that gear (4:56),that 850 dp would not be overkill but the the 750 vac secondary is more than adequate if setup correctly. If you have stick/manual trany and run the car hard or race it i would try out the 850 dp but if your running auto i would stick with the 750 vac.

Scott

Scott, Yes you read that correctly...lol. I talked to my builder today and he said that the first cam tha twas in the motor before i bought it was that cam. I bought it from someone that worked there and went to school. He wiped a lobe and they put a simular cam in he said he thinks its a 550 lift....290 duration....he didnt order it the other guy did so hes not sure. I tried to look at the blueracer cams but nothing goes that high in their cataloge. Oh well....it has a nice lump so i'm guessing its that one. The 850 eats too much gas tho ;) ;) haha.... the 850 needs a rebuild and i have a automatic tranny so i think i wanna stick with it. But still not sure what jets to go with, and where at. Mains and secondaries or just mains. I hate carbs haha

1968 hotrod, where are you from in pa??

I'd go with a double pumper, sounds like a racecar to me with that kind of compression. Not sure where the vacuum sec./auto or dp/stick comes from..for all out performance you want a double pumper.

You'll probably need to pull the front bowl and see what jets are in there. Get some Holley blue bowl gaskets before starting. They are reusable and don't stick. The most economical way to buy jets is in the jet kit, Jegs gets about $40 +handling but you might find them cheaper elsewhere (I got mine from SoCal Speed Shop (they advertise in Street Rod mags) for $25 a few years ago). Pairs of jets are $5-6 each.

With the big cam you have, it undoubtly needs more fuel. I'd be careful driving it until it is jetted right. With a lean condition and very high compression, detonation is a very real possible and could burn a hole in a piston.

Unless it has been converted to a metering block, the carb has a metering plate in the secondaries. The plate doesn't have jets. It is non-adjustable and to change the sec mixture, it needs to be changed. There is a kit available to convert to a block that uses jets.

Take a look on the choke horn and post the numbers you find there. Many here have Holley catalogs and can look up what jets the carb came stock with. However, that may not be what is in there.

<edit> I agree with Bob, a DP would be a better choice with the cam and compression you have. A properly setup DP will get the same mileage as a Vac Sec carb, as long as you stay out of the secondaries. The primary circuits are identical. Also, as I said before, it sounds like a lean condition. That would explain why increasing the timing to 10º causes the problem. The engine is detonating. I suggest getting the distributor re-curved. With the cam and compression, you need to have the ignition under control.

Ok in that case i'm gonna get the 850 then. I dont wanna ruin my motor....the 850 is a chokless carb and its suppose to be jetted up already. I'll get that and work with it from there. Where do i set my idle screws on that? i'm gonna order the clear bowl views so i can set my floats i always had a problem with the rear float in that thing. Incase you were wondering the number on the 750 is 80508-1 and 1B61. I put the carb on right outta the box new. So nobodys touched it. It should have the factory jets.

My distribtor is an MSD billit with no vacume advance. Any advice to what springs to run in that? its suppose to be already done up from its stock form. But i wanna see what you guys think.
Ed

I'm in Warminster.

I'd advise not using the clear sight plugs. They have a tendency for the heads to break off. Also, if you ever had an underhood fire, they will melt and pour gas on the fire. Plus, they fog up. You can use them to set the idle but put the brass plugs in for operation.

Try about 3/4 of a turn on the idle mixture screws to start with. Before you install the 850, turn it over and check that the primary blades show about .040" or less of the transition slot. There is a screw on the underside that adjust the stop for the secondaries. Set the secondary blades to show the same amount of the transition slot as the primaries. If the idle speed is too low set like this, try increasing the spark advance. This will raise the idle speed and allow the transition slots to remain covered.

Does the 850 have a 4 corner idle system. It will be easier to get a good idle when using a large cam. If not you can still get a good idle, it just takes a bit more work.

Jetting is something the factory cannot set for you. Expect to do some fiddling smile.gif .

The MSD probably has a good curve in it. You can check it with a timing tape on the damper or a dial back light.

BTW, the 750 comes stock with 72 jets in the primaries, a 65 PV and .025 primary squirter. There are so many 850s that without numbers there is no telling what jets it comes with. My 350, with an 830 DP and a 296º/305º/108º cam, has 78s in the front and 86s in the rear (no rear PV). Hope this helps.

Hey guys i got the 850 now. I lent it to a friend for his BB. The numbers on the 850 are 4781. Mortec says:
(4781 850 (F)80,(R)80 6.5,6.5 4150)
I'm guessing i'll have this one checked out and run it as is with the 80's in it huh?!
Plus its saying 6.5 twice for the power valve. Does this mean it has two? and if so, do i change them both or just one? I dont know carbs that well...just kinda learning about that. I'm probably gonna take it to a guy that i know around me. Let me know what i will need and i will tell him what to do. Beats me doing it and makin something worse. Plus what does 4 corner idle mean? how can i tell if i have that or not?
Thanks again
Ed

Ed,
If you want a hand email me your phone number since I live close or I can talk you thru this over the phone.igrinditshrp@netscape.net
If you want to tackle this yourself its no problem either

ttt

According to the Holley catalog, there are 8 4781 (-0 through -7) carbs. The primary jetting is 80, the sec could be either 78 or 80. All have 65 PVs front and rear with 31 squirters at both ends. I don't see any other differences in the spec sheet. Looking at the picture, it looks like no 4 corner idle system and it looks like they have downleg boosters. This could cause the carb to be a bit less responsive off idle since DLs have less vacuum amplification. They are much better than straight leg boosters at higher rpm (less restrictive).

There are two 4781's pictured in the catalog. The "C" model and the "S" model. The "S" is a shiny silver color. It says the "S" has a 50cc secondary pump, I would guess the "C" does too but it doesn't say so. Take a look at yours and see if both pump housings are the same size. If the rear is larger, it is the 50cc. If so, you will likely need to use a 1/4" spacer to clear the manifold. As I said, be prepared to re-jet it. You won't know until you run it a bit. You will likely need to dial-in the accelerator pump system, too.