Sorry I don't know all the engine specs (friend of a friend's truck)but here goes-- SB 350, cam w/.500+ lift, Holley carb w/vac sec,MSD 6a w/MSD distributor-no vac advance can.

problem he is having is a very large stumble off idle when he gets on it.Before attempting any carb adjustments, I suggested he verify his timing settings first(35-38 all in by 3000rpm) and go from there.

After putting the light on it and running it up to 4000rpm, his total is reading 48- 23initial and 25 mechanical w/the lightest springs installed.

After doing a little searching through the archives I kinda suspect that the power valve is too big in the carb first off, and it may need some fine tuning also since this guy says he doesn't know how much vaccumm he's pulling through the carb cause "he ain't got no guage"..... :rolleyes:

My question is this--- Is 48 degrees OK if he says it isn't pinging or running hot at WOT or at highway speeds?? I really want to be sure about the timing setting before I hook up my vaccuum guage and help him get the carb set up becuase he's already got a nitrous kit waiting to go on and I've warned him about having to retard the timing after we get it set in order to use the NOS.But that's on him, I just want to help him get his carb issues resolved so he can go from there...

Was that 48 deg reading with or without vacuum advance? If with, it will be lower once it's disconnected when testing and setting.

You mentioned no advance can. 23 initial is going to cause problems hot starting. Back it down to 12 initial and see how it runs.

Mike

try putting a smaller numerical power valve in, i was trying to run a 6.5 in my 800 dp, it did the same thing, wouldnt idle and stumbled, put a 2.5 power valve in and it helped tremendously. My timing is 18-20 initial, and 38* total no vacum advance and no trouble hot starting at all, she turns and fires right now when its warm, cold starts on the other hand are a little tougher, I have a "very big cam" hope this helps a little

DJD- There is not a vaccuum advance can on this MSD...

Mike- Right now we can only back it off to about 20 before it wants to die....

Nickel333- I think your right about the power valve being too big.Do you remember what your timing was at before you went to the smaller PV?
And also now that you changed yours to the smaller one, what kind of vaccuum are pulling through the carb at idle?

Thanx for all the replies so far guys..

Well being that my current cam in my 350 is 268/278 @.050 i dont think im making much for vacum at all, thats why i went straigh to the 2.5 power valve, i didnt even check it. My total timing has stayed the same, my initial backed off some but i dont remember where it was at before the power valve swap. The motor wont idle at all with less than 18* initial but going to the smaller power valve definately helped A TON and i suggest you start there. I believe the the formula for the correct power valve is 1/2 of your idle vacum reading but i may be worng so you will want to confirm that with somone. Also my car still wont idle sometimes in traffic but i think thats due to my carb not being sufficient for my combo, so im going mighty demon this spring, depending on how big your friends cam is this also may be a good route for him.

Nickle333, try richening up the idle mixture a smidgen. I backed out the screws an almost imperceptible amount and now she'll idle all day long. Before it would slowly die.

What I would do is to restrict the mechanical advance to around 15º then set the initial to where it will run. With the set up it, it probably dosent spend a lot of time under 3000 rpm anyways

Pat, ive tried about everything. She idles rich to the point of a harsh gasoline smell everywhere around it, but if i back anything off {jets, idle mix. screws, timing etc.} it just wants to either die, or hesitate once i go drive casually like its not gettin enough. The only reason i can even run this carb right now is cause we drilled .095 holes in all 4 butterflies. This helped tremendously but it still idles way too rich, before that it wanted to fall right on its face if i tried to let it idle in traffic at all. Ive concidered trying .125 holes in the butterflies. What do you think??? I also tried a 750 Proform carb a few months back and that didnt work at all, made things WAY worse. My neighbor has an 850 mighty demon with annular boosters im going to try this spring for a test run, he was having some carb problems with his holley and switched and it will idle all day long, he gets better gas mileage, much better throttle response and feel of more power in the seat of the pants. So were gonna give that a shot. Think i should try drilling bigger holes inthe butterflies first?

Originally posted by Nickel333:
i dont think im making much for vacum at all, thats why i went straigh to the 2.5 power valve, i didnt even check it. Why not check the idle vacuum? How are you setting the idle mixture? With RPM? A vacuum gauge could show lotsa things about whats going on, as well as really help with setting the mixture.....Im sure you know that; it just seems kind of strange to just throw a power valve in there without even knowing what vaccum it has at idle. Who knows- you might be surprised as to what it makes. Are you really sure that your car is only making 5hg at idle? Seems kind of strange to do ALL that work swapping carbs and drilling holes and NOT take a few seconds to check it with a vacuum guage :confused: .

Nickle, I'm using a 4 corner idle system on a 830 annular booster Holley. The butterflies have holes. The cam is smaller at 246/253 @ .050". The 4 corner idle helped tremendously. I get a very strong gas smell at idle, enough to set off smoke alarms smile.gif , but that comes with the territory. No avoiding it with a lot of overlap. I don't know the size of the butterfly holes, my carb guy did them, so I can't make a recommendation. Maybe more timing, 20-22º might help.

Originally posted by speedster:
MSD 6a w/MSD distributor-no vac advance can.

After putting the light on it and running it up to 4000rpm, his total is reading 48- 23initial and 25 mechanical w/the lightest springs installed.

My question is this--- Is 48 degrees OK if he says it isn't pinging or running hot at WOT or at highway speeds?? No 48 degrees is not ok,top end will suffer.

MSD distributors come with an asortment of springs and bushings.Pull out the distributor and remove the advance stop bushing.Look in the bag that contains the extra springs and bushings.Take the largest one (black in color I think) and install it. This will remove some of the distributor advance.

Is there any way you can get the cam spec's?

you will find out that using the lightest springs dont help idle issues any at all as well......they are not strong enough to pull the weights back in at idle.....which cause the timming to jump around........and if you get the idle down low enough where the weights get sucked in then the motor dies cause you just lost alot of your timming.........i would look more into the timming and get that set right.......use your timming light and idle the engine down slowly you might find the timming gets pulled way back all of a sudden and the engine dies.......you also may find at idle the timming moves around..the weights fly in and out since the springs can hold them back for a steady idle........

it dont matter if it dont run at all......chevy's will run with 18 initial and 36 total.....and that is a very good base line to start with..........so get the timming done right then move back to the carb to get it running.............

wasting alot of time by jumping back and forth and your not getting nowhere.........get the dist. set..........and the lightest springs aint the best ones to use.........hope your not using after market weights........use stock weights and the aftermarket spring...... med spring usaully get you were you need to be............

once you have the timming set........then move on to the carb........save your self alot of grief...........and take care of one item at a time.........

Originally posted by ddeennis:
you will find out that using the lightest springs won't help the idle issues at all ......they are not strong enough to pull the weights back in at idle.....

What happens is the weights fly in and out since the springs are not stiff enough to hold them back....This causes timming variations and as a result the idle RPM will jump around a bit.

Once you have the timming set........move on to the carb........save your self alot of grief...........and take care of one item at a time......... True.
Install either both of the medium springs or one heavy and one med.

Hope you don't mind me messing around with your words a bit there D smile.gif

I suppose i should throw a vacum gauge on it. Pat i do agree a cam of that nature is going to probably be "very gassy smelling" but not like this. This is rediculous. My neighbor runs a 268/268@.050 and his has a 108LSA and it dosent have half the odor mine does, his spark plugs also look "new" after he drives his car, i have never seen a plug burn that clean before. He also tried to help me tune mine several times and couldnt get it to work, and if anyone can get a carb to work its him.

Speedster
You need to listen to BuzzBomb
And start with vac gauge the power valve should be about 2" below Idle so if you are pulling 5.5run a 3.5.

Your timming is another thing why will it not run below 20? 12 should be plenty.

What about the carb where are the plates set,is it a 4corner idle system if so do a search I recently posted several of lars Grimsons articals on holly setup and others by him on timming.

I agree with Buzzbomb and Mike that the carb power valve/vaccumm issue should be resolved first.Then I think I will be able to back the timing off to where it should be and still keep the engine running so that I can readjust the carb to where it should be....

This is not my truck and I have no idea wich Holley is on it or how he has it set.I was helping a friend of mine put in another T-5 (temporary 5-speed) in his 5.0 F@#D and this friend of his came by and asked me if I could help him out w/this truck.I told him to come by tonight or tomorrow so we would have time to finish the 5.0, and let me get an idea where to start on his motor since I've never used a Holley carb or a distributor w/out a vaccuum advance setup......

Thanx again for all the help guys!! graemlins/thumbsup.gif