I picked the car up from the mechanic today. It is good to have it back in my hands. Here is the previous post so you can kind of get the jist of what was happening (for those who tuned in late) http://www.chevelles.com/forums/showthread.php?t=88031.

Anyway, he went from 80s to 65s in my primaries. Now even I know when you tune you usually only go steps at a time, but the mechanic was insisting that I was just pouring so much fuel into the engine that 80s were way too rich (and probably were). I was having trouble fouling plugs and therefore getting exhaust backfires.

I get it back and he's telling me what he got the idle to (about 200rpm too low in my opinion), and that there's a flat spot but that it smooths out. He said they drove it the 4 mile stretch, out on the street, then taking all rights to get back to the shop) He knew I was wanting it for prom tomorrow nigiht (which I do), so he basically set it up so I could drive around town.

Anyway, I drive it, and it doesn't really feel that powerful. I also can't really give it gas very fast because it bogs/hesitates pretty bad. I can slowly ease on the accelerator, but too much and I get the hesitation. I could ease the car to 65mph, but by easing the throttle very slowly. If I slowly, but also gradually gave it more and more throttle, I would hit the stumble, then it would pick up, but open too much and I got a pop through the carb (too scared to mess something up to repeat the experiment). My temp gauge reads OK, but I just have a bad feeling it is running hot too.

Would you guys say this is a lean condition? My local parts store has a set of 70 jets in stock (that I have a feeling would still be somewhat lean) that I could buy and try to stick in tomorrow, or I could try to get to a speed shop and buy a kit and mess with jets. I'm planning on not taking it back, though, to him.

What do you guys think? I don't really know that much about Holley's. I've read that maybe the hesitation requires a bigger squirter or something, but I'm guessing this isn't the case since slowly getting on it does the same thing too. It's a mech. secondary double pumper, 850cfm...4781 is the part number. Anyway, I appreciate it, thanks!

I'd put the carb back to bone stock calibration and go from there. Is there a -dash revision number behind the 4781 on the choke horn? I have a Holley book sitting here and I can give you all the jet sizes, etc that came in the carb stock.

Also, you can buy a complete jet kit pretty cheap from Mopar performance. It's MUCH cheaper than buying them piece-by-piece.

primary jet is a 80, and secondary is either 78 or 80 depending on revision. Stock squirters are .031" front & rear. Power valve stock is a 65.

Your bog off idle is likely a lean bog from the throttle blades opening up giving a large shot of air, but not enough fuel to allow the motor to smoothly come up in rpm. It's an easy fix with tweaking the discharge nozzle (shooter or squirter) hole sizes as well as the pump cams. I would probably put .035" in the front and rear to start. If you don't have any sitting around, an easy check is to switch the cams from the #1 position to the #2 position and see if you can feel a difference. If that helps (but doesn't completely eliminate the bog), then buy a set of larger .035" squirters. Install the new larger sqirters and go back to the #1 cam position and test again.

You need to drive the car to verify this tuning, not just sitting in park and whacking the throttle. You need to see how the engine responds to throttle input when loaded against the weight of the vehicle driving down the road...no high speed stuff needed, just loafing around at normal road speeds will tell you if the squirter tuning has helped or not.

Go back to basics and check your float levels front & rear. For general tuning, take the sight plugs out and set the level to where fuel just trickles out if you rock the fender with your leg.

You should also have a vacuum gauge to hook to the carb baseplate which helps with dailing in the idle speed/mixture screws. Eventually, your gonna need to tweak the throttle blade position front & rear so you don't open one pair of blades more than the other and possibly expose too much transfer slot which will definatley make for a rough idle and fuel foul the plugs to an over rich condition.

If your new to tuning Holley carbs, there are plenty of books available that go over all this basic stuff plus have good pictures to explain the components, tools required, parts used for tuning and also diagrams to better understand the theory of how the carb operates and how the systems function.

primary jet is a 80, and secondary is either 78 or 80 depending on revision. Stock squirters are .031" front & rear. Power valve stock is a 65.

Sorry I didn't reply quicker. Yes, it is a 4781-B.

I don't have the jets that came with it, they are with the mechanic. The only problem with the 80 jets is it kept fouling the plugs, or rather, something kept fouling them.

I may end up either trying the '70s for now (he said the pacakge was like $5), see what happens, and then after tomorrow night, park it for a few weeks until finals week is over and I am home for the summer, and then get wrenching. Unless anyone sees a big harm in driving it on the street (no highway) tomorrow, with the 65s in there.

Ah, I just caught the edit to your post. I do have a Holley book, one that is mainly over 4150 models, but I stupidly forgot it and left it at school. My friend has a 750cfm double pumper that he said I'm welcome to try, that he hasn't used for probably about a year or a bit less though. Wonder if it would be okay to put that on without cleaning it a lot and stuff. But, I will try the squirter thing tomorrow and see what happens.

IF the carb is properly sized for the application, the calibration as it came from Holley should be pretty close. Your 850 is not too large for the engine you have and I feel there's no reason the 80 jet should foul the plugs.

Are the plugs fouling by idling, or driving down the road? The main jet really has no effect on the idle mixture, so going to smaller main jets will not lean your idle out assuming it's too rich at idle.

IF the carb is properly sized for the application, the calibration as it came from Holley should be pretty close. Your 850 is not too large for the engine you have and I feel there's no reason the 80 jet should foul the plugs.

Are the plugs fouling by idling, or driving down the road? The main jet really has no effect on the idle mixture, so going to smaller main jets will not lean your idle out assuming it's too rich at idle.

This is what I'm not too sure about. At first, it just started to happen. But then the mechanic said they changed the plugs because they were fouled, and tried to tune it. But, I guess it kept fouling plugs then. That would lead me to think they may have actually been fouling at idle, since would probably be how they were working on it.

I'm still thinking the fouling plugs is due to leaking boosters, or the idle screws being open too far.
80 jets front and rear arent a whole lot of jetting for an 850. Everything is relative, so with more air you need more fuel.

i agree, more then likely they were fouling out becuase he was idleing it while working on it.

I'm still thinking the fouling plugs is due to leaking boosters, or the idle screws being open too far.
80 jets front and rear arent a whole lot of jetting for an 850. Everything is relative, so with more air you need more fuel.

The guy was trying to tell me an 850 was too big, and that maybe I would be more pleased with a...600 or 650. Eh..

I do need to look at the transfer slot stuff so I can take the carb off the engine tomorrow and look at that. I don't even know if the secondaries are open at all at idle. So, just to make sure, if they are closed and my primaries are open to far, does that leave too much transfer slot exposed?

a good chance it would. On my engine i have to open the secondaries slightly to keep the primary at .040"

there is a part you can buy that will allow the secondaries to be ajusted without removing the carb, havent bought one yet, but if you could find one locally tomorrow when buying gaskets, it would be nice to have.

so you could actually set the primary to .040" and adjust the idle by dialing in the secondaries.

850 is fine for that engine, a little on the large side for alot of street driving, however it will be alright to work with for now.

the transfer slot exposer is a small problem though, the floats need to be addressed first.
but if you're taking the carb off, you might as well set up the transfer slots correctly.
I have a few things to try on the idle circuit, maybe nothing is wrong with the floats and the mixture screws are just too far out. 1.5 turns each on a 4 corner idle might be too much for that engine.

I'm heading out to work on a few things, i'll be back later.

Take the carb off and look at the throttle blades bottom side. Look at how much light you can see through the edges and compare front & rear. There's a small verticle slot in each bore just by the throttle blade From underneath, you just barely want to see the tip of the slot (slot is in the main body venture)...not much at all. If you see say 1/16" slot, then the throttle blades are too far open. You can help this by opening the secondary blades a little. The adjustment screw for the secondary side is accessed from the bottom of the carb with it sitting on the bench With a small slotted screw driver.

EDIT: I agree with Brian that your first adjustment needs to be the float levels and making sure there is no fuel dripping from the boosters while idling. Once you accomplish this, then you can move on to other areas of tuning.

Well thanks for the tips. I will get to work on this pretty early in the morning.

if you get tired of screwing with it borrow your buddy's 750. 850 DPs are not a good street carb. A 750 will run better, sharper, cleaner everywhere but the very top end. What percentage of your driving time is spent at 5500--6000RPM? Bet it's very small. Wait'll you see the gas mileage with an 850, they suck in more ways than one.

The 70's that guy put in the carb aren't close, it needs to have 80's. the main jets don't affect the idle, if it's loading up at idle it's for another reason. This guy doesn't know what he's doing, IMHO. If you can't get somebody to fix it right locally you can send it to me, I'll fix it and test it on my truck before sending it back. It'll never be a sharp street carb but it can be made to at least be driveable. Email me via the board if you continue to have trouble.

Hey guys, just wondering something, my friend's 750 I think is a vacuum secondary carb. Am I going to have to 'create' a new vacuum port somewhere to hook them up, or what...just not sure how it works..thanks.

Before you do all the float level checking, rejetting, vacuum tests, etc...

My question would be why would you have to go back and REDO all the work the guy did if he knew what he was doing in the first place? Why would you have to mess with the float levels (CMON! THe guy is a mechanic, right? :eek: ), jetting, idle mixture, etc. if someone who more than likely has sophisticated equipment just "tuned" it? If it were me, I'd take it back to them and make them fix it right. If they just can't do it, get a refund, copy and paste this post, THEN go from here and from the Holley book.

I just can't understand why you would pay someone your hard earned money to fix something, and after they supposedly fixed it, go back and wind up doing it again yourself. Either get them to FIX IT like you paid them to do, or get YOUR MONEY BACK, and send Tom Mobley your carb..

the difference between the mechanical sec and vac sec is internal in the carb. a 750 will go anywhere a 850 is. one possible issue is if the 850 has a sec metering block (it does) and 750 doesn't. some modern 750 vac sec carbs don't have a separate sec metering block, that affects the distance between the fuel line ports. if you have some ridgid dual feed line made for the 850 it might not fit the 750, just depends on how it's made.

Buy a book about Holley carbs,,,by HP books,,,a Holley has to be the most simple carburetor to work on, and tune.

Put that 750 on there now. It will give you a good reference point. I ran an 850 dp on my 454 and it was good with open headers and hammering on it. Around town it tended to load up and didn't idle lean enough. I put a 750 v/s (3310) on that is modified a little and it is unbelievable the difference in basic driveability. I may have lost a little wide open performance, I don't think so, but the basic performance is much better. I do have an auto though.
An 850 is good for hammering but I don't think it is refined for basic cruising, at least mine wasn't.
That 750 will probably leak for a minute or 2 and be fine when the gaskets swell.

Thanks for all the help guys. I decided to just leave it today. I didn't go try my friends carb just yet either. I had noticed that the car sounded worse too just at idle..maybe a big raspy and more sickly. I think they may have adjusted the valves (said they at least checked lash). I have finals week in a couple of weeks, so I think the best thing may be to just put this out of my mind and concentrate on school, and when that's all said and done and I have the summer I can spend a good few days really going through everything. I also think I will try to tackle it myself, so I can try to learn, before I try to send it out to anyone. I really do appreciate the offer though.

Anyway, thanks guys...this may pop up again in a couple of weeks when I'm ready and not busy with a clear mind to do this.

For sure though, I won't be taking it back to this mechanic.

I'm not telling you what to do, but like I said earlier- if he didn't fix it and you paid him to do it, you were GYPPED. That is the reality of the situation. I really think you should try to get your money back for a service that wasn't performed- or at least a refund on his labor or lack thereof.. The parts are alright, its always good to have extra jets and stuff. I hope you don't just throw the money away and take it at a loss..If it runs worse than when you took it, he didn't fix it and NEEDS to do something about it.

" I have finals week in a couple of weeks, so I think the best thing may be to just put this out of my mind and concentrate on school"

That is a great idea. Do the important stuff first, and hit it later with a clear head.

I'll second the suggestion for the HP Dave Emanuel Holley book. Its worth the money.

I think all of you might have missed something that is all too easy to do. When you take the bowls off of any Holley, it is very easy to put them back on with the accelerator pump levers not under their arms. This would absolutely cause a huge stumble when stepping on the gas, and is very easy to check. Just take the air cleaner off, look down into the carb and move the the accelerator linkage. If you don't see gas shooting out of the squirters, good bet the pump levers didn't get put back corectly. Something easy and cheap to fix.

If your anything like me, your still dickin' around on the Chevelle rather than getting ready for Prom, thata boy. I bought my '66 when I was 14, and I drove it to every major, and minor event in my life. I damn sure wasn't gonna get any cruising my mom's 4 doorGranada (guess what year that was). Nonetheless the info will still be valid tomorrow. tpshae

[QUOTE=Tom Mobley]if you get tired of screwing with it borrow your buddy's 750. 850 DPs are not a good street carb. A 750 will run better, sharper, cleaner everywhere but the very top end. What percentage of your driving time is spent at 5500--6000RPM? Bet it's very small. Wait'll you see the gas mileage with an 850, they suck in more ways than one.

Does this apply to the 850 vac sec carbs too?

Man, I hate when that happens. Anyway, tpshae's suggested goof does happen, but I've never seen a carb make it on the car like that, it's possible and I guess your mechanic sounds like the guy to do it. Unfortunately, your carb has been so compromized that all the tuning suggested will inevitably be necessary. In fact it wouldn't be a bad idea to pop the bowls off your buddy's 750 (make sure that he didn't leave it full of gas when it was packed away a year ago), give it the once over with some chem-tool and slap it on your ride. It'll either be fixed or you have another problem to contend with. If it seems to behave a little better, then your problem probably wasn't the carb. It's a little better only because you lessened the degree of complication (smaller carb). I have experienced the same problems, with nearly the same machinery, I have a '66 with a 427 +.060(439) 11:1, Air Gap, 850cfm 4150, Lunati solid roller 242/252 at.050 595/612 lift, 2.19/1.88 semi-closed ovals. In my case, I screwed around with my 4150 (the same carb that I've used since 1983) and my timing to no avail. I found an old 750 Holley 4410 ?(the kind without removeable float bowls), put it on and thought that I noticed an improvement. I washed my hands got online and ordered a Mighty Demon 850 from Jeg's. Later in the afternoon I took off to run some errands and kinda still felt the problem. On the way home I stopped at Sears and bought a dial back timing light just on a hunch. Sure enough, my advance rate was all outta whack. in fact, I had been "tuning" with my initial actually being deep into my advance. I tore into my Mallory Comp9000, pulled the pick-up plate, and sure enough, one of the springs was broken. I've had them come off HEI"s before and should have known to atleast consider it. This was yesterday, and it ran great last night. Let's see, Wednesday I'll have a Demon, sounds like an excellent opportunity to do a head to head comparison, even if the Holley does have 22 years on it, it works better than ever, I credit it to the whistle, you know the whistle that worn out throttle shaft bushings make. Sure glad that I didn't drill my throttle plates back in the eighty's, I'd be posting lean problems.
Have a great night out, hope your back seat is still in good operating condition and your date doesn't mind that you smell like gas.
Rowdy