StevenPud

iTrader: (2)

I've searched this board for days and can't seem to find an answer to my problem. I'm another one with a stock LQ4 short, L92 heads, Comp 281 cam (228/230 571/573), MSD 6010 and carb. The carb is a brand new holley 750hp. I've tried using info from previous posts about timing and carb jetting but i'm getting nowhere. Right now i have the carb jetted 72/78, a 4.5 power valve, and 5lbs fuel pressure. I started at 32 degrees timing and the motor idled ok and was ok at light throttle but if i gave it any gas it would break up. I am now running 36 degrees timing straight across the board and the MAP advances the timing up to 8 degrees at part throttle, which means i'm running up to 44 degrees total timing . Now the motor idles great and runs awesome until i give it enough throttle to open the secondaries then it starts breaking up. If i feather the throttle to get past that point, the car runs like a bat out of hell at full throttle. The more timing I throw at it the better it runs.
Now I know what you're gunna say...its running rich, jet it down. But my issue is this...I haven't seen anyone else on this board run their carb that lean nor has anyone run that much timing. Could there be an issue with the motor or could it be that a 750 is too small and not putting enough air into the engine? If anyone with a similar combo (or anybody actually) could point me in the right direction as far as their carb and timing settings I would appreciate it.

BTW....if it makes a difference the car itself is a '30 Ford street rod that only weighs about 2000 lbs with a turbo 400 and a mustang 8.8 rear w/3.73 gears.

69LT1Bird

iTrader: (1)

I looks like you are building the Carcraft engine. They used an 850 when they put the L92 heads on with the 281 camshaft.
http://www.carcraft.com/techarticles...ads/index.html

Dyno Day
Test 1: is the best power curve from last month's story using the stock LQ4 6.0L iron heads, GMPP Hot cam, GMPP carbureted intake manifold, Holley 750-cfm carb, and Kooks 1-3/4-inch headers.
Test 2: is where we jump directly to the big L92 heads combined with the larger Comp hydraulic roller camshaft. We also increased carburetor size to an 850-cfm Holley to make a little more power at the top while retaining the Kooks 1-3/4-inch headers.

bradyfred77

iTrader: (3)

i think 5 pounds may be a lil low for a holley. it would be good for a carb with metering rods but i think it may be a lil low for a jet type carb. and is ur fuel pressure consistent thru the rpm range? and i would be extremely worried about that much timing. 44 degrees is hard on the mains.

ogermax

You need at least 8 lbs for a carb. Sounds like its starving for fuel.

blackangel327

iTrader: (3)

agreed 5 lbs, too low, not enough fuel hell my old 305 ran more than that

StevenPud

iTrader: (2)

i can assure you all it is too much fuel. the spark plugs are completely black. fuel pressure is consistant all the way through. i started with 7 lbs fuel pressure and 75/80 jets and the car hardly ran at all. i am wondering now if the carb may have the wrong air bleeds. could this be possible on an out of the box carb?

bluethunder364

iTrader: (5)

Your plugs can turn black and foul up in no time, especially since it is not running correct. I agree with the previous post. Sounds like not enough fuel pressure and incorrect jetting. If you are unsuccessful in getting it right, take it to someone that has an inground roller dyno, hook up O2 sensors to sniff the air/fuel ratio. I think the 750 carb is no problem, just needs to be set-up correctly. Hell you can run a 650 if you want, it just doesn't make as much power. If you check the Holley website you'll see that your carb is in the ball park. Hope you get it figured out. Thats hot rodding!!!

rojs234

iTrader: (5)

Since I like streetrods and I have streetrods, I just have to take a shot at this. Don't know that I have the answer, but I might give you a couple things to try. First of all, since you say it runs hard at full throttle, I don't think It's the fuel pressure. And I also don't think the jetting is off enough to cause the problem you are having. From your description it sounds like it is breaking up as you are starting to get more of a load than just part throttle. I am wondering if it may be the way you have you vacuum advance set up. Since the motor will have almost NO vacuum at wide open throttle it also has NO added advance at full throttle. Therefore, you have 36* degrees at full throttle. From my own experience, (6.0 L92 heads,carb, '34 Ford) , I have better luck at 32*, all in at 2000. Any way , my theory is that your vacuum advance is still holding it advanced at the lower RPM because with your very light car it doesn't lose the vacuum as quickly. This can be easily checked by simply disconnecting your MAP sensor. If this does help, I wouldn't do away with the vacuum advance, I would re-configure it so that it takes a higher vacuum to keep it advanced. Another thing to think about is the 4.5 power valve. I know you put it in there because of the big cam, but you need to realize that it doesn't open, thereby enriching the fuel -air mix until the vacuum drops to that point. Again, with your very light car, that could be a lot of your problem. You might try going up at least to a 6.5. Don't know what vacuum you have at idle, but if it is as much as 11-12 inches, you might move up to an 8.5. By the way, sometimes with a big cam, you can run more advance at idle RPM to not only help the idle but also improve the idle vacuum. Try hooking up a vacuum guage or if it doesn't confuse you, watch the PSIG gauge on your MSD screen. In my opinion, being able to see and adjust your advance and vacuum advance with the MSD6010 is the neatest thing since sliced bread!!! Hope this gives you a couple ideas to try. Good luck, and show us a picture of the car. Ron

StevenPud

iTrader: (2)

rojs234....i thank you for the advice. at full throttle the motor runs better with 36 than with 32 so thats why i set it up that way.
as far as the vacuum advance goes, at idle i show 7 psi. currently i have the timing advanced 8 degrees from 0 to 11 psi. then it goes down to 4 degrees at 13 psi and finally 0 degrees at 14 psi. the motor runs perfect until i get to 12-13 psi then thats where i get the stumble. rpm doesn't seem to matter. i started with less advance but the stumble was worse. the more advance i set the better it runs. it just seems that this is a lot of timing for this particular motor. but i know that the richer a motor runs the more timing it will take because the fuel burns slower. i'm starting to think the stumble is coming in when the power valve is opening and i'm getting a temporary rich condition. the power valve is the one that came in the carb...its about the only thing i haven't played with yet.

StevenPud

iTrader: (2)

Oh, i forgot the picture. Here it is:



Its still a work in progress!!

Darracq

iTrader: (1)

I ran 32 total in mine, i had a lot of the same trouble, flat spot when i nailed it, fouled plugs from idling. i tried bigger squirters, pumps, mine was a 750 vac secondary, there seems to be something about these engines that need some different metering, i sold mine before i ever got it fixed right, i did get some good passes out of it, on the street it would spin and not fall on its face, but when i hooked it would just go flat and then run fine after it cleared up.

rojs234

iTrader: (5)

I like the car. You may have to take along a change of underwear when you get it running right!! I think I see most of the problem. From your description of your VACUUM advance curve, I think you have too much total advance at other than full throttle. According to your numbers you still have 44* timing clear down to around 7" of vacuum. This is below where a lot of power valves open!! 44* is too much under this much load. You may be getting confused by the way MSD shows vacuum. PSIG is quite different from inches of vacuum! I had a lot of trouble myself until a member put up a link to a chart to convert PSIG to inhg. Any way, you are still adding 4* at almost wide open throttle. Your idle reading of 7PSIG is almost 14 inches of vacuum and kinda' shows what I said about increasing timing at idle. You probably have 44* at idle if you have your MAP hooked direct to manifold vacuum. Can you hook up your laptop in your car so that either you or a friend can read it as you are driving?? All you need is a small wattage convertor that you can buy at Radio Shack. Then you'll have to wire in a replacement cigaret lighter to plug it into. Very simple. When you get it in and lit up, find the little 3 guage panel and click on the maximize icon. Then you can head out and actually see the RPM where you are having the problem , and see how much actual timing you've got at that RPM, and you can also read the PSIG guage, but I would suggest you get a chart to convert it to inhg. I truly don't think there is anything wrong with your set-up. I think it just needs to be tuned. Although your jetting may not be ideal yet, I don't think it is way rich as you suspect. I would still like to see you unhook the MAP sensor and try it. I have a similar set-up to yours; LS2, L92 heads, 222x234 cam, carb. I have a #1407 EDL. carb with .110 pri. .104 sec. and 75x47 rods. Just for fun ,I sat down and figured the area of your jets and power valve orifices and mine, less the tip of the metering rod. My carb is some richer than yours! Anyway, my timing for now, ( I'm still tweaking) is @0rpm (cranking) 13* - @ 850rpm (idle) 21* - @ 2000rpm up 32*. My VACUUM advance curve, if I remember correctly, starts ramping up at 8PSIG, adds 10* from 7 PSIG (around 13-14inhg) to 0 PSIG (20 some inches of vacuum). I also run my MAP off ported vacuum, so it adds 0* at idle. The vacuum advance is there to add additional timing at cruise and part throttle when the mixture is lean. You want the added timing to disappear when you are under load or especially under full throttle. You should be on your regular advance curve or what we used to call centrifugal advance any time you are under load. Sorry I ran on so long but maybe some of this will help. Ron

Schwanke Engines

iTrader: (1)

Make sure your secondaries are not open to far at closed throttle exposing the transfer slots. This will give you some weird problems when they go to open.
Also, have you checked your secondary accelerator nozzles, make sure they are spraying a good pattern. Also, check your float level. It sounds like a carb issue to me.
Sorry if I missed it, but what type of 750 is it? Mechanical or vacuum secondaries? If it is vacuum operated you may have to play with the opening rate.

Good luck and keep us informed. If you need, I can get you a damn good carb, but it will cost some change. D)

StevenPud

iTrader: (2)

so here's the update guys....

first, for those who asked, its a mechanical secondary holley 750hp.
i went and put the carb back the way it came out of the box except for the power valve. So its jetted at 75/80, 6.5 PV, 31/28 squirters, and 7 lbs fuel pressure. float level is perfect. fuel pressure holds steady all the time. the timing is now set at 36 degrees across the board. light throttle has a 6 degree advance but if i get into the throttle at all the timing goes to 36.

apparently i was wrong and needed more fuel. so here's the deal. the car starts right up and idles perfect. regular driving is excellent. if i roll into the throttle slow it accelerates great. with more fuel its been the best its been yet. now here's the problem....if i stomp on it the motor sputters real bad, backfires out the exhaust a few times then takes off like a bat out of hell. i tried bigger accelerator pump squirters but the problem was worse. 1 out of 5 times i stomp on it the problem doesn't occur. i had the timing advanced more and it was backfiring out the carb & exhaust at the same time.

so what do you think? could i be the exception to the rule and need even smaller squirters? or is there some other adjustment to try? keep the ideas coming....

StevenPud

iTrader: (2)

well it took a few weeks of playing but i finally got it right:

7lbs fuel pressure
jets - 75primary/ 85 secondary
squirters - 35 primary/40 secondary
6.5 power valve
changed high speed secondary air bleeds to #23s
36 degrees timing at full throttle

now it runs great no matter how much throttle i give it and i can flat punch it and it won't stall. now i just need some bigger rear tires...it gets a little scary on 6 inch wide bias plys!!

1cdub

iTrader: (5)

thats a pretty big jump in squirter size....glad you got it fiqured out.

rojs234

iTrader: (5)

Glad to hear you got it sorted out, and yes, you are definately going to need some bigger rear tires!! Now all you need to do is slick it up a little and then get out there and rip the heck out of it. By the way, are you going to run it with the ' boards and fenders, or hi-boy style? Either way it's going to make for a really fun ride!! Ron