Running so lean I think I could lose the motor. Need recommendations. 350 SBB, TA413 ground on 110 LCA with 1.6 roller rockers (.516/.516, 234/244 @ .050), 10.5 CR. Dyno'd with several carbs on 94 gas and ran strong. Pulled the John Osborne carb and put on a fresh QJ 800. Put the motor in the car, hooked up all the accessories, fired it up. Knocking like crazy. Retarded the timing, put in 112 oxygenated fuel. Still knocks like crazy. The car fires right up, idles real nice. Put it in drive and touch the throttle and it knocks like there will be no tomorrow. Any ideas? Could this be a vacuum with the appliances and this cam? Could I possibly have a bad MSD box? Don't want to blow my $8000 motor. Thanks for any advise!
Mechanical advance with silver springs. Originally set at 12 plus 24. Then was set at 8 plus 28 advance. Retarded it to 4 and still had problems. Right now all advance is removed and it is locked down. I'd think this set-up should have 14-16 initial and 36 all in.
Are your connecting rods hitting the oil pan? Is this knock at idle, or at WOT? What spark plug heat range do you have? Do you have an "adjustable advance" device for your MSD? I hate those things. Did it also knock with the other carb? Or just the q-jet? -Bob C.
Assuming your sure it's detonation.. Any chance of copying the jetting from the carb you dynoed with? And what happen to that one?.. always want to dyno with the carb your going to be using, but I understand things happen.. JW
That is very strange. Are you saying that if you put it in gear and literally touch the gas, and it knocks? The only engine that I have ever seen do that is an EGR engine without the EGR. Why did you remove the John Osborne Q-jet? It didn't detonate with that carb? I calculated your dynamic compression with 10.5:1 static, and the 413 cam on a 110 LSA, and installed 4* advanced at 106 ICL, and came up with 8.1. That engine should be a strong performer with lots of cylinder pressure. 8.4:1 DCR is about the limit with pump gas. If you put in 112 octane gas and it still knocked, there is something else wrong. What kind of vacuum does it pull?
step back for a while. rethink your problem. from your post it sounds like it knocks at start up also. to me that is not detonation, but another problem. could the fuel pump lever be knocking, also previously mentioned the rods hitting the oil pan. don't fire it up till you get a handle on the problem. of course i could be wrong. go luck jim
The car idles perfect. I will take a closer look at a few things suggested, but I am almost positive it is detonation. Thought it was just lean because it idled better when you held your hand over the air horn. Had 71 jets and 45 rods so we stepped it up to the 72 and 46. It has 72 jets, 46B rods, CE secondaries; same as the one we dyno'd, but that was a 750 and this is an 800. The original John Osborne carb went to my brother who is putting together a numbers car. His had the wrong carb and that one was the perfect date code. What can you do when your brother comes knocking? I just got another 750 carb from Dan and I will slap it on over the weekend. We already stepped down to cooler plugs. Had a 44, now has a 42.
This is a file of the car idling. No rods hitting the pan, everything just about right. http://www.youtube.com/watch?v=c-DtaOq6ChQ
All of John Osborne's 800 carbs that I have seen have a difference of 30 between the primary jets and primary rods. My carb has 76 jets and 46B rods. BG secondary rods (.0397 tips). My carb is a 7042240, that is a 72 carb. The 72 Stage1 carb (800) had .075 jets and 45B rods.
The John Osborne carb was a 71 750 carb. The 800 is a 75 smogger. I check the set-up and the jetting is the same as a 455 HO Poncho. That should be plenty of fuel for a 350 Buick. Anyhow we are going to open up the primaries by putting in a small rod. I also have the other carb to try. Plenty of options on the carburation. But I am not convinced it is a fuel problem. I am thinking vacuum or bad MSD. But I don't have a spare MSD box to play with. Last choice is to buy another one. I really appreciate your feedback.
Tim, If you put a timing light on it, does the mark jump around? I've never seen an MSD box cause something like that, they usually just won't run the engine. How about the vacuum. What does it pull at idle fully warmed up?
Tim, I do not think you are hearing detonation. I listened to your idle and the motor sounds great. It was fine on the dyno. You have very good vacuum for that cam. Larry calculated your DCR and it should be fine on 93-94 octane. No way should it detonate with 112. It is a new motor so no glowing carbon spots. At low rpm you should be able to run very lean with no problem. I run a 413 cam in my BBB and with the q-jet I can only get 16:1 at idle and have no problems. I know big blocks are a little less sensitive but should not be that big a change. I think we all assume that you ran on and engine dyno and not a chassis dyno. Is that correct? If so you have made 2 changes, different carb, attached a transmission. Logic dictates (thank you Mr Spock :laugh: ) that if it is not one it is the other. Just trying to give you a few things to think about. Good luck.
I did have it run on an engine dyno. I have a new carb and will have it swapped out. It guess is could be the torque converter but the guy that is helping out knows detonation. Plus when things are tweeked it helps a little. If it was purely mechanical with the flex plate or converter it should not have improved when the advance was adjusted. Right now if we baby it around the block it runs fine. It only pings when you get into the pedal.
Just as a temporary fix I would put some good race gas in there. 110 octane at least. Then drive it around. See if the knock goes away. Also, does the car diesel or "run on" after you shut the ignition off?
in line fuel filter or incarb? I put new fuel filter in my carb and two days later the car isnt getting enough fuel to cover the jets. took it out put inline on and no more problems.. just a thought:idea2:
Swapped carbs from the '75 model 800cfm QJ to a '72 model 750cfm carb. Dailed in more intial timing and less total. The ping is finally gone. I owe a lot of thanks to Jim W for helping out on this one. Seems he spotted the '75 model leaning out in transition.
