My recently purchased new street/strip 468 BBB runs very poorly at low RPM, backfires through the carb when starting and shortly after start-up, and stalls when dropped in gear. It settles down a little when water temp reaches 220*. The big cam (TA510) is a contributing factor, but I believe another part of the problem is the tuning and jetting of the carb (******* 950 mechanical secondary, 4-corner idle a Holley 4150 DP knock-off). From the symptoms, I suspect its running very lean on the primary side, at least, and the idle speed screws are adjusted so that about 0.150 of the primary transfer slots and 0.040 of the secondary transfer slots are visible below the throttle plates at idle. Judging from the whacky idle mixture screw settings, the engine was not responding to mixture adjustment efforts. Unfortunately, I dont have an A/F meter at my disposal, yet, so I'm kinda lost. The engine: 468 BBB, 10.5:1 C/R, SPX, TA-510 cam (255*I and 265*E @.050 duration, 40* @.050 overlap), ported TA Stage 2 SE heads, headers & mufflers. Im going to drill a 3/32 hole in each throttle plate (the side closest to the transfer slot). That, in theory, should allow me to back off the idle speed screws and cover all or almost all of the transfer slots at idle. The problem is I have no idea where the throttle plates should be positioned when a virgin carb is installed for the first time. I also have no idea of what the jetting should be. So 1 - What is a good starting point for position of throttle plates in relation to the transfer slots when closed? Should the slots be covered completely? Should the plates be adjusted to uncover a certain amount of each slot (say 0.020 each)? 2 Any idea what good starting point is for jetting (currently #78 primary and #86 secondary as delivered when new)? As always, thanks in advance for your very valuable assistance.
if your valve adjustment is correct, try advacing the timing, large cams require extra initial timing, but also limit the total timing to 36-38 degrees. adjust your float level with the engine running. some of the carb mfg's, edelbrock, barry grant, holley have trouble shooting in there tech section. they also have setup videos. good luck
Dan, we have an asterick 950 carb on our 464. It took a while to adjust. We just went down a step on jetting to 75 and 82. We do have the small hole in the flaps, and only have the adjustment screws out about 1/8th turn. How do your plugs look? Is it smoking at idle? Ours did until we got that all figured out. Make sure you set you floats, and also make sure you don't have any raw fuel leaking in to the center. We also had to adjust the secondary opening. It was slightly open at idle, and had to be backed up to close at idle. Just something you might check on. You are running a power valve in back? If so, you might need to change to a different one. I think we went to 2.5. On your backfiring, sounds like you have fouled plugs, or need more initial timing. Good luck on your &&^(&(%% 950 carb.
In general the starting point for idle is to have enough of the transfer slots covered to look like a square. On a 4 corner idle system one turn out on each screw will be rich but will start ok. Jets are in the ball park. What is your timing. For decent idle you will need at least 20* initial on a 'big" cam. Backfire through the carb is more likely to be a timing or valve train issue than a carb issue. I went nuts with one valve adjusted too tight last year. Good question... who built the engine?
Thanks for all of the great information, you guys. Ill be cutting and pasting it into my Buick Performance file shortly. The specific information I was looking for came from Doug, though (thank you very much). It pretty much matches what a Chevy 540 owner/builder friend of mine suggested: 0.020 of each transfer slot showing, with idle mix screws out 1 & turns to start. I guess theres no reasonable way to predict jetting though. There are a few more Holley DP-related questions floating around in my head, but Ill start new threads for them Yes, plans for the car include continuing the pro-street theme that the previous owner started with his build, with tweaks to satisfy my personal tastes. The first thing I must do is add the basic street-legal equipment, then bring it up to current NHRA requirements for a 10-second car. Then Im going to try to achieve very low 10s (with pump gas) while Im still young enough to drive. (Im 66, so time is running out.) After I achieve that goal, Ill tame it down a little, add more polished aluminum and other do-dads, and turn it into a fairgrounds cruiser.
Dan, I just now found this post,,,, Before you start modifying the carb, call holleys tech line and talk to them,,,, I have run holleys for years and rebuilt a lot of them and most of the time if they are set up to holleys specs they run best... Yes , it is ok to modify holleys , but do it like they say to do it... From what you describe , I would think that the carb is running lean, if fact , super lean,,,, that is why it is back firing when you step down on it and when the engine is cold,,,, and a clue is that it gets better when the engine is warmed up.... when a engine is warm it vaporizes the fuel better...and doesnt need as rich a mixture to run...Dont drill the throttle plates except as a last resort.... and if you do start with very small holes....and work up.... I run a 1050 cfm three barrel on my 401 without any problems,, and I set it up like holley told me to... Doc
Thanks, Doc. Good advice except that I feel kinda funny about getting advice from Holley regarding how to set-up what I suspect is a Chinese-made Holley knock-off carb. All, After receiving input from a number of sources, I've decided to reassemble the carbs without drilling the throttle plates for now. I'll set up the idle speed so that each plate uncovers about 0.020" of each transfer slot, and set the idle mix screws at 1 & 1/2 turns out as a starting point. I also decided to replace or overhaul, as appropriate, all of my fuel system components. While the fuel system is out of comission, I'll determine ignition timing at cranking RPM, and perform compression tests and a cylinder leak-down tests to see if I can find anything fundimentally wrong with piston and valve sealing. Then it all goes together, starts up and purrs like a kitten... I hope. I'll let you all know how this all turns out.
The latest: Pulled the spark plugs. Surprisingly enough, even though I'm aware of a vacuum leak, each plug was black and sooty. That suprised me because I did not see any visable signs of richness (i.e., black smoke) from the exhaust when the the throttle was "winged". I removed the primary metering block in order to check the condition of the power valve (it's a number 45, by the way). After all, even though "power valve protection" is designed into the carb, this engine backfired through the intake quite a few times. Unfortunately for me, I don't know good from bad. I mean, it looked OK from both sides. That is, the diaphram did not appear to be ruptured, and the valve moved back and forth with light finger pressure from the outboard end. I even sucked on the inboard end (luckily no one was looking in my garage at the time), but could detect no air flow. I didn't bother with the secondary side because the Proform carb has the secondary power valve blocked off out-of-the-box. So, two questions: 1 - What does a damaged power valve look like? 2 - For future reference, does it take a special socket or other tool to remove and replace a power valve? It looked like it. That's it for now.
OOOOk, if the plugs are black and sooty, that lets out my theory of being lean....:laugh: :laugh: i will bet that when you find the problem , it wont be with the carb.... it will be ign. or mechanical.... or it will be a combination of each...
On a 4 corner carb I have found that 1 1/2 turns out on the idle mixture screws is too much. All the carb shops and Holley run way to big of an idle feed restriction. Basically they left the idle feed restriction that same as when they had just two feeds for the idle and then doubled the feeds. My experience with them has been to just turn the idle mixture screws out 1 turn and expect to adjust them in to about 1/2 turn on each corner.
Ah-Ha! I had a brainstorm yesterday regarding my black, sooty plugs: The engine in question hasn't had an opportunity to "stretch its legs" since it was taken off the dyno. Since then, it's spent most of its running time (an hour or two, max) idling, or at slightly elevated revs, but still on the primaries. Considering the 40* overlap of the TA-510 cam, I wonder if it's just "loading up"? I haven't delt with an engine with this radical of a cam before, so I really don't know for sure. I guess the only way to know for sure is to replace the plugs and give it a try. I'll let you know. Thanks again, everyone.
We kept changing the jets to get our plugs looking good. With this 950, we are running much smaller jets than you to get there. Otherwise, we were burping out black smoke at idle, and at launch, and even down the track. Now, our plugs look good and it runs better. We have the 286 08hl cam. Not as big as yours. Hope you get it all worked out soon.
I was just watching the Holley video on carbs. and they sate to adjust the idle mix screws 1/4 turn clockwise to lean out and counter to richen. 1.5 turns even 1 turn is to much.
Not to hijack.......but I will:laugh: What did this engine do on the dyno - This is a very similar build to mine and I am just curious:3gears: Awesome car by the way:TU: Thanks
1.5 turns out is the standard starting point. A 1/4 turn is what is the recommended change amount when adjusting. Have yet to see a 4 corner out of the box carb that needs 1.5 turns out. More likely 1/2 turn out on all four corners. I really doubt that this is a carb problem. This is a case of trying to adjust the carb to fix a more basic problem. 1st issue is timing. If it was me I would run a looked out distributor and set the timing at 32* Then run an ignition box that allows start retard. (MSD-6al2) Also double check the valave lash to make sure there is not a valve to tight.
