wallaby Posted December 30, 2006 Share Posted December 30, 2006 Something isn't right about my engine combo, and I'm trying to figure it out. I finally have my carb dialed in (dyno shop had to do that for me) and my HEI distributor has been put on a machine and performance curved. Now comes the technical stuff: when I got the car back from the dyno shop they had the timing set at 16 degree initial with 34 degree total. There is no ported vacuum port on the carburetor so the vacuum advance was left unhooked. This setup works great with big throttle loads, but at cruise it seems lifeless and the idle is crappy. She idles at an unsteady 700rpm and pulls 10-11 inches vacuum. Tailpipe fumes are terrible. . I tried turning in some advance on the distributor and the idle improves, and vacuum goes up to around 14 inches, even with the idle set at 700. Suddenly I have great throttle response and predictible idle, and no terrible fumes from the tailpipe! The problem is that the timing is so far advanced that the starter is dealing with some kickback and the engine pings pretty bad under load. It wasn't smooth at cruise rpm either. The timing light verified that my engine was idling happily with the timing set at 32 degrees initial. . My question is what is this telling me about my engine? Why would it like so much advance to run properly at lower speeds? I'm not understanding how changing the timing alters the vacuum when the engine speed is the same. I have a comp 268 cam, but it's supposed to give fair idle with a stock converter. . My solution was to set the timing back to 16 degrees initial and use manifold vacuum to the advance canister. Everbody says it isn't right that way, so I tried it last, but it seems to be the best setup for me so far. Now I have 16degrees when starting so it's friendly to my starter, 28degrees at idle when the vacuum is high so my engine is happy with the advance, and it backs off timing under load as vacuum drops so it doesn't ping. . Any thoughts, or theories? This could be some valuable clue to what's going on with my engine...if only I knew what it meant. Link to comment Share on other sites More sharing options...
Heckeng Posted December 31, 2006 Share Posted December 31, 2006 What condition is your engine in Wallaby? When was it last rebuilt? Have you ever done a leakdown test? What kind of compression does it have? If I remember right, you had had your carb tuned with a wideband O2 sensor right? I assume the people/person who tuned it understood how to tune your particular carb? What kind of carb is it? I think your initial and total timing sounds excellent for a starting point and it should be fine like that. How soon does your advance come in? In my experience, if you have extremely low compression, the engine will want tons of timing to feel "lively", but it will make it hard to start as well like you are saying. I think that since it works well at full throttle, the compression is not the issue. I would think a/f ratio actually. Lean conditions will cause surging and fluxuations in rpm as well as popping. I would wonder if your carb was tuned to have a good a/f ratio at full throttle, but they didn't tune it for part throttle? Just my guess. I'd love to hear more details! Link to comment Share on other sites More sharing options...
Mike 57 Posted December 31, 2006 Share Posted December 31, 2006 Wallaby Your final setup does not sound bad. If your engine is happy with it run it. Engines have been setup both ways. If you were able to use ported vacuum it would not start to pull in until you crack the throttle and you would not have the advance at idle. I use ported vacuum of my Holley but my set up is different. I have a MSD with adjustable centrifugal advance set up for only 16 degrees so my initial timing is set at 20 with a total of 36. I pull in timing on the vacuum advance from ported vacuum it helps my engine from stumbling right off of idle. I have an adjustable vacuum advance and I able to cut back the vacuum advance timing to eliminate any ping at cruise. I have had guys tell me that that is not the way to go but my big block seems to like it best with my setup. I try to run as lean as possible on the primary side and I think that is why it likes the timing like that. I have aluminum heads and compression is about 10.2 to 1 with a XR276HR hydraulic roller. Link to comment Share on other sites More sharing options...
Guest Posted January 9, 2007 Share Posted January 9, 2007 Mike, Thanks for the info (as I jump onto this post). I am running a 454 with 0.040 over and a set of aluminum heads. The engine was built by a local guy, and he said compression was around 9 to 1. None the less, I was having issues with my timing and vaccum similar to Wallaby. To get the engine to run without stalling and struggling to stay running, I had to set the timing advance at a minimum of 20 degrees (but the engine seems to prefer 25-35 degrees). I'm running an 870 Holley carb from All State which I was told shouldn't be too much for what I have. Anyway, I've heard that chevy's (especially BBC) prefer more advnace than the 8-12 degrees stated in manuals. Even to the point to retard just before pinging and knocking. I guess, I should set the timing up to whatever does best for the engine. Link to comment Share on other sites More sharing options...
wallaby Posted January 10, 2007 Author Share Posted January 10, 2007 My problem isn't solved yet, but it seems better now. I took her out for a drive last weekend and noticed some stuttering just off idle. I also have idle which is generally good, but deteriorates as I try to wait out the signal lights. Baby steps I guess. I just want to be able to drive this car without having to use both feet. Link to comment Share on other sites More sharing options...
Heckeng Posted January 10, 2007 Share Posted January 10, 2007 Alpha, Your engine will definitely appreciate lots of advance. Aluminum heads can take more advance due to the aluminum not retaining as much heat like iron heads do, and having 9:1 compression will also want more advance. Combine those two items with the relatively large diameter of the cylinders that the flame front has to travel and it is screaming for more advance! It would not surprise me if your engine would like 40 or 42 total degrees even. Your cam and amount of vacuum will also affect how much advance it likes. You might look at the MSD ignition site, they have some good things to think about when setting your timing. It is included in most of the distributor installation instructions, and would be good advice regardless of what kind of distributor you have. Check out page 2 of this: MSD Distributor installation instructions, page 2 for timing advance advice Link to comment Share on other sites More sharing options...
Mike 57 Posted January 10, 2007 Share Posted January 10, 2007 Wallaby, I know that you had work done to your carb and distributor so you don’t want to mess with them too much. Are you running a Holley? Have you played with the accelerator pump cams? Here are the colors in order of aggressiveness; White, Blue, Red, Orange, Black, Green, Pink and Brown. A little more pump shot may clear up the stutter right off of idle. Going to the number 2 hole on the pump cam can sometimes help. I run the green pump cam and that took care of most of my off idle stumble I still had a slight bit of stumble occasionally and I did not want to go to the brown cam so I went from a 40 to a 45 discharge nozzle and never had a stumble again. As far as the Idle you mentioned that it smells terrible sometime at idle. Can you lean the idle mixture a little more? It may be loading up. Alpha, Big blocks like timing but be careful 25 to 30 initial plus the centrifugal advance of probably 20 gets you a lot of advance I would not ever run more than 38 degrees total advance at W.O.T. Try using the vacuum advance to pull timing in at idle. That 870 carburetor, is it one of Holley’s “Avenger” series carbs? I noticed that they like to run rich at idle. Good Luck Guys Link to comment Share on other sites More sharing options...
Guest Posted January 10, 2007 Share Posted January 10, 2007 Wallaby, I'd guess that your carb is running too rich at idle as you don't have enough cam overlap to cause your vacuum to be that low. Check float levels, idle mixture screws, and power valves (Holley). Link to comment Share on other sites More sharing options...
72 LS5 Posted January 10, 2007 Share Posted January 10, 2007 MC72 has a good point. If you are running a Holley and the idle is deteriorating at a stop, the floats may be too high or a dirty needle and seat resulting in gas dripping from the boosters. A blown power valve could be a huge fuel leak under the throttle blades. I ran my vacuum advance directly from the manifold for years and years and the car ran great. The reason was I had no spark advance port on a Holley 800 DP. I just recently removed it (after reading Don's posts on vacuum advance) and run no vac advance at all with 38 total. I can't tell too much difference in how it actually runs. It feels livelier at lower rpms with the vac advance and won't stall as easy. Link to comment Share on other sites More sharing options...
Guest Posted January 12, 2007 Share Posted January 12, 2007 Mike/Heckeng, Thanks for all of the help!!!! I set the timing to 18 degrees at idle. You guys are right, the BBCs really appreciate the advanced timing. I think the engine could have stood for more, but I don't want to test my luck until I can test drive her under load. The MSD link was very helpful and worth printing to keep handy. I'll run the car and see how she responds. From there I'll advance/retard accordingly and post the results. The carb is an 870 Holley Avenger. I had an older (real old) 650 DP, but couldn't seem to get the car to run right with it. As a result I just sprung for the 870 ($255 from All State - Ebay). However, I think you are right about running rich at idle. I foresee alot of tweaking during the winter to get her ready for the spring. Thanks again for all of the help!! Man, does this site rock!! Link to comment Share on other sites More sharing options...
Mike 57 Posted January 12, 2007 Share Posted January 12, 2007 Alpha, I could never get the avenger 870 lean enough. I finally went with a proform metering block. It is made for them by quick fuel. It really changed the caracteristics of the carb. I got mine through Summit. Here is a link to the metering block I used. http://www.proformparts.com/mistore/prod...mpany_id=100719 Link to comment Share on other sites More sharing options...
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