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| Cam overlap / Ignition timing. https://www.slantsix.org/forum/viewtopic.php?t=3724 |
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| Author: | V. Tedeschi [ Wed Jun 05, 2002 6:25 am ] |
| Post subject: | Cam overlap / Ignition timing. |
I am having problems with a newly rebuilt 225. It idles nicely, but when "in gear" it starts to stall as it is given gas. Having tried two different Holley (1920) carbs on it (with no success), I now feel that the ignition timing is the culprit. I am using the DC electronic ignition, the compression has been bumped up to 9.0, and I have installed an Isky cam (#152115) which has a 32 degree overlap. From literature I have read, a cam with an overlap of greater than 30 - 35 degrees will have a low rpm response. Can anyone suggest how many degrees the distributor should be turned to prevent this stalling effect (or any other remedes)? vmted@mac.com |
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| Author: | Doc. [ Wed Jun 05, 2002 12:42 pm ] |
| Post subject: | Re: Dialing-in a New Cam |
Quote:
: I am having problems with a newly rebuilt 225. : It idles nicely, but when "in : gear" it starts to stall as it is given : gas. Having tried two different Holley : (1920) carbs on it (with no success), I now : feel that the ignition timing is the : culprit. I am using the DC electronic : ignition, the compression has been bumped up : to 9.0, and I have installed an Isky cam : (#152115) which has a 32 degree overlap. : From literature I have read, a cam with an : overlap of greater than 30 - 35 degrees will : have a low rpm response. Can anyone suggest : how many degrees the distributor should be : turned to prevent this stalling effect (or : any other remedes)? One thing about a cam swap is the need to re-adjust the distributor's advance curve and the carb. to get maximum performance. There are a number of things to set and as always, one system will effect the others. Here is what I do: 1) Find the "best" valve lash setting for the new cam profile, this may be a little different then the factory .010 Int. & .020 Exh. and you may want to try a little tighter / looser then the new cam maker tell you to set them at. 2) Adjust the carb's idle air/ fuel screw to get the best idle at this point. 3) Adjust the initial timing setting, you need a timing light and a vacuum gauge. Shoot for a setting that gives a smooth idle and the highest vacuum reading. Re-adjust the idle A/F as a double-check. (some cams like lots of initial, that's OK but you will then have to take-out some mechanical advance to keep the total advance in the 28-32 degree area) Use an adjustable vacuum advance pod to help get the "tip-in" setting correct. 4) A "soft start" or off idle stalling can be a lack if advance right off idle or it can be that the carb. needs to be recalibrated. If the new cam combo has changed the manifold vacuum then the power circuit (economizer valve) may need re-shimming. Here again, the vacuum gauge is the tool to use to get this "dialed-in". In general, make adjustments that maximize manifold vacuum at idle and this should help get the current advance curve and carb. settings to work better with the new cam profile. DD |
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