Inliners International Forums General Discussion Engines vacuum with m4f cam

Hi all! I'm Bill and live by the gulf coast in texas. I'm 30+ months into what has become a full resto of a 62 1/2t chevy lwb fleetside. It's rolling and running now and has the cab and interior installed; I just lack finishing the rest of the body and I'm done. I'm the second owner and didn't realize when I bought it from a barn (not running and really sorry shape), that it was special ordered as a 5200 GVWR with a 261/4spd. This was the last year for the 261 and the only year offered as an option on the 1/2 ton truck. It even has the original larger Rochester 1bbl. That's why I kept it stock. Anyway, I rebuilt the engine (and everything else), all stock except for the M4F cam. (I have fentons and an offy 1x2 for some time in the future). I have only run the engine stationary for the last year and just in the last week drove it around the block, checking shifting, rear end, brakes etc. It drives O.K. but I thought I'd check everything and found my vacuum is 12+ at 850 rpm. I would think it should be higher. I checked compression with all cylinders between 130 and 145, timing at 5 BTDC, gap .018/ dwell 31, Idle mix 2.5 turns out, idle 850. The engine is still fairly tight and breaking in. I have set the valves several times: 6/13, 12/14, 14/16 and 16/16. 16/16 is on the card that came with the cam and actually the engine seems to like this the best. I have checked for intake leaks , but not as far as spraying wd-40 at the joints. The engine runs fine so i'm kinda stumped. Any ideas? Thanks in advance.

Dear Bill;

This is normal, even though the M4F is a 'mild' cam.

Also; If your 'cam card' says .016 set at .018 till the break in period is complete.

Screw in the mixture screw (at idle)till the engine 'stumbles' then back out 3/4 turn. This may help. Perhaps up to 900 on the speed seting too.

Good luck.

John, Thanks for the quick reply! I had read posts at other places about vacuum at idle, but none specific to the 261 w/ an M4F. I have adjusted the mixture as you said and it seems happy at 2.5, maybe a little less. Nothing I have done (timing,lash,idle, mix, etc.) has changed the vacuum enough to leave it changed. The vacuum pulls down to 3 and up to 21 when I snap the throttle and will do 16 at 2000 rpm. Its pretty steady too, with only a slight twitch at idle. Anyway It does pretty well as far as I have run it and I think it should get better once it sees some good road time. Thanks again!

Have any pictures? Sounds like an interesting project, good luck

I would like to post some pictures. I'll see about borrowing a digital camera and then figure out how to post. I think its looking really good, especially since I spent way more than I thought I would; and that's w/ me doing all the work save machine work, upholstry and paint. I did use both vacuum gauge and ear to set mixture and 2.5 terms seems the best. Thanks.

A good site (free) for hosting pictures would be http://photobucket.com

62RPO293,
If the engine likes the .016 / .016 valve setting as the cam card sais for the Howard's cam I would expect the vac to be higher. The .016 setting was too noisy for me and switched around ending up with .012 & .014, shortening up the duration to better match the rear without the engine heating up. I'm pulling 20" at 600 rpm same cam, however I'm pulling 160 psi compression across the board. Compression will effect vac readings. Valve timing is everything, match to your rear.