Hi fellas, i have a 74 F250 4x4, standard with a 390 i am getting real bad detonation/spark knock under acceleration and i been tryin to get rid of it
.030 bore
.030, 360 flat top pistons
stock rebuilt heads (C8AE-H), guides and most of the seats
stock 2bbl intake
2100 series Motorcraft #57 jets
.010 under crank grind
cam grind .486 ex .512 in
.012 machining on deck
.010 machining on heads
2" dia. maddog fender hedders
points setup
6 degrees initial with vacuum adv disconnected
dwell within range (24 to 39)

im sure the there is more i have done, any knowledge would be helpful and i yet to borrow a vacuum gauge

 

Since 4x4s were all 360s, and you list rebuild parts, I assume you now have a 390 with flat top 360 pistons. Flat top 360 pistons are the same/same spec as car 390 4bbl motor pistons, with 10.5 compression.

Using the other numbers you gave, I can't give you an exact compression number. However, if you are at zero deck with your pistons, and you've taken 10ccs out of the combustion chamber, you could be at 10.93 CR or higher.

You did use the 390 crank and rods, right?

 

Hi 85, b4 the build it was a 390 with dished 4v relief pistons and still had this issue just not as bad, btw all of the ignition parts are new, dizzy with points, (which im thinkin bout getting a premium set), also want to up grade the coil, plugs n wires are also new, my machinist was thinking 9 to 5 or 10 cr, and i tried running 93 octane fuel which did make a difference with out a facecord of firewood. yes it is the 390 crank n rods. should i do a compression check ?

 

Are you positive the harmonic balancer hasn't slipped? That can throw the timing off considerably.

 

Get a Pertronix solid state module for under a C note and be done with points.

 

Not many alternatives for the fix, premium fuel and less timing.. Some times
spark plugs can cause pre-ignition with the higher compression you are running, try using a colder plug.

 

Definately check the balancer like posted before, then set your timing with a vacume gauge, I like to set it to max vacume then back it off about 1lb, also what is your total advance and what rpm is it all in.

Give these guys a call Reincarnation 253-988-6648, they sell rebuilt and recurved distributors built to your engine for $155, best money I ever spent

 

Thanks for the feedback fellas, while the motor was at the machine shop i sandblasted and painted the balancer and it was not loose or anything like that, ya the next purchase is going to be a vacuum gauge, i cant read the harmonic that far advance, might have to find a better balancer that is more legible, it would b awesome to get a pertronics wallet is to thin for that right now

 

A variable timing light would be cheaper then a new balancer. You can't always tell if the balancer has slipped even if it feels tight.

 

You should be able to make your own temporary marks on the balancer, just measure the marks already there and continue those around, this should get you in the ballpark for checking total advance, also don't trust that your new distributor is good, I've installed a brand new Mallory before and it was advancing over 50 degrees

 

holy tamaters thats insane

 

Well I have some good news fellas, i put the carb top back on and warmed er up and set the timing to what I believe is 10-12 degrees and that made a hell of a difference then i took the vac advance off vwalla no noise what so ever, woohoo, now i just need to get my total advance calibrated.

 

hey, I was wondering if anybody might be able to point me in the right direction for getting weights and Springs for a stock
distributor

 

morning everyone, I know it has been a while since I posted any information, but I did find out what was wrong with my truck, my vacuum advance was advancing way too much so I bought a new one and adjusted it and I have no spark knock whatsoever:-)