Spark Knock
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Junior User
Joined: Feb 2004
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From: On a farm
Spark Knock
I have a 1985 F250 4X4 farm truck and I do a lot of towing. The truck was originally a diesel automatic. After the diesel died someone installed a 460 of unknown orgin and a 4 speed. When this engine began burning a quart of oil every 50 miles I installed a rebuilt 460 engine out of a 1976 Lincoln. It works great except I can't get the timing right. It worked fine in the Lincoln but in the truck I have to leave the vacuum advance unhooked to prevent heavy spark knock when under a hard pull at 2000 RPM. If I retard the timing to eliminate the spark knock with the vacuum advance hooked up the engine runs poorly at lower RPM. I used the distributor that was in the truck other than the Lincoln distributor because the wiring was different. It seems to me that I'm getting too big a range of timing advance. It seems to be advaned too much at higher RPM and not enought at lower RPM with the vaccum advance hooked up.
Hope someone has some suggestions. Thanks.
Hope someone has some suggestions. Thanks.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
So check total mechanical advance. Then connect the vacuum and see how much timing you have. You should get up to 8 to 10 degrees more.If it is more than that you need to change out the vacuum unit. Also you need to make sure that the total is about 30-32 degrees, back off the initial until you have that. Car engines quit often can run more timing than a truck.
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Junior User
Joined: Feb 2004
Posts: 59
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From: On a farm
The vacuum advance does advance the timing about 10 degrees so it must be OK. When I set the total advance at about 30 degrees with the vacuum advance connected with the RPM at about 2500 the timing is retarded at idle. I don't recall how much (it has been a while since I worked on it) but it stumbles badly when accelerating from a slow speed. I wonder if the springs are bad on the mechanical advance. When I set the timing at idle and have the vacuum advance connected the total timing at about 2500 is about 40 degrees. Consequently I have left the vacuum advance disconnected but is this hurting my gas mileage?
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Never ever set any timing with the vacuum connected. If you do you will have the retarded timing at idle. I wound suspect that you do not have total timing in by 2500 either (most factory dist. do't except for the HP units). Set initial so you have 30 to 32 degrees without vacuum connected. I would also suspect that you may need to run the motor clear up to 3500 to see what your true total is (unless the dist has been recurved). The vacuum advance is suppose to advance timing beyond your total maximum mechanical advance. That's how it increases gas mileage by advancing the timing more under light load conditions.
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Junior User
Joined: Feb 2004
Posts: 59
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From: On a farm
Thanks, I did not realize that total timing advance at higher RPM is to be set with the vacuum advance disconnected. That is probably my problem. I did know the timing when set at idle is to be with the vacuum advance disconnected. The last time I worked on the truck I ended up setting the timing "by ear". I merely experimented with advancing the timing until I got a spark knock then backed it up a couple of degrees. It starts, runs, and pulls very well (much better than my 350 GMC) but I'm probably hurting my gas mileage. Thanks again, I'll re-tune soon.
