Hi all. I recently posted in the dentside forum with my problems. I'll post here the trouble and suggested causes. Hopefully I can get a handle on where to go from here.
The truck is a 79 f250 with a 351M with a Performer intake and Edelbrock 1406 600 cfm carb to a C6 to Dana 60's front and rear. The engine is apparently not original, or the distributor isn't as it a points type.
Up until a week ago it got 13 mpg at it's best (highway), but it pinged pretty bad on hills or heavy acceleration. I knew it didn't have the hose from the carb to the vacuum advance on the distributor. I bought and installed the hose on the passenger side port of the carb. When I went to install it on the distributor, the whole distributor turned. The hold down bolt had loosened. I put it back where I thought it was before. It didn't run any different. The next day I set the timing to 11 btdc. It was near 20 btdc when I started. I set the points to .017 per the Haynes manual and checked the dwell. It read 15 degrees instead of 24-30 degrees like the manual called for, so I OPENED the points gap until the dwell got to 25 and locked it down. I also put a vacuum gauge on the drivers side port of the carb. The needle bounced between 17 and 13 rapidly. I averaged it at 15 and turned one side screw until the engine started sputtering, then backed the screw out until the vacuum got the highest, around 16 in/hg average due to bounce. After all that, the detonation ping is gone, but I'm down to about 8 mpg. Crap....
So, where do I start?
(I'd like to thank montanahighboy for all of his help up to this point!)
Hi, I have the same intake/carb combination on my 400.
Yes, I agree with your describtion of the adjustments you did - so far.
Guess itīs not the distributor itīs the carb makes the mpg go down.
Have you checked the colour of the spark plugs ? Are they light brown
or more white or black. Plugs colour is an easy indicator to check the engine is running too lean or too rich.
Watch for the vacuum hose, you must connect it to the ported vacuum at the carb ( passenger side ). The drivers side port delivers full vacuum from accelerating over idle. Had this issue by myself and ever wondered why had no power over 2k rpm ....
You wrote the engine had 20 BTDC .... didn`t you have backfires ?
Check your air/fuel ratio, the best A/F ratio should be between 14 and 17.
Start adjusting the carb carefully with both crews, not more than 1/2 a turn on both crews than check again. Screw in = leanm screw out = rich.
The edelbrock homepage has a carb manual for download, this may help.
Remember : lean does not mean high mpg, the right carb-setup depends on your whole engine setup. Youīve changed something by adjusting the dizzy and timing, now it looks like you have to adjust some other components.