74 850 merc

[stormmn]

Well pro's, I have been fighting this 1974 850 long enough and cannot find the problem.
I took rebuilt carbs, rebuilt dist. with almost new trigger from CDI (with proper plate) new CDI ignition & coil, wiring harness, fuel pump, hoses, (even hooked it to a seperate fuel tank) new timing belt, and new plugs all from another 850>1975 that was running fine till it punched a hole in the top cylinder on #4.
the 1973 850 that I swapped the part to had been sitting for a couple years, but had 135 to 142 compression on all cylinders.
I have check all adjustments according to the book and calmato, set the timing at 27 deg. btdc at max. and 2 deg. btdc. at idle. set carbs as directed.
The problem: it starts fine and runs fine at idle and upto full dist. advance, but as soon as it starts into the carb. plates opening it starts crapping out and bucking. I have tried richining out the carbs. as directed but this does not change a thing.

ps.>> all wiring, dist cap, rotor. grounds, fuel, and everything else look good, I ran the voltage tests per CDI specs and voltages are fine, ran a timing light and spark tester and all tested good...
looked at the reed plates inside the intake while I had the carbs off, I did go through the carbs twice and double checked all settings.

what am I missing.

thanks guy's

 

[Chris1956]

Re: 74 850 merc

If your report is accurate, you are on the right track. Idle pickup timing should be 4-6* BTDC. The carbs should therefore start to open at 4-6* BTDC. If not, adjust the brass sliding ring around the midsection of the distributor body, to start the carb opening at that point exactly.

Initial idle mixture setting is 1-1/2 turns open from lightly seated. Proper acceleration usually requires a bit more opening of the idle mixture screws, likely 1/2 to 1 more turn, but must be adjusted in the water, preferrably while in gear. Trial and error is best, since the test is to accelerate smoothly and quickly.

 

[Moody Blue]

Re: 74 850 merc

The problem: it starts fine and runs fine at idle and upto full dist. advance, but as soon as it starts into the carb. plates opening it starts crapping out and bucking. I have tried richining out the carbs. as directed but this does not change a thing.

That is the problem if your description is accurate. You should not be at full advance when the carbs start to open. Full advance takes place ABOUT 3/4 full throttle.