Bob,
Finally I solved the backfiring problem. It was
incorrect valve adjustment on 2 cylinders. One intake
valve was being held open at the wrong time. Worked OK
until I applied power, then KABOOM.
Apparently I am unable to read english.
The book said to mark TDC as "A", 180 degrees as "B"
and 270 degrees as "C". Well I did that, sort of, as I had already
marked my damper at 0, 90, 180 and 270 degrees with punch marks.
The book said to adjust so and so valves at "A" ( on the #1 compression
stroke). Rotate 180 degrees to "B" and adjust some more valves, Rotate
270 degrees to "C" and adjust the rest of the valves. As there was a
diagram of the A,B,C marks on the damper, I noted that "C" was 90 degrees
from "B" or 270-180=90. Did positions "A" and "B" correctly, but
when it came to "C" I just rotated 90 degrees to the next mark. Well
adjusting values at this point gives two valves at the wrong place.
I should have done what the book said and "rotate 270 degrees to "C".
After extensive investigation, I wound up adjusting the valves
every 90 degrees as you had recommended. I just got there differently.
First I have a Clevland, which has a different firing order. Using
Powerpoint on my computer, I physically drew the 4 cycle function for 2
revs and 8 cylinders and overlayed the camshaft specifications .
I then selected the timing mark that was closest to the center of the
gap between the Intake cam openings and likewise between the
exhaust cam openings. This was to be sure that the valve in question
would be on the bottom of the cam lobe. This also allowed me to select
those valves that were to be fully open and measure the lift. The
lift measurments didn't work consistently due to the various leakdown
of the lifters. The valve adjustment did work. I gave 1/2 turn past
the push rod contact. Now when I floor the peddle at 40 mph in high
gear, instead of KABOOM, I get WHAAAAAAAA!
This graphic I did also explains why the Crane Cams say to
adjust the intake just as the exhaust of that cylinder is opening and
to adjust the exhaust just as the intake is closing. That method
may be fine for solids, but hydralic lifter leakdown may cause slight
errors.
The old method of adjusting the intake and exhaust at TDC of
each piston also has errors, depending on the cam design.
>
>Bob Palmer wrote:
...
> When I do my adjusting, I do it every 90 degrees of engine rotation. I know
>> this is overkill as you can get away with doing it every 180 degrees. But I
>> use a hand-held switch that lets me crank the engine over while watching the
>> crank angle and just move it to approximately the 90 degree positions; eight
>> positions in total, then I repeat again and double check the lash. With this
>> procedure, I can set all the lashes to within better than 0.001". Of course,
>> this degree of accuracy isn't necessary with hydraulic lifters. A quarter
>> turn past zero lash is sufficient and also prevents lifter pump-up should
>> you ever over-rev and have valve float. Here is the adjustment sequence I
>> use:
>>
>> Crank Angle Intake Exhaust
>> cyl. # cyl. #
>> TDC 8 5
>> 90 ATDC 1 4
>> 180 ATDC (BDC) 5 2
>> 270 ATDC (90 ABDC) 4 6
>> 360 ATDC (TDC) 2 3
>> 450 ATDC (90 ATDC) 6 7
>> 540 ATDC (BDC) 3 8
>> 630 ATDC (90 ABDC) 7 1
>> TDC 8 5
>>
>> I hope this table comes out legible. Note, of course, that "TDC" refers to
>> cylinder #1; i.e., the valves are closed. In practice, you don't have to
>> start at #1, just which ever #1 or #6 happens to be at TDC when you rotate
>> the crank to the TDC mark.
>>
>> TTFN,
>>
>> Bob
>
James Barrett Tiger II 351C and others
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