Fulcrum Fixture
Posted 14 January 2015 - 02:38 AM
I love these guys who come out and say a 750 is too big for a 308 and a 600 will be more than enough, a 750 what... Vac secondary or double pumper??? Spread bore??? Square bore???
At what RPM and MOST IMPORTANTLY and I see no one has mentioned... What manifold?
Single or duel plane?
A Rochester, factory fitted, is rated around 750...
Fulcrum Fixture
Posted 14 January 2015 - 02:52 AM
Ok that will do, 750 it is.
I have it on the engine at the moment, it idles good and if you slowly bring the revs up it runs good but if i give it a quick rev it backfires big time.
Have i missed anything or done some thing wrong that will shed some light on the backfiring.
Check the accelerator pump shot is coming in soon enough and strong enough, sounds like a lean out to me.
Oh My, Don't you post alot
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Posted 14 January 2015 - 08:11 AM
The 780 that came on my L34 from new works really well.I love these guys who come out and say a 750 is too big for a 308 and a 600 will be more than enough
Fulcrum Fixture
Posted 14 January 2015 - 03:27 PM
No doubt.
Somehow, I think someone who has built probably a Million 308's has somewhat more of an idea than one who has never.
Lotsa Posts!
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Posted 26 February 2015 - 12:06 AM
We found the problem with the backfiring, turns out the timing was way out.
We had it set at 12-14 deg, thats when it backfired, advanced the dissy and it stopped.
Now the timing is about 30+ deg advanced, timing mark on harmonic balancer is pointing to the bolt next to the fuel pump.
Now we have to pull the timing cover off and check why, might be a tooth or 2 or 3 out.
Fulcrum Fixture
Posted 26 February 2015 - 07:28 AM
Balancer slipping?
_oldjohnno_
Posted 26 February 2015 - 08:01 AM
I agree with TerraLX, it sounds lean. I wouldn't be pulling the timing cover off yet, though I would verify that the tdc mark really is at tdc. Super rich and super lean mixtures both burn very slowly, so slow that they are sometimes still burning when the intake valve opens, and that's when they bang back through the intake. Giving it great big handfuls of advance will stop it backfiring (by lighting the fire earlier) but it doesn't fix the mixture problem, just masks it. Fix the mixture and it'll be happy with "normal" spark settings.
Check carefully for vacuum leaks. I once spent several hours looking for a huge leak, changing gaskets and refacing surfaces, nipping up bolts. Somehow I'd overlooked an uncapped hose fitting under the carb... 
Lotsa Posts!
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Posted 26 February 2015 - 10:33 AM
Terra: We changed the balancer with one from another engine and the marks are in the same spot.
Oldjohnno: We set the mixture screws (screw in all the way then back out 1 1/2 turns, is that right).
To check the timing mark do you pull out no1 plug, wind over by hand till it blows out the hole (check with finger over plug hole),
then put a screw driver or something long in plug hole and wind over carefully by hand to get TDC, timing mark on balancer should
be on the TDC mark on the block.
I have checked for vac leaks multiple times and think i have got them all now.
Just for reference if it helps.
Carby is 750 vac sec
New electronic dissy
New Bosch HEC715 coil
New 8.8 mm Top Gun leads
BP6FS plugs, just pulled no1 plug to check the number and found it is cracked in 2 places from the top to the bottom of the porcelain.
Speedmaster raised intake manifold
Cam spec sheet.
Cam Specs for Tracie's engine..jpeg 71.09K
4 downloads
_LS1 Taxi_
Posted 26 February 2015 - 11:33 AM
Hey Mort,
You are only setting idle mixtures when you adjust those screws. 1-1/2 turns sounds fine as a starting point. Fine tune it once you have everything running properly.
To me your symptoms sound like a lean situation. As someone mentioned above, I'd be checking the power valve is operating properly. Should have a nice consistent squirt of fuel as look down the carby with the engine off and throttle opened. On an older carb, this is a fairly common issue.
Fulcrum Fixture
Posted 26 February 2015 - 02:09 PM
What is the present idle speed with the advanced timing?
Try backing off the timing a little then wind each mixture screw out 1 turn and then come back and tell me if the idle speed and base timing come back up and off idle response improve.
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