SBF, lot of cam [242°], 10:1, Holley 1850 [vac sec], timing 18°initail-36°total in by 2200, rear gear 4.11. When decelerating in gear everything is fine until engine rpm goes below 2000. Bucks the most in fifth gear, the least in first. Does it more now than used to in the past as best I can remember. Any clue?
Manual or automatic transmission? One thing you need to keep in mind is that cam will not make decent torque until 2800 rpm....
5th gear under 2,000 rpm under decel? Maybe you should be down shifting a little sooner, or shove the clutch pedal in a bit sooner.
Interested to see what the consensus is on this .. Some I'm sure will say its timing ,others , too much cam Still others will say its too lean while others will say its too rich .....
My guess........with the cam you've got it's getting 'lopey' at lower revs, with enough torque so you can really feel it. I had an engine once that would actually chirp the tires idling along in smooth concrete carparks. Sounded awesome in the underground ones....
I suspect that the engine is lugging due to poor fuel delivery. Venturi that are too large for the demand leads to poor fuel metering. They need to be small enough to induce good velocity. If they are too large, fuel will not be drawn out. The carburetor is too big for the engine, and the cam is not fit for the usage pattern. I run a similar setup with an engine that is all of 200 inches. I don't have this problem, even with a decent cam, because I don't have venturi.
The only thing I've been able to do to " help" this problem is to be sure the throttle plates are as close to completely closed as they can be & still idle . As mentioned , this helps to keep the mixture velocity as high as possible ..
Relatively new, yes. Wasn't as harsh in the past. Carb is only a 600cfm with vacuum secondarys. Trans is Richmond with .76 6th.
.... you must be trying to pull-our-leg on this one. shorten-up the camshaft by 20* @ .050 and that issue will disappear
I know but hate to disturb a 46 year veteran with a maybe problem, youth is not on my side. Here is something I thought of: maybe the secondarys are opening/closing some with the heavy vacuum of deceleration? Maybe the linkage that is supposed to keep them closed when primarys are closed needs adjusting; maybe not but is something to check..
OK .... then push the clutch in and down shift, you know you can't "lug" a small CID engine with that size of camshaft in it.
Running a Isky E3 in my 292 Yblock does the same thing. it has a 3.80 rear gear so I run it in second at about 3000 rpm the little motor loves it . Anything below that i just press in the clutch.
Too slow of rpm for a given cam the engine will run poorly. Doesn't matter if the rpms are climbing or dropping. Get out of that sweet spot it's gonna run shoddy. Want the engine to run smooth, put it in the right gear for the road speed. There is truth to the ole tales of Ferrari tuneups. Up shift too soon, not enough revs, don't downshift when slowing, engine is gonna bog up and get carbon fouled. Run it in the right rpm range, it'll run clean. An engine decelerating in the correct rpm range sounds just as sweet as when winding up in gear.
I can't help but wonder what work, if any, has been done to the op's heads, and how much compression. You just can't throw in a big cam and expect an engine to run well. If he has good heads and compression, he should consider stepping up to a larger carburetor. I don't think he's looking to break any mpg records. Having a manual transmission and gears, he might want a carb with mechanical secondaries as well.