Very slow revving engine

[rewguy]

408 ls1. 4.030 bore, 4.0 stroke. 11-1 (verified). Ported ls3 style head. Carbureted. 750 holley HP carb. (All stock configuration) 242/258 @.050 .636/.606 on a 113 + 3. Single plane super Vic intake. Tried everywhere from 26-33 degrees of timing locked out from 1000 rpm up. Revs up lazily and comes down lazily. We have a nearly identical engine at 370 cubes that is super snappy. Same heads, nearly identical cam and compression. For some reason. ... I'm thinking the carb is not the culprit. .... although we will try a different one that is a known well working one in the next day or two. Any other ideas? I dont believe that the shortblock has any issues. Car seems to make decent power. Mid tens power for sure. But VERY VERY VERY lazy revving. Revs like a worn out 8-1 compression mid seventies smog motor. Car shifts at 7000....no issues going down track. Cranking compression is 185 or so on all holes. Any ideas why it revs so slowly in neutral?

[cgarb]

Vacuum leak somewhere maybe? A 150lb flywheel?

[rewguy]

The intake has been on and off a few times. ....we can triple check for a vacuum leak. ...but I'm 90% sure there is not one. And it is an auto car with a 9" converter. .... very similar to the exact setup in the smaller engine.

[induction apprentice]

Stop revving it in neutral.

[cgarb]

Really, what it does in neutral has no effect on how it runs. If its a long stroke engine it will tend to not be as snappy in neutral. I used to race a friends Torino that was 390 FE powered. By reving that engine in neutral you would swear it wouldn't run 13's. Put it in first an nail it...whole different story. Pulled the wheels up and ran 11.0's.

[rewguy]

Trust me. ...it doesn't get revved up high. ... lol. Just little bursts to 3500 or so. It just doesn't seem anywhere near right. Pure piggish....

[rewguy]

I'm quite familiar with differences in longer stroke, etc. Trust me when i tell you. ..... that is absolutely NOT the case in this engine. ...... this thing revs like a pig. Ive done 408 engines before that were very very crisp. Same stroke. .. etc. This is abnormally slow. Take me at my word on this. ......"wooooooooombahhhhhhh" lol

[Carnut1]

I have found that quench and swirl make a big difference how snappy a motor revs. Do you have quench and swirl numbers?

[John Wallace]

Exhaust partially clogged?

[mag2555]

A quick jetting check can be done with the motor up to fully normal temp and out of gear, then disabling the accelerator pumps ( remove the pump Cam's ) and winging the carb wide open .
The motor should stumble a bit , but not back fire which woukd point to a lean jetting condition.

What do you have for a distributor , is its mech advance hanging up and changing slowly, HEI Dizzys are famous for this due to old dryed up advance shaft grease!

[user-612937456]

A quick jetting check can be done with the motor up to fully normal temp and out of gear, then disabling the accelerator pumps ( remove the pump Cam's ) and winging the carb wide open .
The motor should stumble a bit , but not back fire which woukd point to a lean jetting condition.

What do you have for a distributor , is its mech advance hanging up and changing slowly, HEI Dizzys are famous for this due to old dryed up advance shaft grease!

This is the right track unless you have the cam a timing off somehow

[groberts101]

If you know there isn't a vacuum leak somewhere.. and the timing was locked out to eliminate the mechanical.. then my bet is on carb/fueling issues

[fdicrasto]

Is the distributor advance locked or not? You mentioned it locked out at 1000 rpm and above. What happens below 1000 rpms ? Is there still some mechanical advance mechanism in this distributor? If it is an HEI, as mentioned earlier, the shaft and cam will freeze and not allow the proper free movement of weights or cam. They are notorious for this and whatever position of advance or retard they want to stick at, problems similar to yours happen. Could be something else but I have seen this many,many times.

[rewguy]

The ignition is distributorless. It has a Daytona Speed Sensors box...... which is the overwhelming choice for carbed ls engines. Timing is locked from 1000 rpm up at this point to eliminate any variables.

[cgarb]

This is an LS1...probably has the MSD carb conversion on it, with programmed ignition curves. Below 1000 probably retards for starting and locked out full advance above 1000rpm. No stuck HEI weights or dried up grease.