blower sneezing on dyno

we just started our new motor and tried to pull it on the dyno. 565 chevy, Dart conventional heads (410cc), 10:1 compression, 114 lobe cam installed at 112, 12:71 Mooneyham High Helix std discharge, 27 degrees timing. Autolite 3234 plugs gapped at .018. over drive was at 14%
motor starts and runs great. when pulled on the dyno, as soon as we start to lug the motor with the pump, it quits, pops, sends a flame out the heders, and then a puff through the carbs.
Is this a lean sneeze? any ideas would be great to hear.

Thanks

 

Dyno Sneezing

Off the top of my head, I would say that is is likely to be a too lean condition at idle and not having enough fuel in the intake to cover the area when the throttle blades begin to open. The order in which you stated the symptems might be off, too. Probably more like: loading the engine brings on a flame from the headers, then a pop as the engine quits and then a puff of smoke from the carbs. You might do a compression check on each cylinder to see if you have ring sealing issues now. I experienced about the same issues recently on a run with an experimental fuel management system when it leaned out down track. Video at Youtube search Prairie Hill Dragway Top Dragster Rod Throwing Blown Up Burn Down

 

events

we had the motor up to 5500 rpm prior to loading it with the pump. we were a couple of seconds from idle when this happened.

Thanks

 

Blower Sneezing

I understand but I do not think it matters so much as to how many seconds into the run that you open the throttle as much as when the load is applied and the engine needs enrichment to have the horses to pull the load. The engine will idle forever in a correct fuel air mix but all that changes in the split second that the throttle opens and the load is applied. You might want to run a richer idle mix and see if that helps the sneezing. Just my .02

 

we'll give that a try, thanks

 

blower

I got an idea, get rid of those carbs. But when you put a load on it, it goes into a lean condition. Fix it or it gets ugly. You need enough fuel and pressure to get past that. Don

 

cutout

Had the exact thing happen on 526 kb olds with Mallory III. The cause was the cutoff switch on the dyno couldn't handle the mag's primary voltage. The dyno was a Superflow 2000. Fixed with an external kill switch or you could use a higher voltage relay. Also lowering plug gap to .012 will fix most any primary voltage or secondary cross fire. Lowering plug gap might fix the problem. This all happened when we were running .018 on the Mallory. Later we went to .012 after alot of mag testing and have been there every since.

 

I would think that it is a fuel related issues because you have different fuel requirements revving the engine to 5500 with no load and then trying to do the same thing with a load. Do you have enough fuel up to the carbs? How about the acclerator pump?

 

maybe it getting a cold

 

X2 on leaning out.

 

Double needle and seat bowls?
Bypass regulator?

 

What dyno? Does it have fuel flow readouts? Air hat on it?
Are you even getting the run started? It sounds like not.
Quick test for lean/ ignition... take a bunch of overdrive out.
You will need a BIG PUMP and lines for an alcohol carb piece.
Wags