Looking for a little guidance on next steps in fault finding a stumble with my '37 flathead. The car fires very easily within a split second, regardless as to how long it has been sitting. The idle seems smooth but hitting the gas pedal and it all changes. There is a fairly major stumble. If you gently hit the gas pedal the engine falters but recovers and you can get the engine to rev. Once the engine is at high revs you can rev it without an issue, or at least without a noticeable issue. However, hit the gas pedal heavily from idle and the engine bogs down severely and may even stall or backfire. I rebuilt the carbs (dual Holley 94s) and am happy that they are operating smoothly. The issue is identical both before and after the carb rebuild. I am guessing the next area to look at is ignition but are there are any suspects more likely than others? The engine has a Mallory electronic distributor with power cell. http://www.speedwaymotors.com/Mallo...ModGDYlvFAS_xth5tPUIPS6YDEv7otoXti6dJyRfD_BwE Any input appreciated...
How are your plugs looking? What size jets are you running? If your timing is good, I wouldn't count the carbs out just yet.
Set initial timing and then run it with a timing light...you want to see advance happening right off of idle. For this you can just make a pencil mark on pulley somewhere around TDC #1, accuracy not required, just so you can see if advance is coming on quickly right off idle. Check the advance brake, back it off slowly all the way and then screw in very lightly between one and two turns. If it is real tight it could be a problem here. Next play with choke, running car through stumble with more and more choke added. This will crudely enrich things, and if you find a point of improvement fuel feed may be involved. Look over the long path from bowl, up and over, down to idle holes...be sure passages are open, find the little air bleeds on nozzle bar and check if open, be sure off idle holes are clean. Test with accel pump in riches setting as another diagnostic.
If it winds up right, but stumbles at light throttle, it's probably the transition circuits in the carbs. They might just be clogged, but usually with dual carbs, not enough air comes through each one to properly siphon the gas through the idle circuits at very light throttle settings. Drilling the butterflies, or resizing/relocating the transfer ports will likely fix this issue. I assume everything else is OK, because it both idles & winds up good. It's just the transition from idle to cruise that's ragged.
On the road this weekend and wont be back home till Tuesday but wanted to say thanks for the responses thus far. I will check on the above once back home. Never timed a flathead and it doesn't look to be as straightforward as with others. Don't see any timing marks anywhere on the engine. Probably should have mentioned it but neither carb has a choke fitted.
You said they were "rebuilt" but did they leave out the accelerator pump check ball or check valve? 9A516 or 9576 in the drawing or the pump skirt is folded over.
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