Need advice, jet size..

[jvike]

I'm gonna put on a new (used) carb today, and I'm wondering if anyone could help me with picking the right jet size. The carb is a Demon 750 DP. Engine is a 72' 340, (low comp, small valves). Motor is pretty stock, stock pistons, stock heads (1.88 valves), unknown cam, probably a stocker or close. Eddy RPM Air-Gap intake, TTI headers with TTI x-pipe and flowmasters. Probably a stock converter, 727 with shift kit, 3.23 with suregrip rear.

I'm now running a Holley 650 DP that I borrowed to test, It hesitates when I floor it from zero and it need 13-1400 RPM idle or it stalls when I put it in drive. But the throttle response is very good, and that carb really made my car wake up when I put it on. (Eddy 750 before that ) The 650 seems to be a little fat on the mix, would I be out of the ballpark just to move the jets from the 650 over to the 750?

[matt63]

I would bolt it on and see how it runs.  Basically, your ability to set-up and tune will dictate how well the engine will run. If you can read plugs or have an O2 meter you can fine tune the jetting.  If the carb is properly sized the jetting should be close assuming it is still stock.  Here is a good reference http://www.bob2000.com/carb.htm

I would suggest first setting the float levels and then the idle and idle mixture.  A vacuum gauge is useful for this.  Look at the Road Tuning section in the article for further guidelines.

[71chally416]

DP carbs are great for stick shifts or modified motors with gears and convertors, but I'd never use one on a mostly stock motor with no vert or gears. All it'll do is suck up gas and run worse than a good vac secondary carb will, but if you insist on running a mis-matched combo of parts start with whatever came stock in the carb.

[jvike]

Yeah, I know it's not the optimal combo, but I can only work with what I got at this point.

[71chally416]

My buddy here Tony does real well with a 750 ED carb, that is if you think 12.80's for a stock weight '70 340 Dart with over 100K original miles on the motor and a 3:55 rear, stock convertor/727 and 14" street tires is good. It has headers, a small Hughes cam and RPM Intake.

You can ID the original jets/parts for your Demon here>
http://www.barrygrant.com/demon/default.aspx?page=29

[moper]

The equivolent in going from the 650 DP Holley to the Demon is taking the 650 and making it an 850. It's way too much. I would learn how to make te Edelbrock run as it's just about perfect for your combo. If you think the Holley's fat and slow, wait until the Demon's on there...

[Chryco Psycho]

this is all about tuning , sometimes a bog can be tuned out with more accerator pump shot sometimes it is about tuning t6he driver to jump on it 1/2 way & roll into the throttle , what is happening is when you slam the throttle wide open ther is not enough air flow through the carb due to vacuum drop to pull neough fuel ito the air stream , Vacuum secondaries wait until there is enouigh vacuum to pul th esecondary open eliminating the bog .,Running rich can be other issues from idle mix or lumpy can causing misfire to jetting which is only involved at higher RPM to wrong power valve where th evacuum at idle is too low to keep the power valve closed causing a rich situation

[jvike]

The Demon 750 had leaks all over,so the 650 is back on.. Thanks