I recently had my 400 rebuilt with a larger cam and the works. I have the speed demon 750 VE carb jetted and tuned so that the plugs look good and everything seems about right. The only problem is when I am going along in gear and take my foot off the accelerator to engine brake, the engine surges when the engine gets below about 1800 rpms. So if I was just coasting at like 1500 in any gear, the engine would be lerching a few hundered rpms faster and slower at a rate of about 2 cycles per second. The problem goes away when the throttle is opened a bit. This surging or lerching is worse at jjust off idle, and completely disappears at above 2000 rpms.

Before the bigger cam, i was able to lean the idle screws out enough that the surging went away. I have tried removing the vacuum advance and retarding the timing, but it still happens. My theory is that the intermediary fuel circuit is too rich, although I have read that surging is usually a lean condition. Can anyone describe what a lean and or rich condition feels like? Any alternative explanations? Thanks guys.

 

Maybe it's a blown Power Valve that's letting fuel into the motor as your vac changes. check you vac at idle divide in half, add 1 inch and go buy that size power valve for a Holley. Most car parts store have them. Remove the carb and front fuel bowl and replace the valve. Then use your vac gauge and set the idle screws at the highest vac reading at idle. Maybe both items will fix your problem.

 

Paul Demons don't blow out power valves like holleys. I would tend to look for a vacuum leak which causes low rpm surging. because of lean condition.

If that is not the problem. You need to think about what's going on. When you let off the gas and shut the throttle blades your at highest vacuum. Only the idle and intermeadiate cuircut are supplying gas. My Demons have all had 4 corner idle screws. You start with good timing and the vac adv. hooked up. Set the idle high like 1000 rpm with the motor at normal operating temp. attach your vac. gauge to the full vac port on the carb.

Set every idle screw the same - like 1/2 a turn out from lightly seated. Then turn each one out 1/4 turn increments until you get the highest vacuum reading. As you get the air fuel ratio more correct keep turning down the idle to 1000 rpm. Once you have it set take off the vac gauge and connect up what ever you had attached to full vac port and turn your idle down to what ever you need. My idle is 850 - 900 rpm because of a big solid roller cam and single plane intake.

Even I've got caught inducing my own vacuum leak just changing the rear bowl jetting on my Demon. I have my power brake vac. line connected up to the 3/8th outlet on the back of the carb right under the bowl. Well when you pull the rear bowl of it snagged the vac. line and pulled it off. In my haste to get it going I didn't even check it. Well I fired it up and it ran like poopie :( I was thinking minor jet change :confused: Then I notice the brake petal was rock hard.

 

Thanks for the replies. I have already set the idle using this method, and it idles pretty well at about 700 with 15 inches of vacuum. When the throttle blades close, this creates very high vacuum which I believe is creating a rich condition through the intermediaries. With the old engine, I had the same problem only I was able to work it out using the idle screws. With the more radical engine (same engine rebuilt) leaning the idle out helps to some degree, but I get a lean idle before the surging problem goes away.

 

try turning up the idle to 850 rpm and see if it helps.

I found both of my Demons to be too rich at idle. So I drilled two .125 holes in the primary throttle blades. Which cuases a induced vacuum leak to lean it out. It might also change the high vacuum flow out of the transfer ports.


You don't have enough cam if your getting that much vac. at 700 rpm.

 

I have tried it with the idle as high as 1000, didn't help the situation. I lied, it gets about 12 inches at 7-800 rpms. Its a comp XE274-10.