Blower surge Make it stop
 

Ok Im sure this has been disused allot before/ I have a twin engine Nordic that Ive built two 509 tall decks with 871s 8.7% over driven with two holleys 4150 series 830 cfm and cant get them to idle smooth under 1000 RPMs
The engines
509 Tall decks
Brodix big brodie 344 cc runners
TBS 871 stripped
Chillers
Cam Comp solid lift fixed 114 grind lift intake .561 exhaust 569 Duration @.050 intake 234 exhaust 238
Lightning headers
Hardin Marine max volt ignitions
Timing locked at 34 advanced
Carbs 4150 830 cfm 4 corner idle dubble pumpers anular 88 primary jet 92 secondary 8.5 power valve primary blocked on secondary .067 air bleeds boost reference primary power valve
8.5 compression static
50 bottom pulley 46 top pulley 8.7% over driven

What I have tried/ had the carb shop rework my brand new carbs 1500.00 did nothing/ under driven 8.7 ( swapped pulleys ) nothing/ dropped timing back to 10 degrees advanced nothing/ tried a non stripped 871 blower nothing/ opened my idle air bleeds from .063 to .067 did nothing/ drilled holes in the butterflys nothing

So far ive tried everything under the sun to stop my surge idle and I feel like if I took a dump in the carbs while its idling it would keep surging. I can get them to idle smooth at 1050 rpms and as soon as I try and take it down below that the surge starts. The surge is pretty healthy hunts 600 rpms to 1500 rpms and pretty quick surge. Ive tried most every recommendation I have been told or found on the internet and still no change. Nate at BDS says my cams are too mild and that is the problem. Post vacuum is 20lbs to 12 lbs while surging and when steady at 1050 16lbs vacuum. Pre vacuum under carbs is 14lbs to 25lbs during surge and 20 steady at 1050. Looking for someone who can really answer the question of how to stop this. A real answer or info not just someone spouting what they don't know! Thanks

Oh almost forgot they made 850 HP @ 5500rpms on dyno and 1250 degrees exhaust temps 12 to 1 AFR

I would upgrade to Daytona sensors ignition, it has some pretty advanced tuning options like idle stabilization.

Funny story a friend just ordered his " name brand" 900 blower motors specifically WITH blower surge, the builder hesitated because he thought it might make him look bad! LOL

what sensors are you talking about I don't have EFI they are carbed!

http://www.daytona-sensors.com/CD1.html


unlocking your distributors combined with this should fix it if everything else is in order

In my original post is where I stated that Ive taken the timing all the way back to 10 degrees and it yielded no change at all. And that was with 8.7 over driven and tried that at 8.7 under driven and no change. So dumping the timing off did nothing so don't see how a timing retard would fix it. Timing retards are generally used to help start a locked set up!

Opening up the idle air bleed hole sizes leans it. Did you try smaller ones ?
Are your idle feed restrictions too small ?

I have done testing to see if its too lean or rich and it gets happy when you lean it out. So with opening up the air bleeds .004 made the mix better but still once you drop below 1050 rpms it wants to serge. When I get the mix nice I can get it to idle at 850 to 900 but then its way lean and dies as soon as you put it in gear and makes it next to impossible to start when its cold.

Ok. So, youre idling at 1000rpm with 34 degrees correct? And when you set timing to 10 degrees at idle, did the rpm drop down? It should have, without touching the carbs idle speed screw.

On to the mixture circuit. If you turn the idle mixture screws in, does the surge stop, and engine want to stall ? It should.

Where are the float levels set? Too high of float level, can cause overly rich fuel mixture , and not just from having them high enough to see a visiable fuel dribble. I mean, like 7/8 of the sight glass, to 1/2 of the sight glass.

It sounds to me, like you are simply giving it too much fuel at idle. With that small cam, and locked timing, shes drawing fuel from those idle circuits like crazy.

If you have adjustable idle feed restrictions in the carbs, going smaller can help that. They are much more effective than fuking with air bleeds.

When everything is working properly, the daytona sensors ignitions work beautiful. On the blower motors Ive tuned, i set them up so they idle around 15 degrees in nuetral at 950rpm. Once the rpm falls below 700, which would be when you shift into gear, the idle then kicks up to 36 degrees. It needs the timing in gear, but not when its unloaded in nuetral.

The reason it works, is the reduced timing, helps slow the engine down. When the idle timing is cranked way up, the vacuum goes up, and the engine is sucking on that carb like a baby sucking on his mothers nipple. To calm it down, you end up having to close the throttle blades off completely to get an acceptable idle speed. With the timing backed off, you can now actually fine tune the carb properly, and when its idling at the dock warming up, everybodys eyes arent watering from the raw fuel fumes out the tailpipes

Havasu, the Daytona sensors ignition alters timing quickly (Up and Down) in stabilization mode to keep the idle level. This feature is built into the unit. It also does many, many other things. You leave the carbs on it...