Blower motor surge
 

What am I missing?

I know there's a lot of threads out there and I have browsed them but haven't been able to cure my surge...I have a bored 454 8-71 blower dart heads and if you sit there for an hour and mess with the idle mixture screws it will idle amazing at 950-1000 rpm but....if you even look at lowering the idle screw AT ALL it starts surging BAD...I'm talking 550-1300 surge and the only way to make it go away is to lean it way out which ok tried that and I can get it idling at 800ish but as soon as you try to drop it in gear it dies immediately...I'm pulling my hair out with this thing and looking for options

To summarize...
460 8-71 blower dart heads....blower surge
Dual 800 holleys

have tried...
lean it out to get it to idle at 800....dies immediately when you try shifting
richen it up and the surge goes away at 950-1000 rpm but the drive sounds like its gonna explode when I try to shift
Timing is at 20 degrees with a 10 degree advance still tuning
Tried timing at 30 degrees at idle and it helped but still the above issues

I know there has to be something I'm missing

Mine idle pretty high, higher than I would like but it lets me shift the boat and maneuver

You could;
1. Kick up the idle and live with it (are you running bravos or a transmission type drive? I run a trans so I'm not too worried)
2. Give it some gas when you shift. I had to do this when the idle was turned down.

Someone who is more knowledgeable may have a better solution. I've also read about the Daytona sensors / ignition. That seems to be the ticket for carbed blower engines and usability around the docks.

I have a bravo which is a lot of my concern...I have read a little about the idle transfer slot next to the throttle blades and I'm wondering if that may be my issue...possibly have to pop some holes in the primary blades but before I try that I'd like to get some other opinions on the matter

I would be good with 950-1000 if I had a trans but she clunks HARD when I shift and it drops like 500 rpm and I have to be quick on the stick to keep her going

Gotcha. Maybe someone will chime in on the Daytona sensor.

How about a momentary ignition interrupt switch? They came factory on some engines.

I had kicker solenoids on mine but I removed them because I didn't feel they were needed since I have transmissions. They are tied into the neutral switch so when it comes out of neutral the solenoid is activated and bumps the RPM up. They are adjustable so you can control the amount of increase.

I believe Mild Thunder has the cure.

I should mention that sooner than later I will be going EFI so spending the money on a Daytona setup probably isn't going to happen as the EFI will control the timing I currently have.

I'm not saying that this won't fix it or discredit the suggestions for it as I really appreciate it just want to be totally up front.

I just don't understand how there are hundreds of blower motors out there hooked to bravos and they shift just fine yet mine I am having nothing but fits with and I have the same setup as the ones that run great...I just have to be over looking something

We'll see if Mild chimes in

Thanks again for suggestions everyone keep em comin!!

When I had mine, same thing. I would put it in gear at the low side of the surge. It wasn't a real fix but worked for the time I ran it.

Boost referencing the carburetors will go a long way toward fixing that issue.

I thought that boost referencing only came into play at higher RPM's with the power valves? Could be wrong...probably wrong hahaha

Some people drill small holes in the throttle blades so you're not adjusting the idle screw out of the idle circuit.

That's what I'm kinda thinking..Just watched a video on you tube about it actually....my question to that is...in that video he says an indicator that you are outside of the idle circuit is the low speed enchrichment adjustments won't do anything when adjusted...well mine do a ton when adjusted so does that mean I'm good and should look elsewhere or is it something could still be an issue?

Boost referencing would give you a true vacuum signal versus what you'd get directly under the throttle blades. It would help the tuning process. There's a lot posted in the forum about the process.

I never got to it with mine, but had planned to. I ended up selling the boat before the next season got underway. Locking the timing at 30 degrees helped some too. Still, it would roll a good 500-600 RPM at times. People thought it sounded bad ass, but for me it was a big pain. Mine was a much smaller 250 with 850 Nickersons (dual).

Do your carbs have 4 corner idle adjustment ? how far are the blades open now ?

Yes 4 corner on both carbs and the slightest adjustment of any of the 8 screws can either be heard and seen in the engine or at a minimum observed on the A/F gauge

Wasn't aware that boost referencing had such a big impact all the way around...thanks for the info!!

Are you seeing major changes to the A/F ratio when it surges ?

Id like to know how having a boost referenced carb which opens a power valve after it sees boost has anything to do idle
??

I disagree with all of that . I have 3 years of playing with power valves , boost referenced carbs , and checking EGT and AFRs` . The fuel curve got a lot better when I threw the power valves in the garbage and plugged the boost reference crap.
Does your buddy monitor AFRs thru out the entire RPM range? Running great , not sure what that means. Youd never know mine was pig rich after the power valve opened and dumped a bunch of unmetered fuel . I even played with the power valve restrictors, what a waste of time

Boost referencing shouldn't change anything at idle unless the vacuum is causing the PV's to open.
It sounds like something is off if the carbs are so sensitive to any adjustment of the 8 screws.

Have you tried adjusting the carbs in gear and tied to the dock??

I have adjusted them while in gear...I had someone drive while I was tuning just idling across the lake and I got it running perfect no surge 750-800 in gear A/F 12.5-12.8 and I thought awesome I got it...then I shifted to neutral and the surge was slightly there then went away thought cool another win...then I tried going back into gear and it died immediately....and I was back to square 1....I'm planning to pull the carbs tomorrow and check my blade location in relation to the transfer slots and hopefully find something there...This could be why my 8 idle screws are so sensitive? Possibly getting a "bad" vacuum signal due to the plates not being adjusted properly? just speculating but maybe?

I'm also going to pull the primary power valves and jet accordingly...I don't see a reason to keep them and have numerous people tell me to pull them...my secondaries have always been blocked put going to block the primaries as well

A/F's during the surge are hard to say...the gauge doesn't really react as fast as the surge or at least not at the same rythum at least but you def can see it change while its surging

Make sure your IFR's are in the low spots in the metering blocks as well.

Can you elaborate on that a little more?

A powervalve opening at idle, DOES NOT DUMP FUEL IN THE ENGINE. It is not a hose spigot that just dumps fuel in the engine. I dont understand why people keep blaming powervalves for their problems. The only time a powervalve will fuk your idle is if it is ruptured. End of story!

The power valve orifice simply allows fuel, from the fuel bowl, to enter the mainwell. From there, it flows from the boosters. It simply acts like larger jets were in the metering block when the valve opens.

Boost referencing on a marine supercharged setup is a waste. Sure the PV sees direct manifold vaccuum. Lets say you have a 4.5pv in there. What offshore boat is going to be up on plane, seeing 4.5" of vacuum in the intake? My fountain would be at 0" of vacuum at 3000rpm. If the pv was boost referenced, it simply would be open, the entire time the boat was up on plane. It would dump fuel at 4000rpm, because it would have already been open at 3000rpm, or whenever the pressure in the intake, was greater than 4.5" of vacuum. In a car, they work great. Because we cruise in vacuum in a car. In a boat, we dont cruise in vacuum, at least not a high enough vacuum to keep a pv closed shut. Mercury did not use boost referenced powervalves on their SC engines.

These are not my words below (from this thread https://board.moparts.org/ubbthreads...fr-tuning.html)...but explain it pretty well. How it impacts a blower engine vs. N/A...I dont know.....but I know my N/A (and blow thru turbo) stuff idles much more consistently (AFR stays much more steady) with the IFR in the lower position in the metering block.


If you look at any of the original Holley carbs they all had a submerged idle feed, either a press in restriction at the bottom of the block, a pressed in restriction in the idle well itself, or in the case of the early Dominators an idle feed tube that has the bottom of the tube crimped smaller with the restriction drilled in to the end.

The NACA paper #49 concluded that submerged fuel metering passages are free from instability and irregularity of discharge when the head is small. Head is referring to the signal used to draw the fuel in. This is the basis of why the lower position is best. And I can always make a car run better at idle and light throttle with a low idle feed.

So why do some think the high position is better? At WOT the emulsion characteristics are different between the two, how the circuits reverse to become a bleed into the mainwell is different. It may be they found on a particular combination it improved the fuel curve ON THAT combination. With the variety of emulsion positions available on todays billet blocks it's not necessary to sacrifice low speed and light throttle operation. And if you are not carb restricted booster selection is much better than it was in the past, improves atomization without resorting to emulsion changes that screw up the fuel curve. At WOT on an engine that has a limited RPM range you can do some crazy things that might improve HP a small amount, but on anything with a wide range of RPM and load it's a mistake.

https://www.corvetteforum.com/forums...gine-idle.html
Heres a test a guy did to prove an open powervalve has no effect on idle

Good read..thanks for the info!! Gives me more things to check!!

Read that article yesterday Mild....very interesting...I'm wondering if a ruptured valve could be my issue...I did put new PV's in when I set this all up when I had all the motor issues last season and due to the valve issues I was having I got some pretty healthy backfires through the carbs so I'm wondering if they popped in the process...After talking to a few people like I said earlier I think I'm going to pull them and block em off and jet accordingly...I know you've been an advocate for them in the past however if it's one less variable at the moment I think pulling them to get them out of the equation is my best course of action currently. Can't hurt to give it a try anyway.

Let me ask this..could initial timing(either high or low) present a surging issue?

The reason the daytona box works well, really isnt just the timing. Its the combination of timing, with the carb adjustment. The engine loves lots of timing under load at idle. So, lets say you set the timing to 34 at idle.

The rpms increase, and to get them back down, you close the throttle blades further, which in most cases makes surge happen. Now, when you shift into gear, you tend to get a big rpm drop, because now the engine isnt able to get the air it needs under load. Guys used to drill butterflies for this.

By lowering the base timing, to say 16* at idle, you can now open the throttle blades further, keeping them at the desired amount of exposed transition slot. With the daytona box, when you shift into gear, the box can now add 22* of additional timing, which the engine loves under load. The rpm drop is not as drastic either.

modern efi has an iac, to help control idle airflow . It constantly adjusts airflow to help maintain an idle speed. With carbs, thats not an option. But the fact is, the engine needs air, esp under load.

If you tune it while tied off to the dock, it will simulate the same load as when shifting into and out of gear.
If you tune it while idling and moving with it in gear, the load will be much lower and it won't simulate the same load as when its just shifted into gear.

Makes total sense mild and griff....I'm beginning to think its a combination of small things causing a big thing...we'll see what I can find tomorrow while tinkering and report back...Def all good info to check thanks a ton guys!!

It's a no brainer that if I were to stay with carbs Daytona would be the ignition of choice but I do plan on going EFI sooner than later and I can have timing control with that so I feel like at this point the Daytona would be a waste of money...If I would have researched further before I bought my current setup I would have gone Daytona but is what it is at this point..I'm basically trying to get this thing driveable enough to do final testing and tuning and know I have a good running boat going into winter and I can do the interior this winter and be done and actually enjoy a ride on it next spring for the first time in 2 years lol

My timing issue that I am diving into is that I initially set the timing at 20 with 10 advance yet after running WOT for 2 passes I stopped to verify the timing and it was at 30 initial so either there is a broken spring or the advance bushings bound up or something...however the idle surge did not change with the extra 10 initial timing...if anything it got slightly better to be honest so that's my first course of action is to get the timing issue figured out whether that be lock it or fix the sticking issue and move to the carbs from there....I did watch the timing while someone else was operating the boat and after it stuck at 30 from idle all the way to 4000 when I told them to stop the timing did not change a bit so something is bound up in there just need to figure out what it is.

Will def check into that as well if everything looks right in there...thanks!!

https://www.youtube.com/watch?v=ui2g...ature=youtu.be


I was just playing with the idle stuff in the garage .. I don`t have it in learn mode but the EFI varies that timing like Daytona does.

So after messing with it for another day....I said screw it and bought 2 new 950 holley carbs....much much better!! Still a slight surge when it warmed up but a couple more adjustments and I think I'll have it....not sure if the baseplates are bent or what on the old carbs but they were just way too inconsistent something is not right with them...so easiest way out was to just buy new ones and jet accordingly.

Also locked my timing at 30...I know it's not ideal but to take one variable out of the equation and actually get this thing on the water and run it I decided to lock it...can always change it later

I love my Daytona sensor ignitions! I have multiple friends with successful results!