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custom-cruiser
01-30-2002, 04:05 PM
OK.... I know this subject has been beat to death but lets go over it one more time.
I just switched from dual Carter 600's to dual 4150 Holley 750's. I plan to block the power valves as they will not be boost referenced. I am at 2800 ft. elevation the Engine is 468 Chev. with 8/71 BDS running very mild boost(about 7 PSI). The question is what is a good range of jets(#'s) to try? Squared or stepped up? These carbs sit bowl to bowl not side ways so I want to narrow it down as close as I can because the whole carb(s) has to come off to rejet. This is not any sort of race boat Its a big old(76)Howard daycruiser(but if I do whoop up on a certain 18 ft. Couger jet that would be just fine.) http://free.***boat.net/ubb/biggrin.gif

BradP
01-31-2002, 03:40 PM
Pick up a copy of "Street Supercharging" PAW has it in their catalog. It has a section on blowerr referencing carbs that I found very helpfull. There's more to it than just jetting, the author suggests that as a last resort

BOOSTDADDY
02-02-2002, 12:14 PM
CUSTOM CRUISER- YOU GOT IT BACKWARDS! YOU NEED POWER VALVES AND BOOST REFERENCED CARBS! THE POWER VALVES ARE FOR PART THROTTLE FUEL ENRICHMENT WHEN THE ENGINE IS STILL IN VACCUM AND UNDER A LOAD (CRUISING SPEEDS). THIS IS THE POINT WHERE IT LEANS OUT!
I'M POSITIVE THAT YOU CAN FIND PEOPLE TO ARGUE THE POINT AND THAT IT'S THE OLD WAY OF DOING THINGS, BUT YOU WILL NEVER HURT YOUR ENGINE BY BEING LEAN WHEN YOU ARE ON THE WATER HAVING FUN!

Infomaniac
02-02-2002, 09:41 PM
BOOSTDADDY: No wonder you will find people who will argue that. Power valves are part of the "power enrichment system" Power does not occur at cruise. It is to make sure there is sufficient fuel at high power settings.
This is why they are vacuum operated. vacuum keeps them closed (cruise) (part throttle) when you open the throttles for power, vacuum drops and they open providing the additional fuel needed for higher power. Not old school just basic physics.
If you need to boost reference your carbs. The real problem is not enough carb for the blower. If the blower is sucking enough vacuum under the carb at high power settings to keep the power valve closed. The carbs are not allowing enough airflow to begin with.
Granted boat engines are loaded harder than in a car. But on the other hand a blower is providing the carb a stronger signal and as a result more fuel is being metered. The same jetting is running richer than it would be if installed on a non-blown application.
It is much easier to block off the power valves (and jet up when you do so), than go to the added expense to boost ref the carbs. I am sure the engine builder and carb builder prefer boost reference. Good for the revenue.
custom-cruiser: If you are running a very large boat and require a lot of throttle at cruise, you may consider boost ref carbs. I am pretty sure this is not the case.

Unchained
02-03-2002, 05:45 AM
An easy way to tell if you need to boost reference the powervalves. Take a vaccuum reading at the carburetor base at WOT. If the vaccuum reading is less than the power valve number no boost refencing required.
Example, my engine 540ci, 871 blower, 14 psi boost, Two 850 carbs. Power valve size 4.5 My carb base vaccuum reading at WOT was 1 in mercury. So I concluded that no boost referencing was required,the power valves were still open at WOT. As Info stated, vaccuum closes the power valve, lack of vaccuum opens the power valve. I think that you would at least want power valves in the primarys or you will have a major flat spot. The power valves make for a smooth transition from idle circuit to main jets.

RumRunner
02-03-2002, 07:02 AM
Boost Referencing the power valves is the proper way to set up carburetors for a blown application, not just a way of generating some $ as some would have you believe. There are also other changes to carburetors for blown apllications to help with performmance, and distrabution. The later can save you lots of money on rebuids. Even if your engine doesn't produce enough vacuum at WOT to shut your PV doesn't mean it won't work better referenced. While your at part throttle your engine may be under enough load to use the additonal fuel if you're making boost, but if you PV isn't referenced you can be lean. Taking the PV's out will make the engine run rich at idle, and part throttle since you have to go up on your main jetting to compensate for this !

Infomaniac
02-03-2002, 09:27 AM
Impossible for the main jet to have any effect on idle. The idle jet and idle mixture screws are the only thing metering fuel at idle!
Improper position of the throttle valves in relation to the transfer slot can really make one run rich at idle. Blown (leaking) power valves also.

Snowboat
02-03-2002, 10:33 AM
What does Boostdaddy mean by "BUT YOU WILL NEVER HURT YOUR ENGINE BY BEING LEAN WHEN YOU ARE ON THE WATER HAVING FUN!"? Made no sense to me, unless he's kidding.

BOOSTDADDY
02-03-2002, 12:11 PM
Snowboat, When you are just driving your boat around and not in boost, the engine is under a load. This is the point where boats lean out and hurt the engine.This really shows up in bigger boats, guys with lighter and smaller boat don't seem to have too many problems with this.

Snowboat
02-03-2002, 05:33 PM
OK Boostdaddy, I think I see what you are talking about. My boat is light and I blocked off the power valves because it was easy and I kept the jets rich. The Biesemeyer is quite light and I checked the plugs at various extended rpms. They are fine, but a little rich and thirsty. I can play around a litle more this summer so I have a couple of questions. I see by your profile that your opinion has weight. When I dig my books out it says that I should block the vacuum passage on the underside of the throttle plate, and drill from the outside, into the hole above the place where I put the plug. Insert a tube and run a vacuum hose into where, an threaded hole in the blower manifold? Do you Tee the carbs together or run a separate line for each carb or two lines for each carb? Potentially there can be four vacuum lines. The other thought is if your cam is .700 won't the vacuum signal be so low that you might as well just block off the power valves?

sgdiv7
02-04-2002, 06:41 AM
I have boost referenced carbs. on my bbc w/ 8-71. they really do help because I cruse around most of the time at about 8-10 in. of vacuume. I have no problem with the way it runs at part throttle. It saves on gas and plug color looks better. I like to spend more time crusing then filling the boat with gas.
I teed the vacuume lines together and ran the line to the manifold. You are always going to make vacuume on part throttle.
Bill

Infomaniac
02-04-2002, 09:22 AM
I don't get it. Someone needs to explain it to me.
sgdiv7: boost ref carbs are not helping you with fuel economy. Your powervalves would be closed anyway at that vacuum.
RumRunner: I agree that the engine will need more fuel if the engine cruises under boost. The method used to get more fuel is a matter of personal preference.
BOOSTDADDY: Please explain how boost referenced carbs operate the powervalves while cruising under vacuum.

Unchained
02-04-2002, 01:34 PM
Info, I don't get it either, I asked myself the same questions. I think it is a way to justify boost referencing the carbs when they didn't need it anyway. At cruise speed a blower isn't making any boost. I am afraid some people don't understand what a powervalve is and what it does. Also I think most people have a power valve with an incorrect rating. I always understood that the powervalve rating needs to be about 2in. below the cruising manifold vaccuum.

schiada96
02-04-2002, 03:17 PM
Dosen't holley sell boost refrenced carbs? A simple call to them would help with jetting baseline.
[This message has been edited by schiada96 (edited February 04, 2002).]

Unchained
02-06-2002, 06:37 PM
Some good carb tuning info from partscavengers.com
Power Valves
The power valve?s function is to supply extra fuel for wide-open throttle, high load conditions. When manifold vacuum falls below the vacuum level stamped on the power valve, the power valve opens and enriches the main circuit by about six to ten jet sizes. This occurs under high engine loads such as full throttle acceleration.
The power valve number should be at least 1.5 to 2 in/hg vacuum under engine idle vacuum. If an engine produces 8-9 in/hg vacuum, a 6.5 power valve would be a good first selection. Using a power valve with a rating lower than this will delay the enrichment and can cause hesitation. However, on an oval track car with restricted carburetor rules, using a lower rated power valve can sometimes help performance coming off corners.
Any drag race carburetor with a secondary power valve must be turned sideways to avoid fuel starvation. The power valve is higher than the jets and is the first to be uncovered as fuel is pushed to the rear of the float bowl. Since there is no way to put an extension on the power valve, the carburetor must be turned sideways to eliminate fuel starvation.

Drew
02-06-2002, 08:55 PM
Boost referancing is not that hard if you are into this type of machining. Only way I can try and explain the need is it is possible to run a vacuum below the carburator but still be in a boost or less of a vacuum at the manifold thus possibly causing a lean condition. Mainly with a larger boat or some thing that runs under a load but not WOT. Personaly I referanced but on a light boat probably not required. Running a blower motor in a 18' flat bottom is a big differance than the 29' vee I run now. Hard to cruise any length of time at 4000rpm in the 18' but the 29' it is not uncommon. Sorry not much help on jet sizes.
[This message has been edited by Drew (edited February 06, 2002).]

RumRunner
02-07-2002, 04:48 PM
Infomaniac:
Actually changing your main jets will have a slight affect on your idle circuit, since your Idle Feed Restrictor pulls it's fuel out of the main metering well. If you change the available amount of fuel in the well (changing jets) this will change your idle. Same as changing the fuel in the bowl running the float levels on the high side or low side will change your main fuel curve.
The main issue for safety is two fold one, if your normal cruise RPM (say 3500) you're not making any boost your PV is closed and your running leaner, now say you want to cruise at 4500 RPM where you'll have boost the PV opens adding addtional fuel. You won't be lean and you'll get better fuel economy. If you plug your PV, and go up on your jetting you'll be rich all of the time, and loosing fuel economy. Same thing with not boost referencing the PV you can run in situations under light boost where you'll still have vacuum under the carbs at part throttle and you'd run lean !

JLaughreySS24
02-07-2002, 05:20 PM
Rumrunner, thanks for all your info on this jetting topic. Ive been reading what everyone has to say and it has all been usefull. I have a rich condition at idle that I need to fix and dont know how to battle it. Midrange and WOT is fine and I know this cause I have a ETG gauge to monitor my tune. I have a 502 with a B&M 250 blower, superchiller, and 1150 Dominator. Ken Lane of HTM (also good friend of mine) was going to help me lean it out at idle by drilling the air bleeds. He and I built the motor together but he knew the motors specs better than I. Do you have any idea if that would help my rich condition? Ken seemed to think so. Thanks for the help!!

Drew
02-07-2002, 05:53 PM
I am no expert but I know what worked on a set of 660 and 850 holleys I have expierance with. Dominators may be a different animal. Problem I had was to get idle speed I had to open butterflys to far and they were starting to pull fuel from transfer slots. I had to back off on idle screws and set the gap at what Holley book reccomended. (Don't remember now but I could look it up.) Then used wire drills to open up a hole in each primary butterfly to get the air I needed for idle speed. Worked like a charm just don't drill holes to big or you will not be able to slow idle down low enough. Both motors idled great and would never load up when done. Hope that made some sense.

JLaughreySS24
02-07-2002, 06:24 PM
Thanks Drew, Thats one of the things I myself was going to try first, drilling the primary butterflys. Just wanted to get some input from everybody before I start drilling away on my carb. If thats what I end up doing what size bit do you recommend? 1/8 inch?

Infomaniac
02-07-2002, 10:40 PM
Drew please look it up. I think it will say .030 - .045 of transfer slot showing.
Set the butterflys to this position and see how the engine idles. If it will not idle fast enough open up the existing holes about .040 and try again. Is a bit of work but as mentioned, if you drill them too much you will have to replace the butterflys and and start over.

Drew
02-09-2002, 05:14 AM
H.P. Book I have says NO more than .060 of transfer slot showing. For a 454 hole size in buttery fly is usually between .125 and .140". I would start small and work up. Kind of a pain but better than replacing butterflys.

RumRunner
02-09-2002, 06:08 AM
JLaughreySS24
First start by getting your butterfly's baseline adjusted. Remove the carburetor from the engine, (drain the fuel) and turn it upside down. If you open up the butterflys you'll see the Transfer slots (it's about .030" wide by a little over a quarter of an inch long. As a baseline starting point you want to see about .020" to .030" of the slot, close (or open) both your primary, and secondary butterflys until the slot looks like a little square below the butterflys. This should be a good starting point. Now re-install your carb, and baseline your idle mixture screws at 1-1/2 turns out. Start your engine allow it to get up to temp, then do your final adjustments. The only reason to drill holes in your butterflys whould be if you have to idle your engine at a much higher RPM than this baseline adjustment give you. This can cause a rich condition. If that is the case (not real normal on a boat where you generally want to idle lower) you'd start off with a 3/32" bit .093" DO NOT go bigger than that to start with. If you can idle the engine at the proper RPM with the butterflys where we started then you will be able to lean out the engine by the mixture screws, or by opening up the idle air bleeds (the outside bleeds on the top of your carb) by about .005", or .006". I hope this helps.

JLaughreySS24
02-09-2002, 10:13 AM
Thanks a lot for the help. That should keep me busy for a bit. It will be nice to be able to idle around again and not load up the plugs!!

SOUTHWIND377
02-12-2002, 07:06 AM
I have picked up on alot of good info. but can anyone give me a baseline on what jets you are running. I have two 850's mounted bowl to bowl and I am way rich but my primaries have powervalves and I think they are blown. I am going to block them off and stick with the same jetting since I was so rich to begin with. I have 76 in primaries and 88 in secondaries what is yall's opinion in this. Thanks!

Drew
02-13-2002, 06:44 PM
If your powervalves are blown this is why you are running rich. Personely I would not block them off but install new valves of the correct range. If it was set up correctly with power valves to begin with it will be LEAN unless you jet up. Just remember RICH is better than LEAN. Blower motors do damage very quickly if run lean. http://free.***boat.net/ubb/biggrin.gif

Heatseeker
03-09-2002, 02:25 PM
Originally posted by BradP:
Pick up a copy of "Street Supercharging" PAW has it in their catalog. It has a section on blowerr referencing carbs that I found very helpfull. There's more to it than just jetting, the author suggests that as a last resort
I picked up a copy of the book. It goes into great detail on the history of supercharging internal combustion engines, which is very interesting(did you know the Roots Bros. blower was designed for and first used to force air into forges?). The carb expert contributing to the book seems to favor Carter 750's for blower apps. The Holley section doesn't go into much detail, other than suggesting the use of larger needle and seat assemblies and larger power valves. No mention of boost referencing at all. Save your $16 and read all the threads on this subject here!

JLaughreySS24
03-10-2002, 10:42 AM
I agree, I bought the book also and it wasnt very informative on Holleys. Didnt tell me anything I already didnt know. Plus no info on boost referencing. I would like to find a book on Holley dominators or boost referencing though.