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BBC Hydro
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Discussion Starter #1
454ci BBC, 10.5:1, .660/.680 258/[email protected] on a 108 c/l solid roller, 990 heads, Team G intake (single plane), 750 holley (proform main body, 72's in the front w/4.5 powervalve, 84's in the rear w/o jet extensions), 17 pitch prop, 18 gears, early 70's Howard shovelnose.

When in neutral the motor has excellent throttle response with 0 bog. Put it in gear and wack it and it stumbles around and won't recover until you back off and then roll back into it. Otherwise the carb is working flawlessly-makes a bunch of power and idles perfect. I want to get this bog sorted about before trying to spray it.

Thanks,
Sammy
 

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Sammy, there have been a few similar threads on this subject recently. Take a look through the past few pages of the Dyno section. Lot's of good info. Without getting into all of the different things to dial in on a Holley (jets, squirters, pump shot cams, power valves, etc) , the FIRST THING to check is your float level with the boat in the water. Set the floats at least midway on the clear sight bowl plugs (if you have them). If not, they should dribble fuel when you open the standard bowl plug. That's step one, run it again and let us know if that helped. Jocko
 

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Like Jocko posted, double check the float level first and foremost. You need to post you shooter size, and if possible you accelerater pump cam color. Remeber, the carb was most likely originally set up for a dual plane manifold, with a much better signal strength, and a much small plenum to cover with the accelerater shot.

Also, the PV seems way to low for a single 4 bbl IMO. You have to get the manifold vac below 4.5 to get it to open, yet your running a 12 jet spead primary to secondary. Thats a ton! With a single plane Team G, your looking at some pretty drastic distributions difference front to rear in the engine. Ideally the engine would like you to run square jetting to start, and jet according to plug readings. I understand if your want to run a PV for milage, but you can only take the idea so far.

WHERE IS YOU TIMING, AND IS IT LOCKED OR DOES IT HAVE A CURVE!!!!!!!!



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BBC Hydro
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Discussion Starter #4
Like Jocko posted, double check the float level first and foremost. You need to post you shooter size, and if possible you accelerater pump cam color. Remeber, the carb was most likely originally set up for a dual plane manifold, with a much better signal strength, and a much small plenum to cover with the accelerater shot.

Also, the PV seems way to low for a single 4 bbl IMO. You have to get the manifold vac below 4.5 to get it to open, yet your running a 12 jet spead primary to secondary. Thats a ton! With a single plane Team G, your looking at some pretty drastic distributions difference front to rear in the engine. Ideally the engine would like you to run square jetting to start, and jet according to plug readings. I understand if your want to run a PV for milage, but you can only take the idea so far.

WHERE IS YOU TIMING, AND IS IT LOCKED OR DOES IT HAVE A CURVE!!!!!!!!
The float level is set to 3/4 up the sight glass on both front and rear bowls. I only ran the PV because it was already in there and I needed somewhere to start. I will find out the squirter sizes, and the accelerater pump cam color. I now have a PV block off so I will put that in and put 84's in the front and jet extensions in the rear.

Thanks for the help guys!!!

Sammy
 

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Boat Nut
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I see it was posted (no jet extensions) from what I've read, 0-60 ft in one second or less, will require a j/e, in a single carb application. The bigger jets in the back, reflect the lack of pv in the secondary.
 

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The float level is set to 3/4 up the sight glass on both front and rear bowls. I only ran the PV because it was already in there and I needed somewhere to start. I will find out the squirter sizes, and the accelerater pump cam color. I now have a PV block off so I will put that in and put 84's in the front and jet extensions in the rear.

Thanks for the help guys!!! Sammy
84s square is pretty big for 750. I would think someting in the mid to high 70's would be a better start. Something like 78's maybe. I think who ever set this thing up was trying to cover for a lean primary with a rich secondary jet. Not the best set up unless some thing in the manifold distribution led them that way. That would a huge distribution problem that the Team G isn't know for.
Again, where is your timing. the timing plays a huge part in how well a carb gets on the main circuit. Whats the timing and is the dizzy locked out, or is there a curve and how fast.


I see it was posted (no jet extensions) from what I've read, 0-60 ft in one second or less, will require a j/e, in a single carb application. The bigger jets in the back, reflect the lack of pv in the secondary.
No doubt the secondary is covering for no PV, but 12 jet sizes is huge.



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BBC Hydro
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Discussion Starter #7
84s square is pretty big for 750. I would think someting in the mid to high 70's would be a better start. Something like 78's maybe. I think who ever set this thing up was trying to cover for a lean primary with a rich secondary jet. Not the best set up unless some thing in the manifold distribution led them that way. That would a huge distribution problem that the Team G isn't know for.
Again, where is your timing. the timing plays a huge part in how well a carb gets on the main circuit. Whats the timing and is the dizzy locked out, or is there a curve and how fast.




No doubt the secondary is covering for no PV, but 12 jet sizes is huge.
Sorry gn7 I totally overlooked where you asked about the timing. It is set at 36deg and it is in fact locked out.
 

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BBC Hydro
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Discussion Starter #9

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454ci BBC, 10.5:1, .660/.680 258/[email protected] on a 108 c/l solid roller, 990 heads, Team G intake (single plane), 750 holley (proform main body, 72's in the front w/4.5 powervalve, 84's in the rear w/o jet extensions), 17 pitch prop, 18 gears, early 70's Howard shovelnose.

When in neutral the motor has excellent throttle response with 0 bog. Put it in gear and wack it and it stumbles around and won't recover until you back off and then roll back into it. Otherwise the carb is working flawlessly-makes a bunch of power and idles perfect. I want to get this bog sorted about before trying to spray it.

Thanks,
Sammy
No load in neutral - thus why most motors have 'awesome throttle response' in neutral. This tells you nothing.

72's seem awfully small to me - for almost any 454. But, your the one tuning it so you'll have to find out. Personally, I'd start upping the primary jet size while leaving all else alone- edit in: for now :edit done. If you get within 6#'s to the secondary's and the motor wants more fuel, then start upping the secondary's the same amt you up the primaries.

How do you know when you've added enough jet ? It will start to run slower than the last 'best' tune. "Let the motor tell you what it wants."
 

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No load in neutral - thus why most motors have 'awesome throttle response' in neutral. This tells you nothing.

72's seem awfully small to me - for almost any 454. But, your the one tuning it so you'll have to find out. Personally, I'd start upping the primary jet size while leaving all else alone. If you get within 6#'s to the secondary's and the motor wants more fuel, then start upping the secondary's the same amt you up the primaries.

How do you know when you've added enough jet ? It will start to run slower than the last 'best' tune. "Let the motor tell you what it wants."
I agree CFM, 72 in the primaries are small, which is why I think the secondaries are overly large, in an atempt to richen the total mixture up at WOT. But that just creates a distribution issue. He could richen the pri, and it may very well help the bog issue, but be fat at WOT, even with a PV of 4.5 which is way to low for a N/A engine with a single 4 bbl of that size. He may only be below that for a short time before engine demand will raise the vac back up above that. 750 on that engine is pretty small. I wouldn't get to over large on the jetting. when that thing gets into the upper R's its going to create a pretty good pressure differential, and the jets are going to flow a shit load compared to the same carb on a 350 street engine. The smaller the carb gets, the greater the pressure differential, and the more fuel that will flow thru a given jet size.

Very first thing I would do is dump the 4.5 PV. Its most likely only on line part time. If you wish to blank the PV, fine. But if he square jets it at 84 like the secondaries currently are, I guarantee it will be pig fat. Which s why I suggested mid to high 70's square with no PV to start.

I wouldn't be one bit surprised with a 750 on that engine, it could easily be 72-74 pri with a 6.5 PV and 78-80 in the secondary, and still be a touch fat. Specially as it gets close to max R's. PV with that vac rating is adding nothing to the mix, at least not for very long. So with a non functioning PV, 72 are small. With a funtional PV, not som much. Specially on a 454 with the RPM potential of this thing. The thing will be trying to suck the boosters right out of the carb.



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The thing will be trying to suck the boosters right out of the carb.
LOL.


Well, a vacuum guage will be his best friend. If this thing get's remotely close to 4.5"HG on the top end it's time to throw that 750 on the bench. Costing way too much power in the airflow department. If it's getting to 2.5:Hg or thereabouts, I agree, the thing could be in perfect tune and then as the RPMS get up there, a rich condition can happen because of the increasing pressure differential. Thus why smaller carbs on blow thru's.....always fighting to get more fuel.

FYI: I'd be all over a 850 or tad larger. In my experience, 750's are for small blocks. Only big block I'd consider one for is a 330hp 454. Over 1.5"HG at WOT is too restrictive...in my book.
 

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LOL.


Well, a vacuum guage will be his best friend. If this thing get's remotely close to 4.5"HG on the top end it's time to throw that 750 on the bench. Costing way too much power in the airflow department. If it's getting to 2.5:Hg or thereabouts, I agree, the thing could be in perfect tune and then as the RPMS get up there, a rich condition can happen because of the increasing pressure differential. Thus why smaller carbs on blow thru's.....always fighting to get more fuel.

FYI: I'd be all over a 850 or tad larger. In my experience, 750's are for small blocks. Only big block I'd consider one for is a 330hp 454. Over 1.5"HG at WOT is too restrictive...in my book.
Exactly! It may not hit 4.5 at WOT, and the PV may stay open. But I still believe he would be better served with 6.5 and be sure. Your right, a vac guage would very usefull in telling him he is leaving HP on the table. But at 10 1/2 to 1 and that cam + a Team G, I would say it goes with asking.
Biggest problem I see this thing having is trying to maintain a decent mix across the RPM range. What could be prefect ot 4000 part throttle could go dead rich at WOT and 6500. The carb was never intended to flow the air he is trying to pull thru it, which tends to throw it off its fuel curve as the R's climb. It could be fixed with air bleed correction. Look at 390 Nascar carbs. They work, but they are set up for it. Fighting an undersized carb can be every bit as frustrating as fighting an oversized dominator on a street small block. Just on opposite ends of the RPM scale.

None the less,none of this is why it bogs. In fact it should have pretty decend response on the water until it hits some R's. Its probably the shooter size, or it needs jet extensions on the secondaries. depending on WHEN it hesitates.



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I like to get the basics tuned in first. IE: jetting, idle circuits, PV, and timing. SOunds like everything in that 'basics' list is okay for now, other than jetting. 72 is very low even with a PV. I'd crap myself if 72 is right on. :D

Once that's all set, then playing with all the accelerator pump pieces is next.
 

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I like to get the basics tuned in first. IE: jetting, idle circuits, PV, and timing. SOunds like everything in that 'basics' list is okay for now, other than jetting. 72 is very low even with a PV. I'd crap myself if 72 is right on. :D

Once that's all set, then playing with all the accelerator pump pieces is next.
When in doubt, it never hurts to go back to Holley's original set up. In the case of the 750 DP its all over the map dpeneting on the year of manufacture, or the trailing dash number.Original 4779 were jetted 75/76 with a 8.5 PV! Later ones were jetted 71/80 with a 6.5 PV. Obviously they made a air bleed change and went to larger.

I am going out on a limb here and say something I will probably get hammered for, but it won't be the first time.
Anytime I have a carb with no info except the size, and no carb listing handy to look up the original setup, I take the carb size, and drop the "0" of the end of the CFM rating, and use that as my base jetting for the primaries, and jet the secondaries 1 maybe 2 sizes larger. This only works on 4150 carbs, and DOES NOT include the 950 and 1000 CFM 4150s. But 99% of the time, if the cam is a reasonable duration, and the carb is a reasonable size for the engine package, it wil get you running well enough to tunse from there.

So, a 72 jet in that carb isn't as crazy as you might think. But in the case of this carb, I think some one richened the secondaries, when they probably should have richened the primaries, and let the PV open sooner.



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Sorry for slight thread de-rail everyone.

GN - funny you mention that. I was just mentioning this the other week in another forum. LOL

Posted By CFM 08-01-2011, 05:46 PM

You ever noticed stock Holley 600's come with jets in the 60's, 700's in the 70's, and 800's 's in the 80's ?
 

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I wouldn't say you or I hi jacked the thread. Still talking about jetting, mainly a 750 on a BBC, and what effects the jetting and how. I would say its on topic more than most threads.

One last thing 71Shovelnose, if you get back to this thread. Double check that you have no slop or free play on your accelerater pump levers, or een a very slight amount of preload. Very slight.
See, still on topic:wink2:



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BBC Hydro
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Discussion Starter #18 (Edited)
I wouldn't say you or I hi jacked the thread. Still talking about jetting, mainly a 750 on a BBC, and what effects the jetting and how. I would say its on topic more than most threads.

One last thing 71Shovelnose, if you get back to this thread. Double check that you have no slop or free play on your accelerater pump levers, or een a very slight amount of preload. Very slight.
See, still on topic:wink2:
Ok so this is the game plan... Take the PV out for now and block it off. Put 75 jet's squared in it(someplace to start). Put jet extensions in. Check for "no-free play" on the accelerator pump levers, and mayble only a slight bit of preload. Lastly, get a vacuum gauge and monitor the value at WOT under load (this may be tough to do!!!). Is this everything or am I missing something?

Just a note-remember this is a Pro-Form main body on a 750 baseplate-not a "milled-choke horn" garage job. These main bodies flow well into the 900cfm range. This may not matter but I just wanted to clarify this.

Thanks a ton for the help!!!

Sammy
 

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I haven't used the Proform mainbodies yet (you figure :confused:) but I'm willing to bet you are way light on jet #'s (you stated #75 square) without using a powervalve.

I'd talk to someone proeffecient (sp?) with these main bodies and see where there jetting typically falls with like or near like builds to yours. I've always fallen in the 80's while using a PV with very close builds (some touched 90 in the secondaries), but again, that's with 800's and 850's (including modded one's) and not the Proform body.
 

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Someone said it earler! An 850 would work better than a 750! A stock 850 #4781-2 uses 80s in front and rear with 31 center squrits with a 6.5 P/V! I had a 468 that we set a 750 in stock form on. First was bump up to a 31 front center squriter then added a couple of jet sizes to the front then added some to the rear all this was with stock squriters cams! Ran well and idle great! Get it to idle blip the gas and it would stumble then return! Change to an 850 out of the box problem solved!

?? Will this Pro Form still flow more with 750 baseplate??:hmmm:
 
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