PDA

View Full Version : Carb Jetting



Real Baja
01-04-2010, 07:38 PM
I have a pair of 502s with Dart Pro I CNC ported heads. Original head work was done at Lingenfelter for these injected 540 hp 502s. Comp cam is custom grind, a little more lift and duration than a crane 741.

I purchased Dart single plane intakes and 800 cfm holley double pumpers for the engines.

Carbs came with 72 primary and 87 secondary jets.

I know best thing would be to have dyno tuned, but I cannot do that in the near future.

Any help with suggested jetting/tuning would be greatly appreciated.

Thanks Fred

Blue Thunder
01-05-2010, 07:39 PM
That is stock marine holley calibration. I would go up 5 jets sizes front and rear to 77/92 and run a 6.5pv in the primary only. Then go from there preferably with a O2 sensor in the exhaust to get the actual a/f numbers.

BUIZILLA
01-05-2010, 08:30 PM
I would go 77-78's in front as stated already

but, I would just plug the sec. PV and leave the 87's... for now

walk it up on plane to about 3200-3400, level off for a minute or so, then walk the throttle up moderately... if it bogs slightly bump the sec jets 2 sizes until the bog goes away, run it for a day and read the deep ceramic ring on the plugs, if it doesn't bog, drop it 2 sizes, do the same with the plugs... I think you'll end up with 88-89's at the most

MILD THUNDER
01-05-2010, 08:45 PM
Yea, i'd go 78's up front, with 6.5PV, and 92' s in back, no PV. I'd rather start rich and lean out from there, vs starting lean and richening as needed.

skaterdave
01-06-2010, 11:38 AM
fred, i assume you did the engines yourself or that the builder didn't have access to a dyno.

from past experiences, stock holley cards are 8-10 jet sizes to big. they do this out of fear running to lean. next you add in that your using it in a marine application where the motor runs at very low engine temp (under 140) and most are running low oil temp (under 180). this makes things worst since the engine itself isn't getting hot and hinders the combustion burn.

in race appications i've run holley and holley HP, barry grant, quick fuel and had to make major changes to all of them while dyno testing. now i know this is expected in some cases but both barry grant and the quick were built supposedly to match up with what we were doing.

after spending $$$ tweaking the carbs that i had, we built another motor and that time around listened to the builder and bought a new carb from BLP.

http://www.blp.com/

since then these are the only people i deal with for carbs. the owner is an old nascar guy and these guys only do carbs.

now you already have stock carbs but outside of jetting theres a bunch of other things that affect its performance. i would give these guys a call and see what they have to say before just changing jets.

and one other thing, just to keep up on stuff i recently read a book about performance carbs, which basically clarified what these guys told me. the actual jet size is a small factor in overall performance of a carb. there is many adjustments and modifications that can be made other than jetting. air bleeds, venturi size, accell pumps, floats, ect, ect.

i run there 850 and 650 race carbs and our motors produce better dyno #'s than most the others i race agianst (spec stock type). plus the carbs have never stumbled or given us trouble.

side note, few yrs back i ran into fuel flow problems. boat kept stalling in the turns. a few other racers/mechanics were helping figure out the problem and decided we should tear down the carb. no problems there. but it was discovered that i was running about 10 jet sizes smaller than everyone else. some of the guys thought this was the problem and recommended changing to bigger. it turned out to be a sticking fuel regulator. i left the carb alone. on our next rebuild i brought this to the attention of my engine builder and asked about changing to a bigger size. as you guys know most guys don't like being questioned about this stuff, but to prove a point we ran several dyno pulls and the overall best performance ended up right back at BLP's original size. whereas on other dyno tests with other carbs we constantly were dropping down 4-8 sizes.

so basically talk to as many people as possible before making any changes and know what your doing before making adjusts.

PatriYacht
01-07-2010, 08:17 AM
With jetting like that, I would guess that there is already no power valve on the secondary side. Still, that much difference between primary and secondary has me wondering. Are these new carbs or have they been "modified"? I would probably go up a couple of sizes on the primary side and use a 3.5 or 4.5 power valve to make sure it doesn't open when putting around the docks. Then bolt it on and start doing plug checks at different rpms. It's unlikely you could hurt a n/a engine unless you run it hard for a distance.

PatriYacht
01-07-2010, 08:30 AM
fred, i assume you did the engines yourself or that the builder didn't have access to a dyno.

from past experiences, stock holley cards are 8-10 jet sizes to big. they do this out of fear running to lean. next you add in that your using it in a marine application where the motor runs at very low engine temp (under 140) and most are running low oil temp (under 180). this makes things worst since the engine itself isn't getting hot and hinders the combustion burn.

in race appications i've run holley and holley HP, barry grant, quick fuel and had to make major changes to all of them while dyno testing. now i know this is expected in some cases but both barry grant and the quick were built supposedly to match up with what we were doing.

after spending $$$ tweaking the carbs that i had, we built another motor and that time around listened to the builder and bought a new carb from BLP.

http://www.blp.com/

since then these are the only people i deal with for carbs. the owner is an old nascar guy and these guys only do carbs.

now you already have stock carbs but outside of jetting theres a bunch of other things that affect its performance. i would give these guys a call and see what they have to say before just changing jets.

and one other thing, just to keep up on stuff i recently read a book about performance carbs, which basically clarified what these guys told me. the actual jet size is a small factor in overall performance of a carb. there is many adjustments and modifications that can be made other than jetting. air bleeds, venturi size, accell pumps, floats, ect, ect.

i run there 850 and 650 race carbs and our motors produce better dyno #'s than most the others i race agianst (spec stock type). plus the carbs have never stumbled or given us trouble.

side note, few yrs back i ran into fuel flow problems. boat kept stalling in the turns. a few other racers/mechanics were helping figure out the problem and decided we should tear down the carb. no problems there. but it was discovered that i was running about 10 jet sizes smaller than everyone else. some of the guys thought this was the problem and recommended changing to bigger. it turned out to be a sticking fuel regulator. i left the carb alone. on our next rebuild i brought this to the attention of my engine builder and asked about changing to a bigger size. as you guys know most guys don't like being questioned about this stuff, but to prove a point we ran several dyno pulls and the overall best performance ended up right back at BLP's original size. whereas on other dyno tests with other carbs we constantly were dropping down 4-8 sizes.

so basically talk to as many people as possible before making any changes and know what your doing before making adjusts.

Depending on how the carb is modified, the carb jetting can be smaller. However just changing to smaller jets alone is not a good idea. If you use smaller air bleeds, the mixture will tend to get richer at certain rpms and throttle openings. Then you can change jets to lean it back out. There is definately a science to it. Sometimes having a pro do it is a good idea. Pro-Systems is a shop that does a lot here in Michigan. Nickerson has a good rep too.

skaterdave
01-07-2010, 11:12 AM
patriyatch, let me clarify that i'm not saying to "just go smaller" on the jets.

but i would say that the old school way of looking at plugs shouldn't be used. marine motors run much richer since they run colder and normally have water cooled exhaust. these things greatly affect the plugs. i would be carefull if your going to run the motor hard and relay on how the plug looks as the primary way to make carb adjustments.

one would think that the engine builder should have a pretty good baseline ???

Geronimo36
01-07-2010, 11:28 AM
If you're making huge changes in jet size from what comes out of the box you likely don't have the right size carb on the engine. Too small you need to jet down, too large you need to jet up as a rule of thumb.

you said you have an 800, do you even have change-able air bleeds?

Last NA engine I did was 502" with an 850 race carb (550 hp), 78 and 83's with a single 6.5 pv and it was a little on the rich side.

PatriYacht
01-07-2010, 01:00 PM
Skater Dave, I was trying to go easy on you.:smash: We're giving advice to a guy with a mildly modified BBC who doesnt have a lot of knowledge of modifying carbs. My experience shows that Holley carbs are close right out of the box and usually need to be jetted up 2-4 sizes on engines like his. It's really not rocket science and plug reading is still a decent way to tune an engine. I tune for a light brown on the ring and a touch of color on the insulator which is actually pretty rich, but it's better to be safe. You should see how lean drag race guys are in comparison!. Remember to start with new plugs. I also use oil temp as an indicator. Temps go up fast when you're too lean. I do agree that if you have the money, a dyno tune with a pro built carb is best, but some times money and time is tight. :cheers2:

Blue Thunder
01-08-2010, 08:06 PM
Skater Dave, I was trying to go easy on you.:smash: We're giving advice to a guy with a mildly modified BBC who doesnt have a lot of knowledge of modifying carbs. My experience shows that Holley carbs are close right out of the box and usually need to be jetted up 2-4 sizes on engines like his. It's really not rocket science and plug reading is still a decent way to tune an engine. I tune for a light brown on the ring and a touch of color on the insulator which is actually pretty rich, but it's better to be safe. You should see how lean drag race guys are in comparison!. Remember to start with new plugs. I also use oil temp as an indicator. Temps go up fast when you're too lean. I do agree that if you have the money, a dyno tune with a pro built carb is best, but some times money and time is tight. :cheers2:

I'm with you PY. We aren't all looking for hardcore racer methods....

For the OP which seems to have disappeared.... 77/92 should make this tune rich, which is a good start. An 800cfm carb on a 502cid might need to be leaned some if the airflow is insufficient. Believe it or not, the transom can tell the story on boat carb tune as opposed to the plugs. If it gets black to below the waterline, you need to jet down. Sooty transom above only the waterline means idle is too rich. You can certainly use clues other than plugs and o2 sensor to read tune. I recommend the air fuel meter, but we can't all do that.

BT

Real Baja
01-10-2010, 08:57 PM
Thanks for all the information, I appreciate the feedback. Sites like this, with members that are willing to share are great.

My engines were originally built as EFI 540 horse by Lingenfelter (1997). I know absolutely nothing about EFI, so that is the reason I went with carbs.

My main concern with the beginning tune on the carbs was not to have a lean condition and damage an engine. 72s in the primary seemed a little small, however the carbs have 6.5 primary power valves. There are no power valves in the secondary. I figured that was why the secondary jets (87) were so much bigger than the primary jets.

I purchased these carbs new from CP Performance. They are advertised as the carbs Merc used on the HP500s. I do not know if Merc re-jets these carbs for the HP500, would be nice to know what jets/PV they used. CP Performance tech line did not know if Merc re-jetted or not.

Thanks again (any other pearls of wisdom are appreciated)

Fred

PatriYacht
01-12-2010, 08:36 AM
Mercury uses staggerd jetting in that carb on the HP500. 81/75 primary, 89/93 secondary. You're running a bigger cam so I would guess it needs slightly larger jets.

Real Baja
01-12-2010, 08:59 PM
Mercury uses staggerd jetting in that carb on the HP500. 81/75 primary, 89/93 secondary. You're running a bigger cam so I would guess it needs slightly larger jets.

This is likely a dumb question, but are the jets staggered in a specific way?
Is the larger jet in the primary on the port or starboard side? Is the larger jet in the secondary on the port or starboard side?

Do you know what power valve the HP500 used, primary and or secondary?

Thanks for the information.

Fred

PatriYacht
01-13-2010, 08:46 AM
I believe the larger jet is on the side that is the longest distance to the intake port. I think the power valve is a 6.5. All power valves pass the same amount of fuel when open, a 6.5 just opens sooner than a 4.5 etc. The number refers to manifold vacuum. Because you have a larger cam and different heads, I'm guessing that the manifold vacuum at low speeds will be lower than stock and the stock power valve may open too soon. That's why I think that a 4.5 power valve is a good place to start. Above about 3000 rpm, it's going to be open anyway.

sector
01-14-2010, 08:34 AM
I thought I read somewhere that the staggered jetting was also used to help offset the PCV being plumbed into the intake.