carb jetting

MRHP

 
Supporting Member 1
well i got my old bone shaker out and it did not fix its self over the winter:cry it still will spark knock under acceleration. timing is set at 34 total in by 3000. 474 stroker dual 600 edelbrocks 4 gear 355 posi. 10:1 pistons and msd ignition. my carb jetting is 95 primary 87 secondary with i believe a yellow spring. seems to spark knock when the secondaries start to open. am i going lean on the secondaries? i think so. can a few of you fellas list your jetting and engine combos so i can compare ? plugs seem to be nice and tan but it does not see a lot of high rpm full on driving. any advice or ideas would be helpful. will have pictures soon! thanks Brian
 

Fathead Racing

Well Known Member
Supporting Member 7
Detonation.

Are you sure it's fuel related? Do you run a vacuum advance unit? If so, try disconnecting the vacuum line to the distributor vacuum advace unit, plug the line and take it for a spin. See if this helps. If it does your vacuum advance is not pulling off fast enough. Crane sells an adjustable unit.
 

bluescreamer

Well Known Member
Supporting Member 1
MHRP
Have a 450 cc motor approx 12 to 1 a MSD timing box with 39 degrees of total timing. with timing retards coming in after 6400 rpm. 2 Edelbrock 600 cfm carbs sitting on a Lemar Waldron Modified intake, as of Sunday at US 13 DE. the carbs where jetted 101 with 70X52 meter rods Prim and 86 sec.
Air Density was aprox in the 600 foot area. Thats why the car ran the way it did. 10.93 122mph.
I have since changed the sec to .083 for my next race at Beaver Springs. PA. Which is at a higher altitude.
If You have any other concerns?

Bluescreamer.:)
 

MRHP

 
Supporting Member 1
i forgot to mention i tried disconnecting the vac advance. i also have 68x47 rods. am i barking up the wrong tree or missing something simple? i am also running 93 octane fuel and even tried a couple bottles of octane booster to no avail. if i retard the timing appx 8 degrees the detonation goes away, but so does the power:( . it sometimes also diesels when it is shut off. thank you guys for the help!! Brian
 

dq409

Well Known Member
Supporting Member 3
Try mixing your fuel at a ratio of two gallons racing (110) with eight gal 93. This works well with my .060 over 11.1 600 Edels 34* total 95 primary(don`t have my carb book here so I can`t say what rods i`m running) and 95 secd. I`m fat on the sec`s and will be going down with them.

Question for Allen,,,, Why so high on your primary side? Could you give a lesson on the theory on jetting that high? Thanks,,,dq

BTW,,,FANTATIC TIME !!!!
 

bluescreamer

Well Known Member
Supporting Member 1
DQ
Area= IIr^2
I calculate the area of the jet minus the area of the rod. Which gives me a number. Ex..101 = .008012 - rod .052 .002123=
.005889. That is the amount of area that fuel can flow thru one primary jet. multiphy this number by 4 ( number of primary jets in 2 carbs). ex .005889 x 4= .0023556

Do this for the secondary side than total all the area for 8 jets. this is the total area of jetting that fuel will flow at wide open throttle. ex..086=.005809 x 4= .0023236

.0023556 +.0023236 =.0046792 Total of the area of all 8 Jets.

Back to Question. I must have at least 25 different sets of metering rods. From a small of .037 to a large of.065 on the power portion of the rod. Plus I have jets from .062 thru .114.

So you can see if I install a .101 jet on the primary, I can change just the rod size to increase or decrease the fuel flow. Much quicker than changing jets.

I have a spread sheet that a friend made for me. Called
Carter AFB Primary Effective Metered Area (EMA) at Power Step.
It Shows Primary Jet size Minus Primary rod power size. You don't need a calculator to change jetting. You can change jetting or fuel flow as small as .001 %. It seems like a lot work but
once you work with the spread sheet you have a better understanding of your motors fuel needs.
I hope I answer you question. :)
Bluescreamer
 

dq409

Well Known Member
Supporting Member 3
Thanks Allen,,,Sure makes sense to me !!
I also have a huge supply of jets and rods. Anyway to get a copy of that chart??,,,,dq
 

MRHP

 
Supporting Member 1
quench area

:D to be honest with you i could not say what my quench area is.i bought all my parts from curt harvey. i never did any real research on the motor other than i built it the way curt built his motors. i never changed the engine recipe, so that is why i am suprised of the detonation problem. i did add some 110 fuel and the spark knock is nearly gone. i think i will try to retard the timing 2 degrees and see what happens. do you think i could be going lean on my secondaries and this would be causing the detonation? i really appreciate your input fellas. Brian
 

dq409

Well Known Member
Supporting Member 3
How much of a percent of 110 did you add ?
From my Edelbrock chart you are running lean on your power mode,,,try a 68X42 and see if that helps,,,dq
 

MRHP

 
Supporting Member 1
2 gal 110 to 5 gal 93. not sure what this turns out to be.it did help though. i will see what rods i have and do a little trial and error.:cheers Brian
 

dq409

Well Known Member
Supporting Member 3
Do you have the Edelbrock book? If not let me know what carbs you have and I can send you a copy of the jet page,,,,email me ,,,dq
 

MRHP

 
Supporting Member 1
i gots me a book. i will have to learn how to read though:) :: thank you Brian:)
 

SS425HP

Well Known Member
Supporting Member 3
jetting

I can't help but think your problem is jetting. I'm running 11.0-1 pistons with 1 steel shim gasket. This is a true 11.0-1 motor. I run it on 93 octane, with NO ADDITIVES. 36 degrees oal advance by 3000 RPM. I get NO KNOCK. These motors are not really sensitive to octane numbers or additives. The motor runs cool as well. I'm not sure of the jetting at the moment, and doubt I'll pull the tops of the carbs to see. But, The point is that it isn't compression or octane that is causing your problem. I also have the vacuum advance on my car. It works, I think. The vacuum advance was to get better gas mileage. I get about 7-8 MPG. 4.56 gears don't help! Jetting or maybe an air leak. Sounds lean. Have you checked for an air leak? What gasket are you using under the carbs? Should only have the 4 holes in it. Any other gasket will suck air, and be lean.
There, 2 areas to check.
Good luck

Fred
 

Fathead Racing

Well Known Member
Supporting Member 7
Head gaskets

Unless your block is 0 decked your quench is probably to much, if you are running with the Fel Pro composite gasket which is .041 crushed. If your compression is over 9.0:1 you need a quench distance of .045 or best .035. Quench distances in excess of this with high compression will cause detonation. Find out if you can what your quench is. Also to small a cam with to much compression will cause detonation.
 

MRHP

 
Supporting Member 1
i am running a 1 inch open spacer under each carb and do not have a vac. leak. i have checked for this. i believe i will try to richen my jetting. i did have my deck shaved appx .006 and my heads were shaved appx. .006 as well. i will call curt harvey and see if i can get some additional info on my combonation such as quench area. thank you guys for taking some time to help me out brian.:D :D
 
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