HP500 Headers and bored over jetting change?
06-19-2010, 06:34 AM
#1
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HP500 Headers and bored over jetting change?
Built an HP 500 clone over the winter, all engine specs are too Merc HP 500 specs except:
Bored 60 over, compression ratio is calculated at 8.76 to 1 taking into consideration the overbore.
Engine builder used a slightly longer rod and piston with relocated pin to compensate.
Lightning Headers instead of Gill.
TB IV iginiton total time at 34.
I went with an HP 500 clone to avoid trouble with jetting and I wanted a tried and proven package. Unfortunately I have a lean pop when I accelerate from 3300 to 4000. Sort of a steady spit spit as it builds RPM. I have not pushed it past 4K.
Stock HP 500 jetting is Pri 75 and 81 Sec 93 and 89. I am finding it hard to believe that my engine is far enough off spec to require a jetting change, the Lightning headers may require a bit more jet and the overbore could make a difference but I would not suspect that it would be enough to cause a lean spit when leaning into the secondaries hard.
My plan:
Jet primaries up by 2 and secondaries up by 4 or 5.
Pri 77 and 83 Sec 97 and 93
If I am getting a lean spit in the secondaries because of better flowing exhaust, more cubes etc. then primaries may be a bit lean so bump them up a little and go fairly large on the secondaries to see if it clears up. Darn near impossible to get any decent plug readings due to the fuel type available and brand of plug. I am going to get some AC plugs (they are a little easier to read than the brass colored NGK's) and see what comes up.
Does it sound like I am on the right track or am I missing something else?
Bored 60 over, compression ratio is calculated at 8.76 to 1 taking into consideration the overbore.
Engine builder used a slightly longer rod and piston with relocated pin to compensate.
Lightning Headers instead of Gill.
TB IV iginiton total time at 34.
I went with an HP 500 clone to avoid trouble with jetting and I wanted a tried and proven package. Unfortunately I have a lean pop when I accelerate from 3300 to 4000. Sort of a steady spit spit as it builds RPM. I have not pushed it past 4K.
Stock HP 500 jetting is Pri 75 and 81 Sec 93 and 89. I am finding it hard to believe that my engine is far enough off spec to require a jetting change, the Lightning headers may require a bit more jet and the overbore could make a difference but I would not suspect that it would be enough to cause a lean spit when leaning into the secondaries hard.
My plan:
Jet primaries up by 2 and secondaries up by 4 or 5.
Pri 77 and 83 Sec 97 and 93
If I am getting a lean spit in the secondaries because of better flowing exhaust, more cubes etc. then primaries may be a bit lean so bump them up a little and go fairly large on the secondaries to see if it clears up. Darn near impossible to get any decent plug readings due to the fuel type available and brand of plug. I am going to get some AC plugs (they are a little easier to read than the brass colored NGK's) and see what comes up.
Does it sound like I am on the right track or am I missing something else?
06-22-2010, 07:43 PM
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Join Date: Jul 2007
Location: king george, va
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You don't sound off to me. I'm running a zz502 with thunder bolt IV, single plane intake with a holley marine 800cfm double pumper. I take it you have a 800 crm carb. I was running lean with the factory 72/87 jets. I'm running 78/93 and starting to see some brown in the plug finally. Have you used the carb before?
06-23-2010, 10:27 AM
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Used this carb for a short time several builds ago on a stock 502 but it did not have enough cam. I then went to a blower motor and now a HP 500 clone. Yes it is the HP 500 800 CFM carb. For whatever reason Merc cross jetted that setup. I am keeping a small amount of the crossj etting in but having trouble seeing anything on the plugs.
