carb jetting
06-15-2005, 06:51 PM
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carb jetting
I think my jetting is way off I have a bg 850 with 84,84 6.5 pv my plugs are ngk br6fs the plugs are almost white when checked and I am not turning more than 5100rpm the the motor is a509 647hp at5800 at the cape races last week most of the guys Italked to were running 92,s94,s96,s in there 850,s carbs 502,s 540,s with in500 to 550hp so are my plugs the standard 500hps and jets are wrong help please 
art
art
06-15-2005, 07:33 PM
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Re: carb jetting
Airtime:
Does your carb have two power valves? It should only have square jetting (all 4 one size) if it has 2 P.V's.
Unfortunately, the only way to know for sure is to pull off the fuel bowl(s)
If you don't have a secondary P.V. your secondary jetting should be like 6 -8 jet sizes bigger (like 90 - 92's) to start with. I would start rich and work my way down, by carefully monitoring the plugs.
If it came from Barry Grant that way, it probably had a 2ndary P.V.
If the carb is one of the Demon series, you can go to their web site and they have the jetting (and every other spec.) for all of their carbs on there, so you can start with the factory jetting.
Something to bear in mind is that even similar carbs generate different air (and consequently, fuel) flow on different engines so sometimes even a lower output engine might require bigger jetting from the same carb since the more powerful one is producing a much stronger booster signal.
Hope this is somehow helpful.
Does your carb have two power valves? It should only have square jetting (all 4 one size) if it has 2 P.V's.
Unfortunately, the only way to know for sure is to pull off the fuel bowl(s)
If you don't have a secondary P.V. your secondary jetting should be like 6 -8 jet sizes bigger (like 90 - 92's) to start with. I would start rich and work my way down, by carefully monitoring the plugs.
If it came from Barry Grant that way, it probably had a 2ndary P.V.
If the carb is one of the Demon series, you can go to their web site and they have the jetting (and every other spec.) for all of their carbs on there, so you can start with the factory jetting.
Something to bear in mind is that even similar carbs generate different air (and consequently, fuel) flow on different engines so sometimes even a lower output engine might require bigger jetting from the same carb since the more powerful one is producing a much stronger booster signal.
Hope this is somehow helpful.
Last edited by HARDCORE A/O; 06-15-2005 at 07:37 PM.
06-16-2005, 12:39 PM
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Re: carb jetting
Art:
It's hard to say where to start but if you want to keep the 2ndary PV in there, I would say start like 90ish (which should be way rich at WOT) and take a plug reading. The only way to do this is to get the boat to WOT and hold it there for a while and shut it down quickly. There was a spark plug reading link posted on this site some time ago and it had pictures which is much better than me describing what to look for.
Basically, if the plug looks sooty, you work your way down. Most people look at the insulator around the center electrode but sometimes this can be misleading if you're running like race fuel or blended gas. If you're running regular unleaded gas, you should look for a light tan colouring of the insulator around the center electrode. (This is residue from an additive that they put in unleaded gas)If you're running race fuel, it can be nearly impossible to read a plug sometimes, in which case you need a pyrometer or a good air/fuel (O2 sensor) meter.
BTW: What I found with using 2 PVs was that you can get relatively lean at cruise where you have a high vacuum situation and the PV's are closed. Because the carb uses 2 PV's for final enrichment (at WOT) they have to install smaller main jets which makes it run lean when the PV's are closed. This can mean that you could have a nice reading on the plug if using the above method , because the WOT mixture is good, but a white plug if you take a read after a long cruise.
My solution to my carb-jetting problems: I was running a procharger setup and jetted it up like it was a standard Holley Dbl pump. Little did I know that I had 2 PV's with PV channel restrictions that were monstrous (like .084"!) and it was running waayyy rich and down on power. I eventually, at the suggestion of Tomcat, plugged the 2ndary PV and jetted the rears up like 12 sizes and the front by a 4 and it started to run like a sumb!tch then! (that's much richer than you would need w/ a N/A motor)
It might be easier for you to just plug the 2ndary PV and jet it like to a stock Holley 850 Dbl pump. (I' m not sure what that is) and that should be a good starting point. That would make it easier to tune in my view.
It's hard to say where to start but if you want to keep the 2ndary PV in there, I would say start like 90ish (which should be way rich at WOT) and take a plug reading. The only way to do this is to get the boat to WOT and hold it there for a while and shut it down quickly. There was a spark plug reading link posted on this site some time ago and it had pictures which is much better than me describing what to look for.
Basically, if the plug looks sooty, you work your way down. Most people look at the insulator around the center electrode but sometimes this can be misleading if you're running like race fuel or blended gas. If you're running regular unleaded gas, you should look for a light tan colouring of the insulator around the center electrode. (This is residue from an additive that they put in unleaded gas)If you're running race fuel, it can be nearly impossible to read a plug sometimes, in which case you need a pyrometer or a good air/fuel (O2 sensor) meter.
BTW: What I found with using 2 PVs was that you can get relatively lean at cruise where you have a high vacuum situation and the PV's are closed. Because the carb uses 2 PV's for final enrichment (at WOT) they have to install smaller main jets which makes it run lean when the PV's are closed. This can mean that you could have a nice reading on the plug if using the above method , because the WOT mixture is good, but a white plug if you take a read after a long cruise.
My solution to my carb-jetting problems: I was running a procharger setup and jetted it up like it was a standard Holley Dbl pump. Little did I know that I had 2 PV's with PV channel restrictions that were monstrous (like .084"!) and it was running waayyy rich and down on power. I eventually, at the suggestion of Tomcat, plugged the 2ndary PV and jetted the rears up like 12 sizes and the front by a 4 and it started to run like a sumb!tch then! (that's much richer than you would need w/ a N/A motor)
It might be easier for you to just plug the 2ndary PV and jet it like to a stock Holley 850 Dbl pump. (I' m not sure what that is) and that should be a good starting point. That would make it easier to tune in my view.
06-16-2005, 12:45 PM
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Re: carb jetting
BTW: I wouldn't play with the air bleeds just yet, but they can really affect the air fuel mixture by changing the strength of the signal at the booster. That's advanced stuff and I would leave that alone unless you suspect or know that they've been changed after the carb left the factory. I would say that if jetting doesn't make a difference, give BG a call
Re: carb jetting
