Carburetor modification & performance

[jbschneeweis]

Will try to explain this to the best of my ability. Over the winter I had my 1804 500 CFM carbs modified by a professional to change them to 650 CFM specs. My racing performance decreased. Old carbs were set up with 86 primary & 80 secondary & used 68 x 55 rods. New carbs were changed to 650 specs & using 98 primary & 95 secondary with 75 x 47 rods. I have 1 inch 4 hole spacers & engine specs from Jack are his 4 inch stroker 489 CI, with Edelbrock aluminum heads & matched ports. Last year at the end of the season with the old carbs at a 1/8 mile track at 5000' altitude. Times were 60' 1.61, ET 7.51 & MPH 90.74 corrected altitude 6050. this past weekend with the new carbs setup same track times were 60' 1.67, ET 7.71 & MPH 88.33 not sure of the altitude forgot my meter. A 2 tenth slow down on ET & a little on the 60'. I have a PG trans set up for racing & a trans brake & two step & I launch at 4000 RMPs. Spark plugs look good with a light tan color. I changed the rods to 68 x 37 for 2 runs & not any change to mention except the plugs were a little tanner. I would think with the increase in getting & smaller rods I would think my times would be better not worse. Any ideas or suggestions. Thanks.

Joel

 

[55Brodie]

Explain what was done to increase the carburetors' CFM.

 

[jbschneeweis]

Forgot to mention MSD 6AL 2 digital box with MSD ignition, spark plugs are NGK 45 gap.

 

[Don Jacks]

Try going back to the original jets and rods and see what happens.What was your air temp.and humidity numbers?

 

[jbschneeweis]

Not sure actually what he did. He knew my car, we had met at a drag & he helped me fix a few items. He asked me why 500's & I said that was what Jack indicated. He said he tested them & not flowing good so said he would change them over to 650s. said he would change they should run a lot better. Don't have his sheet on what he did, said he sent it but I don't have it & he is looking for it. I assume he changed the getting, rods & boosters to match those of a 650. the car really sounds a lot better than it did but performance just is not what I expected. the car ran very consistent times during the past weekend.

 

[jbschneeweis]

I forgot my meter, that might of had some effect. It was chilly & very windy. Since I am now running lager jets & smaller metering rods I would think I would dropping more fuel into the system that would make the engine run rich. The plugs were light tan which I thought would almost indicate running lean. I would thing smaller jets & larger rods would lean out the system. Thanks.

 

[Ishiftem]

If the thunder series is like the regular edelbrock, the only difference between the 500 and 650 is the primary booster. A cost cutting measure. Your 1804 500 cfm carbs are "calibrated" for a dual carb setup and have smaller jetting than the 1801 and 1802 500 cfm carbs for a single carb application. The guy jetted your carbs after converting them to 650s using the edelbrock out of the box jetting which is meant for a single four barrel application. My guess is you are way rich.

Also reading the color of the porcelain will tell you nothing about jetting for wide open throttle. You need to start with new plugs and make a pass. Do not drive back to the pits! shut it off after the stripe and coast to the return road and pull the plugs. You need to look down into the plug where the porcelain meets the steel body. You should see a ring going 3/4 to all the way around. If there is no ring it is lean. If the ring is very dark it is rich. The color of the ring will depend on the fuel used.

 

[jbschneeweis]

Thanks Dan: That makes sense to me. With that new getting I would think it would be running rich. I will use a larger metering rods next time & see if I can get a read on the plugs. Hard to do that at Sage raceway, sharp turn at the end of the strip & would be difficult to turn to the return road while coasting. Any idea on figuring rod to jet usage or should I just experiment with a larger rods & see what works. Do you think I should change the secondary getting? Thanks.

 

[Ishiftem]

I think you should change everything. The 650 is 23% larger than the 500. That’s where I would start. The .098 is 33% larger than the .080 you had in the 500. You can probably see where I’m going with this. I would go 23% larger on the primary and secondaries than you had with the 500 as a start. Then try going richer from there first as that is the safer direction.

 

[jbschneeweis]

Dan: Thanks. It is definitely running too rich. I understand what you are saying but don't know how to figure it out. My old getting in the 500 was .086 primary & .080 secondary, so what should I change them to if I want to go 23% larger than the originals? I spoke with my mechanic here who worked on the carbs before I sent them out to get worked on. He suggested changing the primary's to .095 & secondary's to .092 & not changing the booster back.

 

[Ishiftem]

Secondaries figure the area of your .080 jet and pick One 23% larger. Primaries figure the area of the jet and subtract the area of the power side of the metering rod. Go 23% larger. You’ll need to do the math on what you have and what you are planning on changing too. It’s time consuming.

 

[Iowa 409 Guy]

Obviously your figuring percentages works Dan. The last time I messed around with my jets come to find out my coil was going bad...that'll drive you crazy. That's something you may want to take a look at Joel.

I get a little confused due to figuring flow in a pipe back when I had a job. For instance you have a 2" pipe. To get the same flow you would need 32-1/2" 11.7-3/4" 5.7-1"

 

[jbschneeweis]

I can't figure out the formula. So what is the size of the one that is 23% larger than the .080. Thanks. Maybe once I see what that is I will be able to figure it out. When I figure it ,out I come up with .098 & that is already large than my current secondary, so I must be doing something wrong. Thanks.

 

[Jim Sullivan]

If .080 is the diameter of the hole in the jet, should be able to use A = π r(squared)?? the little two did not show up correctly. A=3.14 x .040(squared). The area of the jet would be .005 square inches. .005 x 1.23(increase of 23 %)= .006 square inches. Diameter= 2x square root of (area /3.14). 23% larger jet would be .089. I'm starting t confuse myself. Hope this is correct.

 

[dakota tom]

You need to take into account the metering rod size to figure area available for fuel flow.

 

[jbschneeweis]

OK, thanks. I think I get it. Thanks everyone for your help.