Tuning a Holley 800 mech sec
06-30-2003, 06:21 AM
#21
Charter Member #818
Charter Member
Join Date: Jan 2001
Location: West Michigan & Higgins Lake
Posts: 644
Likes: 0
Received 0 Likes
on
0 Posts
Johno,
I am at 83p & 94s right now. It runs good with just a small amount of soot on the transom. I know I am close on the primaries, maybe even a little rich. I have not had a chance to do a high speed run and do a plug check, though. I am going to try to get it dialed in over the 4th of July weekend. I'll post when I do. See ya. Doug
I am at 83p & 94s right now. It runs good with just a small amount of soot on the transom. I know I am close on the primaries, maybe even a little rich. I have not had a chance to do a high speed run and do a plug check, though. I am going to try to get it dialed in over the 4th of July weekend. I'll post when I do. See ya. Doug
06-30-2003, 07:05 AM
#22
Registered
Vacuum at Idle
F302,
I know you're talking Holleys here but the same principles apply to any carb. I had to drill holes in the throttle plates on the primary side of my Q-jets to allow enough air into the engine for it to idle without opening the throttle blades so much as to uncover the transition ports (leaving the idle mixture screws useless). That got it to idle fine in neutral with a manifold vacuum of approx 10". However, when I put it in gear, my vacuum drops to 5" allowing the primary power piston to raise the metering rods out of the main jets and promptly turning the mixture extremely rich. My fix was to buy the primary piston spring kit for the Q-jet and go to a lighter spring. This is the equivalent of going to a lower PV on a Holley. I, too, changed so many parts at once that it was a nightmare getting it dialed in again. I overshot way rich to keep from hurting anything in the beginning. Now, I am beginning to sneak up on it a little at a time until I get these Q-jets right. Check your "in gear" vacuum figure against your PV number and I'll bet it's opening at idle. Hope this helps.
I know you're talking Holleys here but the same principles apply to any carb. I had to drill holes in the throttle plates on the primary side of my Q-jets to allow enough air into the engine for it to idle without opening the throttle blades so much as to uncover the transition ports (leaving the idle mixture screws useless). That got it to idle fine in neutral with a manifold vacuum of approx 10". However, when I put it in gear, my vacuum drops to 5" allowing the primary power piston to raise the metering rods out of the main jets and promptly turning the mixture extremely rich. My fix was to buy the primary piston spring kit for the Q-jet and go to a lighter spring. This is the equivalent of going to a lower PV on a Holley. I, too, changed so many parts at once that it was a nightmare getting it dialed in again. I overshot way rich to keep from hurting anything in the beginning. Now, I am beginning to sneak up on it a little at a time until I get these Q-jets right. Check your "in gear" vacuum figure against your PV number and I'll bet it's opening at idle. Hope this helps.
06-30-2003, 08:27 AM
#23
Registered
Formula302, your idle vacuum is very low. Do you have big cams? Adding more initial ignition timing can help to bring up your vacuum. I'm at 18 deg. initial, 36 total but I do have solid roller cams. It's probably true that your power valve is opening as soon as you get on the gas but check your timing before you start leaning things out.
06-30-2003, 06:32 PM
#24
Guest
Posts: n/a
I initiated this post, but have been following along with MacDad`s dilema. I am very close now on my jetting. I am running 84 primary / 92 secondary. My idle vacuum is pretty high, even in gear (15"). I`m using a 6.5 PV on primary, plugged PV on secondary. It pulls good with no bog or backfire. I`m still tinkering with timing.
Good luck MacDad, looks like you`re heading in right direction.
Good luck MacDad, looks like you`re heading in right direction.
06-30-2003, 10:48 PM
#26
Charter Member # 55
Charter Member
Good to here you guys are getting close with the tuning. Take some plug readings at high cruise rpm (3800-4000) just before the secondaries start to open to dial in the primaries.
After that, jet up the secondaries about 10 sizes and make some WOT runs and take some plug readings. #8 and #2 are good to take initial readings on as they are usually the leanest.
Depending on if you are running secondary PV's and how lean you run the primaries, you sould end up 6-15 jet sizes higher in the secodaries.
As far as getting the idle lean enough------Good luck!!!! Mine was always a little rich and that was the only way I could keep it running smooth. An MSD box would have been the answer.
After that, jet up the secondaries about 10 sizes and make some WOT runs and take some plug readings. #8 and #2 are good to take initial readings on as they are usually the leanest.
Depending on if you are running secondary PV's and how lean you run the primaries, you sould end up 6-15 jet sizes higher in the secodaries.
As far as getting the idle lean enough------Good luck!!!! Mine was always a little rich and that was the only way I could keep it running smooth. An MSD box would have been the answer.
07-02-2003, 09:38 PM
#27
Registered
Join Date: May 2002
Location: Tampabay Florida
Posts: 1,014
Likes: 0
Received 0 Likes
on
0 Posts
Lean
Iam way lean on Pri (81 & Sec 90) on the HP 500(509cu). I am going to jump up both Pri to 83 & Sec to 95 & see what happens.
I took it up 4200 Rpm & brought back down slowly as my new Eddie Marine exhaust tips have already blown the flapper out of 1 tip( These tips are junk and rattle ) and did't want the any water rush back up the tail pipe. The plugs were very porcelain white & dry. cylinder 1 was a little tan in color. Did a random check & way white/lean.
My question ...by jumping up 5 sizes on Sec. bigger enough?
Thanks John
I took it up 4200 Rpm & brought back down slowly as my new Eddie Marine exhaust tips have already blown the flapper out of 1 tip( These tips are junk and rattle ) and did't want the any water rush back up the tail pipe. The plugs were very porcelain white & dry. cylinder 1 was a little tan in color. Did a random check & way white/lean.
My question ...by jumping up 5 sizes on Sec. bigger enough?
Thanks John
07-03-2003, 05:37 AM
#28
Charter Member #818
Charter Member
Join Date: Jan 2001
Location: West Michigan & Higgins Lake
Posts: 644
Likes: 0
Received 0 Likes
on
0 Posts
Johno, 5 sizes should make a difference that you can notice. While you're at it though you might want to try going up even higher than 95 just to see what happens. I have found that in order to get a good plug reading it takes more than a short 30 second burst. By going way rich to start you can't hurt the motor and you will be able to run it a WOT for a few minutes to get some color on the plugs. Jetting is a time consuming process. Too bad these carbs don't have high and low speed adjustment screws like a chainsaw or weed wacker. It would sure be a lot easier!!! Good luck, Doug
07-03-2003, 08:30 PM
#29
Registered
Join Date: May 2002
Location: Tampabay Florida
Posts: 1,014
Likes: 0
Received 0 Likes
on
0 Posts
Update
Today I rejetted to 83 Pri & 96 on secondaries. Boat ran great and plugs are looking way better. Ran about .5 hour and there was very light film on transom. Never did any flat out runs as my exhaust flapper has fallen out & don't want water rushing back up the pipe. Overall I am happy & too hell with it...jJust on 5 hours with motor + hopefully 3 more hours tomorrow & then the Poker Run Sat.
John
John
07-07-2003, 02:45 PM
#30
Guest
Posts: n/a
Had a family emergency and hav'nt had time to make changes yet that you guys have been posting. I've got MSD boxes already, am at 37 degrees total advance and am considering drilling the holes in the throttle plates when I take the carbs off to rejet this week. Its just plain easier and less mess to do it off the boat. If I drill the holes where should I drill them and what size? My vacuum readings are around 9.5 in gear and idling....
