phughes69

When my starboard engine is warmed up and idling it is surging and will sometimes die after I put it in gear. The engines are draw thru twin turbos. The carbs are Holley 800 cfm double pumpers. I rebuilt both carbs 2 years ago. My fuel pressure is 6 1/2 lbs. My initial timing is the same on both motors and is set at 18 degrees.
The port engine runs perfect and starts easily and idles good. It idles at 900 rpm and when I put it in gear it drops a little and recovers back up to 800 quickly. I found that the best air fuel ratio for a stable vacuum reading is 12.3, and it is pulling about 16 inches of vacuum.
The starboard motor is giving me fits though. Same setting as the other motor, 18 degrees of timing, 800 cfm Holley DP, 6 1/2 lbs of fuel pressure at idle. I replaced the base plate because I thought the throttle shafts were leaking but that did not help much. I also cleaned out all the air passages and fuel passages when I put the new base plate on. I have checked the float level with the boat in the water and it is correct. The engine will idle down to 900 but will stall when I put it into gear. I have found the best air fuel ratio for the best vacuum is 12.5. If I try to go leaner, I start getting a severe idle surge. I have had to raise the idle on this engine up to 1100 to keep it from stalling but when I put it in gear it drops to 500 rpm and has a hard time recovering, and will even stall sometimes. I know that at that idle setting it will sometimes pull fuel from the primaries ( I can hear it whistling past the primary blades and I can see it in the carb). I am going to swap carbs today to see if the problem follows the carb. What other suggestions do people have to try?

ezstriper

1st put a timing light on it and make sure timing is not falling off when you put in gear, seen that many times and the carb is to blame, if that is ok really need a 02 to see where it is...also can try swapping carbs see if problem follows the carb, but the noise you are hearing sounds like secondarys not seating and you are leaning out the idle there..

phughes69

I have wide band O2s on each engine. Ill check the timing. Did not think about that. If I adjust the throttle blade screw, I try to get it closed so the primary blades are not open and pulling fuel out of the primary venturi boosters. The secondaries are shut.

mike tkach

it will never idle right with the throttle blades closed.i try to keep primary&secondary blades as close as i can get them.if blades are closed their is no vacuum to pull the fuel.

phughes69

It pulls fuel from the idle transfer slots in the baseplate.

phughes69

Here is what I tried yesterday. I talked to Holley tech yesterday and they advised me to make sure the idle transfer slots on the primary and the secondary both look like squares which would be open about .020. I checked the baseplate and the secondary was closed and the primary was open about .030. I used a piece of .020 as a gauge and set BOTH idle transfer slots to be open. I made sure all the gaskets for the base plate lined up with the proper hole in the gasket. I also put a file to the main body of the carb where the metering block bolts to. It was slightly warped but not bad. I made sure all the proper holes in the metering block gaskets lined up. Bolted the carb and it would not start. I was only able to get it to start if the idle speed screw was turned up to 1400 but is still ran pig rich (10.0:1) I was actually able to completely turn off the idle mixture screws and the engine still ran and was pig rich. as I tried to get it to idle down closer to just running on the idle circuit, it still stayed pig rich in the 10.0:1 range. I figured that Holleys suggestion of opening the idle transfer slot was letting in too much fuel. I pulled the carb off and readjusted the secondary throttle blade so it was closed all the way now and was just pulling fuel from the primary idle transfer slot. I was not confident the the primary metering block was not the problem, so i installed a metering block PN 134-55 with 70 jets and a 6.5 PV on the primary side. My old jets were a 78. It will idle but only at 1100 rpm (Im going to check the accuracy of the tach today) and at a air fuel ratio of 12.3;1. and at 16 inches of vacuum. The odd thing is that it will idle like this with one idle mixture screw completely closed off and the other only 1/4 of a turn out. The cam is not some big cam shaft that produces no vacuum. At idle this engine should act like a bone stock BBC, but the carb settings and the AFR setting say otherwise. If there was a vacuum leak it would run lean, and this thing like to run rich. I am pounding my head against the wall theorizing what the problem could be. Holley tech confirmed that the carbs are original from 1979. Today's exercise is to swap carbs to see if the problem is the carb still.

phughes69

So tonight I swapped carbs. I just bolted the port carb on the starboard engine. When I twisted the key. it started right up and idled at 16 inches of vacuum, just like it did before. I did learn that the tachs are off by almost 300 rpm. Im going to jet the bad carb and set the throttle angle, and idle mixture screws just like the good carb and bolt it on and see what happens next.

apollard

Didn't see it in the thread, but have you built the bad carb recently? They can get crap clogging passages easily

phughes69

Yes. I have rebuilt the carb completely 2 years ago and then just recently too the whole thing apart and blew out the metering blocks and main body with carb cleaner and compressed air.

apollard

I asked because I had a carb issue that was killing me - going flat at 4500 rpm. Took 3 tries at cleaning the metering block before I got the damn thing to work. Meanwhile, I replaced the entire ignition system because I was sure the carb was ok...