Any Holly Carb guru's here?

M

M99

New Member
Jan 24, 2014
65
Knoxville
Boat Info
SD 300
Engines
5.7 GM Marine
With my 300DA, Marine Power Crate motors, 350, 310HP Carb models, 70 hours on both engines and new Holly 4150 Carbs, the Port Engine suddenly developed a problem that I am pretty certain traces to the Carb. It starts and idles fine, choke works well and comes off properly as it warms up. All linkage checks out properly. I typically idle out of the No Wake Zone at the marina, then go to about 1400 RPM to get farther out to the main channel. All still works fine and smooth. Then I typically go to 2000-2100 RPM for a low cruise speed. As I advance the Port Throttle, slow or fast (doesn't matter which) the engine felt like it completely died the first time, and I started to turn back toward the Marina. Then I realized the engine was running, but at near idle speed. Jockeying with the throttle got it running up around 3000 RPM and again it ran fine, so I continued the pleasure cruise for the day. I could throttle back to 2000 fine and it would keep running smoothly, and back up to higher RPM, but if I brought it back down to 1400, then tried to go up to 2000 again, it would seem to die every time. I have repeated the test several times and it is consistent. Get it past that point jockeying the throttle, and it will run fine again. It also runs fine at full throttle. Its a lot like a HUGE bog, but it doesn't come back the way a bog usually will and it came on suddenly. I checked to see if the vacuum secondary's were opening all of a sudden and causing it by rigging a camera so that I could clearly see the butterfly's as I advanced throttle, and they were not the cause. At the dock under no load, everything seems fine.

I have checked timing and that sort of thing, but since mechanical things don't fix themselves as you go faster, I'm pretty certain it is something with that Carb. Anyone have any thoughts or experience there? I can rebuild it easy enough myself, but wonder what to look for rather than just changing parts. Thanks.
 
Sounds like ether a broken or misadjusted accelerator pump. Check the diaphragm and see if it is OK. Holleys can be tough to tune. I have had them in a few cars and they are finicky.
 
I thought of the Accel pump, but that would not come into play at a very slow rate of advancing the throttle, and it would come into play if I advanced it quickly from 3000 to 4000 RPM, where it shows no problem.
 
Yes, it has vacuum secondary's. As I mentioned in the OP, I had thought that they might be flopping open too soon and causing the engine to die from way too much air at that low throttle setting, so I rigged a camera to watch them as I moved the throttle while running. They weren't opening (as they should not have been) at that low power, and they do open correctly at higher power, around 34-3600 where you need them.

Since it started happening suddenly, something had to change, and that's what I'm trying to figure out. If it kept running rough at higher power, that would make better sense for diagnosis.
 
Sounds like a power valve to me. To be sure it is the carb, swap them between motors and see if problem moves with the carb in question.
 
Thanks. That's what a couple Hot Rod buddies of mine said they thought as well. I think I'll swap it out this weekend as I have a complete Rebuild kit with a new one anyway. I thought of swapping them between engines, but it seems so likely to be the carb, that I guess I'll just pull it apart and at least swap the Power Valve and take a look for anything else plugged. While I'm at it I might also put a vacuum gauge on the engine at idle to see if I have the right size valve, although the current one used to work fine.
 
My bet is fuel pressure and a fuel pump going bad. Before you tear down the carb. Put a fuel pressure gauge on it and have someone watch it when you run the boat.

John
 
I had thought about fuel pressure, or filter, etc, but then it would run worse as I tried to run it at higher RPM or under load where it would need a higher fuel flow. Instead, it runs fine at higher RPM right up to WOT. Its only in that one RPM range from around 1400 to 2000 RPM on the increase that it starts to die. Then it comes on fine when I get past that spot. Its not the vacuum secondary's causing it either, because I put a camera there to check and they are staying closed as they should during that period and beyond. Come back down to 2000 from say 3200, and it runs fine at that RPM as well. Its only during that increase at that spot that it has the problem. Seems odd to me too, and I'm a licensed aircraft mechanic. Holley told me that it is possible that its a blown power valve, but didn't have any other ideas. It uses a 2.5 power valve which is the lowest one they make as well. I'm planning to pull it apart tomorrow to see what I find. A blown power valve should cause it to go way rich rather then lean, but perhaps that is what is happening. Beats me.
 
????? spark advance weights in the distributer possibly gummed up and hanging up.????? a quick check with timing light.
 
I would swap the carbs first before I tore them down. That way you will prove it is the carb or the fuel system.

JD
 
Figured it out

I was able to figure out what was wrong and repair it today. It was not the power valve, I bought one just in case I needed it, but they don’t cost much. When I got the Primary Float bowl off, I could immediately see that one of the two primary jets was contaminated with some sort of gasket material. They were both #68 jets. So I pulled both jets and cleaned them as well as the metering block thoroughly. I tested the 2.5 Power Valve and re-installed the original since it was fine. I also pulled the Secondary Bowl and metering block. That had two #66 jets, but they were fine as well. Put it all back together and tested it on the boat. Runs like an ape in heat now.

However, this is a brand new carb on a brand new crate motor. 69 actual hours on it now. There is a spin on 10 micron fuel filter between the fuel pump and carb, and both of the Holley fuel strainers (dual port) were also installed and showed no sign of any contamination. The contamination had to be in the carb from the time it was manufactured, and finally migrated into the main jets. At least its solved.
 
You must log in or register to reply here.

Forum statistics

Threads
113,186
Messages
1,428,170
Members
61,097
Latest member
Mdeluca407
Back
Top