Bogs down w/ new motor and carb around 2500 rpm

[jgresham75]

I am test running my new (not rebuilt) 350 volvo with a new Holley 2300 carb. (Original was a 5.0GLPWTR, only replaced it w/ 5.7 which the service manaul says the setup is exactly the same.) I am still breaking it in (under 2k rpm). However, If I gas it up around 2500 to get on plane it bogs down. If I gas it but don't get up that high (2k), it does fine. I set the new carb up according to VP manual. I set timing around 8 BTDC (10 if 90 octane, 5 if 87). I use about 90 octane.

The one thing I did change was the accelerator pump cam. The original carb had a 50 cc pump. All indication I have found show the acc. pump should have been 30cc. It also had the red cam on the old carb. On the previous motor/carb setup, I could not gun the engine (like getting skier on plane) because it would bog down. I checked manuals and it showed that engine should have had the yellow cam which was a much faster pumping of the acc. pump. Therefore, I put a yellow on the new carb with the new carb having the 30cc pump. Now I still get the bog. :facepalm:

If I gradually go from 2 rpm to 2.5 or 2.8k, it does ok. It is only when I try to get on plane and gun it.

Sorry if this is complicated. Any thougths? Everything is setup according to the VP service manual. Thanks in advance.

 

[Bondo]

Re: Bogs down w/ new motor and carb around 2500 rpm

Ayuh,.... What was the forensic diagnosis of the old motor,..??

If yer sure it's a carb tune issue,... pull, 'n read the spark plugs at different rpms....

They'll tell ya if they want more, or less fuel, 'n when...

 

[Jlawsen]

Re: Bogs down w/ new motor and carb around 2500 rpm

Sounds a lot like a sudden loss of vaccum.

 

[Maclin]

Re: Bogs down w/ new motor and carb around 2500 rpm

If you can get past the 2500rpm mark after the bog then the rest of this may help you dail in the pump shot. If you cannot get past 2500 then I would suspect main metering is too lean or timing is not advancing as needed. It also sounds like you have had this type of problem on the previous engine also.

I think the 50cc was correct. An incorrectly tuned and timed accelerator pump shot will just cause a hesitation as the throttle is opened. The faster the throttle is opened the worse the hesitation, it could even backfire thru the carb. If you throttle up very slowly the engine should be able to get to the top RPM's even if the accel pump is not working at all. It sounds like to me you need to tune/time the pump shot to optimize low to mid-throttle action, i.e increasing the duration of the shot.

In the Holley world the duration of the shot is determined by the squirter nozzle size and pump housing capacity. Larger nozzle results in shorter duration for any given pump shot volume. Higher capacity pump housings increase length of pump shot accordingly. The cam determines the profile, as in when more or less pump action is delivered. All of this is to cover up the gap from when the throttle is punched and when the main jets can begine metering in enough fuel to maintain the proper air/fuel mix.

The power valve is also a factor in the main metering. For Marine engines that run in lower vacuum situations a 2.5 or 3.5 power valve is needed. A "stock" automotive carb will come with an 8.5 or so usually. A Marine 2300 needs a 2.5 or 3.5 to match engine load characteristics. If the power valve is not right then the main metering is lean and it may appear as the pump shot giving up too soon.

Also the main jets are larger than "stock" automotive, I am not sure of the numbers. Sometimes the jets are staggered, one larger than the other.

If the main metering is too lean then the "standard" pump shot may not be able to cover the gap long enough.

It sounds like you need the 50cc housing with #28-30 squirters and a cam that moves the lever more in the middle of the stroke and not as much at first. The white cam is like that, I think the pink is also. The green one has more action at the first of the stroke and not as much later and is the opposite of what you need.

 

[jgresham75]

Re: Bogs down w/ new motor and carb around 2500 rpm

Thanks for the feedback. Some updates.

The carb is brand new from Holley. I adjusted it according to VP service manual specs which had it using the 4.5 power valve, yellow accelerator cam and did change the jets to ones called for in VP service manual.

With just me, would plane very good keeping rpm under 2500. Could not push through past 3k rpm. I added 3 more people and would not plane at all. Could not even slowly push through to get the rpms up. I am sure it is something simple. This is a brand new motor and brand new Holley carb. Even fuel pump is only one year old.

Any other thoughts before I talk to the shop?

 

[Bondo]

Re: Bogs down w/ new motor and carb around 2500 rpm

Thanks for the feedback. Some updates.

The carb is brand new from Holley. I adjusted it according to VP service manual specs which had it using the 4.5 power valve, yellow accelerator cam and did change the jets to ones called for in VP service manual.

With just me, would plane very good keeping rpm under 2500. Could not push through past 3k rpm. I added 3 more people and would not plane at all. Could not even slowly push through to get the rpms up. I am sure it is something simple. This is a brand new motor and brand new Holley carb. Even fuel pump is only one year old.

Any other thoughts before I talk to the shop?

Ayuh,... Again,... What killed yer old motor,..??

 

[Don S]

Re: Bogs down w/ new motor and carb around 2500 rpm

What's the fuel pump pressure when it's bogging?

 

[jgresham75]

Re: Bogs down w/ new motor and carb around 2500 rpm

Bond-o
The old engine was replaced due to bad valve job. Block deck not flat and water leaked into engine. It did run ok before the valve job.

Using the same fuel pump and distributor, got full rpm range on old engine. Did get new block, carb, alternator, plugs and water pump.

I have not checked fuel pump pressure yet. Fuel pump was bought last year and ran fine on other engine. I will check pressure on pump.