Power lagging

[Brandon777]

Okay so, I have a brand new carb and a new aftermarket intake and a 260h comp cam. My power as I'm driving is really bad. Sometimes it performs pretty good but up a hill or when I make a turn it feels like I'm losing fuel. I stalled when I took my foot off the gas just after I went up a hill and I was in level ground. I tried adjusting the carb but there was no changes. Please help

 

[chengny]

An engine that runs well at low loads but stumbles or bogs down under increased loads generally is not being supplied with a sufficient amount of fuel (or low fuel pressure).

I'd start by checking for proper fuel flow - the pressure test is kind of involved.

Break the fuel line connection at the carb inlet. Attach a hose to the end of the fuel line and run the hose into a small container (a glass jar is good - like a tomato sauce jar).

Crank the engine (it doesn't matter if it starts and runs on the residual fuel in the carb). Whether you crank with the starter or it runs, rotate the engine for about 15 seconds then stop.

Look in the container that you put the hose in. It should have at least 8 ounces of gas in it.

If it has that much (or around that much), you have proper fuel flow.

If it doesn't have nearly that amount, you have a problem in the fuel delivery system.

It could be a number of things. But the most likely reason would be either a fouled fuel filter or a failing fuel pump. A couple of other possibilities are a pinch or a hole in the fuel line - between the tank and pump suction.


But I have to ask the obvious question:

Did this problem exist before you installed the new carburetor/manifold - or did it only begin after you put them on?

 

[Brandon777]

It began when I put it on

 

[chengny]

Uh oh. Forget the bad fuel pump/blocked filter ideas - they didn't go bad while you were installing the new stuff.

Let me think.

You definitely timed the cam correctly right? And stabbed the distributor so that your timing is at least in the right range?

The 260H is a cam/lifter kit right? So you preloaded the lifters as per the instructions?


How about the compatibility between the carburetor and the intake manifold - spread/square bore match, proper base gasket, etc?

Any air leakage under the throttle plates and into the intake manifold?

Any problems with connecting the OEM accelerator linkage to the new carburetor - the throttle plates stroke fully open?

 

[Brandon777]

Yea I made sure the ca.m gear and crank gear dots lined up. I didn't preload the lifters, I didn't think I needed to do that. The intake and the carb should match their both Holley brand, and the bore matches and everything. All the linkage is fine and the gaskets are good. I have a 625 c.f.m street demon carb.

 

[Brandon777]

I'm pretty sure my lifters and flat tappet and not hydraulic so I don't think I had to pre load them

 

[rich weyand]

The lifters are either hydraulic or solid lifters. I doubt you went with solid lifters.

 

[chengny]

I didn't preload the lifters, I didn't think I needed to do that.

I'm pretty sure my lifters and flat tappet and not hydraulic so I don't think I had to pre load them


Flat tappet doesn't mean mechanical/solid. It is just a distinction from roller type liters. I am almost 100% sure that you installed a set of hydraulic flat tappet lifters - that is what is included in a Comp Cams 260H Cam Kit.

It's like: flat tappet vs. roller roller tappet and solid lifter vs. hydraulic lifter

Nowadays, virtually all lifters are hydraulic - I haven't seen mechanical lifters since I used to work on my 1958 Morris Minor and 67 VW (and my small engines - lawn mowers, snowblowers etc.)

This is what a mechanical/solid lifter looks like:

You must be registered for see images attach

You need to pull your valve covers (and spark plugs - to make it easy to roll the engine while setting the pre-load).

Since you have been running the engine without the proper pre-load, I would strongly suggest you back way off on the adjusting nuts and leave them alone for the night. Doing this will let the lifter plungers come all the way back up in their bores before beginning the pre-load procedure.

I almost hate to ask this but; Did you do a proper break-in of the cam/lifters at the initial start up? Basically it involves starting the engine and immediately bringing it up to 2000+ RPM - and running it at that speed for 30 minutes.

Here is the link to CC's cam installation and break-in procedure:

http://www.compcams.com/Instructions/Files/145.pdf

 

[Brandon777]

Yes I did do the cam break in, and okay I'll do the preload, I was doing what my dad told me and I guess he forgot that part, thanks for your help guess

 

[Brandon777]

Okay, my apologies everybody. I did do the preload of the lifters, I don't know what I was thinking. I made sure there was zero lash and than I did a half turn. So I did do the preload of the lifter.

 

[chengny]

That's good. I doubt any damage would have occurred anyway, but now we're back to diagnosing the problem.

Next thing I would do is take a good look around. See if something unrelated to the modification was damaged while the work was being done. Broken vacuum hose, disconnected wire, etc.

Did you do the cam renewal as an in-vehicle procedure - or did you pull the engine, do the cam and then drop it back in?

 

[Brandon777]

Yea, I can't tell if my fuel pump is acting up or what, I don't think any sort of vacuum leak could cause this problem, but than again I don't know

 

[MadOgre]

What did you set your timing to? and is the vacuum port at the back of the carb plugged or connected to the vacuum booster? It also sounds like a float level problem. I have also heard that the street demons need to be almost completely disassembled because sometimes there is a bit of debris from the factory inside that plugs up stuff. They are great carbs if they are clean and set up right.

I have also had really weird things happen like the fuel pump craps out at exactly the same time I replaced something else and kinda leaves you scratching your head because the odds are so unlikely but weird things do happen!

 

[Brandon777]

Both of the vacuum ports are plugged and my timing is set for 4 degrees advanced. I will have to check if the fuel pump is sending fuel, like it works sometimes and than it doesn't work sometimes. So I don't know if it's the fuel pump, it is a mechanical fuel pump if you guys were wondering

 

[chengny]

An air leak in the fuel pump suction line from the tank will cause that behavior - a sudden loss and then resumption of power.