Thread: Engine Surging in slow traffick

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Well, I am finally back to this. I added a thermal wrapping ( here ) to my fuel line close to the engine. Last weekend I vapor locked. Engine stopped and I had to wait until the engine cooled.

I have not had this happen before. But, based on earlier comments in this thread, this is just the worst end of my problem, most likely.

I am tempted to get an electric fuel pump, but earlier in the thread, kcress said that doesn't always work.

I may break down now that I am experiencing this and get one of those temp sensors.

By the way, anyone know the limit on temp in the gas line at which vapor lock will occur? I presume not, but thought I'd ask.

Just thought i should ask,,,how long have you had this problem with surging??
Has it alway's happend?? since you owned/built the car?? or is a recently occuring problem??
Of course taking into acount the same time of year, driving condition's and weather and all that.
If it's only a recently occuring problem:
There is a condition called percolation,,which can happen as a result of air bubbles in the fuel,,,usualy caused by a small crack in the fuel pump diaphram,,,which allow's air to be mixed with fuel prior to delivery to the intake system,, with this condition,,the float loses boyancy and drops momentarily.
The same principal applies to any object which needs to float on liquid,,airiation will cause loss of boyancy.
This condition can also occur due to extreem heat conditions also.

Originally posted by ewingr
My engine is a 350 Chevy engine, and it has a B&M Street Blower (5lbs boost) with a Holley manifold referenced power valve.

If I understand it right you're running a Holley carb, correct?

The Holley manifold referenced power valve throws me.

The power valve is referenced to the intake manifold that sits on the heads under the blower?

Or are you calling the adapter between carb and blower the manifold?

I'm wondering as well what you have to do to reference vacuum for the power valve from an external source.


Perhaps referencing the power valve to an outside source is the correct thing to do, but the Holley's I see on top of a blower operated the same as they do on a naturally aspirated engine.

I wonder if it's something as simple as the wrong power valve selection for the cam intake combo you have?

That happens a lot with naturally aspirated engines where a guy runs a big cam with a stock power valve and the power valve opens at idle & low speed dumping excess fuel into the intake.

Originally posted by DennyW
Yes, you have to match the power valve to the intake vacuum, plus, you have to make sure you have a hole that goes to the vacuum source from way up there.

Ok, but I'm still not understanding.

You source the vacuum from the intake under the blower, right?

And then you have to drill the Holley carb so you can set up vacuum to the power valve?

It sounds reasonable and a little tough to do.

Seems too that just plopping the Holley on the adapter plate on top of the blower would work ok.
That would be under a vacuum at low speeds and once the engine/blower were spun up, the vacuum at the adapter plate would drop close to zero and would allow the power valve to open.

The advantage I can see with an intake sourced vacuum to the carb would be that the power valve would probably come in quicker when the intake vacuum rolls over to pressure when rpm's come up.

Thanks Denny.

I think I have it now.

Dang . . . I gotta get me one of them blower thingy's one of these days....

Just thought i should ask,,,how long have you had this problem with surging??

This has been going on for the past couple years. The 18 years before that I have not had the problem.

In answer to other questions:

I have a Holley 80273S carburetor. That is a carb that is designed with the vacuum hookup to provide the manifold referencing. So, as DennyW says, the vacuum port is already there.

However, if I understand DennyW's replyl correctly, he is saying to put the vacuum to an adapter below teh carb, but above the blower. That is inaccurate. The power valve needs to know what the vacuum is at the manifold. That is what determines how it works. That is what is meant by 'manifold referenced power valve'.

Instructions on that are here on the Holley site.

With regard to which power valve, I have had multiple conversations with Holley tech on this. They say to take idle vacuum and divide by 2. So, my 9.5" vacuum gives me a 45 power valve. I have tried lower and higher. I seem to constantly be over rich, no matter what I try. It doesn't seem too bad right now on richness, but a little rich. I do have the 45 in it now.

Earlier posts indicated that possibly my surging was a 'minor vapor lock'. And, given that I had a hard vapor lock (no fuel getting to engine at all) last weekend, that lends credence to that theory in my mind.

One thing I have changed (don't remember exactly when): I hooked my electic radiator fan that is on the front of the radiator to a temp unit that is supposed to turn it on. Problem is, the radiator has the place for that at the bottom, instead at the top. So it comes on rather late. I seem to have problems when my engine gets to 200, which isn't all that hot. Although it gets quite hot under my hood. (I do have a fan on the front of the engine as well. I had hoped to pull the electric one off the front, but now, I need to get it going all the time I guess.)

So, my immediate problem is "How do I best cure vapor lock?"

Somebody tell Demon I went outside the box, and got shot down

Sorry. Didn't mean to come of as 'shooting you down'. I apologize if I was insensitive in the way I posted.

it doesn't take long to go from neg to pos pressure in the intake

That is exactly right. And I think the point is that when it goes pos at teh manifold, they want the carb to know that, otherwise it will dump the wrong amoutn of gas.

Interestingly I ran that blower from 1985 to 1999 w/o a manifold referenced power valve. It always ran way too rich, and a call to Holley pointed me to this manifold referenced power valve carb. The explanation was that teh carb had no idea that the pressure was pos instead of neg, and dumped too much gas.

Interstingly, I still fight it being somewhat rich, but not near as much.

By the way, wouldn't a surge due to float level being too high happen all the time? It only happens under the circumstances I described at the beginning of this post.

I do feel pretty comfortable with the float level. I actually have it just a smidgen lower than the docs say to do it.

I'm still hoping for some definitive insight on fixing the vapor lock. Should I open a new post titled 'Vapor Lock' for that info?

I just went out and paid more attention to the gas line, looking for places it may be getting hot.

I found that while I am running it on the outside of the frame, I also have my tranny cooler near it at one point, so that may be another place to insulate.

But, I also realized that back when I built this thing, I used copper for the fuel line. I wonder if I replaced that with steel, if it would help.

Thanks

I fixed the "here" link. I should have checked it.

I did another past, and pasted that there too.

I'll open a new thread on Vapor Lock, and on the copper fuel line.