Vapor Lock?

[knish71]

Last Fall, with the advice of my mechanic, it was time to rebuild my '71 350. I asked him if it were possible to get more power out of her without sacrificing reliability. He suggested stroking it to 383 ci. So we went ahead and did it.

I picket it up and the power is outstanding, but on the way home, after sitting at a couple of lights, it started to surge and stall on me. This spring I decided to have the holley rebuilt, that seemed to fix that problem at that time.

Now that hot weather has hit, I'm experiencing higher engine temperatures (approx 210*) when stuck in traffic and the same problem as before has returned.

I then replaced the rad with a new dewitts aluminum rad and now at cruising speeds, the temp stays at 180*, but at idle quickly climbs to 210* and it starts surging and stalling.

I thought that 210* isn't too bad while in traffic as I read other people quoting much higher temps under similar idling situations, so why is this happening and what can be done to fix this problem.

Before the rebuild, there temp rarely went above 180* at anytime.

Sorry for the long winded story, but I thought it's history would help.

Thanks

Robert

 

[WARVET]

I'm no expert, but it sounds like a vacuum leak to me. It's pretty common to get one after a rebuild. Could be any of the hoses, or the seal on the intake or carb. If you spray some starter fluid, or carb cleaner around those areas while the motor is running and the idle gets faster, then you have leak in that area.

 

[kiwi vet]

Hey Knish71,

Not a mechanic, but like problem solving (lucky I drive a vette then ), so will have ago.
Increasing the cubic capacity, must increase the heat energy produced, hence more power from the engine. Logically then with the same set up the engine should run hotter.

But now youve replaced your radiator with an aluminum one, assuming you went for same size or slightly bigger, should improve heat transfer.

Some questions:
When you say mechanic rebuilt the motor, did he recondition water pump?
Where is the fuel filter now placed? Where do the fuel lines run?
Have you considered a flex fan?

Something a little lateral, is it manual or auto. Reason I ask is when I got my 81, had the same surging and stalling problem. Issue resulted from a fairly modified engine trying to drive through a standard torque convertor, the temp would rise as the engine was under load fighting against the brakes. The longer you sat there the more violent the surging got until it stalled. Just a thought.

Good Luck

Kiwi Vette.

 

[Vettehead Mikey]

210 in traffic is normal even for a stock engine. I wouldn't bother trying to fix it, especially by throwing bubba fixes at it.

You need to get to the root cause of the problem, have you gone back to your engine builder to do some basic trouble shooting? A simple error in ignition timing will cause surging like you describe.

 

[knish71]

Thanks guys,


It is a standard trans and the water pump was rebuilt. The original fuel pump and lines where reinstalled right back to to factory spec. The car runs like a champ until I get into traffic. The water temperature rises, then the trouble starts.

I've checked the timing myself as did another mechanic. It is set at (If I remember correctly) 8 or 10* BTC. Even though 210 is kinda on the high side, I don't hear people complain about even higher temps getting vapor lock?

I still suspect that Holley. Can this be the root of my problems?

Robert

210 in traffic is normal even for a stock engine. I wouldn't bother trying to fix it, especially by throwing bubba fixes at it.

You need to get to the root cause of the problem, have you gone back to your engine builder to do some basic trouble shooting? A simple error in ignition timing will cause surging like you describe.

 

[vette-dude]

Thanks guys,


It is a standard trans and the water pump was rebuilt. The original fuel pump and lines where reinstalled right back to to factory spec. The car runs like a champ until I get into traffic. The water temperature rises, then the trouble starts.

I've checked the timing myself as did another mechanic. It is set at (If I remember correctly) 8 or 10* BTC. Even though 210 is kinda on the high side, I don't hear people complain about even higher temps getting vapor lock?

I still suspect that Holley. Can this be the root of my problems?

Robert

I'm wondering if the carb is too lean? If so it would surge and stall and the engine would tend to run a little warmer. Maybe when you increased the CI of the motor the carb may need to be rejetted ( Not normally but it could have been borderline before the rebuild). What color are the plugs? If they are white it is too lean, should be tan or dark gray. I think you are due for a carb tune up. Of course I have been wrong before so just a suggested something to look at.

Randy

Also a thought, fuel filter might be needing changed. If you do rejet only go up 2 jet sizes on the primary plate, run it and then check plugs. Sizes are marked on the jets.

 

[knish71]

Randy, Funny that you mentioned rejetting. When the engine was rebuilt, my mechanic did rejet the carb. I'm not sure what the size is though. I was also thinking of letting out the idle screws to fatten it out a bit. I took the carb off and had it professionally rebuilt. That made it drivable in cool weather, but not so in this heat.

When I set the idle mixture setting, I beleive I started at 1 1/2 turns out, and pretty much stayed right there. There wasn't much change in the vacuum gauge by turning it out. You think fattening out a bit is worth a try? I also dropped the primary float a bit, just in case.

Robert

 

[L48]

Other possibilities:

Too much fuel pressure - a new fuel pump putting out a pound or two more pressure than your needle can hold. Slowly overpowers and could cause the surging you describe.

Not enough fuel pressure - old fuel pump is a little low on pressure and this is allowing gas to boil in the fuel line while sitting in traffic. Yes gasoline can start to boil at about 150 degrees.

I suggest insulating the fuel line between the pump and the carb. This might be a "bubba" fix, but it will eliminate gas boiling in the line as a possibility, and you can remove it easily if it does not fix the problem.

Last year I had problems with the gas boiling in the accelerator pumps of my T/A when the car was sitting. I put a heat shield in there and it seems cured.

Does the car have problems idling after a couple lights when it is not up to 210 yet? If so then I say there is some kind of fuel management problem in the carb.

 

[vette-dude]

Randy, Funny that you mentioned rejetting. When the engine was rebuilt, my mechanic did rejet the carb. I'm not sure what the size is though. I was also thinking of letting out the idle screws to fatten it out a bit. I took the carb off and had it professionally rebuilt. That made it drivable in cool weather, but not so in this heat.

When I set the idle mixture setting, I beleive I started at 1 1/2 turns out, and pretty much stayed right there. There wasn't much change in the vacuum gauge by turning it out. You think fattening out a bit is worth a try? I also dropped the primary float a bit, just in case.

Robert

Idle mixture screws do nothing for anything except idle on a holley. I would take a look at the plugs first and determine if the car is running lean. If so fatten the primaries up by two jet sizes and try again. The only other reason it would be running lean is a vacuum leak or the thottle shaft is bad. I still would bet it is the carb. Keep your float level just below the weep hole.

 

[goertz]

it sure sounds like a vapor lock to me, caused by boiling fuel. the engine temp issue sounds normal to me. make sure you've got all your radiator foam seals in place, and a good shroud, I think you're fine there. Check the routing of the fuel lines. I'd seriously suspect it's the fuel getting hot. if you have headers, they definately add a ton of excess heat into the engine compartment. personally, I think holley carbs suck a ss. I'd put a nice edelbrock carb on there. they are designed after a carter, and work exceptionally well.

 

[knish71]

Assuming I have a vacuum leak, approximetly how many inches of vacuum should I have? When I had a gauge on a few months ago, the max I was able to acheive at idle was about 10".

Robert

 

[L48]

Assuming I have a vacuum leak, approximetly how many inches of vacuum should I have?

You will need to tell us your cam specs - lift and duration at .050 inch before that question can be answered. IMO 10 is a little low for a mild cam.