427 dart block with 750 quick fuel, cold start trouble
 

Hey guys, hoping some can provide some guidance for me.

Have a 5 month old backdraft, with a Smeding 427, quick fuel 750 double pumper and mechanical fuel pump. Having big time cold start issues, I'll try to document them below.

1) here's what I've confirmed, when I park the car for more than 2 days the fuel in the bowls (visible through windows) evaporates. I know it's not leaking because I've sent in an oil sample for analysis and there is no fuel in the oil. If I park after a hard drive I can actually see the fuel percolate out within 30 minutes. But it will still start hot without issue.

2) now after a couple of days the bowls are dry. I crank the engine for 5 seconds and notice the bowls fill up.

3) I give it 1-2 pedal pumps, gas jets nicely in the carb, choke closes and car fires up nice and strong. Then 2 seconds later starts to stumble and stalls after 3-4 seconds.

4) I wait 30-60 seconds and crank for 10 seconds and it won't start.

5). Wait another 30-60 seconds and give it a full pedal shot again and this time it fires up strong and a big cloud of gray smoke from the pipes like its flooded and the engine stumbles like its flooded. Get out and smell the pipes, there's strong gas smell and gray vapor from pipes. Definitely seems flooded.

6). Wait 3 minutes, push pedal to the floor and hold it, fire it up and again starts strong with a big gray cloud, RPMs struggle around 600-800 as the car works through the gas and after 1 minute the RPMs stabilize at 1400 rpm and car runs great thereafter.

So why is the engine flooding after the first start? This doesn't happen during hot starts and won't happen if I drive the car every day. Only after sitting for more than 2-3 days.

Frustrated!

Art

We need to work out whether the fuel is leaking internally or evaporating (or both).

If you cold start the engine and run for a short period without putting a lot of heat into the engine, then let it sit for 2 days, do the float levels stay correct or drop down as stated?

If they do drop, I'd look at the power valve gaskets and/or the power valve diaphragms for leaks.

If they don't drop, then I'd be looking at adding a phenolic or wood carburettor spacer to help insulate the carb from the engine temperature at hot shutdown.

Wood spacer? I have not heard that before. Wood grows and shrinks with moisture content, that could be a vacuum leak waiting to happen.

Here's one:
Moroso Wood Carburettor Spacer 1 2" Single Holefits Standard Holley Carb MO65015 | eBay

Good post thank you. So here's my experience based on the two scenarios you provided

1) hot shut down, engine oil temp 180-200F. Within 1 hr fuel levels in the bowl are no longer visible and if I take the air cleaner off I can see the vapor rising from carb. But at this 1 hr mark if I give it 1 pedal push it will crank for 1 second and start up with no stumble or smoke

2) cold shut down, ambient temp. With in 48 hrs the fuel in the bowls is no longer visible. It's at this point where the problem to start begins to occur.

But the question is why is it not flooding when it's hot but it floods on the 2nd and 3rd try when cold. I should also note that when I got the car in October I had none of these issues, but beginning in November we switch to winter grade blend in CA. Not sure it's the cause but that is when this cold start issue began.

I would say that your intake combo gets a little warmer than normal (acceptable) especially after engine shutdown, heat soak.

By the sound of it, you have both issues.

It has been compounded by the change to winter blend which has more low end vaporising composition. The fuel vaporises at a lower temperature.

The engine starts ok hot because most has evaporated. When cold it is more than likely flooded with liquid fuel, hence the flooded starting.

I would take the carb off, drain the bowls, take the bowls off carefully to inspect the back of the power valve AND the area within the main body where the manifold vacuum is presented to the back of the power valve from the base of the throttle body. Hope that makes sense.

There is a recommended torque figure for the power valves.

I fitted a QFT 750 to a mates car after I stripped it for inspection. Has never played up.

I have a Backdraft also with a dart based block. The 427 built by Total Engine Concepts and installed by Speed Fanatics. I had the very same problems as you describe both hot and cold. I checked timing, idle screw settings with a vacume gauge, idle speed, choke settings and still did not correct the problem. I noticed the fuel level in the site glasses were in the middle of the glass just as the instructions for my Holley Advenger carb instruction recommend. As you describe the fuel would evaporate until the level was at the bottom of the glass and percolate when hot. I went on the internet and found that the recommended level for the fuel bowels was just at the bottom of the sight glass not the middle. I set the level at the bottom of the site glass and rocked the car from side to side and gas should slosh onto the bottom of the sight glass. Now the level is correct. I did this and I have no more problems. I push the gas peddle when cold once to set the choke and car starts right up and fast idles until I kick it down. When hot I just turn the key press the start button and it starts right up. Hope this helps.

Gary this is starting to make some sense. So when hot the fuel is vaporizing faster than it is potentially "leaking" hence not flooding. But on cold start when I crank and fill the bowls up, then the fuel potentially leaks and hence the cloud of smoke on cold start and no smoke on hot start.

Ok so when I take off the carb what exactly am I looking for? I'm handy but I don't have lots of carb experience. Am I looking for a gasket leak or tear somewhere?

thanks!

Hi Tim

thanks for the suggestions. I spoke directly with Quick Fuel and my fuel bowls are properly calibrated. They also suggested it may be leaking some based on my explanation of the problem

Hopefully I can figure this out, it takes me 5-10 minutes to get the car going on cold start, so frustrating! (and my neighbors love the loud rumble, stall, rumble, stall LOL!)

This article may be of some guidance.

Carburetor Valve Problems - Tech Article - Chevy High Performance Magazine


If you see any fuel in the cavity of the mainbody behind the power valve, (refer pic 6/10), the power valve gasket is leaking or the diaphragm is leaking, or even possibly the metering block gasket.

Before you tear everything apart, consider the simple solution:
Back off on the choke tension (turn it a few marks toward lean).

A phenolic or wood spacer would still be a good idea if you have the hood clearance.

427BDR,
Have you ever noted a backfire up through the carburetor. They are famous for blowing out the power valve described earlier.

so the carb was running lean for the first 500 miles and I had at least a dozen back fires through the carb (possibly more). After speaking with smeding and quick fuel I was instructed to richen the idle mixture 1/4 turn and haven't had a single carb back fire over the past 200 miles.

I Did a quick google search and from what I found it seems a blown power valve would make idling very difficult at all times (is this not the case?). But once I finally get the beast properly started it idles great and runs perfectly.

Thanks!

blown power valve
California Winter blend gas

after it has been setting a couple of days, take out one of the corner float bowl screws and catch wathever amount of fuel is in there with a small cup

after you have cranked it over to fill the float bowls up to the site windows ( don't run it and get it hot) do the same drain and compare the amounts of fuel (do both primary and secondary)

As for the spacer...I have BDR #1230 and had heat soak issues. Now using a "Cool Carb" spacer which has eliminated my problem. I don't have a link but you can find them easily.

Will try this, but what will we learn from this exercise? I assume there would definitely be lots more fuel after the bowls are full. Thanks!

Unfortunately no room under the hood for even a 1/4" spacer. But even with the bowls evaporating I am still able to fill them up by cranking the engine for 5-6 seconds. But for some reason the process of filling the bowls floods the cold engine. Have to guess that it must be as others have stated, some kind of leak somewhere

we won't learn nothing-I already know-you will learn how muck gas leaks into your engine-you will then determine that its maybe the power valve and replace it-----

got it, I think the ideas are good and the carb probably needs to come off and the PV replaced. But before I do this what if I have a couple of badly fouled spark plugs? could that cause potential flooding on a cold start if 1 or 2 cylinders are not firing because of dirty spark plugs and just pushing gas into the engine?

I'll pull the plugs and change the fouled ones and try a cold restart later this week. If that doesn't improve things then the carb comes off and we attack the PV.

Thanks all, very much appreciate the insights!

I'd sort the carb first.

The rough running does eventually clear, and the engine runs fine after that.

Fit your new plugs after you have sorted the fuel leak.

Your problem is with the carb, not the plugs.

It sounds like a fuel percolation problem. Somehow, you've got to insulate that carb from the engine temp during the winter months when our gas turns to crap.
You can always get a drop base air cleaner to fit under the hood so you can use a phenolic spacer to help insulate the carb.

Also, try opening the hood as soon as you get home and leave it open. Helps dissipate the heat quicker.

And, finally, most newer carbs have blowout proof power valves, so I doubt that's your problem.