QFT 750 Help

[straightlinespeed]

Hi guys, I need to pick your brains regarding my QFT Slayer 750 vacuum secondary carb. This is on my LA 360, stock pistons, with Magnum heads, XE268 cam, air gap. Static compression is around 9:1. Timing is 21* initial and 36 total.

I got this carb brand new in about August of last year. The car seemed to idled and drive great with it. I laid the car up for the winter with 92 non-oxy gas. I did my first start up about a month ago and it was carbon fouling plugs, with in 5 minutes of running. This set of plugs where black and sooty.

I swapped in a new set of plugs, started it and it acted very cold blooded, almost like a misfire. Until the engine was up to about 160* then it smoothed out and idled ok, but not great. I took the car out on the road and drove it. Once home I let it cool down and pulled the plugs, the ceramic had carbon on it, but not as sooty as the first set.

Curious about it, I called QFT and talked to there tech support. They told me the carb was slightly big for my engine but increasing the idle air bleed size from 70 to 74 in the front and 39-43 in the rear would take care of the carbon fouling that was going on.

I did this, installed a spare set of new plugs I had and started it. It started like before, cold blooded and misfiring type issues. Once up to around 160* again it smoothed out and idled better this time. I readjusted the carb because of the IAB change and took it for a drive. Once home I let it cool again and pulled the plugs. This time the plugs where whitish/brown on the ceramic and no soot. So that change did help. Oh and I have a fresh tank of gas in it.

Although curious about the cold bloodiness and misfiring on a cold engine I called QFT again today to discuss this further. They said that is a symptom of being to lean. Which my buddy RRR mentioned to me as well. (Thanks Rob, you havent been wrong yet). They asked if I was running a PCV valve, I told them yes. The guy I talked to said that could be part of my problem with the issues Im having. He said running a PCV valve is for emission purposes on a stock engine. He then said that aftermarket cams are not designed for use with PCV valves. Saying, that is like running a smog pump on the engine and could be what is causing the cold engine symptoms. I mentioned I was running it to save the gaskets and rings etc... He said pull the PCV and put a additional breather in it place and that would have the same affect. Then plug the port on the back of the carb, retune it and see how things are.

So a few things.....

-Any thoughts on why my engine acts the way it does, until its warm?
-Removing the PCV goes against everything that I was told and why I should run one, thoughts on what this guy told me? Try, not try?

 

[RustyRatRod]

He's an idiot regarding the PCV valve. You should keep it.

 

[Inertia]

He's an idiot regarding the PCV valve. You should keep it.

^^^^ Xs2

That's a sure way to get a moisture accumulation inside the engine.

If you try running without a PCV,, pull the valve covers off after a few miles,, and see how much condensation is on the inside of the valve covers,, that moisture ends up in the oil..

Member Cannucky found this out.. lol

hope it helps

 

[straightlinespeed]

He's an idiot regarding the PCV valve. You should keep it.

^^^^ Xs2

That's a sure way to get a moisture accumulation inside the engine.

If you try running without a PCV,, pull the valve covers off after a few miles,, and see how much condensation is on the inside of the valve covers,, that moisture ends up in the oil..

Member Cannucky found this out.. lol

hope it helps

Yep, that is what I was thinking about the PCV as well. Didnt make any sense to me hence why I asked about it.

So any thoughts on why the cold start issues? Possibly the gas went bad or some plugged passages in the carb? Although that doesnt make sense to me since it clears up when the car warms up. Im kinda stumped

 

[RustyRatRod]

The PCV also introduces manifold vacuum to the crankcase and helps the rings seal by applying a vacuum behind them. That's well beyond "just" an emissions device. In fact, you "could" say it even enhances performance. I don't think that's a stretch at all.

 

[AJ/FormS]

Are you running a vacuum advance unit.
Cold engines like extra fuel or extra timing. Or both.
My LA360 runs real sweet with the fat Holley and 25* to 30* @ 800rpm, during warm up.

This is one of the reasons, I love the v-can. You can plumb it in to run manifold vacuum when cold, and then switch to ported as the engine comes up to temp. You can use one of those Mopar emission devices that plumb into the water jacket, right beside the T-stat. Its called a thermo-vacuum switch. They come in different temp. calibrations. Find one that drops out pretty late, but at a lower temp than your stat. That should be easy, as most are switched by 110*F or less.
The can is able to bring in from 12 to 20* or so depending on the can, on the cold engine. Thus the cold idle timing can be varied from base timing,which on your 284 cam is 21*, to, up to 41*.
If you go this route, I would suggest to reset the idle-timing to 18*, and find a 14* to 18* can for a cold timing of 32* to 36*. (With iron heads ,shoot for the smaller can. With aluminums, Id go closer to the 18 or 20* can).Then set the rate of advance in the can to about the middle of its adjustment. Then after the engine is warmer, retune the can to come in as quick as possible without detonation under various light load/part throttle settings. This will fatten up the low-speed torque curve. At least it did on mine(Hughes 232*@50, with Eddies).
Then finally, if you need to ,fatten up the low-speed circuit.If your float level is standard,I would try a slightly higher float level,and/or a slightly higher curb idle setting,so long as that doesnt get too far into the transfer slots.
All the best to you

 

[straightlinespeed]

The PCV also introduces manifold vacuum to the crankcase and helps the rings seal by applying a vacuum behind them. That's well beyond "just" an emissions device. In fact, you "could" say it even enhances performance. I don't think that's a stretch at all.

Correct! I totally agree with you and that is why Im stumped when he said that.. Just didnt make any sense to me.

 

[straightlinespeed]

Are you running a vacuum advance unit.
Cold engines like extra fuel or extra timing. Or both.
My LA360 runs real sweet with the fat Holley and 25* to 30* @ 800rpm, during warm up.

This is one of the reasons, I love the v-can. You can plumb it in to run manifold vacuum when cold, and then switch to ported as the engine comes up to temp. You can use one of those Mopar emission devices that plumb into the water jacket, right beside the T-stat. Its called a thermo-vacuum switch. They come in different temp.calibrations.Find one that drops out pretty late, but at a lower temp than your stat. That should be easy, as most are switched by 110*F or less.
The can is able to bring in from 12 to 20* or so depending on the can, on the cold engine. Thus the cold idle timing can be varied from base timing,which on your 284 cam is 21*, to, up to 41*.
If you go this route, I would suggest to reset the idle-timing to 18*, and find a 14* to 18* can for a cold timing of 32* to 36*. (With iron heads ,shoot for the smaller can. With aluminums, Id go closer to the 18 or 20* can).Then set the rate of advance in the can to about the middle of its adjustment. Then after the engine is warmer, retune the can to come in as quick as possible without detonation under various light load/part throttle settings. This will fatten up the low-speed torque curve. At least it did on mine(Hughes 232*@50, with Eddies).
Then finally, if you need to ,fatten up the low-speed circuit.If your float level is standard,I would try a slightly higher float level,and/or a slightly higher curb idle setting,so long as that doesnt get too far into the transfer slots.
All the best to you

Yes I am, running a MP Dizzy unit with the adjustable mechanical and vacuum pod on it.

Im assuming that the idle was to rich hence the carbon fouling the plugs, but it may be something as simple as increasing the timing like you suggested. I'll have to re-read what you wrote to process it all but I think I know what your getting at.

Im assuming your using one of those thermal vacuum switches? That is how you get the higher timing when cold?

 

[crackedback]

If you went from ethanol laced fuel to straight gasoline, the car will run richer.

 

[straightlinespeed]

If you went from ethanol laced fuel to straight gasoline, the car will run richer.

I've always run this 92 non-oxy. I don't think that was the issue unless it went bad over winter

 

[Lonewolf3165]

The guy is an absolute DOPE! He's probably confusing EGR with a PCV. Check your float height to see if the level is high. It may have sunk a little during storage.

 

[straightlinespeed]

The guy is an absolute DOPE! He's probably confusing EGR with a PCV. Check your float height to see if the level is high. It may have sunk a little during storage.

I've checked it out and it is about 3/4 of the site glass in the front and 1/2 in the rear. Is that to high?

 

[straightlinespeed]

One other thought.. I did have a backfiring issue thru the carb and exhaust due to my foulded plugs early on.. Could that have taken out the power valve causing more fuel than needed?

 

[AJ/FormS]

If it smooths out when warm,the PVs are probably fine.

There is such a thing as too much idle timing, for a given cam size.
What happens is; with the extra timing the rpm starts to climb. So then you back out the curb-idle screw to slow it down,and the carb gets out of the transfers. Well that makes an ok hot idle but the cold idle goes lean, because the transfers are not flowing,as much as they should be.
Theres only one way to prove this. You have to take a look underneath the T-blades. The T-ports should be square to slightly rectangular. If yours are not, then make them so. Start her up,get her warmed up good, do not touch the curb idle screw. Now if its idling too fast, back off the idle timing to get the idle where you want it.Reset the mixture screws. If you get a tip-in stumble that cant be cured by pump-shot or pump-timing, NOW you can increase the curb-idle adjustment about 1/4 turn at a time, with accompanying timing changes as may be required from time to time. Finally, re-engineer your power-timing,and roadtest.
--I have a Hughes 232*@50 cam which is very similar to your XE268. I only run 14* to 16* idle-timing. But my Vcan is 22* and fast.I no longer use the thermo-vacuum switch. I installed a dash-mounted dial-back timing device. It has a working range of 15 degrees.Which I set close to the middle; so I get about 9*advance/6*retard. Slick as ice.When starting it up, I set it to max advance(16+9=25*). If its still cranky I increase the throttle slightly to bring in some of those 22* in the Vcan. At 1600 or so, theyre all in, so that could be as much as 25+22=47*. Slick as ice.
Theres just one caveat;I have to remember to dial back the advance as the engine warms up,as Im driving down the road. My power-timing is set to 34*. If I forget to dial it back, its 34+9=43* Oh-Oh. Its (the device)been in there now for around 10 years. So far so good.
Another thing I like about the DB device is this; The 232 cam is pretty tame idling once you get the tune into it. I really miss the idle of the 292/509 I had.Well it turns out, that if you get the idle speed and timing retarded just right, that 232 goes into rumpity-rumpity mode. So I tuned the idle to 14/16 degrees at 750ish. And when I back out the 6*with the DB, to about 8* initial,the idle speed drops down and there-it-is.Rumpity-rumpity. Its still kinda subdued, but with the 10.7 static C/R, at least I can hear it.

 

[straightlinespeed]

Thanks again for the insight AJ.

What is odd about this is, it ran better last year, prior to the winter lay up. So something while it was sitting over the winter caused the change. Now I just need to figure out if it is the carb that's the issue or if its the timing. I had my motor tore down over the winter, to take care of various other issues, but I never pulled the dizzy out.

I'll check into what you suggested, as a start to try to figure it out.

 

[AJ/FormS]

That engine with a 268,should purr like a kitten. Its just a matter of time.
Mine with a 232*, sure does.One second its idling; the next second, the tires are screaming for mercy.Then its cruising along at 1800, on high alert, ready to explode again. Just thinking of the memories has me all gooey inside.I cant wait for the street-sweepers to come around and get rid of the winters sand and gravel. Then it will be GO-TIME, again.

 

[Lonewolf3165]

Timing won't change just from sitting. Probably can rule that out. Carburetors & fuel systems on the other hand are very subjective to their environment. I'm sure you stored it ok, but conditions like weather and fuel change all the time. Look down the top of the air horn while it's running & see if it's dribbling fuel into the venturies. I don't know for sure if the QFT carbs have backfire protection built in or not(although I have seen even the newer Holleys PV's get blown out even with it) so a PV change might not be a bad idea. Try tweaking the float level down on the one that's at 3/4. Just go in small increments to see if there is any difference. Hard to help over the web, wish I could be there to see it & help you out more!

 

[straightlinespeed]

That engine with a 268,should purr like a kitten. Its just a matter of time.
Mine with a 232*, sure does.One second its idling; the next second, the tires are screaming for mercy.Then its cruising along at 1800, on high alert, ready to explode again. Just thinking of the memories has me all gooey inside.I cant wait for the street-sweepers to come around and get rid of the winters sand and gravel. Then it will be GO-TIME, again.

hahaha Glad to make some memories for ya. :burnout: Yep, it did idle all purrrrrrty like at one time.. I just have to find that missing link!

Timing won't change just from sitting. Probably can rule that out. Carburetors & fuel systems on the other hand are very subjective to their environment. I'm sure you stored it ok, but conditions like weather and fuel change all the time. Look down the top of the air horn while it's running & see if it's dribbling fuel into the venturies. I don't know for sure if the QFT carbs have backfire protection built in or not(although I have seen even the newer Holleys PV's get blown out even with it) so a PV change might not be a bad idea. Try tweaking the float level down on the one that's at 3/4. Just go in small increments to see if there is any difference. Hard to help over the web, wish I could be there to see it & help you out more!

Its all good, any little bit helps. I appreciate the kind gesture.

I've looked down the carb while it was running and didnt see it dribbling fuel. Since I have spare Pv's here I'll change that. Oh and for your knowledge according to QFT's website, they do have backfire protection. I had to look it up since I wasnt sure either. LOL I'll also be checking all the passages on the carb, just in case something is plugged. I'll lower that float bowl level also. I was thinking it was slightly on the high side.

 

[AJ/FormS]

Heres a quickie for ya, before you pull the carb off.
Do you know how to clear the idle-air bleeds out with compressed air, through the mixture-screw ports?

 

[Lonewolf3165]

Don't change everything all at once! Step by step one at a time so you know what worked & what didn't make a difference. Remember your adjustments starting point before you begin in case you have to set it back the way it was.
Good Luck!

 

[straightlinespeed]

Heres a quickie for ya, before you pull the carb off.
Do you know how to clear the idle-air bleeds out with compressed air, through the mixture-screw ports?

I assume your talking about pulling the metering blocks off and "back blowing" to say thru the ports on the block?

 

[straightlinespeed]

Don't change everything all at once! Step by step one at a time so you know what worked & what didn't make a difference. Remember your adjustments starting point before you begin in case you have to set it back the way it was.
Good Luck!

Check!!! :thumbup:

 

[AJ/FormS]

Mostly correct.
This can be done without removing the blocks. I remove the mixture screws and their springs or seals.(remembering their turns-to-seat adjustments).Then I take my blowgun with the tiniest little orifice, and give them two or three short gentle blasts. I do not seal the gun to the ports. I dont like to buy floats to replace collapsed ones.The idea is to make the fuel in the standpipes to blast into the airhorn taking any dirt that may be in them, out.This takes but a short, low-volume blast. If you blast the fuel out of the float bowls,that would be waaay too much air. If you remove the air cleaner, you may catch the spurts.