Need help tuning 28/28 DFTA

Road-race engines and ancillaries - general discussion
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Need help tuning 28/28 DFTA

Post by turbofiat »

I need help troubleshooing an issue with my Weber and I think the problem lies in the the primary idle jet but I'm not so sure. Wanted to get some feedback.

Here's the specs:

1987 Yugo , Fiat 1500cc engine, 28/28 DFTA carb from Yugo, Fiesta mk1(USA version), Escort (USA version), blown through with a T25 running 6 lbs of boost. Choke is set to be fully open around 180F. Accelerator pump appears to be working.

I built this car 6 years ago and it has run fine but I seldom drive it in the winter because it doesn't run as well in colder temperatures. I'd like to change that.

The car runs like a champ and makes plenty of power in the middle of summer when it's 95F. But as the temperature decreases (usually below 60F and high humidity) I start developing driveability issues and sometimes backfiring which I think indicates a lean mixture. This is even after the car has reached normal operating temperature. I can understand if the engine is still cold and the choke has not fully opened but not with a hot engine.

The car starts up fine and runs fine under cruise until I start to accelerate if below 2700 rpms in top gear. If I accelerate slighly the car starts to buck and carry on and sometimes backfires. Now if I am 3000 rpms, this usually doesn't occur. Downshifting into 3rd and raising the RPMs by 1000 sometimes makes the engine accelerate smoother. That is until the ambient temperature drops another 10F. Then the problem occurs even in 3rd gear.

In the mid 30s and 80% humidity, the car runs horrible during slight acceleration. So much so I sometimes I have to downshift to the point the engine has to be turning 4K rpms before I can accelerate.

Here's what I got:

Primary: idle jet : 50, Main F22/250/??? The numbers are too marred to tell
what jet side I have. But I think I have a 100 primary main.

Secondary : idle jet : dummy idle jet replaced with a 47, Main F22/250/150

I can't tell much difference after replacing the dummy plug with the 47 idle
jet in the secondary. But it was 73F at the time so the car is going to run better
anyway.

I decided to hook up my air/fuel meter to see if it tells me anything. When the bucking occurs under slight acceleration the meter goes unacceptable from lean to rich. I realize short band O2 sensors are not that accurate but this leads me to beleive when transitioning from cruise to acceleration, it's got something to do with the primary idle jet being too small.

I beleive the reason this driveabilty issue is showing up worse the colder it get's is because colder air is more dense so the mixture is going to be leaner. So if the idle jet is already too lean, colder weather is just going to make it worse.

I realize there is a method for determining idle jet size based on the amount of turns required to smooth the engine out. I'll check that.

One more thing this car does not idle very well at Fiat's recommended idle speed (800 rpms). At 800 rpms, the engine idles really rough and shakes the whole car. In order to smooth the engine out, I have the engine set to idle at 1100 rpms. Does that indicate anything? Or could that be something else like valve lash needs to be checked/adjusted?

Do you think i need to step up from a 50 idle jet to something like a 55, 60 or larger?
124 Spider, Yugo,131
WhizzMan
Posts: 459
Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: Need help tuning 28/28 DFTA

Post by WhizzMan »

In general, make sure the rest of your engine is set up properly, before adjusting your mixture. Timing and dwell of the ignition and indeed valve lash. Do a compression test to make sure your rough running is not caused by engine internals. Check spark plugs, plug wires and coil for wear and cracks.

A lambda probe (wide band) will be all over the place on misfires. It measures oxygen content in the exhaust fumes and a misfire will cause fresh air to enter your exhaust from the engine side. A narrow band lambda is not suitable to actually measure A/F mixture, only to determine if the mixture is rich or lean.
Book #348
Brit01
Posts: 825
Joined: June 28th, 2011, 4:54 pm
Location: Uruguay

Re: Need help tuning 28/28 DFTA

Post by Brit01 »

+1 with whizzman. If you don't have a compression tets kit, they are readily available and cheap.

Remember to do a dry and wet test on a hot engine with all plugs out, and with full throttle until the gauge reads it's peak value. Unplug the coil and maybe the fuel lines also so as to not wash off the oil from the cylinders too much.

Shouldn't be more than 10-15 psi difference between cylinders.

Leak down test would be more accurate also.

check distributor/cables/timing etc as whizzman stated.
What condition are the plugs in?

Come back with you results from the compression and timing/plug tests.

Then if all looks good tell us when your carbs were last serviced. No air leaks, all clean/good gaskets, o-rings etc.

Inlet and exhaust manfolds in good condition with no leaks or holes?


Chris
WhizzMan
Posts: 459
Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: Need help tuning 28/28 DFTA

Post by WhizzMan »

Given the fact that he probably built this car himself and is asking the right questions about his single carb, I'm assuming his carb is clean, non leaking and functioning correctly.

What I'm wondering is if the 28/28 is big enough for a 1500 with a turbo on it. I'd expect it to be a 100+ BHP engine this way, stock being at 75-85 BHP already.

Is the bowl pressurized by the manifold pressure, or is the carb set up before the turbo? If you have the bowl not pressurized by the turbo, you get weird pressure differences that will make adjusting the carb more or less impossible once your turbo starts pumping. All sorts of things happening once you start pressurizing your inlet and you're working with carburetors.

Is this the same car as found on youtube, with an X1/9 engine in it?

If this was my own car, I'd be looking at a different carb configuration for this car, with pressurized bowls, or some other form of adaptation to the turbo pressure. I know Renault used Solex 32DIS carbs with some adaptation for this, maybe that is something you could be looking in to? The Fuego Turbo had 130+ BHP and is 1600CC. The R5 turbo has a slightly smaller engine and also used the 32DIS, so I'm guessing this is a carb that is at least sized more or less properly for this kind of setup. Conversion to EFI will probably be a hassle, but it's something you can look into as well.
Book #348
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Re: Need help tuning 28/28 DFTA

Post by turbofiat »

>>Given the fact that he probably built this car himself and is asking the right questions about his single carb, I'm assuming his carb is clean, non leaking and functioning correctly.>>

I fully rebuilt this carb myself about 5 years ago. I don't think there is anything actually wrong with the ignition or carb itself. I just don't think I have it jetted correctly because it was originally jetted for a 55bhp 1100cc engine. I'm estimating I'm pushing twice that now.

>>>What I'm wondering is if the 28/28 is big enough for a 1500 with a turbo on it. I'd expect it to be a 100+ BHP engine this way, stock being at 75-85 BHP already.>>

That's what I've been wondering. Most guys over here prefer the 32/34 DMTR on stock or camed up 1500 motor with HP pistons. If not using IDFs. I haven't put this car on a rolling road but I'm estimating around 110 to 115 BHP. Just based on the math. 75 BHP + 7 lbs of boost should give me at least 110 BHP.

I dynoed my fuel injected 124 Spider and my estimation was spot on at the rear wheels based on 102 BHP pushing 7 lbs of boost.

>>Is the bowl pressurized by the manifold pressure, or is the carb set up before the turbo? >>

This is a blow through setup. The fuel bowl is pressurized. The fuel bowl vent port is connected to the carb bonnet.

>>>If you have the bowl not pressurized by the turbo, you get weird pressure differences that will make adjusting the carb more or less impossible once your turbo starts pumping. >>

Actually the carb won't even function unless the fuel bowl is pressurized. As soon as the fuel bowl becomes pressurized, fuel will stop flowing through the jets.

>>>All sorts of things happening once you start pressurizing your inlet and you're working with carburetors.>>>

Not as hard as you might think. The trick is getting it jetted properly. Something I'm not a pro at. Not to mention Weber jets are overpriced. An idle jet over here is like $4.50.

>>Is this the same car as found on youtube, with an X1/9 engine in it?>>

Yes it is.

Image



>>>If this was my own car, I'd be looking at a different carb configuration for this car, with pressurized bowls, or some other form of adaptation to the turbo pressure. I know Renault used Solex 32DIS carbs with some adaptation for this, maybe that is something you could be looking in to? The Fuego Turbo had 130+ BHP and is 1600CC. The R5 turbo has a slightly smaller engine and also used the 32DIS, so I'm guessing this is a carb that is at least sized more or less properly for this kind of setup. Conversion to EFI will probably be a hassle, but it's something you can look into as well.>>>

This carb was originally jetted for a 55 bhp 1100cc engine. Now it's on a 110 bhp engine. So looks to me the jetting that came with the car would not be correct. I seem to have the primary and secondary mains setup OK. The car makes plenty of power and doesn't ping even in the hottest weather. Runs quite well in hot weather.

Not to sound mean but I was hoping to get a few basic questions answered concerning idle jets. I'd like to at least know if I even have the correct size idle jet. But I can see how having an air leak could throw a monkey wrench into the system.

First of all like I mentioned the car runs like a champ when it's 95F outside. But the colder it get's (below 60F) the progressively worse it runs between cruise and slight acceleration in lower RPMs. The car starts bucking, sometimes backfires during this progression. And my air/fuel mixture goes full lean. Like it's running out of fuel. Now if I go from cruise to full throttle, it doesn't stumble or hestitate at all. Based on what I've read, the idle circuit not only controls the idle mixture but 80% of the carb's operation.

1) Would ANY carburarted car run terrible in cold weather? If not see next question.
2) Would a too lean primary idle jet show up worse in colder weather because the air is denser so the mixture would be leaner?
3) Is backfiring associated with a lean mixture?

Yesterday I played around with idle mixture screw to see how many turns it would take before the engine would no longer idle smooth.

I determined that anything below 3/4 turn causes the engine to stumble. Anything past 4 turns makes the car stumble. I orginally had the mixture set screw set at 1 turn. I increased it to 2 turns and drove the car this morning to work. It was 35F this morning. Still some slight bucking between cruise and slight acceleration but not as bad. Maybe because it was not as humid this morning. I don't know if enrichening the idle mixture would make the car run better or not.

It appears the engine idles smoothly between 3/4 and 4 turns. Since I can go as far as 1 or 4 turns without much effect, based on the link before, the range seems to be too wide compared to what it should be. The link below seems to indicate the mixture screw should not require more than 2 turns in any direction and I need to step up to a richer idle jet.



Also as far as the idle speed is concerned. As I mentioned the engine seems to run it's smoothest around 1100 rpms. However I am assuming my tach is accurate. I have an Equus tachometer in the dash and a Sunpro tuneup tach. At high RPMs both seem to be in agreement with each other but not at low RPMs. I seem to think 1100 rpms is actually 800 rpms. I'm going to get a VDO tach because VDO instruments seems to be more accruate.
124 Spider, Yugo,131
WhizzMan
Posts: 459
Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: Need help tuning 28/28 DFTA

Post by WhizzMan »

It's not just about the jet size, it's also about the air speed inside the carb. With such small venturi openings you will be pushing the air through at max speed way before you hit max power. I wouldn't be surprised if a larger venturi carb setup would instantaneously give you 10-20 BHP more. You are using a high pressure carb setup, so you need less venturi opening to push through a lot of air, but you need more fuel at essentially the same pressure difference between the inlet and outlet of the carb. It's all a balance and everything influences everything else in some form or way.

You seem to have a lot of knowledge or searching skills yourself already. Given the fact that the DFTA 28/28 is not the most common carb to be used on turbo engines, you probably won't find a lot of people in the know about your application, if any at all. I wouldn't be surprised if you already found several solutions, using other more common carbs like the IDF you mentioned yourself. Sometimes, it's easier to just do what everyone does, because it seems to work for them. True, sometimes you have to try something new to find out it's actually better, but the numbers on the 28/28 don't look promising for your application in my opinion. Then again, I'm just blabbing on a forum, if you find a way to make it run properly, you deserve a lot of credit. If you would be asking here what IDF with what chokes, what emulsion tube and what jets to use for your application, I wouldn't be surprised if more than one member would be able to give you a good starting point from their own experience.

The price of jets may sound high, but think of what you'd pay for having this car built by someone else. Compared to the price of building the engine, making it fit in the car and getting it set up on a rolling road, a box of plausible jet sizes isn't really significant. If you're lucky, you have a rolling road shop in your area that stocks all relevant jets and has experience in setting up carbs. It may cost you a few hundred, but the reward of a properly tuned engine may just be worth spending the money vs. tinkering on yourself?

I love what you've done so far to your "Lemon". Keep it up and post plenty of photo's of tech bits here. Folks here really enjoy craftsmanship like yours, so please share.
Book #348
timinator
Posts: 116
Joined: March 9th, 2011, 5:20 pm

Re: Need help tuning 28/28 DFTA

Post by timinator »

Hi turbofiat,

Any chance that your carb is just icing up. High enough venturi speed that the constriction is forming ice in the carb. I often see ice forming on the tunnel ram intakes of the big block boat engines that we tune on 100 deg. days at the Colorado river. Our 4 cylinder circle track engines with 350 cfm Holley carbs that we have blocked off the water to the intake manifold will also form ice on the outside of the carb body when held at 3000 rpm when breaking in a new cam.

You can add a heated spacer under the carb, either electric or water, to test if icing is the problem. Or maybe just hook up a heater blower motor behind the radiator with ducting to the carb. Going to a larger carb may also solve your problem. Good luck.

Tim
robert kenney
Posts: 161
Joined: July 11th, 2007, 2:23 am
Location: La Verne Calif, USA (A)
Contact:

Re: Need help tuning 28/28 DFTA

Post by robert kenney »

Some good points made already but here is my take on it.
Assuming for the time being that you have a rising rate fuel pressure regulator and at least 10 psi (15 psi perfered) of pump output pressure to the regulator. Your fuel pressure to the carb should track the manifold pressure when boosted MAP +4 psi at all times.

First we need to look at the regime at witch the engine begins to run poorly.
Runs great and makes good power under warm ambient conditions save the idle.
Tells me it is very close on mixture. cannot hurt add a tad .005-.010" more main jet for kicks and for a safety margin. once you get the bugs ironed out lean it back if needed Off boost the pilot circuits should behave similar to NA applications

Low temps and especially humidity is the key. High humidity will increase the density altitude while the low temps act to lower the AD. These will offset each other so it is not a big factor for fueling.

What is however effected by low temps and humidity is the ignition system. From the tendency of condensation in and on the cap causing flash over carbon tracking and the dielectric degradation of wires ETC allowing cross firing.

Your rich reading on the exhaust meter it due to the misfire and nothing more. Misfire = unburnt fuel in the exhaust and a rich reading.. Don't trust the readings until you fix the poor running and or miss fire.

With a turbo the intake air temperatures will be well above icing conditions. Not impossible but not likely.

I recommend checking or changing the cap. Start it up at night under the same conditions that it runs poorly under and in the darkest conditions you can find watch for a blue ghosting aurora on the wires and cap. A good indicator that the wires are bad and at a to high a resistance.
Verify the rotor to cap phasing if you have a modified distributor. The after market conversion kits are notorious for putting the rotor tip out of position from the cap tower during the firing point.
Robert Kenney # 111
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
Contact:

Re: Need help tuning 28/28 DFTA

Post by Guy Croft »

Turbofiat please do not post links without my permission.

Links deleted.

Respect the forum protocols please.

GC
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Re: Need help tuning 28/28 DFTA

Post by turbofiat »

Guy Croft wrote:Turbofiat please do not post links without my permission.

Links deleted.

Respect the forum protocols please.

GC
Sorry. Is it OK to post photos of my car from Photobucket?
124 Spider, Yugo,131
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Re: Need help tuning 28/28 DFTA

Post by turbofiat »

timinator wrote:Hi turbofiat,

Any chance that your carb is just icing up. High enough venturi speed that the constriction is forming ice in the carb. I often see ice forming on the tunnel ram intakes of the big block boat engines that we tune on 100 deg. days at the Colorado river. Our 4 cylinder circle track engines with 350 cfm Holley carbs that we have blocked off the water to the intake manifold will also form ice on the outside of the carb body when held at 3000 rpm when breaking in a new cam.

You can add a heated spacer under the carb, either electric or water, to test if icing is the problem. Or maybe just hook up a heater blower motor behind the radiator with ducting to the carb. Going to a larger carb may also solve your problem. Good luck.

Tim
Since it's been years since I actually messed with this carburator, I forgot what size it was. for some reason I was thinking it was a 28/28 DFTA but It's actually a 32/32 DFTA. There is no 28/28 DFTA. There was a 32/34 DFTA used on the US version of the Escort GT but only for a couple of years before they went to fuel injection.
124 Spider, Yugo,131
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Re: Need help tuning 28/28 DFTA

Post by turbofiat »

robert kenney wrote:Some good points made already but here is my take on it.
Assuming for the time being that you have a rising rate fuel pressure regulator and at least 10 psi (15 psi perfered) of pump output pressure to the regulator. Your fuel pressure to the carb should track the manifold pressure when boosted MAP +4 psi at all times.

First we need to look at the regime at witch the engine begins to run poorly.
Runs great and makes good power under warm ambient conditions save the idle.
Tells me it is very close on mixture. cannot hurt add a tad .005-.010" more main jet for kicks and for a safety margin. once you get the bugs ironed out lean it back if needed Off boost the pilot circuits should behave similar to NA applications

Low temps and especially humidity is the key. High humidity will increase the density altitude while the low temps act to lower the AD. These will offset each other so it is not a big factor for fueling.

What is however effected by low temps and humidity is the ignition system. From the tendency of condensation in and on the cap causing flash over carbon tracking and the dielectric degradation of wires ETC allowing cross firing.

Your rich reading on the exhaust meter it due to the misfire and nothing more. Misfire = unburnt fuel in the exhaust and a rich reading.. Don't trust the readings until you fix the poor running and or miss fire.

With a turbo the intake air temperatures will be well above icing conditions. Not impossible but not likely.

I recommend checking or changing the cap. Start it up at night under the same conditions that it runs poorly under and in the darkest conditions you can find watch for a blue ghosting aurora on the wires and cap. A good indicator that the wires are bad and at a to high a resistance.
Verify the rotor to cap phasing if you have a modified distributor. The after market conversion kits are notorious for putting the rotor tip out of position from the cap tower during the firing point.

I'm using a Malpassi 1:1 regulator from a Maserati Bi-turbo and Diesel pusher pump making 20 PSI dead head pressure. 3 PSI at idle.

Your mentioning of the ignition system makes me wonder. I have all new ignition components but it's been a while since I checked the plugs.

During the tuning process I glazed the plugs. Chunked them and went with a colder plug. Fouled them when I altered the advance to get rid of pinging. When to a hotter plug (actually the one Fiat recommends) and have been running those ever since.

When I fouled the plugs, the engine would idle really bad when cold but would smooth out at normal operating temperature.

I'm guessing I have maybe 6K miles on these plugs. Which maybe time to replace them. Or blast them if they are not glazed. Impossible to blast a diamond coating off a spark plug.

So the off idle misfire would be bad plugs. But how could high humidity and ambient temperature show up during cold weather? I could understand bad plug wires.

I'll replace the plugs and see what happens.
124 Spider, Yugo,131
turbofiat
Posts: 67
Joined: November 30th, 2006, 10:09 pm
Location: Kingsport, Tennessee, USA
Contact:

Re: Need help tuning 28/28 DFTA

Post by turbofiat »

One more thing or two:

During one of my Google searches, I found an X 1/9 forum and these guys say they are running 150 air correctors. I'm not sure if these are 28/30 carbs, 1300 or 1500 engines or what.

I'm running 250 air correctors. Not that I chose these for any particular reason. They just came with the carb. The 32/32 DFTA I am running is supposed to be an "upgrade" from the carbs 28/30 carbs installed on the 79 and 80 1500s.

Based something I read years ago, changing an air corrector will make the carb do different things are different RPMs. So would going to a smaller air corrector give me more fuel at lower RPMs? That seems to be what I am "feeling". Not enough fuel below 3000Ks. The thing is if I accelerate through the gears, the problem doesn't happen.

It just happens if I am tooling along below 3000 rpms for at 10 seconds and then try to accelerarte slightly. Going from cruise to full throttle is no problem.

Still wondering about the range of the mixture screw. If this is an indication the primary idle jet is too small. I'd like the rule that out first. I think the idle mixture screw should be sensitive enough to where 1/4 to 1/2 a turn would make a difference in how smooth the idle would be. Not 3 turns like what I am seeing.

Once had an issue with my 124 Spider where it wouldn't track right under braking or acceleration. The alignment shop told me if was in the brakes and the steering was spot on. I questioned if that was the case then why does the car do the same thing under acceleration? He still said it was in the brakes. I changed everything on the braking system to no avail.

Took it to another alignment shop and they diagnoised it as a bad ball joint. Problem fixed.
124 Spider, Yugo,131
robert kenney
Posts: 161
Joined: July 11th, 2007, 2:23 am
Location: La Verne Calif, USA (A)
Contact:

Re: Need help tuning 28/28 DFTA

Post by robert kenney »

turbofiat wrote:Based something I read years ago, changing an air corrector will make the carb do different things are different RPMs. So would going to a smaller air corrector give me more fuel at lower RPMs? That seems to be what I am "feeling". Not enough fuel below 3000Ks. The thing is if I accelerate through the gears, the problem doesn't happen.
Main air correctors actually have the opposite counter intuitive effect to what may seen logical at the low end or progression end. The corrector will help induce the emulsion formation of the main circuit. Bigger AC = earlier emulsion formation and main jet fuel delivery along with that is a better progression smoothness. Smaller AC = later emulsion and opposite main circuit effect ETC.

Along with the AC increase you should realize that it will lean the main circuit at the high rpm end. I would go up .005 on main jet with a .01 increase in AC.

I doubt plugs are the cause but they are cheap and if they are dirty/glazed a change is in order.

I can't hear the misfire but from a symptom perspective, an erratic misfire that comes and goes quickly is usually ignition related while a misfire that rolls on synchronous with throttle tip in is fuel related. These are base diagnostic theory and need to be thoughtfully applied.
Robert Kenney # 111
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK
Contact:

Re: Need help tuning 28/28 DFTA

Post by Guy Croft »

"Is it OK to post photos of my car from Photobucket?"

Thank you for asking but no it's not - use the site hosting service please - may I respectfully suggest that you study the Protocols again? This is stated there.

http://guy-croft.com/viewtopic.php?f=4&t=1194

Thank you,

GC
Post Reply

Who is online

Users browsing this forum: No registered users and 28 guests