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Crossflow Problem (breaks down at 3500 rpm)

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Hi,

       I am looking for some ideas to help fix an alloy head 250 crossflow that breaks down at 3500 to 4000 rpm. Beautiful till that point.  This is an on going problem and have thrown a lot of money and parts at it with no success. I will give a brief description of engine mods and what has been tried so far.  Maybe someone knows what I have missed.

 

Alloy C2 head , Ported with large valves fitted. Hydraulic Camtech cam (CT324 528) intake lift .534" ex lift.534"  lobe sep 110 , Dur 230 and 230 ( cam dialled in ) , Johnson anti pump lifters and hardened  push rods , roller rockers and heavy valve springs.   Aussie speed 4bbl intake , Holley Carb , Electric fuel pump from 5-8 psi , 10.5:1 comp ratio , balanced bottom end. Ignition is Bosch electronic dizzy module removed and running off MSD 6al digital box and MSD Coil.  Dizzy was graphed to suit engine. Vaccume advance locked off.

 

Has had the breaking down issue since engine was built. originally had high volume oil pump , been swapped out for standard ( no change) , tried various Carbs from 465 to 650 DP Holley no change. tried TFI dizzy no change , tried various leads , plugs , coils even swapped 6al no change , have had fuel pressure all the way as high as 8 psi no good. Have changed valve springs retainers and collets. Started with valve seat pressures on the conservative side and have now increased probably a little on the heavy side and no change. ( my son is an engine machinist he did this so , don't know seat pressures off the top of my head )

 

Engine is in a TE Cortina. I have run out of ideas and will probably swap the lifters out for a new set and only other thing I haven't paid much attention to is plug gaps just set them to whatever MSD recommended. I have probably missed some things I have tried , but if anyone has some ideas I am listening.

 

 

Cheers

 

 

 

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is the fuel line up to the task in the cortina? kinked hose/pipe/pick up screen etc.

 

fuel flow, not necessarily pressure

 

done a fuel flow test? how many Litres per minute comes from the fuel pump?

 

just a few ideas outside of the square i can think to check

 

otherwise i'd be sus on ignition still


 

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Hi,

       has an alloy drop Tank fitted and a new 3/8 fuel line all the way through. I will check the fuel outlet and see. I don't expect to find anything but every idea is worth a look. It has to be something simple. Whats a simple fuel flow test , just run it into a 10 liter bucket and time it I guess. Do you have a recommended liters per minute in mind ? I keep going back to the ignition because that's where I think it is but no amount of swapping components has helped. Thankyou for you ideas.

 

cheers

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Mate this one's a deusie...

As I was reading each thing you tried, I would think of another thing and you'd already tried that too.

You say "breaking down" as if it feels like coil breakdown.

Maybe instead of continuing throwing nice expensive parts at it, you could just book a bit of dyno time with someone who knows carbies. You can load up the engine and watch what the ignition is doing on an oscilloscope and/or timing light, also AFRs will tell how the carby is tuned. Often, carby problems are ignition problems.

Being a mechanic myself, am no fan of paying someone else to spin spanners on my old jalopies, but sometimes you just need someone with the right gear to get to the heart of the matter.

How have you got the MSD hooked up? They want a direct supply to and from the battery, not from your key switch. At peak output, they do draw quite a few amps. Plug gaps aren't anywhere near as fussy with MSD. It generally fires them no matter how fouled or worn they are, so you can pretty much rule that issue out.

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Yeah timing is correct , Didn't trust markings on new balancer so was checked using TDC tool.   MSD is Running 12 volts direct from battery. It is running through a relay triggered by the original Cortina ignition wire.  Fuel filter is Glass reusable type and is clean. I am not sure about the premium fuel lost track of everything I have tried but definitely fresh fuel. MSD set up I am using was brought second hand but as new condition , I swapped it for MSD out of My Falcon and made no difference. The MSD had previousl been used on a 4cyl and so the cylinder calibration loops were cut. I rejoined one of the loops to set it to 6cyl. I might try cutting that loop and joining the other loop instead. I don't know if this could make a difference. when it breaks down it is similar to a rev limiter cutting in. It runs sweet as till then not a miss nothing.

 

Cheers

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1 hour ago, winton said:

Hang on, was it a points dissy originally? Then yeah not 12v

its' triggering a relay though, then 12V direct to the MSD, good thought though, it would have been resistor wire originally as to drop the coil voltage with points.

 

i'm not up with what fuel flow is needed for a hot crossflow at full noise,  but as gerg said above, stick it on a dyno you'll be able to monitor O2 readings if its losing fuel, or probably be also able to monitor spark disappearing as it has to be one or the other.

 

other than lifter preload/valve lash.. could be holding valves open at certain revs? no idea tbh, just another thought to consider

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If you are talking about the wire I think you are its not a 12v wire.


If it's only triggering a relay, then it will be as good as 12 volts. Being a resistance wire in series with another resistor (the coil), the voltage drop at the coil + is dependent on the resistance of the coil itself. A resistor resists current, not voltage. It results in going from about 8 amps of draw from a coil to about 20mA of a relay. This gives the resistance wire bugger all current to resist, so voltage barely changes.

its' triggering a relay though, then 12V direct to the MSD, good thought though, it would have been resistor wire originally as to drop the coil voltage with points.
 
i'm not up with what fuel flow is needed for a hot crossflow at full noise,  but as gerg said above, stick it on a dyno you'll be able to monitor O2 readings if its losing fuel, or probably be also able to monitor spark disappearing as it has to be one or the other.
 
other than lifter preload/valve lash.. could be holding valves open at certain revs? no idea tbh, just another thought to consider




The MSD had previousl been used on a 4cyl and so the cylinder calibration loops were cut. I rejoined one of the loops to set it to 6cyl. I might try cutting that loop and joining the other loop instead. I don't know if this could make a difference. when it breaks down it is similar to a rev limiter cutting in. It runs sweet as till then not a miss nothing.
 
Cheers


You might be onto something there... Not sure how these calibration loops work as I run a programmable MSD but if it thinks it's a 4 cyl, and you have a 6000 limit, that makes it a 4500 limit on a 6.

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Hi,

       been playing around and no luck yet. I am going to play with plugs , I ran the engine to running temp and put an infra red heat gun on each extractor pipe and discovered a big difference in temp on 3 and 4 they were much hotter. 3 especialy about 70 deg c higher than 1 2 5 and 6. I  put a picture on here of the plugs to show number 3 is clearly very grey/whit coloured and 4 is not quite as bad but all the rest are black. Would a lean condition cause it to break down at just 3500 or is this a separate problem. I will check for a leak but has just in past couple days had new intake and carb gaskets fitted. I might try changing primary jets in carb up a few sizes. would this be how these intake run ? these two lean cyl are the 2 shortest runners on the intake. Its an Aussie speed intake.

DSC02062.JPG.a87b6b7afa86967f2073bc6a86630c0e.JPG

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Hey, I forgot to mention I did a fuel flow test.Carter rotary Pump filled a 4 liter container in 1 minute 12 seconds. Had the container above carby. I have no idea if that's good or bad. That pump runs at 5 psi at all revs. I have had a holley blue dummied up to test the fuel pressure theory and it made no difference.

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Sounds remarkably like the engine someone messaged my business page about the other day.

To answer some questions;

. yes 3&4 will run hotter (leaner) than the other cylinders due to their location.  We have played with the plenum to address this issue and had some success. 

. As you've tried everything from the ignition side of things and most other mechanical things I'd be looking at tune, especially looking at those plugs. As I told the bloke that messaged me - get a $200 o2 sensor and gauge from Just Race Parts and then you'll have half a chance of getting the tune close.  

. If the carb is east/wested it's not uncommon to have to pull fuel out of the primaries and a good amount, as your miss is occurring early on I'd say your lucky to be cracking the secondaries so it may well be that.  But without an o2 sensor - your in guess land. Oh and usually the fuel you take out of the primaries ends up going back into the secondaries. 

. I agree with gerg- the best money you ever will spend on your car is on a dyno session with a competent operator who knows carbs. 

. You mention that your running the Hall effect dizzy (which is good). Have you tried another dizzy?  I have had Hall effect sensor go walk about and cause this condition. 

. Has coil condition been checked? Not "it's brand new" - has primary and secondary resistance been confirmed as in spec. Both when the coil is cold and after being driven.  Even new stuff fails - especially in the made in China era. 

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Hi mate,

                  The car belongs to my son and I reckon he was talkin to you as you said. I have a new O2 sensor kit that I was going to set up on my Falcon to tune tunnel ram carbs , We intend to weld a bung onto Cortina extractors and give it a go.  I have had a breakthrough with the crossflow. I decided to play around with the carb tune and change jet sizes. I went a full 5 sizes bigger on primaries to see if I could make something change. It did exactly that it made it much worse , it caused it to breakdown at 2200.  Next I went back a heap of jet sizes on primaries from where I started and it improved the thing a whole heap. It is twice as responsive down low and revs straight past 4000 without a problem. it does still however play up a little higher up. Next I will fit a decent Tacho as Not sure how accurate Cortina one is , O2 sensor , and get some more jets to play with.  thanks for all the good advice everyone.

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Sounds like it might need some air bleed tuning. If jetting alone can't get the right AFRs throughout the fuel curve, then something else is out. Sounds like there isn't enough emulsion air to correct the mixture at high flow (high rpm).

 

Does the carby have replaceable bleeds?

 

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Hi,

       My carb tuning experience is limited to mixture screws and power valve replacement using vaccume gauge . I have a couple pump cam kits that I brought to experiment with my tunnel ram carbs. I don't know much about the air bleeds. But I like learning so I will check and see if they are replaceable or just a set thing. I feel confident that I am on the right path now so I will get it as best I can and then it will go to a dyno.

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Has a screw slot = replaceable

No screw slot = not replaceable

Although some old school tinkerers will get the pressed-in ones out with a pen torch and a seal pick, etc.

Air bleeds are the equal and opposite to jetting, but going larger affects the fuel curve specifically higher in the rpm.

They are also referred to as emulsion bleeds. They feed the emulsion circuit and determine how much air is bubbled through the fuel (to break up the droplets) before it exits the booster. The harder the boosters suck, the more air will be introduced in relation to fuel. This is due to fuel being a much thicker fluid than air, so will become harder to pull through the jet as flow increases. Go introducing more air, and the amount of fuel decreases.

In short, going with smaller bleeds richens up the top end, going bigger leans it out.

Going bigger also delays the main circuit slightly, which might need some pump tuning or float adjustment to cover up any flat spots.

I'm in the middle of playing with air bleeds right now. So far with number drills, I've punched mine out from 0.028 to 0.040 (a huge jump) and the results are promising. Did the idle bleeds as well, out to 0.043 and the result of that has been good also. My carby is a Street Demon.

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