Crossflow poor low speed running
Discussion
I am at my wits end with my 200 mile old crossflow.
It will not pull smoothly on light throttle at all, is very snatchey on the over run and there is a big hole for a second on opening the throttle. Above 2500 with moderate throttle it pulls ok but seems a little flat.
It is a 1640 with ported mexico head, Piper 285 cam, L+B etc. The carb is a 32 dgv. It uses the std breather system.
I have tried timing swings, different jets larger and smaller different pump jets. And it fundamentally stays the same. advancing the timing makes it pull better but makes the roughness worse on light throttle.
Am I expecting too much from a 285, previous engines have always used bcf3's and have been fine. Or can some one offer some suggestions of places to look please I an all out of ideas.
It will not pull smoothly on light throttle at all, is very snatchey on the over run and there is a big hole for a second on opening the throttle. Above 2500 with moderate throttle it pulls ok but seems a little flat.
It is a 1640 with ported mexico head, Piper 285 cam, L+B etc. The carb is a 32 dgv. It uses the std breather system.
I have tried timing swings, different jets larger and smaller different pump jets. And it fundamentally stays the same. advancing the timing makes it pull better but makes the roughness worse on light throttle.
Am I expecting too much from a 285, previous engines have always used bcf3's and have been fine. Or can some one offer some suggestions of places to look please I an all out of ideas.
waxaholic said:
getting the car set upon a rolling road is the way forward
Certainly agree with that,but dont expect much low end from the 285 cam thats quite extreme for the road,well it is if you want good low speed manners 
it certainly not made for overhaul tractability like say the 270,as the cam been timed up properly with an adjustable vernier? cam timing as with most engines is important but in my opinion is critical on the X Flow.Edited by That Daddy on Sunday 7th December 16:02
The accelerator pump diaphram does not affect slow speed running(constant throttle)only inital pick up when you stab the gas,it can cause flat spots on pick up,that does not sound like the OP problem though but agree it could be an air leak or just as likely a main jet or air correction problem on the primary choke.
but agree it could be an air leak or just as likely a main jet or air correction problem on the primary choke.Using a long duration cam with a single carb will always cause problems like this, due to exhaust gases getting into the inlet manifold.
With the cam you've got, (292 deg ?) there will be some dilution of the of the inlet charge by exhaust gasses, this is not that much of a problem with one choke per cylinder, (you may just get the odd pop) but when using a common plenum, each cylinder will interfere with other cylinders causing the problem you have.
I put a Pinto 2.1 with a 292 deg cam into a Tiger kit car, and as there was already a hole cut in the bonnet for a dgas carb, tried to get it to run on the dgas.
Although it produced excellent power on full throttle, I could never get it to run right on small throttle openings, although I did find increasing the idle jet sizes by 10 helped to mask the problem a little.
I gave up and put a pair of 40s on it and it then ran superbly.
With the cam you've got, (292 deg ?) there will be some dilution of the of the inlet charge by exhaust gasses, this is not that much of a problem with one choke per cylinder, (you may just get the odd pop) but when using a common plenum, each cylinder will interfere with other cylinders causing the problem you have.
I put a Pinto 2.1 with a 292 deg cam into a Tiger kit car, and as there was already a hole cut in the bonnet for a dgas carb, tried to get it to run on the dgas.
Although it produced excellent power on full throttle, I could never get it to run right on small throttle openings, although I did find increasing the idle jet sizes by 10 helped to mask the problem a little.
I gave up and put a pair of 40s on it and it then ran superbly.
oakdale said:
Using a long duration cam with a single carb will always cause problems like this, due to exhaust gases getting into the inlet manifold.
With the cam you've got, (292 deg ?) there will be some dilution of the of the inlet charge by exhaust gasses, this is not that much of a problem with one choke per cylinder, (you may just get the odd pop) but when using a common plenum, each cylinder will interfere with other cylinders causing the problem you have.
I put a Pinto 2.1 with a 292 deg cam into a Tiger kit car, and as there was already a hole cut in the bonnet for a dgas carb, tried to get it to run on the dgas.
Although it produced excellent power on full throttle, I could never get it to run right on small throttle openings, although I did find increasing the idle jet sizes by 10 helped to mask the problem a little.
I gave up and put a pair of 40s on it and it then ran superbly.
Just re-read your post studog, and to add to above, my Pinto also had a big hole on applying full throttle. (if you applied throttle slowly, it wasn't too bad).With the cam you've got, (292 deg ?) there will be some dilution of the of the inlet charge by exhaust gasses, this is not that much of a problem with one choke per cylinder, (you may just get the odd pop) but when using a common plenum, each cylinder will interfere with other cylinders causing the problem you have.
I put a Pinto 2.1 with a 292 deg cam into a Tiger kit car, and as there was already a hole cut in the bonnet for a dgas carb, tried to get it to run on the dgas.
Although it produced excellent power on full throttle, I could never get it to run right on small throttle openings, although I did find increasing the idle jet sizes by 10 helped to mask the problem a little.
I gave up and put a pair of 40s on it and it then ran superbly.
Edited by oakdale on Sunday 7th December 20:26
Echo Thatdaddy and Oakdale... don't expect a Crossflow with a hot cam and single carb to behave nicely, 'cos it won't.
Properly jetted twin Weber DCOE 40's will work better (it still won't have a hell of a lot of torque until it comes on cam, but it will at least be smooth and tractable) and fuel injection/mapped ignition will be even better (albeit very expensive), but you're onto a bit of a loser with a single carb, to be honest.
750 Club kit car championship used to have a class that was limited to sngle carbs, and most of the crossflows that ran in that were pretty ill-mannered things at low revs.
Properly jetted twin Weber DCOE 40's will work better (it still won't have a hell of a lot of torque until it comes on cam, but it will at least be smooth and tractable) and fuel injection/mapped ignition will be even better (albeit very expensive), but you're onto a bit of a loser with a single carb, to be honest.
750 Club kit car championship used to have a class that was limited to sngle carbs, and most of the crossflows that ran in that were pretty ill-mannered things at low revs.
Edited by Sam_68 on Monday 8th December 12:26
toohuge said:
Ive got a hot cam on a 1700 x-flow with twin 40's, it's not that smooth at low revs.
You'll get a lumpy idle, for sure, but if the carbs are set up correctly it should pull smoothly, if not strongly through the lower rev range, until it comes on cam.If it's not jetted correctly, you may well end up with stuttering and flat spots so bad that you've got to slip the clutch to let the engine pull into the power band.
...and my experience of single carb race Crossflows is that the fluffing and stuttering at low revs can make them pretty much undriveable in traffic on public roads.
Air leak!
It now runs fine after draw-filing the baseplate of the carb totally flat. Resealing the mainfold faces and reinstalling water heating to the manifold
Oddly the idle is now better than my previous engine that ran lesser cam (BCF3)
I'm not covinced the cam timing is spot on yet. But it is now perfectly driveable
and pulls well from around 2500 upwards, even if it is a little dull below that.
Some fine tuning on the rollers and it looks like it will be a good one
Whilst I agree that a high spec cam with low spec carbs will be more of a challenge this cam was originally sold as an upgrade for the escort sport with no other changes so I was convinced there was something else. Just as it turned out.
Sam what cam are you using in your "race spec" Sylva?
It now runs fine after draw-filing the baseplate of the carb totally flat. Resealing the mainfold faces and reinstalling water heating to the manifold
Oddly the idle is now better than my previous engine that ran lesser cam (BCF3)
I'm not covinced the cam timing is spot on yet. But it is now perfectly driveable
and pulls well from around 2500 upwards, even if it is a little dull below that.
Some fine tuning on the rollers and it looks like it will be a good one
Whilst I agree that a high spec cam with low spec carbs will be more of a challenge this cam was originally sold as an upgrade for the escort sport with no other changes so I was convinced there was something else. Just as it turned out.
Sam what cam are you using in your "race spec" Sylva?
studog said:
Sam what cam are you using in your "race spec" Sylva?
Kent 254. It's a reasonably mild 'race spec' (about 245-255 bhp at a guess - the rolling road I used to use - Dave Bogg's - over-reads dramatically; it was climbing off the rollers with 2 of us sat on the back and 255bhp showing on the dial, but still climbing fast, when we set it up!). The rev limiter is set to 7,800rpm.It was built for the single carb class of the 750 MC championship and was fitted with a single Weber when I bought it, but tended to fluff if you asked it to pull below 2,500rpm and didn't start to come on-cam properly until nearly 4,000rpm.
It's much more tractable now, on twin 40's, though I'm probably giving away a little top-end power by not using 45's. It still won't pull the skin off a rice pudding below 3,500rpm, though!
I was thinking the same myself 
Sorry, guys - mis-type; that'll teach me to try and touch-type!
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