22 replies to this topic

[bwi_bwi_12]

So there seems to be something strange happening with my engine. Idle is rough and sounds as if it isnt running on all four cylinders. This is confirmed by removing HT leads from cyl 1 and 4 at idle. When 1 is removed, there is no change. When 2 is removed, the engine starts to run worse. The effect is the same on number 3. when removing number 4, the effect is the same as when removing number 1. In fact, at idle i can remove both 1 and 4! The idle is the same, but with all four leads attached the engine revs nicely where as with 1 and 4 removed it is more reluctant.

The problem now is that the outer two cylinders seem to be running lean. This is evident when looking at the colour of the plugs. 1 & 4 are light mustard colour while inner two are dark brown. What i am beginning to suspect is that the outer two cylinders are running leaner, so if I richen the idle mixture the inner two cylinders start running too rich. Either excess fuel is coming out the exhaust from the inner cylinders because they are too rich, or the outer two are too lean to combust and the fuel ends up coming straight out the back. It is smoking at idle but perhaps this is because the engine still needs to be run in.

The whole skewed mixture vibe (as im beginning to suspect) may be from using a 4.5" manifold. Who can shed light on this for me?? Please!
ive tried putting the original points back in (was suspecting ignitor kit to be at fault) but to no effect. I am also confident that my cam timing is correct (dot to dot) and the valve clearances are set properly.

Brief engine details:
piper 270 cam, 10.3:1 compression, timing at 10' using strobe light, firing order is all correct too.

Weber details:
DCOE 40
needle + seat 2.25mm
float levels
vertical 8.5mm
horizontal 15mm
choke tubes 32mm
auxillary venturi 4.5mm with inserts for increased air speed
pump jets 40
idle jets 50f8
main jets 1-38
emulsion tubes f16
air jets 1-80

please see my previous thread for more details

http://www.theminifo...robbing__st__15

[The Matt]

Valve clearances are OK and adjusted to suit the cam?

[Sam Walters]

Certainly sounds fuel related to me.

The inner cylinders will always run richer. Try richening it up a little bit.

I dont know all too much about webers, but it may not be atomising the fuel correctly.

[Sam Walters]

Certainly sounds fuel related to me.

The inner cylinders will always run richer. Try richening it up a little bit.

I dont know all too much about webers, but it may not be atomising the fuel correctly. Blocked Jet?

[The Matt]

I'm copying this from a book I have "how to build and tune weber and dellorto carbs...."

A-series, 1293, 11:1 CR, 648 camshaft, ported head, LCB, high lift rockers and 45 DCOE (slightly higher spec than yours I guess):
38mm chokes
4.5 aux. venturis
F2 emulsion tubes
165 main jets
170 air correctors
45 accel pump jets
45 F9 idle jet
50 accel pump inlet
7.5mm float level shut off height
15mm full dropp float setting
idle screws at 7/8 turn out.

Dizzy electronic, full advance at 35deg, idle speed advance at 18 degrees (1500 rpm idle speed)
Engine has lumpy idle at 1400rpm, but smooths out above 2500......

[The Matt]

I only put that up for reference, the 270 cam is milder than the 648, but the advance of 10deg seems a little retarded to me (from memory, I had my advance with my 1380, 11.5:1CR, MJ, 286 ang 1.5:1 rockers) at more like 14 degrees. BUT, that idled at more like 1350-1400 I guess.

The above values were on a 45. But the reason I posted up that paragraph from the book is to compare what I used as it's the book that I used to set mine up.

[The Matt]

and yeah, on a shorter manifold the fuel is more prone to enter the inner two than the outer two cylinders. But you can normally fine tune that out enough to get it running better than that, even if it's never quite 100% balanced.

I'd say it needs a little more advance and a little more fuel personally. But I'm no engine tuner by any means!

[bwi_bwi_12]

Valve clearances are OK and adjusted to suit the cam?

I have adjusted them to 0.30mm. Maybe i should try a bit looser, but as far as i can tell they are definitely rocking on the compression stroke!

[The Matt]

if all cylinders are set the same, then that shouldn't be the cause of the imbalance I guess

[The Matt]

basic one, what is your idle speed? Not sure on the spec. of the 648, but I'd assume it's gonna have a higher idle speed than sorta 276 cams?

[bwi_bwi_12]

and yeah, on a shorter manifold the fuel is more prone to enter the inner two than the outer two cylinders. But you can normally fine tune that out enough to get it running better than that, even if it's never quite 100% balanced.

I'd say it needs a little more advance and a little more fuel personally. But I'm no engine tuner by any means!

okay im going to try a bit more idle advance and set the idle to 1400. does a 4.5" manifold sound too short? Also, what sort of thing needs tuning on the carb? bigger idle jets? it has all been professionally rebuilt so there are no blockages...

[bwi_bwi_12]

idle is at around 1100. when i drive it, she pulls really nicely at higher revs but hesitates slightly at lower revs

[The Matt]

You've balanced them too, yeah? Then again, your imbalance is inner versus outer, not 1+2 versus 3+4...

[sledgehammer]

Check compression's

try a (known to be good) spare manifold & SU carb , & get engine to warm up & settle down

check manifold air leak on outer lug areas ? / manifold gasket fault

disconnect & blank off servo connection and any other take offs

I would be surprised if the mixture varied so much as 2 cyl's (in different ports) wouldn't run , what is it like on full choke ?

Edited by sledgehammer, 07 February 2012 - 09:03 PM.

[bwi_bwi_12]

compression seems fine. cyl 1: 1050 kPa, cyl 2: 1200 kPa, cyl 3: 1300 kPa, cyl 4: 1050 kPa on an engine which hasnt been run in properly yet...
my previous SU ran like crap so i cant use that one... not sure what it is like on full choke.