Well my car was registered on xmas eve and has been going great. Oil pressure is still a little on the low side (can drop to 10 PSI at idle, but jumps up as soon as you touch the throttle).
I have a fairly aggresive cam (35647) and the head flows around 265 HP. Triple Webers, Roller rockers, Counterbalanced crank by crankys - all the good stuff. Up until tonight the engine has made good power right through to 7000 (occasionally - but normally not past 6500) but now is misfiring at around 5500. Anything below this is fine. I hadn't changed anything on the engine.
I've had a running on the rich side until it gets on a dyno (plugs are sooty with a white tip) and fuel pressure shouldn't be a problem (carter 7psi pump)
Dizzy is Electronic from a commodore rebuilt and regraphed by Scorcher.
Like I said, I have changed nothing - just can't figure out what it is...
Any tips would be great.
High RPM misfire
Started by
_PeteXU1_
, Jan 26 2011 09:29 PM
24 replies to this topic
#3
_CraigA_
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Posted 26 January 2011 - 09:42 PM
Everyone including me is going to throw guesses at you but best bet is trip to a dyno.
Three areas to check - electrical, mechanical, fuel.
My guess - check leads are seated both ends and are known good and visual inspect plug insulator for carbon tracking to ground. Check dizzy end is seated and clean, rotor and carbon contact, cap for cracks and tracking. I have a magnifying glass for this sort of stuff and review under a bright light.
Ohm check leads is easy to do but not always a true test as they need to be checked under load. Spark will chase path of least resistance to ground so a dud plug will also cause issues as cylinder pressures rise.
Good luck :-)
Three areas to check - electrical, mechanical, fuel.
My guess - check leads are seated both ends and are known good and visual inspect plug insulator for carbon tracking to ground. Check dizzy end is seated and clean, rotor and carbon contact, cap for cracks and tracking. I have a magnifying glass for this sort of stuff and review under a bright light.
Ohm check leads is easy to do but not always a true test as they need to be checked under load. Spark will chase path of least resistance to ground so a dud plug will also cause issues as cylinder pressures rise.
Good luck :-)
#4
debkar
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Posted 26 January 2011 - 09:51 PM
Well my car was registered on xmas eve and has been going great. Oil pressure is still a little on the low side (can drop to 10 PSI at idle, but jumps up as soon as you touch the throttle).
I have a fairly aggresive cam (35647) and the head flows around 265 HP. Triple Webers, Roller rockers, Counterbalanced crank by crankys - all the good stuff. Up until tonight the engine has made good power right through to 7000 (occasionally - but normally not past 6500) but now is misfiring at around 5500. Anything below this is fine. I hadn't changed anything on the engine.
I've had a running on the rich side until it gets on a dyno (plugs are sooty with a white tip) and fuel pressure shouldn't be a problem (carter 7psi pump)
Dizzy is Electronic from a commodore rebuilt and regraphed by Scorcher.
Like I said, I have changed nothing - just can't figure out what it is...
Any tips would be great.
Hi Dan,
Good news on the rego. Just wanted to ask, do you run a fuel pressure regulator?
I wasn't sure if Webers are as sensitive to fuel pressure as Dellortos?
I always run a regulator with Dellortos set at 2.5 psi,
Not too sure about your mis fire, but one car I had with first set of Dellortos used to do the same until I fitted pressure regulator, but it always did it. Sounds like you had decent rev range that has now changed,
Regards Simon
#5
rodomo
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Posted 26 January 2011 - 10:14 PM
I'd be rechecking the pump pressure, leads, especially the coil lead, and maybe even broken valve springs.I've had a running on the rich side until it gets on a dyno (plugs are sooty with a white tip) and fuel pressure shouldn't be a problem (carter 7psi pump)
#6
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Posted 26 January 2011 - 11:41 PM
maybe fould a plug try new plugs
^^^ +1
What spark plugs are you running and what gap have you set them at?
#7
_PeteXU1_
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Posted 27 January 2011 - 12:18 PM
In answer to a few of the questions;
Fuel reg is fitted and running at 3.5 psi
All the leads, coil, distributor are new.
Plugs are BP6ES
They didn't look fouled when I took a couple out, but when I rechecked them all this morning, a couple were a bit sooty.. One of the gaps had also closed up to less than .4mm
I suspect I didn't check it properly when I put them in a week ago. It was fine when new, but obviously this is the one that has fouled up...
Anyway, I regapped them all to 1mm, gave them a good clean, and tightened the little end screw things on them (one was almost off) and she seem to be going fine again.
I guess I better get used to fouling plugs if I drive it too much in traffic...
Fuel reg is fitted and running at 3.5 psi
All the leads, coil, distributor are new.
Plugs are BP6ES
They didn't look fouled when I took a couple out, but when I rechecked them all this morning, a couple were a bit sooty.. One of the gaps had also closed up to less than .4mm
I suspect I didn't check it properly when I put them in a week ago. It was fine when new, but obviously this is the one that has fouled up...
Anyway, I regapped them all to 1mm, gave them a good clean, and tightened the little end screw things on them (one was almost off) and she seem to be going fine again.
I guess I better get used to fouling plugs if I drive it too much in traffic...
#8
_Drag lc_
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Posted 27 January 2011 - 07:07 PM
moght be time invest in a proper ignition unit?.
#9
_Quagmire_
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Posted 27 January 2011 - 08:16 PM
moght be time invest in a proper ignition unit?.
can i read between the lines and suggest a msd or ice ignition?
or is that going to far?
#11
_torbirdie_
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Posted 27 January 2011 - 08:40 PM
Does the motor always missfire at 5500rpm, even with no load on it.
If its only under load, then it points to a spark leak. Run the engine in the dark, and have a good look around the dizzy cap, plug ends, coil and spark leads for any leaking spark, it should identify any pin prick defects in spark leads with small tracer trails going to the defects.
.
If its regardless of load its something mechanical in the dizzy causing sparkus interuptus.
I did read somewhere(perhaps on this forum) that using a steel dizzy drive in a bosch electrical engine can get harmful resonances at higher rpm and cause missfires(not that I can understand how this eventuates) and that the problem is alleviated by using the original plastic type. which gear are you running?
If its only under load, then it points to a spark leak. Run the engine in the dark, and have a good look around the dizzy cap, plug ends, coil and spark leads for any leaking spark, it should identify any pin prick defects in spark leads with small tracer trails going to the defects.
.
If its regardless of load its something mechanical in the dizzy causing sparkus interuptus.
I did read somewhere(perhaps on this forum) that using a steel dizzy drive in a bosch electrical engine can get harmful resonances at higher rpm and cause missfires(not that I can understand how this eventuates) and that the problem is alleviated by using the original plastic type. which gear are you running?
#12
rodomo
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Posted 27 January 2011 - 09:41 PM
Anyway, I regapped them all to 1mm, gave them a good clean, and tightened the little end screw things on them (one was almost off) and she seem to be going fine again.
#13
_gtr-xu1_
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Posted 27 January 2011 - 10:07 PM
my bike misfires under load and i pull plugs they look fine then i put in new ones and it runs perfect. its got a BIG drag race carb and eats plugs for breakfast.
#14
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Posted 28 January 2011 - 06:58 PM
Are you running the steel gear on the dizzy? If so get a plastic one as the steel gear can cause a chattering at high RPM which can cause a missfire.
#15
_PeteXU1_
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Posted 28 January 2011 - 08:27 PM
Since I regapped all the plugs and gave them a good clean, the misfire has totally gone and she is revving again beautifully.
Would one plug with a really small gap, coupled with a rich fuel mixture, be enough to cause the misfire?
I currently run a commodore HEI ignition with a Bosch coil pack to suit. Rebuilt, vacuum removed and regraphed.
Can anyone explain to how much of an improvement an MSD or ICE system is going to deliver? It's the one area I haven't really delved into yet. I understand the bigger spark concept, and more reliable timing, but will there be a power gain?
Would one plug with a really small gap, coupled with a rich fuel mixture, be enough to cause the misfire?
I currently run a commodore HEI ignition with a Bosch coil pack to suit. Rebuilt, vacuum removed and regraphed.
Can anyone explain to how much of an improvement an MSD or ICE system is going to deliver? It's the one area I haven't really delved into yet. I understand the bigger spark concept, and more reliable timing, but will there be a power gain?
#16
_Bomber Watson_
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Posted 28 January 2011 - 08:37 PM
Probably not a huge power gain as such, although if it solves a missfire then yes there will be!!
Its just better, will start better, run better etc.
Price some MSD units up from www.summitracing.com
So cheap from the US.
Cheers.
Its just better, will start better, run better etc.
Price some MSD units up from www.summitracing.com
So cheap from the US.
Cheers.
#17
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Posted 28 January 2011 - 08:41 PM
Are you running the steel gear on the dizzy? If so get a plastic one as the steel gear can cause a chattering at high RPM which can cause a missfire.
#18
rodomo
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Posted 28 January 2011 - 10:56 PM
A theory that might of originated here:
Are you running the steel gear on the dizzy? If so get a plastic one as the steel gear can cause a chattering at high RPM which can cause a missfire.
http://www.gmh-torana.com.au/forums/topic/17346-msd-for-holden-6/
#19
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Posted 28 January 2011 - 11:27 PM
A theory that might of originated here:
Are you running the steel gear on the dizzy? If so get a plastic one as the steel gear can cause a chattering at high RPM which can cause a missfire.
http://www.gmh-toran...d-for-holden-6/
Thanks for that Rob.
I was just a little bewildered why anyone would put the crappy nylon drive gear in a high revving engine.
I'll have to have a look through my Dealer service letters but I'm sure I saw one about problems with the nylon gears stripping or adversely wearing the camshaft gear teeth or something like that.
Edited by S pack, 28 January 2011 - 11:28 PM.
#20
rodomo
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Posted 28 January 2011 - 11:39 PM
Plus with an alloy timing gear I reckon it would be worse. A lot of velocity fluctuations between all the shafts involved.
Edited by rodomo, 28 January 2011 - 11:40 PM.
#21
Heath
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Posted 29 January 2011 - 10:29 AM
The nylon one wearing the camshaft? I doubt that. I haven't heard of an alloy one but you can get brass ones can't you? May be the way to go. It doesn't really make sense to me that you would need a very strong gear there, if a lot of torque is being transferred then the shaft probably isn't spinning nicely in the dizzy.
#22
S pack
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Posted 29 January 2011 - 10:54 PM
You can decide for yourselves about the pros and cons of nylon drive gears, but here is what GMH learned about them.
Extract from GMH Dealer Service Letter December 1971.
NYLON DISTRIBUTOR DRIVE GEARS - FIELD TRYOUT - HQ MODELS - V8 ENGINES
A number of V8 engine assemblies produced recently, incorporate nylon distributor drive gears. The purpose being to test this new type gear in Field conditions; to achieve this twelve thousand engines incorporating the nylon gear have now been released to the Field.
The brackets of engines affected are:-
308 V8 - from engine No.'s QT 60361 to QT 62360
308 V8 - from engine No.'s QT 92943 to QT 94942
253 V8 - from engine No.'s QR 65055 to QR 73054
In the event that Service complaints are experienced, regardless of the vehicle mileage, the distributor gears should be returned for examination.
Extract from GMH Dealer Service Letter February 1972.
NYLON DISTRIBUTOR DRIVE GEARS - HQ WITH 253 AND 308 ENGINES.
An item under the subject heading of the December 1971 Service Letter, advised that trial production runs of V8 engines incorporated nylon distributor drive gears.
The engine number brackets listed in the earlier Service Letter are not correct and should be amended as shown below:-
253 cu. in. From QR 65055 to QR 108404
308 cu. in. From QT 60361 to QT 95000
SOME ENGINES IN THESE BRACKETS ARE EQUIPPED WITH NYLON DISTRIBUTOR DRIVE GEARS AND SUCH DISTRIBUTOR ASSEMBLIES ARE IDENTIFIED BY A SPOT OF YELLOW PAINT ON THE VACUUM DIAPHRAGM HOUSING.
It has been established that some of the nylon gears wear excessively giving rise to conditions such as engine misfire, erratic engine performance, excessive 'ping'.
As vehicles come in for service, check the engine numbers against the brackets listed above. If the distributor has a nylon gear, replace it with an iron gear.
A kit is merchandised by NASCO as M37137 for this purpose.
Extract from GMH Dealer Service Letter December 1971.
NYLON DISTRIBUTOR DRIVE GEARS - FIELD TRYOUT - HQ MODELS - V8 ENGINES
A number of V8 engine assemblies produced recently, incorporate nylon distributor drive gears. The purpose being to test this new type gear in Field conditions; to achieve this twelve thousand engines incorporating the nylon gear have now been released to the Field.
The brackets of engines affected are:-
308 V8 - from engine No.'s QT 60361 to QT 62360
308 V8 - from engine No.'s QT 92943 to QT 94942
253 V8 - from engine No.'s QR 65055 to QR 73054
In the event that Service complaints are experienced, regardless of the vehicle mileage, the distributor gears should be returned for examination.
Extract from GMH Dealer Service Letter February 1972.
NYLON DISTRIBUTOR DRIVE GEARS - HQ WITH 253 AND 308 ENGINES.
An item under the subject heading of the December 1971 Service Letter, advised that trial production runs of V8 engines incorporated nylon distributor drive gears.
The engine number brackets listed in the earlier Service Letter are not correct and should be amended as shown below:-
253 cu. in. From QR 65055 to QR 108404
308 cu. in. From QT 60361 to QT 95000
SOME ENGINES IN THESE BRACKETS ARE EQUIPPED WITH NYLON DISTRIBUTOR DRIVE GEARS AND SUCH DISTRIBUTOR ASSEMBLIES ARE IDENTIFIED BY A SPOT OF YELLOW PAINT ON THE VACUUM DIAPHRAGM HOUSING.
It has been established that some of the nylon gears wear excessively giving rise to conditions such as engine misfire, erratic engine performance, excessive 'ping'.
As vehicles come in for service, check the engine numbers against the brackets listed above. If the distributor has a nylon gear, replace it with an iron gear.
A kit is merchandised by NASCO as M37137 for this purpose.
#23
Peter UC
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Posted 30 January 2011 - 02:11 PM
Yet the same company released nylon gears only 9 years later. Things have moved on from 1971.
#24
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Posted 31 January 2011 - 04:01 PM
Yet the same company released nylon gears only 9 years later. Things have moved on from 1971.
I suppose that's why the Bosch HEI distributor in the 1980 WB tonner I used to own had a steel drive gear.I think the point we can learn about distributor drive gears is that whether they are steel or nylon, if they are worn they can cause your engine to misfire, not run properly or excessive "ping".
Cheers
Dave
#25
_brett_32i_
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Posted 01 February 2011 - 11:47 AM
any electrical connection on ignition system.
we had/have a high rpm miss on the speedway motor. found a loose connection on the ignition switch going to coil. hopefully that does it.
dyno showed mixture going rich, i guess as its not burning properly.
will find out saturday night.
we had/have a high rpm miss on the speedway motor. found a loose connection on the ignition switch going to coil. hopefully that does it.
dyno showed mixture going rich, i guess as its not burning properly.
will find out saturday night.
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