Help needed - Misfire, Off Idle Stumble not present at cruising speed
04-22-2017, 10:15 PM
#1
Member
Thread Starter
Join Date: Mar 2011
Location: Chicago, Il
Posts: 114
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1986 Iroc Z28
Engine: 305 TPI
Transmission: 700R4
Help needed - Misfire, Off Idle Stumble not present at cruising speed
Hey all -
I recently started to get a misfire that I cannot narrow down. Vehicle idles fine. Slight misfire when reving in park. In drive, itll drive fine on slow accelerations. Anything more than slow acceleration it starts to hesitate and misfire / buck until I really get on it and then it powers through. Cruising at all speeds is fine once you get there. I will randomly get a consistent popping in the exhaust, usually after driving.
Engine is a 305, relatively recent tpi carb swap - new carb/manifold, new distributor, timing is set. Fuel Pressure regulator set at 7psi (needle on gauge flutters at idle - normal? Or not?). Has headers and a comp HE 268H Cam (.454 lift, 268 dur).
With all the new parts I have on it, Im worried its something more, like a burnt valve, but I woulnt think Id see the random issue - my guess is it would be consistent at idle, off idle, and during cruising / accelerating.
Help.
I recently started to get a misfire that I cannot narrow down. Vehicle idles fine. Slight misfire when reving in park. In drive, itll drive fine on slow accelerations. Anything more than slow acceleration it starts to hesitate and misfire / buck until I really get on it and then it powers through. Cruising at all speeds is fine once you get there. I will randomly get a consistent popping in the exhaust, usually after driving.
Engine is a 305, relatively recent tpi carb swap - new carb/manifold, new distributor, timing is set. Fuel Pressure regulator set at 7psi (needle on gauge flutters at idle - normal? Or not?). Has headers and a comp HE 268H Cam (.454 lift, 268 dur).
With all the new parts I have on it, Im worried its something more, like a burnt valve, but I woulnt think Id see the random issue - my guess is it would be consistent at idle, off idle, and during cruising / accelerating.
Help.
04-22-2017, 10:33 PM
#2
Supreme Member
iTrader: (1)
Join Date: Sep 2005
Posts: 26,115
Received 1,688 Likes
on
1,283 Posts
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: Help needed - Misfire, Off Idle Stumble not present at cruising speed
Sounds like a combination of an obsolete cam that is a poor choice PERIOD for a 305, and lack of carb calibration to match the engine's new inappropriate and crippled configuration.
"Fuel pressure" makes very little difference to the situation. Carb part # might be AHELLUVALOT more useful as a guide to tell you what to do next. "Timing" being "set" is equally irrelevant: we ALL "set" timing whenever we drop in a dist, just, some of us ASK the engine what IT wants before we tighten the bolt, rather than assuming we can TELL the engine what it's GONNA like with the particular dist we have in hand and THE ENGINE is gonna agree. Too bad some people are so short-sighted and arrogant.
"Fuel pressure" makes very little difference to the situation. Carb part # might be AHELLUVALOT more useful as a guide to tell you what to do next. "Timing" being "set" is equally irrelevant: we ALL "set" timing whenever we drop in a dist, just, some of us ASK the engine what IT wants before we tighten the bolt, rather than assuming we can TELL the engine what it's GONNA like with the particular dist we have in hand and THE ENGINE is gonna agree. Too bad some people are so short-sighted and arrogant.
Last edited by sofakingdom; 04-22-2017 at 10:37 PM.
04-22-2017, 10:54 PM
#3
Member
Thread Starter
Join Date: Mar 2011
Location: Chicago, Il
Posts: 114
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1986 Iroc Z28
Engine: 305 TPI
Transmission: 700R4
Re: Help needed - Misfire, Off Idle Stumble not present at cruising speed
Carb is a HLY-0-83570 if that helps you. Was not trying to sound arrogant with saying timing "set" I was trying to provide as much detail as possible to make for accurate recommendations. I read too many posts on here where contributors waste time with recommendations only to be met with "yeah, did that, checked that"
...Just trying to be helpful when requesting help...
...Just trying to be helpful when requesting help...
04-23-2017, 08:31 AM
#4
Supreme Member
iTrader: (1)
Join Date: Sep 2005
Posts: 26,115
Received 1,688 Likes
on
1,283 Posts
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: Help needed - Misfire, Off Idle Stumble not present at cruising speed
OK, pretty typical vac sec Holley 4150... should be eeeeezy enough to whip it into shape.
Visit the Carb forum on this site. Go through the "Holley Tuning" sticky process, starting at the beginning, doing each step in the order given, skipping none. Should make a YUUUUUJJJE difference. Set fuel level properly; roughly set the idle; find the right jet size by shrinking them until it leans out, without worrying about ANY other effects; increase the PV # until the Holley bog goes away; adjust the sec throttle stop until the pri throttles expose .030" - .050" of the transition slot when the idle speed is set right; might take a couple of iterations of all this to get it perfect, since even though by this method you are "isolating" each circuit as much as possible to tune its effects, there's still a small degree of interaction; then go to work on the secondaries.
Visit the Carb forum on this site. Go through the "Holley Tuning" sticky process, starting at the beginning, doing each step in the order given, skipping none. Should make a YUUUUUJJJE difference. Set fuel level properly; roughly set the idle; find the right jet size by shrinking them until it leans out, without worrying about ANY other effects; increase the PV # until the Holley bog goes away; adjust the sec throttle stop until the pri throttles expose .030" - .050" of the transition slot when the idle speed is set right; might take a couple of iterations of all this to get it perfect, since even though by this method you are "isolating" each circuit as much as possible to tune its effects, there's still a small degree of interaction; then go to work on the secondaries.
04-23-2017, 09:23 AM
#5
Member
Thread Starter
Join Date: Mar 2011
Location: Chicago, Il
Posts: 114
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1986 Iroc Z28
Engine: 305 TPI
Transmission: 700R4
Re: Help needed - Misfire, Off Idle Stumble not present at cruising speed
OK, pretty typical vac sec Holley 4150... should be eeeeezy enough to whip it into shape.
Visit the Carb forum on this site. Go through the "Holley Tuning" sticky process, starting at the beginning, doing each step in the order given, skipping none. Should make a YUUUUUJJJE difference. Set fuel level properly; roughly set the idle; find the right jet size by shrinking them until it leans out, without worrying about ANY other effects; increase the PV # until the Holley bog goes away; adjust the sec throttle stop until the pri throttles expose .030" - .050" of the transition slot when the idle speed is set right; might take a couple of iterations of all this to get it perfect, since even though by this method you are "isolating" each circuit as much as possible to tune its effects, there's still a small degree of interaction; then go to work on the secondaries.
Visit the Carb forum on this site. Go through the "Holley Tuning" sticky process, starting at the beginning, doing each step in the order given, skipping none. Should make a YUUUUUJJJE difference. Set fuel level properly; roughly set the idle; find the right jet size by shrinking them until it leans out, without worrying about ANY other effects; increase the PV # until the Holley bog goes away; adjust the sec throttle stop until the pri throttles expose .030" - .050" of the transition slot when the idle speed is set right; might take a couple of iterations of all this to get it perfect, since even though by this method you are "isolating" each circuit as much as possible to tune its effects, there's still a small degree of interaction; then go to work on the secondaries.