Issues with slight missfire under load
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Hm. Carb jetting?
Maybe, though stumbling upon acceleration is usually a matter of the size or duration of the squirt shot from the accelerator pump. You did mention the plugs were not fouling and were a nice tan color. Excessively lean air/fuel mixtures can cause what is called "lean surge", a kind of bucking or stumbling at steady cruise.
Maybe, though stumbling upon acceleration is usually a matter of the size or duration of the squirt shot from the accelerator pump. You did mention the plugs were not fouling and were a nice tan color. Excessively lean air/fuel mixtures can cause what is called "lean surge", a kind of bucking or stumbling at steady cruise.
Cargo Master
Joined: Oct 2005
Posts: 2,084
Likes: 94
From: Stanton, Michigan, USA
But if you're still concerned about it, consider asking your shop to perform a Cylinder Leakdown Test (CLT). Here is an explanation about the difference between the two tests: Compression-leakdown test
BarnieTrk
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Junior User
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So, still got the stumble at cruising speed. Got the truck back from the garage this evening. They checked it over again and swapped the distributor cap and rotor (the one that I replaced had a weird mismatch, was firing at the top of the pins inside, all charred etc).
The trucks not been on the rolling road yet, not been able to get the truck there due to work commitments and the rolling road being fully booked when i'm free (typical).
The garage did deduce that if the choke was pulled out a little during cruising speed the stumble almost disappeared. So maybe it is due to fuel starvation at cruising speed, although when accelerating hard all is well, when revving up on the driveway all is well. The truck idles well etc.
So I guess the next step is the rolling road, changing the jets. The truck has a new Holley 670, which I belive is man enough for the engine size.
Any leads chaps? would love to get the engine sorted
The trucks not been on the rolling road yet, not been able to get the truck there due to work commitments and the rolling road being fully booked when i'm free (typical).
The garage did deduce that if the choke was pulled out a little during cruising speed the stumble almost disappeared. So maybe it is due to fuel starvation at cruising speed, although when accelerating hard all is well, when revving up on the driveway all is well. The truck idles well etc.
So I guess the next step is the rolling road, changing the jets. The truck has a new Holley 670, which I belive is man enough for the engine size.
Any leads chaps? would love to get the engine sorted
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
OK.
"Lean surge" is a possibility. Carb tuning is one of those things that requires sorting out as you put it. In fact without an O2 or exhaust analyzer, jetting down till it stumbles at cruise is about the best way to do it. Jet up 1 or 2 sizes from whatever size are in there now.
Did you ever get the engine timing sorted? Remember we're looking for 19" or 20" at least with a stock motor and cam. Also you absolutely want to use vacuum advance and want that enabled and dialed in when making jetting adjustments.
"Lean surge" is a possibility. Carb tuning is one of those things that requires sorting out as you put it. In fact without an O2 or exhaust analyzer, jetting down till it stumbles at cruise is about the best way to do it. Jet up 1 or 2 sizes from whatever size are in there now.
Did you ever get the engine timing sorted? Remember we're looking for 19" or 20" at least with a stock motor and cam. Also you absolutely want to use vacuum advance and want that enabled and dialed in when making jetting adjustments.
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Junior User
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Hi Tedster9,
Yep the garage have re-checked the timing. The vacuum advance was always running correctly. We were concerned about the vacuum reading being low, but I couldn't find any vac leak, nor can the garage. A compression test was checked again today, all well there also. I does seem to lean towards Lean Surge. I guess the best move from here is to get a some jets and get the truck on a rolling road
I did also find some details on a Mustang forum. Makes for some good reading:
/http://forums.vintage-mustang.com/vintage-mustang-forum/435175-holley-tech-response-no-help-carb-surge-stumble-kinda-long.html
Yep the garage have re-checked the timing. The vacuum advance was always running correctly. We were concerned about the vacuum reading being low, but I couldn't find any vac leak, nor can the garage. A compression test was checked again today, all well there also. I does seem to lean towards Lean Surge. I guess the best move from here is to get a some jets and get the truck on a rolling road
I did also find some details on a Mustang forum. Makes for some good reading:
/http://forums.vintage-mustang.com/vintage-mustang-forum/435175-holley-tech-response-no-help-carb-surge-stumble-kinda-long.html
Moderator
Joined: Aug 2002
Posts: 6,103
Likes: 384
From: Southern California
I would also consider the fuel pump diaphragm at this point....as they can develop pinholes that allow lower rpm operation just fine but as rpm goes up, it starts with a slight miss and eventually gets worse as the pinholes get larger...but I do agree, likely a lean out condition.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I'd still want to investigate why that engine is not pulling a steady 19" or 20" of vacuum at idle. A stock engine at sea level in good health will do this, always. You will be much happier when it is setup this way. Trust Me.
Ted
Lead Driver
Joined: Aug 2003
Posts: 6,305
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From: Boulder Creek, Ca
Not an authority, IMO the 600 CFM should be fine, unless the engine is running into the 6K to 7K RPM range. Can not recall if the Holley 4160 secondary metering plate is preset or if there are metering jets, I believe they are preset? Another possible option may want to discuss with mechanic is pull a couple of spark plugs to determine if the engine is running lean; if so, would increasing the primary jet size possibly solve the misfire at the higher RMP? Anyhow, I do not have a clue but had an urge to chime in?
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Junior User
Joined: Aug 2015
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So, next step I guess, change fuel pump (worth changing to cross it off he possible issue list) and book the truck in for some rolling road action. Jet the carb if needed and cross fingers all sorted.
Also looking into swapping the manual transmission for a C6 Auto, also got the truck booked in for a new interior and ordered a new dash (old dials need to be changed). Already got a rewire kit as the electrics on the truck are shocking.
Plan to have it all done and sorted for next summer. The weather turning here in the UK, there forecasting heavy snow this winter, so the truck will be stored over the winter.
Also looking into swapping the manual transmission for a C6 Auto, also got the truck booked in for a new interior and ordered a new dash (old dials need to be changed). Already got a rewire kit as the electrics on the truck are shocking.
Plan to have it all done and sorted for next summer. The weather turning here in the UK, there forecasting heavy snow this winter, so the truck will be stored over the winter.
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