Backfire? Transmission issue?
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
Read your plugs. I am assuming you had a new "high flow" dual exhaust put on it. A lot of times this requires a richer pair of jets in the carb. Usually 2 steps richer if headers were installed also. I think you will find your plugs showing a lean mixture. The certain speed is actually a certain RPM I believe. Once you reach that spot the idle circuit has stopped helping. And the power circuit that was helping get to that RPM has stopped working. So the carb was left with only it's cruise circuit. Which I believe is now too lean.
Lead Driver
Joined: Aug 2003
Posts: 7,592
Likes: 10
From: **** hole San Jose ca.
2x with Mr Jeff with running lean.
If you have a manual choke pull it out a little an see if it runs any better.
You can do the same but with a warm engine slowly cup your hand over the carb intake air filter as your cutting off the air going in to the carb if it run a higher RPM then it needs lager jets. If the engine bogged or if it loads up with gas. Check timing..
And a person who makes it to King is not from being wrong.
Orich
If you have a manual choke pull it out a little an see if it runs any better.
You can do the same but with a warm engine slowly cup your hand over the carb intake air filter as your cutting off the air going in to the carb if it run a higher RPM then it needs lager jets. If the engine bogged or if it loads up with gas. Check timing..
And a person who makes it to King is not from being wrong.
Orich
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More Turbo
Joined: Aug 2016
Posts: 504
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Read your plugs. I am assuming you had a new "high flow" dual exhaust put on it. A lot of times this requires a richer pair of jets in the carb. Usually 2 steps richer if headers were installed also. I think you will find your plugs showing a lean mixture. The certain speed is actually a certain RPM I believe. Once you reach that spot the idle circuit has stopped helping. And the power circuit that was helping get to that RPM has stopped working. So the carb was left with only it's cruise circuit. Which I believe is now too lean.
Lead Driver
Joined: Aug 2003
Posts: 7,592
Likes: 10
From: **** hole San Jose ca.
I put on a used set of 92 factory ford fox body short headers, and 2.5” pipe. Not sure if that’s “new high flow” by your definition. Lol. Ok I’m tracking with you above, but wouldn’t this happen at idle under the same RPM situation? I can sit in my driveway and throttle up to around 3k which is were I’d be at 65 mph with no issue. My work schedule hasn’t permitted me to check the plugs yet. Hopefully tomorrow we can sort some of this out.
Not the same ...
Orich
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
I put on a used set of 92 factory ford fox body short headers, and 2.5” pipe. Not sure if that’s “new high flow” by your definition. Lol. Ok I’m tracking with you above, but wouldn’t this happen at idle under the same RPM situation? I can sit in my driveway and throttle up to around 3k which is were I’d be at 65 mph with no issue. My work schedule hasn’t permitted me to check the plugs yet. Hopefully tomorrow we can sort some of this out.
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More Turbo
Joined: Aug 2016
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Read your plugs. I am assuming you had a new "high flow" dual exhaust put on it. A lot of times this requires a richer pair of jets in the carb. Usually 2 steps richer if headers were installed also. I think you will find your plugs showing a lean mixture. The certain speed is actually a certain RPM I believe. Once you reach that spot the idle circuit has stopped helping. And the power circuit that was helping get to that RPM has stopped working. So the carb was left with only it's cruise circuit. Which I believe is now too lean.
As this relates to the backfire/misfire/whatever it is at higher speeds, the plug with speed glazing makes sense, but only one has this condition? I’ve never checked the timing but sure I can do so. After I get this sorted out we may head there next.
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Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
Well, I would put a timing light on it. Check the timing to be set right and if the timing mark is steady or jumps around. Then hold it at about 1500 RPM and see if the mark bounces around then. If it does chances are your timing chain is shot.
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More Turbo
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Wanted to do a few week later follow up. I ended up changing the spark plugs which were questionable at best, the rotor cap, and the dizzy cap. After doing so, I reset my air/fuel dials on the 2150 and have not had the issue reoccur. I did notice on the dizzy cap that I took off that there were very minor burn spots on the plastic from where the spark was obviously not hitting the plug points on the cap but the spark was still going off. Less than 1/8th in off all the way around. Almost looks like at some point the dizzy cap shifted or became loose. Nonetheless I haven’t had an issue so far.
Lead Driver
Joined: Aug 2003
Posts: 7,592
Likes: 10
From: **** hole San Jose ca.
Did you go to a colder heat rang that should have fixed the an autolite plug.
Reading through you reply's reminded me of when most teenager owned a Modal A or a Model T.
As those were the days of just clean the old plugs an make sure points & cap timing were close:
Over 60 yrs ago, I read in Hot-Rod Mag it was stated when installing headers you may half to change carb jets 2-3 sizes due to running leaner with less back pressure. .
Reading through you reply's reminded me of when most teenager owned a Modal A or a Model T.
As those were the days of just clean the old plugs an make sure points & cap timing were close:
Over 60 yrs ago, I read in Hot-Rod Mag it was stated when installing headers you may half to change carb jets 2-3 sizes due to running leaner with less back pressure. .
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