midrange stumble?
Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
midrange stumble?
350 , roller cam,demon 750 dp .msd dizz , master blaster coil, anything else you might need to know just ask but transission from idle is fairly smooth, pulls well to mid range revs then seems to hesitate and flatten out...once im passed it then it pulls hard all the way to full revs.
i have never had a problem like this till recently when i blew the top radiator hose at the thermostat end. i predicted that the airfilter got a big gulp of water when this happened so i have replaced the oil and drained gas out of carb and its running night and day better but i still have this flat spot. it used to be smooth all the way from idle to full power. to explain it i can really feel the sensation of my secondaries opening exagerated. it shouldnt be like this. where should i start?
i have never had a problem like this till recently when i blew the top radiator hose at the thermostat end. i predicted that the airfilter got a big gulp of water when this happened so i have replaced the oil and drained gas out of carb and its running night and day better but i still have this flat spot. it used to be smooth all the way from idle to full power. to explain it i can really feel the sensation of my secondaries opening exagerated. it shouldnt be like this. where should i start?
Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
whats PO? ............... possibility of?
it never gets above 190 and i dont think much water got in . when i drained it i didnt find any water/coolant but my airfilter did have some dripping off it.
it never gets above 190 and i dont think much water got in . when i drained it i didnt find any water/coolant but my airfilter did have some dripping off it.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
PO= previous owner.
Look at your secondary ignition. cap and coil specifically. Get a spray bottle full of water with a couple drops of dish soap (soap keeps the water from running off so fast) and soak everything down. I had a Mallory coil at one time and it'd die if I went thru a puddle in the road. Wipe the coil off and it's start right up.
Just thinking out loud here, but did the engine backfire thru the carb at anytime? A midrange stumble I'd be thinking power valve. Especially if it happened all of a sudden.
Look at your secondary ignition. cap and coil specifically. Get a spray bottle full of water with a couple drops of dish soap (soap keeps the water from running off so fast) and soak everything down. I had a Mallory coil at one time and it'd die if I went thru a puddle in the road. Wipe the coil off and it's start right up.
Just thinking out loud here, but did the engine backfire thru the carb at anytime? A midrange stumble I'd be thinking power valve. Especially if it happened all of a sudden.
Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
haha thanks for the tip on PO timat - matt had me racking my brain on that one....
P.O. was 8 years ago in cali and alot of water has passed under the bridge since then . the only thing left of that motor now is the block and rotating assembly. it was rebuilt fresh 30 over with flat tops in sacramento so i think it would have given me grief by now.
can you explain to me the coil trick with some more detail. you want me to soak it while its idling and see what it does? the msd dizz and coil is only 2 years old and only gets driven every few weeks so im not expecting it to have given up on me but you never know.
you mention backfire....possibly through the carbat some time or another , but i dont think it should matter. i think demons have some inbuilt bypass that stops any damage from occurring. but since you mentioned backfiring my dizzy did rotate after a few hard launches a while back advancing my timing too much and it would backfire through the pipes.i remedied that when it happened but this is slightly different that at 1000-2500 rpm everything is normal. around 300-3500 it has a dull spot with slight crackling in the exhaust then 4000-6000 it pulls hard as it used too.
P.O. was 8 years ago in cali and alot of water has passed under the bridge since then . the only thing left of that motor now is the block and rotating assembly. it was rebuilt fresh 30 over with flat tops in sacramento so i think it would have given me grief by now.
can you explain to me the coil trick with some more detail. you want me to soak it while its idling and see what it does? the msd dizz and coil is only 2 years old and only gets driven every few weeks so im not expecting it to have given up on me but you never know.
you mention backfire....possibly through the carbat some time or another , but i dont think it should matter. i think demons have some inbuilt bypass that stops any damage from occurring. but since you mentioned backfiring my dizzy did rotate after a few hard launches a while back advancing my timing too much and it would backfire through the pipes.i remedied that when it happened but this is slightly different that at 1000-2500 rpm everything is normal. around 300-3500 it has a dull spot with slight crackling in the exhaust then 4000-6000 it pulls hard as it used too.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
If your engine has swallowed enough coolant from the air cleaner, even during a short period,
it may have created a hydraulic lock in the combustion chamber and damaged pistons / rings...
Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
thanks guys -im hoping nothing has gone that wrong , but you give good advice so a leak down test is on my things to do list. in my msd im only running 1 light and 1 medium spring. do you think it might be advancing too fast and possibly 2 medium springs might be a better option. im going to try it as well so ill report back once i have done both those things.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
With it running at idle, spray the whole secondary ignition with a water/soap mix. If the secondary is leaking or breaking down the water will exaggerate it. Might lead you to the source.
I agree that the Demon and late Holley carbs have a blow-off to protect the power valve. I just tossed that out as a possibility. Something is causing soft spot in your midrange and that's one possible cause. After it swallowed a load of coolant thru the intake there are a load of possibilities.
I agree that the Demon and late Holley carbs have a blow-off to protect the power valve. I just tossed that out as a possibility. Something is causing soft spot in your midrange and that's one possible cause. After it swallowed a load of coolant thru the intake there are a load of possibilities.
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Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
i did leak down test - no problem
i sprayed electrics with soapy water- no problem
i got another of the same carb of my other car- bolted it on and the 81 is now hammering again !!!! i have pulled the other carb apart and gave it a thorough cleaning. ill see how it goes when i start up the other car later. i am very relieved that i have narrowed it down to the carb and nothing major.
i sprayed electrics with soapy water- no problem
i got another of the same carb of my other car- bolted it on and the 81 is now hammering again !!!! i have pulled the other carb apart and gave it a thorough cleaning. ill see how it goes when i start up the other car later. i am very relieved that i have narrowed it down to the carb and nothing major.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
i did leak down test - no problem
i sprayed electrics with soapy water- no problem
i got another of the same carb of my other car- bolted it on and the 81 is now hammering again !!!! i have pulled the other carb apart and gave it a thorough cleaning. ill see how it goes when i start up the other car later. i am very relieved that i have narrowed it down to the carb and nothing major.
i sprayed electrics with soapy water- no problem
i got another of the same carb of my other car- bolted it on and the 81 is now hammering again !!!! i have pulled the other carb apart and gave it a thorough cleaning. ill see how it goes when i start up the other car later. i am very relieved that i have narrowed it down to the carb and nothing major.
Thread Starter
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
could be a coincidence that the carb took a crap at the same time you blew the hose...sounds like the carb is too lean during transition when the secondaries are opening...is there a power valve in the secondaries??..could be not working or the air bleeds are blocked or maybe the linkage connection is fubar for some reason..hows the base gasket.....accelerator pump??good luck..maybe it's the fosters.....
