Im completely stumped

****Eric****

New member
Hopeing someone out there has come accross this and can help shed some light. I have a animal big bore stroker with billet head that since the start of this season cannot get to run right. It makes power like crazy and has tons of torque but once it nears 3/4 throttle falls on its face and stutters like its jetting is off or there is something wrong with timing. I have tried 3 different carbs, with jetting and even switched from methanol to gas. I have had 3 different coils on it and tried different gaps as well as different flywheel with no luck. Cam timing is correct i checked 3 times. Has great compression. New roller rockers, new valve springs, checked push rods they r straight. We have been building our own motors for years and have never been able to not diagnose something but this one has me and everyone else i know stumped. So im reaching out to the internet looking for any ideas or direction what i should try or look for next. Thanks guys
 

flattop1

Dawg 89
First thought is valve seat coming loose.
Second is a tight or stuck ring.
 

Jimbo

If you talk the talk you should walk the walk
What valve springs are you using, what is the seat pressure and what is the weight of the valves in grams?
 
If taking a one shot guess.......I'd say valve float is your problem. Put on a new set of the white stripe dual springs, and go again.
 

****Eric****

New member
Thanks for all the responses. I can tell you its not valve float. It did run with the current setup last year great with no issues. Since then i had replaced valve springs with exact replacements that i had in it last year. It seems like it progressively has been getting worse as i have been trouble shooting it. When its cold it does it less than when it is warm. Almost non existant when its cold. Take it for a couple laps and it starts to do it. I have the head off and had to stop due to needing to leave to go look at a new vehicle. I am going to pull the valves and springs out tonight. Will update u all and get more spacifics on valve springs, seat pressure and valve wieght that jimbo was asking for. I know the springs are dual spring just not sure exact ones.
 

sundog

Member
Do springs get stronger in the cold? the head stays cooler so the valve seat stays in? Sounds to me like it's just too rich.
 

****Eric****

New member
Its not a tilly and its not too rich. Ive tried jetting and im no stranger to changing jets or carbs. Used 2 different 28mm mikuni flat slides running methanol. I have a gas mikuni on it now running vp race fuel. went all the way up to a 500 main which was so rich all the way down to a 150 main which was way lean but runs best at a 200 main. Ive tried it all incrementally.
 

Jimbo

If you talk the talk you should walk the walk
I've seen some weak magnets in the Briggs PVL flywheels lately.
Not new ones but ones that have been in use for some time.
 

****Eric****

New member
I was wondering the same with the flywheel so i switched my pvl flywheel and coil out for a flathead arc flywheel and coil which i used on it before season before last and has same exact problem
 

paulkish

Premium User
"I can tell you its not valve float. It did run with the current setup last year great with no issues. Since then i had replaced valve springs with exact replacements that i had in it last year. It seems like it progressively has been getting worse as i have been trouble shooting it."

what's to say your new springs haven't gone bad or were bad from the git go? If you didn't have the problem last year and still have the springs, you might want to try throwing in the old springs to see how it does. and ... ????????????


When ever I have a tough problem which crops up after working on something the first question I ask myself is "What's the last thing I touched?".
 

flattop1

Dawg 89
not to say this is it just an experience.
similar issue rebuilt engine late in the year . ran like a top.
put it up for the winter, inside storage. ran great but high speed miss.
could not fix so switched engines .
when I got around to it disassembled the engine the only thing I could find was a sticky ring .
Like Mr. Angrey said may have been corrosion from sitting. I never reassembled it, used the parts to build a different one.
 

Jimbo

If you talk the talk you should walk the walk
NO OFFENSE
But if you have no idea what your spring pressure are my guess is you probably don't know the other important values that could be causing your problem.
Just switching parts is what i call the hope and poke method. I'm not going to say i have never done things that way, I have.
Sometimes at the track when a problem occurs and there isn't enough time to figure out what the actual problem is i will replace everything that could be causing the issue.
Example: If i have a fuel related issue i will replace the carb, fuel pump, fuel filter, clean the vent hole in the fuel tank, etc.
I will clean and adjust the carb later.
Paul has some excellent advise for you also!
In the automotive business they have what's called the Ever since syndrome.
It may go like this:
Ever since you changed the oil in my car the air conditioning (or what ever) doesn't work.:confused:
 

****Eric****

New member
None taken jimbo heres what i know. The springs are .850 at installed height and 48lbs. The cam calls for 42 to 44 lbs. It's the dyno cam F275 with 1 to 3 ratio rockers. I believe at the moment i have a valve seat coming loose. I should be more spacific that i had the problem before i put the new valve sprongs in it. In my original post i thought that is what i was relaying. Its not since i changed them ive been having the problems. Everything else seems to check out so far. I had the piston out previously this season when i tore it all down trying to see what the heck is making this thing runs like this but the rings were not stuck. I hope to have the topend completely tore apart by friday night. Its been a bit crazy the last few nights with other life things and i work a more than full time job. Thank you everyone for your input and thoughts and ill update when i find more myself.
 

Jimbo

If you talk the talk you should walk the walk
Refresh my memory. Doesn't the F275 cam have lots of duration. Isn't it basically a .356 cam that has had the lobe lift ground down to .275" If so it will have a very radical valve acceleration rate even with out 1.3 rockers.
Anyhow, that combination gives you .3575" lift at the valve if you set the valve lash at zero.
Are you saying your installed height is .85O"? Are you sure you don't have spring coil bind?
If the cam calls for 42 to 44 pounds and you only have 48 with 1.3 rockers i'm betting you are way short on valve spring pressure.
The 1.3 rocker ratio will increase the valve acceleration rate even more and will require considerably more valve spring pressure.
A real simple test would be to put standard 1 to 1 rockers on it and see how it runs.
 

Spinner

New member
The way to tell if it's valve spring float is it always happen at the same rpm or going down the straight at the same place. It has never failed me and it makes perfect sence.
 
JMO but I would look very closely at the head gasket. A small leak will act just as you described many times. May be small enough to not notice easily. Good luck
 
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