[ckanderson]

I recently put in a Thumpr cam in my 350. spare me the "oh what a ****ty cam" crap. i dont care.

Im trying to figure out the surging problem I'm having. held at about 1500 rpm the engine will rise to 2000, down to 1000 in about 3 second cycles. idleing it does the same thing, except it will go so low it will almost die (and die in some cases) I have an edelbrock 1400 carb.

My timing is set @ 24 initial with no vacume advance at all (disconnected) and heavy springs on the hei mechanical advance, which wont allow timing till 3500 rpm (2 degrees). I dont BELIVE i have any air leaks, and used new gaskets when reinstalling the intake manifold, which I torqued to the proper specs.

I have full exhaust and the engine is a stock gm, heche in mexico.


Suggestions?

Thanks in advance

 

[Chori]

i am not sure how the eddies work do they use a float and bowl system? Do you have a fuel Pressure and/or regulator?

Just today i cured my dieseling problem caused by too much pressure in my fuel line, so much that it would push fuel through the needle and seat assembly and flood the motor.

 

[batman09]

I recently put in a Thumpr cam in my 350. spare me the "oh what a ****ty cam" crap. i dont care.

Im trying to figure out the surging problem I'm having. held at about 1500 rpm the engine will rise to 2000, down to 1000 in about 3 second cycles. idleing it does the same thing, except it will go so low it will almost die (and die in some cases) I have an edelbrock 1400 carb.

My timing is set @ 24 initial with no vacume advance at all (disconnected) and heavy springs on the hei mechanical advance, which wont allow timing till 3500 rpm (2 degrees). I dont BELIVE i have any air leaks, and used new gaskets when reinstalling the intake manifold, which I torqued to the proper specs.

I have full exhaust and the engine is a stock gm, heche in mexico.


Suggestions?

Thanks in advance

Why is your initial so high?It sould be somewhere between 8-14 degrees....34-38 degrees at 3000rpms

 

[ckanderson]

Why is your initial so high?It sould be somewhere between 8-14 degrees....34-38 degrees at 3000rpms

@ 10 it wont idle much at all. just peters out. Did a bunch of research today on various forums and this is what people were suggesting for this cam, and ones with a small LSA. high initial with low mechanical. This made a huge difference and now my car will idle (badly).

 

[ckanderson]

i am not sure how the eddies work do they use a float and bowl system? Do you have a fuel Pressure and/or regulator?

Just today i cured my dieseling problem caused by too much pressure in my fuel line, so much that it would push fuel through the needle and seat assembly and flood the motor.

its a float. i dont have a separate regulator, just the stock mechanical fuel pump.

Car idled fine before this.

 

[64PRONOVA]

Put a decent distributor in it and see if it runs. One that is fully-functional with vacuum advance and a correct mechanical advance curve.

 

[ckanderson]

Put a decent distributor in it and see if it runs. One that is fully-functional with vacuum advance and a correct mechanical advance curve.

What would you suggest?

the dist. is a stock GM HEI. I only disconnected the vacume advance and put heavier summit springs on the mechanical advance. @10 degrees BTDC the car would just die out. I read more then one post where people recommended the setup I'm (attempting) to run.

 

[ckanderson]

This would be the thread where I got lots of my info. notice they mention setting it as high as 36 locked out, and then connecting the vacume.
http://www.hotrodders.com/forum/reccomendation-between-two-cams-148691.html

 

[64PRONOVA]

That's a race setup. You usually don't use vacuum advance with it either. Not recommended for the street (unless you drive around at WOT all day long). Your distributor isn't locked-out anyway from the sound of it. That's why it's probably surging.

I'd put the HEI back to stock. You'll be a lot better-off.

 

[ckanderson]

That's a race setup. You usually don't use vacuum advance with it either. Not recommended for the street (unless you drive around at WOT all day long). Your distributor isn't locked-out anyway from the sound of it.

I'd put the HEI back to stock. You'll be a lot better-off.

with the hei stock, it wont idle period. once it gets below 1500 it just drops like a rock. whats different from his setup over mine?

"reisntall the rotor and retime the motor with 36deg at idle.
The advance does not move with rpm when its locked out.
This cam requires a full 36deg of timing at idle. Not when you rev it up.
(therefor the locked out mechanical advance curve)

All the carb and timing setting up (fixed 36deg ) is done with the vacuum advance disconnected. when you are all done, reconnect the vacuum advance to the carb."

looks like they are recommending 36 plus vac. with no mech.

"The more overlap the cam has the more egr effect at idle/ needs a ton of idle timing."

these (and other) comments are driving me to set my idle timing that high. I'll fully lock out the dist. tonight i'll try adding more timing.

Not trying to disagree with you, I do appreciate your input, but I cant get it to idle with 10-14 degrees base timing. just falls flat on it's face. increasing it to 24 without vac. helped a ton, now I wonder if adding 6-8 degrees more will be the final thing to make the car idle....

 

[levisnteeshirt]

valve adjustment

is this a hydraulic cam , and how do you adjust your valves ??

 

[ckanderson]

is this a hydraulic cam , and how do you adjust your valves ??

yes, hydraulic flat tappet.

I adjusted the valves before starting the engine. get it on the base of the lobe and tighten it 3/4 of a turn past zero lash, and due to the inability to get it to idle, i havent been able to readjust them.

after the engine gets past 2000 rpm or so, it runs great, just that low rpm it is surging bad.

 

[72GreenRally]

Exactly which thumpr cam did you install? Do you know the compression ratio in your engine? How did you install the cam?

 

[ckanderson]

Exactly which thumpr cam did you install? Do you know the compression ratio in your engine? How did you install the cam?

just the normal thumpr, not the mother thumpr or anything.

compression ratio is 8.5

installed the cam like your suposed to. tons of grease, broke it in properly. used the kit, so i replaced valve springs, locks seals and timing chain.

 

[The Big Al]

A stubble or surge in most cases is fuel problem.
a lean condition, poor fuel pressure, bad float adjustment, vacuum leak.

Ignition is misfire problem most of the time.
get off this timing amount. If the engine at first wants more timing, do it. There is no magical timing setting number! It's a reference and more trial and error tunning to find what your engine will need. And get the vacuum advance hooked back up if your driving on the street.


IMO
AL

 

[72GreenRally]

I believe this is your cam: http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=216&sb=0

and the cam being discussed the other thread is this one: http://www.compcams.com/Cam_Specs/CamDetails.aspx?csid=218&sb=0

Your cam is not as radical as the cam you are trying to follow the instructions for on the thread on the other hotrodding site. Your idle problem is most likely a lack of fuel. Your idle curcuit most likely needs to be richened up. A 1400 Edelbrock is 600CFM which may not deliver enough fuel for your engine to idle properly with the cam. You can't expect the carb set-up that worked with the old cam to work with the new cam. You need to set the timing and advance approriately for a mild cam, then address the fuel issue. You will need to get a tuning kit for the Edelbrock carb which will have various jets, rods, and metering springs.

 

[levisnteeshirt]

if it wuz me

yes, hydraulic flat tappet.

I adjusted the valves before starting the engine. get it on the base of the lobe and tighten it 3/4 of a turn past zero lash, and due to the inability to get it to idle, i havent been able to readjust them.

after the engine gets past 2000 rpm or so, it runs great, just that low rpm it is surging bad.

i'd set them about 1/4 turn if not just taking the lash out then lock them down right there with a poly lock. That will help it idle better by doing that. Did you check spring height when you changed springs ,, have to shim them or change retainers or locks to get the right height ??

 

[dave's 62]

I vote clogged idle circuit. sounds like you are running off the metering circuit.If you can't get it to idle lower then 1500+ your timing will not be set correct becuse the mecanical adv.is kicking in at the rpm. good luck.

 

[Dan_Lebherz]

I bet the valves are misadjusted to the point that some are not closing all the way. Re-check everything, starting witht he valves. Put the distributor back to stock with 12-14 degrees of initial.

 

[ckanderson]

thanks guys, looks like I have a few things to check. ill be checking it over this weekend