hey guys i just rebuilt a junkyard hei and im having problems at high rpms. Ive changed almost everything exepts pickup coil, which read fine on voltmeter. Anyways when i have the computer controlled hei it would backfire when cold then after it was warm it would run just fine, no backfire. Today i replaced the new vac hei and everything is fine until i hit 3400rpms then it stuters backfires(the more gas the worse). It will do it not matter how much load i put in it full throttle or light, its just the 3400 rpms barrier.

My question is could a non working or badly working centrifugal advance do this. Right now dont have vac advanced hocked up, which would make a difference in light thottle but not wot. The more I advance initial the higher in the rpm range it starts sputtering. When i had it set at6 degress it started pooping at around 2400rpms i raised it to around 14 initial and now is fine until 3500rpms.

The only part that i put in the distributor that i had doughts was a mr. gasket hei advance kit. Which according to the diagram i installed right, im thinking im not getting full advance, i would turn the weights to full open(anvance) and the thing seemed to only travel half way through travel. I also put in the lightest springs,(the set has three from light to heavy) could this be it? Will hevier springs limit total centrifugal advance a little. I put in the ones that are very very very soft. I also run 87 octane, for now. Which i figured i could take detonation by retarding it a bit, but it instead lower the rpm at which its sputters.

Oh yeah and by the way, i just finished doing a tbi to carb swap if it helps any! Edelbrock 600cfm, and performer rpm manifold..

Anyhelp would be appreciated...THANKS

 

A few things come to mind.

Check the pickup wires inside the distributer. They may be cracked or broken and when the advance comes in, the pickup plate is rotated and the wires are being seperated causing lose of ignition. ** Took me 3 hours to find that problem on a car once **

Make sure you have the proper coil. GM produced 2 types of HEI coils. One has a red and yellow wire. The other is red and white. Chevys use the red and yellow style.

Also check your fuel system. Maybe the filter is plugged and the engine isn't getting enough fuel for higher rpms. A fuel line could also be pinched since you just did a conversion.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

I'd start by playing with different sets of springs on your advance weights. I put one of those Mr. Gasket advance weight kits in my HEI, and I ended up using the original stock springs with the new weights to get the best results out of mine. If your spring tension is too light, you can get too much advance too early, which can cause preignition. When this happens, your intake valve is still open, and you get a popping up through the carburetor from too early of a combustion. Oh yeah, one other thing. You might be one tooth off if you've removed and reinstalled your distributor. You won't be able to get the proper amount of advance or retard on your timing if this is the case. You can either pull it back out and move it a tooth, or jog the wires one post in either direction on the cap of your distributor if you don't want to remove it again.

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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."

[This message has been edited by Pat Hall (edited December 28, 2000).]

 

I like the theory of the pickup coil it would definetly make sense, but if i advance the distributor initial timing the poping will be at higher rpms. If it were the pickup coil (wires) which did look pretty beat up. why would it change relative to timing. The centrifugal advance will come in due to rpm spin, in which case the pickup theory would be fine but every time i advance initial timing the sputter is at higher and higher rpms. Im sure if i advanced it enough it would stop completely but i dont want to do that. Will no contrifugal advance cause this, currently i dont even have the vac advanced hooked up. I also have the yellow wire coil.

Could i be running lean due to the intake swap, the motor is pulling 22 inches of vacuum at idle if it helps. Would having on advance help a lean condition(not saying i would do this) but when i had the computer controlled distributor it would stutter when cold, but thats probably normal since i have stock cam, fully open exhaust and performer rpms manifold with carb. but after warm it wouldnt do it. Could to rich do this? I smell fuel when i floor it.

If its causing preignition due to it coming in to early then why is it that when i advance the timing it runs better at higher rpms progresively. could i be getting no advance? I personally saw no way that i could get full advance out of those mr. gaskets weight spring sets. they seems not to let the advance go in all the way...

And also today i found an oil leak on the floor...great....Im moding my car and it wants to fall apart, i mod something and something comes back and breaks..


and thanks again in advance for the help!

 

bttt

 

TBI,

I think your problem sounds like a lean condition coupled with incorrect timing. You need to get the vacuum advance connected, then set the idle timing correctly. You should also make sure the spark plugs and wires are in good condition. High RPM missing and backfire can be caused by a weak secondary ignition system and weak spark.

A lean condition can cause popping in the intake due to higher chamber temperatures and a small fuel/air charge being burned too quickly and too soon. Make sure there are no hose or intake leaks, and that the float level is set correctly. Make sure the power circuit on the carb is scaled correctly for your engine displacement.

Since you've made a few changes at once, you'll need to diagnose the problems and their effects simutaneously. This is one reason that it's usually better to perform mods one at a time where possible.

If you want to test the timing advance theory, move the distributor while at the RPM where the problem begins. If you can advance the timing to reduce the problem, you are probably not getting adequate advance for your engine. And don't be so sure the timing marks on the harmonic damper are absolutely correct. They do tend to move over time, so use them as a guide only.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

 

Muahahahahah..

First off. THANK YOU GUYS..DAMN. I love you people its fixed.

You where right stephen. it was the bloody pickup coil. I didnt replace it against my will cuz a friend told me it was OK. MAN she doesnt even backfire when cold. I finally have my car, its been 1 month and a half. I was getting tired of borowing imports. That when you trully apreciate a v8. BIG DIFFERENCE.

Im putting in the weak spring later on.. To get the advance in earlier. DAMN SHE RUNS..

Thank you all again youve all been alot of help. If any off you are in florida ever..you got yourself a guide...

THANKS AGAIN!!!

 

Oh by the way Stephen. Im building a short rod 385(.40 over, 400block crank 350) in the garage. How did your old one run??

Im having a problem getting short rods. Any ideas.

I have an edelbrock rpm cam. 234 244 dur at .480 and .510 lift. Ported polished heads by me 333882. rpm intake 600cfm carb, headers, etc etc. What do you suggest/think?

Thanks in advance!!

 

Mine ran fine as you can see from the sig (above) except the stock bottom end didn't like those 6400 rpm shift points. If you check my racing page you can see everything I did to it. It looked like a lot of wrong parts but when you consider how well it performed I'd say I got it right. The only thing that wasn't set up right was the rear end gearing and that was because I ran a 3.27 9 bolt for 2 years.

My 383 (400 crank in 350 block) used the short 400 rods because it was an easier buildup. Stock 400 crank and rods machined for the 350 block. Stock cast 350 pistons.

I'd never suggest using 882 heads to anyone. If they only have one port for the exhaust crossover then you might be ok. If not then the heads will probably crack. 992 or 997 heads would have been a better choice. And when you consider the cost to put in bigger valves, springs, machining stud bosses down for screw in studs etc, it's even cheaper and better to get some Dart heads.

The 600 cfm carb will also be too small for that cam and intake. A 750 would be better.

Edited because of some errors. I misread it as a 350 block. Going .040 over on a 400 block is an extream limit. When you put the 350 crank into the 400 block, you need special destroke pistons to use the short 400 rods or even the longer 350 rods. Either way, building a destroked 400 (377) is more expensive than a simple 383.


[This message has been edited by Stephen 87 IROC (edited December 31, 2000).]

 

TBI...

Short rods shouldn't be that hard to come up with, except you might have to buy a whole motor to get them. Other than haunting the boneyards I don't know of much else to do. Anyway, if you do have to buy a whole motor, you shouldn't have too much trouble turning the block over, seems people are always looking for those.

This sounds like a true budget build... stock 400 shaft turned down, stock 400 rods, cast rebuilder's 350 pistons since you can't get anything decent in .040", that's a size those guys use to refresh warrantied .030" over blocks. I couldn't agree more with Stephen about the heads, those are just a bad choice all the way around, unless you are so budget limited that you aren't going to get something good.

IMHO that is not a good cam, period. Look at the "advertised" duration and compare the .050" lift and duration to some of the better cams, like Comp's XE series. I think you'd do alot better with the XE268H than that other one. Also, either of those cams will cause stock-diameter valve springs and press-in studs to be short-lived; that sends you heads to the machine shop for about $400 worth of work right there, which makes those castings an even pooorer candidate for a budget build, because the bang-for-the-buck ratio is so low when you spend your money on garbage castings.(You still have garbage at the end.) If you'll get a set of 186 castings, which are not that hard to come up with, you'll have a totally better combo, especially if those pistons are dished.

I also agree with Stephen that the carb is too small, it's a serious mismatch with the other induction components. I'd suggest a Holley 650 at least. As long as you go with that head/induction combo though you're going to end up with a soggy bottom end and no upper RPM benefits to offset that.

If you're set on using those heads, I think you'd be alot happier with a XE268-H10, the Performer intake, and the Holley 650; especially if you're sticking with exhaust manifolds.

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"So many Mustangs, so little time..."

 

I misstyped it sorry, the block is a 2 bolt main 350(1972). and a 400 stroker nodular iron crank. I have already ported and polished ive done these heads before on a 350 (with a milder cam). After port and polish they ran pretty damn impressive for what they are. Oh and by the way im building a torque motor, so its not going to see over 5500 rpms. I do know that the 600cfm is restrictive, but its gonna go in until i get the 750(in the works). The valves are gonna stay the same. 1.94 1.5, there is almost no bloody gain from them(2.02 1.6) unless you take special precausions when you port polish on 882 heads. Thats why alot of people get disapointed with these heads, they just port polish like a normal head and then put 2.02 1.6 valves which actually reduce flow on THESE heads. ive seen this happen. They become dogs. Ive since become an expert on these heads, they work extremely fine on 350s, i wanted opinions on 385 cuz ive never done it before. Im not expecting 450 lbs of torque and 400 hp.

It actually cost the same for me to get a 350 crank and a 400 crank, so i thought of doing the 385 instead even though is just a little extra torque in my case. All i want is around 350 honest horsepower and about 380 lbs of torque on a slightly detuned camshaft.

I asked about the edelbrock cam because it was given to me practically new, i also thought it might be tooooo much even on a 385, specially with push in studs. Also they 882 heads are the slighly thicker casting with only one port for the exhaust crossover these heads have been checked and are not cracked.

Ok put in a smalled cam on the 385, the same as stated above what do you guys think on torque production. which is what matters to me. Ive seen to many street races to know that in the street(were this engine is gonna be) torque is very important.

Thanks again guys!!!