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Old 06-17-2003, 11:01 PM
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Question Cam Size / Carb Selection Relationship, Four Corner Idle

Hi Guys,

This is my first post here, and I apologize for jumping in just to ask a question, but I've already searched though every Usenet post I could find plus everything here, all to no avail. Hopefully someone here has something of an answer.

Here's the problem: I just can't get the car to run decently, if at all, and I'm almost sick of trying. It's rich, rich, rich no matter what I do. The problem is with the carb. I assume to get any idea, pertinent engine information is needed:

Heads are 64cc ProActions with 220cc runners, without any major port work done. Intake is an Edelbrock Victor Jr. port matched to the heads and cleaned up a good deal. Cam is a Comp XE274H-10, which is 230/236 @.050 with .49 Lift and 110 degree LSA. Pistons are flat top, so ~9.5 or 10:1 compression.

I've tried two carbs: A Holley 80508, which is just a 3310 with an electric choke, and an Edelbrock/Carter 750. Neither works out of the box, but I didn't really expect that. The Holley was particularly bad. What I need to know is if either of these carbs can be made to work with this engine, and if not, what would, and if so, what needs to be done.

The Edelbrock has a horribly high idle no matter what I do to the thing unless I shove the choke horn shut. Other than that, it could be made to work, although right now it idles pretty rich, and likes to backfire out of the carb when I hit the gas.

The Holley is just terribly rich no matter what I try. I dumped the original power valve and put in I think a 4.5, so that's not the problem. Idle vacuum isn't great, but I still pull at least 10, if memory serves. Even if I lean it out, its still pretty rich. No matter which screws I touch, nothing works. It also backfires and generally runs like crap.

Here's my guesses: Perhaps the transfer slot is overly exposed and dumping excess fuel? Fixes? Actually, that's about my only guess right now. =)

I've been advised I may need a four corner idle adjustment to have any hope of getting this to work. However, I'd prefer to keep one of the carbs I have, if possible. If 4 corner is necessary, would modification be possible/feasible? I've also thought about drilling some holes in the primary throttle blades, but I'd prefer to leave drastic stuff like this to a last resort, unless there's a really high probability of success.

Any suggestions are welcome and would be most appreciated. I haven't run the car in over a year due to the last engine making like a grenade after the machinist gave me a bent rod. Then the same shop scrapped up this motor by giving me standard slot rocker arms. A lifter nicely shredded and took the cam with it. This is the third shot at it, now with a different builder, and I'm getting a little eager (and frustrated).

Best regards,

Ryan

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Old 06-18-2003, 01:00 AM
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My new motor(mild 350)was popin out the carb,runnin rich,missing and fartin out the tail pipes too.I jetted down from 75s to 68s and took it up the road(5:15am with no plates lol)and it cleaned out and runs great now.
By the time I got back from around the block the carbon had blown off the plugs and it was idling very high.Idled it down set the air/fuel and tweaked timing and it seems OK now.
Choke adjustment and jets would be were I would go.
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Old 06-18-2003, 06:10 PM
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Perhaps I'm missing something here, but how would the primary jets affect the idle characteristics?
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Old 06-18-2003, 06:54 PM
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You are not far off in my estimation on the slots being exposed. I am not sure why you are apprehensive about drilling, I do it to all of mine that are on performance engines. I would try it.


Second, if you are popping through the carb, chances are timing is too slow. You did not mention what kind of ignition you are running. I have found with out a hot coil and ignition amplifier of some sort(msd perferred) engines with a cam that large tend to load up on idle. If you dont drive them and clean them out a bit, it gets worse. This is especially true with a weak ignition or too cold of plug. What are the specs of the ignition?


Chris
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Old 06-18-2003, 09:02 PM
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Whats the timing set at idle speed (under 1000 rpm)?

I would pump it up to 12-16 first and then try to get your idle down and stable with the idle mix screws, that cam is not that radical that you need to get a four corner idle carb.

Whats the fuel pressure?
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Old 06-18-2003, 09:20 PM
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I would try a 1" carb spacer to see if it cleaned it up first, or even a two inch spacer, if it cleans it up, you can either run the spacer or go to a little larger carb, I had thiss problem once and I added the spacer, (acts like a bigger carb, by allowing more air/fuel available) and I actualy had to increase the jet size because it started running lean, I have found that not only do to big carbs run rich, but to small ones do also....
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Old 06-19-2003, 11:17 PM
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My reply regards the timing issue, but first, as for the guy who posted about the carb spacer: It sounds like you've got other issues and you're tossing a Band-Aid on that motor at best, IMHO. A spacer will cool the fuel charge and provide a cleaner transition path for the air into the intake, but how this relates to fixing genuine idle problems confuses me. Hot fuel is not the problem here, and air velocity is not the problem either. That aside..

Anyhow, I have no idea where the timing is right now, to be honest. The engine shop set the timing, and I haven't checked it. I assume that he did it correctly, but assumptions are dangerous things. In any case, I tried adjusting the timeing to no avail on the previous (identical) motor that ate a cam to no avail. That's not the immediate problem.

Ignition is a GM HEI, but still with the stock coil. So this suggestion is certainly something to think about. MSD is planned, but I just haven't gotten around to it. I had assumed the HEI would be fine to about 5500-6000 RPM or so. The carb backfires only really happen when I get it started and either smack the gas or let the motor kill itself. My guess it that it gets loaded up on gas and ignites it right back out the carb because there's so much of it.

Now that I've heard from someone else who's had good luck drilling the primary throttle blades, I'll give that a shot ASAP. Further research also yielded that cracking the secondaries open with the adjustment screw might help, so I'll give that a shot before drilling holes. I'll post an update on how it works.
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Old 06-20-2003, 02:46 AM
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Quote:
Originally posted by rmhuizenga Perhaps I'm missing something here, but how would the primary jets affect the idle characteristics?
Guess I shouldn't have answered this one.When it comes to carbs after I rebuild a Holley I just sort of,play around with everything till it works the way I want it to.My knowledge of the inner circuits is limited.I seem to get um workin,but I shouldn't be giving advice.
What happened with mine or what I think it was is during the initial breakin I fouled the plugs with the big jets and I couldn't get it to idle.So it took leaning the jets and a quick shot up the road to clean it out.After I blew the crap out I was able to time it better and set the A/F right.
I figured this might be your same situation but I guess not.

Last edited by 78 monte; 06-20-2003 at 04:02 AM.
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Old 06-20-2003, 09:51 AM
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The first thing to do is set the idle to 800RPM, pull the carb off and remove the primary float bowl and metering block and check the gaskets to see if they are sealing the power valve vacuum side from the fuel side cause this is a place where gas can get through and cause a rich carb.there's at least 2 different metering block gaskets and usually both are included in a rebuild or trick kit.Make sure you are using the correct one.also there's two different power valve gaskets that goes under the power valve when it's installed,make sure you are using the correct one here.picture window power valves use the one with the nibs on ID and the holed style the round ring gasket.Next, look at the underside of the carb and see the relationship between the primary throttle plates and the transfer slot,the slot should not be exposed and if it is then only the lower edge only.next check the secondary's to ensure they're closed all the way,just to make sure the secondaries aren't working at idle.
If these are correct then I'd drill a 3/32" hole in each primary throttle plate along the front side of the shaft 1/2 way between the shaft and the leading edge of the plate.
reinstall the carb and tune it for idle speed and mixture,always recheck your accellerator pump linkage when ever you have a float bowl off of a holley also.
test drive for 10 to 15 minites minimun to heat up the engine and clear the plugs out,then take a new plug reading and readjust the jets in the carb to correct the mixture.normally the 3310 comes with 71's and 76's.you shouldn't have to change more than 2 sizes up or down to get it right

lots of work but should fix it for you,good luck
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Old 06-20-2003, 09:54 AM
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I think I got the power valve gasket styles backwards? must recheck that sorry
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