serious carb issues

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mshred

mshred

The Green Manalishi
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hey guys,

im having some pretty big problems with a carb that i had made for me. Its a 750hp carb and i have it on top of a mild 340 mopar engine.

My problem is getting the car to stay alive. Before the lowest i could get it to idle was 1200 (much too high for a street car...with the old carb it idled down by 700) so the guy i bought it from told me to close the secondaries all the way at idle, which has seemed to help the problem...However, now even when warm the car wants to die, especially after i rev it up, it just stumbles like crazy and shuts down unless i throttle it...When adjusting the 4 corner idle, unless i have the driver side primary screw turned in all the way, the stumbling gets worse...as of right now the transfer slot is not exposed, and i was told by the builder that in order to fix this problem i would have to drill bigger holes in the blades until the problem stops and i get correct idle...i dont know how well this solution would work and id rather not drill any holes

The timing is set at 15 initial, 36 total, with 13 inches of vacuum..The car is also backfiring quite a bit (thinking i might have bad valves, but im hoping its just a carb issue)...its got 10.5:1 pistons, but im running 89 octane and the cam specs out 230 duration at .50, 480 lift (single pattern)...There is no pinging, but im not sure if the backfire is a sign of the gas not being high enough octance...

Any help anyone can provide i would really appreciate...the car is running like a total bag of crap right now, and its pissing me off since this carb was supposed to be a set the idle and go kinda deal (though nothing really ever is in this hobby).

thanks!
Matthew
 
Sounds like the carb was modified off the car and needs to be tuned on your car by someone very familiar with them with some squirters, jets, power valves on hand to play with.
If it ran okay with old carb, you should be allright once you get this one dialed in right would be my best guess unless this carb is just plain too modified or worn out already.
Got any buddies good at tuning carbs with lots of tuning parts to play with?
 
well the thing is the carb was setup to my specs regarding power valves, jets, squirters, etc....a company like pro systems did it for me

in regards to tuning, my dad is actually really good at tuning carbs, and we have a ton of holley parts, but this one is really stumping us...we've tried everything, not sure what it is...the old carb was a 650dp that hadnt been run in years, it would idle, but it would die off rev as well...im hoping its not an engine issue and just a carb one
 
I've been dealing with carb problems for over a week and using the info fellow FABO members have given me is working.
You need to open up the secondarys till you see the slot.
Adjust you primary to lower the idle.
Check your vacuum at idle, sounds like you need to change the power valve, if vacuum at idle is say 6 you need to run a 4 power valve in order to get any adjustment on the mixture screws.
I think you need a little more intitial timming like 17
Good luck carbs can take a lot of time to get tuned, I'm no expert but all the members here have givin me advice that works.
 
If its 4 corner idleing, i would start off with a baseline on all 4 (all 4 even), then go from there, set throttle blades even F&R. Sounds to me that if its stalling after you rev it up, then its going "lean" in the idle curcuit, most likely from the idle mix screws not being far enough out.
 
When choosing a power valve, make sure that it is at least 2 # lower than the vacuum reading in gear.

As for idle mixture screws, I have found that after you turn all 4 corners out the same amount to get the best idle in neutral, turn them out about a 1/2 turn more for best idle in-gear. Then you can turn down the main idle screw to try for the lowerst rpm.

As was mentioned, the secondary idle slots should be visible, but not more than about .060". That takes a while of playing with to get it right for me.

Wouldn't hurt to totally disassemble the carb and blow through ALL the passages in all the parts with a can of carb cleaner and nozzle, including the main body air bleeds. (Watch your eyes.)

What about applying as much of the tune-up as possible from the 650, like power valve, pump cams and adjustments? You could need bigger squirters and jets than the 650 because the 750 wouldn't have a vacuum signal as strong as the 650.
 
It sounds like you already have holes in the throttle blades? If so, you probably don't need them with a 4 corner idle setup.
I just ran into this on a BBF I built for a customer using his "modified" carb. I could close both front and rear blades and it idles at 1400rpm.
I epoxied the holes up, opened the blades up some and now it idles at whatever I want it to. This was a mild 500HP build, so with a 4 corner idle carb it didn't need the holes in the blades.
That's what it sounds like is happening with your carb to me anyways....
Brian
 
kocuda said:
I've been dealing with carb problems for over a week and using the info fellow FABO members have given me is working.
You need to open up the secondarys till you see the slot.
Adjust you primary to lower the idle.
Check your vacuum at idle, sounds like you need to change the power valve, if vacuum at idle is say 6 you need to run a 4 power valve in order to get any adjustment on the mixture screws.
I think you need a little more intitial timming like 17
Good luck carbs can take a lot of time to get tuned, I'm no expert but all the members here have givin me advice that works.
Click to expand...

vacuum at idle is good, but i will recheck it...my dad is away on business for a few days, and he's the carb guru...im just learning right now, but these problems are definatly helping me learn faster lol...as far as timing goes, if i want to go higher im going to need to modify the distributor (Mopar performance smallblock distributor) which id rather not do...would the low timing actually be hurting it? even if i was to advance it, then the total would be higher wouldnt it?

JoeDust451 said:
If its 4 corner idleing, i would start off with a baseline on all 4 (all 4 even), then go from there, set throttle blades even F&R. Sounds to me that if its stalling after you rev it up, then its going "lean" in the idle curcuit, most likely from the idle mix screws not being far enough out.
Click to expand...

the guy who built the carb mentioned that holley's have a real touchy idle circuit, and your comment about the idle screws not being far out enough is exactly right...the driver side primary one is in all the way right now, but thats because if we turn it out at all, even starting with them all even, it dies right away...with it turned in all the way it runs, but hardly

ou812 said:
It sounds like you already have holes in the throttle blades? If so, you probably don't need them with a 4 corner idle setup.
I just ran into this on a BBF I built for a customer using his "modified" carb. I could close both front and rear blades and it idles at 1400rpm.
I epoxied the holes up, opened the blades up some and now it idles at whatever I want it to. This was a mild 500HP build, so with a 4 corner idle carb it didn't need the holes in the blades.
That's what it sounds like is happening with your carb to me anyways....
Brian
Click to expand...

Brian, i posted this same question on another forum and got a similar response as yours...he said that as far as he knows there was never a 750 offered that has holes in the blades except for annular booster carbs, which mine isnt, and he said i have a problem on my hands...my dad said the same thing, and you are saying it too...i have emailed the guy who built it telling him about this and im waiting for a reply, so we will see what he says...this is really frustrating me right now
 
well i talked with the carb builder about the issue and he said that there is a high probability that there is some sort of dirt or debris in that section of the idle circuit, so for me to clean it out, and if it still does it he will send me a new metering block...he said he is almost certain this is what it is, and others have suggested this as well..hopefully its an easy fix like this...thanks for all the suggestions and help so far, if it doesnt work i will be back here again asking for further help!
 
well i took the carb apart today, blew air through all the passages and put it back together. Set the 4 corner idle screws each at 1.5 turns out and fired the car up...started right up!!! and adjusted from there! the car is running strong now, and doesnt stumble off the revs either, but i do have a slight vacuum leak however, and the lower the rpm at idle the worse it gets. It seems to be coming from the throttle shafts as they did have some play to them when i had the carb apart, and when the car is running if i push up on them to snug them the whistling goes away...whats the fix for this? new baseplate?
 
mshred said:
well i took the carb apart today, blew air through all the passages and put it back together. Set the 4 corner idle screws each at 1.5 turns out and fired the car up...started right up!!! and adjusted from there! the car is running strong now, and doesnt stumble off the revs either, but i do have a slight vacuum leak however, and the lower the rpm at idle the worse it gets. It seems to be coming from the throttle shafts as they did have some play to them when i had the carb apart, and when the car is running if i push up on them to snug them the whistling goes away...whats the fix for this? new baseplate?
Click to expand...

This carb was just rebuilt & it has sloppy shafts WTF, When i was rebuilding holleys i always re-did the shafts with new bushings, with those loose shafts, you'll never get that carb tuned in "right", you may get by, but it won't be right, plus it'll always stay out of adj., At this point, i would just get a Billet base & be done, there set up for 4 corner, there nice, & it won't have holes drilled in the blades.

Heres one. http://cgi.ebay.com/ebaymotors/HOLL...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
 
JoeDust451 said:
This carb was just rebuilt & it has sloppy shafts WTF, When i was rebuilding holleys i always re-did the shafts with new bushings, with those loose shafts, you'll never get that carb tuned in "right", you may get by, but it won't be right, plus it'll always stay out of adj., At this point, i would just get a Billet base & be done, there set up for 4 corner, there nice, & it won't have holes drilled in the blades.

Heres one. http://cgi.ebay.com/ebaymotors/HOLL...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Click to expand...

hey Joe, your probably right about it never being "right" with the blades like this....today i had a tough time getting it to idle again and had to reset the screws once more. I emailed the builder asking him what to do next and if HE could send the billet base plate since in reality i shouldnt be having these problems, plus i paid for mods on the blades and such, so it should atleast not be sloppy...Ill see what he says and go from there...i really dont want to spend anymore money, but if it means the car will run better than i definatly will
 
I think JoeDust has something and I agree. It will never be right. Have you tried spraying water around the shaft ends? I bet the idle tightens up. Just ask the guy what the shaft to bore clearance was. I bet you get a big fat "uhhhhhhhh". Not that there's a set number really, but it will at least get him thinking.

On the flip side, if your guy was right and it is indeed dirt in the idle circuit, either way he should have caught it. Time to buy the book and do it yourself.
 
JoeDust451 said:
This carb was just rebuilt & it has sloppy shafts WTF, When i was rebuilding holleys i always re-did the shafts with new bushings, with those loose shafts, you'll never get that carb tuned in "right", you may get by, but it won't be right, plus it'll always stay out of adj., At this point, i would just get a Billet base & be done, there set up for 4 corner, there nice, & it won't have holes drilled in the blades.

Heres one. http://cgi.ebay.com/ebaymotors/HOLL...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Click to expand...
jet performance makes bushing kits or send it to holley or proform billet
that screwed all the way in it had to be a vacume leak
 
JoeDust451 said:
This carb was just rebuilt & it has sloppy shafts WTF, When i was rebuilding holleys i always re-did the shafts with new bushings, with those loose shafts, you'll never get that carb tuned in "right", you may get by, but it won't be right, plus it'll always stay out of adj., At this point, i would just get a Billet base & be done, there set up for 4 corner, there nice, & it won't have holes drilled in the blades.

Heres one. http://cgi.ebay.com/ebaymotors/HOLL...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
Click to expand...
jet performance makes bushing kits or send it to holley or proform billet
that screwed all the way in it had to be a vacume leak
 
bad440 said:
jet performance makes bushing kits or send it to holley or proform billet
that screwed all the way in it had to be a vacume leak
Click to expand...

the 4 corner screws regulate air not fuel? wouldnt the screw all the way in be leaner. seems you should have to screw it out to give it more fuel with the xtra air.someone school me i,m learning carbs.
 
The idle screws on Holleys only meter fuel. If things are tuned and the carb is sized right, the engine will die simply by turning the mixture screws all the way in. If it continues to run (4 corner idling) the float level may be a little high, the secondary plates may be open too far, or the power valve(s) may be opening, or a gasket leaking, the ide speed may be too high.... In primary circuit idling (no secondary screws) the secondary does flow a tiny amount of fuel at idle but it shouldnt be enough to run the engine.
 
eh guys,

i dont know the builder personally. He goes by DRC&I on ebay. I talked to a few different customers of his who all said he builds pretty good stuff, so i thought id take the chance, but the amount of money i have paid i shouldnt have had to take the whole carb apart. I emailed him last night and this morning again about the vacuum leak coming from the throttle shafts since thats something he should have checked before sending it out (something i unfortunatly didnt notice until now...and no, its not from me either) and asked him what the remedy would be, hoping he'll send me the base plate, since i shouldnt have had to go through all this b.s. for a carb that i paid to have "ready to run" in one way or another...he still hasnt answered

i set the secondary screws one and a half turns out and set the primaries the same, but only adjusted the primaries...seems to run better this way (had a major rich condition before...burned the eyes and throat, but thats gone now thankfully). The car does idle but that vacuum leak (severe whistling) is driving me nuts...the floats are set correct, the secondaries are opened just ever so slightly at idle (was idling too high before with how he sent it so i had to close em up), power valve is not opening, new base gasket and nonstick gaskets all the way around, idles around 750rpm

I figure that there must have been something in that secondary idle circuit, even if its only flowing a small amount of fuel. The builder told me that if there is a blockage there, because it cant get enough air, as soon as you turn the screw it will pull a massive amount of fuel, thus causing it to run like crap. I just hope that with changing the base plate (if thats what i have to do) i dont run into a whole other slew of problems...I have heard horror stories of guys changing their holley carbs to billet base plates and never getting them to run right thereafter
 
mshred said:
just saw this...would this work to fix the problem? how hard would it be to install? i have heard some say that putting in new bushings is quite hard and can require machine shop tools i.e. lathe

http://www.summitracing.com/parts/QFT-10-2/
Click to expand...

Just installing throttle shafts may not be the fix, the base is aluminum, its most likelt worn too, i would just bite the bullet & get a new base, NOW, AED has stock type base for around 100.00 bucks includeing 4 corner idleing, but for the little extra, you can get a billet like the one i showed you, installing bushings is not that bad, but the tool is about the cost of a billet base, Hmmmmm lol.
 
JoeDust451 said:
Just installing throttle shafts may not be the fix, the base is aluminum, its most likelt worn too, i would just bite the bullet & get a new base, NOW, AED has stock type base for around 100.00 bucks includeing 4 corner idleing, but for the little extra, you can get a billet like the one i showed you, installing bushings is not that bad, but the tool is about the cost of a billet base, Hmmmmm lol.
Click to expand...

hey Joe, thats not what i was hoping to hear lol (more money)...but i guess that makes sense...i've had another guy say the same thing, that its probably the baseplate itself thats worn, not the shafts...Hopefuly i can get a response today from the builder as to whether HE is going to send me a baseplate or not (imo he should be since that should have been checked) and then go from there...The car is SOOOO close to being on the road, and this carb issue is just a stupid headache i dont need to worry about

thanks all for the help and suggestions so far!
 
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