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Old 07-17-2019, 08:34 PM
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Carb tuners : I screwed it up

The back yard mechanic strikes again. Here it is from the beginning. ..

400 sbc compression is higher than it should be. 9.9 - 1 ..retro-roller.. RHS 202 cast heads ..Yes I built it and Yes I put too much compression in it. Anyway, I put a Summit brand 750 on it and was running it out of the box with a pretty bad hiccup at throttle delivery drove it like that for a couple years..when it took it would go like hell.. Idle was great, cruising at any speed was smooth. Then I got real busy and it sat for a few years. Now I'm back into it and put a red holley pump cam in. It helped quite a bit but still had that fart right off the line. I had an intake leak ( oil out the back ) so while I had that off I switched out the (all 4 ) jets from a 35 to a 40. NOW it doesn't want to idle great and stutters at any cruising speed. ( but when I mash it it get the hell out of the way ) I really don't know what I'm doing but am confidant in my abilities to understand and fix the problem.. I just need some good advice on where to go with this.. any takers ? Did I go too far with the jets ? should I only do 2 of them ? air adjusts ?? HELP !

Thanks

Matt

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Old 07-17-2019, 10:22 PM
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Big cam strikes again. The usual problem with a long duration cam is getting enough idle speed. The usual solution is to crank the curb idle screw down till it idles. What this does is to expose too much transfer slot. Then when you crack the throttle the mixture drops lean for a moment before main metering comes on and you get the off idle bog then it goes like hell when the mains come on.


The bog is actually from you using the transfer slot to assist the idle it pulls fuel from that circuit. So when you open the throttle that fuel isn't available anymore for the transfer function.


The answer is to get some idle air from somewhere else other than over cracking the throttle. This ranges from buying a carb with four corner idle circuits, to cracking the secondary stop screw a hair, to drilling a very small hole into the primary throttle blades.


Bogie
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Old 07-17-2019, 10:57 PM
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This is the Summit Racing M2008 750 cfm aluminum carb, correct?? No separate float bowls and metering blocks on each end of the carb, carb body and float bowls are one big casting, with a top lid held on by 10 screws??

If so, this is Summit's rethink re-introduction of the Holley 4010/4011 design from the late 1980's 1990's that was then discontinued by Holley.

Anyone who is familiar with Holley adjustments can deal with this carb, but you are going to have to be specific as to exactly what you did change....because I can't find a single specification listed for a feed restriction or discharge orifice or jetting sized at 35 or 40.
The only exception is this....accelerator pump discharge orifices on the primary side are .040".....but the secondary side is .031". Neither started at .035 as delivered from Summit, but .035" and .040" are in the Pump Discharge restrictions tuning kit you can get.
Main jetting is 77 front, 79 rear
Idle feed restrictions are .052" front and rear.

If you can post pictures, or give better description of exactly what orifice you changed or where it was located on the carb we can help you sort this out.
Be dead clear and certain about those orifice sizes too, as just .002" difference in size can make a big difference in tune depending on what orifice/jet it is.

Summit offers idle feed restrictor kits and Pump Discharge restriction kits....were the parts you changed from one of those tuning kits.??
Idle feeds kit #SUM-M08041
Pump discharge kit #SUM-M08040

Numbers 104, or 105 in this exploded view?? Is one of these locations what you changed??
https://static.summitracing.com/glob...08500vs_bp.pdf

Original pump cams are listed as Dark Green on the primary side and Pink on the secondary side....which one did you change to Red??

How old is the fuel you have?? Anything over 3 months old is no longer going to burn correctly and will give all kinds of tuning nightmares.

Listing all your cam specs will help greatly with what and where to adjust for much of the carb tuning feeds and restrictions, so list them up.
Do you have an idle vacuum reading off the engine?? This will also affect which direction to take on tuning.
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Old 07-18-2019, 12:14 AM
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Truth starnger than fiction, I had a Early '85 Crown Vic 302 with the variable venturi carb. Did everything right and it ran just as you describe. I pulled one of the small vac lines off of the manifold on the fire wall and it ran absolutely fine.
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Old 07-18-2019, 08:56 PM
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Eric .. Thank you for your response. Yes its the Summit carb described and yes I have the pump discharge tuning kit #SUM-M08040
I did not have the manual with me and apparently replaced the idle feed restrictions ( part #105 ) and NOT the discharge restrictions ( part# 104 ) refer to diagram you posted. So would it be accurate that I should put the original idle feed restrictions back in there where they belong and replace the pump discharge nozzles with one or two sizes up than what it came with in the box ? Boy do I feel stupid..

So that explains the rough idle and why it runs like crap at constant cruising speed .. So back to square one with just that flat spot off delivery.. Please advise

And one more thing I screwed up ( so that makes both things that I tried to better) The part I changed that I said was RED.. is the fast idle cam .. that is stock..I did not change that. It was the pump cam that I changed and I believe it was the largest one that came in the holley kit ( summit uses the holley pump cams ). It really helped but still has that flat spot just not as bad. I will check the color next time I get to the shop.. I think it might be brown but not sure... Could be a couple days..

Thanks again,

Matt

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Old 07-18-2019, 09:00 PM
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And thanks Bogie / Foper.. Clearly I gave bad information and my apologies for that.. you cant do anything with bad information...
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Old 07-18-2019, 11:36 PM
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I don’t remember all the colors any more, but with the cams, you have to think of slope and depth, and remember, from human point of view, the cam works upside down as it pushes on the pump arm.

Some of the cams have a gradual slope and some are almost instant on. The height of the curve is the final amount the pump arm is moved in its travel. You also have the adjustment on the pump linkage itself you can play with. I don’t know if the 80CC pump is still out there, but one of the things I did with my TA was put the 50CC pump on front and back. I was running the mechanical secondary double pumper spread bore on an 81 Turbo TA. I had to modify the manifold to accept the secondary linkage and add a spacer to get it to work.

Point being and for a different reason, I originally though the change of the carb would fix the poor performance, but because of turbo lag and poor static compression it still had flat spots all over the place.

For the off idle problem; I was able to correct with bigger pump shot. I had the front unit come on fairly gradual, but got the pump arm to full travel with a 50CC pump. For the rear pump I had it very aggressive and full travel on a 50CC pump.

For the off cruise to WOT problem; I changed the power valve to a large window 10.5 this got rid of the off cruise acceleration flat spot. I also changed out the OEM HEI to a 1979 unit that had vacuum and mechanical advance. With this I played around with the weights and springs. I ended up with heavy weights and medium springs, for cruise nights I put in bigger primary jets and light springs.

For the flat out running WOT; This final issue I didn’t fully understand back then and the issue was that the carb was just too small. Even with the 301 I needed an 850 when it wound up to 9 psi boost, I only was running a 650 CFM and anything above 4,000 RPM it started leaning out.

It didn’t really matter though because the crappy heads started to valve float around 4,500 RPM. I redlined one night racing a Z28 down Route 53 and the slapstick OEM shifter skipped from 1st to 3rd, I blew flame out my hood scoop, and that was that. But one thing I was sold on was the spread bore carb, small primaries for around the suburb driving with large secondaries when you put your foot in it go go juice.

Last edited by Foperfoauto; 07-18-2019 at 11:43 PM. Reason: line spacing
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Old 07-19-2019, 02:26 AM
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Quote:
Originally Posted by Fast 4 Door View Post
Eric .. Thank you for your response. Yes its the Summit carb described and yes I have the pump discharge tuning kit #SUM-M08040
I did not have the manual with me and apparently replaced the idle feed restrictions ( part #105 ) and NOT the discharge restrictions ( part# 104 ) refer to diagram you posted. So would it be accurate that I should put the original idle feed restrictions back in there where they belong and replace the pump discharge nozzles with one or two sizes up than what it came with in the box ? Boy do I feel stupid..

So that explains the rough idle and why it runs like crap at constant cruising speed .. So back to square one with just that flat spot off delivery.. Please advise

And one more thing I screwed up ( so that makes both things that I tried to better) The part I changed that I said was RED.. is the fast idle cam .. that is stock..I did not change that. It was the pump cam that I changed and I believe it was the largest one that came in the holley kit ( summit uses the holley pump cams ). It really helped but still has that flat spot just not as bad. I will check the color next time I get to the shop.. I think it might be brown but not sure... Could be a couple days..

Thanks again,

Matt
Yes, put the idle feed restrictions back to stock, as the carb was previously happy at cruise with them, and increase the size of the pump discharge restrictions to fix the off idle/throttle move to WOT lean hole that is the stumble you've got when mashing it to the floor from low speed or from a stop.

If you added the Brown pump cam, you'll need to change that back to something smaller...the Brown and the White cams are for use with the 50cc pump upgrade or on the Holley dominators which come with 50cc pumps on them....those two cams have too much pump travel for 30cc pumps and bottom out the pump stroke, which can be hard on the linkage and the pump diaphragm.

A little info on pump shot....the cam decides how much volume the shot will have.....the pump orifice restrictions determine how fast or slow that amount of fuel is discharged - the larger the orifice the bigger the initial pump shot is but the less time it lasts.
By having both cam and orifice adjustable both the rate of discharge and the volume discharged are both adjustable....a good thing

Backfire "sneeze" or stumble in the intake manifold when throttle is moved quickly from off-idle or to WOT is a lean hole not being covered by the size or duration of the pump shot in almost all cases.

Another adjustment which can affect this is the idle mixture needles.... a lot of guys try to adjust mixture to highest vacuum at idle, or "best lean idle" like many have learned or read about for tuning....which only works on stone stock smooth idle engines....nearly all performance engines with any type of larger duration camshaft will be too lean at that setting and can add to the problem you are describing.

I typically tell guys who are vacuum gauge tuning a performance engine that if adjusted to best lean idle, then go back out richer by 1/16 to 1/4 turn each on the idle mixture screws.....the increased overlap and idle mixture dilution that results on a performance engine needs a richer idle calibration to avoid hiccups.
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Old 07-22-2019, 07:45 PM
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Quote:
Originally Posted by ericnova72 View Post
Yes, put the idle feed restrictions back to stock, as the carb was previously happy at cruise with them, and increase the size of the pump discharge restrictions to fix the off idle/throttle move to WOT lean hole that is the stumble you've got when mashing it to the floor from low speed or from a stop.

If you added the Brown pump cam, you'll need to change that back to something smaller...the Brown and the White cams are for use with the 50cc pump upgrade or on the Holley dominators which come with 50cc pumps on them....those two cams have too much pump travel for 30cc pumps and bottom out the pump stroke, which can be hard on the linkage and the pump diaphragm.

A little info on pump shot....the cam decides how much volume the shot will have.....the pump orifice restrictions determine how fast or slow that amount of fuel is discharged - the larger the orifice the bigger the initial pump shot is but the less time it lasts.
By having both cam and orifice adjustable both the rate of discharge and the volume discharged are both adjustable....a good thing

Backfire "sneeze" or stumble in the intake manifold when throttle is moved quickly from off-idle or to WOT is a lean hole not being covered by the size or duration of the pump shot in almost all cases.

Another adjustment which can affect this is the idle mixture needles.... a lot of guys try to adjust mixture to highest vacuum at idle, or "best lean idle" like many have learned or read about for tuning....which only works on stone stock smooth idle engines....nearly all performance engines with any type of larger duration camshaft will be too lean at that setting and can add to the problem you are describing.

I typically tell guys who are vacuum gauge tuning a performance engine that if adjusted to best lean idle, then go back out richer by 1/16 to 1/4 turn each on the idle mixture screws.....the increased overlap and idle mixture dilution that results on a performance engine needs a richer idle calibration to avoid hiccups.
SO I made it back to the shop this afternoon and put the original idle feed restrictions back in and replaced the 35 pump discharge nozzles with the 40 on the primary side.. . Here is where it got weird for me.. when I pulled the secondary side out I noticed there were NO nozzles in there at all. It IS threaded but no nozzles. So it came out of the box like that. I replaced the idle feeds and put it back together without any discharge nozzles in the secondary.
The result was back to square one. Same flat spot ( backfire ) off the line but super smooth idle and cruising.
The pump cam I have in there is BLUE not brown.

Any advise on where I should go from here ? Why are there no nozzles in the secondary bar assembly Could that be part of the problem ?

..I feel like we are getting closer..
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Old 07-22-2019, 11:19 PM
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The lack of rear nozzle orifices could be a big part of the problem.
Looks like they just got left out by mistake during the initial build of the carb, and the listed size is .040" on the front, and .031" on the rear according to the current ad listing in their catalog.

You have the Blue cam on the primary side. correct??
Blue cam is good, but needs to be in the second hole to really be that much bigger than the stock cam was.

Another you can try is the Green cam, in either hole.

What cam is on the secondary side??

I probably should have asked already...this is the double pumper, right?? Or are we dealing with the Vacuum secondary model??
Because if it is vacuum secondary there would be no orifices installed in the secondary cluster. The Booster Cluster would be drilled for them because the same cluster is used front or rear, but the passages leading to it in the carb body wouldn't be drilled.

If this is the vacuum secondary, then we have to approach tuning a little differently.

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Old 07-23-2019, 06:43 PM
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Yes it is the vacuum secondary. I apologize again for the confusion.. I did find the manual last night and gave it a good once over.. It appears as though I should be experimenting with the spring inside the secondary diaphragm. Do you have any other suggestions ? I do have a vacuum gauge if you think I should go that route but the idle is great and the cruising is great at steady throttle and no trouble at WOT. Just that horrible stumble /backfire/ sometimes even stall if I hit it hard enough.. That being said, if I roll into it steady it also is fine.. but that's not much fun.
Do you have a suggestion in what color spring to start off with ? or any other suggestions ?

Thanks for bearing with me.. I did know my way around the carb at one time but its been a few years since I've touched it..

Matt
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Old 07-23-2019, 09:18 PM
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I don't know what spring comes in it stock, but for most 750cfm vacuum secondary performance applications I find the Purple spring is the most common one I use.

Still, a stumble like that from just quick move to WOT says work on increase idle feed fuel(idle feed restrictions) or larger pump cam and pump nozzle orifices.

Another thing I forgot to ask...there is absolutely zero clearance anywhere in the accelerator pump linkage at idle, correct??
Even the slightest clearance delays the pump shot and causes a stumble.
Nozzle should discharge fuel with even the faintest throttle movement.

Verify float levels too, low fuel level in the bowl can lead to stumble problems also.
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