MasterDave

Got a part throttle/off idle bog or stutter that I've been fighting. The new engine only has an hour on it. This problem was evident in the previous engine also but not as bad. See sig for engine/carb. This is what I've done. (1) I have the vac can on ported side.
(2) Installed .31 shooters (stock is .28). No help, backfire thru carb
(3) Installed larger main jets, prob gone but fouled plugs. Went back.
(4) Installed different pump cams, no help, backfire thru carb.
(5) Switched to manifold vacuum using the (B28 can). Prob 90% gone.

Where do I go now?

Dist specs; Initial timing is 12 deg.......mechanical is 21 deg starts at 1000 rpm, all in by 2500.....vac can has plastic sleeve limiting it to 9 deg.

The slight stumble is almost liveable now but would like to have it gone. Would I take the vac can sleeve off for more initial advance or mess with the Holley plastic accelerator pump cams for a quicker shot? Or put in a little more initial advance? :confused: Sorry for the length, just trying to get enough info here for ya.

Crazyhorse

How are the throttle shaft bores on your carb?

Vetterodder

Make sure that the accellerator pump starts pumping (not just the liinkage moving but that gas is actually squirting) as soon as the throttle moves. A delay in the pump shot when the throttle starts to open will cause a sudden lean condition which in turn causes a stumble and backfire.

MasterDave

Holly 650 DP was bought new and has about 1000 miles on it. I did open the secondary butterflys slightly due to the previous engine had a cam with very low vacuum, like 6-8" and would not idle very well. Maybe I should close them back down now as this engine (different cam) pulls a fairly steady 11" at 650-700 rpm?

MasterDave

Thanks, I did preload the accel pump a little but did not check to see if I get an immediate shot of fuel. Do I need an immediate hard shot or just need to see SOME fuel coming out? Got the whole cam kit to change the shot profile. Some look like they'll give a quick shot, dunno though.

Vetterodder

The rate of flow should be fairly consistant but I think a little less at the start and at the end of the travel would be normal. If the pump is in good condition (and yours should be considering it's age), there shouldn't be much "slop" before the arm movement results in fuel being squirted. You could try more preload to see if it helps but it sounds to me like you already have that covered. Is it more prone to stumbling or backfiring under gentle throttle opening than a rapid opening? While your stumbling/backfiring strongly suggests a sudden lean problem, and assuming that there are no vacuum leaks, I'm starting to wonder if the problem isn't elsewhere.

Quote "Dist specs; Initial timing is 12 deg.......mechanical is 21 deg starts at 1000 rpm, all in by 2500.....vac can has plastic sleeve limiting it to 9 deg."

I don't see where those specs would create an off idle problem or your backfiring but I suspect that they aren't optimum. Are you sure that the mechanical isn't starting before 1000 rpm? Most spring sets that I've used that were light enough to allow full advance by 2500 were also light enough to let advance start below 1000 rpm (my MSD is set up for all in by 2500 and mechanical starts at about 750 even though I idle higher than that). 33º total sounds a little low to me also but I don't have any experience with your heads so that might be optimum for them. 42º including vacuum also sounds low to me. 50º-52º is closer to optimum with decent fuel.

MasterDave

Thanks Vetterodder for the info. It stumbles way more under moderate/heavy throttle pressure, if I slowly roll the throttle on it's almost gone. Maybe I need to bump up the initial timing some to get closet to 50-52 total. I did (last night) install the black and then the green accel pump cams from the Holley kit and noticed no real help. I figured what the heck and started preloading the pump even more. With the black cam (which looks identical to the stock one with the exception of a slightly higher bump on the initial side), the engine started to come around. I preloaded some more and then had to reduce the idle some. Even better, but still stumbles some. Maybe I need the 50 cc accelerator pump? Would also like to know how to set the 2500 rpm timing (vac unhooked) to get to 36 deg without an adjustable timing light. Mine's the old Craftsman style. I have the timing tab on the drivers side not straight up like the newer engines. I do know that at 2500 rpm the TDC mark on the balancer is straight up. Thanks.......


[Modified by MasterDave, 7:06 AM 2/13/2003]

jerrybramlett

I know you want to just fine tune the combination you already have, but I would take a different approach. I would install a vacuum secondary carburetor like the Holley 80457 or an Edelbrock 1406. Perhaps you have a local car friend with one of these who would let you try it out for test drive. You could easily sell your double pumper on eBay to recover the cost of the other carburetor. In fact, I know a local drag racer who will probably trade you a near-new 1406 for your current Holley.

Anyway... here's what I'd do to sort out your current set-up.

First, I would check the location of the timing mark on your balancer to see if it is really indicating TDC with the timing tab you have. I would then set your total initial plus mechanical advance to be about 36 degrees. This is with the vacuum advance temporarily disconnected. If you can make it reach the max advance below 3,000 rpm, that's great. However, this usually requires weight springs so light that you get some mechanical advance at idle. That makes your idle speed vary. You can check total advance with a conventional timing light by putting a timing tape (available at any speed shop) around your balancer.

If the bogging problem is still there, I would test the carburetor to see if leaning the power valve would help. Baseline the carburetor before changing the power valve. I would go back to fully closing the secondaries at idle, using the original accelerator pump cams, and putting no significant preload on the cam levers. Then try a power valve with a number lower than what you have now. By pre-loading the accelerator pump levers you are reducing the amount of gas the pumps supply when you goose the throttle. This implies that the carburetor is supplying too much gas under heavy acceleration. Your previous test where the problem was worse with a larger shooter nozzle seems to confirm this conclusion.

I'm curious about the cam you're running. Would you please post the duration at .050" lift and the max lift specs?





[Modified by jerrybramlett, 9:48 AM 2/13/2003]

Ironcross

Get a carb with vacuum secondaries and a total lead 36-38 degrees. You probably do not need a double pumper. As far as a vacuum advance, modified engines can run without it. FI engines and L88`s do not have vacuum advance cans. :thumbs: Since this is in C1`s and C2`s the OEM style of distributor is with points. Dual points were the rage for these engines and all performance engine aftermarket distributors {Mallory} did not have any vacuum advances on them and most were street driven with out any problems at all. Refered to as centrifical advance. I have three early big block Camaros, A 70 Hemi Cuda, A couple of Drag {ski} Boats, 23 T roadster, and two Vettes without vaccum advance. However, I know this is not correct for everyone without the experience to do this correctly :lol:

I`m not one to say it can`t be done, it`s that the combination has to be correct and I can not see a DP carb with a low RPM engine. A hydraulic cam is not a high HP set up. :thumbs: He`s probably dumping more fuel than the engine can take, a very common mistake :cheers:


[Modified by Ironcross, 4:04 PM 2/13/2003]

JohnZ

I agree on getting rid of the DP and using a VS instead - DP's are a poor street carb, and difficult to tune for normal operation. I do NOT agree on no vacuum advance; vacuum advance is essential for stable idle, good idle cooling, progressive throttle response, and decent fuel economy. The last non-vacuum advance F.I. distributor disappeared at the end of C1 production, and subsequent '63-'65 F.I. engines used vacuum advance, for good reason. L-88's didn't have it because they were never designed for street operation - "race" motors don't need it, because they're at high rpm and wide-open throttle most of the time. Contrary to the Summit/Jeg's catalog hype, most "race" parts suck for street use, especially non-vacuum advance "whizbang" distributors.
:cheers:

MasterDave

Jerry, it's an Isky 270/280 Megacam. Specs are;

Intake timing Exhaust timing

Duration: 270 Duration: 280
Open: 23 Open: 72
Close: 67 Close: 28
Cam lift: .310 Cam lift: .323
Valve lift: .465 Valve lift: .485

Lobe center 112 deg. Overlap 51 deg. 1.5 rockers.

Intake duration: 221 deg. Exhaust duration: 232 deg.
Taken at .050 lifter rise.
Hope this is enough info......... :cheers:

Kid_Again

...whoa, just noted the bit about the double pumper...i have to pretty strongly agree that you should sell the dp and go with a correctly sized, vacuum secondary carb...at the very least, if that chokes the chicken, you should disable the secondary pumper......

...my L36 came to me with a double pumper and i ran rich ALL the time and i ain't no lightfoot.......

...there is no street application that suits a double pumper - they are not street carbs - no how, no way and it doesn't matter how many cubes you are running on the street..it is NOT a street carb...

...NOW that you know how i feel, i'll lie down 'till this passes :sleep:

...good luck, looks like good advice in this thread


[Modified by Kid_Again, 12:47 PM 2/13/2003]

Brutal64

I don't know, maybe I have some sort of freak motor. I run a DP, no vacuum advance, and a single plane intake. Initial timing 16 degrees, total timing 36 degrees, all in by 2500 RPM. This car runs like a scared ape, or is docile enough for the wife to drive. Idles at 850 RPM, and doesn't run any hotter than 190 degrees. So it can be done.


[Modified by frannsteve, 12:16 PM 2/13/2003]

MasterDave

Whooeee!! :yesnod: I was hopin' someone would chime in here and say it could be done. :thumbs: I'm gonna get a timing tape this afternoon and stick it on. OR....could I just put initial at 16 deg and run it? Nope, better get the tape, my timing tab only goes to 12 deg. :D :cheers:

Barry's70LT1

I just read all the replies. Here are my opinions, based on having a 750DP Holley installed for over 30 years.
The Holley DP DOES make a great street carb.. It will work great and be very reliable when set up properly.

The problem is that it can take awhile to get it tuned to your driving/engine.
If you are not prepared to take the time to understand a DP, and tune it, then by all means buy a vacuum secondary carb.

All the problems that people have discussed here, I think I have had most of them.

The off idle stumble/bog took the longest to find.
The problem on mine was that the transfer slots in the primary bores were not being exposed soon enough. I tried jets, power valves, big squirts, little squirts, however could not eliminate it.

Here's what I did.....
- Make sure the secondaries are closed. If open some, then the primary needs to be closed to compensate. This delays the exposure of the transfer slots.
- Make sure you have a squirt happen the instant the throttle is moved.
- A Holley has three circuits it uses.
Idle - Used for idle mixture up to about 1000 rpm.
Transfer slot - Comes in at the time the idle circuit is maximized.
Main Jets - Depending on the engine combo, not until 1500-2000 rpm.

If the transition from idle, transfer slot, main jet is not correct, you will get a bog when passing from one to another.

On mine, if I very slowly increased the rpm of the engine, at about 1000 rpm, the engine would stumble, then pick it up again at about 1200, as the transfer slot became exposed.

I managed to "tune" and fix about 80% of the problem.
To "completely" solve the problem, after exhausting all the options, I made the transfer slot "slightly" lower, so it would become exposed sooner.

The problem you are having may not be the same as mine, however I would "not" replace the carb. The DP is a fine carb.
Regards, Barry

Mel Foye

Look down the carb--eng. off. You should see the transition slots above the plates as being as long as they are wide. Same on both sides or a plate bent?
My guess is you will need to close the primaries down some. If so open the secondaries up the amount you closed the primaries.
The pump shot should squirt the instant the throttle is moved.
The shorter you make the pump screw the less TOTAL shot you will have. The longer the screw the more pump bore travel you will get. Getting to the longest screw setting while keeping the WOT clearance seems to work best. As you change pump cams you may/will need to readjust this screw and setting WOT clearance.
My plan of attack would be to check and correct as above if needed. Adjust the idle mixture mix and speed. Leave all else unchanged and drive it to compare. If better then focus on nailing the timing. Mel

Kid_Again

...this goes to show how good you guys really are, and i'm being very serious here, with your engines....you can actually make the wrong application function well for street use.....i didn't say my L36 didn't run well the DP, it's just obvious with all the tweaking that needs to be done to compensate for the over-rich condition this carb generates, there are much more appropriate solutions for the street.........

...you will not see advertising from holley that recommends the DP for the street.....some tend to ignore what the factory did because they say well, that was a long time ago and GM was only interested in making lots of cars, not the "best" HP cars for the street....if the General thought the DP was a good street carb at any time, it would have bolted it on in the interest of making even more money..........

....i take you point that your cars run very well with the DP....in my hands, over a number of years, with small blocks and big blocks, many different cam profiles, i have solved a LOT of performance problems by removing DP's (some being highly modified $1,000 carbs) and replacing them with a properly sized vacuum secondary carb - the only casualty in this process is usually the owner's ego and sometimes that's why i can't get them to change...

...i'm being honest here, removing the DP, in my hands, is the way to go

Barry's70LT1

I guess some of us fit into the "greedy" category.
We want it all. A good driving street machine with the performance of a DP carb...
Unfortunately, the penalty is the requirement to "tune" the carb to your application.
I enjoy doing this and don't view it as a problem.

I agree with you Kid_Again (100%), if obtaining the maximum performance is not of a major concern, and you want a good street performer, then the DP is not the way to go.
There are many carbs that would "bolt on" and work great right out of the box.


[Modified by Barry's70LT1, 11:47 AM 2/14/2003]

Mel Foye

Day in day out for the majority of cars and the majority of folks you are soundly correct Kid Again. The logic is correct. However intinsic to this hobby is the reality that "playing with the cars" is on going. Often have been it your spot where you know there is an easier road. See you around. Mel

JohnZ

Just measure the circumference of your balancer, and divide by ten; make a mark that distance clockwise from your existing index mark, and when that mark lines up with the "0" on your timing tab, that's 36 degrees :thumbs: