Hey Guys,
I will keep the initial bit short and sweet - Having just dropped a 454 LS6 in place of the L82 SB, its now running and on the road but has a slight hesitation.

The ignition system was set up on the SB by talking to the guru himself Lars and ended up at 12 initial, 36 mechanical at 2,500 and 51 (i think from memory) total with manifold vacuum. It ran perfect on the SB, so should be somewhere near for the BB?

I am running a new Holley Street Avenger with vac secondaries and choke flap removed but choke tower still in place.

The hesitation is present at approx 2,500 to 3,000 rpm and occurs at anything above 1/4 to 1/2 throttle. The car holds back for a second or two and then goes like a scalded cat.

Other than setting float level I have very little knowledge on setting up/tuning a carb.

After reading various posts on the forum I am thinking about changing the pump cam as it seems to have been the cause of hesitations in the past (would this effect it as high up the range as 2,500 or just off idle?), it currently has the pink cam installed. Any help you can offer would be much appreciated. Having only covered 600 miles at the moment I have not yet run a full throttle or taken it above 4,000rpm.

Thanks in advance guys,


Lester

 

sounds like timing ad issue could be carb related with power valve but i would try alighter spring and weight first

 

Which spring and weight? Mechanical advance springs and weights on the distributor? That would suggest that your thinking the timing curve is not advancing early enough? I am all in on mechanical by 2,500rpm.

Planning to have a play with a few things at the weekend, figured I would try to get some direction before I start.

Cheers for the reply,
Lester

 

Do you have a copy of my SA Carb Hesitation paper? If not, send me an e-mail request for it:

V8FastCars@msn.com

Lars

 

Email sent - Thanks Lars... Who would have thought you could set up and engine in the UK from the USA, slowly as I tune each bit using one of your papers its becoming a Lars motor, trouble is I am doing you out of the beer each time....

Lester

 

So, is it a hesitation or a bog? If it's a bog, and the engine dies out for a second or two, then you have a problem somewhere and there have been a lot of good suggestions to follow to solve the problem. If it's a hesitation, that's what comes from a vacuum secondary setup. Those secondaries don't instantly open. There is a very slight time lag, and then they kick in. If you're running an automatic, then the vacuum secondaries are the way to go. If you're running a 4 speed, you'd be much happier with a double pumper carb setup. Someone will chime in here and say that your fuel consumption will suffer, but if you're driving a C3 and you're worried about mileage, you should sell and buy a moped.
My 72 has a 350 with a 4 speed, and I'm running a Holley 600cfm D.P. and the response is instant. Before that, I had the original Q-jet and then tried a Holley with vac. secondaries but there was that slight hesitation before the sec. opened up. Good luck and hope you find the solution.

 

i agree with duane 4238

 

The hesitation means it's lean. Also, you should be full in on the timing at around 36 depending on the chamber design of your heads, but 36 is a good number in most cases. If you have 51 total, you're way to high with the timing.

Go up 2 jet sizes in the front.

 

The 51 total he's talking about is full centrifugal with full vacuum on top of that. The 51 is a good number. For "total timing," most people refer to max centrifugal only, which is usually around 36. That's what he's got.

Lars

 

I installed a ZZ454 last year and had some stumbling issues. Try 18 degrees on your idle timing and another 18 on your advance. Also, the BB likes fuel, try a 50cc pump on your primary.

 

Was hoping someone wasnt going to say that. The Street Avenger was not my choice of carb. When I bought the engine I bought a new distributor, carb, manifolds, pulleys, etc as part of the deal so they were sensible money. The carb is the only part that I actually used and now its the only bit Im not happy with on the builb. lol. If im going to be changing the carb as the solution I guess I might as well have a play with this one and see where I end up, will be a good was for me to learn about fuelling.


Thanks for the confirmation that its somewhere near. I havent tried tweeking it eaither way yet. I will have a look at your carb paper first and make sure the carb is somewhere near.

Thanks for the suggestion, but I am guessing changing the timing to 18/18=36 rather than 12/24=36 mechanical wont effect this side of it as the hesitation/bog is occuring from 2,500rpm upwards by which time I am at 36 in either of the cases, or am I missing something there?
I am swaying towards it being fuelling, guess I will set up/tweek the carb if not start looking at getting more fuel through it.

Whats the general feeling on Street Avenger carbs? Is it worth spending the extra on a better carb or can I get the street avenger working properly for a street motor?

Cheers for all the replies and help guys, all the suggestions, giving me a few things to look at and try at the weekend.

Lester
P.S. Lars - did you get my e-mail for the carb paper?
P.P.S - I should have mentioned driveline, 5 speed manual transmission (getrag) with 3.08 rear end.

 

one of the best ways to determine what your engine is doing is to read your plugs. The plugs are the first indicator of whether or not you are getting too much or too little fuel. They can also diagnosis a vacuum leak and many other conditions. Drag racers use to shut down after passing a critical point and immediately check there plugs.

 

Plug colour appears to be good, But I have never shut down whilst driving and checked them, only after a drive and letting it idle for the shortest time possible, so probably not really a true reading of whats happening at the revs where the hesitation is present.
I have got Lars paper on the SA now and have a few things to check. I will report back with my findings at the weekend and go from there.



Lester

 

Only people that can gain anything from looking at plugs are drag racers after a 1/4 mile run and that is suspect now too because of the lousy gas.

On a street car checking the plugs means nothing as it runs anywhere between idle and WOT so the color ain't gonna matter. Unless they are pure white with specs of aluminum in them

 

For anybody confused 36 degrees before top dead center is a good timing number for most heads on chevys. The vortecs like a couple of degrees less. For old style chamber low compression heads with a popup piston for more compression, you need 40 or so degrees because the dome obstructs the flame travel. Gapping plugs from 40 to 45 is like adding one or two degrees timing(or going the other way opposite), since you have 12% more initial spark area, and so on.

You need full on timing by 2000 or so for street apps, and if you dont know what your timing is because the spring confuses you, set you initial timing where you want it, then take the springs out and use a dial type light or equivalent open it up to around 2 g's and see what you have.

For you folks having trouble with the vac secondary holley carb, never use the lightest spring, as it will bog. Whatever jets is in the carb, take them out and put a 71-72 in the front and a 78 in the back with the standard spring in the diaphram, do not take the check ball out. You can probably go slightly lighter to the purple spring without a bog on mild sb motors, but anything lighter will cause a bog unless you have a highly modified motor.

Save yourself the hassel and remember one thing, the motor makes the carburator work, not the other way around.

 

the 72/78 is for sb's, your mild BB would probably require 75/76 front and 79/80 rear.

for 4150 style carb.

 

BB,
Where's the check ball?
Thanks

 

in the diaphram body, but the easy off lid doesnt disturb it. take the whole thing apart and you'll find it, unless they changed the design.

 

Thanks, I was worried there for a minute!

 

Just figured I would post an update with the findings.

I have been working with Lars, who I have to say is an absolute legend and very patient with all the stupid questions I kept asking lol. The story goes as follows:

First job was to get the SA Carb working the way it should and rectify the faults with it that were present straight out of the box.
1. Pump cam lever not sitting on the cam, and sitting on the throttle return spring instead. This was removed, trimmed and the angle modified slightly so it sat on the cam rather than the spring. Prior to modification this was resulting in the pump not operating for the first few mm of travel.
2. Secondary throttle plates (butterflies) fully closed. There was actually daylight between the adjustment screw which acts as the stop. Carb was removed. Primaries then set at 0.020 of the transition slot exposed and then the secondaries set up to match the primaries. (to keep this right means all future idle speed adjustments are made equally on the primary and secondaries to ensure they stay in time/calibrated with each other).
3. Lean jetting - The standard jetting on my 870cfm SA is 78 primary and 82 secondary. This was changed to 80 primary and 88 secondary.

Steps one and two will apply to all SA carbs of any size, obviously Jet size will vary with carb size and application.

The carb was then refitted and all of the basics set up - Idle, Idle mixture, float levels.

The hesitation was greatly improved with this set up but was still present slightly but slightly higher up the range and present between 3,000 and 3,500rpm.

I decided to take another look at the timing. I ended up with 17 base at idle with no vac, 38 all in @ 2,500rpm, with the vac can pulling an additional 13. Lars closing words were '90% of carb problems are timing problems' - He wasnt wrong!

Hesitation is now gone completely and running sweet. A big thanks to Lars and to everyone else for their suggestions and help . Hopefully this post might come in useful for someone in the future (nothing worse than a long post with no conclusions!). I havent yet opened it up fully as I am in the final stages of running in. I am sure it will still require some adjustment, but for now its all good.


Lester