Glad to hear it worked for you so far - you do work quick though.

Back to the stumble, first guess is too lean and/or not enough timing.. You are probably moving a lot more air at all RPM ranges, plus you can run a lot more inititial timing with these heads. For comparison, I run around 23-24 at idle, 36-37 total timing all in by 2500. Run my vac advance ported (no vac advance at idle) - vac source from the side of the metering block.

 

I can't stand to be down long!!! Besides, shows and cruise ins start here in a couple of weeks so I gotta be ready!

I hadn't thought about upping the timing that much. I did try upping it one degree when I first fired it up to get the idle back where it was. I run a Mallory comp 9000 with mechanical advance. With the other heads I had to go with 12*-13* initial which gave me 35* total, any more than that and the spark knock was pretty pronounced, any less and it ran like a slug. I'll try upping the initial timing and see what that does, if it works I can reset the advance to keep the total at 36-37. I did notice that the engine seemed to like more intial timing when I first fired it up and started adjusting it but until I went on the shakedown run didn't realize I had a little tuning issue to resolve.

Last time it was on the dyno the AF ratio was a steady 13-1 so I'm reluctant to mess with that till I'm sure it needs fattening up.

Could be an initial timing issue for sure though because once it gets to about 3 grand it really starts pinning me back in the seat!!!:thumbsup:

And thanks for the help, and the deal!!!:thumbsup::beers::thumbsup:

 

Looks like you are onto it. These modern heads can tolerate a lot of timing early, yet don't need as much total as the early closed chamber heads.

 

As mentioned James, give it as much initial timing as it can stand and make up the balance of total with centrifical.
If that doesn't get rid of the stumble, increase the carb squirters. Is it a double pumper carb?
You may even have to go with 50cc pumps.

 

It's a 750 Holley that I added a rear jet plate to and cut the choke horn off, not a double pumper. It worked pretty good with the 290 heads I was running and as I said the AF ratio was dead on. Even so this motor has always begged for more carb but I'm trying to keep the fuel milage from dropping out the bottom by using the smaller carb.

It may very well need the bigger pump now though. I'm going to give the initial timing a bump up first and see what that does before making changes to the carb.

Thanks for all the input guys!!!:thumbsup:

 

What Bill said, once you get your timing straight.

The shooters are cheap, like $5-$10. Depending on what number you have and how bad the stumble is, you'll want to go up 3 to 6 numbers. Trick to getting them back on: put a piece of electrical tape on the shooter so you can drop it in place, once screwed down pull the tape off. Nothing worse than losing a screw down the carb.

 

James,you need to 1st get your ign timing curve dialed in a little better before going after carb issues,timing 1st then carb tuning 2nd because timing issues can create seemingly carb issues at times,not all the time but at times yes.

Most aftermarket perf cams need approx 16-18 initial timing ,then you recurve the dist for 18 mech in by 2800-3krpm.

Then 16-18 initial + 18 deg mech = 36-38 total.(if can tolerate 38,if not us Kemco booster or back off timing to 16 inital)

And for street you need vac adv,with mild cam ported is ok but with fairly lrg cam for size motor its installed in you may need to run vac adv from full intake vacuum all the time to help it idle better. It just depends on setup when it comes to running ported or non ported for vac adv,8 of 10 street motors with perf cams like non ported/vac all the time for vac adv due to longer cam duration requiring more timing for better idle & also may run a few deg cooler on hot day in traffic with non ported.

If your running a mild to fairly hot aftermarket perf cam (like most of us ) running the inital timing approx 6 deg retarded like your at with only 12 deg inital when it should be approx 18 deg is substantially retarded resulting in a little more fuel used,little more sluggesh throttle responce,less power/trq,and possible motor running a smidge hotter too.

If it pings with a full pref ign curve like i suggested then try 93 oct fuel if your not already running it. But if you'r currently running 93 fuel and it runs/feels better with the additional 6 deg inital timing but still pings you have a few choices without getting into anything major to lower comp.

1,If it only pings when under a load/WOT back of the timing 1-2 deg at a time untill the ping goes away with 93 fuel,that will loos some fuel mileage ,throttle responce,power ,etc.

2,If it only pings while at light load/part throttle lowload cruise and when lightly accelorating get vac adv that has less adv or mechanically limit it/reduce its total adv by a few deg because some vac adv is still god ifyou can get it to stop the ping.

3,Simply run kemco lead supreme octane booster esp if it's not a dialy use vehicle . The additional cost to treat 18 gals of 93 fuel to get 97+ octane is approx 55-60 cents per gal if you buy 2 cases /24qts ofKemco at the discounted price. In that case using the kemco booster will retain the full power perf ign curve and also give a few more hp if the motor truely needs the additional octane.

Using the kemco is the easy way to run a fully perf ign curve to retain full power in many setup's (that ping at times) without having to retard the timing to stop ping reducing fuel mileage,loosing overall perf from idle on up,and running hotter in traffic.

Now if your carb seems to still be acting up after getting the timing curve dialed in better then by all means dig into it now that the timing is squared away.

NOTE:ENSURE MOTOR IS AT A LOW 600-650RPM IDLE WHEN SETTING THE INITAL TO ENSURE THE MECH ADV IN DIST ISNT PARTIALLY ACTIVATED UPSETTING INITIAL TIMING ADJ,ALSO ENSURE VAC ADV IS UNHOOKED/PLUGGED WHILE SETTING INITAL OR CHECKING TOTAL.

Scott

 

Before the head change it would only ping under a light load part open throttle. That's with the timing at 35 total/12 initial using the mech adv Mallory, I do not have any vacumn advace and and don't want any. I could put 5 gallons of VP110 to a full tank of 93 octane gas and the problem went away. I tried a full bottle of the Kemco additive to a tank of 93 octane gas with some improvement but still some pinging.

On my shakedown run yesterday I could lug the engine harder than I ever have with the total at 35-36 and it never pinged at all but it is still on the tank with the Kemco in it.

So I'm going to up the timing, run out this tank of gas, and go from there.


Thanks!!!:thumbsup:

 

NP James,it seems you have a real good handle on how to tune/dial in your motor so let us know what tune works out the best timing & fuel wise to get rid of the ping if it detonates again and also let us know what gets rid of the stummble if adv the base timing doesnt help.

BTW,when the motor pinged with kemco what was the mix of booster to 93 fuel ?

1qt or # ounces kemco to how many gals of what grade fuel (91 or 93)?

I know 1 qt kemco to 18 gals 93 fuel is roughly 97.5 octane (+ - .5) so if you were close to that mix & octane lvl your motor must have had fairly high compression/cyl pressure to still ping in the past with that much octane/97.5 .

Good luck....Scott

 

I just got though setting the Mallory up to give me 20* initial and 36*-37* total and I'm bettin the hesitation is gone. Just blipping the throttle now the engine wakes right up and it's got a very healthy rumble at idle. I wont get to make another shakedown run till tomorrow though.

I used a full bottle of the Kemco to 18-19 gals of gas like you figured, that's with 93 pump gas. It did help but not enough. That's why I jumped on the ZZ502 heads from Joe when I saw them in the classifieds. I figured the only way to lose enough compression to stop the problem was another set of heads with more than 103cc like the 290s I was running. I'm sure I had at least 10.5-1, I don't remember what the DCR worked out to be but the 280/288 Lunati is as big of a cam as I want to run.

I'll keep you posted as the tuning progresses!

Thanks for the input.:beers:

 

I would run as much initial as I could right up to where the motor will still turn over without issue when hot. You're at 20, could probably go to about 24 (maybe more, but you have to trial & error). Then back off mechanical advance to keep the total to 36-37.

 

I figured 20 to be a good starting point. It's cool here now and the 160* thermostat is keeping the temp right there. When the weather gets warmer I'll see what it can take. I'm still pushing 9.6/9.7-1 compression so I don't want to get to much till I drive it a bit.

Thanks for the input!!!:thumbsup:

 

James,your timing is setup just right with 20 base/36-37 total,motor should feel very responsive with the additional base timing and ping should hopefully be gone with the larger chamber heads to lower comperssion .

As suggested in an above post your motor may in fact respond ok to a few deg addtitonal base timing but since your already at 20 deg base (which is plenty with your cam) i would not push it any more on pump gas and to avoid ping or hot start issues with more strain on starter due to base timing being pushed over 20 deg. If your looking to get every ounce of perf out of it try a few more deg base timing as suggested and as alway's listen for ping & over working the starter for hot starts and back it off if you you suspect either of those problems with over 20 deg base..

Let us know how it runs when you give it a shakedown run.

Scott

 

Update, 2nd shakedown run.


I've moved the timing all the way to 24 intial about 38 total and Ol Paint loves it!!!

It's about 65* here this afternoon so it might not be able to take that much in the 100* summer or with less than 93 pump gas but I haven't heard any ping/knock at all so far!!!

It could probably stand a little more from the accelerator pump now but I'm going to drive it some more, run this tank of gas out and see what happens with some 87 pump swill.:thumbsup:

 

Sounds good. I can hear it roaring from here. You really won't know for sure until you get into the dead heat of the summer, but I would think you will be fine. Key is the modern heads like a lot of initial but don't need as much total. My guess is you picked up a good 50HP with the swap, not to mention the throttle response from having the timing up where it should be.

Bet it feels like you dropped another 100CI under the hood!

 

It's pinning me in the seat harder for sure!!!:beers:

 

Big James glad to hear it responded well to the additional base timing,i knew it would.

Anyway,dont jump sall the way down to 87,will likely ping ,try some 91 1st and if thats ok then try try 89 but if i remeber correctly your motor is at low 9's for compression so i would not chance not hearing light detonation running 87 or even 89 in hot weahter.

I have seen 1st hand what detonation can do to motors and it aint pretty,brokend pistons,bent valves,ruined cylenders & heaqds,etc so why risk it.

If your setup is in the 9's compression i would not chance less then 91 in hot weather and maybe 89 in coole weather by thats pushing it IMHO.

But i would keep the base timing at 20-22 max to avoid any starter issues and or detonation.

Scott

 

I hear ya Scott!!! When this tank is about gone I'm going to get $5 worth of 87, if it pings then I'll step it up a grade and try again to find the threshold.

I'm well aware of the damage detonation can do. Learned the hard way with a stroker Harley I had. Stayed on the throttle till the pingging stopped. The rear piston looked like I had blown a hole in it with a tourch, needless to say shrapnel filled the bottom end killing everything in it.

I'm not going to crowd the throttle more than I need to till I know what it can take.

So, any speculation as to what will happen to my fuel milage now, driving conservative with the old combo I could get almost 15mpg on the open road ?

 

Keeping your foot out of it, you have two things working in your favor. Running a lot more timing is more efficient. Better breathing produces more power per throttle input so less throttle should be needed.