Factory Five Racing Forum banner

1 - 16 of 16 Posts

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #1
The engine in my roadster stutters when accelerating. Some background info: 1989 302 GT engine with a Holley 650 cfm ultra carb with mechanical secondaries and electric choke, stock cam, stock fuel pump with pressure regulator set at 6 psi, 4X4 headers, stock T5 trans, 3.73 rear gears, MSD 6AL ignition with MSD distributer without vaccum advance. The engine stutters when accelerating, usually occurs around 1500 rpm and lasts until 2500 rpm or so. After 2500 rpm it clears up and runs fine. It happens in all gears, and if I'm crusing around 2500 rpm it will stutter until I speed up. I was thinking maybe the distributer without the vaccum advance is the problem? I didn't build the car, I bought it and am trying to get it sorted out. Another thought is maybe the carb needs rejetted? Anyone else had a similar experience? Any thoughts are welcome. Thanks in advance.

ewh123
 

·
Registered
Joined
·
585 Posts
Sounds like a fuel issue. That RPM range is fed by the transition circuit. When you smooth out at 2500 you are probably just coming on the main jets. You adjust the transition circuit by changing the IFR (idle feed restrictor) jets (You'll need to drill and tap the IFR so you can use #6 set screws as jets) and / or the IAB (idle air bleed) (will also need to drill and tap for a #8 set screw).

Do some research. You will find plenty on the internet about how to do this properly. You also need to determeine if you are going lean or rich. Research will help best determine the symptoms of each.

You are likely rich. the holleys are usually set up a bit rich and also setup for a more aggressive cam than stock. With a stock cam you don't have as much overlap and will be more efficient (i.e. needs less fuel) in the lower RPMs.

It's not likely your accelerator pump, as that only affects things for a couple of seconds after you press the pedal.

Larry
 

·
probably insane but lovin life
Joined
·
550 Posts
mine was too lean...

I had nearly the same setup as you, even the gearing...
I at first suspected starvation/filter issue, or vac. leak.

Went up 4 jet sizes, hesitation/stuttering went away, all good.
 

·
Registered
Joined
·
585 Posts
You are not on the main jets at 1500 RPM

You can lean up your idle mixture to test the rich theory. Get the idle as lean as you can without killing it and see if the stutter improves or gets worse when you accelerate. You can do the same test rich.

The idle circuit flows at all RPMs. While the transition circuit is delivering fuel so is idle circuit. Adjusting the idle one way or the other will make a difference that is noticeable and will help in your diagnosis.

Larry
 

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #5
Thanks for the info guys, I'll try to lean the idle mixture first to see what happens, then richen it up if that does'nt work. I'll then try the idle circuit modifications next. These would be cheaper than buying a new distrubitor first and then still having the problem. I did buy a Holley rebuild and modify book at Jegs, this should help me to understand what I'm attempting to do.

Thanks again

ewh123
 

·
Registered
Joined
·
585 Posts
Your timing would have to be WAY off to create a stutter. Classic symptoms of poor timing are low performance, missing, pinging, even backfire, very poor running at ALL RPMs, etc.. I've near heard anyone complain of a stumble that was caused by timing. Classic carb tuning here.
 

·
Registered
Joined
·
585 Posts
Thanks for the info guys, I'll try to lean the idle mixture first to see what happens, then richen it up if that does'nt work. I'll then try the idle circuit modifications next. These would be cheaper than buying a new distrubitor first and then still having the problem. I did buy a Holley rebuild and modify book at Jegs, this should help me to understand what I'm attempting to do.

Thanks again

ewh123
That book will help some. Be sure to use Google to research this. There is a lot of great content out there that will help you to better understand how your carb works as well as how to make adjustments and read symptoms. SOunds like that book may be focused on rebuilding more than tuning and adjusting. Do this research before you attempt changes to the IFR or IABs. You want to understand what you're doing and what to look for.

The idle mixture test should help give you some clues.

Good luck

Larry
 

·
Snake Farmer
Joined
·
10,321 Posts
Another FFR builder I know (running a Holley as well), seemed to think that it was the fuel being sloshed back in the float bowl, due to acceleration, and starving the engine. No problems on smooth acceleration, just when he stepped on it hard from a stop, or travelling fairly slow.

I don't know if this is possible or not, it's just what he figured was going on..
 

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #9
AC Bill

I guess I should have given a little more info. The engine will stutter on light throttle acceleration, if I floor it it will briefly hesitate the accelerate normally, well normally for a car that will flat fly! If I'm under steady throttle around 2000 to 2500 rpm it will stutter until I give it enough throttle to pull higher rpm's. So it looks like you guys are pointing me in the proper direction. I'll have some time early next week to fiddle with it. I'll post what I find and what I am doing to correct the problem. Thanks again to all.

ewh123
 

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #10
I tried the lean out the idle mixture screws, no change. Then richened them up, no change. I then changed the jets, upping them by 4 sizes as dirtfarmer69 did. No change. Still has the stutter. I'm still thinking the distributor without the vacuum advance may be the culprit. I suppect the timing isn't advancing mecanically fast enough with the current distributor. Any thoughts on this? I'm going to MSD's website to see what I can find out about the distributor I have now. I didn't get any info on the ignition system when I bought the car. I'm hesitant to drill the carb air bleeds since adjusting the carb didn't have any positive results.

ewh123
 

·
Registered
Joined
·
20 Posts
While I know others will disagree, I personally have not had much luck with mechanical secondaries for street driving. It sounds like your issue may be with the accelerator pump. Holley has an cam/arm with a bolt that allows you to taylor the way the shot comes it. If you're getting a stumble under light acceleration but not under hard, the shot may be too big for light accleration. I guess look to see if you give a puff of black smoke while it's stuttering, i.e. too rich. As other point out, while transitionsing between circuits, the accelrator pumps help make up for the difference.

To check this, pull the air cleaner and look down the throat or the carb and pull the throttle open as you would when you are driving. you'll see the accelerator pump squirt fuel. By pulling the throttle open slowly and then again quickly, you'll get an idea how the pump is set. Actually, now that I think about it, with mechanical you've probably got a 2 pumps (i.e. double pumper).


A way to make sure it's the carb is to find a buddy nearby with an extra carb and swap that on and see how it runs.

One CRITICAL thing with tuning a carb is to make only ONE change at a time!!! mulptiple changes can lead to missing the issue.

Regarding your Vacuum advance, I doubt that is your issue. VA was added to increase fuel economy and improve emmisions. I highly doubt that is the reason for the stutter.
 

·
Registered
Joined
·
20 Posts
one more thing, 99% of the holley's I've dealt with like having the idle screws turned out 1.5 turns from seat.

there may not be enough idle fuel to affect a change over the accerlator circuit. also, there are different squrter nozzles you can get to help tune the accelerator circiut.
 

·
Registered
Joined
·
585 Posts
I tried the lean out the idle mixture screws, no change. Then richened them up, no change. I then changed the jets, upping them by 4 sizes as dirtfarmer69 did. No change. Still has the stutter. I'm still thinking the distributor without the vacuum advance may be the culprit. I suppect the timing isn't advancing mecanically fast enough with the current distributor. Any thoughts on this? I'm going to MSD's website to see what I can find out about the distributor I have now. I didn't get any info on the ignition system when I bought the car. I'm hesitant to drill the carb air bleeds since adjusting the carb didn't have any positive results.

ewh123
Then increase the timing manually and see if your problem goes away. Maybe this will help you better understand ignition timing's relationship to your problem.

Larry
 

·
Registered
Joined
·
5,950 Posts
I'm still thinking the distributor without the vacuum advance may be the culprit. I suppect the timing isn't advancing mecanically fast enough with the current distributor. Any thoughts on this? I'm going to MSD's website to see what I can find out about the distributor I have now. I didn't get any info on the ignition system when I bought the car. I'm hesitant to drill the carb air bleeds since adjusting the carb didn't have any positive results.

ewh123
This is pretty easy to check. I'm using a MSD Pro-Billet 8579 Distributor with mechanical advance on my 302. You can clearly see the advance working with a timing light, and this is necessary for setup. Base timing: Stock around 10 degrees BTC. 12 - 14 degrees for performance, if no pinging. Max timing: 38 BTC. Any more (40+) is typically only for racing and higher revs. To adjust: Set base at idle. Increase RPM in 500 increments. Should see timing moving slightly each step. At 2500 - 3000, should be at max. 3000 and beyond, timing should not increase any further (e.g. mech advance on stops). You will need to do some research and also test to find the best settings for your engine.

MSD sells a kit that includes various springs and bushings to set the curve, along with instructions that show how the various combinations work.

http://www.summitracing.com/parts/MSD-8464/
 

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #15
How I got into this situation is that when I bought the car it had a Holley 600 cfm vacuum secondary carb. It did the same thing and also leaked fuel out of the fuel bowl gaskets that I could not get to stop. The car has an electric fuel pump and no matter how I set the pressure regulator the carb would leak, even with new gaskets. So I bought a new carb and regulator. The carb is an Ultra series 650 cfm with mechanical secondaries and electric choke. So with the new carb I don't have any leaks but still can't get it to run right. I was under the impression these carbs were set up to run "out of the box" as Holley puts it. I am discovering just the opposite. I have around $600 in the carb and regulator and am getting a little frustrated. When I make an adjustment to the carb I only change one thing and then go for a test ride, then back to try another adjustment. I do change back each adjustment before adjusting something else. Thanks to all who have offered advice and input but I'm at a loss on how to get the car to run right. I have even requested help from Holley's tech page on their website and did not even get a response from them. With carbs being replaced with fuel injection it's next to impossible to find someone to repair it, and it seems I don't have the experience to fix it either.

ewh123
 

·
Blues n Brews
Joined
·
495 Posts
Discussion Starter #16
Sorry about my rant in my last post but it is frustrating. I can report now that I have been successful in getting the engine to stop stuttering. I tried a combination of things which worked. First I advanced the timing a little and also set the idle mixture screws to 11/2 turns out as suggested. This made an improvement but the engine was idling worse. So I backed the timing back just a tad and now it runs fine. I left the jets which I replaced in for now. The timing is advanced a little more than the stock settings but it seems this is what the engine likes. I watched the coolant temp and its where its always been at 195 degrees. The plugs look fine, just a light tan color. I guess I just had to get away from it for a while and get my mind back on track instead of letting myself get frustrated. But all is good now. One last thing to do which is fix the speedometer gear. Maybe tomorrow. Thanks to all who gave input, this really did help and I am grateful.

ewh123
 
1 - 16 of 16 Posts
Top