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Quadrajet Hesitation

E

ejkob

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"I have a 1972 Classic Glastro

"I have a 1972 Classic Glastron speed boat with a rebuilt 350 small block powering a 12JCB Berkeley jet. Over the years I have rebuilt the boat and this year added a new (re-built) quadrajet marine carb, and a new MSD marine distributor. I finally had the boat in the water and it ran fairly good up to half throttle. Beyond half throttle, the boat hesitated terribly up to full throttle where the motor evened out but just sounded poor. I set up the MSD advance with the lightest springs to full advance at about 2200 RPM but did not change the bushings. The old distributor was an HEI (vehicle) distributor and was in poor shape. What I want to know is will the timing cause this hesitation? How important is the timing and to what setting should I be looking for. I have not completely timed the motor yet, but am going to set it to 34 deg BTC at 2200RPM or full advance. Is 34deg enough? Do I also need to look at the carb and where should I start since the carb is a rebuilt marine quadrajet? Any advice would be great. Thanks"
 
"That timing curve is a bit ag

"That timing curve is a bit aggressive....

I'd shoot for 3000-3200 for the full advance point. Until you set the timing and get that dialed in, I wouldn't go modifying the carb. That much advance, that quick could cause some issues but frankly, I've never seen a distributor setup that way.

The typical q-jet hesitation (the quadrabog syndrome) is due to the air valve spring not being tight enough (or broken) or the vacuum break being damaged/disconnected.

There could also be a jetting issue if the carb was a generic rebuild."
 
"Thanks, I have a post at MSD

"Thanks, I have a post at MSD and they also suggest changing the springs, leaving the bushings as this gives me 21deg plus the 10 at idle will put the timing at 32 deg with a full advance at 3000RPM. So, what about that carb, any idea where I can get instructions on how to tune a quadrajet? Also, I run the boat in fresh water but do not have the thermostat installed. I am now reading that the engin might also be running too cool. What temp thermostat should I put in, 140deg or 160deg. The cooling comes from the jet direct from the lake. Please advise. Thanks in advance."
 
"Unless you do most of your ru

"Unless you do most of your running at 3000 or above, yes, put the t-stat back in. Idle and off-idle will be sooo much better as the manifold will warm up and help vaporize the fuel.

Two books come to mind - the classic is by Doug Roe and is still available. A newer one is by cliff ruggles - www.cliffhighperformance.com. Cliff also has a comprehensive kit and many other desirable parts. There are also several 'muscle car' discussion forums on the internet....as long as you understand the difference between marine and auto, they may be of benefit, too."
 
"Ed,

Check the metering rod

"Ed,

Check the metering rods on your rebuilt unit. Your rebuilt Q-Jet most likely has various parts from many different Q-Jets. The metering rods in Q-Jets are all different and are based on engine specs and in most cases are matched to specific engines. These rods allow you to precisely tune your carb for your specific case. This is the big advantage of Q-Jets over others. It's also a curse because it's make Q-Jets more complicated. As such it is always better to have the existing carb rebuit to ensure the metering rods are kept the same. The metering rods determine when the secondaries begin to open and how much total. This will affect the air/fuel flow. Sounds like maybe in your case the secondaries are not opening up fully, restricting the amount of mixture getting to the cylinders."
 
Thanks for the advice. I am r

Thanks for the advice. I am really hoping that the timing will correct most of the problem if not all of it since I really do not want to mess with the Qjet. But will take it a step at a time.
 
"greg:

"The metering ro

"greg:

"The metering rods determine when the secondaries begin to open and how much total." Hold On here...

The rods have nothing to do with the secondaries opening...its the air valve, the vacuum break, and the air valve's spring that control the opening rate. Also, it is the air valve that varies the position of the rods' tapers doing the metering."
 
"Mark,

You are correct. My

"Mark,

You are correct. My main point was the metering rods my be different on the rebuilt carb which will effect performance. I wasn't trying to cause confusion. I guess I was looking at how the system works together. Most Q-Jets use a vacuum operated piston to move the primary metering rods to control the air fuel ratio. Q-jets do have mechanical secondary throttle plates operated via linkage, but as you said, they also employ air valve plates above the secondary throttle plates. Those air valves are connected by linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts the secondary metering rods.

I wasn't trying to get into a heavy discussion on carbs. I was just merely trying to point out a problem with generic rebuilt carbs and that they may not work very well when installed. It's better to have the original rebuilt if possible."
 
"Agree with the drawbacks to t

"Agree with the drawbacks to the generic rebuild.

For Ed, it's probably OBE as it appears to have been done. As long as the rebuilder didn't hack it, it shouldn't be too hard to get tweaked to his needs. It'd be even easier if he still has the original.

I've found that the OEM ones can usually use some work in the idle circuit. If they are installed in a light ot medium weight vessel, they can usually be leaned out a bit, too.

Let's see what Ed reports back with once the timing curve is calmed down a bit."
 
"Lots of great discussion and

"Lots of great discussion and thanks! I wish I knew more about carbs and more so the Qjet. I did actually have the "old" card re-built, but I had other issues with the engine that I had drilled down to the old carb. Basically the old card at full throttle would case the engine to stall out... So this past year I put in a new marine distributor and a "new" rebuilt marine Qjet. By the way, the old Qjet was not a marine from what I could determine and the distributor was a "vehicle" HEI... So I figured I would start over with marine parts. I will certianly report back what I find out. Thanks again."
 
"I quess to say more, the moto

"I quess to say more, the motor in this boat is a rebuit 350 block a friend had used racing. The heads are polished 400's and the top end (manifolds and cam) are from the original engine. The original engine had a cracked block. But, the old distributor and carb are not marine build.. So bascially I am tring to get this thing tuned in. The boat is an old 3000lb fiberglass 18ft speed boat and back in the day, fairly fast. So it is heavy and old and fun. I have done a lot of work to make it more fun, but just want it to run great. So this weekend I will put in a new 160deg thermostat, put in the new distributor springs, time the motor to 32deg BTDC at 3000RMP and then hope the marine Qjet was rebuilt right. Otherwise I will start looking at how to tune a Qjet."
 
"Those Glastron hulls were the

"Those Glastron hulls were the ticket in their day.

You'll find the engine mods will have shifted the power band up a bit but coul be impacted by retaining the stock cam. If you know which cam was retained, that would help.

Putting on the marine grade parts is a prudent idea. The auto stuff will work until something goes wrong. thats were having the marine stuff really makes a difference (all safety related items).

You using an advance timing light or a tape on the balancer?"
 
"Really not sure what cam it h

"Really not sure what cam it has and almost put in a new cam and intake! Maybe next year. But to answer your question on timing, I plan on using an advance timing light. Will see how it all comes together on Friday. Thanks"
 
"Well, I did want to provide a

"Well, I did want to provide a little update! Had some time tonight and put in the stiffer springs in the distributor. This will give me full advance at around 3000RMP. With the "B" impeller in the Berkeley jet, the engine in the past has topped out at 4200RPM at full throtle. I also installed a new 160deg temp thermostat. I did some checking around the rebuilt carb and discovered that the vacuum actuaor that connects to the top plates of the secondaries was not working! Any vacuum applied to the actuator did not move the linkage. It acted as if the acuator input port was plugged. So I took a tiny drill bit and was able to drill into the port and freed whatever was plugged into the port. NOw the actuator works with vacuum applied. When there is a vacuum, the linkage now moves and will hold until the vacuum is removed. So, what infact does this actuator do? Could this cause hesitation at mid throtle? Friday I plan on timing the engine and then we will see from there."
 
"The 160F thermostats are the

"The 160F thermostats are the usual for FWC; if raw water cooled, they usually call for 140(143)F.

That actuator is called the vacuum break - dual purpose: 1) opens the choke slightly after the engine starts & 2) controls the opening of that "top plate", which is known as the air valve.

Could that cause the hesitation - absolutely! That's what I was getting at in the inital response. Do you know what sized you drilled the opening? Too much ( 0.025"+) and you'll see the same symptoms - don't worry, there's an easy fix."
 
"I run the boat in fresh water

"I run the boat in fresh water so yes it is cooled by raw water. I decided to go with a 160 deg as that is what I have read in other blogs. As for the opening in the vacuum break, I used a 1/16" drill bit (.0625) and quite frankly this bit almost fit in the original hole. I did time the boat today and well, it was really at retard until I dialed it in. Will see how it runs tomorrow. At idle (700RPM) the vacuum break is operating really well (considering it was not operating at all), the secondary air valve is closed pretty tightly by the linkage controled by the vacuum break. So, I may need a little advise on how to adjust the secondary air. Thanks for all your advise and I will certianly let everyone know how it goes. I will also try to post a few photos of my project."
 
Sounds like progress.

1/16&

Sounds like progress.

1/16" bit is too much. Lets just hope it was flashing from the casting that you removed and not the regulating hole that may have been enlarged.
 
"UPDATE: Got the boat out tod

"UPDATE: Got the boat out today and it ran... Seemed to be OK, but still some hesitation after half throttle. I am going to guess that the regulating hole that I opened may have been too much. I did however order a new vacuum break so we will see if that may make a differance. Will check the timing again to make sure it is on target. The motor seems to run great up to half throttle, but after that she seems to just labor, like she wants to take off, but there seems to be more noise than action. Anyway, will continue to push forward"
 
I'd concur with your thoug

I'd concur with your thoughts....not being there to hear and feel doesn't help at all.

Do you have any idea which secondary rods and hanger have been installed?
 
"Been traveling and have not h

"Been traveling and have not had much time to run the boat, but did want to provide an update. I purchased a new vacuum break and installed it since the one that I tried to repair was incorrect. I also adjusted the air valve spring approx 1/4 turn and will now see how it runs with the new adjustments. As for the secondary rods and hanger, I have no idea what is installed."
 
"I'd be inclined to start

"I'd be inclined to start with a 1/2 turn from no tension on the windup spring....you can always bring the hex key and the screwdriver to experiment. Just remember to keep good notes on what you changed and when.

The hanger will have a letter stamped in it; very close to the mounting screw. If you remove that screw and lift the hanger, both secondary rods will come out with the hanger. Each rod will have a two letter code stamped into it - in the 'fat part', towards the top.

Another item to note is the release time on the vacuum break. Easiest way is to apply full vacuumm and just remove the hose. Time it to see how long it takes to fully release. 2-2.5 seconds is a good starting point."
 
"Hey and thanks for the messag

"Hey and thanks for the message! I had a chance last week to get the boat back in the water and did do some adjusting on the spring. But after a quarter turn, the spring slipped off the hanger and so I lost all tension. So I did have to remove the carb to fix the tension. I did also look at the rods and the letter stamped is "AK" I think! Getting there but its taking time! I do have another problem as I am leaking oil somewhere. Thought at first it was the valve covers, but I am inclinded to think elsewhere! Hopeing its not the rear bearing and could be from the timing cover. Fun, Fun, Fun!! Will let you know!"
 
"Sorry, but I have a correctio

"Sorry, but I have a correction, the first set in the rebuit carb were "CH", and the ones in the old carb were "AK". I did put the "AK" in to see what might happen. So I guess we will see."
 
"They are both short tip rods,

"They are both short tip rods, both should be the same taper. In other words, there's no difference between them.

I wouldn't go further w/o fixing the spring.

Sometimes, just letting gravity work with some paper towels will help finding those pesky leaks."
 
"Thanks, the sping is fixed an

"Thanks, the sping is fixed and pre-adjusted one full turn from no tension. Will leave the "AK" rods in and great idea using paper towel. I will run the boat at home hooked to a hose to see where oil may be dripping. Will let you know how it goes. Thanks"
 
"If the towels don't help,

"If the towels don't help, one of the posters on here uses a dye with a black light...He says it is very fast. if needed, I'll hook you up."
 
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