Mid-Range Hesitation Flat Spot

I have flat spots in acceleration when I accelerate from 2K+ RPMS to WOT. I have no problems accelerating from idle or 1,800 RPM to WOT in or out of gear. The boat is a 1994 19? Chaparral Bow Rider with a GM 5.0LX engine 4bbl. The carb Rochester QuadraJet purchased Nov 2008 from I-5. I have replaced the coil, plugs (gapped at 0.035?), wires, cleaned arrestor, fuel pump, rotor & cap, both fuel filters, and changed the oil. I had the flat spot before I changed all of this and I am still getting a flat spot in acceleration. The choke is almost closed 3/16? from Horn Wall when starting and the boat starts fine with a few pumps of the throttle and with the throttle at half open. The boat starts fine and warms up within 5 minutes and the choke fully opens. I can obtain WOT and boat runs within spec 4600 RPM at 45MPH. The accelerator pump is squirting fuel in both sides of the 4bbl carb at good stream. With the boat in the garage I can pump the accelerator pump a few times and fuel is squirted around the 4th or 5th time it empties the fuel bowl.

I can steady up the throttle to WOT with no problems. I also can pump the throttle to WOT with no problems while in idle. The problems occurs under load i.e. in the water in gear moving at 2K+ RPMs. Idle mixture screws out 1-1/2 turns. Pretty smooth idle. Plugs looks good slightly tan in color. I recheck plugs wires to make sure they were on correctly. Prop is 14 x 19 Mercury Marine Tempest SS.

I have not checked the anti-siphon valve (replaced 2 years ago) at the fuel tank or the pickup screen in the tank.

Any help would be greatly appreciated,
Greg

Re: Mid-Range Hesitation Flat Spot

what kind of carb? if it is a q-jet it is possible that the secondary air valves are set too loose. try taking off the spark arrestor and watch them under acceleration when under load. if they slam open they will need setting, they should open slowly as rpm increases

Re: Mid-Range Hesitation Flat Spot

Yes, Q-Jet. Thanks for the suggestion I will do that tonight while I am enjoying the lake tonight. I will report back.

Re: Mid-Range Hesitation Flat Spot

Not sure what ignition you have but I had a similar problem on a 1999 5.0 gl (Prestolite )and it turned out to be rusted advance springs/ weights on the distributor. Take a look and see how they appear.You can try spraying the whole mechanism w/ wd 40 or similar and working the weights back and forth. I had to replace the springs and the problem went away

Re: Mid-Range Hesitation Flat Spot

I have a Thunderbolt IV ignition it has 87-892150Q02 ignition sensor I do not remember seeing any springs or weights in the distributor.

Re: Mid-Range Hesitation Flat Spot

Update I found that the secondaries were opening up as soon as the throttle was advanced quickly to WOT and the engine bogged and then the secondaries would close back the engine would accelerate. So I adjusted the windup spring for the secondaries and it helped but it hesitation just occurred at a higher RPM. So I adjusted a little more again it helped where the bogged occurred at around 2,500 RPMs. Then I tried to adjust more and the windup spring came off the secondary arm which required to take off the Carb.

I decided to hold the secondary butterflies closed with a wrench and at 2,500 RPMs quickly to WOT and their was no hesitation. I cannot tighten the windup spring any tighter or it will pull off the arm again.

Any ideas besides making it so the secondaries do not open at all i.e. tying them shut?

Thanks in Advance

Re: Mid-Range Hesitation Flat Spot

check the vacume diaphram on the side of the carb with the linkage that runs to the secondary air valves it should hold vacume indefinetly and release slowly, the spring setting is usualy 1/2 to 3/4 turn from when the spring contacts the arm with the air valves closed

Re: Mid-Range Hesitation Flat Spot

ok I just had a brain storm, when the air valves open is the secondary rod hanger actually lifting the rods? I have seen the cam that lifts it break and fall out ,then when they open there is no extra fuel, check it out

Re: Mid-Range Hesitation Flat Spot

Yes when the valves are open the rods are being pulled up. I did some more testing last night and found that between 2,000 and 3,200 RPMs is the range that has a flat spot or hesitation. If I am cruising at 3,400 RPMs and I advance the throttle to WOT it takes right off with no hesitation.

Still not sure what is causing the problem.

Re: Mid-Range Hesitation Flat Spot

I might have found the problem. The carb's secondaries had DL rods when the manual calls for CL. After looking at the specs on both the DL rods are much different on the closed position to the 70? position. I ordered the CL rods and will install them by this weekend and report back. I also removed and checked the anti-siphon valve which was working and clean. I reinstalled it with pipe dope on the threads and two clamps and a new 3/8" fuel line to the f/w filter. The only things after that I have to check is timing and float level. I have not checked them yet because I do not have a float tool or timing light.

Re: Mid-Range Hesitation Flat Spot

The CL rods did not make a difference. Tonight when I was out on the lake I played with the accelerator pump and found that it only sprayed fuel through half throttle. I pulled a pump out of my old carb and it looked different. The cup is bigger on the old one. I pulled the top of the carb and put in the other pump and it sprayed fuel through the whole cycle of the throttle. I cannot wait to test it now but it is a little too dark. I will wait till Saturday.

Re: Mid-Range Hesitation Flat Spot

Hopefully the accelerator pump will cure it. If not, have you checked all the carb adjustments per the factory manual (under the "Adult Stickies")? Check the Air Valve Dashpot and make sure it's working right. If it's still bogging, the Rochester section in the V8 manual has the handwritten notes shown in the pic below. Sounds exactly like your situatiuon. I don't know where the notes came from, but it's an easy adjustment and worth trying if nothing else has worked. Good luck!

Re: Mid-Range Hesitation Flat Spot

Yeah I have played and checked the dash pot and adjusted the tension on the secondary butterflies many times.

Re: Mid-Range Hesitation Flat Spot

Sounds like you need a new primary pulloff, as this will slow down the opening rate of the secondary airvalve flaps. Tightening up the secondary air valve spring too much will break it.

Re: Mid-Range Hesitation Flat Spot

The pulloff should be new as I bought it from I5 last November. If I push in the lever that comes out off the pulloff it takes a second for it to come back out. It holds vacuum if I pull off the hose that goes to the carb and push in the lever and hold my finger on the hole.

I ordered a 'G' Secondary hanger from Summit Racing the carb came with 'J' and tested it tonight with CL rods it had a bog still so I put in DL rods that came with the carb and the bog was almost gone a whole lot better. I adjusted the tension on the secondary butterflies a little more 1/8 turn and the bog was pretty much gone. I tried one more adjustment on the tension and ended pulling the spring off the rod which means I had no acceleration back to the dock. It was getting dark anyway. So I went home and pulled the top of the carb off and hooked the spring back up and adjusted it to 7/8 of a turn from first contact and I will test tomorrow.

Re: Mid-Range Hesitation Flat Spot

does the primary power valve piston move smoothly down from finger pressure and pop back up from spring pressure when the top is off the carb, I have seen the power valve piston stick down (closed) ,this will give you a flat spot

Re: Mid-Range Hesitation Flat Spot

Yes on the primary power valve.

One other thing re: Rochester carbs. Check your accelerator pump spring. Many of the new kits coming out use and automotive spring on this assembly. This is way too weak. If this is your case find and old stock spring and use which ever you can find that is stronger.

Re: Mid-Range Hesitation Flat Spot

Check that the accelerater pump is working. had the same problem on one of my boats.

Re: Mid-Range Hesitation Flat Spot

When I had the top off last night the PP moved up and down smoothly. If I pushed it down it would pop right back up. The accelerator pump pumps 2 strong stream into the primaries. I will be taking it back out either tonight or Thursday and retest / tune. I was pretty close last night in getting the flat spot out completely. Thanks for all the suggestion hopefully I get it tuned this week it is embarrassing to have friends on the boat that are boaters and have that flat spot when pulling a tuber.

Re: Mid-Range Hesitation Flat Spot Updated

Re: Mid-Range Hesitation Flat Spot Updated

During my Winterization of the boat I checked the timing and it was way off. The light was showing the timing mark was not even on the the marks. Looking down on it the mark was way left of the timing plate maybe 1/2" from it. I rotated the distributor CCW a little and it dialed right into 8? BTDC. I also noticed the gasket between the carb and intake was thick 1/4" so I went to the local boat store the gasket should have been 27-52457 which is about 1/32" thick.

So maybe between this two finds maybe I have solved my problem. I will not know until Spring which is end of March here or maybe longer because it has been really cold the last few weeks here in Atlanta.

Thanks for everyone's help so far