Hesitation / Bog under acceleration
 

Just installed a new motor in my 24' Sunsation... and had to pull the intake manifold and replace a set of lifters due to one of them being bad and knocking.... Prior to that the motor seemed to idle and be great under throttle (though only went for a quick 15 minute ride till I found the knock).... Threw it all back together, and while in neutral, if I rev the motor up to 2500 or 3000.... and then if I rev it some more it will hesitate momentarily and then rev up.
Took it for a ride and then it doesn't hesitate under throttle... It actually runs killer and accelerates great.
However it does hesitate, kinda almost like bog like a car would... when coming off a dead idle (when in gear).
I dont think its something with the timing, BC I tried adjusting timing from 8-12 btdc at idle... and 32-36 at 3000... and still same result.

Is this probably a bad power valve or something else I should be looking at??

TIA - Chris

In a car or my bike I would look at accelerator pump.

sounds like acc pump or incorrect power valve.did you put a vacuum gauge on it and check how much vacuum it has at idle.if you have 12 inches of vacuum you need a 6.0 power valve.whatever your idle vacuum is half of that is a good starting point for the power valve.maybe mild thunder will chime in,he is a master at tuneing carbs.

I can't tell you how many times I've seen carbs where the accelerator pump linkage is not touching the accelerator pump arm, after someone has taken apart a carb.

ie: the arm from the floatbowl is pointed up, and the linkage (w acc pump spring) is not setting on that arm.

edit in: double pumpers - and thus why named so, have two of these. One in front, and one in rear of carb.

Easy to do if you don't know to look for it when re assembling.

http://image.mustangandfords.com/f/m...olley-carb.jpg

What kind of carb is this? Holley, quadrajet, etc?

Typically sounds like an accelerator pump issue. But, if you're "easing" up on the throttle and it falls on its face, or sneezes back thru the intake, it could be a lean condition as well. I'd also make sure you don't have a vacuum leak since you just had the intake off too.

Thanks for the all the replies guys....

This is a Demon Carb... I think a speed demon. After reading your posts, I went and checked out the accelerator pump... and it was connected properly as SB suggested to double check. Actually I found that the adjustment screw was pressing down on it while at idle. Where as the secondary acc. pump had no tension and could be slightly jiggled around, which I believe is the correct adjustment?

I backed the primary acc. pump adjustment screw 1 full turn back, but it still seemed to be putting some slight tension on the arm... After that I ran it and still seemed to have that hesitation! Also worth mentioning, that it still does the same thing at 2500-3000RPM, only when in neutral.

I guess I really need to back that screw off till its just got some slack in it... is that correct!? Do you guys think that its dumping too much fuel too quick?? I wonder why I didnt notice this problem when I first took the boat out... maybe I need to check the intake for leaks, but I silicon'd the **** out of the front and back of it...

Im also going to hook up a vacuum gauge and make sure power valve is correct... Tho from what you guys are saying and a Holley video I watched, doesnt seem like it would be the issue?

Check the float level. I just had to readjust mine. My thought is we are getting more than 10% ethanol these days and it's less dense causing it to dump fuel into the carbs. Just went through this last weekend. Take the flame arrester off after running and see if it is dumping into carb.

What is the cfm? What motor? Specs of motor?
Maybe too large of a carb? Wrong squinters? Are you running 30 or 50 accelerator pumps?

Did you check to see if when advancing the throttle lever, you are getting a good squirt of fuel from the squirters in the carb?? Adjustment is one thing, a bad accelerator pump diaphram, clogged orifice to the squirters, or clogged squirter, can be a whole separate thing.

Siliconing the front and back of the intake doesn't really have anything to do with a vacuum leak.

When you had this intake manifold off, to replace the lifters, what was your procedure for adjusting the valves after replacing the lifters?

850cfm, BBC Mark IV - 489 Stroker, 9.0:1cr,
Intake Duration at 050 inch Lift: 236
Exhaust Duration at 050 inch Lift: 244
Intake Valve Lift .588
Exhaust Valve Lift .595
Lobe Separation (degrees):114

I was thinking about maybe trying larger squirters after watching a Holley video saying too small of squirters can cause a lean condition and bog the motor.... Not sure what size the acc pumps are though...


Forgot to mention that I did have a good deal of gas when pressing on the lever...
To adjust the valves, I took the longer more painful method, but knew itd be good... I watched the cam through the block, and waited till the lifter was on the flat part of the lobe, then tightened the nut till the pushrod had just a bit of tension, then gave it an additional 3/4 of a turn.

Where should I be looking for a vacuum leak then??

Thanks again.... Chris

It's not a vacuum leak. If it had a vacuum leak it would be difficult to keep it idling. This is what I would do. Run the boat up til it starts to stumble. When it starts to stumble vigorously pump the throttle. You have to do it very quickly. What this will do is keep activating the accelerator pump. If the stumble quickly goes away (faster than normal) you will know that you have a lean condition in the carb causing your stumble. I would do this and see what happens because you very well could have an ignition issue.

Black Baja... I was doing something similar the other day and it actually caused the motor to back fire.... That and it really only does it if you are initially on the throttle quickly. If you ease into it, it barely does it...

That said Ill try to give it a shot next time im there...

I've had many vehicles with vacuum leaks that had no trouble idling. Sometimes a little high, or a lot high , depending on the leak.

Every engine has a vacuum leak

a large vacuum leak can cause the engine to die at idle.a small vacuum leak will increase the idle speed.

Mine seems to idle quite well.... nothing can ever be simple... lol

I didn't read that it idles ok. Never mind my post lol.

two things that are probably common here. Throttle position. In nuetral, at 2500-3000RPM, you are just off the idle circuits. IN GEAR, just above idle, you are just off the idle circuits.

The idle circuit still plays a part in the transition until the main circuit becomes active. If the idle circuit is too lean, you can get a chitty transition, or "dead" spot. I've seen guys tune carbs on boats for the cleanest idle in nuetral, by turning the mixture screws in too far. Then, as they are trying to get on plane, the engine coughs back thru the carb. in these scenerios, backing the idle mixture screws out to enrich the idle circuit, solved the "lean sneeze" problems they had.

you may need an acc pump cam that starts the shot earlier.

Damn... might be my issue MILD THUNDER. I had adjusted the 4 corner idle screws after going for that first ride... I turned them in a 1/4 turn each BC it seemed to he running a lil rich... that said after I ran into this hesitation I turned them 1/4 back out... maybe I'll try going out some more

It really sounds to me like you have an ignition issue.

You can take the screws out and throw them in the lake/river or whatever it's not going to make it backfire. When it backfire it's because you have ignition when an intake(s) valve is open.

a lean condition can cause a backfire.my money is on poor carb adjustment.the engine can also backfire from not enough timing at low rpm.

Some guys skipped the high school automotive class and went straight to the spintron.

A lean condition CAN CAUSE A BACKFIRE thru the carb.

Yeah excessively lean condition. 2500-3000 rpm just coming off idle circuit? Yeah ok.

joe,doe you know what PPIOFAC stands for?

Yea, in NUETRAL I don't know about the engines you build, but I can rev my engines in nuetral, to 2500 RPM with very little throttle blade opening.

cdanial525,post a pic of the plugs.

Have fun with it guys. Wait a minute why don't you tell him to go and buy a wideband and a Quickfuel carb. And he will be good to go...

Why dont you tell him how to melt your pistons several times and lose control of your overpowered 20ft boats and crash them into the rocks, barrel roll them, and then tell everyone about your nascar, world of outlaw , plug reading expertise.

Joe, please more research get that facts straight please!

Here we go again.....:nhl_fight:

you are amazing.why don,t you look at a pic of his plugs for a real assessment,lol.just more of your wrong information.spintron testing for a pleasure boat engine,REALLY SMART.

Chris , here's a quick article explaining acclerator pumps. Black Baja, you may want to get your magnifying glass that you read plugs with, and take a look at this as well.

http://www.stockcarracing.com/techar...mp_adjustment/

No, I wouldn't do that Mike. What I have learned from you and Joe on OSO is. Bolt on a Quick Fuel carb run a wideband and go. No need for anything like that. Why would you ever pull a spark plug and read it? Makes no sense.

Joe, sorry but this is a Marine site. Please, that's stock car. Got anything Marine oriented?

I'm actually enjoying this one.

Ha !

I'd definitly try that. I've seen it before .

Lets say you find the stumble to go away by doing that, but the idle is too rich. You may try , keeping the primary's backed out further than the secondarys to even things out. Help clean the idle up, but yet still keep the primary idle circuit rich enough not to sneeze back at you.

:food-smiley-007::food-smiley-007:
edit you all know a lot more engine wise than me so I do like the different perspectives, carry on.

you will never learn anything from myself or joe because you already know everything,no need to learn.in this day&age only a fool would tune by plug reading only when afr gauges are cheep and accurate.we recommend quick fuel carbs because they are quality american built carbs that are very tunable.we recommend the use of a wideband because they are acurate.we also look at the plugs,but that is just one tool in the toolbox.and in one of your posts did or did you not say you plan on spintron testing for valvetrain for your boat engine build?

bb,you seem to have a dislike for quickfuel carbs,can you spread some of your wisdom and tell us why?